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A project of Volunteers
in Asia
. .
Technology for Solar Enerav Utilmt
ion
UNIDO Development and Transfer
of Technology
Series No. 5
Published by:
United Nations
Organization
P.O. Box 300
A-1400 Vienna
Austria
Industrial
Development
Available
in English,
French and Spanish.
Paper
copies are free to local groups in developing
countries,
$10 to others.
Ask for document number
ID/202.
Available
fram:
United Nations
Organization
P.O. Box 300
A-1400 Vienna
Austria
Industrial
Development
Reproduced by permission
of the Department of
United Nations Industrial
Public Information,
Development Organizaion.
Reproduction
of this microfiche
document in any
form is subject to the same restrictions
as those
of the original
document.
JNIT33D
NATIONS
INDUSI’RIAL
Development
DEVELOPMENT
.
and Transfer of Technology
NO.
GY
UNITED
NATIONS
OF$GANIZATION
Series
UNITED NATIONS INDUSTRIAL DEVELOPMENTORGANIZATION
Vienna
Development and Transfer of Techndogy Series Ns. 5
TECHNOLOGY
FOR SOLAR ENERGY
UTILIZATI
UNITED NATIONS
New York, 1978
The designations employed and the presentation of the material in this publication do not imply the
expression of any opinion whatsoever on the part of the Secretariat of the United Nations concerning the
legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its
frontiers or boundaries.
Mention of firm names and commercial products does not imply the endorsement of the United
Nations Industrial Development Organization (UNIDO).
Material in this publication may be freely quoted or reprinted, but acknowledgement is requested,
together with a copy of the publication containing the quotation or reprint.
reface
As a result of the energy crisis, attention has been focused on the need, not only to conserve
conventional sources of energy, but also to explore non-conventional sources of energy such as
solar radiation, winds, biomass, tides, geothermal sources etc. There is no reason why these
important natural resources should not be exploited for the benefit of the developing countries,
particularly
solar radiation, which is relatively abundant in most developing countries.
co-ordinated activity in the exploitation of non-conventional sources of energy :vould contribute
considerably towards the developing countries’ achieving their target of a 25 per cent share in
world industrial production by 2000. The role of the United Nations Industrial Development
Organization (UNIDO) would be, in keeping with a recommendation oT the Round Tabl$:
Ministerial Meeting on Industrial and Technological Co-operation among Developing Countri.-5.
held at New Delhi in January 1977, to launch programmes of co-operation in applied rese:t:ct;
and development activities in the energy sector of industry, drawing upon the machinery an!,
capabilities already available in the developing countries.
This volume deals with the technology of exploitation of solar energy for the benefit :,f the
developing countries. It is hoped that with the further improvements in this technolog;
-*jar
energy will supply not only the certain needs of industry but also the everyday needs 0;. ;he
population in the rural and remote arcas of those countries. The text of the volume is primarily
based on the contributions made to the Expert Group Meeting on the Existing Solar Technology
and the Possibilities of Manufacturing Solar Equipment in Developing Countries, organized by
UNIDO in co-operation with the Austrian Solar and Space Agency (ASSA) and held at Vienna,
14-18 February 1977.’ In addition, it incorporates information obtained by UNIDO as a result
of its field contacts in many developing, as well as industrialized, countries. Because of the
complexity of soIar energy technology and the accelerated research and development taking place
in solar energy in many parts of the world, this treatment of the subject cannot be exhaustive.
Nevertheless, as a first step by UNIDO in this field, it should be an important aid to interested
Governments of developing countries in acquiring the latest trends in the area of solar technology
and in providing a basis of national action.
The first part of this vcLume contains two papers: a recommended programme for solar
utilization in developing countries, prepared by the Senior Interregional Adviser on Engineering
Industries at UNIDO, and a background document on the utilization of solar energy in
developing countries, prepared for UNIDO by a consultant. The second part consists of
summaries of work being done in some countries and institutions, and the third of 17 technical
papers dealing with the conversion of solar energy into mechanical or electrical energy, the design
of solar collectors, the utilization of solar energy in heating, cooling, distillation, drying and
cooking, and the transfer of technology.
Throughout the volume, the views expressed are those of the contributors and do not
necessarily reflect the views of the secretariat of UNIDO.
In the context of the forthcoming United Nations Conference on Science and Technology
for Development, to be held at Vienna in August and September 1979, several developing
countries are devoting attention to the application of technology for meeting some of their
energy needs. This study, along with others to be prepared by UNIDO, is intended as a
contribution to the conference and its preparatory activities.
’ The recommendations of the Meeting are in annex I on page I47
V
CONTENTS
Part one
Utilization of solar energy in developing countries
Page
Solar energy: A recommended programme of action for developing countries
Secretariat of Uh?i?DO . . . _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Development of solar energy utilization in developing countries
AssadTakla . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
7
Part two
Summaries of country and institutional progwnmes on solar energy
Country programrnes ...............................................
41
Institutional programmes ..............................................
48
Part three
Selection of technical papers on the utilization of solar energy
A IO-kW solar electric power plant
Hans Kleinrath . . . .
. . . . . . . . . . . . . _ . . . . . . . . . . . . . . . . . . . . . . . . .. .
Conversion of solar into mechanical or electrical energy: Indian experience
V.GBhide _....................................I.........
Utilization of solar energy in the development of arid zones: Solar water pumps
Jean Paul Wand, Max G. C’lemot, J. Pierre Girardier and Marc Y. Vergnet
53
55
. . . . . . . .. .
62
Theoretical conditions for maximum power from the sun
Kamal-Edin Hassan . . . . . . . . . . . . . . . . . . . _ . . _ . . . _ . . . . . . . . . . . . . . .
65
Solar flat-plate collectors
A.A.M.&yigh
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
66
Aspects of solar-heated swimming-pools
Cangolf Brtit&ich . . . . . . . . _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
72
A solar energy system for greenhouses
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
M.Posnansky
75
Solar timber kilns: Their suitability for developing countries
RA.Rumptre
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
78
Solar refrigeration and cooling
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . _. . . . . . . .
ErichA.Farber
89
Solar space heating and cooling and solar water heaters
V:G.Bhide...............................................
92
vii
3,
‘1.
.“1
.?j: ;,:
Part three (continued)
,c
,_
.’
,
Page
Preliminarydesigndata for a solarhousein Riyadh
. . . . . . . . . . . . _. . _. . . .
101
A. Eggers-Lura . _ . . . . . . . . . . . . . . . . . . . . . _ _ . . . . . . . . _ . . _ . _ . . , . .
107
A.A.M.SayighandE.M.A.El-Salam
.. .... .. . .
Solar refrigerationin developingcountries
Solar distillation: The state of the art
B. W. lleimat
. . . . _. . _ . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . ,
113
. ., ...... . .. .. . ....... .. . .. .. . ..
119
Solar water distillation
Gvio Mustacchi and Vincenzo Cena
The potential of solaragricultural driersin developingareas
T.A.Luwand
. . . . . . . . . . . . . . . . . . . . .._..........._...__...._
The potential of solar cooking in developingartas
T.A.Luwand
. . . . . . . . . . . . . . . . . . . .._._...._._........._....
Assessmentof solarapplicationsfor technologytransfer
Jyoti K. Parikh . . . . . . . . . . . . . . . , . . . . , _ . . _ _ _ . _ _ . _ . . . . . . . . . . _ .
125
133
137
Annexes
I.
Recommendationsof the Expert Group Meeting on the Existing Solar Technology and the
Possibilitiesof ManufacturingSolarEquipment in DevelopingCountries ...............
147
II.
Current IEA projects in solarenergy ....................................
150
III.
Solarenergyinformation systems .....................................
152
IV.
Institutions involvedin solarenergydevelopment ............................
155
_.f
Vlll
EXPLANATORY NOTES
Referencesto dollars ($) are to United States dollars, unlessotherwise stated.
A slashbetween dates (e.g., 1970/71) indicates a crop year, financial ye&r or academicyear.
Use of a hyphen between dates (e.g., 1960-1965) indicates the full period involved, including the beginning
andendyears.
A full stop (.) is used to indicate decimals.Thousands, millions etc. are ser fCby spacesunless a symbol for a
monetary unit precedesthe number.
Referencesto tons are to metric tons (tonnes), unlessotherwise specified.
The following notes apply to tables:
Three dots ( . . . ) indicate that data are not availableor are not separately reported
A dash (-) indicates that the amount is nil or negligible
A blank indicates that the item is not applicable
Totals may not add precisely becauseof rounding
Besides the common abbreviations, symbols and terms and those accepted by the International System of
Units (SD, the following have been used:
Technical abbreviations and symbols
(with approximate equivalents)
one thousandth of an inch
inch (1 in. = 2.54 cm)
revolution per minute
;pd”
horsepower (1 hp = 746 W)
atmosphere (I atm = 1.013 bar)
atm
board foot (1 bd ft = 2 360 cm3)
bd ft
gallon (US) per minute (1 gpm = 0.063 l/s)
am
pound per squareinch (1 psi = 0.069 bar)
psi
pound per squareinch (absolute)
psia
pound per squareinch (gauge)
psig
gal (US) United States gallon (1 gal (US) = 3.785 1)
gal (Imp) British Imperial gallon (1 gal (Imp) = 4.546 1)
foot (1 ft = 30.5 cm)
ft
British thermal unit (1 Btu = 1.055 kI)
Btii
calorie (1 cal = 4.186 I)
Cal
weight per cent
Wt%
polyvinyl chloride
PVC
polyvinyl fluoride
PVF
polytetrafluoroethylene
PTFE
acrylonitrile-butadiene-styrene
hBS
R and D researchand development
mil
in.
Organizations
United Nations bodies
Food and Agriculture Organization of the United Nations
FAO
UNEP United Nations Environment Programme
rUNESCOUnited Nations Educational, Scientific and Cultural Organization
UNICEF United Nations International Children’s Emergency Fund
Other
E&WA
IEA
IIASA
Energy and ResearchDevelopment Administration
International Energy Agency
International Institute for Applied SystemsAnalysis
ix
Part one
UTILIZATION
OF SOLAR ENERGY
IN DEVELOPING COUNTRIES
., :
.
Solar energy:
A recommended programme
for developing countries
Secretariat
of action
of UNIDO
As will be seen in the next article in this volume,
there are definite possibilities of solar technology
application for water distillation, water heating,
drying, cooking, refrigeration and air-conditioning,
and the conversion of solar radiation into mechanical
and electrical energy. The essential need is for the
development of a programme for applied research and
development and eventual manufacturing activity.
with emphasis on technology transfer from industrialized countries, domestic promotion of research
and development capabilities and co-operation among
developing countries, in the field of solar technology.
Although the basic concepts in solar technology are
centuries old, their industrial and commercial
application have only recently received any significant amount of interest in industrialized and
developing countries.
The economic analysis of solar technology and
equipment requires a different philosophy and
approach. As commercial manufacture is still in the
early stages and not at all widespread, the optimum
economic level of production cannot be established.
Certainly, the initial cost of solar equipment is higher
than that of similar, conventional products. However,
the operating cost of solar equipment is far less.
The major problems in solar technology development, product design and product manufacture are:
(a) The technology is new and there is a lack of
knowledge on what it can do and how it can be used;
(b) Solar products and equipment are not yet
readily available on a commercial basis on an
extensive scde; and
(c) The equipment initial cost is high, probably
because the technologies are still mainly experimental
and no economy of scale can be realized.
industrialized countries that have engaged in applied
research, developed sound technological concepts,
and tested and transformed them into manufacturing
prototypes, with significant emphasis on the needs of
developing countries. Therefore there is a need for
twinning these research and development institutions
in selected industrial and developing countries in the
field of solar technology. Thirdly, there is a need to
develop integrated programmes of action within the
framework of co-operation among developing
countries. In this connection, certain institutions in
developing countries should be selected for strengthening and transformation into “centres of excellence”
that can provide assistance to other developing
countries.
In connection with the need to develop a
programme of information extension and popularization of solar technology in the developing
countries, the Governments of the developing
countries might take the necessary steps to initiate
and support a modest programme in solar technology
in a local institution, with a view towards launching
an effective research and development programme
with co-operation from other institutions and
appropriate developing countries, as well as from
bodies of the United Nations.
Any programmes that are so established should
suit the needs of the individual developing countries
and promote regional and interregional co-operation
in the field of solar technology. That applies to
information collection and analysis, information
extension, applied research and development, evaluation and manufacturing promotion.
A limited number of solar products are
commercially manufactured, and these in only industrialized and a few developing countries. There is a
need for more information, evaluation of performance on site in interested developing countries, and,
eventually, investment promotion. In addition, there
are a number of institutions in both developing and
Although there is an awareness in the developing
countries of the need to develop alternative sources
of energy, there appears to be an absence of
integrated policies and work programmes directed
towards that end. It seems that in most developing
countries research and development in this area is
carried out on an nd hoc basis.
The prerequisites
3
.4
In most developing coun:, j, the major problems in the utilization of nonconventional sources of
energy fall into three groups: One is the lack of
information on the state of the art; the second relates
to the appropriateness of the methods used to apply
those sources of energy in the conditions that prevail
in the developing countries; and the third concerns
the reliability of the technology that the developing
countries would have to import. In addition, the
formulation by developing countries of integrated
practical policies for these alternative sources of
energy has to cover the establishment of appropriate
institutional facilities for applied research and
development and the promotion of local manufacture, with due emphasis on technoeconomic
analysis, field testing and extension services.
UNIDO, in consultation with experts in this
field, has identified these areas about which
developing countries lack adequate information:
(a) The criteria on which policy should be
based, particularly in regard to energy utilization in
rural areas;
(b) The appropriate governmental structure and
machinery for development planning with respect to
non-conventional sourcesof energy;
(c) Indigenous technology relating to the
application of nonconventional sourcesof energy;
(d) Local potential for engineering development
and manufacture of production equipment, as well as
product demand analysis;
(e) Appropriate institutional facilities, work
programmes and technical manpower for (c) and (d).
Special equipment to harness solar radiation
must be made available. As with other products, the
equipment may be obtained by importation or local
production. However, solar energy exploitation
provides a unique opportunity for co-operative
programmes between industrialized and developing
countries, aswell as among developing countries.
Therefore, if eventual local manufacture of solar
energy equipment is the ultimate goal of developing
countries, then applied research and development is
an important integral element in eventual ,local
manufacture, operating in two ways, by local design
and development and by adaptation of imported
technology. In either, co-operation among deve1opin.g
countries and technology transfer from industrialized
countries to developing countries plays an important
part. In addition, international assistanceis required.
There are many prerequisites for international
co-operation in solar energy research and development. Three require action by the Governments of
developing countries themselves:
Elaboration of a national energy policy
Evaluation of the role to be played by
non-conventional sourcesof energy
Realistic analysis of the contribution of solar
energy to that role
Technology for Solar Energy Utilization
As the next step, a narional institution should be
designated for c>rrying out the applied researchand
development, tpchnolc a’ transfer, extension and
manufacturing promotion activities. Such a programme should consist of not only technical
evaluation of imported technology and hardware
products but also local development of domestic
technology and products. Such an integrated activity
should concentrate on adaptation, prototype fabrication, testing, techno-economic analysis, and eventual
local manufacture of appropriate products. The
programme should also include technical manpower
development, technology transfer, co-operation
among developing countries, and the physical
facilities should include technological research,
engineering design and development, laboratory and
field-testing facilities, prototype manufacturing facilities and a manufacturing extension programme.
Recommendedprogramme
Taking into account the world-wide state of the
art and the capabilities of the developing countries,
there is a need for developing countries to draw up a
the immediate
three-phase programme for
(1980.1990), intermediate (1999-2000) and long
(2000 and beyond) term.
To elaborate the programme the developing
countries need the answersto two questions:
What is the state of the art in solar energy in
industrialized countries?. What may be achieved,
and when?
Which technologies could be applied to the
immediate needs of developing countries and
which require more time for effective realization,
commercial exploitation and utilization?
Based on the analysis of the available information, it is recommended that the programme of
applied researchand development and manufacturein
developing countries in the three phases be as
follows:
1980-1990. Programmes for immediate local
commercial exploitation and local manufacture of
solar energy equipment based on well known and
accepted low-temperature technology. This may be
flat-plate collectors and solar equipment such as solar
water heaters, grain driers, spaceheaters, cookers and
water stills. These are the only products that can be
considered for local manufacture. The applied
research and development programme in this case
would involve production of commercial manufacturing prototypes and manufacturing promotion.
The international assistancein applied research and
development may be information dissemination,
prototype exchange, assisting applied research and
development institutions, and manufacturing promotion, with emphasis on cost, realizability and
acceptability.
&: ,;‘I, ‘:’ *
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.-
S&r energy: A recommended programme of ae,tion for developing countries
199Z-Z999. To be ready for possible commercial
exploitation of solar energy by 1998-2000, the
developing countries may have to start applied
research and development programmes on mediumand high-temperature technology now. This will be
primarily based on concentrating parabolic collectors
with or without a tracking system. The solar products
may be solar coolers, refrigerators, water pumps and
small electric generating plants. Although the
principles are known, the work is still in the
experimental and prototype stages even in industrialized countries. The solar pump and generating
systems that have been installed in a few developing
countries by industrialized countries are still prototypes, and the exact commercial opportunity with
special reference to local manufacturing potential in
developing countries is yet to be assessed.Therefore,
the applied research and development programmes of
developing countries will have to be carefully
evaluated. They will have to concentrate on keeping
track of research and development work being done
in industrialized countries; evaluation, adaptation and
absorption of imported technology; and regional
co-operation among developing countries. In this
connection an effort should be made to prevent
duplication of work, and a mechanismwill have to be
established to develop exchange of research and
among developing
development information
countries. As researchand development is expensive,
the developing countries should be selective, within
the framework of national requirements and priorities. It may be worthwhile to consider co-operative
research and development programmeswith emphasis
on effective prototype fabrication at selected regional
centres of selected developing countries. In addition,
such results may be made available free to other
developing countries through a network system. That
could result in local product performanee evaluation
and possibly commercial manufacturing promotion.
2000 and beyond. In order to be ready to
effectively utilize the commercial opportunities that
may arrive in the “solar era” of 2000 and beyond, the
developing countries should establish a judicious
research and development programme now. However,
it must be understood that it is industrialized
countries that are engaged in this sophisticated
high-temperature technology. The eventual products
are optical transmission technology, solar power
towers and complex technology involving solar cells,
semiconductors, photovoltaic conversion and material
SCieii~* This research and development is expensive.
The industrialized countries themselvesdo not know
the answers, although the direction is known. The
role of developing countries is therefore complex.
However, this field has a great potential for
international co-operation, especially between industrialized and developing countries. There is a need
for sharing of efforts and results; twinning of research
and development institutes in industrialized and
5
developing countries on specific applied research
programmes should be established. The prerequisites
for such a programme in developing countries are:
(a) Identification of local institutions that have
the potential for undertaking such a co-operative
research and development programme under a
twinning of institutions scheme. Existing institutions
may have to be strengthened and personnel trained;
(bi Exchange of information, initiation of
fellowships and organization of seminarsand training
programmes.
Integrated action in the United Nations system
Taking into account the three-phase programme
detailed above, the most irnportant aspect for
initiation of applied research and development
programmes is to establish realistic targets based on
the present and probable future state of art. There is
a need to be selective in applied research and
development programmes. The eventual aim is to
promote local manufacture. But that requires various
prerequisites; international co-operation in this field
will be in the strengthening of existing institutions,
promotion of technology transfer, training and promotion of co-operation among developing countries.
In principle, the integrated programme of action
.in the United Nations system incorporates the
following two areas:
(a) Evaluation of the existing technology in the
field of solar energy in order to define the
for t& developing countries;
appropriate teChnOkJgy
(b) Elaboration of a programme of technical
assistanceaiming at developing the utilization of solar
energy, intensifying research and development,
initiating the manufacture of equipment and
strengthening the transfer of technology among
developing countries.
The programme will require the following steps:
Identification of institutions in developing
and developed countries
Practical research and development programmes
Development of prototypes, specifications
and standards
Manufacturing promotion
Technology transfer from developed
countries to developing countries
Co-operation among developing countries
UNIDO activities in non-conventional sources
of energy
The UNIDO programme in non-conventional
sources of energy is in the areas of solar, wind,
pyrolysis, biomass energy now and may incorporate
micro-hydropower, geothermal energy and photosynthesis later. The emphasis in assisting the
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developing countries is on the adaptation and
extension of imported technology, the promotion of
applied research and development, and the extension
of domestic technology with special reference to
promotion of eventual local manufacture of equipment together with development, of the necessary
infrastructure. In this connection, significant
emphasis is given to the promotion of co-operation
among developing countries.
In early 1977, UNIDO organized, in co-operation
with the Austrian Solar and SpaceAgency, an expert
group meeting on the existing solar technology and
the possibilities of manufacturing solar equipment in
developing countries. (See preface and annex I.) The
conclusions and recommendations of this expert
group may be regarded as the basis of the UNIDO
programme of action and technical assistance
activities in the field of solar technology.
Technology for Solar Energy Utilization
PossibleUNIDO assistanceto developing
countries
UNIDO, at the request of the Governments of
developing countries, is prepared to assistthem in the
promotion, development and utilization of solar
energy along the lines recommended above. The
components of UNIDO technical assistancemay be
experts, equipment or fellowships for specific,
approved projects. Normally, these components are
financed by funds allocated by the United Nations
Development Programme (UNDP) according to the
Indicative Planning Figure (IPF) for each developing
country for a given programme cycle. (The next cycle
is 1978-1982.) Therefore it is recommended that the
Governments of the developing countries allocate a
priority for UNDP-IPF technical assistanceprojects in
the field of solar energy. In addition, UNIDO has
modest funds availablefor direct assistance.
Development of solar energy*, utilization
in developing countries
Assad Takla
Afamia Consulting Engineers, Abu Dbabi, United Arab Emirates
Introduction
History shows that solar energy has been utilized
for a long time, but it was only in the last century
that such equipment as boilers fitted with mirrors,
steam engines, hot-air engines and cookers came into
being. The intensive development of thermal and
electrical engines and the extremely low cost of
energy, especially that imported from the third
world, discouraged research in the field of solar
energy to someextent. Now that the cost of energy is
reaching a normal level and the discovery of new oil
resources is becoming rare, industrialized countries
are launching intensive research programmes in solar
energy. For example, -he Energy and Research
Development Administration (ERDA) in the United
States of America had a budget of $115 million for
the fiscal year 1976. The projects in this field of the
International Energy Agency (IEA), whose member
States are all industrialized countries, are described in
annex II on page 150.
Some methods of utilizing solar energy have
reached a stage of development where they can
compete economically with methods of using
conventional energy sources. Since developing
countries are often situated in sunny regions, it is in
their own interest that they should develop the
utilization of solar energy, which is free, inexhaustible, omnipresent (no transport or distribution problems) and non-polluting. This energy could
be converted into mechanical, electrical or chemical
energy to be used in various fields, such as the
production of electricity, desalination of water,
irrigation, cooking, food preservation by means of
refrigeration, drying of fishery products, fruit and
vegetables,spaceheating, and air-conditioning.
The purpose of this study is to give an account of
the development of researc’hon solar energy and its
utilization from the techno-economic point of view.
It aims mainly to throw light on the principal issues
related to the utilization of solar energy by
developing countries, and it is hoped that it could
serve as a first guideline for technicians, economists
and policy makers in those countries.
.3ere has been a proliferation of commercial
companies in the field of solar energy. Unfortunately.
some of them have asked extremely high prices for
the transfer of solar technology of doubtful value.
Most developing countries therefore need a tool that
can help them to improve their position in
negotiating the transfer of solar technology for R and
D purposes. This study is the first attempt at
providing such a tool. For more details concerning
one aspect or another of the study, more specialized
references should be consulted. An annotated
bibliography of important sources of information is
provided. Annex III, on page 152, which describesinformation systems,and annex IV, on page 155, a list
of institutions involved with solar energy, should also
be consulted
Attention has here been focused on the shortand medium-term prospects becauseavailabledata are
not good enough to serve as a basis for valid
long-term projections. However, because of the
accelerated change in technology,it is also felt that a
study of this nature should be repeated periodically
and that the specific field of utilization of solar
energy in developing countries should be discussed
periodically in expert group meetings.
This study is neither a manual nor an extensive
and detailed survey of all aspects related to solar
energy utilization. Its chapters are not balanced; in
general more importance has been given tQ fields
which have not yet been popularized.
Chapter I describes the general applications of
existing technology and includes information gleaned
by the author in visits to R and D centres and at
international meetings. In the second chapter some
general techno-economic comparisons are made to
show which solar equipment could be economically
utilized in the short- and medium-term in developing
countries. General equations for the comparison are
introduced and an example of their use is slaborated.
The author has visited some important centres of
solar and wind energy researchin developedcountries
(Canada, France, Germany, Federal Republic of,
Netherlands, United States of America) and developing countries (Greece, India, Mexico, Trinidad and
Technology for Solar Energy Utilization
I..
_
- -.
“_
Tobago). Some findings and evaluations based on
these visits constitute chapter III. Problems and
possible solutions and the general trend for
co-operation between developing and developed
countries and among developing countries are
discussed.
Except for the original work and the personal
appraisals, the author does not claim credit for the
information included in this study. Such information
is based on availabletechnical literature, brochures or
statements by manufacturers and on direct contacts
and &scussionsheld in specialized institutions.
“Medium” and “high” temperatures will therefore
refer to temperatures above IOO’C; in this case,
focusing solar collectors, which track the sun and trap
only direct solar radiation, are used.
Air can be heated to a relatively high
temperature by solar energy and used as the working
fluid in a solar engine. Two cycles can be used:
(a) closed (Stirling). The air is compressedin a
cold space. Then it is put into contact with a hot
source, where its pressure increasesand expands in a
power cylinder. From there it flows to the cold space
and the cycle is closed;
(b) Open (Ericson). Compressed air is introduced into a hot space. It then expands and exhausts
into the atmosphere.
I. DEVELOPMENT AND STATE OF THE ART
Low-temperature solar engines
Conversion of solar energy into mechanicalenergy
General considerations
The term “solar engine” designates an engine
operated by solar energy. The thermodynamic cycle
of such an engine may be as follows: Vapour is
obtained when a liquid working fluid is heated by
solar radiation. This vapour expands in a reciprocating or rotating engine, doing work. From the
engine it flows to a heat exchanger, in which it
condenses. The condensate is reinjected by a pump
(usually operated by the solar engine itself) to
another heat exchanger, in which it evaporates,
closing the cycle.
The efficiency of the engine depends first on its
Camot efficiency:
Tl - T;
k=
Practically speaking, the low-temperature solar
engine is restricted to temperatures lower than 80°C.
A working fluid (Freon 22, Freon 12, Freon 11,
Freon 114 or butane) is evaporated directly in
flat-plate solar collectors or by hot water obtained
from solar collectors and circulating in a heat
exchanger (e,vaporator). (See figure 1.) In its gaseous
phase, the u.orking fluid flows to and expands into a
reciprocating or rotating engine. From the engine it
flows to an air- or water-cooled condenser, from
which the working fluid, now a liquid, is reinjected
into the evaporator by a pump operated by the solar
engine itself. In some applications, when hot water is
used to evaporate the working fluid, a circulating
pump also operated by the solar engine is used to
accelerate the circulation of the hot water and
improve the heat transfer in the evaporator. In this
case,manual starting is necessary.
Tl
where T1 is the thermodynamic temperature of the
hot source (the evaporating heat exchanger in the
example) and T2 the thermodynamic temperature of
the cold source (the condensing heat exchanger).
It appears from this equation that, theoretically,
one should use the highest temperature possible for
the hot source and the lowest temperature possible
for the cold source. In a practical sense,however, T,
depends on the performance of the solar collectors
and on how high a pressurethe materials of which the
engine is made can withstand; for example, the
pressure of Freon 22 is already 20 bar at only 50°C.
And, T2 cannot be lower than the temperature of the
fluid used for cooling-water or air with natural or
forced convection.
No standards defining the range of low, medium
and high temperatures exist. In this study, however,
“flow temperature” means a temperature below
100°C. Flat-plate solar collectors capturing direct and
diffuse solar radiation operate in this range.
Reiniection
pump
Figure 1. Low-temperature solar engine with working fluid
evaporating in an evaporator heated with hot water from the
solar collector
Direct evaporation of the working fluid in the
solar collectors can be economical in small installations, but it would be very difficult to use the
method in large solar collectors because of the
difficulty of maintaining trouble-free circulation of
the working fluid in large installations.
9
Development of solar energy utilization in developing countries
SOFRETES engine
The Societi franqaise d’etudes thermiques et
d’energie solaire (SOFRETES), has already installed
or is installing about 50 solar pumps, most of which
are rated at 1 kW.
The technology, however, is not yet fully
developed. SOFRETES has tried butane and many
kinds of Freon, especially Freon 12 and 11, and now
seemsto be changing to Freon 114. Their technology
with respect to heat exchangers has changed.
Shell-and-tube condensers and evaporators were used
first, then tube-in-tube (coaxial) condensers,and now
plate heat exchangers similar to those used in the
food industry.
In one of the 1-kW solar-pump installations using
butane as the working fluid and 60-m’ flat-plate solar
collectors, the water outlet temperature of the solar
collectors is about 70°C. The temperature at the
outlet of the evaporator and at the entrance of the
solar reciprocating engine is about 67’C, and the
outlet temperature of the engine is about 50°C. The
condensing temperature in the condenser, cooled by
the pumped water, is about 30°C. In a good solar
radiation regime, such an engine could function about
6 h a day without solar storage, but it would not give
full power all this time. In another 1-kW solar-pump
installation, the engine entrance temperature is 55’C,
the engine outlet temperature, 40°C and the
condenser outlet temperature, 30°C.
SOFRETES, in collaboration with the Government of Mexico, has installed a 25kW solar power
plant in San Luis de la Paz. The electric generator is
operated by a turbine of 7 200 rpm driven by the
evaporated Freon 11 at a pressureof about 3 bar. The
working fluid entrance temperature is 57°C and the
outlet temperature is about 30°C. The Freon 11 is
evaporated in an aluminium evaporator with an
exchange surface of about 350 m2 and rate of
1 740 MJ/h with a water entrance temperature of
62°C and a water exit temperature of 58°C.
The e?aporator is fed with hot water coming
from solar collectors with a net effective surface of
I 200 m2. The gas is condensed in a stainless steel
condenser exchanging 1 590 MJ/h with an exchange
surface of about 100 m* . The condensed Freon is
reinjected into the evaporator bqha 3.kW reinjection
pump driven by electrical energy from the electric
generator.
The installation has been in operation for about
one year and does not present serious technological
problems, but the control system is very sophisticated.
V-2 solar wpour engine
Erich A. Farber of the Solar Energy and Energy
Conversion Laboratory of the University of Florida
(United States of America) has developed the V-2’
solar vapour engine, which uses Freon evaporating
directly in the solar collectors. It consists of two
cylinders at right angles to each other, each having a
bore of 5 1 mm and a stroke of 39 mm. Slid*- valves
control the vapour flow in and out of the cylinders
admitti.lg vapour for 90” of the flywheel rotation and
exhausting it for 140”. The engine, 25 cm high,
35 cm wide and 23 cm deep, is mounted in a housing
40 cm in diameter and 25 cm deep. The total
displacement of the engine is 305 cm3 per revolution.
The vapour is fed to the engine through the
housing and, after it has produced work, is exhausted
into the housing surrounding the engine. Thus any
leaks that may be present are not critical, since the
housing catches all exhausted and escapingvapours.
From the housing, the vapour flows to the condenser.
The speed of the engine is controlled by an
adjustable centrifugal flywheel governor, which regulates the vapour flow to the engine.
The water-cooled condenser used in connection
with this engine is a cylinder 76 cm in diameter and
61 cm long containing seven coils of 3.5-cm pipe,
giving a total length of 13.5 m. The vapour is
condensed in this pipe.
Entropy
S
Figure 2. ideal temperature-entropy
diagram
of the V-2 solar vapour engine
The operating conditions and the ideal temperatureentropy (T-S) diagram for the system are shown
in figure 2. Path l-2 represents the expansion of
vapour through the engine, which converts some of
the thermal energy into mechanical work, path 2-3,
the changes of state that occur in the fluid when it is
moving through the condenser, path 3-4, the pump
action raising the pressure to that of the solar vapour
generators, and path 4-1, the changes that occur in
the evaporator, completing the cycle. This samecycle
is presented for Freon 11 as the working fluid on the
pressure-enthalpy (p-H) diagram in figure 3. Conservative operating conditions that can readily be
obtained by such systemswere selected. Vapour at a
temperature of 72°C is delivered by the flat-plate
solar collectors; the liquid from the condenser has a
temperature of 28°C. The pressurescorresponding to
these temperatures are moderate and do not require
special design.
..
Technology for Solar Energy Utilization
I
239.6
57.9
Enthalpv
459.7
H (kJ/kg)
Sun Power Systems engine
Figure 3. Pressure-enthalpy diagram of the V-2 solar vapour
engine using Freon 11 as the working fluid (not to scale)
The changesof state of the air inside the engine
cylinders, on one side of the piston, are indicated in
the idealized pressure-volume (p-V) diagram of
figure 4. (In reality, the corners are rounded.)
3.35
-5
0
P
ii
- -I--,
1.78
:
------A.--------
lrh
1
/
I
!I
9.09
9.09
1
I
I
#
22.9
22.9
I
I
1
42.7
Volume
II
I
75.8
V (cm’)
Figure 4. Pressure-volume diagram of the air
inside the cylinders of the V-2 solar vapour engine
150
Supply pressure (bar):
g
z
2
100
B
50
0 3.46
Cl 3.09
2.74
A 2.39
0
0
riot)
Rotational
rprrd
Figure 5 shows the actual performance of the
engine with supply pressures held constant at 2.39,
2.74, 3.08 and 3.45 bar, pressures corresponding to
temperatures of 51°, 56”, 60” and 65’C, respectively,
for Freon 11. The curves are typical of engine
performance. Maximum speed is reached at no load,
and as the load is increased the speed drops. If the
power output is plotted against rotational speed,each
curve exhibits a maximum. Temperatures and
pressures higher than those shown can be obtained,
but only for a very short part of the day.
The combination of two cylinders in a compact
V arrangement makes this engine self-starting, which
is a distinct advantagewhen cloud cover is intermittent.
1000
(rpm)
Figure 5. Power-speed curves for the V-2 solar vapour
engine
Sun Power Systems has developed a rotating
engine designed mainly to use industrial waste energy.
However, the working fluid, namely, Freon, could be
evaporated by hot water obtained from flat-plate
solar collectors. The engine is based on the Rankine
cycle, A IO-kW power generation plant was tested at
Albuquerque, New Mexico (United States), by a
team of consultants working for the United Nations
Environment Programme (UNEP), before being sent
to a project executed by UNEP in Sri Lanka. This
unit is now operating at full power with 12 m3 of
water at 90°C entering the Freon evaporator hourly.
It is expected that a 276.m2 net effective surface of
solar collectors would provide the energy necessaryto
heat the water. In this plant two standard heat
exchangers manufactured by a refrigeration and
air-conditioning firm are used, one, with a 23-m2
heatexchange surface, as evaporator and the other,
with a 35 .2-m2 heatexchange surface?as condenser.
The engine runs at 1 800 rpm and its weight is about
80 kg. It drives a 60-Hz electric generator. The
engine, including the heat exchangers,the reinjection
pump and the electric generator, is quite compact.
The lo-kW power is for a 55°C difference
between the evaporating and the condensing temperatures, but in practice such a difference cannot be
obtained with a flat-plate solar collector; only a
difference of about 40°C can be expected with the
usual collectors of this type. The maximum expected
power will then be about 7 kW. As in the caseof the
SOFRETES engines, lubrication is ensured by a
lubricant dissolvedin the Freon. The actual surface of
the evaporator seemsto be insufficient, particularly
when the temperature of the hot water entering the
evaporator is about 70°C. According to the
manufacturer, one of his small prototypes has been
tested for lo4 h without significant problems.
However, the test was undertaken near the factory
and not in the field.
Gironnet-ENSAM engine
The Ecole nationale superieure des arts et metiers
(ENSAM) has developed a prototype low-speed
reciprocating engine delivering less than 1 kW and is
now negotiating with an industrial firm to undertake
the m&rufacture of a 2-kW prototype. The only
technical problem that has not yet been resolved Is
lubrication. The same system of lubrication as that
used by the SOFRETES engine or the Sun Power
Systemsengine could be used, but an attempt is being
made to develop a dry lubricating system, which is
believed would be better.
The cost of construction of the 2-kW prototypes
is estimated at $4,000, not including the solar
collectors, of which the required surface is estimated
at 50-60 ml in a favourable solar radiation regime.
The first prototype is being tested with
compressed air. Such a test will not permit a valid
evaluation. However, the design of the engine is
simple and its expected cost is relatively low.
Messerschmidt-Biilkow-Blohm
(MBB) eng’ne
Messerschmidt-Biilkow-Blohm GmbH, Ottobrunn,
Federal Republic of Germany, is working on a lo-kW
solar electric power plant. (See figure 6.) This plant is
to be an independent power station for remote rural
communities. Besides the required peak electrical
power of 10 kW, an energy reserve for night
operation is planned that has been specified to be
12 kWh at the rate of 1 kW. This requirement implies
an optimal energy storage system. The flat-plate
collectors used for about two years by MBB in
preliminary work on solar space heating will be used
as solar collectors. To achievethe desired peak power,
a total collector surface of approximately 700 m* is
required. MBB expects to be able to reduce the net
effective surface of the solar collectors to about
350 m*. However, the final specification of the
required surface depends strongly on the climatic
conditions and consumer requirements at the place of
installation and must be harmonized with the
required storage capacity over the 24-h working cycle
A
/Y
0
m
taking into consideration the partial-load behaviour
of all the plant components. A screw motor developed
by the Linde company, using R114 as the working
fluid, will be used because of its expected high
efficiency in the partial-load range. Other advantages
are low specific weight (weight-to-power ratio), small
bulk and absenceof valves.
The MBB flat-plate collector is a two-glass
collector of modular design. The outer dimensions of
the absorption surface of each module are
60 cm X 180 cm = 1 m*. The absorber is made of
roll-bond aluminium and is protected against
corrosion by an inhibitor. The outer absorber layer is
a thermal paint with a high absorption factor (0.96).
(The paint was developed for space applications.) The
rear heat insulation consists of a protected polyurethane-foam cover. The temperature of the hot water
could be as high as 95°C.
MBB has already assembled a prototype, which
has been given several short tests with hot water
supplied by an electric boiler. Some modifications are
now under consideration. The engine itself is very
compact and is used in the air-conditioning of trains.
(It has been modified to be included in this piant.) To
reach the evaporating temperature of Rl14 with
conventional flat-plate solar collectors would be very
difficult. Sophisticated technology, including the use
of selective surfaces in the collectors, would have to
be used, and it has not yet been proved that such
collectors can be manufactured easily at a reasonable
cost. However, MBB is open-minded about all
possible changes regarding temperatures used or
modifications in the design of the plant.
High-temperature engines
To obtain steam or vapour is the main problem
with the high-temperature solar engine. The Carnot
efficiency TJCis relatively high, but other efficiencies
-...r
(hot water)
Throttle
valve
m
11
11
II
II
11
II ) II
1 IEveporatorl
~preheater
Expander
1
1/
fl
n
11
Elec~ri~~rtorage
Electrical pnsrator
L&APL
(cold water)
Condenser
Pump
(cold wear)
Feed pump
(Freon)
Fipre 6. Schematic diagram of the MBB solar electric power plant
Technology for Solar Energy Utilization
should be taken into consideration. In the caseof the
heliostat, for example, the overall efficiency is
the individual efficiencies other than Q-, which
depends on T, and T2 (see above), having the
following typical values:
diffuse-direct solar radiation factor
reflectivity of mirror
sunset-sunrisefactor
focal absorption and geometrical losses
heat losses
transient clouds factor
qh
heliostat spacingfactor
‘It
Bm mechanicalefficiency ratio
lid
Ilr
Jls
t7a
rll
0.80
0.80
0.70
0.70
0.70
0.80
0.6
OS-O.8
It can be seen from those figures that the overall
efficiency cannot be more than 5%.8%of the Carnot
efficiency. For example, if the steam temperature is
200°C (TI = 473 K) and the condensingtemperature
is 30°C (T2 = 303 K) the Camot efficiency is about
36% and the overall efficiency of the system is
1.%3.0%. In a favourable solar radiation regime
about 40 m* of heliostat will be neededto obtain an
average power output of 1 kW during the daytime.
Some firms seek to obtain an area per unit output of
IO-15 m*/kW but this generally refers to peak, not
average, power. (Peak power is the power delivered
by the engine when solar radiation is maximal.)
Many institutes zre working on very large
thermal power plants. For example, the Centre
national de la recherche scientifique (CNRS), in
collaboration with ElectricitC de France, is working
on a high-temperature power station of 10 MW in
which the pressure could reach 80 bar. A lOO-kW
boiler being developed jointly by CNRS, BabcockWilcox, Heurtey, St. Cobain and Renault (SERI) and
using heliostats was to have begun operation at
Odeillo in early 1977.
Among the firms which have already realized a
prototype of a small steam engine is Maschinenfabrik
Augsburg-Niimberg AC (MAN’) in the Federal
Republic of Germany, which, in collaboration with
the Deutsche Forschungs- und Versuchsanstah fiir
Luft- und Raumfahrt, Stuttgart, is constructing a
plant consisting of 12 collector rows that track the
sun with 6 parabolic trough collectors (concentrating
factor, 30) in each row. The length of a collector is
2.5 m, the aperture 1 m * . The collectors are arranged
on a platform inclined at an angle corresponding to
the latitude. The total effective mirror area of the
prototype is about 180 m* , the working temperature
is 200°C and the mean thermal energy output per day
is about 700 kWh (working time from 7 a.m. to
5 p.m.). With a steam motor and electric generator
(10 kW peak), the electric power output is about
70 kWh/d (overall efficiency about 6%). The
condensed water has a temperature of 95”C, and it is
planned to use it for hot-water suppiies, spaceheating
or air-conditioning. To increase the electrical output
while decreasingthe effective collector surface, higher
working tempertilures are envisaged. Table 1 shows
the specifications planned for different stages of
development of the plant.
TABLE 1. SPECIFICATIONS
POWER PLANT
AT
DEVELOPMENT
Specifica riot1
Effective area (m’)
Working temperature (“0
Thermal capacity (kWh/d)
Electrical capacity (kWh/d)
Efficiencies (%)
Collector
Cycle
Motor/generator
Overall
OF THE
DIFFERENT
Pro towe
MAN SOLAR
STAGES OF
Improved
we
Op timired
series
type
-
180
200
700
70
130
250
480
69
100
3oi)
390
68
58
20
50
6
54
24
60
8
59
28
63
10
According to t:he design, this plant could be
extended in modular construction to larger plants
having electrical power outputs up to several
hundreds of kilowatts.
MAN is also working on a screw motor, and it is
planned that this unit will work with superheated
Freon 114 vapour.
The following quotation is taken from a MAN
report.
“The chance of success with a lowtemperature solar engine is very small. Contrary
to concentrating collectors conventional flatplate collectors utilise partly the diffuse radiation. This part however, is on the averagelower
than 10 per cent for the regions considered and
plays therefore a minor role. Flat-plate collectors
have the crucial disadvantage of strongly
decreasing efficiency with increasing collector
temperature. Furthermore, the insolation on fix
tilted collectors is smaller in the morning and
afternoon. Thus the value of efficiency decreases
stiil more. An additional disadvantage is that a
low-boiling working fluid such as Freon must be
used. This demands expensive heat exchangers.
“The thermal efficiency increasescorrespondingly for higher collector temperatures. However, sufficiently high efficiencies can be
achieved only if envelopes reflecting the infrared
radiation are used (which are expensive) or
selective coatings are applied (which show
degradation).
“Focusing collectors consist for instance of a
parabolic trough or a Fresnel lens concentrating
the direct solar radiation on an absorber pipe
mounted in the focus line. These collectors have
very high efficiencies-about 50 per cent-already
for low concentrating factors between 20 and 30.
‘/.:~c
>,
‘:
n
Ed,
*
,“._
,,
,‘.
_:
!._
Developm’mt
of solar energy utilization in developing countries
-.
“Focusing collectors must track the sun.
Thus the high efficiency remains nearly constant
in the morning and afternoon. A further crucial
advantage-contrary to flat-plate collectors-is
that conventional, available steam enginescan be
used because of the higher working temperatures.”
The steam engine used in the MAN plant is a
conventional one that was manufactured in the
1960s. Its power capacity is greater than that which
the available set of solar ioliectors can give.
The expected breakdown of the cost (1976) of
the plant per unit of electrical power output is
($/kW):
Steam engine and generator, including
frames and controls
Condenser
Pumps. pipelines, insulation and cycle controls
Stora,%, insulation and storage-water container
CoUe&or
Total
810
140
200
530
1110
2 790
In the United States,many small companieshave
emerged whose aim it is to construct focusing solar
collectors that track the sun automatically. One of
them, Sun Power Systems Corporation in Tempe,
Arizona, has developed cylindro-parabolic solar
collectors. One of these has the following performance data and specifications:
Description: Aluminium parabolic troughs are
arranged in series;the number of troughs needed
per specific installation is determined by energy
requirements. Troughs are kept constantly
focused on the sun by an electronic device that
incorporates a high-temperature defocusing
capability, low-temperature freeze protection,
and a temperature comparator that guarantees
that the unit will only heat water in the storage
facility
4 ft x 10 ft
dimensions-standard:
Trough
(1.22 m X 3.05 m; effective area 3.41 m’)
Trough surface: anodized aluminium, guaranteed
for over five years. Dust has no significant effect
on efficiency
Energy produced daily (assuming latitude N 32’
and 100 % sunshine):
Per unit area
21 June
21 December
Average
Per trough
iMJIm’l
(BWft’J
IMJI
(Bml
21.41
13.05
17.23
1 885
1 149
1517
73.0
44.5
58.7
69 190
42 170
55 680
Concentration ratio: 44 to I
Absorber fluid: water
Absorber fluid flow rate: 0.3 l/s (5 gpm),
although the system works equally well with
faster or slower flows
13
Water temperature: 177°C (350°F) (closed-loop
system circulating water from eight collectors
through a 150-l (40.gal), insulated storage tank)
Maximum operating pressure: 20 bar (300 psi)
weight:
specific,
7.3 kg/m’
Collector
(1.5 lb/ft’);
per trough, 27.2 kg (55.1 lb)
(includes all framing, components, water)
Absorber material: l-in. hard copper pipe with
selectiveblack coating
Framing material: tubular steel (rectangular),
0.065-in. (1.65-mm) wall
Tracking motor: 2.8 rpm; gear ratio, 1 780 to 1;
current load, 1 A; accurate within 10 min of sun
time
Collector end-fittings: adaptable
Recommended storage per unit area of collector:
60 l/m* (1.5 gal/ft*)
Orientation: north-south orientation is preferred,
but not necessary
A flat roof is preferred, bu,t not necessary
Aesthetics: system is very low profile; it can
easily be placed behind a parapet wall and thus
be unobtrusive
Adaptability: system can be fitted to any
existing structure and can be expanded by adding
extra troughs should energy demands increase
Maintenance: None required. Collectors may be
washed occasionally, but it is not necessary
Storm-damage susceptibility: in overcast conditions troughs are automatically returned to
nighttime position to minimize storm damage
Warranty: one year on all materials and
components except those under warranty limitations imposed by other manufacturers
The problem with these simple focusing collectors, which certainly work, is that the short duration
of experience is insufficient to evaluate their lifetime,
their performance and the effects of climatic
conditions and dust. The present (1976) unit cost of
such simple solar collectors is about 100 $/m2.
Summary
Medium- and high-temperature solar engines can
use conventional steam engines and steam turbines.
(That is nothing new; in the early years of this
century, a successfulsolar steam engine was installed
in Egyptj. However, small turbines do not yet exist
on the market.
The thermal efficiency of these engines is better
than that of the low-temperature engines becausethe
Camot efficiency is higher.
The engines require direct solar radiation, which
is not always available.
Focusing solar collectors should track the sun;
the tracking problem can be technologically solved at
;i”:,*:,‘,’
”
:, I,)
Technology for Solar Energy Utilization
14
a reasonable cost. (However, no systemsof this kind
have yet been test:d over a long period.)
Medium- and high-temperature solar energy
plants are expected to be more successful in large
rather than small installations.
The problem of energy storage is still the most
important problem; it must be satisfactorily solved
before many of the other problems can be overrome.
PlrtonT n
D~rplacar
----
section
------
Hot-air engines
IJ
i%e Stirling engirv
Figure 8. Simple representation
In a conveiitional combustion engine, heat is
supplied by burning a quantity of fuel inside the
working chamber of the engine. In the Stirling engine,
heat is added to the working gasinside the engine by
an external flame and a heat exchanger(heater head).
First, a volume of ccol gas, entrapped in a
cylinder by a piston, is compressed (figure 7a) and
then heated by an external heat source (figure 7b). As
the gas is heated, its pressure increasesand the piston
is driven downward, turning the crankshaft. After
expansion (figure 7c), the gasis cooled by an external
cooling source (figure 7d). Its pressuredecreases,and
the gasis once again compressed(figure 7a). Since the
pressure during the hot expansion is much higher
than during the cool compression, there is a net
output of work from the engine. The complete cycle
takes place in one revolution of the crankshaft
instead of in two revolutions as in conventional
engines.
The cumbersome exchange of the heating and
cooling sources shown in the simple representation of
figure 7 is, of course, impractical. Stirling’s key
invention was to achieve the exchange by adding a
mechanism called a displacer piston, which servesto
move the gasbetween a stationary hot chamber and a
stationary cold chamber (figure 8). These chambers
(represented by coils in figure 9) are connected to
opposite ends of the displacer section of the cylinder.
When the displacer piston movesupwards (figure 9a),
1
b
c
d
Figure 7. Simple representation of the operating principles
of the Stirling cycle
of the displacer piston
Displscer
piston
Low pnsure
High pressure
I
b
Figure 9. Action of displacer piston
the hot working gas from the upper portion of the
cylinder is first moved through the heating coil. The
gas then flows through the cooling coil, where it is
cooled until most of the working gas is in the cold
section below the displacer piston. Becausethe gasis
cool, its pressure is low. Moving the piston downward
(figure 9b) forces the working gas back through the
cooling coils and into the heater tubes, where it is
heated and forced into the hot section above the
displacer piston. Since the gas is hot, its pressure is
high. There are no valves in the flow path, so that
when the upper chamber is at high pressure,the lower
chamber is also at high pressure.
One more addition is required to make the
Stirling engine practical: the regenerator (figure 10).
Located between the fixed heating and cooling
sources, it stores otherwise wasted heat during the
cooling process and permits recovery of that heat
during the heating phase. The amount of this stored
heat is actually equal to several times the amount of
heat added from the external heat source.
Figure 11 shows the regenerator and the
displacer section combined with the power section to
form the basic Stirling cycle power unit. (Not shown
is the mechanical linkage between the pistons that
maintains the proper phase relationship between
them.) Figure 1la shows the cooled gas being
compressed by the power piston as in a conventional
internal combustion engine. In figure 1lb, the
@?, ”
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Development of solar energy utilization in developing countries
n
b
I
Figure 10. Action of regenerator
--
k
I
b
J,
c
d
Figure 11. Stirling cycle complete with displacer section and
regenerator
compressed gas is being heated and its pressure is
being increasedbecausethe displacer piston is moving
a portion of the gas into the upper (hot) part of the
displacer section. The pressure increase is felt on the
lower piston, driving it downwards. In figure 1lc the
hot, high-pressure gas has completed its heating cycle
through the action of the descending displacer piston,
and the power piston has completed its power stroke.
Figure 11d shows the displacer piston moving upward
to force the working gas into the cooling chamber,
thus decreasing its pressure. The power piston is now
ready to repeat the compression stroke of figure 1la,
and the cycle is completed.
Closed-cycleho t-ak engine
Philips engine
Philips has developed small hot-air engines in the
past; one engine of 750 ‘W (1 hp) at 1 500 rpm has
been modified by KHANA in India for experimentation with solar energy. The heating system, in
the form of a cylindrical head 6 cm in diameter and
designed to bum kerosene oil, was removed and
concentrated solar energy used to heat the engine. A
set of mirror reflectors with a surface of 8 m2 was
used; the engine was able to operate a 200-W electric
generator, but the theoretical efficiency of the
Stirling cycle could not be reached.
15
Farber engine
Several prototypes of the closed-cycle, hot-air
engine type have been developed by the Solar Energy
and Energy Conversion Laboratory of the University
of Florida (United States). One interesting prototype
is a supercharged, water-injected solar hot-air engine
in which an adjustable check-valve allows the engine
to supercharge itself by drawing in fresh air or water
during the part of the cycle that is below atmospheric
pressure.
The engine can be “fuelled” with solar energy or
used directly without modification to burn wood,
coal or liquid fuels. If used with solar energy it is only
necessary to concentrate the solar energy upon the
end of the displacer cylinder inside the furnace box.
The engine can be built with very simple machine
tools.
A displacer cylinder with a bore of 70 mm and
an internal length of 257 mm is mounted at the top
of the engine. Inside this cyliner moves a displacer
with an outside diameter of 68 mm and a length of
203 mm. The displacer, with a stroke of 50 mm has
enough end and side clearance to move freely in the
cylinder.
The displacer cylinder is designed so that it can
be heated at one end by gas, oil or solar energy and
cooled at the other end by air or water (closed or
open circuit). The displacer is moved by a 12.mm rod
entering through a sleevebushing.
The displacer cylinder is connected by a 3/4-in.
pipe nipple to the power cylinder, which has a piston
60 mm in diameter and a stroke of 38 mm.
The linkage between the displacer and the power
piston allows timing of the engine. For normal
operation the displacer leads the power piston by
about 100”. Regeneration occurs along the displacer
and the displacer cylinder walls. Heat is stored in
those walls during part of the cycle, to be released
and used during another. The working fluid,
streaming back and forth, alternately giving off this
heat and then absorbing it later and thus preventing it
from leaving the system, provides internal regeneration.
The engine is started when the pressure inside is
equal to atmospheric pressure. Thus, during
operation, the internal pressure will dip below
atmospheric pressurefor part of the cycle. During the
operation of the engine under normal conditions this
dipping is enhanced by leakage of the working fluid
(air) through the displacer-rod bushing, out during
the high-pressure part of the cycle and in during the
low-pressure part.
It has been found quite difficult to prevent or
minimize this leakage without increasing the friction
losses considerably. Two methods of solving the
problem have been developed:
(a) Air injection. A small, adjustable ball-check
valve, installed as shown in figure 12, makes it easy
for fresh air to enter the system quickly during the
;.--
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Technology for Solar Energy Utilization
Opetz-cycle hot-air etzgine
Qej
Displacer
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Power cylinder
Figure 12. The Farber engine with air and water injection
below-atmospheric-pressure part of the cycle. This
very simple addition allows the engine to operate
with a larger average amount of working fluid,
resulting in higher power output.
(b) Water injection. If the inlet to the check
valve is dipped in water, water is injected into the
system rather than air. This procedure allows even
larger amounts of fluid to be added to the sjrs;em,
since it is added in the liquid phase,resulting in even
greater increasesin power output. Another advantage
of injecting water (or other liquids) is that it greatly
enhancesthe heat transfer at the hot end.
Thus, self-acting air or water injection can
considerably improve the performance of the simple
closed-cycle hot-air engine. (See figure 13.)
Engines of the type described here can be
classified as “hybrid”, since they combine the
advantagesof the Stirling cycle with those of others.
Power (kW1
1.5 -
The open-cycle hot-air engine takes atmospheric
air, compressesit, then heats it by solar energy; the
compressed air expands and exhausts into the
atmosphere. A compressor is combined with the
engine (which can also be a turbine). The advantage
of this system is that the speed of the engine is
independent of the air-heating cycle.
KHANA engine
A small open-cycle hot-air engine taken from an
old kerosene-operated fan was overhauled. Its
worn-out parts were replaced and it was suitably
modified before use. It operated at an averagespeed
of 250 rpm. Heat at the cold end was dissipated
through large. thick fins cast along with the body of
the engine. To give smooth and continuous running a
38-mm thick hollow disc, which formed the false
bottom, was slipped over the bottom of the expander
cylinder. The disc was made of copper sheet and the
empty space was filled with dry sand. It formed 2
perfect fit and ensured complete contact between the
metal surfaces. The entire cylinder length (216 mm),
including the false bottom, was enclosed in a Pyrex
glasstube of slightly larger diameter and closed at one
end. Both these arrangements helped to raise the
temperature of the hot end and to achieve
uninterrupted and steady running of the engine.
Coupled to a small reciprocating water pump,
this engine was suitably mounted with the three
metal reflectors described above and used for
experiments on pumping water from different depths.
The coupled unit developed only about 45 W, half of
the power expected.
Later, another hot-air engine of nearly double
the capacity of the one used earlier was procured,
modified and mounted in the vertical position on an
iron tripod. It was used with plane-mirror concentrators. Coupled to the water pump, the engine
developed about 95 W. A small parabolic cylindrical
metal reflector was placed behind the cylinder to help
heat the hot end of the engine uniformly and thereby
ensure its smooth running.
Solar pumps
o 1
0
i
,
loo
200
7
300
Noryaloperdon
400
Engine speed (rpm)
Figure13. Power-speed curves of the Farber engine,
showing the increase in performanca obtained with injection
Conventional pumps can be operated by solar
engines; however, prototypes of installations for
pumping water without moving parts are being
developed by the Birla Institute of Technology and
Science, at Pilani, India. The principle is described
below.
A mixture of petroleum liquids with a boiling
temperature range of 35”-40°C is evaporated in
flat-plate solar collectors and then flows to a closed
tank full of water situated in 2 well. The pressure of
the working fluid permits the water to rise to an
upper level, depending on the pressure of the
mixture. The vapour condenses during the night in
the solar collectors. This discontinuous mode of
Pumpedwatar
Hi+pressure
vapnur hna
he
-ndsnrare
return
lme
Water tank
Figure 14. Solar waterpump
pumping is very simple, but only a small quantity of
pumped water is obtained. The vapour can also be
condensed by allowing it to flow to a condenser
cooled by the pumped water. By using two water
tanks and a set of control valves, semi-continuous
pumping is possible. Besides the collector and the
flash tank, there are two water tanks located close to
the water source and a condenser at ground level. The
pipe network interconnecting the tanks is shown in
figure 14.
The working fluid drawn into the collector is
vapourized and returned to the flash tank. The
vapour from the flash tank is let into one of the water
tanks, displacing the water. (It is assumed that the
water tanks are full of water.) The displaced water
condenses the vapour in the shell side as it goes
through the condenser coils. After the first tank is
emptied, the vapour is switched over to the second
tank. Simultaneously, the first tank is condensed by
the water being pumped from the second tank. As
condensation proceeds, the pressure in the first tanks
is reduced and water enters through the non-return
u&e. Thus, as the second tank is emptying the first
one is being filled. On reversing the cycle, by
manipulation of the valves, the first tank will pump
while the second one draws water. In this way, water
can be pumped continuously.
To prevent working fluid from going into the
water line, a water seal is always maintained inside
the water tanks. The working fluid that is condensed
in the water tank can be pumped to the condenser at
the start of each cycle. The condensate is pumped by
the condensate retrieval valve, which is similar in
principle to a steam trap. Further, the condensatecan
be transferred to the flash tank, periodically or at the
end of the day, by pressureequalization.
The capacity of the pump can be increased by
the addition of more collectors, which affects only
the cycle time.
The working fluid should be immiscible with
water; have a normal boiling point slightly higher
than the atmospheric temperature; and be non-toxic,
non-flammable, cheap and readily available. Pentane
fulfils all the requirements except for its flammability .
A petroleum fraction having a close boiling range
with properties similar to that of pentane would be
cheaper and more readily available than pentane. The
petroleum fraction, a mixture of hydrocarbons, offers
an additional advantage. It can be tailor-made to suit
the atmospheric conditions of a particular region. For
example, in a region where the night temperature is
around 2°C and the day temperature is lS”C, by
choosing a mixture having more light hydrocarbons
with a boiling range of 15”-2O*C, water can be
pumped to a considerable height even at very low
collector temperatures. In regions like Pilani (located
at the edge of the Thar Desert) where extreme
climatic conditions occur, the working fluid
properties can be modified to suit seasonalvariations
by adding small amounts of light or heavy
hydrocarbons to obtain high pump performance.
Practically speaking, any two fluids are always at
least slightly soluble in each other; hence continuous
contact of the working fluid with fresh water in each
cycle will result in some loss of working fluid.
Fortunately, the working conditions in the pump are
such that the masstransfer rates close to interface are
extremely low most of the time. As a consequence,
the loss of working fluid will be negligible.
The following is a proposed specification for a
solar water pump of the type described above:
Flat-plate collecror area
Pumping rate
Head
Water tank dimensions
100 m’
150 m”/d
18m
150 cm high X 90 cm diameter
The cost of the installation, assuminga collector
unit cost of 35 $/m*, would be $6,000.
In developing prototypes many technological
problems remain to be solved, in particular the
contra! of the system of valves.The present proposed
electrical control does not meet the requirements of a
rural solar pump, which should be independent of
any external source of power.
Direct conversion of solar energy into
electrical energy
Photovoltaic cells produce an electric potential
vhen they are illuminated by solar radiation. Those
utilizing the semiconductors Si and CdS are the best
Technology for Sdar Energy Utilization
solar energy will be discussed,This kind of machine
can function either continuously or intermittently. In
the continuous regime an external source of power is
necessary, at least to operate the pumps and control
system. The best known fluids used are aqueous
solutions of ammonia and of lithium bromide. The
first is more suitable for use with flat-plate solar
collectors.
known on the market, those utilizing Si having the
longer lifetime. Important R and D programmes are
being undertaken to improve the performance,
simplify the technology and reduce the cost. In 1976,
the cost per unit of power was about 15 000 $/kW
(peak). Researchprogrammes seek to reduce this cost
to 8 000 $/kW in 1980 and to some hundred dollars
per kilowatt in 1985.
At present, the available technology seems still
too sophisticated for most developing countries, and
manufacture even of a small series of cells cannot be
planned for the medium term. For these reasonsthis
subject will not be discussed further in this study in
spite of its very promising future.
f+inciple ofoperation (figure 15)
Mar refrigeration and air-conditioning
Air-conditioning means the treatment and
handling of air to obtain well-defined values of
temperature, humidity, velocity and purity of the air
in a given space. Only the cooling aspect will be
considered here, so that solar refrigeration and space
air-conditioning can be associated.The temperature is
generally lower in the caseof refrigeration, especially
if one is speaking of ice production or most casesof
food preservation. Welldefined valuesof temperature
and humidity could not be obtained in the case of
airconditioning without an external source of energy
to operate fans, pumps and control systems.
Solar refrigeration can be achieved through a
solar engine operating a conventional compressor, but
in this report only absorption machines heated by
The ammonia-water mixture is heated directly by
solar energy in the generator (boiler) or indirectly by
water heated by solar energy. When the temperature
of the mixture rises,the ammonia begins to evaporate
because its solubility in the water decreaseswith
increasing temperature. The vapour flows to a
condenser, which is water- or aircooled, where it
condenses. The operation now becomes similar to
that in a conventional refrigeration system with
compressor. A subcooled liquid is available at the
outlet of the condenser. The pressure of the
ammonia, which increases when the temperature
increases, is controlled by the condensing temperature. The liquid expands through the expansion v&ve
and begins to evaporate; its temperature and pressure
decrease. The cold, low-pressure vapour reaches the
evaporator, in which it absorbs the heat of the
material being cooled (e.g., chilled water for
air-conditioning purposes and brine for ice production). The temperature of the vapour increasesand it
flows to an absorber where it meets a spray of a weak
solution of ammonia in water and is absorbed. The
GWW~tC.r
(boilw)
Condenser
,
kl Block dia#mn
Condanter
1
-Cooling
Bottom
twdw
Pump
fbl Schrnutic diagram
Figure15. Ammonia-water
refrigerationcycle
wter
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.’
Development of solar energy utilization in developing countries
absorption is exothermic, and to keep the temperature of the mixture in the absorber within the limits
permitting the desired concentration to be obtained,
the absorber should be cooled. The concentration of
the mixture increases,it is pumped to the generator
and the cycle is closed.
Hence:
(u) Cooling is necessaryin the condenser and in
the absorber; if the cooling is by natural convection,
the heat exchangers must have quite large surface
areas;
(bj Powered pumps are necessary, at least to
pump the mixture from the absorber to the
generator; a thermosyphon, if applicable, would
greatly decreasethe productivity;
(c) Operation should be continuous in solar
air-conditioning;
(d) A heat exchanger between the condenser
and the absorber can improve the productivity.
Some experiments are taking place (for example
at the Refrigeration Institute of the Technical
University at Delft, the Netherlands) for producing
4 kg of ice per day with 2 m2 of flat-plate solar
collector, with an intermittent ammonia-water
absorption machine independent of any external
source of power. Condensation of the ammonia
vapour is obtained during the night by sky radiation
through the flat-plate solar collector.
First casestudy:
An ammonia-water solar refrigeration system, Solar Energy
and Energy Conversion Laboratory, University of Floridu
A 1.2 m (4 ft) square flat-plate solar collector
acts as generator. It consists of l-in. steel pipe
running from a 1.25~in. bottom header to a 2.5in.
top header. The steel pipes are spaced on loo-mm
(4-m) centres and soldered to a 20.gaugegalvanizediron sheet. This element is then placed into a
galvanized sheet-metal box between a single glass
cover and 25 mm (1 in.) of Styrofoam insulation.
The whole unit is inclined 30” with respect to
the horizontal; the angle is a compromise to provide
both good solar collection and good two-phase flow
and heat-transfer characteristics in the inclined tubes
running from bottom to top. Since the unit is
stationary it is faced south to give the best average
orientation for the whole day.
The condenser consists of a 3-in. pipe shell
containing four standard &in. black-iron pipes,
1.2 m (48 in.) long providing a heat-transfer surface
of 0.325 m2 (3.5 ft’).
The evaporator is made from a 4-in. pipe shell
containing seven standard $?&i. black-iron pipes,
1.2 m (48 in.) long providing a heat-transfer surface
of OS72 m2 (6.15 ft’).
19
The absorber is fabricated from a 6-m. pipe shell
containing eleven standard M-in. black-iron pipes,
0.9 m (36 in.) long, providing 0.573 m2 (6.16 ft’) ot
heat-transfer surface, which in addition serves as
support for the liquid film in which part of the
ammonia vapour is reabsorbed.
The ice production and storage unit is a cubical
galvanized sheet-metal box 46 cm (1.5 ft) on an edge
with 100 mm (4 in.) of Styrofoam insulation around
it, protected by a thin plywood outer layer.
The heat exchanger is a simple, single-pass,
counter-flow, double-tube type.
The circulating pump for the water anti-freeze
solution is of the standard centrifugal type, while the
ammonia-water solution circulating pump is of the
rotary, nylon-roller type. A bypass loop on the pump
permits control of the amount of solution distributed
to the various parts.
Four valves in the system in addition to the
expansion valve permit control of the flow rates in
different sections of the system. Numerous pressure
gauges,thermometers, thermocouples and liquid-level
sight-glasses permit constant monitoring of the
conditions. The ammonia concentration in the
solution varies from 48 to 60 wt%.
The system has been hydrostatically tested to a
pressure of 20 bar and is considered safe for
operation at 13-l 5 bar. This corresponds to a
temperature in the generator of about 66’C (150°F).
To freeze the water in the metal container in the
ice box, the design temperature of the evaporator is
-6” to -9°C corresponding to a pressureof about 3 bar.
Figure 16 presents the data of a test run on
6 July 1968. It was a perfectly clear day, although a
considerable amount of solar energy arrived as diffuse
radiation owing to the relatively high humidity. An
indication of this was the slight temperature increase
of the absorber-generator even before the direct
sunshinehit the front surface of the unit.
At the start of the day the expansion valve was
closed and the system was allowed to warm up. (The
heat capacity of the combination solar absorber and
ammonia generator was about 1.9 kJ/“C (46 Btu/
OF).) Part of the energy used for warm-up was
returned to the system in the late afternoon hours
when the stored heat was released owing to the drop
in temperature.
The pressures and temperatures in the solar
refrigeration system were effectively controlled by
varying the flow rates in the different parts of the
system by adjusting the various valves. About 41 MJ
(39 X lo3 Btu) fell upon the absorber-generator
during that day, and 18.6 kg (41 lb) of ice were
produced (from 24°C (75°F) water), a rate per unit
area of collector surface of 12.5 kg/m2 (2.56 lb/ft2).
The following comments may be made:
(a.1 A compact solar retrigeration unit that will
give satisfactory performance can be designed and
constructed;
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Technology for Solar Energy Utilization
20
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0700
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,
lnsolatmn
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OBm
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1100
1200
13M
14cm
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1600
-r1600
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Figure 16. Data from a test run of a solar refrigerator performed on 6 July 1968
(b) A considerable arnoi;zt of soiar energy can
be collected even on cloudy day< by the flat-plate
solar collector; operation of a sc&r refrigeration
system is possible at solar absorber temperatures as
low as 43°C (110°F);
(c) Utilizing solar energy to produce ice solves
the problem of storage;
(d) Combining the solar collector and the
ammonia generator into one unit eliminates the
rather large heat losses between the solar absorber
and the ammonia generator observed in previous
systems. However, the advantage is realized only
when small units are involved; in large units, problems
of circulation of the two-phase fluid and of tightness
could arise;
(e) The extensive theoretical analysis of the
system, especially for the combined absorbergenerator reported elsewhere, has shown that the
design can be theoretically determined and the
desired performance obtained;
(f) Since the maximum production rate of ice is
about 20 kg/d for a system including two pumps
operated by external sources of power and a
water-cooling condenser and absorber, the system
delivers only about 10.5 MJ (2.4 Meal) of cooling per
&Y.
Second casestudy: An air-cooled ammonia-water
absorption air-conditioner with
new generator temperatures,
Energy
and
Environment
Division. Lawrence Berkeley
Laboratory,
University
of
Gdifornin (1976)
Part of an ERDA*upported project, the system
is designed to operate at generator temperatures of
80”.99°C (175”-21 O”F), compatible with the temperature range of flat-plate solar collectors.
The air-conditioner uses as a base the condenser,
absorber, pre-cooler evaporator and solution pump
from an A&la Model ACB-60-00 gas-fired ammoniawater absorption water chiller, which has a nominal
cooling capacity of 3.5 kW (5 tons of refrigeration).
The total power of the pumps and fan is 1.6 kW. The
following components were added: generator, preheater, rectifier, storage tanks, adjustable expansion
valves and measurement instruments (6 pressure
gauges,25 thermocouples, 2 rotameters and 2 samp
ling tubes for concentration measurement).
The generator is a packed-tower, counter-flow
heat exchanger. The strong solution drips down
through the steel pall-ring packing, -making contact
with four hot-water coils in parallel. ‘Thetotal outside
surface area of the water coils is 3.3!m2 (36 ft2). The
condenser and absorber are aircodled, finned-tube,
cross-flow heat exchangers with a total outside tube
area of 1.3 m2 (14 ft2) and 2.0 m2 (22 ft”),
respectively. The fiis were 0.01 in. (0.25 mm) thick
aluminium sheetsspacedat 14 fins per inch (1.55 mm)
apart.
The unit is started by pumping hot water at
constant inlet temperature through the generator
coils. The condenser-absorber fan and the solution
pumps are turned on when the generator pressure
reaches about 10 bar (150 psig). It takes about
15 min to warm up the system to approach running
conditions (with a chargeof solution in the system of
about 29 kg (65 lb) at 55% ammonia concentration).
The flow rates of ammonia and of the weak solution
are then readjusted, by meansof expansion valves,to
the desired values.
All runs made during the initial testing stage
served to confirm cycle calculations based on the
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Development of solar energy utilization in developing countries
assumption of equilibrium states. That is, given the
measured mass-flow rates and the measuredpressures
and temperatures, the energy balances between the
components can be satisfied within experimental
error (about fS.70) by using the thermodynamic
equilibrium enthalpies. The mass balances can be
satisfied by using the equilibrium concentrations.
The system operation is very stable. No
appreciable changes are observed after hours of
operation. The stability of operation extends a
circulation ratio (mass of absorbent per unit mass of
refrigerant) as high as 27.
Operating the system at near cut-off conditions
(i.e., Ax close to zero. where x is the mass
concentration of ammonia in the solution) demands
more power and a larger pump to circulate the
solution. Therefore, imposing a limit on the pumping
power to say 48 mW per watt of cooling load
constrains h to valuesabove 0.03, or the circulation
ratio to values of less than 16 (assuming a pump
efficiency of 40%). Figure 17 shows graphsthat serve
to summarize the possible operating temperatures for
an ammonia-water absorption air-conditioner operating under these conditions.
For acceptable cooling and dehumidification
using reasonably sized chilled water coils, CE is
limited to cE < 8°C (47OF). Inexpensive flat-plate
collectors may reasonably limit the generating
temperature to fG < 90°C (I 95’F).
,‘I
The above practical constraints combine to
require condensing and absorbing temperatures below
43°C (1 lOoF). This constraint can be met by
doubling both the cooling-air flow rates and the size
of the colldenser-absorber, compared with those used
in convention21 gas-fired systems using a finned-tube
condenser and absorber. (These conditions are
essentiall!~ satisfied already since the condenser,
absorber, and fan are over-sized.)
Doubling the cooling-air flow rates is essential,
but new designs of more efficient condenserabsorbers may reduce the requirement of doubling
the size (and cost) of these heat exchangers.Doubling
the cooling-air flow rates typically increases the
power of the fan from 24 mW to 48 mW per watt of
cooling, giving an overall performance of 10 W of
cooling per watt of electrical power input (as
compared with a rating of about 2 W per watt for a
mechanical compressor unit).
The coefficient of performance (COP) depends
strongly on the pre-heater efficiency vpH and AX and
is quite insensitive to the values of the remaining
paraneters. To have COP> 0.65 with LLY= 0.03,
VpH must be at least 90%. This value of qpH is not
expensive to achieve, since there is sufficient pressure
in the weak solution KOpromote high heat-transfer
coefficients. A high value of QPH is essentialto avoid
dumping the heat contained in the weak solution into
the absorber.
The experience with the air-conditioner described above shows that it is technically feasible to
use the ammonia-water absorption cycle for cooling,
with a heat-source temperature below 93’C (200°F)
and a heat-sink temperature (using air cooling) below
43°C (110°F).
Third casestudy:
Possible operating
zone for a*
an ammonia-water
ammonia i-water
Arc=.5
30
9!5
mrbing
I p’P__‘B.100
temperature
-- -.-?--.105
rA or condensing
110
temperature
115
fc (OFI
figure 17. Evaporating temperature tE of an ammoniawater absorption air+xmditioner as a function of the
absorbing, condensiq and generating temperatLrc tA, tC and
fG for the following conditions:
pressure drop across absorber p = 0.2 bar (3 psi), concentrrition difference Ax = 0.03, and rA = tc
bromide solar absorpsystem.
tion air-conditioning
Ohio State University (I 9 75)
Lithium
The system was designed to provide air-conditioning for a single-storey laboratory with a floor
area of 204 m2 (2 200 ft’) built over a 1.2-m (4-ft)
deep crawl space. In addition, there are three
enclosed, but unconditioned, courtyards. The perimeter window area of the building is limited to six
vision strips, or 1.7 m2 (18 ft2) of glassarea. Natural
lighting orginates primarily from the courtyards. The
building wall and flat-roof framing membersare steel
and support, in addition to the conventional building
toads, over 1 500 kg (3 300 Ib) of collector, framing
and piping load. The walls and roof are insulated with
50mm (2 in.) and 75 mm (3 in.) of talked-joint
Styrofoam to minimize infiltration and heat transfer
with the outside. The I/ values for the composite wall
and roof sections are 0.07 and 0.05 Btu h-’ ftV2
“F-’ (0.40 and 0.28 W m-’ “C-l) respectively.
The roof-mounted coHectc,rs are oriented due
south and are tilted up 45” from the horizontal. The
collector array consists of 37 baseline collectors
Technology for Solar Energy Utilization
22
connected in parallel (total area: 61 m2 (660 ft2)).
The collectors are constructed with two sheets of
l/8-in. (3-mm) thick tempered glassover a flat black
aluminium roll-bonded absorber. Heat loss from the
back of the collector is controlled with 3% in.
(90 mm) of fibreglassinsulation. The plumbing in the
system is copper with dielectric unions at each of the
two connections per solar collector. Great care was
taken at start-up to flush the system and fill it with
distilled water. The system was operated this way for
one year, then modified to accommodate a
glycol-water mixture as the collector working fluid.
The thermal-energy storage system consists of
two steel storage tanks each 1.5 m (5 ft) in diameter
and 3.7 m (12 ft) long and lined with a Tinkolite
coating. The horizontally mounted tanks are in the
crawl space under one of the courtyards. The entire
crawl-space volume around the tanks was filled with a
foam-in-place polyurethane to minimize energy
losses.
An Arkla 3.ton (10.5 kW) lithium-bromide,‘water
absorption direct-expansion machine, modified by
Arkla Industries to operate on hot water, is the solar
cooling machine. Circulation within the machine
occurs as a result of the thermal syphon pump effect,
which requires a generator inlet temperature of 88’C
(190°F) or higher to start circulation. In addition to
the hot-water heat source, the cooling machine
requires a cool-water heat sink. The heat rejection is
accomplished with a 7.5-ton (26.kW) Marley cooling
tower. The presence of this unit adds two
energy-consuming motors to the operation, a ?&hp
(375-W) cooling-water pump and a l/a-hp (250 W)
cooling-tower fan.
Figure 18 gives the collector efficiency as a
function of the ratio Ar/HR, where At = tin-t, is the
difference between the inlet and ambient temperatures and HR is the insolaticn. The absorption
cooling machine requires a steady heat input at the
rate of 16 kW (55 X lo3 Btu/h) to cool at the rate of
10.6 kW (36 X lo3 Btu/h). To achieve that, the
collector array must collect energy at the rate of
262 W per square meter (83 Btu/h per square foot). If
the ambient temperature is 32°C (90°F) and the
collector inlet fluid temperature is 85°C (185”F), we
have At = 53°C (96”F), and from figure 18 we find
that HR must be 946 W mm2 (300 Btu h-’ ft -2) and
the collector efficiency 28%, which clearly pushesthe
array to its limit.
The storage-tank temperatures in the early
summer of 1975 never exceeded 71°C (160°F) and
consequently no solar cooling was done. A check
revealed that there were serious heat lossesfrom the
storage tank. With an adequate system of control, it
was possible to obtain a water temperature of 93°C
(200 F) periodically; however, the system was
extremely sensitive to passingclouds. A passingcloud
would cause the flux to drop radically, and the
collector lossescaused the loop temperature to drop
rapidly. The motor-operated control valves were
unable to respond to such operating conditions;
therefore this mode of operation was abandoned.
The comparison below between the energy
required with a conventional vapourcompression
machine and with a solar absorption machine shows
that absorption cooling requires three times as much
power as cooling by vapour compression.
Conventional
vapour compression
Solar absorption
Power input (W)
Electrical
Collector pump
Mechanical room pump
Generator pump
Arkla blower
Cocling-water pump
Cooling-tower blower
Subtotal
100
Solar
t
Cooling output (WI
COP (output/input)
‘0
0
‘0
0
0
\
Figure 18. Collector efficiency as a function of the ratio of
the difference between the inlet and ambient temperatures
and the insolation
2 195
Compressor
Blower
Condenser fan
5 400
640
620
6 660
16 100
Total
t
560
125
350
560
350
250
18 295
6 660
10 560
0.58
11 720
1.76
A significant advantage of the solar absorption
machine can be seen from the above comparison: its
six electric motors require only one third as much
power asthe three motors of the conventional vapourcompression machine. That is important when it is
necessaryto conserveelectrical energy.
These are the disadvantages of the system
described:
(a) Lithium bromide requires a minimum
temperature of about 9O”C, which is very difficult to
obtain with flat-plate solar collectors, unless very
expensiveselectivematerial is used;
(b) The system is bulky;
i<,>i :L .,
;“(
: I ‘,
/
‘3
Development of solar energy utilization in developing countries
(c) With six independent motors, the system is
complicated; it requires a control system just as
complicated as that of the conventional machine.
Such a simple design is easy to construct. The
walls cnnstitute the storage system. ,4t Odeillo in the
south of France, such systemshave supplied 605%70%
of the total energy necessaryfor heating purposes.
Solar space heating
Many studies have been made on the economics
of solar space heating. The energy saving varies from
30% to 80% of the total consumed heat. It depends
on, among other things, climatic conditions; building
shape, location and orientation: number of floors;
area of glass in the external walls; materials used in
construction; kind and method of insulation; and
mode of utilization of the premises.
Much development work on solar heating has
been undertaken in both developed and developing
countries, for example France, the Federal Republic
of Germany, India, the Netherlands and the United
States. From the technological point of view, solar
space heating is already operational. Nevertheless,R
and D programmes are being undertaken to improve
the performance, find better architectural solutions,
and reduce the cost. In many cases,as in conventional
heating systems, domestic hot water can be obtained
from the solar spaceheater.
Two modes of heating are usually employed
when solar energy is used to heat space,passiveand
active_
Active system
Water is heated in flat solar collectors and is
circulated through a storage tank into radiators or
convectors located in the space to be heated. Hot
water can also feed coils, on which air is blown; thus
the space is heated by hot air. In normal climatic
conditions an auxiliary source of heat may be used
(figure 20).
Roof-mounted
heat-absorbing
panels
To heated areas
Water storage tank
Return
Passivesystem
PumpNo.
The principle of the passive system is shown in
figure 19. Solar radiation heats the absorbing surface,
the dark outer surface of the wall which is oriented
towards the south (in Fe northern hemisphere). The
distance between this absorbing surface and the
double sheet glass constitutes a duct in which air
warms up and rises by the thermosyphon effect and
then enters the space to be heated by an aperture in
the upper part of the wall. As the warm air heats the
space,it cools, descendsand returns to the duct by an
aperture at the bottom of the wall.
c
North-w
Absorbing
surface
Figure19. Passive
sdar spaceheatingsystem
1
u
line
Ll
Figure 20. Active solar space heating system with auxiliary
heater
Solar water heating
Solar water heaters are already used on a
relatively large scale in Australia, Cyprus, Japan and
the United States. They have already passedthe phase
of R and D and pilot projects, and the techn,o!;@ is
reliable and well known. Several models exist on the
international market. The most common has a 2-m2
flat-plate solar collector and a 200-l storage tank.
In many countries solar water heaters could
supply all the hot water necessary for domestic
purposes all year round. In other countries with a less
favourable solar radiation regime, a conventional
auxiliary heater is necessary.
The development of solar water heaters depends
on the development of flat-plate solar collectors for
other purposes, such as solar space heating or solar
engines.
The solar water heater developed by the Brace
Research Institute of McGill University, Canada,
could be used in a wide range of developing countries.
A diagram of this heater is given in figure 2 1.
Technology for Solar Energy Utilization
24
(a) Schematic-pictorial
Vent
diagram
of heater
circuit
-ml
Float valve
Cold-water
r
Latitude
+ 100
feed tank
Collector-absorber
L
Collector-absorber
L
casing
from
Cold water
to showers
Hot water
(b) Installation
Ordinary
Cold water
window
mains
to showers
etc.
etc.
diagram
glass
Cold water from mains
Cold water to showers etc.
Hot water to showers etc.
Figure 21. Solar water heater developed by the Brace Research Institute (Canada)
Solar water distillation
General considerations
Distillation of sea water or brackish water can be
obtained by solar stills or by conventional methods
such as multiple effect, thermal compression, inverse
humidificationosmosis,
electrodialysis,
dehumidification and freeze-desalination. In ?his
article, only direct distillation in solar stills that do
not require external power sources will be discussed.
The principle of solar water distillation is based
on the greenhouseeffect. A layer of brackish or salt
water is put in an air- and water-tight container
covered with sheet glassor other transparent materiai.
The bottom of the still is black; it absorbs a large part
of the solar radiation and heats the water, which
begins to evaporate. The vapour reaches the cooler
transparent cover, on which it begins to condense. A
system for collecting the condensed water is
provided.
Many distillation plants of various sizes have
been cnnstruc?ed in several countries. In 1973,
Dolyannis of the Greek Atomic Energy Commission
published the list given in table 2 and illustrated in
figure 22. Figure 23 givesthe annual variation of the
productivity of the Nisiros (Greece) plant over the
period 1969-1973.
Development of @ar energy utilization in developing countries
25
The number of variables influencing the productivity of solar stills is very high and they are often
interdependent. Among the most important are:
Solar radiation regime
Wind velocity
Rainfall
Design of still
Depth of the water layer
Filling and flushing regime
Maintenance
lb) Inflat& ,,ta‘nc cover
Materials of construction
The Office of Saline Water (United States) has
this to say about the materials of construction of
solar stills:
“Since the building of the first large
commercial solar still in Las Saiinas, Chile,
around 1872, the most significant gains in solar
still technology have come by way of improved
materials of construction. The productivity has
not been increased much, but the maintenance
and operating expenses have been reduced
appreciably. For example, the 4 800 m2 still at
Las Salinas was constructed of wood, glassand
putty, and its operation required a clerk, a
glazier, two full-time labourers, and occasionally
a carpenter. By contrast some recently built stills
require only one full-time attendant and a few
are designed to operate unattended for long
Icl CSIRO IAurtralml
Figure 22. Solar still designs
4.5
.tm
FfJb
Mar
AW
Mw
June
,
July
Aw
sem
dr
NW
OK
Figure 23. Annual variation of productivity (amount of fresh water produced per unit area of still per
day) of solar distillation units at Nisiros. Greece, 1969-1973
TABLE
2.
Country
or area
DATA ON THE MOST IMPORTANT
Location
Australia
Cape Verde
Design a
Muresk I
Muresk II
Coober Pedy
Caiguna
Hamelin Pool
Griffith
SOLAR DISTILLATION
Year
builr
Area
Im’I
1963
1966
1966
1966
1966
1967
.;;2
3 :ci
372
557
413
PLANTS THAT HAVE BEEN BUILT. 1872-I 97 3
Feedwater
Cover
Sra*trts
in I 973
Brackish
Brackish
Brackish
Brachish
Brackish
Brackish
Glass
Glass
Glass
Glass
Glass
Glass
Re Juilt
Operatmg
Operating
Operating
Operating
Operating
Sea water
Plastic
Santa Maria
Santa Maria
C
C
1965
1968
743
Chile
Las Salinas
Quillagua
e
e
1872
1968
4 460
100
Brackish
Sea water
Glass
Glass
Abandoned
Operating
Greece
Simi 1
Simi II
Aegina I
Aegina II
Salamis
Patmos
Kimolos
Nisiros
Fiskardho
Kioni
Megisti
b
d
:
C
f
f
f
c
i
f
1964
1968
1965
1968
1965
1967
1968
1969
1971
1971
1973
2 686
2 600
1 490
1 486
388
8 600
2 508
2 005
2 200
2 400
2 528
Sea water
Sea water
Sea water
Sea water
Sea water
Sea water
Sea water
Sea water
Sea water
Sea water
Sea water
Plastic
Stretched plastic
Plastic
Stretched plastic
Plastic
Glass
Glass
G1S.b
Glass
Glass
Glass
Rebuilt
Dismantled
Rebuilt
Abandoned
Abandoned
Operating
Operating
Operating
Operating
Operating
Operating
d
1969
223
Sea water
Glass
Operating
Bhavnagar
c
1965
377
Sea water
Glass
Operating
Haiti
India
Abandoned
Mexico
Natividad
d
I969
95
Sea water
Glass
Operating
Pakistan
Gwadar I
Gwadar II
1969
1972
306
9 072
Sea water
Sea water
Glass
Glass
Operating
Operating
Spain
Las Marinas
f
!4
a
1966
868
Sea water
Glass
Operating
Tunisia
Shakmou
Mahdia
d
d
1967
1968
440
1 300
Brackish
Brackish
Glass
Glass
Operating
Operating
USSR
Bakharden
e
1969
600
Brackish
Glass
Operating
United
States
Daytona
Daytona
Daytona
Day tona
Beach
Beach
Beach
Beach
a
a
b
b
1959
1961
1961
1963
228
246
216
148
Sea water
Sea water
Sea water
Sea water
Glass
Glass
Plastic
Plastic
Rebuilt
Dismantled
Dismantled
Dismantled
Windward
Islands
Petit St Vincent
b
1967
1 710
Sea water
Plastic
Operating
Source: AEC.
“See figure 22.
periods of time. Glass, concrete, and asphalt
materials appear to require only a minimum of
maintenance.
“Indigenous materials are usually preferred.
However, in selecting materials, the overall
economics must be carefully considered, including maintenance and rebuilding in?ervalsas well
as initial capital cost. The present Trend is toward
materials that will last 20 years with minimum
upkeep. Such materials include concrete, glass,
butyl rubber, and stainlesssteel.
“The foilowing
lists still-component
materials that have proved to be reasonably
satisfactory in actual use around the world. For
each component, the materials are listed in order
of preference from the standpoint of durability.
When a solar still is to be built directly on the
ground using a basin liner, it is advisableto first
use an insecticide and a weed killer to reduce the
possibility of punctures.
Materials
Basin liner
Cover
Butyl rubber (0.015 to
0.030 inch thick)
Asphalt mat.s (0.12 to
0.25 inch thick)
Black
polyethylene
(0.008 inch thick)
Roofing asphalt (over
concrete, etc.)
Window glass (0.10 or
0.12 inch thick)
Wettable [PVF film]*
(0.004 inch thick)
,&‘-*,‘i,;
‘a ““:,:;
I.3 )I “<< d “r,
,*‘.>:,,
‘,.,‘.‘“’
/I.
I
‘dy+
/
s
_, _
:
. . !
I>
;,‘.,
,
i_
27
Development &solar energy utilizatiott in delleloping countries
Component
Materials
Support structure
Concrete
Concrete.
block. Aluminium.
Galvanized metal. R:dwood*
Stainless steel. Butyl rubber (lining)
polyethylene
Black
(lining)
Silicone rubber. AFghalt
caulking compound
Butyl-rubber extrusions
PVC (polyvinyl-chloride) . Asbestos cement
(for saline water). ABS
(acrylonitrile-butadienestyrene)
Distillate trough
Sealant
Piping and valves
Water storage
reservoi;F:
*Relatively
Concrete. Masonry
short lifetime.’
”
UNIDO/UNICEF solar distillation plant in Somalia
UNIDO, with UNICEF financing, is implementing a solar distillation project in Somalia, the main
component of which is a solar water distillation plant
of about 2 000 m2 net evaporating surface. The
expected production of the still, including rainfall
collected, is 5-6 m3 of fresh water per day.
The design of the plant was adapted from a
design that had been prepared by the Central Salt and
Marine Chemicals ResearchInstitute (India) for use in
India. Modifications include improving the piping
design, reducing the passages between the stills,
changing the inclination of the cover glass to 1.5”
instead of 2C’, using aluminium instead of wood for
the supports of the upper side of the sheet glass,and
changing the location of the sea-water,distilled-water
and blending-water tanks.
The plant is composed of 15 blocks, each
consisting of 6 symmetrical and intercommunicating
basins about 13 m long and 1.5 m wide. The main
construction materials are bricks, cement, sheet glass,
plastic, tar, mastic tank paint, electrical cotton
insulating tape to cover the sheet-glassjoints and
support the putty, aluminium profiles and sheet, and
galvanized pipe. The estimated cost of these materials
and two hand pumps in India is about $23,000 and
the estimated cost of labour is about $4,600. The
cost per unit area of net evaporating surface is thus
about 14 $/rii2.
It has been suggestedthat in another distillation
plant of 200 m ‘, 12 experimental units of 3 m* each
will be constructed to compare behaviour of
mate&&, effect of insulation etc. One of the units
will serve as the reference unit; each of the other 11
will be identical with the reference unit except for a
-’ Office of Saline Water, ReportNo. 546.
change in one of these variables: inclina?ion, depth 01
water, wall construction, liner material, basin
insulation.
Design w~siderariom
A consensus does not yet exist on the optimal
design of a solar distillation plant. However. the
following considerations stem to meet with general
agreement:
(a) Local materials and simple technology
should be used as far as possible;
(b) Plastered brick or cement blocks able to
wlrhstand weather conditions and the effects of salt
water and to ensure tightness should be used:
(c) An aluminium structure with sheet glassfor
the construction of the walls and the cover represents
a good, but expensive. solution and would not often
be available;
fd) For the absorhing black surface, two
solutions could be envisagedat the moment: concrete
with special bituminous paints, which has been
successfully experimented with in India, or thin butyl
rubber sheet about 1 mm thick, which has also been
successfully experimented with in Australia, Greece
and other countries. The latter material, while it
requires rather more sophisticated technology (sealing
by vulcanization or with adhesives),is not affected by
solar radiation, high temperature or’ dry spots;
however, it is often not available ;:I developing
countries;
(e) Insulating the still under ihe basin is not
justified when the surface is large becausethe ground
acts as a semi-infinite medium through which there
can be very little heat loss, and insulating the sidesof
the still should not significantly increase the
productivity owing to the relatively small heattransfer surface involved. The cost of the insulation
materials and its installation is therefore not
economical in terms of increasing the productivity of
large stills. Nevertheless, the use of available local
insulation materials should always be studied. It is
essential that any insulation used be kept dry. A layer
of dry earthv material beneath the basin liner is
usually sufficient for insulation purposes;
(j] Regarding the cover, sheet glassabout 3 mm
thick still seems to be the best solution. Use of a
cover of two sheet-glasslayers is not justified. The
experience with plastics is not yet conclusive. Several
kinds (PVC, PVF, PTFE, polythene, polyester,
polytrifluoromonochloroethylene, and nylon) have
been tried in conditions less severe than in full-sized
stills. The only materials that will last as long as five
years are 0.005.in. (0.13-mm) PTFE, 0.004~in.
(0.1 O-mm) PVF and 0.005in. (0.13-mm) weatherable
polyester. In general, the lifetime of plastic material
exposed to solar radiation is very short. In addition,
very thin covers do not withstand the effect of wind;
_’
,L
:
.__
.
1
28
Technology for Solar Energ)* Utilization
(g) The tightness of the still is very important
and at present no efficient, cheap sealant materials
are on the market. Silicone rubber is very good, but it
is also very expensive. A cold-application mastic
bituminous compound intended for repairing leaky
roofs has been successfullyused on stills in India, but
it is too soon to forecast what the lifetime of the seals
may be;
(k) Regarding the depth of the layer of water in
the still, it is generally agreed that shallow layers give
better productivity, but if the ground is not level, dry
spots on the absorbing bottom can appear if butyl
rubber or equivalent material is not used; a depth of
5 cm seemsto be a realistic solution;
(i) No general agreement exists as to the best
inclination of the sheet-glasscover. It is important to
ensure the formation of a film that permits both
adequate condensation and adequate transmission of
solar radiation; IO”-20” seemsto be acceptable. For a
given horizontal surface to be covered, increasingthe
inclination increasesthe surface of the cover and thus
its cost, and may decrease the productivity of the
still;
(j) Any effect of the geometry of the covering
on the fraction of incident energy entering the still is
negligible in large units, where the area of shadow
causedby the sidesis small compared with the area of
the evaporating surface;
(k) A basin still is usually oriented with its long
axis aiong the E-W or N-S direction. The orientation
of the symmetical-cover or low-slope still does not
affect productivity: asymmetrical and single-sloped
covers should be oriented with the long axis in the
E-Wdirection, with the low-slope or single-coverplate
facing towards the equator (S in the northern
hemisphere, N in the southern);
(1) The still should serve also as a rainfall
catchment surface;
(m) The optimal regime of feeding the still with
fresh water (continuously or by batch) has not yet
been determined. It is certain, however, that periodic
flushing of the still with fresh water can prevent the
deposit of salt and the growth of algae,both of which
decrease productivity. Salt deposition should be
avoided, as it increasesthe reflectivity of the black
surface and thereby decreasesproductivity;
(n) The addition of a few parts per million of a
copper salt should help prevent the growth of algae.
solar drying
Drying food, agricultural products or fish by
solar energy is a very old practice. However, in recent
years there has been intensive R and D in developed
and developing countries to systematize the process
and to protect products from rainfall, dust and
insects. The Brace Research Institute’ (Canada) has
prepared a very useful study of the process; most of
this section is based on it. Data on solar driers are
presented in the form of casestudies which deal with
descriptions of the driers, experimental results and
drawings. Economic information has also been
included.
Casestudy 1. Solar cabirwt drier: general design
The solar cabinet drier is essentially a solar hot
box in which fruit, vegetablesor other matter can be
dehydrated on a small scale. It is insulated and is
covered with a double-layered transparent roof. Holes
drilled through the basepermit fresh ventilating air to
be drawn into the cabinet by convection.
The construction of such a drier can t%e man).
forms. Nevertheless, certain specifications can be
recommended for all driers of this type (figure 24;.
The length of the cabinet should be at least three
times the width so as to minimize the shading effect
of the side panels. The angle of the slope of the roof
covering should be taken from figure 25, which gives
the recommended angle for drying seasons as a
function of latitude. The graph is equally applicable
to areasnorth and south of the equator. Note that for
latitudes less than 20°, the slope of the transparent
roof is constant at 6”. This is to allow a minimum
difference in elevation from one side of the collector
to the other to permit adequate convective air
circulation over the drying area and to allow rain to
run off the cover. The transparent cover should be
made from two layers of either glass panes (2 mm
thick) or plastic film (about 0.13 mm thick).
In general, covers made with plastic film have a
limited life and therefore films that have been treated
to give protection against ultraviolet radiation must
be used. They can be of polyester or PVC; films of
polyethylene or cellulose acetate should not be used
becausethey would have to be replaced at the end of
each drying season and might not give favourable
results in service. Although it may be advantageousto
replace covers seasonallyin certain cases,trouble may
occur with films that are not able to withstand the
high cabinet temperatures generated, which may
reach SO”-100°C in some driers. It is adljisablein this
type of unit to use ordinary window glasssupported
by a suitable frame.
The frames of portable models may be of wood,
metal or hardboard. Plywood may be used for the
more sophisticated units, basketwork, wicker or
bamboo for the more primitive units. Perforated
cabinet bases and side panels may be fabricated by
placing insulation between layers of blackened wicker
‘A Surveyof Solar Dryers.Technical Report T99, Brace
Research Institute, McGill
Quebec, Canada (1975).
University,Ste.
Anne de Bellevue,
‘.,&‘., ‘_‘(
a,y. ,’ /
_/
.
:,
Development of solar energy utilization in developing countries
29
hole for
: air
SECTION
1
2
3
4
5
6
7
8
9
10
11
12
13
14
AA
Description
Material
Dimensions lcml
Side panel
Rear panel
Front panel
Base
Hole liner
Wood
Wood
Wood
PlvNood
Polythene pipe
Wood
Wozd and wire mesh
Wood
Glass
Wood
Fibregless
Wood
Wood wool
Aluminium
foil
2 thick
2 thick
2 thick
198X66X2
12.7 diam. X 6 long
Leg
Tray
Door
Transparent
Tray runner
cover
Aperture screen
Cover frame
Insulation
Internal side-wall
covering
62 X 62
61 X62
Number
required
2
1
1
2
27
4
3
3
3
4
27
1
5 thick
Figure 24. Drawings and specifications for a solar cabinet drier
2
b.,
:
;
‘.
_:.,,
I’..,,>
..,
_
..‘,,
-i
_,
Tecfrnology for Solar Energy Utilization
30
00 i
.._.-------100
71
200
30-7
400
Latitude, Nor S
Figure 25.
Optimum slope of soiar drier cover as a function
of the latitude of the location
or open basketwork. This would cut down costs and
make use of local industry.
Permanent structures may be made of adobe,
bricks, stone or concrete.
The insulation should consist of locally available
materials, such as wood shavings, sawdust, bagdsse,
coconut fibre, reject wool and animal hair. in areas
affected by wood ants, termites or other noxious
insects, the susceptible materials should be properly
protected before being placed in the base.
The hot box should be constructed along the
lines outlined in figure 24. (The dimensions shown
are those actually used in the drier of case study 2
below.) The insrrlating layers lining the baseand sides
should be at least 5 cm thick. Holes should be drilled
in the insulated base and fitted with short lengths of
pipe (plastic or rubber garden hose, bamboo etc.).
Where insect infestation is prevalent, all cabinet
apertures should be covered with fine mosquito
netting (preferably fibreglass) or gauze.Generally the
high temperature of the cabinet interior discourages
insects and rodents from entering and feeding on the
drying produce. Furthermore, in arid areas where
there is a high concentration of airborne dust and
debris, the transparent cover eliminates product
contamination.
The transparent cover can be attached to a frame
that can then be fixed to the chassisof the cabinet. If
glass is used, a sealant may be used to hold it to the
frame; otherwise, the glass should be held firmly in
place by a suitable moulding. Care must be taken to
ensure that the cover is completely watertight to
prevent the interior from deteriorating and the
‘insulation from getting wet. All components of the
cover framework should be painted black or some
other convenient dark colour to absorb the maximum
amount of solar radiation. Hold-down strips should
be secured to the upper exterior rim of the cover
frame to protect the film against excessivelift caused
by wind.
Once the cover and chassis are secured, several
holes should be drilled in the rear and side panels.
Theseprovide the exit ventilation ports to remove the
warm, moist air. The number of holes depends on
climatic conditions and the nature of the drying
material. A satisfactory method is to provide the drier
initially with a minimum of side ventilation ports and
to drill further holes as needed to prevent internal
moisture condensation. This method ensuresthe right
number of ventilation ports are drilled.
The rear panel should be fitted with accessdoors
to give entry into the cabinet. All doors should be
placed on the rear side to prevent excessive
shadowing of the drier during handling operations.
Trays should be constructed, as indicated, of
galvanized chicken-wire or some similar material.
They should be placed on runners a few centimetres
high so as to ensure a reasonable amount of air
circulation under and around the material being
dried.
The interior of the cabinet should be painted
black. The exteriors of the side, rear and basepanels
should be painted with aluminium paint. If desired,
the interiors of the side and rear panels can be
covered with a layer of aluminium foil. If the foil is
not available, these surfaces should be painted black.
Drier operation is not complicated. The produce
to be dried is pretreated in the usual manner (i.e.
blanched and fumigated) and placed on the
perforated trays, at a loading rate of about 7.5 kg per
square metre of drying area. A small thermometer
inserted into one of the ventilation ports, its bulb
shielded from the direct rays of the sun, is a
convenient accessory; the upper temperature limits
that agricultural produce can withstand vary greatly.
Where the produce being dried might suffer from
the direct rays of the sun or where the light colour of
the produce reflects much of the incident radiation, it
is advisable to cover the loaded trays in the drier with
a black plastic mesh or black gauze. This covering
should not inhibit the flow of air tllrough the trays,
but absorb the radiation and transmit the heat to the
produce by conduction and convection. The resultant
temperature increase can be controlled by opening
the rear accessdoors. This approximate temperature
control system can be easily mastered with
experience.
Casestudy 2: Solar cabinet drier at Kanpur, India;
experimental results
Climatological data
The pertinent climatological data for the test site
are:
Maximum temperature in summer
Minimum temperature in winter
Hours of sunshine per year
Days with no sunshine per year
45°C
10°C
4 000
30
It was found that the optimum tilt of the drier was
13” in summer and 40” in winter.
-31
TABLE3.
EXPERIMENTALRESULTSONADRIER
AT KANPlJR.lNDIA
Moisture cotlterl t
Amount
dried
MI
Produce
Prunes
Peaches
Peas
Cauliflower
PretreatmeN t
Sulphuring
None
Blanching
None
7110
215
420
200
M2simum
allowable
Iemperatlrre
(“Cl
(%,J
hitial
Preftrred
jittal
85
80
80
85
I s-20
5-6
5-6
5-6
Dr>-irrg
time
01I
71
-7
5
18
11
5
2.5
--
Drying data
The drier was used on an experimental basis from
July 1971 to July 1972. Table 3 gives results
obtainqd.
Fqr purposes of comparison, table 4 gives some
indica$on of the temperature limits and possible
throug!.lputs of other produce with a drier of the
same size and specifications in the. dry, arid, cloudless
Mediterranean climate.
TABLE 4. PROBABLE PERFORMANCE
OF THE DRIER
INTHEMEDITERRANEAN
CLIMATE
Produce
Apricots
Garlic
Grapes
Okra
Onions
AunL.-.
[fresh) dried
per unit
_ .~
Processing
time
(“Q
NJ
/kgJ
Maximum
allowable
temperature
Casestudy 3: Solar wind-ventilated
drier
(Syrian
Arab Republic)
66
60
88
66
71
2
7
4
2
2
4
2.6
5.1
3
3
by flying stones t
‘Xscounting the breakagehazard,
the glass would
,nally have a longer life, say
r is installed on the roof of a
IO-20 years if th
analysis, the farmer himself
b$lding. In the
must decide which .A the more economical and
acceptable cover for him.
Since the drier is simple to construct, the farmer
can build it himself using simple hand tools, thus
lowering labour costs.
The drier can also be used to warm foods and
other materials. It is particularly advanta :ous as a
self-contained source of heat at 70”-80°C 3J
’ the field
and in isolated farm areas.
I
Several wind-ventilated driers of the kind
described below were used in the Syrian Arab
Republic on an experimental basis during 1564-1968.
Characteristics
Operating conditions
It was observed that an average temperature of
75°C was attainable inside the drier as compared with
an averageoutside temperature of 35”.
The drier was built according to the specifications of figure 24 in two days and cost about $20 in
1973. Annual operating expenditures were as follows
(rupees):
Depreciation
Interest on capital (at 10%)
Contingencies
Total
16
16
8
40 (about
$5)
Conclusions
The esfirnated life of the drier is 10 years. Solar
drying saves considerable time as compared with
simple drying in the open. Also, the final product
obtained from the drier was found to be superior in
taste and odour and was not contaminated by dust or
infested by insects.
Sometimes it may be preferable to have a plastic
cover rather than a glasscover, which can be broken
The drier can be described as a drying chamber
through which warm air, heated in a solar collector, is
drawn by means of a rotary wind ventilator. (See
figure 26.)
The solar air-heating collector used consists of a
blackened hardboard sheet, insulated at the bottom
and covered by a plastic (or glass)sheet. The collector
is mounted facing due south and tilted at the
optimum angle for the area and season.
Air enters through the open bottom end of the
collector. It passes up between the hardboard
blackened bottom (absorber) and the cover. The
effectivenessof the collector is increased by placing a
black mesh screen midway between the cover and the
absorber; solar radiation that passes through the
transparent cover is then absorbed by both the mesh
and hardboard. The mesh provides an additional heat
transfer surface area, and increased heat is supplied to
the passing air. Collector efficiencies of over 75%
have been achieved using this system.
The warm-air outlet of the collector is connected
to the base of the drying chamber, which holds
adjacent stacks of six trays each. Hot air circulates up
through the drying produce. Additional heating is
obtained from solar radiation transmitted through
rj.
,’
F_
e II
:
.’
i
,
Technology for Solar Energy Utilization
32
FRONT
VIEW
SIDE VIEW
Ventdator
slack
Drymg chamber
Drying
trays
----------1
!hhl
-
Solar wind-ventilated
transparent sheets covering the east, south and west
sides of the drying cabinet. The rear vertical and
bottom horizontal panels of the drier are of
blackened hardboard insulated to reduce heat losses.
A rotary wind ventilator is placed on top of a stack
above the drying chamber. An adequatelength of this
stack is required both to achieve a chimney effect and
to catch more wind.
The rotary wind ventilator is a corrugated vane
rotor. As it spins in the wind, it expels air from the
ventilator stack. The rotor is mounted on a
ball-bearing suspension. The friction is low, and
momentum keeps the head spinning even in sporadic
winds. Quantitative tests carried out using the
ventilators indicate that the rotary ventilator yields a
constant exhaust of high volume in spite of
intermittent winds. A stationary eductor placed on
top of a chimney can also be used; however, it would
rely solely on natural convection during periods of no
wind.
The materials of construction are as follows:
Insulation
Trays
Solar air heater
Transparent cover
Absorber
Insulation
Frames
Collecfor
-
83cm
122cm
Figure 26.
Drying chamber
Transparent cover
Frames
wheaung
Polyester film
Blackened hardboard for the
back wall and bottom panels
Straw
Wire mesh
Polyester film
Blackened hardboard bottom
sheet with a black plastic mesh
2 cm above it
Straw
Blackened hardboard sheets
on bottom and side walls
drier
Climatological data
The drier is located at Dima (near Damascus)at
latitude N 33”33’ and longitude E 36”24’. The
climate is characterized by dry, cloudless summers,
and cool, partly rainy winters. There is a substantial
annual variation in monthly mean temperature that
divides the year into four distinct seasons.Nevertheless, the spring and autumn periods seem to blend
partly into the traditional desert summer climatehigh daytime temperatures; low relative humidities;
clear, cloudless days; no precipitation for nearly six
months. In most of the country the percentage of
sunshine during the period May-October is over 85%.
During the period June-September, the percentage is
generally above 95%.
Drying data
The unit successfully dried okra, cousa (Baladi
variety), squash, Jews’ mallow, egg-plant, tomato
paste and yams. As an example of the drying yields
attained by the unit, drying times for okra and cousa
were reported to be, respectively, 20% and 58%
shorter than with sun drying. In addition, the final
product quality obtained using this solar drier was
reported to be superior.
The efficiency of the solar air heater was
reported to vary between 64% and 88% (ratio of
useful heat absorbed into air stream to the energy
transmitted through glazing).
,,‘jf’F’;
,“: ‘,,*.-I
,>#‘ ‘,
>-
:4
,I’
\
33
-Development of solar energy utilization in developing countries
The operating conditions during the tests the
results of which are reported above were as follows:
The day was fairly sunny, and the ambient air
temperature was 30”.34°C. The temperature of the
heated air entering the drying chamber was
36.2”-58.4”C.
Comments
For solar drying purposes, the greater the air
flow within the drying chamber, the greater the yield.
If the average wind speed is high, the use of a
stationary eductor instead of a rotary ventilator will
be just as practicable. When higher temperatures are
desired for drying particular crops, dampers installed
in the ventilator stack will permit control over the
air-flow rate and thus the temperature of the drier. If
the ventilator diameter is small, it seemsbetter to use
a stack of a larger cross-section with a smooth
transition to the ventilator section. That will reduce
air friction and ensure adequate air flow.
Evaluation
These solar driers are simple to construct and
economical to use. However, local imaterials and
technology should be used to the m&mum. Farmers
may be encouraged to construct their own solar driers
based on models designed by local institutions and
available for demonstration purposes.
The only possible external source of energy in
small applications is, in the present conditions, the
wind. When using forced convection obtained by fans
operated by thermal, electrical or solar engines, a
feasibility study should be undertaken to find out
what percentage of the consumed energy can be
delivered as mechanical energy by the engines. Many
research projects are being undertaken in various
developing countries, and it seemsthat the following
suggestions could be included in these research
programmes when forced convection is used:
(a) Study of the influence of the temperature of
solar-heated air. This temperature could be varied
until it reaches the maximum compatible with the
conservation of the quality of the dried products by
varying the performance of the solar collectors or the
velocity of the heated air;
lb) Study of the influence of the Reynold’s
number of the flow of air acrossthe dried product by
changing the dischargeof the fan;
(c) Reduction of the useful fluid power ‘by
optimizing the pressure drop between the solar
collectors and the outlet of the dryer. In the
intermittent drying regime, calculation and measurement of the moisture should be made at the end of
each period.
II. TECHNO-ECONOMIC EVALUATION OF
SOLAR EQUIPMENT
Comparison of solar and internal combustion engines
The term “solar engine” designates in this study
a complete installation of an engine powered by solar
energy, and the term “internal combustion engine”
will be limited to diesel and gasoline engines (gas
turbines of very small power do not exist yet on the
market).
Many factors could be considered when undertaking a techno-economic comparison of this nature,
but to simplify the study, these will be limited as
much as possible without significantly altering the
validity of the comparison. The cost of the installed
equipment, depreciation, maintenance, repairs and
consumed energy and the impact of solar-energy
technology on industrial development will be
discussed.
A general equation to find the maximum cost per
unit of installed power after which the solar engine
begins to be more economical than a diesel or
gasoline engine will be presented below. To keep the
equation simple certain assumptions were r:;ade;these
should be kept in mind in reaching conclusions based
on the equation.
Assumptions
A solar engine in favourable solar radiation
conditions may work about 6 h daily, but it does not
give its maximum output all of that time. Hence, an
operating time at nominal power of 1 500 h a year
seems to be reasonable. This duration could be
extended in the caseof extensive heat storage.
Except for experimental purposes, the use of
fractional-kilowatt solar engines does not seemto be
justified in developing countries in the short and
medium term. For instance. a ‘55kWsolar pump could
be replaced by a %-kW animal-powered pump
working 8 h daily. The energy saving in using small
solar engines is small; for example, a H-kW solar
engine will save only about 250 kg of fuel per year.
The transport of such a quantity of oil would not
pose difficulties. Solar engines in the 2-l 5 kW range
would be more economic and would sell better.
A diesel or gasoline engine can easily work 8 h
daily (3 000 h in a year). A set of two engines each
having half the power of the solar engine will be
considered, so that a stand-bv will be available and a
I
reliable performance can be expected.
Most current small diesel engines in the speed
range 1 000-l 5 000 rpm have a power of 2 kW or
more. Therefore, in the low range only gasoline
engines, and for the range 10-l 5 kW only diesel
engines,will be considered.
It will be assumed that the annusl cost of the
loans made to finance the installation will be equal to
j$‘;(&
S,”
__.
_
“_ ~‘&‘I,
a’, * ,,
- /
I
34
Techmlogy for Solar Energy Utilizatior~
half the rate of interest multiplied by the amount of
the loan. This simplification will not significantly
affect the accuracy of comparison becauseit applies
to both solar and conventional installations and is
relatively unimportant as against the other terms.
This comparison omits the effect of inflation.
Cost comparison equation
Based on the above considerations, the following
equation can be written:
(4+b.+~)K,=(~+b~+~K,+q,c,+q,c,
(1)
where the symbols have the following meanings:
Variables
a annual rate of depreciation (equal numerically to the inverse of the expected lifetime
of the engine in years)
b ratio of the annual cost of maintenance and
repair to the cost of the installed engine
rate of interest on the loan used to finance
i
the cost of the installed engine
/.
K Cost of engine per unit of installed pows~ ”
4 Annual rate of consumption of diesel fuel,
gasoline or lubricant (per unit of installed
power)
i
c Unit cost of diesel fuel, gasolineor lubricant
Subscripts
5 solar engine
t thermal engine (specifically:
g = gasoline)
f diesel fuel or gasoline
1 lubricant
system, the material used, the accuracy and the
process of manufacture, and the operating conditions
(duty cycle, level of maintenance, climatic conditions). A lifetime of 6 000 working hours will be
assumedhere. This seemsto be a reasonable lifetime
in isolated places where changing the engine before
major repairs are required makes economic sense.The
rate of depreciation will then be at = 0.25.
Gasoline engine. A lifetime of 3 000 working
hours will be assumedfor the gasoline engine, giving
ut = 0.5.
Maintenance and repair
Solar engine. Commercial companies sometimes
refer to free (or insignificant) maintenance and repair
of solar engine installations. A machine to be
operated by a solar engine as well as the solar engirlr:
itself will always need maintenance and rtipair
(replacement of broken sheet glassof solar collectors,
replacement of deteriorated insulation, repainting of
containers of solar collectors, possible repainting of
the black surface of the solar absorbers, replacement
of working fluid, correction of leaks, plumbing). The
cost of a full or partial salary of a person to take care
of the day-to-day maintenance will not be included
because it will also be required for the diesel and
gasoline engines. Taking the expected lifetime into
account, it will be assumed that b, = 0.02-0.03 for
solar thermal engines and b, = 0.01 for solar
photocells.
Thermal engine. The maintenance and repair
ratio for both diesel and gasoline engines will be
taken asbd = b, = 0.20.
Rate
d = diesel;
.Numerical application
:
Values of the v+?ables
The probable value of the variables in equation
(1) will now be discussed.
Expected lifetime of the engine
S&r engine. Commercial companies speak of a
lifetime of 20 y for the solar engine. In view of the
present technological conditions, that figure seems
optimistic. Here we shall assume a lifetime of 10 y;
hence we have a, = 0.1.
Diesel engine. The lifetime of a diesel engine can
be as long as 20 000 working hours depending on the
power, the design and especially on the speed of
rotation, the piston speed, the type of cooling
of interest
The rate of interest will be taken as 8% (i = 0.08)
even though it is higher than that now.
Fuel and lubricant consumption
Diesel engine. The consumption of diesel fuel is
about 0.25 kg/kWh and that of lubricant about
0.007 kg/kWh.
To combine the cost of fuel and lubricant, it will
be assumed that the cost of the lubricant is 5 times
that of the fuel. Hence the equivalent fuel
consumption is 0.25 + 0.007 X 5 = 0.285 kg/kWh. If
the engine runs 1 500 h annually, we obtain qf
(equiv) = 0.285 X 1 500 = 427.5 kg/kW as the annual
rate of consumption per unit of installed power. As
the conditions of exploitation of the engine will not
always be optimal, it would not be unreasonable to
increase this figure by about 17% to get a round
500 kg/kW, or 0.5 t/kW. as the value of qf.
Gasoline engine. The annual equivalent fuel
consumption for gasoline engines will be assumedto
be 20% greater than that of diesel engines, giving
qf = 0.6 t/kW.
Development of solar energy utilization in developing countries
Comparisons
A IO-kW solar engine working 1 500 h a year will
be compared with a set of two diesel engines each
having 5 kW and 1 500 rpm and working 6 000 h over
4 y. A 2-kW solar engine will be compared with a set
of two gasoline engines each having 1 kW and
working 3 000 h over 2 y.
Taking into consideration the above assumptions,
the following equations c?n be written:
For the diesel engines:
(0.10 + 0.03 + 0.04) K, =
= (0.25 + 0.20 + 0.04) Kd + 0.5 Cf
or
K,z3K,+3c,
(2)
and for the gasolineengines:
0.17 K, = (0.50 + 0.20 + 0.04) K, + 0.6 cf
or
K, = 4.35 K, + 3.53 cf
(3)
where the K are expressed in currency units per
kilowatt and cf is in the samecnits per ton of fuel.
Since taxes vary from country to country, it will
be assumed that no taxes will be placed on the
equipment and its installation and on the consumed
fuel.
The unit cost of diesel fuel is difficult to
estimate; it depends on many factors, such as the
price of crude oil, the distance between the refinery
and the crude-oil supplier, and the cost of internal
transport and distribution.
The unit cost of installing a diesel engine of the
size being considered in the present (1976) conditions
of the international market may be assumed to be
150 $/kW. In the same conditions, the unit cost of
installing a gasoline engine in the I-kW range is
50-70 $/kW.
Substituting Kd = 150 $/kW, K, = 50 $/kW and
cf = 150 $/t in equations (2) and (3). we find that the
cost of installing a solar engine should not exceed
900 $/kW if it is to compete with a dieselengine and
834 $/kW if it is to compete with a gasolineengine. it
is important to note that these figures are the cost of
the solar engine without heat storage.
The present cost of solar engines varies greatly;
amounts as low as 3 000 $/kW and as high as
20 000 $/kW have been asked by manufacturers. The
price depends strongly both on the size of engine and
on the number of identical units produced, as can be
seen from the example provided by one manufacturer
presented in figure 27.
Comments and conclusions
(a,J Calculations of the cost per unit of energy
(kwh for example) is not useful becausethe variables
vary too much from country to country. A general
equation, such as equation (1) above, is more useful:
35
/bj A solar thermal engine within the range of
2-10 kW begins to be economically feasible when its
unit cost does not exceed 1 000 $/kW. Thd present
cost on the marhet is 3-N times higher ( 1976);
(L’j Solar photocehc would be economically
feasible at 7 000 $/kW becausethe cost of repair and
maintenance and depreciation are lower. Also, the
cost of an electric generator was not included in the
costs of the diesel and gasolineengines.
(d) Economy of scalecan not be fully applied in
the case of solar thermal engines, since the required
surface area of the solar collector is approximately
proportiona! to the power of the engine. (The
improvement of the efficiency of larger engines can
be neglected in a first approximation.) The solar
collectors represent an important part of the total
cost of the installation. However. the cost per unit
power of other components, transport and installation will be greatly reduced when the power
increases:
(ej The investment is higher for solar engines;
(f) The solar engine installation will occupy a
ground surface of 20-50 m2 for each kilowatt of
installed capacity; this extra land cost should be
taken into consideration. However, if solar collectors
are used as roofs for buildings, the cost per unit of
installed capacity will decrease;
(gj If there were a feasible way of storing solar
energy, the cost per unit of installed power capacity
would increase, but the cost per unit of energy
deliveredwould decrease;
(12) It is difficult to obtain a constant speed on
the shaft of a solar engine unless a costly and possibly
sophisticated system of control is provided. A
constant speed may be necessary for some applications;
(i)
The machine that will be coupled to a diesel
or gasoline engine giving the same energy will have
half the power of that coupled to the solar engine, so
its cost, transport and installation will be cheaper and
easier.
Solar refrigeration and air conditioning
Solar refrigeration and air-conditioning systems
are still bulky, complicated, too expensive and
require as much as 30% of their input energy to come
from an external source. It was determined in the
third case study of a solar air-conditioning system
above that, with the cost of fuel at 480 s/t and
increasing 20% annually and an interest rate of 9%.
20 years would be required for a IO-kW solar
air-conditioning unit to pay for itself, and that
calculation was rather optimistic. It is obvious that
further R and D is necessary,particularly to decrease
the proportion of external energy needed.
,‘,
,I,
::
36
Technology for Solar Energy Utilization
(a)
Unit cost as a function
of size of plant, with
production
10
run as parameter
100
1000
Electnc power capacity IkW)
(I9
0’
1
Unit cost as a function
of production
run, with
size of plant as parameter
-----I10
Production
run humber
100
of units)
1000
Figure 27. Unit cost of installation of solar power plants
III.
SOLAR ENERGY UTILIZATION IN
DEVELOPING COUNTRIES
Evaluation of experience in developing countries
rubber. This technology seems to be reliable, but it
requires materials that are not often availablein most
developing countries and its cost is relatively high.
Such stills can be reproduced only if good technicians
are available.
Greece
In Greece,the major development in solar energy
is water distillation. The most important installations
in the world are here, at Nisiros and Fiskardho for
example (see table 1).
The basic design of the stills is an aluminium
structure with sheet glas for the walls and roof
(figure 22 (I)). The black absorbing surface is made of
thin sheets of butyl rubber sealed with silicone
India
India has an ambitious programme in solar
energy R and D:
Solar water desalination
One installation for solar water desaiination is
located at Bhavnagar with a capacity of 0.9 m3/d,
and another of 5 m3/d is under construction. The
principal materials are bricks, cement, glass, wood
Development
of so&
37
energy utilization in developing countries
and cotton electrical insulating tape. A small quantity
of aluminium is also used. The black absorbing
surface ir concrete painted with mastic. The total
estimated cast for this installation is about $28 000
or about ‘~$13 per square metre of effective
evaporating surface. The expected average rate of
production of this installation is about 2.5 1rne2d-’ .
The technology seems to be reliable and easily
transferable to other developing countries.
Solar water heaters
An electrical equipment company is experimenting with solar collectors with a view to mass
producing solar water heaters. It plans to use existing
facilities for the aiuminium roil-bond technique to
construct the collectors.
Solar space heating
A system of heating by hot air has been
developed, but the costs of such a system cannot be
evaluated until the results of the R and D programme
on solar collectors are known.
Solar pumps
The Birla Institute of Technology and Science at
Pilani has already constructed two prototypes of a
solar pump without moving parts. These two
prototypes are now under experimentation. The first
evaluation shows that the water pump is promising
and that the cost per unit power of the installed
prototype could be about 2 000 $/kW. Many
technological problems are still to be resolved.
Other work
Work is just beginning on solar drying, solar
air-conditioning, solar refrigeration and conversion of
solar energy into mechanical energy. The National
Aeronautical Laboratory is working on windmills
tising a technology similar to that used in developed
countries. This work is conducted in three directions:
Irrigation: Improvement of the existing
multiblade model
Electric power generation: mode1 with two
or three blades and flap control, with a
nominal output of l-5 kW at a wind speed
of 5 m/s
Pumping: windmills to ;ower pumps for salt
works
An attempt is being made to combine the
pumping of water with the generation of electricity.
The hope is to achieve a cost per unit power of
1 000 $/kW.
Mexico
The most important solar energy programme in
Mexico is solar pumping, which is based on the
SOFRETES technology. A low-temperature solar
turbine of 25 kW driving an electric generator, which
in turn supplies energy to one or more pumps, is
operating at San Luis de la Paz, about 350 km from
Mexico City. The cost. however, is still prohibitive.
Eleven I-kW pumps are already installed, eight are
under construction and one solar pump to be driven
by a rotating solar engine is planned.
interesting work has been done on the
construction of solar collectors using a container
made of asbestos cement, imported aluminium roll
bond as water ducting and absorbing surface, double
silicone rubber sealant and fibreglass
gluing,
insulation. A solar collector using a fibreglass
container is in the R and D stage.
Mexico is planning to produce low-power solar
pumps in large numbers for export to other
developing countries. However, with the present
reciprocating Freon engine and the high cost of the
other components of the system (solar collectors,
heat exchangers), it will be very difficult to withstand
the competition of diesel and gasoline engines.
Discussion of problems and solutions
Applications of solar energy that are feasible for
developing countries under present conditions are:
(a) Solar distillation of sea water and brackish
water at low and medium fresh-water production
rates; about 30 m3 ,‘d seemsto be reasonable;
(b) Solar domestic water heating on an individual or collective basis. Solar water heaters hnve been
proved competitive and are already very popular in
Australia, Cyprus, Japan and the United States. The
technology of this application is easy and can be
adapted to local conditions;
(cl Solar drying. Heretofore, this type of drying
has always taken place in the open air. Now, with a
simple apparatus, it should be possible to obtain
clean, healthy, products of better quality. The
equipment is simple and can be manufactured
without great difficulty. Local institutions should
prepare adequate designsfor demonstration purposes.
Farmers could use these models to manufacture their
own driers. The use of solar driers with an additional
source of energy could be envisagedwhere energy is
cheap and maintenance and repair can be carried out
easily. But it will not often be a good solution to
transform the mechanical or electrical energy into
heat to drive the necessary fans. Combining wind
energy with solar drying may be 3 good solution.
Applications that are not yet feasible are:
(a) Solar engines.Many prototypes already exist
on the market. Some of them have been tested over
years, some for months, and others are only now
ready for testing. It would be inadvisable for
developing countries to use these solar engines for
purposes other than experimental. Their cost is still
prohibitive and the technology has not yet been
thoroughly tested;
(b) Solar electricity generation by photocells.
Although photocells are claimed to be reliable, their
cost is still very high and it is not advisable to use
them in developing’ countries for purposes other than
experimental. The technology of photocells is still
being developed, and it is hoped that in the not too
distant future these cells will have the same success
that transistors have had. In such a case, small
developing countries could envisage the assembly of
such units, and large ones could envisage partial or
total manufacture;
{cl Solar refrigeration. Unit< independent of
external sources of power are being developed, but
even for small cooling capacity, they are very large
and heavy. F-tir intermittent or continuous absorption
machines, the solar energy equipment will be a
supplement to conventional machines. The existing
technology seems to be feasible; however, an exact
feasibility study should be prepared to determine to
what extent external sources of power would be
acceptable. The manufacture of ice is often a good
solution because the ice can be transported and used
to preserve food or other products.
Brinkworth, B. J. Solar energy for man. Salisbury,
Compton Press, 1972.
Eggers-Lura, A. Flat plate solar collectors and their
application
to dwellings. Low temperature
conversion of solar energy. Commission of the
European Communities.
The documentation section of this report
surveys the present state of the art, and
thereafter a summary is given of solar R and
D work undertaken so far in the individual
EEC countries, in other European countries,
and in countries outside Europe that have
made important contributions
to solar
energy research.
International Solar Energy Society, U.K. Section.
Solar energy-a United Kingdom assessment.
London, May 1976.
An analysis of all aspects of solar energy
systems, made to assess the potential for
solar energy utilization and R and D needs in
the United Kingdom of Great Britain and
Northern Ireland and for export.
Kredder and Kreith. Solar heating and cooling. New
York, McGraw Hill, 1975.
BIBLIOGRAPHY
Messel, H. and T. S. Butler, eds. Solar energy-a
course of lectures. Oxford, Pergamon, 1975.
American Society of Heating, Refrigeration and
Air-Conditioning Engineers. The 1972 handbook
of fundamentals. New York.
The 1972 handbook of systems. New York.
The 1972 handbook of applications. New
York.
The 1972 handbook of equipment. New
York.
Noyes Data Corporation. Solar energy for heating and
cooling of buildings. Patton, 1975.
Austrian Solar and Space Agency. Small solar power
systems. August 1976. (FA-3) The Austrian Solar
and Space Agency (ASSA) was charged by the
Federal Ministry of Science and Research to
carry out an inquiry amongst all member
countries of IEA concerning national activities in
the field of small solar power systems. This
inquiry includes work being carried out in
member states on R and D on solar power plants
and their subsystems as well as information
systems covering this area.
Backus, Charles E., ed. Solar cells. New York, IEEE
Press, 1976.
Brace Research Institute, Macdonald College of
McGill University. A survey of solar agricultural
dryers. Ste. Anne de Bellevue, Quebec, Canada,
December 1975. (Technical Report T99)
This report describes some of the solar
agricultural driers actually built by experimenters and practitioners in the field.
Photovoltaic power and its applications in space and
on earth. Paper prepared for the International
Congress: The Sun in the Service of Mankind,
Paris, 2-6 July 1973.
Solar desalination-status
and potential.
&J
C. Mustacchi and others. Rome, ASIS, May
1976.
Prepared for the Commission of the
European Communities. The aims of this
study include: a summary of the state of the
art in the field of solar desalination; a choice
in priorities for future programmes of
research and development; and indication of
the potential benefits of technological
advance and of the appropriate siting of the
research efforts.
United States. Library of Congress, Science Policy
Research Division. Survey of solar energy
products and services, May 1975.
This survey was conducted to obtain
descriptive information on solar energy
hardware and related services.
Vaillant, J. R. Les problbmes du dessalement de l’eau
de mer et des eaux saumltres. Paris, Eyrolles
Editeur, 1970.
Part two
SUMMARIES OF COUNTRY
AND INSTITUTIONAL
PROGRAMMES
ON SOLAR ENERGY
Country
programmes
Austria
Austrian Solar and Space Agency
The Austrian Solar and Space Agency (ASSA)
co-ordinates solar energy research and applications in
Austria. Its main tasks are:
(n) To serve as a contact with foreign and
international solar organizations:
(b) To advise the Austrian Government on solar
research, technology and applications of interest to
Austria, taking into account international developments in this field;
(c) To process and distribute information and
data on solar technology to Austrian industry and
scientific institutes; to organize lectures and conferences; to train Austrian specialists in solar energy.
ASSA has carried out three studies so far. One
deals with solar energy systems in member countries
of IEA, particularly in the field of solar power plants
and their subsystems.
The study Kleine Sonnenkrajtwerke fiir Entwicklungsltinder (FA-4) (Small Solar Power Systems
for Developing Countries) considers existing small
solar thermal power stations up to a capacity of
50 kW and suggeststhat a prototype with a capacity
of 10 kW (flat collectors or concen:rating collectors)
be established.
The study Meteorologische Messdaten jiir die
Nutzung der Sonnenenergie (FA-5) Meteorological
Data for the Utilization of Solar Energy, discusses
meteorological facts for direct use of solar energy in
view of their importance for setting up solar technical
plants.
ASSA wishes to promote R and D projects in
scientific institutes, universities, researchcentres and
industry in close co-operation with other countries.
Austria is participating in the IEA programme and has
initiated a co-operative programme with UNIDO.
Activities of other institutions
Several Austrian working groups at universities,
research centres, and in industry are concerned with
solar energy R and D activities, especially in the area
of solar heating and cooling systems and in the
production of electrical or mechanical energy.
Solar heating and cooling
To promote the economic use of solar energy in
Austria, severalprojects, e.g. houses. swimming pools,
collector test stations in various parts of the country,
have been established with the support of the Federal
Ministry for Science and Research. The essential
factors for the economic use of solar energy plants in
given geographical, meteorologicai and other environmental circumstances are being summarized and
evaluated. The possibility of integrating solar
techniques with conventional systems is investigated
under this programme. From the experience gained,
guidelines are to be established with regard to
equipping solar technical plants for producing warm
water and heating houses with the simplest possible
control systems.
Design of solar houses
The designing of buildings to have the lowest
primary energy demand is one of the most effective
methods of saving energy. For over five years the
Institute of Building Construction has advocated the
adoption of a low-energy philosophy both in building
construction and design method and has promoted a
“heavy-weight” integrated design approach that has
resulted in significantly improved thermal response,
energy savingsand economy of building systems.
Production of solar collectors
Austrian firms produce various kinds of flat-plate
and concentrating collectors and special test stations
have been established to make comparative tests of
them.
Solar heating systems
In 1976, the manufacture of complete solar
installations for heating homes and swimming pools
was begun. Approximately 100 installations of this
type are in operation. A special collector control
system has been developed that enables the
collector-fluid flow rate to be regulated depending
upon the temperature level.
41
Technology for Solar Energy Utilization
42
When considering application of solar heating in
Austria, the following possibilities must be investigated:
Off-seasonheat storage
Combination of solar energy and heat pumps
to increase the temperature of solar-heated
water or the output of the solar collectors
Combination of solar heating with conventional stand-by heaters during co!d spells and
cloudy periods
For economic reasons, seasonal storage is
unrealistic at present in Austria. Concerning the two
other possibilities, the Institute of Applied Physics of
the University of Vienna carried out a study to design
a solar energy installation for the newly founded
Institute of Molecular Biology, in Salzburg, of the
Austrian Academy of Science. This installation is to
serve as a demonstration plant for the use of solar
energy under the specific climatic conditions in
Salzburg. It is the first large-scale solar-heated
building in Austria. The main purposes of this
demonstration plant are:
(a) To test on a long-term basis the performance
and ability to resist corrosion of the solar collectors;
(b) To gain experience on the joint operation of
the solar collector and heat pump;
(cl To collect and evaluate extensive data to
provide the technical and economical basisfor further
application of solar heating (and cooling) of buihiings
in Austria.
Solar cooling systems
Research on space cooling with an absorption
machine and flat-plate collectors is being carried out.
The first installation will be put into an apartment
building m Spain. It is planned to have a heating-fluid
temperature of 80°C and a cooling-fluid temperature
of [email protected]
Conversion of solar energy into electrical or
mechanical energy
Photovoltaic systems
Four university institutes are carrying out
theoretical studies, and a government testing
laboratory is implementing an extended field test
programme to evaluatesolar celI panels.
The Institute of Physics of the University of
Vienna is actively interested in this field and is
planning research on the preparation and properties
of solar cells for earth applications. CdS and
poIycrystaIline Si are regarded as the most promising
candidate materials.
The Institute of Physico-Chemistry of the
University of Vienna deals with material characterization of semi conductors.
At the Institute for Applied Physics of the
Technical University of Vienna, investigations have
been carried out since 1965 on the photoconductivity
of single crystals of CdS and the enhancement of
their sensitivity by doping with Te. In 1972, the
investigations were expanded to the study of thin
films of CdS. The work was finished in 1974 with a
research paper about evaporation techniques and
photoconductivity of the thin films. Investigations on
photovoltaic cells are planned.
A research team at the Institute of Solid State
Physics, University of Linz, has been engaged for
severalyears in research and preparation of infrared
detectors. The researchers are experienced in
prp,raring thin-film devicesusing epitaxial growth and
ior. implantation techniques. The research could be
shifted to solar cells.
The Federal Testing and Research Institute, at
Vienna, has installed four solar cell panels at a test
site in eastern Austria. The panels, consisting of Si
solar cells, will be tested for two years. Each panel
has a maximum power output of 1.6 W. The test
programme includes recording of power output as a
function of solar radiation, weather conditions etc.
Solar thermal-electric systems (including collectors)
Solar-thermal conversion systems. In recent years
universities, research institutions and industries have
co-operated closely in developing solar collectors. The
possibility of obtaining higher concentration factors
in flat collectors by change of surface structure or
selective coatings is being investigated. Water
temperatures of 300°-4000C seempossible. For small
power plants such types of collectors may be
versatile. The co-operating institutions include: in
Vienna, the Institute of General Physics of the
Technical University, Institute of Physico-Chemistry
of the University of Vienna and Federal Testing and
Research Institute, in Graz, Institute of Thermal
Power and Nuclear Engineering of the Technical
University and the Research Centre, Institute for
Environmental Research.
Solar power plants. The International Institute
for Applied Systems Analysis (IIASA) at Iaxenburg
near Vienna is evaluating large-scale solar-thermal
power plants of the power-tower types.
The .!lstitute of Thermal Power and Nuclear
Engineering of the Technical University, Graz, has
developed thermal storage systems (steam storage by
means of pressurized hot water) for energy storage in
power plants. The proposed scheme is particularly
suitable for load balancing in solar-thermal power
plants. Depending on size and operating pressure,
either welded steel vesselsor prestressed cast-iron
pressurevesselswill be employed.
Denmark
About 50% of all the energy consumed in
Denmark is used for domestic water and space
13
Country programmer
heating. Hence, the solar R and D effort is directed
towards those purpdses.
In 1975, the Zero-Energy house, a one-family
house with a floor area of 125 m2. was completed. It
is heated by solar energy, the waste heat from the
electrical installations in the house (e.g., light bulbs,
refrigerator and washing machines), and the body
heat of the inhabitants. The Zero-Energy house has a
vertical solar collector of area 42 m2 and a water
storage tank of capacity 30 m3. The Zero-Energy
house is uneconomic, but does conclusively prove
that even at the high latitude of Denmark (56”) a
house can be heated by solar energy alone, provided
that it is well insulated.
In 1976, a new building code was introduced
requiring increased insulation of new buildings.
Through proper insulation alone, the annual energy
requirements of a single storey, one-family house can
be decreasedfrom aboiit 25 000 kWh to 10 000 kWh.
Further work has been directed especially
towards space and water heating by solar energy in
combination with heat pumps. A solar house at Skive,
North Denmark, was constructed in 1977. The total
heat requirements of the house are covered by a solar
collector of area 28 m2 ccjmbined with two heat
pumps that recover heat from the exhaust air and
waste water of the house.
Finally, significant R and D wcrk is being done
on solar refrigeration by a solid absorption plant. The
preliminary study covers a plant producing ice at the
rate of 500 kg/d.
There are many solar-heated housesin Denmark,
and three or four firms are producing flat-plate solar
collectors for spaceand water heating.
France
Since the energy crisis in the early 197Os, the
French Government has shown a great interest in new
energy sources. A special ministry “La delegation aux
energies nouveiies” was created to co-ordinate
research. development and industrial programmes.
About 300 researchersin severalnational laboratories
are engaged in fundamental research to fiid ways to
use solar energy (biological, chemical and physical
processes). The industrial application of these
developments is very different for developed and
developing countries, from the viewpoint of both the
objectives and the needsof t!le countries.
French industry has develop,zd solar heaters for
domestic and industrial water and for heating houses.
About 50 experimental houses have been built using
flat-plate collectors (area 20 m’). A total collector
surface area of about 15 000 ,m2 has been installed in
industry.
The Government expects to reduce total energy
consumption by 5% by 1980. French industry is
developing possibilities of converting solar energy
into electricity by a high-temperature thermal process
through a programme carried out jointly by the
national laboratories and international partners. At
Odeillo, a converter using the concentration system is
in operation. Day and night, it supplies electrical
energy at the rate of 60 kW. The working
temperature is around 350°C.
For developing countries, French industry is
engagedin a programme concerned with:
Irrigation pumps using small collectors
Supply of electricity
Refrigeration (of foods)
Desalinization
Air-cooling
For greater feasibility of technology transfer, the
flat-plate collector technology with low-temperature
cycle conversion has been chosen. Around the world.
about 50 prototypes of I-kW solar pumps are being
operated. The surface of the collector is about 50 m2
and the overall efficiency is about 2%. It is hoped to
increase performance further. having in mind the
same principal objective: the possibility of transferring technology.
Federal Republic of Germany
The R and D programme of the Federal Republic
of Germany for the utilization of solar energy was
launched as a part of the non-nuclear R and D
programme at the beginning of 1974, immediately
after the oil crisis. Up to then, solar energy had been
considered uninteresting because of the extreme
northern latitude of the country, with a mean
insolation of only 110 W/m2 and a mean sunshine
duration of 1 300-2 000 h a year, depending on the
region under consideration. However, studies and
experiments with test houses carried out in recent
years have proved that utilization of solar energy is of
interest even to countries situated in mid-Europe.
For the time being, activities are focused
primarily on the possibilities for supplying hot water
or heating buildings and not on generating electricity
because of the structure of the country’s energy
consump!ion. About 40% of final energy in the
Federal Republic of Germany is required for space
and water heating, 36% for generating process heat
and merely 24% for generating light and power. Thus,
the heating sector is the major area of interest in
terms of securing the energy supply in the long run.
The expenditure on solar R and D was DM 6.2 million in 1974, DM 15.9 million in 1975, and
DM 27.0 million in 1976.
Based upon the experience gained since 1974. a
new solar energy research programme (1977-1980)
has been launched, with a budget of about
DM 150 million. The major objectives of the
programme are discussedbelow.
Technology for Solur Ettergy Utilization
44
Titerma uses
Systems for producing hot water using solar
radiation are aheady commercially available in the
Federal Republic of Germany. However, the main
components, such as collector and hot-water storage,
have to be improved in order to become competitive
with oil, gas or coal heating systems. In addition,
long-term behaviour and maintenance of solar heating
systems have to be evaluated. Therefore, the
following activities are of major importance in the R
and D programme of the Ministry of Research and
Technology:
Measurement of data to assessthe efficiency
of hot-water supply systems
Short- and long-term testing
Standardization of components and systems
Development of new absorbers and minimization of collector losses
Solar systems for heating buildings are being
tested in several solar demonstration houses. The
major objective is to improve the economy of already
existing systems,so that the housescan be introduced
on the market within 10 years. Activities are
predominantly concentrated on:
Improving collectors and storage technically
to facilitate massproduction
Expanding knowledge of system behaviour
Strengthening the capacity of adequately
trained architects, engineers and skilled
workers
In addition to these activities, data will be
collected on climatic conditions, types of building
that can be equipped with solar systems,effects on
the environment and construction practices.
Processheat is another sector of interest for the
application of solar power. For the Federal Republic
of Germany, the ranges below 200°C and around
2 000°C are important. At present, R and D activities
are directed mainly towards temperatures below
200°C for processesof drying, distillation, cooling,
petrochemistry or evaporation.
Electrical utilization
Developments in the sector of electrical utilization are focused at the moment on systems with a
closed thermal cycle. The collectors that have already
been developed for heat supply purposescan be used
in these systems. Turbines and expansion machines
are available, operating with Freon as a working
of success in generating
medium. The PiOSFCtS
power by means of photocells are poor, on account
of the high cost of solar cells, the low efficiency and
the very considerable area requirements in Central
Europe. However, in the long run, it may be possible
to introduce this method in the sunny regions of the
world.
The generation of electricity with the aid of solar
energy-whether by thermodynamic or photovoltaic
means-is one of the most promising possibilities for
supplying energy to rural areas in the developing
countries. R and D activities are oriented particularly
to this aim, namely, to develop well-functioning
systems in the IO-kW range that are easy to maintain
and to produce lessexpensivefacilities.
There are plans to construct solar power stations
of 500 kW capacity within the framework of the
European Communities (tower concept) and IEA.
The wind-energy potential for the area covered
by the Federal Republic of Germany is, in theory,
extremely high-the average power of the winds is
about 200 X lo9 kW, but in practice wind energy can
be utilized only to a very limited extent. On the one
hand, new kinds of technical effort must be applied;
on the other hand, the problem of storing energy
ariseo,as it inevitably does when solar energy is used
on a broad scale. Both these limiting factors prevent
the wide use of wind energy in the Federal Republic
of Germany, which would suggest that it could be
used for special purposes only, or where local
conditions are particularly favourable. R and D
activities are now primarily focused on the designof a
1-3 MW windmill plant, to go under constru:~ IOI: !‘I
1979 and a 10-kW vertical-axiswindmill.
by Kiv2
iiiOiiOl1
II.!:I
The energy piO\iJtd
utilized for years to supply electricity for ZLVYS
and lighthouses. Such facilities have a low capacity,
however, usually under 100 W. They operate where
the utilization of another form of electricity would
be both difficult and costly, meaning that even the
high costs of power generation by wave energy can
still be accepted. The switch from small facilities to
use in industry entails severalproblems. To date, no
large-scale facility has been built. Analyses have
invariably shown that such facilities would not be
competitive with other power plants. It has been
estimated that, for the Federal Republic of Germany,
the power of the ocean waves along the entire North
Sea coast is modest: 3 600 MW. Without doubt, it
would be possible to utilize only a small proportion
of this capacity for practical purposes. In this respect,
the situation for countries with long coastlines, such
as Ireland or the United Kingdom, certainly holds
more promise. Becauseof the low energy potential in
this sector, R and D acttvities in the Federal Republic
of Germany are minimal.
Biological and chemical utilization
Another brngd
- Y.. field is the biological and
chemical utilization of solar energy. As a result of
natural biological processes,a vast amount of solar
energy is converted into chemically combined energy
and stored. This energy can be releasedagain with the
aid of thermal fermentation processes.The efficiency
of these systems is, however, very low. The
4s
--
technically and economically usable potential is so
small in the Federal Republic of Germany that
corresponding R and D projects are being promoted
to a modest extent only. The investigation of
biological processesoccurring in photosynthesis and
photolysis constitutes the focal point of the R and D
work currently in progress.
International co-operation in R and D
The new solar energy R and D programme is
being carried out in part through international
co-operation. Major partners in multilateral co-operation are the European Community, IEA and
UNESCO. In addition, numerous bilateral agreements
on solar energy utilization have been signed, including
technology transfer agreements with developing
countries, such as one for the construction and
operation of a IO-kW electricity generation plant in
Egypt and a sil;:.ilarplant in India.
India
Interest in solar energy utilization was revived in
1973 in the wake of the energy crisis. In that year,
the National Committee on Science and Technology
appointed a committee to compile a status
report on solar energy and to recommend to the
Government action to be taken. The Committee
brought out, in 1974, a comprehensivereport entitled
“Solar energy-hope and challenge”. The report
critically evaluated the technology available in the
country and also the state of the art in the world. It
examined the energy consumption of various sections
of society and identified urgently needed R ana D
programmes. Basedon these recommendations, solar
energy programmes are being pursued in the country.
Collector development has been given highest
priority.
The Indian energy scene is characterized by low
per capita energy consumption, about 100 kWh per
day, and hence low per capita gross domestic
product. India is essentially an agricultural country
with 70% of its population living in rural areas.There
are as many as 350 000 villages with a populatjon of
less than 500. Becauseof their low load factor and
their distance from the industrial belt, it is most
uneconomic to electrify these villages. It is equally
difficult to reach them with fossil fuels. Therefore
locally available energy sources must be found. Thus,
a solar energy programme in India is heavily biased
towards meeting the energy needs of the rural sector.
India has a well co-ordinated and broad-based R
and D programme in solar energy with targets to be
achieved within certain periods. The Energy Research
Committee, which monitors this programme, consists
of Secretaries in the Government and presided over
by the Union Minister for Energy.
The Government is deeply committed to
developing solar energy utilization. It is liberally
funding R and D. The Government has also permitted
all expenses incurred on solar applications ill industry
to be treated as researchexpenditures and therefore
eligible for tax reduction. Among the organizations
actively engaged in solar energy utilization or in
funding R and D are the Tata Energy Research
Institute, Kirloskar’s. Jyoti Ltd., Metal Box and
Khatau.
R and D is being pursued in relation to the three
approaches to the utilization of solar energy; (a)
low-grade thermal devices, (b) direct energy conversion and f~) biological conversion. In each of the
areas, a project co-ordinator has been appointed to
monitor the projects and prepare to pioduce these
devices. In addition, extensive work is in progress in
the India Meteorological Department to establish a
network of radiation stations and collect solar energy
data at as many places in the country as possible.
Some 36 stations have been set up, of which 13 are
principal stations measuring total, diffuse and
normal-beam radiation and hours of sunshine. Other
stations measure total radiation and sunshine hours.
The necessary instruments (pyranographs, pyrheliometers, sunshine recorders) are being manufactured in
India.
Collector developmelrt
Various selectivecoatings are under investigation.
Stagnation temperatures as high as 18O’C have been
achieved. Punjab Agricultura! University is developing
low-concentration, modified Winston collectors.
Steam up to 5 atm has been obtained. Very
high-concentration, high-temperature collectors are
under development at Radio Telescope Laboratories,
Ootacmund. Bharat Heavy Electricals Ltd (BHEL) is
responsible for manufacturing these collectors and
co-ordinating the programme.
Solar pumps
Solar pumps have received very high priority.
The National Physical Laboratory (NPL) has
developed an organic Rankine cycle engine using
spiral expanders. Prototypes of the 1-kW
ABHIMANYU pump have been in operation since
mid-l 976. Auroville Centre for Environmental
Studies is developing organic Rankine cycle engines
similar to SOFRETES pumps. The Birla Institute of
Science and Technology has developed a lowtemperature direct-contact vapour pump for lift
irrigation. Feasibility and cost analysis studies for
heads up to 30 m and a capacity of 10’ I/d havebeen
completed. Similar pumps have also been developed
by the Central Mechanical Engineering Research
Institute. Medium-temperature 200-W hot-air engines
based on the Stirling cycle are under development at
46
Techology for Solar Energy Utilizatiotl
the Central Salt and Marine Chemicals Research
Institute (CSMCRI). One private enterprise, Metal
Box Ltd., has entered into an agreement to
manufacture Fluidyne pumps developed at Harwell.
Jyoti Ltd. is developing I-kW pumps using
cylindro-parabolic stationary concentrators and steam
Rankine cycles.
Solar drying
Cabinet driers have been developed by the
Central Arid Zone Research Institute at Jodhpur,
RRL at Jammu and the Indian Institute of
Technology (IIT) at Kharagpur. In 1976 a major
portion of the apricot crop in Kashmir was dried
using cabinet driers developed at RRL. Annamalai
University has built a prototype of a l-t/d paddy
drier that has been in operation for over a year. The
National Industrial Development Corporation has also
fabricated a I-t/d paddy drier that is under test. The
Forest Research Institute has undertaken a pilot
project to establish four timber drying kilns in four
parts of the country. Amul Ltd. has undertaken the
project of making powdered milk using solar energy.
Desalination
CSMCRI (Bhavnagar) is active in desalination. It
has provided drinking water facilities for several
lighthouses and distilled water facilities for many R
and D laboratories and educational institutions. It is
providing two villages in Gujarat with drinking-water
facilities. The laboratory has also built a 103-l/d solar
still. Based on a humidification-dehumidification
technique, the laborat.ory has set up a pilot plant
producing at the rate of 2 X lo3 l/d.
Space heatiq and cooling system
In 1975, BHEL undertook a major project in
technical collaboration with NPL to provide space
heating in one wing of its factory at Harwar. The
system has a collector area of 200 m2 and stores
lo4 1 of water at 60°C during the day. This hot water
is passedthrough fan-coil units to provide heating to
the workers in the factory.
IIT (Madras) has developed a space cooling
system that has been on trial for several months. NPL
has provided space heating in one room in the
building and is setting up a combined space heating
and cooling system. The cooling cycle based on the
vapour absorption refrigeration principle will operate
during the summer. TIT (Madras) and BHEL have
developed l-t air-conditioners based on the ammonia-water absorption cycle.
Direct energy conversion
A well co-ordinated R and D programme
concerned with various aspects of direct energy
conversion has evolved. Various options are being
tried. The Solid State Physics Laboratory is making
single-crystal Si solar cells of the quality used in space
programmes. T,1-0
.v efficiency of these ce!!s is around
10%. The Central Electronic Engineering Research
Institute, Pilani, is exploring the possibility of using
high concentrations to reduce the cost of solar cells.
The National Aeronautical Laboratory is studying the
feasibility of growing single-crystal Si ribbons. IIT
(Kanpur) and NPL are exploring the possibility of
making Si solar cells from polycrystalline films.
Cds-CuzS solar cells are being developed by IIT
(Delhi) and NPL. Programmes in the area of direct
conversion are being co-ordinated by Central
Electronics Ltd., a Government of India enterprise.
Solar power plant
A IO-kW solar power plant is being established at
Madras as a joint venture between the Government of
India and the Government of the Federal Republic of
Germany. BHEL, NPL, and IIT (Madras) are the
executing agencies on the one side and MBB the
executing agency on the other. This solar power plant
will have a flat-plate collector array and an organic
RanIcine cycle engine with a Linde screw expander.
Feasibility studies on another IO-kW power plant
using spiral expandersare in progress.
Biological conversiott
Relatively little attention is being paid to
biological conversion. Biogas systems are becoming
very popular in the country. Solar energy is being
used to heat the slurry with a view to increasing the
efficiency of biogas conversion. In addition, some
attempts are being made to convert agricultural waste
into methane, ethanol etc.
My
Solar water heater
Several laboratories have developed solar water
heaters. These are now commercially available in the
country. A few other commercial enterprises are
entering the field. Industrial solar water heating is
also being attempted in two or three industrial
enterprises.
Nat-plate collectors
Over 20 manufacturers are already on the market
with standard flat-plate collectors, mostlv 2 m X 1 m,
with single or double glass and galvanized-steel.
fibreglass or painted steel case. Commercial prices
range around 100 $/m2 and installations to provide
hot water for one-family houses or apartments range
between $600 and $800, made up with a collector
area of 4 m2, a 100-l tank, piping, pump, insulation
and controls. The manufacturers have scarcely been
able to penetrate the market.
Research on collectors, begun in 1958, is carried
out in five universities and by six private manufacturers. It concerns, among other things, sealants,
surface treatment and the use of plastic honeycombs
to increase efficiency. Few funds have been budgeted
for it.
Italian ResearchCouncil, a third solar boiler was built
(150 bar, 500°C) with 121 cone-mirrors clockoperated by an appropriate mechanism and producing
steam at an averagerate of 40 kg/h from 11 a.m. to
3 p.m.
In the period 1946-1977 a new mechanism with
271 flat mirrors 80 cm in diameter (205 m2 total
surface) was developed. The steam was produced at
the rate of 130 kg/h at 150 bar, 600°C with an
efficiency 0.73 at an isolation of 800 W/m’. Tests
carried out at 800°C showed equally good efficiency.
Solar pumping units
Research was carried out at the University ot
Milan in the 1940s on a solar engine using sulphur
dioxide as working fluid. This engine, coupled with a
pump, has been produced on a small scale by a
private company and installed in several tropical
countries.
Soiar power statioiu
The first solar boiler working at high temperature
and pressure was built in 1961. With conical
concentrators and manual tracking, this installation
could produce steam at the rate of 7 kg/h at a
pressureof 100 bar and a temperature of 500°C.
In 1963 a second solar boiler supplying 38 kg/h
at 100 bar and 450°C with an efficiency of 0.6 was
built. This installation was later assembled in
Marseille in 1964. In 1965, with support of the
Desalination
The Universities of Cagliari, Bari and Rome are
carrying out a limited amount of work in
desalination. During the past 20 years, solar stills have
been studied. Several pilot-scale installations were
built (mostly by Nebbia University of Bari, with the
financial support of the Italian ResearchCouncil) arid
have operated successfully. Feasibility studies and
engineering estimates are being carried out 011 the
completed flat collector and multistage flash units at
the University of Rome.
Silicon cells
Very limited work on radiation damageto such
cells is being carried out at two universities.
.
.
Institutional
programmes
Austria: Institute for ;Environmental Research
Since 1974, a team of experts has been exploring
possibilities of using sc’lar energy in Austria. The
lnstitute for Environmental Research at Graz
developed a solar collector that is now commercially
manufactured in Austria.
In 1976, a 600-m2 solar collector system was
installed for heating a swimming pool in the village of
Eggersdorf. The design criteria were based on
achieving a water temperature of 24°C from May
until September. Other projects involving the use of
solar energy include the reduction of heating costs for
greenhouses,crop crying (tobacco barns) and cooling
(air-conditioning). The design for an Austrian solar
house is under way.
A contract has been signed for co-operation in
the field of solar energy with the University of
Dhahran, Saudi Arabia.
Canada: Brace ResearchInstitute
‘The activities of the Brace Institute in Quebec
are devoted particularly to problems in rural areasin
developing countries. The Institute is active in
development and application of solar distillation,
solar agricultural driers and solar cooking.
The Institute has a number of co-operative
projects in development, fabrication, installation and
testing of solar stills, driers and cookers in several
developing countries. The Institute also has assisted
institutions in developing countries in formulating
solar energy programmes.
Egypt: The Industrial Development Centre
for Arab States
The Industrial Development Centre for Arab
States (IDCAS) located in Cairo, is affiliated with the
Arab League. The 21 Arab States are membersof the
Board of Directors. One of the main activities of
IDCAS is to further co-operation among Arab
countries in the various branches of industry.
IDCAS has held two meetings on solar energy in
the Arab countries as a start towards collecting the
48
information required on steps that the Arab countries
must take to make the utilization of solar energy
economic. IDCAS is ready to co-operate with any
organization inter;*<tcd in solar energy and is willing
to prepare comprehensive studies on possibilities of
using solar energy in the Arab countries.
India: the National Physical Laboratory, New Delhi
In the early 1950s the National Physical
Laboratory began work in tile field of solar energy
and continued until 1958 or so. During this period,
the Laboratory developed solar water heaters, solar
cookers and solar stills. A few solar water beaters
were made and installed, and they are still working.
Many solar cookers were also fabricated. However,
becauseof the plentiful supply of conventional forms
of energy there was little demand for appliances run
by solar energy. Consequently. the work in this field
was largely abandoned.
NPL is responsible for developing high-efficiency,
medium-temperature flat-plate collectors. In the
process, it has investigated a variety of selective
coatings such as oxides of copper, nickel and
chromium, lead sulphide, cadmium telluride, and
cermet films of MgO and gold. It has also developed
structural selective coatings and selectivewindows on
the glazing surfaces using SnOz or In2 OJ . Stagnation
temperatures as high as 180°C have been obtained.
NPL is also responsible for establishing testing
procedures and standards for these collectors. It has
set up extensive facilities for testing them. Emissivity
and absorptivity of selectivecoating and transmission
characteristics of glazing surfaces can be measured
over the spectral range 300 nm to 20 m. All the
parameters of a flat-plate collector can now be
measured. Testing of assembled collectors to
determine their efficiency can also be done. Facilities
are set up to study the influence of environmental
conditions, humidity, salt spray etc. on the
performance of solar collectors. The field testing is
done by BHEL. NPL is in the process of establishing
helio centres where artificial sun conditions in respect
of solar insolation will be created to test solar
collectors and solar cells.
NPL has developed 75-l and 150-l solar water
heaters that are now being tested.
The Libyan Arab Jamahiriya:
Arab Development Institute
The Arab Development Institute was established
in the Lib>an Arab Jamahiriya in 1975 to mobilize
the efforts of Arab scientists for the benefit of Arab
countries. Its objective is development through
scientific and technological research.
The Conference on Physics of Solar Energy, held
in 1976 at Car Yumo University in Benghazi, made
recommendations to the Institute on the establishment of a centre for solar energy studies. The
Institute is now in the process of establishing this
centre.
Research is concentrated at present on solar
collectors. Work will begin soon on cycle optimization for solar engines and on radiant exchange
through glasscovers.
Parallel to the work of the Institute, research is
being carried out on solar stills by a group at El-Fateh
University (Tripoli).
Saudi Arabia: College of EngineeringUniversity of Riyadh
From 1969 to 1973, several solar water heaters
and solar stills were made and tested at the College of
Engineering of the University of Riyadh. Since 1973
work has been carried out in the following areas:
radiation estimations, single-slope concrete stills,
night cooling, solar homes and their prospects.
convection suppression and honeycomb structures.
effect of ultraviolet light on skin, thermal pile storage
and other types of storage, flat-plate collectors and
their environments, and air heaters.
United States: Solar Energy and Energy Conversion
Laboratory of the University of Florida
Over the years, the qolar Energy and Energy
Conversion Laboratory of the University of Florida
has been measuring the amount of solar energy
available and its characteristics and has developed
tables giving data for design purposes. Solar
properties of material are measured in a solar
calorimeter, developed by the Laboratory, which is
the only instrument of its kind described in the guide
of the American Society of Heating, Refrigerating
and Air-conditioning, which is the main reference
book in this field.
The laboratory has developed collectors for
water heating, space heating and cooling, swimmingpool heating, fresh-water generation and liquid-waste
recycling. The work also includes conversion of solar
energy for cooking purposes, baking or in solar
furnaces. The Laboratory has developed two solar
electric cars, two solar electric buses and a solar house
in which over 9070 of the energy used is provided by
solar energy.
Part three
SELECTIQN OF TECHNICAL PAPERS
ON THE UTILIZATION
OF SOLAR ENERGY
A lo-kw
solar electric
.Hans Kleinra th
Institut fir Elektrische
Maschinen, Tech&he
Universitri’t, Vienna, Austria
There are many options open to the designer of a
solar power plant with an output of some 10 kW of
electricity. Some of these are set out below.
Component
of plant
Working
fluid
Water
Organic
Mixture
Collector circuit
(low concentration
(high concentration
liquid
of systems
Storage
Thermal
Electrical
Prime mover
Steam engine
Turbine (low-speed)
Turbine (high-speed)
Primary
High-frequency
machine
Machine with a frequency of roughly
50 Hz with or without gears
Electrical
generator
network
to build a plant that had the lowest pcissiblenumber
of control devices and could operate automatically.
Figure 1 is a schematic diagram of the plant.
OptiOllS
Flat-plate
Concentrating
factor)
Concentrating
factor)
Collector
power plant
The concentrating collectors selectedwere of the
parabolic-trough type with a concentration factor of
10. Compared with fixed-direction flat-plate collectors, parabolic-trough type collectors can have a
higher working temperature without the efficiency of
the collectors decreasingstrongly, and hence a better
Carnot efficiency; they are also better at collecting
energy throughout the day, since they can be made to
track the sun.
Due to the relatively low concentration factor
the receiver pipes produce hot water at 135’C. This
implies a relatively low pressure and consequently
low stressesfor the fittings, valvesand insulation. The
total area of the collectors is about 300 m2.
D”
AC
Freon circuit and prime mover
The current projects of several manufacturers
consequently employ a wide range of ideas and many
different permutations of the above-mentioned
possibilities. This paper describes an Austrian project
The primary circuit is connected by a pre-heater
and an evaporator to the secondary Freon circuit. A
Freon turbine is used as prime mover. Unlike screw
Prr-hi
Figure 1. Schematic diagram of the Austrian I CbkWsolar electric power plant
53
.,“_,
~:-:
.(“,
,~-;..,
_I
‘
Technolog,v for Solar Energy Utilization
54
expansion engines, a turbine maintains an approximately constant speed even under partial load. It is
thus easy to obtain a constant voltage from a directly
coupled AC generator.
The turbine specifications are as follows:
Working fluid
Inlet temperature
Outlet temperature
Inlet pressure
Outlet pressure
Rated output
RI13
120°C
73°C
7.0 bar
0.7 bar
16 kW
By placing a recuperator between the turbine
outlet and condenser, about 11 per cent more
primary energy can be utilized. This reducesthe total
collector area needed, as well as the amount of
cooling water needed in the condenser.
Generator and electrical network
Figure 2. Calculated characteristic curves of the plant (June)
A synchronous generator is directly coupled to
the turbine. When an AC machine of the homopolar
type is used, the rotating part consists of solid steel
without any windings, slip rings or brushes.
The voltage of the high-frequency generator is
rectified and then connected to a battery. This
battery operates both as ati electrical store and as a
buffer for the rotating unit, keeping its speed
approximately constsnt. The third aim of the battery
is to feed all auxiliary drives during the starting
procedure.
Performance of the plant
As the collectors track the sun, the energy
produced increasesvery quickly to its peak value. As
soon as the energy produced can cover both the
auxiliary drives and the losses of the whole
equipment, the pumps for the three circuits start.
After the turbine has started, equipment utilizing tl!e
electricity is switched on automatically according to
the amount of electrical energy available.
Figure 2 shows the characteristic curves calculated for operating conditions in Austria in June.
Curve 1 is a plot of the variation of direct insolation
as measured by a meteorological station. The
absorbed energy is plotted on curve 2; this is reduced
by shadows cast by one coilector on another when
the inclination of the collectors is more than 45”.
Curve 3 is a plot of the efficiency of the
collectors and curve 4 of the useful electrical output.
Supplementary equipment
The simplest network is the DC network
described, with the battery acting as a store. The
main advantagesof this network are:
No need for a high-precision speed control
No gear between the prime mover and the
generator (this would be indispensable if 50
or 60 Hz AC were to be produced, because
prime movers of only 10 kW are of higher
speed than 3 000 rpm)
No additional equipment is needed for the
starting phase
The simple control system has E high degree
of safety
A power plant with an output of only 10 KW will
in most cases be enough to power lighting,
broadcasting and television equipment. Electrical
drives can also be fed if DC motors are used.
Nevertheless, AC motors may sometimes be used.
Then a supplementary converter of the single
armature-converter type is needed. In the classic
single-armature converter with its commutator and
slip rings, the commutator needs cleaning and the
brushes need changing from time to time. Nevertheless, it has some advantanges over the thyristor
converter: it is much cheaper; it has very low losses:
and it is very insensitive to overload, for example
during the starting period of AC motors.
Conversion of solar into mechanical
or electrical energy : Indian experience
V. G. Bhide
National Physical Laboratory,
New Delhi, India
The total area of land under cultivation in India
is about 143 X lo6 ha, of which 30% or
43 X IO6 ha, is irrigated. Of this irrigated area,
23 X lo6 ha are served by tube wells, ponds or other
minor irrigation schemes. The Fifth Five-Year Plan
(1974-I 979) envisagedan increase in irrigated area of
11.2 X 1Oh ha, of which 6 X 10” ha were to be served
by minor irrigation schepes.
It was originally envisaged that 1.S million
1S-kW (2.hp) pumps would be installed during this
period. However, because of the oil crisis, not only
have these additional pumps not been installed but
even a fair fraction of the existing 2.5 million pumps
do not have an energy supply (diesel or electricity).
Consequently, the development and production of
solar pumps in the range 1S-4 kW (2-5 hp) have
received highest priority. The pumps will preferably
be in modular form so that when pumping is not
required, the same system can be used to produce
equivalent mechanica! or electrical energy for minor
industrial operations or for lighting.
Another approach that ?he
in India have
adopted is to initiate research, development and the
installation of solar power stations in the range
lo-100 kW, each of them meeting the total energy
needsof a village or a cluster of villages.
Photovoltaic or thermoelectric conversion of
solar energy does not appear practical, and hence
efforts have concentrated on directly utilizing heat
from the sun.
Abhimanyu solar water pump
Figure 1 shows the Abhimanyu solar water pump
developed by the National Physical Laboratory
(NPL). Its primary components are a flat-plate
collector array and a closed-cycle organic Rankinecycle engine. During operation, a heat-transfer fluid
(water) flows through the collector array and is
heated to a temperature of 80*-95’C, depending
upon the collector efficiency and configuration and
the solar fiux. This hot water is used to vaporize an
organic liquid with a low boiling-point in a
reverse-flow heat-exchanger (boiler). The hot, highpressure organic vapour is then used to drive the
pisnners
Control
valve
Boiler
Condenser
Figure 1. Diagram of Abhimanyu solar water pump
55
:. j,~”
_
‘;.
.?‘i
.; :
,.
Tecitrtologv for Solar Energy Utilization
56
expander of the Rankine-cycle engine. After leaving
the expander, the vapour is condensed in a condenser,
where the water being pumped is used as the heat
sink. The condensed organic liquid is pressurizedand
then pumped back into the boiler with the help of a
reinjection feed pump mounted and driven by the
shaft of the expander. For some organic fluids, there
may also be a regenerator that utilizes exhaust vapour
superheat to pre-heat the fluid coming into the boiler.
remjection teed pump and workmg fluid. Nevertheless, it is obvious that the higher the efficiency of the
collector array and the higher the temperature it can
produce, the higher the system efficiency. It is all the
more necessary to optimize the collector assemblyin
a large-scale power plant because a substantial
fraction of the total cost is accounted for by the
collectors.
The organic Rankine-cycle engine is particularly
suitable for solar pumping and power generation and
for waste-heat utilization for severalreasons:
(a) High thermal efficiency even when operating
with the low to moderate temperatures (SO”-95°C)
achievablewith flat-plate collectors;
(b) Low-cost components owing to the use of
commonly available construction materials and
simple mechanical components;
(c) High reliability because of its sealed
construction, which protects it from harmful effects
r\e-zmir;rrrr.mr~t
r:,
1, ilJ
00 cLL,,
%3rl dust and moisture;
ui
iii:ii-CLiiiiiiii
iiic.L
(d) No problem with freezing, since the working
fluids have very low freezing-points.
Collector arrays
Flat-plate collectors were used in the Abhimanyu
pump (see figure 2). The absorber plate is made of
aluminium alloy with channels built into it by the
pressure-bonding technique. The complete mechanical design of the collector was optimized.
Selective coatings ot’ oxides ot copper, nickel,
chromium, as well as PbS, CdTe and other materials
were tried. With the best collector, it was possible to
obtain a stagnation temperature as high as 180°C for
an insolation of 1 000 W/m* (figure 3). Figure 4
shows the annual variation in the daily useful gain in
thermal energy from a typical c-ollector assembly
operating at 90°C. Although it was attempted to
create selective windows by coating glasscoverswith
SnOz, In2 O3 etc., the cost involved in the processdid
not justify the improved efficiency.
Since it is advisable to operate the expander at a
constant input temperature, a reservoir is needed to
store hot water: the reservoir is connected to the
array by meansof a thermostatically controlled bypass
valve. Under operating conditions nearly one half
of the heat is required to pre-heat the organic liquid
and the other half is used during the process of
boiling. Some saving on the collector area requirement can be effected by having one collector array
for low temperatures and one for high temperatures.
The former is used to provide pre-heating whereasthe
latter servesto boil and superheat the organic vapour.
The efficiency of the system, which to a great
extent determines the collector area requirement and
hence the cost, is defined here as qs = qc X 7),, where
qe is the engine, and qc the collector, efficiency.
Whereas the engine efficiency increases as the
collector temperature increases, the collector efficiency decreases on increasing the collector
iemperature. It is therefore necessaryto determine
the temperature range for each collectorengine
combination that produces the maximum system
efficiency. As we shall seelater, this optimum system
efficiency depends upon several factors such as the
insolation, the condenser temperature, and the
characteristics of the collector array, expander,
80 mm
Extruded
aluminium
__-
-. - --Glass wool
channel
v
Figure2. Cross-section of a collector assembly
- Glass or aluminium
backing
/
0
10
20
30
40
50
60
70
80
TEMPERATURE
90
100
110
OF THE ABSORBER
120
130
140
150
160
170
180
I00
Figure 3. Efficiency of the collector chosen for the Abhimanyu pump
/
3.6 -
/
/
3.2 -
0
/-\
\
A
\
/I
I
\
O/
\
/
/
4’
I
t
“E
g
Y
z
a
z
=!
\
0--#
/
2.0 -
\
//
1
2.4 - ;
:
2.0 -d
%
1.6-
1.2
0.8
’
i
L-,
Jatl
Feb
I
I
I
Mar
APT
May
1
JWle
I
July
I
Aug
I
SPt
I
Ocr
1
NW
I
Dee
Figure 4. Annual variation of the daily useful gain in energy by a typical collector operating at 90°C
Expander
The heart of the pump is the expander, in which
high-pressure vapour does mechanical work and
generates shaft power during the process of
expansion. To a large extent, the expander
determines the reliability and the efficiency of the
system. The expander could in principle be a
reciprocating machine, a turbine or a positivedisplacement rotary machine.
m a reciprocating machine, linear motion is
converted into rotary motion by a crankshaft
mechanism. The reciprocating machine needs inlet
and outlet valves to control the flow of vapour under
pressure through the engine. For low-power pumps,
the use of reciprocating equipment makes the system
complex and inefficient.
Turbines can be used when the power is high.
For low powers the turbine size is smaller and its
speed higher, which creates a variety of problems. A
Techr~~logy Jar Solar Etwrgt Uiilrzatiotl
58
Orbmng sptral hub
,con”ect~ to arbttal
drtvel
f Gas exhaust
r)rbmng
sprral
190
u
270
Mwnq
rp~ral
Figure 6. The positive-displacement
rotary machine operating as a compressor, at different phases of the orbit of the
moving spiral
Figure 5. Typical spiral expander configuration
gear mechanism has to be used to reduce the speed to
match either the pump or the generator. Since
turbines are not positive-displacement machines, the
efficiency of the turbine drops as the vapour fiow is
reduced. Furthermore. the response of a turbine to
variable load is rather poor. For low power it is
therefore preferable to have positive-displacement
machines mnning at a modera?e speed of
1 000-8 000 rpm.
The expander used in the Abhimanyu pumps is a
positive-displacement rotary machine. It consists of
an assembly of two oppositely cut spirals, as shown in
figure 5. One of the spirals is fixed to the cover plate,
and the other orbits round the centre of the inlet port
with a slight degree of eccentricity. When these two
spirals fit together a number of pockets are formed.
As the moving spiral orbits, the volume of the
pockets changes (figure 6). Depending upon the
direction of the orbital motion, the pocket size either
increases or decreasesduring the orbital cycle. The
TABLE I.
Fsxed r~,ral
COMPARISON
machine can thus be used either as a compressoror as
an expander. When it is worked as an expander,
high-pressure vapour enters the spiral assemblyat the
centre and after expansion leaves the assembly from
thti periphery.
When it is used as a compressor. the inlet is from
the periphery, and compreqed gas leaves the
assembly through the centre. This expander has no
valves to regulate the flow and there are very few
moving parts. The volume displacement of the
expander per revolution is a function of the volume
of the pockets formed between the two spirals,which
in turn is determined by the pitch (distance between
successivespiral loops) and depth of the spiral, and
the thickness of the spiral walls. The cover plates seal
these pockets, thus serving the same function as the
cylinder walls in a reciprocating machine. Table 1
compares the characteristics of the Abhimanyu
positive-displacement rotary machine, a reciprocating
machine and a turbine.
OF MACHINE
TYPES
Chnracteri3tic
Abhimanyu
pump expander
Reciprocating
machine
7trrbine
Efficiency (%I
Valves
Moving parts
Operating speed (rpm)
Connection to feed pump
Variable speed capability
Partial load operation
Noise level
Reliability
70-80
None
Few
900-l 800
Directdrive
Good
Good
Low
Good
70-80
Inlet and exhaust
Many
900-3 600
Direct drive
Good
Good
High
f:air
60-80
None
Few
3 000-80 000
Large gear reduction
Poor
Poor to fair
Fairly high
Fair
p,* i<-‘.,.w~ ‘-3,
+.c’“.
12,
2 ,.
‘( ‘,
_;
5Y
Conversion of solar into rneehaanicalor electrical energy: Indian experimce
Since the spiral expander operates at low speeds,
it can drive a pump or generator directly and use
fairly effective shaft seals.Since the expander has few
moving parts, there are low rubbing velocities and
contact pressures: the spiral expander wears in
instead of wearing out. These attributes result in
highly reliable operation, with measuredefficiencies
in the vicinity of 80%.
The volume ratio that can be obtained is a
function of the diameter and the pitch of the spiral.
The dimensions of a typical spiral expander used in a
I-kW Abhimanyu pump are as follows:
Pitch
Axial length
Wall thickness
Outside diameter
Orbital radius of the moving spiral
Inches
Millimetres
0.625
0.875
0.125
6.5
0.1875
15.88
2?.22
3.18
165
4.76
The volume displacement per revolution of such an
expander would be 5.7 in.3 (93.4cm3), and the
volume ratio iuouid be 3.0.
Working fluid and cycle description
The size of the expander is greatly influenced by
the choice of working fluid. Various working fluids,
including the well-known refrigerants Rl 1, RI 13,
RI 14, were considered. Rll was eliminated because
of its low pressure (and consequently poor
performance) at temperatures that can be achieved
with flat-plate collectors. The drawback of R113 is
that it needs a regenerator. Eventually, Rl14 was
selected, since it was ideally suited to the expected
temperatures and power generation.
The working cycle of the engine is shown in
figure 7. Hot water from the storage tank at about
I
.:I
Pi
i
20.
Item
Specificatiotl
Inlet pressure
Inlet temperature
Condenser temperature
Working fluid
Enthalpy at expander inlet
Pressure at outlet
EnthaIpy after expansion
Work done during isentropic
expansion
Specific volume of working
fluid at inlet
Rate of circulation of working
fluid needed to give shaft
powerofl
kW(1.3hp)
Rate of flow of vapour into
expander at inlet pressure
Volume of high-pressure
vapour drawn by the
expander per revolution
Expander speed
Boiler-generator
9.3 bar (120 psig)
80°C
30°C
R114
223.3 kJ/kg (96 Btu/lb)
2.4 bar (20 psig)
202.4 kJ/kg (87 Btu/lb)
Temperature of water entering
the boiler
Temperature of water leaving
the boiler
Rate of flow of water through
the boiler
Enthalpy of working fluid
vapour at 37.5”C
Enthalpy of working fluid
liquid at 30°C
Total heat that has to be
removed in the condenser
Collector area required to
operate the pump for 4 h a day
I
loi
i:;“i
-a
90°C exchanges heat with the working fluid Rl14 in
a counter-flow heat-exchanger. The working fluid
leaves the boiler at a temperature of 82°C and
pressure of 9.3 bar ( 120 psig) (AB in figure 7).
High-pressurevapour enters the expander at 82°C and
9.3 bar (120 psig) pressure. After expansion, the
exhaust vapour has a temperature of 37.5”C. The
vapour leaving the expander is slightly superheated
and is at a pressure of 2.4 bar (20 psig); the condenser
temperature is 30°C. This isentropic expansion is
shown by BC on the figure. The work done by the
vapour per unit mass of vapour expanded is
20.9 kJ/kg. The exhaust vapour from the expander is
condensed in the condenser by cold water. This
process is shown by CD on the figure. The condensed
liquid is pressurized and reinjected into the boiler by
the feed pump (DA on the figure), completing the
cycle. The Camot efficiency of the cycle is 14.7%,
and its thermodynamic efficiency is 12.5%, giving an
expander efficiency ratio of 85%. The detailed
specifications of a 1-kW Abh.imanyu pump are set out
below.
Ch*lpl:plY!.
kyc
-____-__
0 10 80 j 120 --xiiv3
140
EntiulDv
kmgabove
ururaed
llQ”ld
,t4oel
Figure 7. Engine working cycle
20.9 kJ/kg (9 Btu/lb)
0.015 m’/kg
(0.24 ft3/lb)
168 kg/h (370 lb/h)
2.49 m3/h (88.8 ft’/h)
22.9 cm3 (1.4 in.‘)
1 827 rpm
Reverse-flow finned-tube
heat exchanger
90°C
81°C
3.5 l/min
202.4 kJ/kg (87 Btu/lb)
53.5 kJ/kg (23 Btu/lb)
24.9 MJ (23.6 X lo3 Btu)
10 m’
System optimization
The engine efficiency, which is the product of
the efficiency of the expander and the feed pump,
was of the order of 70% of the Camot efficiency.
Technology for Sn!ar Energy Utiliraticm
60
mind in designing the system. The system is versatile
in that, when pumping is not needed, it could be used
to generate electrical power or to drive a mechanical
system such as a thresher or a small lathe. The
collector assembly has an area of 10 m’: the
dimensions of the other subsystems are given below.
Condenser
Boiler
Expander
Storage tank
Pump
Height
/em j
Diameter
{cm)
50
so
35
200
25
25
30
30
65
20
The I-kW Abhimanyu pump has been in
operation for the last six months. Pumps of up to
12 kW can be designed following the sameprinciples.
“EAT lNLET TEMPEFmTURE
PC,
--I
Figure 8. Engine efficiency against inlet temperature at
different condenser temperatures
IO-100 kW power generation
Two IO-kW solar electric power plants are at
different stages of development in India. One is a
joint venture between the Government of India and
the Government of the Federal Republic of Germany.
Bharat Heavy Electricals Ltd., the National Physical
Laboratory and the Indian Institute of Technology at
Madras, are the executing agencieson the Indian side.
Messerschmidt,Bolkow and Blohm (MBB) of Munich,
are the executing agenciesfrom the Federal Republic
of Germany. The objective of the project is to set up
a IO-kW demonstration plant that utilizes solar
Figure 8 gives the engine efficiency as a function of
the expander inlet temperature for various condenser
temperatures. The lower the condenser temperature,
the higher the engine efficiency. Combining the
engine efficiency data of figure 8 with the collector
efficiency data of figure 3, one obtains the system
efficiency shown in figure 4. For a given insolation
there is an optimum range of temperature of the
collector for which the system efficiency is a
maximum. These considerations have to be kept in
ul
Em
m
70
a0
00
1W
COLLECTOR TEMPERATURE
110
120
130
140
150
It1
Figure 9. Variation of systemefficiency with collector temperature at insolations of 800 W/m’
and 1000 W/m’
Conversion of solar into mechanical or electrical energy: Indian experience
Hot-water
storage
tank
Generator
I
Condenser
Pre-heater
B
Hotwater
CIrcuIatIon
+--Coalmg
Inlet
Coolin~Z
outlwt
pump
warn
Pump
1
Llquld
Gi
Freon
purn~
Figure 10. Diagram of a IO-kW power station
energy. Figure 10 is a diagram of the power station.
The Linde screw expander, a flat-plate collector array
and a conventional electric generating system are to
be used. The working fluid will be R114.
The second project to install a lo-kW solar power
plant is totally an Indian effort, which envisagesthe
use of spiral expanders. Both projects use the organic
Rankine-cycle enginesin conjunction with a flat-plate
collector array. Someof the design parametersof this
system are given below.
Item
Specification
2.4 bar (20 psig)
5.7 cm (2.25 in.)
7.6 cm (3 in.)
6.4 mm (0.250 in.)
22.2 mm (0.875 in.)
SO.8 cm (20 in.)
Item
Specification
Exhaust pressure
Spiral pitch
Spiral axial length
Spiral wail thickness
Orbital radius of the moving spiral
Spiral outside diameter
Volume displacement
per revolution
Volume ratio
Expander speed
Rate of circulation of working fluid
Boiler-generator
Shaft power
Expander
10kW
Positive-displacement
spiral expander
R114
80°C
9.3 bar (120 psig)
37.S”C
Temperature of water entering
the boiler
Temperature of water
leaving the boiler
Rate of flow of water through
the boiler
Collector area
Working fluid
Inlet temperature
Inlet pressure
Exhaust temperature
524.5 cm’ (32 in.‘)
3.5
1 500 rpm
49 kg/min (108 lb/min)
Reverse-flow finned-tube
heat exchanger
90°C
81°C
180 I/min
400 ml
Utilization
of solar energy
in the development of arid zones:
Solar water pumps
Jean Paul Durand, Max G. Clemot, J. Pierre Girardier
and Marc Y.Vergnet
SOFRETES
et Mengin, Montargis, France
(a) A battery of flat-plate collectors in which
water or another heat-carrying liquid circulates in a
closed circuit;
Basicdesign of a s&r pump
The first goal of SOFRETES, working in
co-operatio.1 with Dakar University in Senegal,was to
develop water-pumping equipment with no external
fuel requirements, initially for domestic use in small
villages and in raising livestock, and then for irrigating
crops in arid regions. Since the equipment would have
to operate at isolated sites where there are usually no
specialists able to provide maintenance for sophisticated equipment. it had to be simple, rugged and
reliable. It was decided as a first step to use a
low-temperature themodynamic cycle between a hot
source supplied by solar energy and a cold source
maintained by the pumped water.
A solar pumping station includes the following
components (see figure 1):
(bJ A heat exchanger inside which heat is
transferred from fluid circulating in the collectors to
the fluid circulating in the motor circuit, causingthe
latter fluid to evaporate;
(c) The motor circuit, which, in addition to the
heat exchanger, includes the expansion motor, a
condenser and a reinjection pump;
(d) The pumping circuit itself, which for
low-power installations of about 1 kW includes a
hydraulic motor driving a well pump or, for higher
powers (25-50 kW), an alternator and an electric
motor driving one or severalpumps.
_.-.-.-.-.-.-.-.-.-.
Solar energy
j
‘1,
’
y&5j”eate::::;
:,
i
I
.
i
i
i
use
I!
c
i
I
Hydraulic
II
prers
Controlpiping
Well pump
Figure1. Schematic diagram of a solar pumping station
62
To operation
,
/
@,‘.’
J-’ L
,,c,
,.<’
,/
,+t
3
I,!;,:”
’ ^. ,-_
;~~+,,-<~. s
,
,,c
.;: ”
.,
.:.,
I
63
Utilization of solar energy it1 the development of arid zones: Solar water pumps
Ptzctical applications of solar pumps
Lhnwstic water jbr villages
To ensure a water supply for isolated villages in
arid regions is essential. Hand pumping consumesthe
time and energy of the inhabitants Conventional
pumping by internal combustion enginesis subject to
all the uncertainties of fuel supply, and the
equipment requires maintenance, which is made
difficult by severeclimatic conditions and the lack of
skilled manpower and a stock of spare parts.
The integration of a solar pumping station into a
village is a practical means of providing a water
supply. In addition, the installation of the collectors
on the roof of a building makes the building cooler
inside, since a large portion of the heat received by
the collectors is conducted away by the fluid. This
building can be used to house a school, a cattle
market, a dispensaryetc.
Water for livestock
In livestock grazing regions, the uncertainty of
the wells often means that flocks and herds must
depend on a single well, sometimes with disastrous
results. The multiplication of small-volume watering
places by means of reliable equipment powered by
solar energy would make it possible to supply water
rationally to nomad tribes and their herds and flocks.
About 40 I-kW stations are now being tested in
12 countries: Brazil, Cameroon, Cape Verde, Chad,
India, Mali, Mauritania, Mexico, Niger, Senegal,
United Arab Emirates, and Upper Volta.
In Mexico, with the support of the Government,
solar hydraulic pumps have been set up in villages
scattered around the country. A 30-kW station to
supply drinking water and water for irrigation has
also been set up. These pumps are being operated
under varied climatic, economic and social conditions.
l-kW solar pump characteristics
The standard equipment now used in the I-kW
installations has the following characteristics:
Collector
Active area
Area available underneath collector
Fluid circulation
70 Ill'
100-l 20 m’
Thermosiphon
circulator
Fluid
Expansionmotor
Rotationalspeed
Pumping circuit
Well pump
Tubular or plate
elements
Butane or Freon
Two-cylinder:
displacement 12
200 rpm
Hydraulically
driven
X00 W/m’
20°C
2O”-30°C
5-l
11
3Om’
1 500
750
Solar pump for irrigation in Mexico
The first large solar unit devoted to irrigation has
been established at San Luis de la Paz, in the state of
Guanajuato in Mexico. It is part of a long-range
government programme, run by the agency responsible for environmental improvement in the Health
Secretariat. The 30-kW installation. which delivers
electricity for pumps, has been operating since
September 1975. San Luis de La Paz belongs to the
semi-arid zone of Mexico. It has an average mean
temperature of about 17’C (62”F), a maximum of
41°C (106’F) and a minimum of about -5°C (23’F).
In an averageyear, rain falls on 39 days and the town
has sunny weather on 250 days.
The installation operates according to the same
principle as the I-kW stations, except that the
expansion motor is replaced by a turbine that drives
an alternator.
The general characteristics of the installation are
as follows:
Surface area of collectors
Working fluid
Turbine rotation speed
Pump
Pumping rate for a discharge
height of 40 m
1 500 m’
Rll
7 400 rpm
Electric centrifugal
1SO m’/h average
This solar power station delivers about 900 m3
of water per day. It is presumably the most powerful
solar unit working in the world. The water, pumped
from a depth of 40 m, is delivered to 15 000 village
inhabitants and will eventually also be used to irrigate
20-30 ha of experimental crops. The room u’nder the
1 500-m’ solar collectors is to be used for the
facilities of an experimental farm.
or
Motor circuit
Heat-exchanger/condenser
Operating conditions
Mean insolation
Air temperature
Pumped water temperature
Daily operating time
Daily output for a total discharge
height of 20 m
Number of inhabitants that can be
supplied with 20 I each
Number of head of cattle that can
be supplied with 40 I each
Future development
At the present stage of solar energy technology,
it is quite feasible to use solar pumps in remote arid
zones. However, these techniques and systemsmust
be adapted individually to local conditions. The
experience of SOFRETES is that solar pumps using
the low-temperature thermodynamic cycle with
flat-plate collectors can indeed be so adapted.
Future work anticipated in this area includes:
Improving existing equipment in the I-kW
range and in the 25100 kW range; research
on collectors, heat-exchangers, motors and
turbines, fluids etc.
Applying the low-temperature thermodynamic cycle to the use of geothermal
energy
Solar refrigeration for preserving food and
medicines
Solar air-conditioning and spaceheating
Economic aspects
In choosing an energy source, the advantagesand
disadvantagesof using it must be analysed and each
component of the total cost calculated. In this way, a
comparison can be made between a conventional
energy source (e.g. a diesel engine) and solar energy
on the basis of the volume of water that needs to be
pumped. The comparison in the table below can be
used to make such an analysis. It should be realized,
however, that the efficiency of solar energy
utilization and its competitiveness with that of
conventional sourcesdepend on local conditions and
that therefore a special study of these conditions
must be included in the analysis.
Energy
Disadvan rages
or ~ons;,ahi ts
source
Advantages
Diesel
Low investment
Great flexibility in
installation and use
High operating cost
Need to import, transport and store fuel,
lubricants and spare
parts
Consumption of
non-renewable
resources
Localized pollution
(exhaust gases)
Skilled manpower
Uncertain prices for
fuel and manpower
(50% of cost)
Low operating cost
In situ availability
High investment
Discontinuous supply
of energy
Long lifetime of
equipment
( 1O-20 years)
More extensive civil
engineering
engine
Solar
radiation
(storage tank required)
ElIerg)
source
Solar
radiation
(con 1.)
Dhadvan rages
Advan tagcs
nr constraints
Reliability
Possibility of
technology transfer
No degradation of
natural resources
No pollution
Refrigeration possible
underneath the
collectors
No skilled manpower needed
Maintenance reduced
to care-taking
Operating costs known
A comparison of the relative cost of pumping
water using a diesel engine, electricity and solar
energy is illustrated in figure 2. It can be seenthat the
distance to the conventional energy source is the
determining factor. It is estimated that in the Sahel a
50.kW solar energy station used for irrigation will
begin to be competitive with electricity when the
distance from the electric power plant reaches
100 km.
It has been calculated that for solar pumps
already installed in Africa the cost per unit of water
pumped is about 0.60 $/m3 for I-kW stations
(assuming an operating time of 1 800 h a year and a
water depth of 30 m) and approximately 0.05 Urn3
for 25kW stations (sameoperating time, water depth
10 m). A similar calculation for a pump system using
a diesel engine and doing the samejob as a 1-kWsolar
station, givesa cost of about 0.47 $/m3.
Dlrtsnce to energy ~ourca
Figure 2. Cost of pumping water using various forms of
energy as a function of the distance to the energy source
Theoretical
conditions
for maximum power from the sun
Kamal-Edin
Arab Development
Hassan
Institute,
Tripoli, Libyan Arab Jamahkiya
A solar heat-engine, like any other heat engine,
has limits imposed by the second law of thermodynamics. Its maximum efficiency is thus the Carnot
efficiency (7jc):
vc = 1 - (TJT,)
where TL and Tc are respectively the ambient and
the collector thermodynamic temperatures.
The maximum power w’ that can be generated
per unit area of the solar collector is given by
j&J”= qcQ”
where the power 0” supplied per unit area of the
collecting surface to the engine is the difference
between the n-radiance I of the collector and the
power losses. For relatively low collector temperatures, the radiation and convection lossescan both be
collected into a single overall heat-transfer coefficient
U; the losses per unit area are then U(Tc
TL).
Therefore, we have
@=I-
U(T,-
TL)
(1)
and
ti” = [l - ( TJTc)]
[Z - U (T, - T,)]
(2)
For a given TL. a high Tc would improve the
Camot efficiency, but would also increasethe losses.
The maximum work would, therefore, be obtained at
some optimum collector temperature Tc.opt, at
which the efficiency gain balances the heat loss. This
optimum condition is attained when dl$“/dTc = 0,
which happens when
T c. opt = I/Tit1
The maximum fraction of the solar power that
could be converted to mechtinical power is obtained
from equations 2 and 4:
@,l,,l~ = l- ( W&,p,)
,__~
W
= (l-I/M/(M+l))(l+M-1/M(M+
1)) (5)
The figure shows a graph of equation 5. At
M = 0. we see that ril%,,/I = I, an unattainable
condition, becauseM = 0 implies that I/ = 0, i.e., that
there is no heat loss from the collector and its
temperature would approach infinity, whereas it
cannot be hotter than the sun itself (5 750 K).
Indeed, the present analysis fails at very high
collector temperatures where the losses would be
mainly due to radiation and could not be expressedin
the linear form used in equation 1. However, except
in the vicinity of M = 0, the graph shows the trend
properly: an increase in M, due either to an increase
in II or a decreasein I. decreasesthe useful fraction
of I; decreasing M, either by using a good insulating
system to decrease U, or by concentrating the solar
energy to increase 1, results in an increase in the
maximum usable fraction of the incident energy.
If one considers flat-plate collectors, it is difficult
to imagine conditions in which M would take values
less than unity. At M = 1, the maximum possible
power that could be obtained is only about 17% of
the incident solar power. This limit seemsto rule out
the use of flat-plate collectors for power generation.
‘01
+ u T,J/U
084
or
Tc..,,/TL = vm
04
Defining the dimensionlessvariable M as
M = U TL/I
we have
l- WYHT,..,,-
.;
(31
T c,opt!Z-L= I/(M+1)IM
(4)
Equation 4 gives the source temperature for
which a Carnot heat engine should be designedto give
the maximurr; possible power under initial conditions
given by I, TL, and U.
f
(
,,i t
o L -,
_. .._ ..__...
_____-_
-..-~.05
H~UT,”
_.
10
-.--._
15
Ratio of maximum usable to incident solar power as a
function of the design parameter M (graph of equation 51
Solar flat-plate
collectors
A. A. M. !Sayigh
College of Engineering, Riyadb, Saudi Arabia
Of all the applications of solar energy, the use of
flat-plate collectors in heating is the most practical.
The solar liquid heater was invented by H. B. Saussure during the second half of the 17th century;
Herschel ( 1837) and Tellier (1885) also experimented
with solar water heaters. Even in earlier times the
indigenous peoples of Africa, the Arab countries,
Australia, China, India and Pakistan used their
ingenuity in heating water by placing a specially
shaped copper pot filled with water in the sun during
the winter. Air heaters, however, are of recent
invention. K. W. Miller introduced the overlapped
glass-plateair heater in 1943. Nowadays, it is cheaper
to use solar water heaters for domestic applications,
and as such they are used all over the world.
TABLE
I.
PROPERTIES
From 1960 onwards, flat-plate collectors have
had the biggest share in research and development.
This paper outlines the capabilities and limitations of
such devices, with the intention of promoting the
proper use of flat-plate collectors, especially in
developing countries.
Characteristics of the components of fiat-plate
collectors
A flat-plate collector normally consists of an
absorber, which is made of blackened metal-usually
copper-and a grid of pipes soldered to the absorber.
The assembly is placed in a box with insulation at the
OF SOME SURFACE COATINGS
Coating
Black enamel paint
Tar
Lamp-black
Nickel black (oxides and sulphides of
Ni and Zn) on polished Ni
Nickel black on galvanized-iron (experimental)
Nickel black on galvanized-iron (commercial)
Nickel black, two layers on electroplated
Ni on mild steel (after 6-h immersion
in boiling water)
010 on Ni (made by electrode deposition
of Cu and subsequent oxidation)
Co,O, on Ag (deposition and oxidation)
CuO on Al (by spraying dilute Cu(NO,),
solution on hot Al plate and baking)
Copper black on Cu (commercial treatment
of Cu with solution of NaOH and NaCIO,)
Ebanol C on Cu (commercial Cu blackening
treatment giving coatings mostly consisting
of CUO)
CuO on anodized Al is treated (Al with hot
Cu(NO,), - KMnO, solution and baked)
Al, O,-MeAl, O,-MO-AI, O,-MO-AI,O,
interference layers on MO
PbS crystals on Al
COLLECTORS
Solar
energy
absorptatvx
01
I. 011~ wave
radia tiot,
admittancea
c
Reference
0.83
0.86
0.95
0.91-0.94
0.83
0.86
0.95
0.11
Sabbagh, J. A. et 01.
Sabbagh, J. A. et of.
Sabbagh, J. A. PI 01.
Tabor, H. et al.
0.89
0.12
0.16-o. 18
Tabor, H. et al.
Tabor, H. et ul.
Tabor, H. et al.
0.94
0.07
Schmidt. R. N. et 01.
0.81
0.90
0.17
0.27
Kokoropoulos,
Kokoropoulos,
0.93
0.1 I
Hnttel, H. C. and
Ungcr, T. A.
0.89
0.17
Close, D. J.
0.90
0.16
Edwards, D. K. er al.
0.85
0.1 I
Tabor. H.
0.91
0.89
0.085b
0.20
Schmidt, R. W. ef ul.
Williams, D. A. e! al.
‘At temperatures typical of flat-plate solar collectors.
bMeasured at 260°C (500” F).
66
USID IN FLAT-PLATE
P. et al.
P. et al.
‘* ‘3,;.‘, ,_
j&j :,
je ‘_
?“‘,2,.
,:,;
1:
Sohr jlat-ploie
collectors
67
100
A-common
90
window
glass
60
I- Outside box
- - Insulation
Figure I. Cross-sectionof a flat-plate collector
M/M
.----C-X-in.
Copper
Aluminium
Brass (60/40)
Steel
heat-absorbing
pIat,\
b
back of the absorber and one or two transparent
covers at the top to allow sunlight in (seefigure 1).
Solar absorbers are commonly made of the
following metals:
Metal
(6 mm)
\\ \
I-
Thermal conductivity
at 100°C (W m-’ ‘C-‘)
393.56
217.71
121.42
66.99
Notes about various coatings and where they can
be purchased are contained in McDonald’s work.
Table 1 shows the properties of various surface
coatings used in flat-plate collectors.
The properties of commonly used insulation
materials are shown in Table 2. Figure 2 shows curves
for three types of glass used for the transparent
covers of collectors. It is clear that type A is the best.
Figure 3 shows the properties of a typical clear,
drawn sheet-glass at various wavelengths. Plastic
materials such as PVF, and fibreglass sheets, have
been Iled in solar heaters, but they are in general
inferior to glass because they deteriorate with time
20
I
----\
I--
------B--%-in.
(6 mm)
plain
plate
10
-----\.
--
-m-m.--A-common
window
20
ANGLE
‘\
glass
I
1
40
60
a I-
OF INCIDENCE
60
90
(degrees)
Figure 2. Transmissivity and absorptivity of common glass
materials. Solid curves are transmissivity; broken curves,
absorptivity
TABLE 2. PROPERTIES OF COMMONLY USED INSULATION MATERIALS IN FLAT-PLATE COLLECTORS
Material
Mineral wool (clay wool,
fibreglass, rock wool)
Hair felt
Granulated cork
Re-granulated cork
(0.474~cm particles)
Compressedcork
Straw
Sawdust
Vermiculite (granulated)
Polyurethane foam, rigid
Polystyrene, expanded
Approrima
density
fkglm I
te
7’hermal
conductivit
(W rn-‘?T-$
tP
560
i-y
c /
ti:
i
T~l”lflll*ll”ll”
fkl
12-14
80
120
0.0332-0.0404
0.03 89
0.0476
30
136-176
[email protected]
13-240
128
24
16
0.0447 1
0.04 18-0.0462
0.05 76
0.0649
0.0721
0.0245
0.0303
i
%
*
1
40
0.2
,
,
0.4
0.6
~
08
10
WAVELENGTH
12
14
I6
16
2.0
@ml
Figure 3. Transmissivity and reflectivity of a typical clear,
drawn sheetgIass with respect to wavelength. Glass is type
DME INT 528, 3.96 mm thick
Technology for Solar Energy Utilizatiotz
68
and at high temperatures. Moreover, ultraviolet
radiation discolours them. The plastic cover is,
however, easier to handle than the glasscover.
The theoretical analysis for a flat-plate collector
is well established and can be summarized in the
equation 0, = & - i)~, where 0, is the power
output, & is the power absorbed by the collector
and & is the power lost to the surroundings. The
value of 0, dependslargely on the materials of which
the collector is made, and its coating and the solar
angle of incidence. Figure 4 shows the effect of angle
of incidence on the surface absorptance.
09
1
00’
1.0
0.8
8
2
p
0.6
2
$
t
5
_._.
0
I--
0.4I-
002
I
004
r
.~~
000
008
,.
..-~
0.10
NORMALiZED
5
UY
013
,
0.14
016
o;f)
0.2
?-
~--,O(:W.lm’,
TEMPERATURE GAIN
Figure 5. Collector efficiency as a function of type and
number of covers
0.2
a I-
1
I
20
40
ANGLE
Figure 4. Effect of
OF INCIDENCE
1
!
60
80
90
(degrees)
of incidence on the surface
absorptance
angle
A full theoretical treatment of the mathematics
relating to heat lossesfrom the top, bottom and edges
of collectors is given in Hattel and Woerts (1942).
Klein (1973) also discussesheat lossesfrom the top
of collectors, and covers corrections for the angle at
which the collector is tilted from the horizontal.
By considering the distribution of temperature
across the absorber and the efficiency of the fin-tube
arrangement, &, can be determined, as can the
collector efficiency. Figure 5 shows how collector
efficiency varies with normalized temperature gain,
i.e. the temperature gain of the collector per unit
insolation, for different kinds of covers.
, As the collector temperature rises,the efficiency
falls, because the heat losses to the surroundings
increase. The use of more than one glass cover
improves the efficiency at high collector temperatures, but reduces the amount of heat collected. In
theory the optimum performance curve is that which
is shown dotted in figure 6; but it is obviously not
feasible to realize such a curve in practice, since it
requires different numbers of covers at different
temperatures.
NORMALIZED
TEMPERATURE
Figure 6. Collector efficiency with
covers
GAIN
--
various numbers of
Optimization and comments
The best design will take into consideration the
optimum air gap between the absorber and the glass
covers. The problem was studied by the author, who
found that the best gap was over 4 cm and, owing to
the side-shading effect caused by the collector box,
less than 8 cm. In figure 7 the gap conductive-
top half of this figure shows how the temperature
varies across the plate: the bottom half shows how
the water temperature lags behind the absorber-plate
temperature at the centre of the fin by almost 10°C.
This temperature lag is of the same order over the
whole temperature distribution in the flow direction.
Most commercially made flat-plate collectors
have an efficiency ranging from 50% to 67%. The
breakdown of heat lossesis as follows:
3:
L
7
E 1
r
$2:
Percentage points
Type of loss
I
1
I
0 i ---
y-r-.t
2
3
-7.---J---5
4
GAP WIDTH c knll
--J----,
0
I
Figure 7. Gap conductive-convective coefficient C, as a
function of gap width for T = 7O”C, tilt angle 40”
convective coefficient C~is plotted againstgap width L.
This coefficient is related to the Nusselt number Nu,
which has the value 1 for pure conduction and higher
values when convection is important, by the
equation CL = NuK/L, where K is the thermal
conductivity of the fluid in the gap. The performance
of a collector absorber plate is shown in figure 8. The
l-3
5-10
5-7
Edge
Back
Radiation
Convrction
0
22-30
It can be seen that. in order to improve the
collector efficiency, lossesdue to convection must be
reduced to a mimimum. If there is to be no
convection, Nu must equal 1 and the Rayleigh
number Ra should be less than 1 708 (see figure 9).
There are two ways of reducing convection. The first
is to produce a vacuum (partial or total) in the
collector spacing. A partial vacuum is only effective
with selective surfaces; it is not effective in collectors
with painted surfaces.
(bl
103
105
104
10”
ua COIf
Figure 9. Nusselt number versus Rayleigh number multiplied by the cosine of the tilt angle
0
i
l/6
DISTANCE
Fire
5/B
l/2
FROM BOTTOM
lh.cWm
1
o! tube length)
8. Temperature distribution over the absorber
plate of a colbctor:
(uj transverse temperature variation of the plate;
(b) plate and water temperature variation in the flow
direction
The second way to reduce losses due to
convection is to place transparent honeycombs in the
air gap. Figure 10 illustrates the effect of this type of
structure on Ra. The parameter a mentioned in the
figure is defined as II V’s(dlw) where d is the depth and
w the width of a honeycomb cell. To suppress
convection, a honeycomb should be chosen such that
the collector operates in the stable region of
figure 10.
_ ‘,’
,‘.
‘;,
.i
70.
Practical points
, ---r--------___
I
5
10
------r-----y20
RAYLEIGH
Firure 10. a&
50
NUMBER
loo
XKI
too0
Ra 110’1
stability chart for honeycombs in the air
gap of a collector
An example of the effect of temperature gain on
collector efficiency is shown in figure 11, while
figure 12 shows how the glass temperature of an
actual flat-plate collector varied during a short period
on a particular day.
0%.I
/
0
Figure 11. Collector efficiency as a function of temperature
gain and insolation. Temperature gain is normalized by
dividing the difference between the mean fluid temperature
and the inlet temperature by the mean msolatlon
The folio-Gng points should be noted with regard
to the manufacture of flat-plate collectors:
(a) Poor adhesiveis often used between the glass
covers and the collector box. This is because it is
preferable to let the pressure inside the collector gap
be atmospheric so as to reduce convection and
air-conduction losses. This can only be achieved by
not making the covers airtight. However, as a result,
dust and moisture penetrate the collector and erode
the surface of the absorber plate;
(b} Insulation materials may contain moisture
before they are used in the collector. As the collector
gets hot, this moisture evaporates and condenseson
the inside surface of the glassand affects the incident
radiation. It also corrodes the absorber plate;
(c) Allowance must be made for giass expansion, and the edges of the collector must be bevelled
so that no rain water collects on them;
(dj The collector should not be used without
any liquid inside it. Otherwise, the high interior
temperatures generated cause abnormal expansion of
the covers, which become distorted or break:
(ej In order to alleviate problems due to the
freezing of water in tubes, a water/ethylene-glycol
solution can used;
(f) The maximum area of a collector should be
2 m2;
(g) To reduce the amount of infra-red radiation
escaping from the collector, specially coated glass
covers can be used. This coating should be on the
inside of the covers.
BIBLIOGRAPHY
INSOLATION
(W/m’1
Ackerman. The utilization of solar energy. Annual
report. Washington, D.C., Smithsonim Institution, 1915. p. 141-146.
T, = 27 7 %
Basic studies on the use and control of solar energy.
WIND SPEED Imfd
f3y D. K. Edwards nnd others. University of
California, Department of Engineering, October
1960. (Report 60-93)
2.0
Chinnery, D. N. W. Solar water heating in South
Africa. NBRI information
sheet (Pretoria) 44.
10.36.lOa.m.
10.58.20a.m.
Dare: 6375
Figure 12. Variation of the glass temperature of an actual
flat-plate collector during a short period on a particular day.
Shown also are the variations of insolation and wind speed
over the same period
b
Close, D. J. Flat plate solar absorbers: the production
and. testing of a selective surface for copper
absorber plates. Melbourne, Commonwealth
Scientific and Industrial Research Organization,
1962. (Report E.D.7)
Duffie, J. A. and W. A. Beckmann. Solar energy
thermal processes. New York, Wiley Interscience,
1974.
Further studies on selective black coatings. By
H. Tabor and others. Proceedings of the United
Nations Conference on New Sources of Energy,
Rome, 21-31 August 1961. (Solar energy: I, v.4,
p. 618 (S/46))
Sales no.: 63.1.38.
Hottel, H. C. and B. B. Woertz. The performance of
flat plate solar heat collectors. American Society
of Mechanical
Engineers. Quarterly
transactions
(New York) 64:91-104, 1942.
Hottel, H. C. and T. A. Unger. The properties of a
copper oxide-aluminium selective black surface
absorber of solar energy. Solar energy (Elmsford,
N.Y.) 3: 10:3, 1959.
-.
Klein, S. A. The effects of thermal capacitance upon
the performance of flat-plate solar collectors.
M.S. thesis. University of Wisconsin, 1973.
Kokoropoulos, P., E. Salam and F. Daniels. Selective
radiation coatings-preparation and high temperature stability. Solar energy (Elmsford, N.Y.)
3: 19:4, 1959.
Kondratyev, K. Ya. and M. P. Fedorova. Radiation
regime of inclined surfaces. Paper prepared for
the Solar Energy Symposium, Geneva, 30
August-3 September 1976.
McDonald, Glen. Research highlights choice of
coatings. Solar engineering (Dallas) 23, October
1976.
Meinel, A. B. and M. P. Meinel. Applied solar energy:
introduction. London, Addison-Wesley, 1976.
Sabbagh, J. A., A. A. M. Sayigh and E. M. El-Salam.
Solar water heaters in Saudi Arabia. Paper
prepared for the Seventh Arab Scientific
Conference, Cairo, 1973.
Sayigh, A. A. M. Investigation into solar heating with
some methods for convection suppression. Paper
prepared for Solar Cooling and Heating, A
National Forum, Miami Beach, Florida, 13-15
December 1976.
The uses of solar energy. Paper presented to
the Cultural Activity Programme, College of
Engineering, University of Riyadh, Saudi Arabia,
4 April 1974.
Sayigh, A. A. M., ed. Solar energy engineering.
London, Academic Press, 1977. Academic Press,
1977.
Schmidt, R. N., K. C. Park and E. Janssen. High
temperature solar absorber coatings. Hoaeywell
Research Centre, September 1964. (Part two,
technical report ML-TDR-64-250)
Tabor, H. Selective surfaces for solar collectors. Low
temperature engineering applications of solar
energy. New York, American Society of Heating,
Refrigerating, and Air-Conditioning Engineers,
1967.
Solar energy collector design with special
reference to selective radiation. Research Council
of Israel, 1955. (Bulletin 50
Stationary mirror systems for solar collectors. Solar energy (Elmsford, N.Y.) 2:3-4:27-33,
1958.
Williams, D. A., T. A. Lappin and J. A. Duffie. Selective radiation properties of particulate coatings.
American
Society of Mechanical
Engineers.
Quarterly transactions(New York) 85A: 213,1963.
&rem, A. M. nnd D. D. Erway. Introduction to the
utilization of solar energy. New York, McGrawHill, 1963.
Aspects of solar-heated
Gangolf Brknlich
Institut -fir Umweltscbuta,
Grao, Austria
Basic concepts
A project intended to popularize solar energy
should meet some of the following requirements:
(a) It should be economically advantageous;
(b) As many people as possible should share the
profit;
(c) It should be seenby many people.
Projects involving the heating of swimming-pools
by means of solar energy are thus an excellent way to
introduce the public to solar energy technology.
Compared with the many other possible usesof solar
energy, such projects have several important advantages, including:
(a) Working temperatures are relatively low;
(b) Auxiliary heating is not needed;
(c) There are no problems with thermal storage.
colIectors
Glazing
The problem of glazing can be reduced to two
questions: should the cover used be glass or
synthetic? Should it comprise one or more layers?
Synthetic glazings are easier to handle than glass;
they are light, elastic, and resistant to thermal shocks.
However, they are sensitive to overheating and to the
optic properties of the surface; they are inflammable
and their chemical stability over long periods is not
proven. The type of glazing chosen will depend on
the requirements of the customer and the particular
situation.
As the working temperature of swimming-pool
solar collectors has to be low, only collectors with
one glazing should be used. In regions where the air
temperature tends to be the same as the water
temperature in the pool, glazing can be omitted. Two
glazings should never be used, since the reflection
losses due to the second glazing will be greater than
the reduction in thermal losses it produces, provided
the working temperature is below 50°C.
72
swimming-pools
Specific
heat capacity
The collector massimpedes energy gain through
lossesof rest energy from the collector system after
operation. It also influences the speed of temperature
increase.
There are three different methods to avoid losses
of rest energy:
(a) The thermal mass of the collector system
can be made approximately zero;
(b) All the energy in the collector system can be
transferred to the consumer;
(c.) The collector system can be designed to
incur nearly no thermal losses during times of nonoperation.
Most designers of solar-energy equipment consider only the first rilethod. However. in every case
where solar collectors transfer their energy to a very
low-temperature system (temperatures near ambient)
the thermal energy of the collector system is
completely delivered to the user system. This
condition is met by collectors heating a pool. In this
case, there are no losses of rest energy after
operation. The third method to avoid these lossesis
to prevent the collector system from cooling down
after operation. This requires good thermal insulation
during times of non-operation, which although it may
be feasible, is too expensive.
For some applications, rapid increases in the
temperature of the collector system may be
necessary;but a “quick” collector loses more energy
than a “slow” one if the heat-transport and
heat-transfer system cannot handle the thermal
peak-power without undergoing an unnecessary
temperature rise. A typical flat collector loses l%-2%
of its efficiency for every degree Celsius of
temperature rise. A “slow” collector avoids these
lossesin systems with a low-heat removal factor (i.e.,
the thermal power transported per unit of tempera
ture difference). For swimming-pool heating, “quick”
collectors provide unnecessarytemperature peaks and
need high-quality pumps to withstand the continual
on-off switching.
Aspects of solar-heated swimming-pools
Hydraulic features
The most important design goal is certainly to
ensure low power consumption by the circulation
pumps. A good collector system needs only about
5 W/m* to drive the water circulation at sufficient
speed to avoid a temperature increase in the
collectors of more than 8°C. The consumption of
such a system during the summer is about 1% of the
net energy gain. However, there are collector systems
that use more than 10% of the energy gain to power
the circulation pumps; this reduces the amount of
electricity that can be produced by the collector
surface.
Pump control
The circulation pump of the collector system
should be controlled by an electronic device that
switches the pump on when the temperature of the
pool water exceeds the temperature of the collector.
More important than the temperature difference at
which the system starts to work is the temperature
difference at which the system stops the pump. The
collector temperature increases with the increasing
rate of heat transfer through the heat exchangers.
Thus there is a minimum temperature difference
between collector water and pool water at which the
collector pump should still work. in order that the
heat transferred to the pool corresponds to the.energy
gain at low insolation. For a well-d&gned pool. this
temperature difference will be in the range
0.5”-1.5”C. Depending upon the thcarmalmass of a
collector, the operational starting-point of the system
should be selected at a temperature difference of
2”-6°C. Otherwise a continuous switching on and off
occurs.
The heat exchanger
For collectors operating at high temperatures,
small heat-exchangers may be sufficient. However, a
high collector-temperature reduces its efficiency and
so more collectors need to be installed. Depending
upon the relative values of collector cost and
heat-exchanger cost, one can find any optimal
heat-exchanger size. Given the current price conditions in Austria, an economically viable heat
exchanger should transfer heat at the rate of 50 W per
degree Celsius for every square metre of collector
surface.
Control of pool temperature
The best way to prevent overheating of the pool
is certainly to cool the pool water by adding fresh
water. If this is not possible, there is another way: if
73
one reduces the heat transport either in the collector
loop or in the pool water circulation, a higher
collector temperature ensues, and the efficiency of
the collector system decreases.As the piping of the
collector system for an open-air swimming-pool does
not need thermal insulation, the energy gain by the
collectors can be reduced to zero via the piping
without the risk of overboiling.
Conclusions
If solar energy is to be used for spaceheating in
Austria with reasonable efficiency in the near future,
then collectors must be combined with heat-pumps.
The sameoperating conditions are necessaryto feed a
heat-pump from solar collectors as prevail in the case
of pool heating.
Only when the system design is non-optimal is
the return worse than that obtained in the heating of
swimming-pools. The relationship between costs and
available insolation is very unfavourable in the caseof
spaceheating, and therefore the development of this
technology is an important step in the development
of the use of solar energy.
Practical experience
The Institut fiir Umweltschutz has planned and
built several solar-heated swimming-pools and has
provided consultants for several other projects. Its
main efforts were devoted to the planning and
construction of Austria’s largest collector system, the
solar-heatedoutdoor swimming pool at Eggersdorf.
Approximately one year was required to plan
this pool. It was necessary to compute the energy
conversion and severalother factors. Much value was
derived from practical experience. Construction
began during the first week of May 1976, and
consisted of building a new hall and redesigning the
roof. The original saddle roof was rebuilt to make it
slant in one direction and corrugated asbestoscement
was laid down. During the construction, the normal
functions of the swimming-pool were not disturbed.
The official opening of the solar-heating system was
10 July 1976. The use of only local craftsmen
contributed to the short construction time.
The main data for the Eggersdorf pool are:
Surfaceareaof pool
Water capacity of pool
Total open space
Attendance capacity
300
1 300
15 000
1 500
m’
m3
m3
persons
The pool is heated by three separate systems,
with a total of 360 collectors, each with an absorber
surface of 1.55 m’. or a total net surface of 558 m2.
74
This allows a maximal thermal power gain of some
450 kW.
These items were -used in each system:
circulation pump (1.1 kW), one-way valve,expansion
tank, pressure valve, heat-exchanger, differential
thermostat.
Clear weather predominated for several days
after the pool opened. The wateri temperature
Technology for Solar Energy Utilization
reached a record high of 27°C. During the period of
bad weather that followed, the temp-rature fluctuated between 22°C and 24°C. The swimming-pool
operated with the same frequency as other pools.
Attendance decreasedonly when the air temperature
was cold, The water temperature could always be
maintaine’d above air temperature after the solarheating s$stem was installed.
A solar energy system for greenhouses
M. Posnansky
Polisolar Ltd., Bern, Switzerland
The functions of greenhouses are to sur;?lj;
plants with the light and warmth required f‘o~
optimum growth the year round. Although greenhouses consume a large amount of thermal energy,
they draw a substantial portion of the necessaryheat
directly from the sun. The effect by which the
thermal balance in greenhousesis maintained is well
known as the greenhouse, or glasshouse,effect (see
figure 1).
greenhousebracing structure. The solar collectors are
parabolic cylindrical models designed, unlike
conventional concentrating collectors, to reflect light
on both sides. A special control system automatically
positions the collectors towards the sun using an
electric drive system (see figure 2).
The thermal energy withdrawn by the absorber
pipes helps keep temperatures at a lower level than
that in greenhouseswith conventional shading. Since
Losses
due to CI
contact
with coo
cooler outside
Irradiation
from soil
3
Loss via soil
Soil
heating
Figure 1. The greenhouse effect
For most types of plants, shading is necessary
during the summer months. Formerly, almost all
greenhouses were permanently shaded during the
summer. To some extent this is still true today,
although the idea of movable or variable shading has
met with growing acceptance. Shading has become a
practical means for the gardener to control the
greenhouse atmosphere, particularly its temperature.
The system described here collects and removes
heat, keeping the growth area cool. It shades the
plants, heats the room during cold weather and
insulates it at night.
The heart of the ;y;tem consists of concentrating
solar-collectors in a parallel-series configuration
2.20-2.80 m above the floor and at the height of the
both collector surfaces reflect, most of the indirect
light can be routed into the greenhouse for almost the
entire day.
Figure 3 shows the collector lamellae in the
position for collecting diffuse radiation during times
of poor weather. Also, when a certain irradiation
angle is reached, the collector lamellae are put into
this vertical position by the electric drive system. In
this position, light entry into the greenhouse is
practically uninhibited.
Figure 4 illustrates the installation at night. At
dusk, the lamellae automatically assumea horizontal
position over the absorber pipes. This is achieved
using a sell-or that, when darkness falls, causes the
electronic control system to activate the drive motor
75
._,
-_
S,“,
_.
Technology for Solar Energy Utilization
76
Figure 2. Greenhouse with concentrating solar collectors
Figure 3. Collector position for collecting diffuse radiation
k
‘1
Figure 4. Collector oosition at nkht
“291 ,_
,‘: ,\ -, )
,,
/I I, ..>~,,
.$“,,$’
:
“..
;“::$&;;~‘;
_i :. ,>,,: ..) , /’
:
.,
._
A solar energy system for greenhouses
77
Maximum solar
irradiation
(21 June, noon)
‘\
“\;
\
\
\
,
Figure 5. Greenhouse for solar energy exploitation
that positions the lamellae horizontally. in the
nocturnal position, the collectors inhibit outgoing
radiation. The heat from the plants is reflected on to
the collectors and stays inside the greenhouse.
A conventional accumulator (a tank or pond) can
be used to store surplus thermal energy. When the
room requires additional heat, the recirculation pump
routes the warm water from the accumulator back to
the absorber pipes, which then supply the additional
energy to the plants via the lamellae.
Some comments should be made on the selection
of the collector system.When a collector system with
radiation-absorbing elements is installed movably or
even permanently on the surface of the roof, the loss
of light would be so large in the diffuse position
(vertical arrangement) that there would be insufficient light for optimum plant growth. For this
reason, the reversesidesof the collector lamellae were
designed to reflect light also.
Many homes and schools, hospitals and other
public institutions operate their own gardens either
for internal supply or for educational purposes. These
facilities also require relatively large amounts of
thermal energy for heating water, even in the summer
months. The optimum solution would be a
greenhouse with heating that exploited solar energy.
A special type of greenhousehas been developed
for this purpose (see figure 5). In comparison with
conventional greenhouses,the southern roof area is
larger, making the northern side of the roof steeper.
The inclination on the latter side is 65’, so that no
direct sunlight can enter the greenhouse even at
midday. The collectors are aligned parallel with the
southern roof surface. Energy recovery is approximately 30% over the year. In those caseswhere the
recovered,energy can be used to the full extent in the
summer, this represents a very interesting advantage.
This kind of greenhouse fulfils its objective as a
plant-growing room just as well as a conventional
greenhouse. The cooling area is not larger, and ideal
shading is guaranteed. Ventilation possibilities are not
restricted, and the collectors can also be used for
heating purposes.
Solar timber kilns:
Their suitability for developing
countries
R. A. Plumptre
Commonwealth Forestry Institute,
and Northern Ireiand
Oxford,
United Kingdom of Great Britain
Unless solar kilns are competitive with or
complementary to conventional methods of drying
timber, solar seasoning will never be used commercially and it will remain what it has been up to
now, an interesting research exercise. Any discussion
of solar seasoning should therefore begin with a
description of the main conventional methods.
Forced-air drj$zg
Air is forced through a stack of timber, normally
under a shed, by placing one or more fans at one side
of the stack. No extra heat is applied. There is an
added cost of fans and the power to drive them. The
fans normally have to be large to give an even flow of
air through the stack.
Prciirrers
Conventional methods
The conventional methods described below are
all likely to compete with solar kilns except the last.
which is very expensive.
Air dr?,ing in stacks
Timber is stacked on supporting bearers in an
open yard either with or without waterproof or
sunproof covers. Drying is by circulation of air
through the stacks;the air picks up moisture from the
surface of the timber. There is no control of
temperature, humidity or rate of air flow other than
through correct orientation and spacing of the stacks
and good methods of stacking. Air drying, a widely
used method, is apparently cheap and simple to carry
out, but the costs of holding stocks of timber for long
periods and maintaining yards can be considerable.
Pre-driers are similar to conventional kilns but
with less complicated heating, ventilating and control
systems. They are larger than normal kilns and have
been used extensively in cold climates to dry large
quantities of timber down to about 20% moisture
content, after which drying is completed in
conventional kilns.
Converltional kilns
There are many designs and degrees of
sophistication of conventional kilns, but in all of
them the temperature, humidity and ventilation are
controlled to give optimum drying conditions for any
given species of timber. For the kilns to be used
efficiently, timbers must be sorted into individual
species before being dried, and each speciesmust be
dried according to a specific schedule.
DekumidijTers
Air drying in sheds
The timber is covered by an open-sided shed.
Protection against rain and sun is normally better
than with open stacks, but it is more difficult to
design sheds with as good a circulation of air as can
be obtained with a good layout of stacks. The cost is
greater than with open stacks, since the capital costs
of sheds are greater.
78
Dehumifiers are kilns where no external heat is
applied, but instead humid air from the kiln is
refrigerated, which causesthe water to condense.The
dried air is then allowed to reheat and is returned into
the kiln or exhausted. This method of drying timber
has proved successful in drying small quantities of
timber in warm climates, but it has the disadvantage
that, since no heat is normally applied to the timber,
the rate of drying within the timber is slow.
+_
-_
1
Solar timber kilns: Their suitability
?V
for developing countries
Radio-frequency heating
Timber can be dried by heating it internally,
using radio waves. It is, however, expensive and
impracticable for large quantities of timber.
Kilrl
capacit>~
or
dimemiorrs
I,ocotio:r
-
Lk-w~l,
Ligandu
I 400 hd ft
(3.3 m’)
North->outh oriented,
duuhlr layer of plas;ric,
tuo 18-111.(46-cm)
fanr. . reflectors
For1 CoIlin\.
Colorado.
Llnitcd State\
I 200 bd ft
(2.8 m’)
South-facing roof.
fibreglacs-reinforced
polyester covering.
two 24-in. (61-cm) fan\
Phillpplries
480 bd tt
(I .I ma)
South-facing roof.
fibrrglass-reinforced
polyester covering.
24-in. (6 l-cm 1 funs
Kuma\l,
Ghana
I 700 hd ft
(4 m”)
Converted prccnhou\c. single fan
Tananarlvc
Madagascar
I 200 bd ft
(4 m’)
North-south orientrd,
translucent fihreglach
covering, Iwo lam
Mo%h!,
United
Republic
of Tanzania
4 000 hd ft
(6 m’)
North-houth oriented,
tlat glass rooi. polythrnr
\vall>. three fan>
Liganda
4 000 bd I’t
(9.4 Ill31
North-south oriented,
pltched roof, ueatherahlc
polycslcr covering (two
layers), four !O-in.
(Sl-cm) fans. reflectors
Llganda
8 000 bd ft
(18.9 mJ)
North-south oriented.
pitched roof, sis 20-m.
(5 I -cm) fans, wcattierahlc
PVF covering (two layers)
Review of researchon solar kilns
The table gives information on experimental
solar kilns. Researchon solar drying appears to have
started more or less simultaneously in India and the
United States.
Debra Dttrt, hdiu
Rahman and Chawla’ worked on a laboratory
scale and tried out nine designsof miniature kiln. The
first used a small pump to cause air to flow through
the timber placed inside one chamber. Another
chamber was used to heat the air and connected by a
pipe to the chamber containing the timber. The other
kilns used convection as a means of circulating air
through the timber, some by having the heating
chamber underneath the chamber containing the
timber and some by using a chimney. Small gains in
speed were found over normal air drying, but all
models suffered because there was no method ot
recirculating heated air, which consequently was lost
after a single passthrough the timber. Regrettably the
research appears to have stopped there; as far as is
known, no further results have been published.
INl~ORMATION
f.ocution
Dehra Dun,
India
ON EXPERIMENTAL
K ilrr
capacitv
or
dimensions
Nine small
Iahoratl~ryscale kihls
SOLAR KILNS
Wisconsin. United States
Design
Various designs using
separate chambers for air
heating and timber and
convection movement
of air
Colorado,
...
United States
South-facing roof, double
layer of plastic,
wind-powered fans
425 bd ft
Wisconsin,
United States
(I m’)
South-facing roof, double
layer of plastic, single fan
Rio Piedras
Puerto Rico
2 000 bd ft
Rio Piedras.
3 000 bd ft
South-facing roof, double
layer of plastic, four
Ih-in. (4~cm) fans.
roof covered with glass
Philippines
480 bd ft
North-south oriented.
single layer of plastic,
one 24-in. (61-cm) fan
PuertoRico
(4.7 m’)
(7.1 m3)
(I.1 m’)
South-facing roof. double
layer of plastic, four
I h-in. (40-cm) fans
M. A. Rahman and 0. P. Chawla, “Seasoning of timber
using solarenergy”,hdian Forest Bulletin, No. 229 ( 196I ).
’
-
Peck.2 working at the Forest Products Research
Laboratory at Madison, Wisconsin. designed a small
kiln similar to a conventional dry kiln but with a roof
sloping towards the south and a double layer of
weatherable polyester to trap solar radi;ltion and keep
it inside the kiln. A black-painted corrugatedaluminium sheet was stretched across the top of the
kiln 15 cm below the inner layer of polyester to
absorb heat, and air was passedby means of one fan
over both surfaces of the aluminium sheet. Inlet and
outlet vents were used for letting in and evacuating
limited quantities of air. Speeds of drying were
appreciably faster than those of air drying, being
about 60% of air-drying times for timber dried from
green to air-dry. Drying defects :ve:: found to be less
than for air drying. Although costs were roughly the
same as those of air drying, they were half those of
kiln drying. However, the kiln was a small on?,
capable of taking only 1 m3 of timber.
-z E. C. Peck, “Drying
Products
Research
pp. 103-l 07.
4/4 red oak by solar heat”, Forest
vol. 12. No. 3 (1962),
Journal,
80
Technology for Solar Energy lJMi:a~inn
.--
Colorado, United States
Johnson3 built a small kiln in Colorado to dry
timber for his private use. The kiln consisted of a
chamber in which boards were stacked in racks and
solar heat yas absorbed by wind.ow-type absorberson
the south-facing side. Air was circulated by
wind-powered fins. The kiln dried l-in. (25mm)
timber in two to six weeks, depending on weather
conditions.
Rio Pie&us, Puerto Rico
In 1962. Peck and Maldonado published a paper4
describing a kiln built in Rio Piedras, Puerto Rico,
capable of taking 4-5 m3 of timber. It was similar to,
but larger than, the one built by Peck in Wisconsin.
Air was circulated by four 16-in. (40cm) fans
powered by a 1?4-hp ( 1.I -kW) electric motor.
Otherwise it was almost identical in design to the
Wisconsin kiln.
In 1966, Chudnoff. Maldonado and Goytia
published a detailed account of the operation of an
enlarged version of Peck’s and Maldonado’s kiln at
Rio Piedras,’ an account that is one of the main
contributions to the literature on solar kilns. The kiln
had been enlarged to a capacity of 7.1 m3 by
lengthening it, and the original I-mil PVF sheet on
the inside of the kiln had to be replaced with 2-mil
sheets after one year’s use. The roof panels of 2 mil
PVF sheet had to be replaced by glassafter two years’
use. The deterioration of the sheet was attributed
mainly to flexing, but some brittleness owing to
degradation from ultraviolet radiation was observed
on the roof panels. Trials were subsequently started
using 4-mil PVF sheet snd S-mil weatherable
polyester sheet.
The speedsof air and solar drying under various
weather conditions, including solar radiation, were
compared. Temperatures inside the kiln were 28°F
(15S”C) above the outside temperatures, and the
humidity was 21% lower. The equilibrium moisture
content for air-dried timber varied from 13%-l 5%
according to the time of year, while the equilibrium
moisture content within the kiln varied 87’1%.
Trials of different timbers were carried out, and they
were grouped into mahogany (Swietenia mucrophylln) and mixed hardwoods varying in specific
gravity at 12% moisture content from 0.48 to 0.82.
“C. L. Johnson, “Wind powered solar heated lumber
dryer”, Southern Lumberman, vol. 203, No. 2532 (1961).
‘E. C. Peck and E. C. Maldonado. “Drying
by solar
radiation”, F’oresr Producrs Research Journal. vol. 12, No. IO
(1962). pp. 487-488.
’ M. Chudnoff. E. C. Maldonado and E. Goytia. United
States Forest Service Kesearch and Paper No. ITF-2 (R;3
Piedras, Puerto Rico, Institute of Tropical Forestry, 1966).
One-inch (25mm) mahogany dried from 50% to 12%
moisture content in 18 d, while 1K-in. (28-mm) took
25 d and 2-in. (50-mm) took 41 d. Mixed hardwoods
l!&n. (28-mm) thick took 43 d to dry from 60% to
12% moisture content. The quality of timber
produced was asgood asor better than timber dried by
air drying. Some trouble was experienced with case
hardening, and mist sprayers were installed to
humidify the atmosphere inside the kiln during the
early stages of drying and for conditioning casehardened timber. The water for the sprayers was
heated by a solar water heater before being sprayed
into the kiln. Drying was two to four times as fast as
air drying.
Philippines
Casin in 1967,6 and Casin, Ordinario -and
Tamayo in 1?68’ described a small portable kiln
built in the i’hilippines with a capacity of 1 m’. It
was oriented north-south, unlike most of the previous
kilns. It had piywood end-walls and a door in the
north wall. Circulation of air was produced by a
single 24-in. (61-mm) fan powered by a 3/4-h?
(560-W) motor. Three 4-in. X 4-in. (10 cm X 10 cm)
vents were placed on one side of the kiln. Trials were
carried out on four species, Shorea polyspermn.
Pterocarpus indicus, Shorea negrosemis and Dipterocurpus grandiflorus. The rates of drying were
considerably faster in the solar kiln, indicating that
the kiln not only dried timber in about half the time,
but also dried it to a lower moisture content than was
possible with air drying. Temperatures in the kiln
were 7.2”-11.7”C higher than comparable outside
temperatures. Case hardening was severe in some of
the timber dried in the solar kiln, and humidification
was considered necessaryin future trials.
Solar kiln No. I
Trials with a smali 3.3 m3 solar kiln were begun
in 1954, and the results of these trials were published
in 1967.* The main features of the kiln are a double
layer of S-mil weatherable polyester film, which at
the time of writing, had deteriorated only in the
6R. F. Casin, Solar Drying for Lumber. Technical Note
No. 76 (Los Barios, Philippines, Yo,:st Products Research
Institute, 1967).
‘R. F. Casin, L. B. Ordinario and G. Y. Tamayo, “Solar
drying of Apitoi:v,
Narra, Red Luan and Tanguile”,
Philippine Lumberman, vol. 15, No. 4 t 1969). pp. 23-30.
‘R. A. Plumptre, “The design and operation of a small
solar seasoning kiln on the equator in Uganda”. Cotntnonwealrh
Foresrty Review. vol. 46, No. I30 (I 967).
I-.--‘%
^,
7
.’
I,‘~:.::,,<
~I,,
_
-,-.
/
,’
,‘~
,(.,_
.~
:
’
,,,
‘,
.,
‘L.
Solar timber kilns: Their suitability for developing countries
outer-roof layer. The inner roof and walls were still
sound. The kiln is oriented north-south and has two
small vents on the north and south walls. Two 18-in.
(46-cm) cross-shaft fans are powered by an externally
mounted 3/4-hp (560-W) motor. Curved aluminium
reflectors are used on each side of the kiln to reflect
solar energy into the kiln from the sides. The- false
ceiling, central partition in line with the fans and
other internal parts, including the floor, are painted
with a matt-black blackboard paint. Nine speciesin
differing thicknesses of timber were tried, and air
drying in a shed and in a small open stack under a
cover, solar kiln drying and steam kiln drying were
compared. The quality of timber produced by drying
in the solar kiln was as good as or better then that
produced by air or steam drying. Timber could be
dried to 12% moisture content in the solar kiln in a
reasonable time, whereas air-dried timber never dried
below 157020% (normally about 17%), which is not
low enough for furniture or high-grade joinery use.
Drying speeds in the early stages of drying were not
much faster than those of air drying, but the gain was
decisive below 30% moisture content. Normal
medium-weight furniture timbers such as Chloruptloru excelsa I-in. (25cm) thick could be dried from
green to 12% moisture content in a month or less.
Since 1967, the kiln has been in almost continuous
use drying timber for the Forest Department or for
government and private furniture workshops. Possibly
because of the small vents, little or no trouble has
been experienced with casehardening.
GENERAL
Alummwm
ahrorber,
Solar kiln No. 2
Solar kiln No. 1 was so useful and remunerative
that it was decided to design a larger kiln, one large
enough to interest joinery and furniture workshops
throughout the timber trade in the use of solar kilns.
The result was kiln No. 2, of which the major design
details are shown in figures 1 and 2. The main
principles are the same as for the kiln No. 1, except
that the air is circulated down the side of the kiln and
then turned by a curved sheet of aluminium through
180” to go up and through the stack. Four 20-in.
(5 I-cm) fans, powered by two 2-hp ( 1.5-kW) motors
mounted on the roof of the kiln are used to circulate
air. The fans are reversible. The kiln is capable of
taking a stack of timber 7.2 m X 2.3 m X 1.6 m,
approximately. Improved reflectors the same length
as the kiln are polished with silicone car polish to
reduce, as far as possible, dulling of the surface
through oxidation. The kiln covering material is 5-mil
weatherable polyester sheet in two layers 4.4 cm
apart. The sheet is stretched vertically. rather than
along the length of the kiln, and held along the joints
by timber battens. Wall frames and roof trussesare
thus spaced to coincide with the width of the sheet
( 107 cm).
Four vents (see figure 2) are placed opposite the
fans, just under the eavesof the roof on each side of
the kiln. and are controlled as shown in figures 1 and
2. The reflectors are placed beside the kiln. The holes
in the adjusting quadrants are marked with the time
END VIEW OF KILN
pamted black
Ttmber stack
Figure 1. Cross-section of Uganda kiln No. 2
Technology for Solar Energy Utilization
82
VENT
FAN
SECTION
A.A
a
k3
TROLLEY
Steel channel
/’
Wheel bearmg
Iwo-ad)
REFLECTOR
AND STAND
Figure 2. Uganda kiln No. 2: details of fans, vents, trolley and reflectors
of day. When the adjusting pin holding the reflector is
in the proper hole, a band of light is reflected at right
angles, in through the side of the kiln and on the
black-painted aluminium absorber. Reflectors are
2.4 m wide and 3.6 m long, and there are two on each
side of the kiln. The floor of the kiln is concrete that
was laid free of charge by a building firm wanting
timber dried for use in a hotel. The trolley and rails
were fabricated from old sugar trolleys and rail, steel
channel, rolled-steel joints and 1%-in. (38-mm)
shafting in the Forest Depaitment’s Utilization
Section workshop. An extra source of heating has
been added in the form of a home-made solar water
heater connected to a single coil of 3-in. pipe painted
black and fitted to the wall of the kiln between the
inner 1a:;er of plastic and the aluminium absorber.
This forms a colt,nuously circulating hot-water
system. It is coubtful, however, whether it
contributes much to raising the temperature in the
kiln, owing to its small heat capacity compared with
that of a full load of green timber.
This second kiln w,13compared with the first kiln
and found to be about 10% slower. The quality of
timber seasoned was better than in kiln No. 1. The
vents are too large for optimum seasoningand need
to be kept almost closed throughout the whole
seasoningperiod.
The cost of building the kiln is estimated at
about USh 12,000 ($1,700).
Since 1968 the kiln has been used almost
continuously for commercial seasoning of furniture
timber. Normally. charges have been made up of a
single species at a time, but often, where small
quantities of any one species were required, this
species was mixed with others in the kiln. When
species were mixed, or timber thicknesses were
mixed, the slowest drying species or size was placed
at the bottom of the kiln so that the quicker drying
material could be removed without breaking down
the whole stack. It was found inadvisable to mix
small quantities of green timber with a charge cf
semi-dry timber unless it was a timber that dried fast
with little degrade, since sudden drying in an
atmosphere of low humidity is liable to cause
case-hardening and other degrade. There is no doubt
however, that, because of its slow drying rate, the
Solcir timber kilns: Their suitability for developing countries
83
kiln is much more versatile than a conventional kiln
with respect to its capacity to dry a mixed lot of
green timber.
Only two kilns of this design, other than the one
built by the Forest Department, were built in
U:Aa.,da, one by Prison Industries to supply dry
tii*rber for their furniture workshop and one by a
private company manufacturing furniture. Neither
became fully functional becausethe first was covered
with light-gauge polythene that disintegrated rapidly
and the second was never completed because the
manager interested in the kiln left the firm concerned
without taking steps to obtain the correct covering
material.
Solar kiln No. 3
The second kiln was complicated and timeconsuming to b&i. Although it has been successful
in that it has operated well and is in continuous use,
it wastes space and material in relation to its
timber-holding capacity and, therefore, in terms of its
potential for earning money. It is also not geared to
being mass-produced. A third kiln was designed in
1970 and built in the first half of 197 1 with the aim
of doubling the kiln capacity while at the same time
keeping the cost and the complications of building it
to a minimum. (See figures 3 and 4.)
One difference in design is that the air is not
taken down the side of the kiln and then up and
through the stack as is the case with the secondkiln.
It passes,mainly on the inside, but also partly on the
outside, of the absorber, which stretches along the
side of the kiln. Vents are positioned so that they are
partially covered by the upper edge of this absorber,
and only a small proportion of the air passingthrough
the fans can come in at one vent and out at the
opposite vent. They are smaller and better sealed, if
lined with felt or similar material, than the vents in
kiln No. 2. The main structure of the kiln remains the
same, but the plastic sheet (in this case 4-mil PVF
sheet for the roof and ?-mil PVF sheet for the walls,
since the weatherable polyester became unobtainable)
is stretched over standard-sized panels, which are
Flap of PVF fjlm under rtdge
B”YI %” board full lengh of kaln
’
coverd wth PVF t!lm
Black alummum
sheer :o 46 cm from rade uprlghts
Black slummwm
sheef 10 loveI wth
bottom of venf and down to 30 cm from
bortom of baffle
Timber
,Atr
stack
battle
1. nliev
ar for No. 2 kiln except
wulder
Figure 3. Cross-sectionof kiln Kvo.3 with details (31component parts
_1:,/
I I.
:‘.
.‘,!,
Technology for Solar Energy Utilization
84
8 tans: 3 pwered
JY ana 2 hp alec,r,c
mwnred
at each end of lrdn
more,
DETAIL
PLAN
“lEti
OF VENT
SECTtON
OF VENTS
VENT
OF VENT
To own
CONTROL
LEVER
or C!OIC venfr
Figurr 4. Details of fans and vents for kiln NO. 3
bolted together before the sheet is finally fixed on
the outside of the panels. In all, 22 such panels are
used in the construction of the walls, the door and
upper sections of the roof. Only four non-standard
panels are required for the lower panels on the roof;
the panels on the fan-shaft side are smaller than those
on the opposite side. There are six 20-in. (5 1-cm)
fans, instead of the four in the previous kiln, to cope
with the greater stack width and height, but they are
still powered by the same two 2-hp (1.5-kW) motors,
which have ample power to drive three fans. The
motors in this kiln are mounted at the end of the kiln
to avoid shading the roof and reducing the amount of
radiation entering the kiln. The door designhas been
altered to enable the door to be made from
standard-size panels, and it can also be removed
completely to one side. To lower the cost, the floor is
made of rammed gravel covered by tarmac. The
trolley rails are laid on pressure-treated timber
sleeperswith a concrete beam under each rail to give
added bearing strength to prevent the rails from
sinking.
The fans and motors have worked virtually
trouble-free apart from the occasional need to tighten
and grease bearings. The roof of the small kiln has
required a new outer layer, but only after six years of
operation. Occasional cleaning or repainting of the
black surfaces inside the kilns is required but,
normally, only once every two years. No permanent
operator is necessary,since the kilns are switched on
in the morning and off in the evening and the
reflectors are moved hourly by a timber inspector
doing other work during working hours and a
watchmar at other times. The Ministry of Works has
its own electrically heated, automatically controlled
dry kiln capable of taking about 2 m3 per charge.
Fort Collins, Colorado. United States
Troxwell and Mueller reported in 1968 on a
2.8 m3 capacity solar kiln at Fort Collins.g A single
layer of fibreglass reinforced corrugated polyester
sheet was used to cover the kiln. This translucent
material was tested against glass, to compare the
properties of the two materials in transmitting solar
radiation, using an Eppley pyrheliometer. It was
9 H. E. Troxwell
and L. A. Mueller,
“Solar lumber
drying in central Rocky Mountain region”. Forest Prodtrcts
Rrseurch Jnrrmnl, vol. 18, NO. I (I 9681, pp. 19-24.
82
Solar timber kilns: Their suitability for developing countries
found only slightly inferic,r to glass in this respect.
Previously, PVF film had failed in the high winds
common in the area.The kiln was equipped with two
24-in. (61-cm) fans giving an air velocity of
100-300 ft/min (30-100 m/min). It was east-west
oriented with a north-south slope on the roof.
Differences in temperature were found in the middle
of the day in summer. It was reported that vent
control was important in obtaining optimum
conditions. Using the pyrheliometer it was calculated,
from the radiation reaching the inside of the kiln and
the timber moisture content, that 25%-45% of the
solar energy reaching the inside of the kiln was used
in drying the timber. The rest was dissipated. Trials
with Engelman spruce and lodgepole pine gave
satisfactory rates of drying.
Wengert, in an interesting paper,’ ’ identified
sources of energy loss from the Fort Collins solar kiln
and indicated that only some 16% of the solar energy
reaching the outer surface of the kiln was effective in
evaporating water from the timber; the rest was lost
approximately as follows (70):
Through walls and roof
Reflected solar energy
Through the floor
Heating fabric of drier ‘)r stored therein
As long-wave energy
Through air vents
Total
30
17
II
7
I3
14
G
The sum of the above lossesplus the 16% used
exceeds 100% owing to an unmeasured input of
energy from the fans and to experimental error. The
kiln used had only a ;ingle-layered fibreglassreinforced polyester coverin,::,and Wengert suggested
that a double layer be used’,he also suggestedbetter
insulation of the floor, baffles in the roof of the kiln
painted black to absorb more of the energy falling on
the roof, and improvements in the system of venting.
He proposed coating the walls with infrared-reflecting
chemicals, such as titanium dioxide, to reduce
long-wave radiation loss from the kiln.
Ghana
Martinka’ ’ used an old glass-coveredgreenhouse
in Kumasi, with spacefor about 4 m3 of timber as an
improvised solar kiln to compare solar drying, with
pre-drying in a converted oven and with air drying.
The pre-drier was capable of taking 96 bd ft
(0.23 m3). A single fan was used in the kiln to
circulate the air, and black-painted aluminium sheet
was used to absorb heat insicle the kiln. Using Naucka
’ ’ E. M. Wengert, Improvrments in Solar Dry Kiln
Design, United States Forest Service Research Note
FPL-0212 (United States Forest Service, 1971).
’ ’ E. Martinka, Predrving of Some Chanaiarr Timber.
Technical Note No. 11 (Ghanaian Forest Research Institute,
1969).
and Entandroptvagnm arlgolensr he found that the
speed of drying in the solar kiln lay between that of
air drying and the pre-drier, possibly hecause the
insulation of the greenhousewas poor and less good
air circulated than in the pre-drier. ‘The pre-urier was
also much smaller.
Madagascar
In 1970, Gueneau’ 2 described a solar kiln at
Tananarive similar in design to the Fort Collins (see
above’) kiln but oriented north-south and with the
slope on the roof facing west. It was covered with a
single layer of corrugated fibreglass-reinforced
polyester sheet.mo fanswere used for circulation and a
hygrometer was used to monitor the humidity inside
the kiln. The cement on the floor was painted black.
Polystyrene was used to seal the joints between the
fibreglass sheets. The cost of building the kiln was
FMG 417,000 (approximately $1,500). Two vents
were placed on the high side of the kiln near the roof
and two more near the floor on the opposite side.
Trials were carried out using Pinus kesiya and
Dalbergia barorzi, and these showed reductions in
drying time using the solar kiln compared with air
drying of 34%68% for pine and 48% for the
Dalbergia. Savingsin time were particulr?rly apparent
below 20% moisture content. It was recommended
that the solar kiln be used after a preliminary period
of air drying to bring timber down to the required
equilibrium moisture content of 10%.
United Republic oj’ Tanzania
In 1968 a solar kiln was built at the Forest
Department Utilization Section at Moshi in the
United Republic of Tanzania. It was flat-roofed with
a single layer of glass on the roof and had
polythene-covered side walls. A galvanized-iron
absorber placed 6 in. (I 5 cm) inside the walls was
painted black, and air was circulated by means of
three fans placed above the absorber; the latter was
carried across the kiln as a false ceiling. The kiln
capacity was 6 m 3. Unfortunately, no information is
availableon its performance.
important factors in solar drying
Instrlaticm
Wengert’ 3 demonstrated that a large proportion
of the heat loss in solar kilns was due to poor
insulation of the roof, walls and floor. He was dealing
’ *P. Gueneau.“An
experiment
in solar drying wood”.
Bois et For& des Tropiques. No. 13 I (19701. pp. 69-70.
’ ‘Op. cit.
Tcchologv fbr Sdar Etterg>*Utilizatiotr
86
with a kiln that had only a single layer of
fibreglass-reinforced polyester sheet. Kilns with
double layers of covering material are almost certain
to be more efficient. Where flexible plastic sheet is
u$ed, the double layer is likely to be less effective
than a rigid sheet, which retains a constant air space
and reduces the movement of the air within the space
during the vibration of the kiln caused by the fans.
Doors could, almost certainly, be made to fit better
with little extra cost, and vents need to be efficient
and easily adjustable to be able to control, more
exactly, the quantity of air entering and leaving the
kiln.
The type of covering material used is controlled
by the limited number of transparent or translucent
weatherable materials available. Since the covering
material forms a large part of the cost of building a
kiln. a compromise has to be made between the idea!
material and economy. It is unfortunate that the
weatherable polyester sheet used on the first two
Uganda kilns is now out of production, and in
Uganda any suitable covering material, other than
glass, which is expensive to use, has to be imported
specially from abroad, usually with an attendant
delay of six months. Undoubtedly, this, more than
anything else, has prevented the wider use of solar
kilns in Uganda.
Air circulation
The rate and evennessof flow of air through the
timber stack is important in controlling the rate of
drying. If kilns are only partially filled, a great deal of
the air flow will go over the top of the stack. A piece
of plastic sheet attached at one side to the roof of the
kiln (or false ceiling) and rolled round a piece of
timber that is lowered on to the top of the stack,
provides an easy method of preventing this. Correct
design of absorbers to give an even flow of air
through the timber and good heat absorption is
imnnrtant
-**-I---
-----.
The control of vents is a&o important.
Over-ventilation has been a fauit of many solar kilns
and a correct balance has to be found between
humidity and temperature. In the early stages of
drying and particularly in the region of the fibre
saturation point, the limiting factor is the rate at
which water will passfrom the centre to the outside
of the timber. Too large a moisture gradient
case-hardens the timber and, in the early stages of
drying, high temperatures and high humidities are
required. Only in the latter stages of drying is it
possible in a solar kiln to obtain high temperatures
and low humidities. Thus, further study to improve
vent design and control is needed.
developing country. where furniture and joinery
workshops are run on a smaller scale than in the more
developed countries. Most of the experimental kilns
have so far been small ones, and more work is
required with sizes of kiln where the ratio of surface
area of kiln to volume of timber is smaller. The third
Uganda kiln was set up to study this problem. If the
kiln operates successfully, it could provide a basisfor
a commercial kiln design suitable for conditions in
Uganda.
Reflectors
With kilns of lo-20 m3 or larger, as much heat is
required as possible, and it should be collected from
as wide an area as possible. In Uganda, which lies
across the equator, it is easy to design and operate
simple reflectors that reflect a considerable quantity
of so!ar energy throuti the sides of the kiln. Away
from the equator this would be mord difficult. Simple
air heaters might be designed to preheat air before it
is drawn in through the vents.
costs
While it is possible, no doubt. to increase the
efficiency of solar kilns greatly, the cost of doing so
must be considered. :‘he main justification for solar
kilns is their low cost and ease of building as
compared with conventional kilns. There is no point
in losing this major advantage in order to employ
some sophisticated method of increasing kiln
efficiency.
Quality of solar-kiln-dried timber
Almost a!! researchworkers in solar drying have
reported on the high quality of timber produced. The
reason is almost certainly that the lack of solar
heating at night and the consequent lower temperatures, higher !iumidity 2nd lack of air f!ijw aiiow a
nightly “conditioning” period, where internal and
surface moisture contents have time to even out. This
is particularly important with the harder, heavier.
slower drying species. Good conventional kiln
schedules properly applied can give the same results,
but skilled operation is required.
Competitivenessof solar kilns
The competitiveness of solar kilns in relation to
each of the main conventional methods described
earlier will now be discussed.
Size of kilns
/1ir dr?Mg
To be of interest for commercial use a solar kiln
needs to have a capacity of lo-20 m3 even in a
Costs of drying by solar kiln are generally
accepted as being as high as, or higher than, air
drying, even when the extra length of time during
which capital is tied up in air-drying timber is taken
into a,:count. Solar drying has three major advantages
over air drying. First, it is quicker, normally about
twice as fast to equilibrium moistllrc cpntent of
air.dried timber in the climate concei-l1.d.Secondly,
it is possible to dry timber to a moisture content
lower than the equilibrium moisture content of
timber within closed buildings, which means that it
can be dried sufficiently to be suitable for furniture
and high-grade joinery manufacture, which is
undoubtedly the greatest advantage of solar drying
over air drying. Thirdly, solar-dried timber is almost
always superior in quality to air-dried timber, since
humidity is more uniform in a!! stagesof drying. The
timber is also not subjected in the sameway to more
air flow and more sunlight or heat at the ends of the
timber than in the centre of the stack, which is
common even in the best air seasoning. Thus, less
chance of degrade in the form of end split exists than
with air-dried timber.
The major disadvantageof solar seasoningis the
small capacity of the kilns.
Forced-air driers
c -
With research. forced-air driers may be made
competitive with solar kilns. Large fans are required
unless complicated baffle systems are designed;to be
effective, they rn*usistbe covered by a roof and
mounted in a wall or partitioh. The capital costs for
these driers are, therefore, considerable; and they do
not give the advantagesof the higher temperatures
and humidit} control of the solar kiln. At present,
therefore, they do not threaten to compete with solar
kilns in tropical areas.
5
he-driers
If pre-drier? are used only for the purpose for
which they were designed,it is unlikely that they will
compete with solar kilns, since the latter are most
efficient and can best be used for the final stagesof
drying. If, however, pre-driers are modified to form
low-cost conventional kilns with a simple form of
heating suitable for developing countries, the chances
of their competing with solar kilns are much greater;
it may well be possible, for instance, to augment solar
heat with a simple hot-water system basedon an open
fire fed on wood waste and a Gga! (Imp) (200-l)
drum.
Conventional kilns
Solar kilns have proved in all cases, where
castings have been carried out, to be cheaper than
conventional kilns. Within the limitations of their size
they are, therefore, competitive. but, since they are
slower and must be placed so that they are exposed
to direct sunshine for most of the day, they require a
much larger area of ground per unit of timber
seasoned than conventional kilns. In their present
form, therefore, they are more suitable for seasoning
small quantities of timber than for mass production
of seasonedtimber. This indicates an initial use for
small joinery and furniture concerns. Later they may
be used in batteries for seasoninglarger quantities of
timber, but development on a comparatively small
scale is required first. One advantage of solar kilns is
that they do not require highly skilled operators and
need only periodic attention.
Dehumidifiers
Becauseof their low efficiency, especially in the
drying of large-size timber. it seems unlikely tb,lt
dehumidifiers can compete with solar kilns.
Remarks on cost comparisons
The only castings that have been carried out
comparing solar seasoning with other methods are
those of Peck.’ 4 In l.Jganda,neither the steam kiln
operated by the Forest Department nor the electrical
kiln operated by the Ministry of Works is
represeritative of potentially competitive commercial
kilns, since they are too small and expensive to run.
No castings are available for the only commercially
operated kilns in the countql, which are located
50 miles (80 km) away from the solar kilns.
It is not considered valid to compare the costs of
air drying with those of solar drying, since the best
way fo use a solar kiln is almost certainly for bringing
air-dried timber down to a moisture content that
cannot be reached by air drying alone.
_-.
Suitability of solar kilns for developing countries
In developing countries, the individual units in
the timber industry are normally small and often
separated from each other by considerable distances;
communications are often poor. Requirements for
seasonedtimber are correspondingly low in any unit,
which favours the solar kiln with its low capital and
running costs and small output of seasoned timber.
At the same time, most developing countries have
climates that favour solar seasoning. A certain degree
of sophistication of the industry is necessary,since
the “bush” carpenter will not be able t<) pay for a
solar kiln, nor does it matter to him or his’customer if
the timber used is not absolutely -dry. Once the
industry has developed to the stage where individual
units use IO-40 m3 of timber per month or where
’ 4Lot. cit.
there are co-operatives of smaller units, there is likely
to be a demand for solar kilns. At this stage of
development it is often nqt the cost per unit of
timber dried that determines which method of drying
should be used, but the initial capital cost of the
method. Thus, initial investment must be kept as low
as possible, Kilns should be designed so that they can
be easily made from readily available materials. In
this respect modular building systems and standardized components are an advantage if they come with
good drawings and instructions.
Recommendations
Co-ordination and centralization of basic research
Research along the lines suggestedby Wengert’ ’
is required to discover the nature of energy lossesand
to test alterations in design intended to reduce them.
Research directed towards finding the best covering
materials is essential. This research can be done
wherever there are staff, funds and facilities to do it;
the results will be applicable to all areasof the world.
The design of the best size and shape of kiln, which
will vary from country to country, should be left +o
’ 5Op. cit.
individual research workers in interested countries so
that they can suit kiln designs to the particular
conditions in their countries.
Funds for research and development
In many developing countries, funds for research
are short, especially for the kind of researchrequired
For solar kilns, where immediate financial returns will
probably not be great. Since “seeing is believing”,
commercially operated kilns are not likely to become
a reality in any country until a solar kiln of
commercial size can be demonstrated in that country
or a near5, zne, in operation. Thus, funds are needed
not only fc)r research but also for prototype
commercial models.
Dissemination of information
Much of the research on solar kilns has been
carried out without knowledge of research done
elsewhere. A corresponding committee composed of
personsworking with solar kilns in different parts of
the world would solve that problem. A convener or
sxretary would be required to keep the information
circulating.
Solar refrigeration
and c
Erich A. Farber
Solar Energy and Energy Conversion Laboratory,
Refrigeration is available in the more industrialized countries through the availability of
electricity produced by fossil fuel but is not readily
available in the major part of the world and to the
great majority of people. Solar energy, however, is
available in most areas where people live, and the
conversion of solar energy into other forms which can
provide the refrigeration or cooling needed could be a
tremendous benefit to mankind. This paper discusses
in detail some of the methods that can be used to this
eni and makes recommendations for their implementation.
University of Florida, United States
Expansion
valve
Condenser
I
1
1
7
a
’ ’ 0’ ’
O”
0
O 0
Basic methods
l!
The most widely used methods of refrigeration
are the compression system, consisting of a
compressor requiring high-grade energy such as
mechanical or in most cases electrical, and the
absorption system, in which the compressor is
replaced by an absorber, a generator and a small
pump. (See figures 1 and 2.) Both methods require a
condenser, expansion valve or system, and an
evaporator. In an absorption system with three fluid
components, even the pump can be eliminated.
Detailed descriptions of the methods are given below.
Expansion
valve
Condenser
-
Figure 1. Conventional comprewion
tion system
7
refrigera-
Evaporator
5’
n
”
Fump
Generator
I 1
‘6
Figure 2. Continuous absorption refrigeration system
Solar refrigeration systems
When solar radAation is used as the energy source
for refrigeration, many methods can he employed.
So’larenergy can be converted into mechanical energy
and the compressor of a standarc compression system
be driven in this manner, or. electricity can be
produced either by a solar engine driving a generator
or through a solid-state device, which then operates
standard electrical refrigerators. However, since we
can convert solar energy more efficiently to heat than
to mechanical or electrical energy, the use of an
absorption system with solar energy seemsto be the
best solution at this time.
In higher-temperature applications the so!ar
energy must be concentrated, which requires in most
cases systems that can track the sun. which makes
them more cumbersome and more costly. Wind
loading, life expectancy of the reflecting surfaces,the
tracking mechanism etc. are sometimes problems. It
must also be realized that concentrating solar
collectors can only utilize the direct rays of the sun
and not the diffuse portion, meaning that even on
89
90
Technology
clear days some of the energy is lost, and of course in
cloudy weather these systems cannot collect any solar
energy.
In lower-temperature applications, which can
utilize flat-plate collectors, both the direct and
diffuse radiation of the sun is utilized. Refrigeration
systems in this range are not commercially available
at the present time and must be specially designed
and manufactured. Examples of both of the above
approacheswill be described.
!’
Operating characteristics
Most people are quite familiar with the operation
of compression refrigeration systemsbecausethey are
so common. There is less information about and
familiarity with absorption systems, and so they will
be briefly described.
The intermittent absorption system indicated
schematically in figure 3 is simpler in construction
and consists of two tanks, each of which does double
duty. During the charging mode, one of them acts as
the generator, driving refrigerant (ammonia) out of
solution (in water) when heated, and the other as
condenser, changing the state of the refrigerant from
gasto liquid.
for Solar Energy Utilization
the tank which was the condenser now becomes the
evaporator, producing the refrigeratior. temperature
needed.
The continuous absorption refrigeration system
is similar to the compression system, with the
compressor replaced by an absorber, a generator and
a pump. The refrigerant-absorber solution is heated in
the generator, driving the refrigerant from solution,
which flows to the condenser where it is cooled and
becomes liquid. From there it passes through the
expansion valve and into the evaporator where the
cold temperatures are produced. After the refrigerant
has done its job it passesto the absorber, where it is
reabsorbed into solution. The solution is pumped
back to the generator to repeat the cycle. In other
words the absorbent acts only as a carrier. In figure 2,
the carrier circulates from 5 to 6 to 7 and back to 5,
loading up with refrigerant between 7 and 5 and
unloading it between 6 and 7. The refrigerant
circulates through the system from 1 to 2 to 3 to 4 to
5 to 6 and back to 1.
Thermodynamic analysis
Many substances can be used for the absorber
and the refrigerant; ammonia and water is the most
widely used and best known combination and for this
reasonwill be the only one discussedhere.
Figure 3. Intermittent
absorption
system
refrigeration
The charging mode usually takes a short time
compared to the cooling mode, during which the tank
which was the generator becomes the absorber and
Since the continuous system is more complicated
than the intermittent, thi: analysis will be carried out
for the continuous system. What are needed for the
analysis are pressure-temperaturecharts for ammonia
and enthalpy-concentration charts for ammonia-water
solutions. These charts are readily available in
refrigeration handbooks. The table gives data taken
from such charts for the seven numbered points in
figure 2, assuming the generator operates at a
temperature of 160°F (71 “C), easily obtainable with
solar energy, the evaporator at 20°F (-7’(Z), and the
condenser and absorber at 80°F (27°C).
OPERATING CONDITIONS IN THE CONTINUOUS AMMONIA-WATER ABSORPTION REFRIGERATION SYSTEM
Pain I
in
figure 2
Tempem ture
OF
Oc
1
2
3
4
:
160
80
20
20
CO
80
7
160
Gauge pressure
Ammoniu
concentmtion
(weight fraction)
Enthalpy
Brtr/lb
kJ1k.x
psi
bar
27
-7
-1
27
140
140
35
35
140
35
9.6
9.6
2.4
2.4
9.6
2.4
1
1
I
1
0.50
696
132
L??
618
-17.6
-18
71
140
9.6
0.47
67
11
Flow rate
lb/h
kglh
! 620
307
307
1 438
-42
-40.9
!
1
1
1
20
0.5
0.5
0.5
0.5
69.1
9.1
156
19
8.6
~~Yw*ew~w~<.~
,$e&&&~~“3~*~r$ ,y\.
I%,
~~~~~~~~~~~~~~~~~:~;~~~~~.~~~~:~~,-,I.,
,
j
J~~~~,~~?;~~~~~-~~,~:!~~~~,
;‘<‘; ; “‘+. j) _
.
a&&@.
~~~~~y?~“““~~~~~~~~~:
(@‘,“,,, - /‘ [email protected],“,
..$*“,,f
,,i” ‘-&, ,!I 3
,-_A /.,a. +,....>
“;y
“;:- ;:‘f;
^. ,ii,
I., ’ ..i
j r’.
;,,.
i‘
: ‘j’L(
1,
I.>.
.,
.;
,,.
..,“.,,
,_‘.
:,s,’
. .
: i 1 Sqlar refrigeration
and cooling
91
For a typical 12-ft3 (340-l) refrigerator and
continuous operation, the quantities of heat which
must be exchanged are as follows:
: .Bn4/h
kJlkg
Generator
Condenser
Evaporator
Absorber
2 321
564
486
2 251
5 399
1 312
I 130
5 236
added
rejected
added
rejected
The above values are within the accuracy which
reading charts the size of a book page allows. For
more exact values tables or larger charts must be
used.
The coefficient of performance for this system
under the above operating conditions is about 0.21
and the pump power needed for circulating the fluid is
about 0.003 hp (2 mW).
A complete description of a system basedon the
above principles is in the article by Assad Takla, on
page7.
Solar space heating and cooling
and solar water heaters
V. G. Bhide
Natimai
Physical l!,aboratory, New Delhi, India
Introduction
It has been fairly well established that human
performance, in terms of productivity, is optimal
under certain environmental conditions of temperature and humidity. The optimum temperature range
is believed to be between 20” and 25’C.
India has a very broad spectrum of climatic
conditions. In certain parts of the country, winter
temperatures may drop below freezing, whereas in
certain other parts, summer temperatures may rise to
45°C. To make these conditions more comfortable,
spaceheating and cooling is normally resorted to.
Because of the energy crisis, there is an attempt
all over the world to explore the possibility of
providing space heating or cooling utilizing solar
energy. Indeed, results over the last few years have
shown that of all the applications of solar energy,
space heating and cooling is not only technically
feasible but is becoming economically competitive. In
India also, some attempts have been made to study
the feasibility of solar spaceheating and cooling. This
paper presents the results of Indian experimental
projects in this field.
Spaceheating
The first major experimental project on space
heating was conducted by the National Physical
Laboratury (NPL). The project sponsor, Bharat
Heavy Electricals Ltd., (BHEL), was finding that the
productivity of its factory at Haridwar was very low
during the winter months. In order to see whether
space heating could improve productivity at the
factory, BHEL proposed that NPL,collaboiate with
them to design, install and eva!uatethe spaceheating
facility.
The chief objectives of the project were:
(a) To establish the feasibility of providing
spaceheating to workers;
{bi To design an effective- and economtiial
system for factory heating wiih soiar energy;
92
(c) To determine which of the severaloperating
modes minimized auxiliary energy requirements;
(d) -1’0 estimate the pertrlrmance of the
camplete heating system and ea& of its principal
ccbmponents;
(e) To appraise the utility by comparing its
actual performance with result;; predicted by a
mathematical model;
(f) To modify the design if necessary.
The factory consisted of a large building with big
doors, windows and ventilators. Huge machines were
installed in various places and there was a provision
for a crane to move across the building. In view of
these conditions, the idea of heating the entire space
was abandoned and it was decided instead to provide
1ocaliPPdheating to those working at the machines.
Thz space heating system is shown schematically in
figure 1. It essentially consistsof an energy-collection
cycle and an energy-radiating cycle. During the
energy-collection cycle, water is circulated through an
array of solar collectors mounted on top of the
building. Water thus heated is stored in an insulated
storage tank. During the radiation cycle, hot water is
circulated through the fan-coil units. The design
criteria for the two cycles are discussedbelow.
Energy
radiation
cycle
Assume an energy radiation system consisting
only of the storage tank, pump and radiator in
figure 1. To analyse and mathematically model the
system performance, the following reasonablesimplifying assumptions are made:
(a) The heat lost from the storage tan&: is
negligibly small compared with the heat radiated by
the fan-coil units;
(b) The hear-transfer coefficients of the different radiators are the same:
1~1 The specif;.c hsats of water and air are
1-j
constant over the temperature range involved;
,. _I
.‘,_,
:
Solar space heating and cooling and solar water heaters
Solar radmtion
93
\
Cold water
Hot water
Figure 1. Solar space heating system
(dl During the cycle, no water is either added to
or subtracted from the storage tank:
fe) The circulating pump does not alter the
temperature of the water pump.
Let the massof hot water in the full storagetank
be M and the temperature of the watLr within it at
the start of the radiation cycle (t = 0) be To. Let the
massrate of flow of water through the fan-coil unit be
rit, and that of the air blown over the fan-coil unit be
ha. As the water circulates through the fan-coil units
and exchanges heat with the blown air, the
temperature of the water in the tank decreases.Let
the temperature at any instant of time t be T. If the
specific heat of water is C,,, and the temperature of
the water after it has passedthrough the fan-coil units
is T, *, we can write
M+=Xk,C&,
- T)
(1)
where N is the number of fan-coil units.
Tw,) = fi, C,(Ta, - Ta)
MC,
(2)
where C, is the specific heat of air. We now define
two parameters R and E:
+Nrir,C,cR(T-
TN)=0
(5)
which has the solution
(T-
T,)=(T,--
T,)exp(-Nrh,ERt/M)
(6)
The above equation states that the temperature
of the tank falls exponentially with time. Using the
above equations, it is possible also to determine how
the temperature of the hot air will change with time.
Using equation (4), we see hot-air temperature Ta2
will be given by
(T,2-
To find how T,z varies with T, we must
consider the heat balance during the heat exchange
process. As the hot water exchangesheat with the air
that is blown over the fan-coil unit, the temperature
of the air increasesfrom the ambient temperature T,
to T,*. Under these conditions we have
fiir, GtT-
The product ER is the ratio of the actual heat
exchanged to the maximum heat that could be
exchanged, which we regard as a constant characteristic of the fan-coil unit, even though E is a ratio
involving the two variablesin our problem.
Combining equations (I), (2), (3) and (4), we
have
T,) = E(To - T,) exp ( - Nti,
ER t/M)
(7)
This equation shows that the hot-air temperature will
also fall exponentially with time. Defining a
parametera z h,eR/M,
and plotting the variation
of the temperature of the tank and the hot-air
temperature as a function of time, the graphs shown
in figures 2 and 3 are obtained. Figure 2 provides the
design data. Depending upon the temperature one
would like to have after a certain length of time (at
the end of an eight-hour shift, in this case), the value
of a is fixed. Knowing tiz,, E and i-Z. one can then
proceed to fix the capacity of tb? YS~!~Mlr.
Table 1 sets out the obser & n(:rto; mance of the
fan-coil unit .
Assuming that the ?ernpec,,!u;:. required at the
end of the secnnd shif: --after air :I~H;;; of operation of
the plant-is 65°F (18.3’C\. oii+ obtains a value ofa
of 0.150. With thik vaiue one carI determine the
-_. Technology for Solar Energy Utilization
storage tank capacity M. For tiqv = 450 kg/h
(990 lb/h) and ti,=408 kg/h (900 lb/h), the value of
M is 700 I per fan-coil unit. In this case the tank
temperature and hot-air temperature will vary as
follows:
(T- T,)=(T,T,)exp(-0.15r)
and
T,)
(Ta:,,- T,)=c(TThesecurves are shown in figure 4.
50
40
TIME (hl
Figure 2. Variation of tank temperature over one radiation
i,,cle with time for various values of 0
25
2c
1c
1
1-1
I
I
0
2
3
TIME
4
5
6
(h)
Figure 4. Variation of hot-water and hot-air temperature
with time of utilization for u = 0.15
10
TIME
lhl
Figure 3. Variation of hot-air temperature over one
radiation cycle with time for various values of II
Thus, to obtain the required temperature at the
end of the shift, the storagetank temperature at I = 0
of the radiation cycle has to be 130°F (54.4’(Z).
Given these parameters, the collector area requirement call now be determined.
TABLE 1. PERFORMANCE OF THE FAN-COIL UNIT
Measurement
number
:
3
8 4
Fall
Massflow
lktrth)
Water
450
180
180
450
450
Air
Hot-wuter
tempem lure
PC)
408
212
408
272
408
272
‘Measurements taken in closed room.
54.4
54.4
54.4
60
60
Ri&?
in water
temperature
PC)
4.4
5.8
10.0
I.?
4.1
3.3
in air
tempercirurt
(“Cl
19.0
19.4
17.8
18.3
20.3
21.1
c
R
ER
0.50
0.49
0.218
0.145
0.545
0.343
0.218
0.145
0.106
0.072
0.250
0.161
0.098
0.068
0.46
0.47
0.45
0.47
!‘.‘,
.,
Solar space heating and cooling and solar water heaters
Energy-collectiort cycle
Dvlring the energy-collection cycle, water from
the storage tank is circulated through the collector
array and hot water is stored in the tank. For given
insulation Hr and heat-loss coefficient (UL), it is
possible to determine for a given flow rate the
temperature of the tank as a function of time. It can
be shown that under these conditions the tank
temperature will vary as follows
(‘f-
T,)=-$(l
-e-Y’)+
- 9.5
-
pe-fl’
the efficiency of the collectors. as a function of the
collector temperature for various values of insolation.
From these curves, it is possible to determine the
efficiency of heat extraction at the required
temperature (54.4”C) for various values of insolation.
With that information and a knowledge of the diurnal
vari&tion of insolatl, 1, it is possible to determine the
useful gain in thermal energy. Figure 6 shows the
annual variation of daily useful gain in energy when
heat is extracted at 60°C. From this curve, it is easy
to determine’ the area of collectors required to
provide 700 1 of hot water per fan-coil unit.
L
where
Qsten? desigrl parameters
p=qjf(I --e-q
Design parameters for providing comfort heating
at 15 stations in the factory were as follows:
and
P=f,T-
T,! at r=O
In the above equation, pL is a factor determibcd by
Hf, UL, specific heat etc. Since the insolation Hr is
not constant throughout the day, this model is not
physically realistic. However: there is another method
to assessthe area of the collectors.
The flat-plate collectors used by NPL in its
experiments on space heating are the same as in the
solar pump experiment (page 55). Figure 5 shows
Ambient temperature
Initial hot-water temperature
Initial hot-air temperature
Temperature of hot-air after 6 h
Radiator specification
Number of radiators N
Flow rate of water per radiator &,,
Flow rare of air &a
Value of L
Value of R
\a!ue of II
Collector area
50°F (10°C)
130°F 154.4”C)
80°F (26.7”C)
65°F (18.: L’)
Fan-coil
KEY
Insolation (W/m21
200
400
600
800
Curve
8G
‘1
1
2
3
4
5
70
60
-
-
1 000
-
-
Smgle glass cover
Double
glass cover
50
3
is
I’,
g 43
0
ii
ii
30I-
20
10 l-
i
0
lb
i0
TEMPEtiATURE
3b
40
OF THE COLLECTOR
50
C”c)
Figure 5. Variation of efficiency with collector temperature
60
unit
15
990 lb/h (450 kg/h)
900 lb/h (408 kg/h)
0.49
0.218
0.15
105 m2
i0
96
Technology for Solar Energy Utilization
-
3.6
“E
1
s
s 3.2
2
z
u
?I 2.8
2
0
2.4
2.c
1.6
I
I
Jan
Feb
Mar
Apr
I
Mav
I
June
I
July
I
Aw
I
Sept
I
act
1
Nov
Dee
Figure 6. Annual variation of the daily useful gain in energy by a collector operating at 60°C
lnrnle the shop
Solar radwmn
ill111
Steam line
mride the
*OP
T
L
KSV
Thermowell
F
I
2-D
To tank.2
ziJ+-y
Storaga
tanks
Z.Zmhi#tr
2.8 m diameter
Main mndenlrts
line inside the shop
Figure 7. Solar spaceheating system at Haridwar; plumbing diagram
x
Gate valve
Z
Non.rewrn
Y
Priming
!?
Pressure gauge
valve
arrongament
Solar mace heatinn and coolina and solar water heaters
97
Tank temperature
5
T
40
e
s
5
30
Comfort
temperature
i!
f
20-1
Ambient
temperature
T,
10 1
1600
1900
2000
2100
2200
2300
2400
Time
0100
0200
0300
0600
0700
0800
0900
(h)
Figure 8. Radiation cycle at Haridwar, 6 February 1976
The plumbing of the whole system is shown in
figure 7.
During the first 18 months of operation, the
system has given very satisfactory performance.
Actual performance characteristics on a typical day
are shown in figure 8.
Combined spacecooling and heating
Spce heating cycle
Encouraged by the success in providing heating
to workers in a factory, we started to design and
install a combined heating and cooling facility in one
room at NPL. The room was of the dimensions
6.1 m X 3.7 m X 3.0 m, with two doors (each
2.i m X 1.2 m) and three windows (each 1 m’). In
designing the space heating system, it was assumed
that the ambient temperature was 10°C, and that a
temperature of 25°C was comfortable. In designing
the space cooling system, it was thus assumed that
the ambient temperature of 35°C was to be reduced
to 25°C. It was calculated that approximately
9 430 MJ/h (2 250 k&/h) needed to be suppl.ied to
the room during the heating cycle. The corresponding
refrigeration required during summer months was
nearly 12 600 MJ/h (3 000 kcal/h). The difference
between the heat load and the refrigeration is due to
the necessity to adjust for humidity during summer
months.
The space heating system is based on figure 1 and
consists of a collector array on top of the building, a
small insulated storage tank, and a fan-coil unit. The
fan-coil unit described earlier was used in the present
experiment. The E: and R values calculated for this
fan-coil unit were 0.49 and 0.2 18 respectively
In a typical experiment, hot water was passed
through the fan-coil unit at 60°C at a rate of flow of
400 kg/h. The air flow rate was nearly 280 kg/h. This
gave a rise in air temperature of about 21°C. The
temperature of the water decreased to 54°C. Thus the
loss of heat by water, equal to the heat supplied to
the room, was 10 055 MJ/h (2 400 kcallh). Thus, the
area requirement of the collector array should be
such as to provide 10 055 MJ/h (2 400 kcal/h) at
60°C. A thermostatically controlled bypass valve
allows water to enter the storage tank only at 60°C.
High-efficiency flat-plate collectors were used. The
dai!y energy gain over a seven-hour period during the
Delhi winter is as follows:
Man fh
November
December
January
February
Average
Energy gain
(k Wh/ma)
3.5
3.1
3.3
4.0
3.5
The average hourly energy gain was thus
0.5 kWh, or roughly 1 800 MJ h-’ rnv2. Thus, to
obtain the required useful energy gain of nearly
9 430 MJ/h, 5 m* of collector area would be needed.
The design parameters of the space heating system
can be summarized as follows:
Collector area
5 m’
Fan-coilunit
Radiatorspecification
value of f
Value of R
Rateof flow of water through collector
Inlet temperatureof water to the fan-coil unit
Rateof flow of air through the fan-coil unit
Outlet temperature of water
Rise in temperature of air
0.49
0.218
400 kg/h
60°C
280 kg/h
54°C
21°C
The satisfactory performance of the heating
system on a typical day is shown in figure 9.
Teciwolog,v for Solur Energy Utiliznriotl
--98
850
8W
j
7w
Figure 9. Performance of solar space heating system at NPL,
1 February 1977
Space cooling
cycle
The space cooling cycle is more complicated and
involved than the space heating cycle. There are
essentially three approaches that can be taken to
achieve cooling:
Dehumidification and evaporative cooling
Vapour compression
Vapour absorption
In the dehumidification and evaporative-cooling
system, the room air is dehumidified by an absorbent;
adiabatic evaporative cooling follows, with the solar
desorption of the absorbent in a flat-plate collector
open to the ambient air. This system is efficient at
places where the humidity is very high, but rather
inefficient where humidity is low. This system has the
further drawback that it cannot be integrated with
the heating system.
A vapour compression system is ruled out
because it requires the use of a compressor.
In a solar coohng system. energy is available in
the form of heat at temperatures of up to 100°C.
This makes the vapour-absorption cycle the most
appropriate.
The main components of a vapour-absorption
cooling system are shown in figure 10. A refrigerantsolvent mixture is used in the vapour absorption
cycle. This refrigerant-solvent mixture is heated in the
generator by water, itself heated in solar collectors.
The heating of the mixture in the generator releases
refrigerant vapour, possibly mixed with small
amounts of solvent vapour. This vapour mixture.
together wit!1 a small amount of solution, enters the
separator where the solvent vapour is condensed and
separated out. The refrigerant vapour then enters the
condenser where it is condensed into liquid. The
condensed liquid passes through the throttle va%
and is evaporated in the evaporator. Here it takes
latent heat from the surrounding bath and delivers
the reTWed refrigeration. The vapour is carried from
the evaporator by the carrier gas (usually hydrogen)
to the absorber where the refrigerant vapour is again
absorbed by the solution. The resultant solution goes
from the absorber to the generator and the cycle
repeats itself.
Particular care needs to be taken with the choice
of the refrigerant-solvent combination and with the
following operations: the flow of the solution from
the absorber to the generator, the separation of the
refrigerant vapour from the weak solution in the
separator, the condensation of the refrigerant vapour
into liquid. and the transmission of vapour from the
Cl-&- rCYr”C
Pump
Figure 40. Solar space rzoling system
Solar space heating and cooling and solar water heaters
evaporator to the absorber. The product of the
separate efficiencies of these operations determines
the system efficiency.
The refrigerant-solvent combinations that are
commonly used are ammonia-water, water and
lithium bromide, or fluoralkane-refrigerant/tetraethylene-glycol. The most important requirement of
the refrigerant-solvent mixture is that the solvent
should be able to absorb a large proportion of the
refrigerant. For a given refrigeration to be delivered,
the larger the concentration of the refrigerant in the
solvent, the smaller the flow required from absorber
to generator. It is weil known that the higher the
absorption of the refrigerant in the solvent, the higher
the negative deviation from Raoult’s Law; this also
implies that there is increased binding between the
refrigerant and solvent molecules. This increased
binding would mean that the generator would need to
produce more heat to vaporize the refrigerant. The
most extensively used refrigerant-solvent combination
is ammonia-water, which offers several advantages
over the other combinations. The refrigeration
capacity of ammonia is fairly high. The sort of
temperatures obtainable using ammonia-water with
flat-plate collector.: are enough to operate the system.
T!-Pse and sevela! other advantages prompted the
selection of ammonia-water for the cooling system
constructed at P:PL.
99
fan-coil unit as was used in t!:e heating cyc!e was used
to provide cooling. Cold water was passed through
the coil and air blown over it. The rate of flow of
water through the coil was maintained at 4C0 kg/h,
the same level as in the heating cycle. Cold water
enters at 10°C and leaves at 18’C. The air-mass flow
rate is 280 kg/h, :vith an air temperature of 35°C
initially and 25°C after cooling. The change in
relative humidity consequent on cooling of the air is
accounted for.
In the vapour-absorption cycle, heat is absorbed
at the generator and evaporator, and evtilved at the
absnrber and condenser. The temperatures of these
subsystems lead to other design constraints. In
northern parts of India in summer months, cooling
water c available at 30°C even during extreme
con&, I.-.,,J. Therefore, absorber and condenser
temperatures of 30°C were chosen. Since the
ammonia-water mixture is heated in the generator by
hot water available from the solar collectors, the
maximum temperature of water heated by solar
means is another constraint. The maximum temperature was taken to be 90°C. Once the generator.
condenser and absorber temperatures are fixed, the
evaporator temperature is automatically fixed by the
system. In this system the evaporator temperature
will be 10°C.
Design parameters
lksign
CO): itraints
As mentioned earlier, the heat load of the room
was 12 600 MJ/h (3 000 kcal/h). The system was
designed to deliver this refrigeration. The same
The system provided is shown in figure 11. Water
heated by solar energy is stored in an insulated tank
at a temperature of 92°C. This is achieved by putting
a t,hermostatically controlled bypass valve in the
Technology for Solar Energy Utilization-
100
collector circuit. This hot water is then supplied to
the generator. Hot water enters at the top and leaves
at the lower exit at 85°C. The rate of flow is
maintained at 5 I/mm. The coil in the generat,or
contains a strong ammonia-water mixture. The rate of
flow of the ammonia-water mixture (51% ammonia)
is 2 kg/min. In the generator, 0.2 kg/min of ammonia
gas is liberated, and this gas, together with 1.8 kg of
the solution (41%, or 0.82 kg of ammonia), enters the
separator at 85°C. The weak ammonia solution drams
from the bottom of the separator to the heat
exchanger. Ammonia gas (0.2 kg/min) from the
separator enters at the middle level of the analyser,
which contains a strong solution of ammonia liquor.
In the analyser, any trace of water vapour in the
ammonia gas is filtered out. The dehydrated ammonia
vapour leaves the analyser at a temperature of 78’C
(and a rate of roughly 0.2 kg/mm) and enters the
condenser. The condenser is cooled by cold water at
3O”C, and the ammonia gas condenses roughhly :it
the process of condensarion
35°C. During
242 000 MJ/min (57 700 kcal/min) of heat is removed by the cold water in the condenser. The liquid
ammonia flows from the condenser to the receiver.
The liquid ammonia is then throttled through a valve
and the throttled liquid mixed with hydrogen in the
junction box. The hydrogen-ammonia mixture enters
the evaporator where the ammonia evaporates. The
ammonia-gas/hydrogen mixture is heavier and therefore flows down into the absorber at the central inlet.
The absorber is cooled by water at 3O’C. The
incoming ammonia mixes with the solution, and ‘the
concentrated solution (5 1% ammonia) collects at the
bottom of the absorber. This strong solution enters
the heat exchanger, where it exchanges heat with the
weaker solution that came from the separator at
81’C. As a result of this heat exchange, the
temperature of the strong solution rises to 75°C; the
strong solution enters the analyser, and an equal
amount of solution is transferred from the analyser to
the generator to repeat the cycle. The entire system is
kept at a pressure of 12.1 bar (175 psia). Hydrogen
acts as a carrier gas and helps to move ammonia
vapour from the evaporator to the absorber and also
to reduce the partial pressure of ammonia to 5.2 bar
(75 psia). It is because of this reduction in partial
pressure that the boiling point of ammonia in the
evaporator coil is about 5°C.
Five litres of water at 90°C needs to be passed
through the generator per minute. This determines
the collector area requirement. In the space cooling
system, the demand for refrigeration is in phase with
the insolation.
The daily energy gain over a seven-hour period
during the summer is as follows:
Month
April
May
June
Average
Energy guiv
(kWhfm’)
3.G
3.4
3.8
3.4
The average hourly gain in useful energy is thus
roughly 0.5 kWh/m’. On this basis one would need
7 m* of the collector array to:produce the required
cooling. Thus, the collectrir array needed is
approximately the same for both the space heating
and cooling cycles. The fan-coil unit is also a common
feature of both the cycles. Such a combined space
heating and cooling system& under trial. The heating
cycle has been tested successfully and it is hoped that
the cooling cycle can soon be tested successfully.
Preliminary
design data
for a solar lhouse in Riyadh
A. A. M. Sayigh and E. M. A. El-Warn
College of Engineering, Riyadh, Saudi Arabia
One of the major applications of solar energy is
in the home. The heating and coolTiig of houses and
the use of household appliances accounts for over
20% of all the energy consumed in Europe, Japan and
the United States’ and for about 50% of the total
energy consumption in the developing countries. The
need for solar houses is the greatest in the developing
countries, mainly to save energy for other more vital
purposes.
Since Saudi Arabia is the richest country in the
world with regard to solar energy availability,’
designing and building solar houses in this country
would not only save energy on the nation21 scale but
mean that houses could be built in remote areas of
the country where the conventional forms of energy
do not exist or are not economic. Therefore a study
was carried out to find the best means of building
solar houses in the Riyadh area. It dealt with building
materials, air-conditioning systems, electricity generation and costs. The findings are discussed below.
Riyadh is almost in the middle of the country at
latitude 24” 42’ N and longitude 46’ 43’ E. It is
about 600 m above sea level and has a population of
roughly three quarters of a million. It has little
rainfall per year (about SO mm), and the annual mean
relative humidity is 30%. Figure 1 shows the mean
maximum and mean minimum temperatures and the
total insolation at Riyadh3 and the months in which
heating or coohng is required. In summer electricity
’ C. A. Berg, “A tee- nical basis for energy conservation”, American Society I r Mechanical Engineers. Quarterly
Transuctions. vol. 98, No. 5 (May 1974). pp. 30-42.
’ A. A. M. Sayigh, “Saudi
Arabia
and its energy
resources”, Paper prepared for the International Solar Energy
Conference,
COMPLES,
University
of Petroleum
and
Minerals, Dhahran,
1-5 November
1975; “The energy
prospects in the Arab World”, Paper prepared for the
International
Conference in Mechanical Engineering, University of Engineering and Technology, Lahore, Pakistan,
6-11 October 1975.
“J. A. Sabbagh, A. A. M. Sayigh and E. M. A. El&lam,
“Estimation of the total solar radiation from meteorological
data”, Paper prepared for the ISES Conference, Los Angeles,
28 July-l August 1975.
100
500
-E
500 g
c
2
‘loo $
15-
300
IO-
200
I
5-
/
I’M
f
I
OI
J4”
Feb
-r
M.rch A,ml
i
MW
JU~C ,ulv
Aug
i
Scpr 0.x
NO”
net
Figure 1. Heating and cooling seasons at Riyadh
consumption increases owing to the cooling load
while in winter gas consumption increases owing to
the heating laod.
Building materials
Several local building materials were tested for
their strength and suitability for the solar house-clay
bricks with and without straw, clay bricks baked at
1 000°C and partially hollow cement blocks.
Tables 1-3 show the properties of such materials.
Technology for Solar Energy Utilization
-.
TABLE
Number
1
2
3
4
5
1. PROPERTIES OF BRIG XS MADE OF NASLRIA CLAY (SAMPLE A)
Type of
samplea
Weight
(kg)
Dimensions
(cm)
crosssectionnl
area
(cm’)
Cube without straw
Cube with straw
Cylinder with straw
Cylinder with straw
Cylinder without straw
5.119
4.503
6.612
6.519
7.895
14.5
14.8
15.0
15.0
14.5
201.35 (
207.2
116.63
176.63
165.05
x 14.3
X 14.0
diam.
diam.
diam.
‘The mixing proportions,
taken from an old formula used in building Riyadh
clay. 73.4; water, 25.5; straw, 1.1; (b) without straw: clay, 77.5; water, 22.5.
Num her
1
2
3
4
5
TABLE
Sample
C
D
Type of
sample
Weight
(kg)
Dimensions
@ml
Cube without
4.465
4.26 I
6.976
6.315
6.155
13.8
14.2
13.9
14.5
14.5
187.68
198.8
151.67
165.05
165.05
straw
Cube with straw
Cylinder without straw
Cylinder with straw
Cylinder with straw
X 13.6
x 14.00
diam.
diam.
diam.
3. PROPERTIES OF SAMPLE C (CLAY BAKED
Water
content
(wt7ol
3 700
920
570
1 480
700
17.84
4.44
3.22
8.31
4.24
2.85
2.61
3.34
3.4
5.41
(a) with straw:
CLAY (SAMPLE B)
Ultimate
crushing
load
(kg)
4 920
1 820
2 570
770
630
Compressive
sfrength
lkglcm2)
27.03
9.154
16.94
4.66
3.82
(wt
Water
con tent
2.82
2.71
2.81
2.9
2.9
AT 1 000°C) AND SAMPLE D (HOLLOW CEMENT BLOCK)
Cr0.w
sectional
area
(cm’)
Dw of
sample
Weight
(kg)
Dimensions
(cm)
Cube
Biock
0.372
23.25
6 X 5.8 X’5.9
40 x 20 x 20
Several building materials surfaces were exposed
to solar radiation all day, and their temperatures were
recorded as shown in figure 2. The emissivity and
absorptivity
for such materials were obtained
experimentally as shown in table 4.
Compressive
strength
(kglcm =I
houses in the past, were (wts):
TABLE 2. PROPERTIES OF BRICKS MADE OF OLD MANFOHA
Crosssectional
area
(cm ‘l
Ultimate
crushing
load
(kgl
Ultimate
crushing
load
(kg)
34.8
800
Compressive
strength
(kglcm’)
4 200
18 160
120
22.7
10 12noon2
TIME
OFDIY
d
Water
contenf
Iwt%)
40
30
TABLE 4.
AND
EMISSIVITY
VARIOUS BUILDING
Building material
Fire-bricks at 1 000°C
Clay bricks
Cement blocks
Roof tiles
ABSORPTIVITY
MATERIALS
Emissivity
0.75
0.67
0.90
0.9
OF
Absorptivity
0.75
0.65
0.90
0.9
g
(E
23 20.
fl
$
e
!O
To test for the effects of rain and erosion on
building materials, water was dripped continuously
for a day on all samples. Also, the samples were
blasted for a day with dry sand from a sand-blasting
machine at moderate speed (30 km/h). The effects
are shown in table 5.
o-
Dam
a
Figure 2. Surface
temperatures for
materials
6
various
R tOOIll
building
Preliminary
design data for a solar house in Riyadh
TABLES.
EFFECTS
OF
ON BUILDING
Sample
RAIN
AND
MATERIlLS
103
project .’ Numerous papers describing other project:
can be found in the literature.6
Night cooling PIas studied by measuring the
temperature of different surfaces at night as shown in
figures 3 and 4. Jones’ suggests that for comfort the
EROSION
Amount of rain absorbed
Erosion
A
With straw
Without straw
Fair
Sizeable
(disintegrated)
Little
Fair
SiTeable
Little
Y.ittle
50-
Little
B
With straw
Without straw
c
None
None
D
Very little
None
‘loG
L
k!
2
2
L
Air-conditioning systems
E
Of all the uses to which solar energy might be
put in tropical countries that of heating and cooling is
the most feasible. During the summer, solar radiation
is intense and it can be utilized for cooling; in winter,
a system of flat-plate collectors can provide more
heat than is actually needed.
30
/
I
I
20
L
19
21
23
1
3
5
I
9
TIME OF DAY Ihl
Cooling
Figure 3. Temperatures of various surfaces during the night
of 18 June 1975
Several kinds of cooling system have been tried.
A 3-ton lithium bromide absorption cooling unit was
used by Ward and LX’ in their residential system.
The unit was modified to utilize hot water instead of
natural gas as the source of heat supplied to the
generator. The ammonia-water absorption system was
used for cooling in a California (United states)
4D. S. Ward and G. 0. G. L&f, “Design and construction
of a residential solar heating and cooling system”, Solar
Energy, vol. 17, No. 1 (1975),pp. 13-20.
’ University of California, Lawrence Berkely Laboratory, Control System for Combined Solar Heating and
Cooling Systems, Progress Report No. 1, January 1975.
W. A. Beckman
and
’ For example,
R. L. Oonk,
J. A. Duffie, “Modeling of the CSU heating/cooling i>‘stem”,
Solar Energy, vol. 17, No. 1 (1975), pp. 21-28; G.O. & Laf
and R. A. Tybout,
“The design and cost of opfimlLq<
systems of residential heating and cooling by solar energy ‘,
Solar Energy, vol. 16, No. 1 (1974), pp. 9-18.
‘W. P. Jones, Air Conditioning Engineering, 2nd ed.,
Arnold, 1973.
1 Black surface with 3mm plaa cows
1
’
201
15
17
19
21
23
1
/
3
I
5
I
1
TIME OF DAY (hl
Figure 4. Temperatures of various surfaces during the night of 2 September 1975
I
9
104
Technology for Solar Energy Utilkaiion
\
Cold-ai; vents
one in each room
66.4 m 3 of rock
30 m’ of watet
Figure 5. Night cooling system
indoor temperatures should be lower than the
outdoor temperature by 4”-1 l”C, with a relative
humidity of about 50%. The best coinbination for
night cooling is that of using surfaces with a thin
layer (2-5 cm) of water as shown in figure 3. In this
method an average temperature of 10°C below
ambient temperature was recorded for seven hours
during the night. Bahadori’ reported that by
circulating the outside air in Shiraz for two hours at
night (2 a.m.-4 a.m.) at an average velocity of
9.1 m/min through a bed of rock having a volume of
39.4 m3 the rock could attain the average minimum
air temperature, which was about 17.8’C. Using the
minimum temperature in the collector with a layer of
water in Riyadh, which is 20°C during the summer,
and if the rate of flow is 10 m/min, a l-o& pile of
160 m3 can be cooled to this temperature during the
night. If the heat is stored partly in water and partly
* M. N. Bahadori, “A feasibility study of solar heating in
Iran”. Solar Energy. vol. 15, No. 1 (May 1973).
in rock, and if the water tank has a capacity of
30 m3, then 66.4 m3 of rock will be required as extra
storage.
The proposed house has a 100-m’ ground area
and a height of 3 m. It consists of two rooms besides
a bathroom, a kitchen, an attic and a basement 3 m
deep under the entire house.
If the outside and inside design conditions, the
kind of structure, the doors and windows, the lighting
and other equipment inside the house, the number of
occupants and the ventilation requirements are taken
into consideration, the cooling load needed for such a
house will be 3 refrigeration tons (10.56 kW). In the
Riyadh area this represents an extraction rate of
about 40 W/m3 at peak load. After 10 hours of
operation, the total energy dissipated is 3.3 X 10s J
for an average rate of 9 kW. From these data, it can
be calculated that the temperature in the house will
be 21.56’C. i.e., a rise of l.S6’C, which is tolerable.
Figure 5 illustrates the night cooling system.
Preliminary
design data for a solar house in Riyadh
105
with a melting point of So-10°C and a heat capacity
28 times that of an equal volume of water, can be
used as storage in a cooling system;
(bj The latent heat of melting, such as that of an
inexpensive solution of salt in water. A major
disadvantage is the occurrence of stratification effects
in melting and freezing;
(c) The heat of a solid phase transition, such as
in vanadium oxide or ferric sulphide. In the latter, the
transition takes place at 138°C; storage capacity is
230 kJ/l;
(dj The heat of a reversible reaction, such as in
the two-chamber system with sulphuric acid at a
weak concentration in one chamber and strong in the
other. Use is made of reversible reactions between the
acid and water to store and regain heat.
Figure 6 illustrates the solar heating system,
which uses water and rock for sensible heat storage
and a solar battery to generate electricity.
Electricity generatim
Fiie
6. Solar heating system
Hearing
A good storage material (a) has a high heat
capacity; (b) is highly kinetic, i.e., heat can be
rapidly extracted from it: (cj does not freeze; and (d)
does not corrode or rust the containers in which it is
stored.
Heat can be stored in the form of:
(a) Sensible heat in a liquid or solid. Examples
are: water, providing that the temperature remains
between 0” and 1OO’C; rock, which is inexpensive
but has only one fourth the specific heat of water and
hence requires a mass of rock four times the mass of
water for the same storage capacity; C1 5 wax, which
\
The electrical power load for the house, taking
into consideration all needs of electrical appliances,
lighting, water pumps and blowers was estimated at
1 kW. The solar power supply equipment (solar
battery, storage battery, control box and inverter)
was designed to meet this need. The following design
factors were used:
Average number of hours of sunshine per day
Sunshine ratio = (fraction of time sun shines) X
(incident anglefactor)
0.2
0.834
0.65
0.9
Storage-battery efficiency
Inverter efficiency
General loss factor
Four 24-V silicon solar-battery arrays, each with
a power output of 89 W, were used in parallel. The
area of each array was about 3 m2; they were
Solar radiation
\\
1 i
I I
I
I
1 i
I
Y
1
t
I
Load
lnverter
i
I
I
-
i
J.
i
Solar battery arrays
1
IL------------
Control box
2I
I
L-----w----
10
I
________________
-_--------
-I
Figure 7. Diagram of tie solar-battery power supply; CD is a diode for blocking reverse current, and
M,, M, and M, are volt-ammeters for measuring charging and discharging currents, terminal voltages etc.
Technology for Solar Energy Utilization
106
mounted on the top of the house facing south, with a
tilt of 35” from the horizontal. The storage battery
used was a 24-V sealed lead-acid storage battery with
a capacity of 12 kWh. The inverter produced
single-phase, 50-Hz alternating current at 110-l 20 V
and was rated at 500 VA.
A diagram of the solar-battery power supply is
shown in figure 7. When the sun shines, the output
current passesthrough the diode to charge the storage
battery and drive the inverter. The blocking diode
prevents the storage battery current from being
returned and consumed by the solar batteries when
they are not receiving solar energy at night or in bad
weather.
Cost analysis
A solar house still costs more than a conventional
house, mainly because of the high cost of the
electrical energy required in a solar house. Costs for a
solar house are given below (thousand Saudi riyals): 9
Building
Heating
Storage
Electric
materials and construction
and cooling system (night cooling)
and light fittings
power generation
Total cost, excluding
the land
50
20
10
140
220
A conventional
house would
cost about
SRls 100,000. The cost of solar and conventional
heating and cooling equipment is almost the same.
This finding is corroborated by other investigators.’ O
9$1 = SRls 3.5.
C. Swaminathan,
’ OR. K. Swartman,
V. Ha
and
and ammonia-sodium
“Comparison
of ammonia-water
thiocynate
in a solar refrigeration
system”, Cooperation
mediterraneenne
pour I’energie Solaire (COMPLES), first
semester 1974.
Conclusions
A study of all the parameters for designing a
solar house in Saudi Arabia indicates that construction of such a house is not only feasible but practical.
Of the various building materials shown in tables l-5,
the best combination is sample C. The wall should be
40 cm thick, with bricks 10 cm thick on the outside
and inside and a 20-cm foam-filled cavity between
them. The cooling system may be either the
night-cooling system of figure 5 or a solar-operated
lithium-bromide/water
absorption air-conditioning
system.’ ’ To use night cooling or evaporative
cooling, the amount of water evaporated per day
must be estimated because of the water shortage in
Riyadh. The average pan evaporation during June,
one of the hottest months in Riyadh, is 300 mm,”
an average rate of 1 cm per day. The area of the pan
is SOm”. Therefore, the amount of water lost
through evaporation per day will be 0.5 m3. As for
heating, several flat-plate collectors covering an area
of 80 m2 with selective surfaces are suggested. The
amount of solar energy that can be used for heating
during January was calculated to be 59 X 1Gs J per
day. This can raise the house temperature by 5°C
during the day. As for the solx battery, it can be
utilized for heating, which will result in an additional
energy gain of about 8 X lo8 J per day. As for the cost
ana!ysis, the major cost is due to the solar battery.
Apart from this, the ctists of a solar house and a
conventional house are about equal. Using solar
heating and cooling will result in a saving of about
SRls 14 per day.
’ ‘J. A. Duffie
and W. A. Beckman,
Solar Energy
ThermalProcesses 1st ed. (New York,Wiley Interscience 1974).
’ ’ Raikes and Partners, “Hydrological
service for the
period 1 August 1968 to 31 August 1970”, Report prepared
for the Ministry of Agriculture and Water, Saudi Arabia.
Solar refrigeration
in developing countries
A. Eggers-Lura
International
Solar Power Co. Ltd., Gentofte,
Denmark
Solar refrigeration technologies that may be
applied economically in developing countries must
meet the following requirements:
(a) The refrigeration equipment must be
simple and cheap, so that it can be manufactured in
developing countries using materials and working
skills available in those countries;
(b) The equipment must be socially acceptable
to the population and conform with their living and
working habits;
(c) No auxiliary power or equipment should be
required.
Most of the basic research on solar refrigeration
took place in the 1950s and was reported on at the
United Nations Conference on New Sources of
Energy in Rome in 1961. Then research virtually
came to a standstill, and only since the “energy
crisis” has it been actively resumed. However, from
the work that has already been undertaken, it is fairly
easy to conclude which of the technologies available
at present would be suitable or economical for
application in the developing countries.
Only two refrigeration technologies can be
chosen at present for application in the developing
countries: evaporative cooling and the intermittent
absorption refrigeration process. All other refrigeration processes are either too complicated or too
costly to be considered. The continuous absorption
refrigeration process requires too high working
temperatuLes and therefore the use of concentrating
collectors, which are too expensive to build and too
complicated to operate.
Evaporative cooling
The evaporative food cooler, in principle,
consists of a container surrounded by a suitably
shaped piea of cloth, the lower part of which is
submerged in a tray containing water. The water is
absorbed by the cloth, and through capillar; action it
acts lie a wick, the water moistening rncjst or all of
the cloth. If the climate is dry and the cooler is kept
in a breezy spot in the shade. the food will be cooled,
as the water evaporates, to a temperature that lies
considerably below ambient.
Three simple types of evaporative food coolers
that could be constructed cheaply-perhaps even
mass-produced-from materials available in almost
any developing country are described below. To
introduce such food coolers on a large scale in rural
villages in developing countries would, more than
anything else, entail education of the population on
how to build and utilize them.
Basket coolers
A basket with a loose fitting cover is woven
from bamboo or other slender elastic wooden
branches. The size depends on the needs of the
family. A square or round pan, of earthenware or
metal, is made in which the basket can be placed. The
bottom part of a clean oil drum may be used. The
pan should be 25-30 cm high and wider than the
basket. The pan is placed in a cool, shady place in the
kitchen, away from the stove, and with a regular
breeze. A number of bricks or flat stones are placed
in the pan so that the basket can balance evenly on
them.
Burlap, of the soft type, or other suitable fabric,
is sewn around the rim of the basket. It must hang
loosely around the bottom and extend into the pan.
Likewise, burlap is sewn loosely over the cover of the
basket. Then, the’ basket is placed on the bricks, food
put into it, and the cover placed on top of it. The
bottom of the pan is filled with water, and the burlap
cover of the basket is wetted with water the first time
the baket is used, and occasionally thereafter.
The basket itself should not be in the water, but
the burlap should hang down in the water. The burlap
acts like a wick, and through capillary action the
water wets the burlap. As the water evaporates, the
food is cooled. An added advantage of this cooler
construction is that it keeps flying insects away from
the food, and cockroaches and other crawling insects
cannot enter the food through the water.
107
.
1
108
Technology
Cupboard coolers
Utilizing the same cooling technology as the
basket cooler, a large cupboard type of evaporative
food cooler may be constructed. A wooden frame is
nailed or screwed together and placed in a pan of
water. A pan of water may also be placed on top of
the frame. A hinged, framed door may be arranged to
allow easy accessto the food, which is placed inside
on wire-mesh shelves. The frame is covered with
burlap or other suitable fabric, and part of the burlap
is immersed in the top and bottom pans, which are
filled with water.
Jar coolers
Evaporative coolers may be manufactured
entirely from earthenware, in the form of jars or jugs,
a system that has been used for hundreds of years in
Asia for cooling drinking and bathing water. Such
coo!ers are ubiquitou in the villages of Burma, China,
Democratic Kampuchea, Indonesia, Malaysia, Philippines, Thailand and Viet Nam. Until the Second
World War, small earthenware evaporative coolers
were used extensively in Europe to cool butter and
cheese.
Intermittent absorption refrigeration
While evaporative cooling is suitable only for
small domestic coolers, the intermittent absorption
refrigeration process may be applied both in
individual domestic households and on a community
basis. This process has the advantage that the
generating and absorption cycles follow the solar
cycle.
Intermittent absorption refrigeration equipment
is somewhat more complicated and expensive than
evaporative coolers. Therefore it is necessary to
determine whether the refrigeration equipment in the
rural villages should be used on an individual or on a
communal basis.
For a start, it may be preferable to concentrate
on plant and equipment that can be used
communalIy, because it would be easier to educate a
few technically minded persons to operate a
collectively owned plant than a large number of
individual household members. If the plant produced
block ice, and if each household had an insulated ice
box for food storage,some member of the household
could fetch a portion
of ice every day from the
community plant and place it in the box. It would
probably be possible to find local entrepreneurs, for
example storekeepers, who could organize the
production, sale and distribution of the ice, possibly
with some sort of government support.
It is, however, also possible to construct small,
simple, and cheap intermittent absorption refrigera-
for Solar Energy Utilization
tors for individual households, but for most areas
they would likely prove to be too costly at present.
Both small and large intermittent absorption
refrigerators can be powered by a flat-plate collector,
perhaps with slight concentration added in the form
of flat metal mirrors.
Individual refrigerators
For the individual household absorption refrigerator, the type to be considered would be the
“icy-ball”, a 1920 invention that uses water as an
absorbent and ammonia as a refrigerant and may be
heated by kerosene, paraffin, gas, or even firewood.
Icy-balls were used extensively in Canada and the
United States and in Europe between the First and
Second World Wars. With the advent of the electric
compressor refrigerator use graduall/ dwindled. Very
few of these refrigerators are still in existence today.
There are six in the United States but only two of
them are still in working order. A United States firm
is resuming production of an icy-ball type of
refrigerator.
The icy-ball was cheap and simple. It consisted
of a generator-absorber connected with a pipe to a
condenser-evaporator. In the generator-absorber there
was a mixture of water and ammonia. It was heated
by some means, and at the same time the
condenser-evaporator was immersed in cold water.
When all the ammonia had been boiled out of the
water, which took 1-2 hours, the cycle was reversed.
The condenser-evaporator was placed in an insulated
refrigerator box, the generator-absorber was cooled
by the air, and during the next 24-36 hours the
ammonia would flow bpck from the condenserevaporator into the generator-absorber, at the same time
producing cold in the refrigerator. There was only
one drawback. During the boiling out of the
ammonia, some water vapours were carried over
simultaneously to the condenser-evaporator, reducing
the efficiency of the process.
Chung and Duffie suggest that by redesigning
the icy-ball slightly it may be driven by solar energy
and preliminary experiments have shown that it is
feasible to do so.
In 1957, the Wisconsin group (Duffie, Ltif,
Williams and Chung) reported on experiments they
had undertaken with an icy-ball type of domestic
refrigerator. The generator was heated for about 2
hours in the focus of a small parabolic reflector,
whereafter the unit was transferred to a “refrigeration
box”, which remained cool for about 24 hours.
Apparently, no further work has been done on this
project.
It should be fairly easy, by obtaining one of the
icy-ball. units that remain and studying the available
literature, to begin manufacture of a simple and
cheap icy-ball type of domestic refrigerator, powered
by the sun, which can be used for individual
Solar refrigeration in developing countries
Figure 1. General view of solar-powered ice-making plant
households in the developing countries. There would
be an immense market for them. The generator
should probably be constructed in such a way that it
can also 02 heated by a fossil fuel when there is no
sunshine. In the case of SOIX generation, a flat-plate
collector with slight mirror concentration or,
alternatively, a cylindrical concentrating collector
with a pipe-shaped generator placed in the focal line
should be used.
Communal refrigeration
Glamg
/’
plants
A communal sola*: refrigeration plant could be
either a cold-storage building in which the inhabitants
of a village store their food on shelves, each family
being allocated a small volume of storage space, or a
plant producing block ice for ice boxes in individual
households. A suitable plant for a village should be
able to produce, on the average, 500 kg of ice per
day. A plant of that size has been designed by the
author. It uses calcium chloride/ammonia as the
absorber/refrigerant combination, but in ;Irinciple
many other combinations could be used.
A prototype in reduced scale is at present being
made in Denmark and will be sent to the Sudan later
for testing under tropical climatic conditions. To
allow for periods of cloudiness, the system has been
designed for a production of 720 kg ice on a clear
day. The length of the generating period was set at 5
hours around noon, that of the absorption period,
which is also the freezing period, at 12 hours during
the night.
Figure 1 gives a view of the proposed plant. A
stagnant-water condenser and ice generator are
underneath the solar collector. Figure 2 shows an
exploded view of the combined solar collector,
absorber and generator, which is constructed from
steel tubes and separated by steel-plate fins. Except
for the glazing, the collector is painted witn a black
selective coating.
The insulation on the back of the collector is
mounted so that it can be removed during the
absorption period. This is necessary because the
Barge board
Concr;te
base
Mmaral wool insulation
huhlion
bon
Figure 2. Exploded view of one section of the combined
s&r collector, generator and absorber
-l
L
II
,_4
Jl
II
II
II
Figure 3. Stagnant-water condenser
I,
110
Technology for Solar Energy Utilization
selective collector surface and glazing do not allow
for adequate cooling by radiation during the night.
!f ihere is no means for providing water
circulation (apart from natural convection), a
stagnant-water condenser (figure 3) is the best
solution. Simulation studies show that an average
condensing temperature of 40°C can be achieved even
with a small basin.
The principal dimensions of the ice-making plant
are as follows:
Solarcollector
Total projected area: 156 ma (10 sections, 3 m X 5.19 m
each)
Tube size: 33.7 mm OD. 28.5 mm ID
Number of tubes per section: 60
Condenser
Water basin: 6 m X 2.8 m X 0.8 m
Total heat-transfer area: 18 m’
Tube size: 21.4 mm OD, 16.1 mm ID
Total tube length: 270 m
Ice generator
:
Brine basin: 1.7 m X 1.5 m X 0.85 m
Total heat-transfer area: 16.5 rn’
Number of ice cans: 60
Dimensions of ice block: 120 mm X 180 mm X 600 mm
Weight of ice block: 12 kg
The ice generator (figure 4) operates without
forced circulation of the brine, which means that the
evaporator area must be fairly large. The best solution
appears to be a plate-type evaporator with the plates
between the rows of ice cans. The rather small
block-size chosen means that freezing can be
completed in 12 hours with an evaporation
2c mperature of approximately - 1 1“C.
The typical operating conditions for the plant
are as follows:
Dire3
Diffuse
Ambient temperature during generation
Efficiency of solar collector
Condensing temperature
Evaporation temperature
Final absorption temperature (liquid absorbent)
Duration of generating period
Duration of absorption period
Estimated specific mass of generator vessel
(mass of vessel divided by mass of
refrigerant generated)
890 W/m’
150 W/m’
31°C
0.55-0.45
40°C
-10°C
40°C
5h
12h
10
Based on the preliminary design, the following
cost estimate for the plant was made (cost in Denmark
in dollars):
Solar collector, generator and absorber
Condenser
Ice generator
Piping and installation
Total
21000
500
2 500
2 000
26 000
In a developing country the plant could probably be
built for approximately S16,000.
Figure 4. Ice generator
Based on an annual production of approximately 180 000 kg of ice, an interest rate of 6% per
annum, and a working life of the plant of 12 years,
ice could be produced at a price of 0.02 $/kg. This
price does not appear unreasonable, judging from
figures given in the literature.
As the working susbtances for the absorption
system of the plant, several absorbent/refrigerant
combinations could be considered, both liquid and
solid, but those most likely to prove successful are
the following:
Lithium bromide/water (LiBr/Hz 0)
Water/ammonia (Hz O/NH,)
Sodium thiocyanate/ammonia
(NaSCN/
NH31
Lithium nitrate/ammonia ( LiN03 /NH,)
Calcium chloride/ammonia (CaC12/NH,)
Strontium chloride/ammonia (SrC& /IQ-Is)
In all but one of the above cases ammonia is the
refrigerant, and the first four absorbents are liquid,
while the last two are solid. There are possibilities of
using other refrigerants, alcohol or methylamine, for
example, Niebergall has provided extensive data for
most of the absorbent/refrigerant
combinations
that might be considered.
,,> I /
‘,T);
.~
,;.,
!&>,,
_‘.I,
‘;
,’
I
Solar
rcfrigration
-
in developing countries
I11
Research results
The following remarks are based on the research
of the author and others in Denmark.
The use of liquid absorbents is advantageous,
since the concentration of refrigerant in the
absorbent can be adjusted at will to suit the lower
temperatures met with in solar refrigeration. On the
other hand, with solar liquid systems it is difficult to
obtain adequate circulation, especially during the
regeneration cycle, without utilizing pumps; also, the
risk exists that part of the liquid absorbent will
vaporize together with the refrigerant during the
generation cycle. This can be remedied either by a
better choice of absorbent/refrigerant combination or
by installing a fractionation column, which, however,
is costly. (Fractionation is imperative when using the
water/ammonia combination.)
The solid absorbent systems have the advantage
that the absorbents cannot evaporate at all; however,
owing to their density, some difficulties
are
encountered during the regeneration cycle, and also it
has proved difficult to keep the layers of solid
absorbent in the generator-absorber porous and to
prevent them from clogging the piping.
The generation temperatures required in connection with solid absorption refrigeration are at the
upper limit of what flat-plate collectors can yield
namely, 100“-12O”C, depending on the absorbent/
refrigerant combination used. However, the technical
problems encountered in this respect could probably
be solved by means of available technology, for
example, selective surface coatings and slight
concentration by means of metal mirrors. The real
problems are economic rather than technical.
For the condensation process, temperatures of
about 4O*C are required. Such a low temperature
may be difficult to achieve in certain tropical regions.
Using a stagnant-water condenser, where the temperature of the condensing water is kept down by
evaporative coc&ng, has been suggested.
Experiments have shown that with a mixture of
85% calcium chloride and 15% Portland cement an
absorbent granulate can be made having reasonable
mechanical strength and sufficient porosity so that
when filling the absorber-generator an even distribution can be obtained without difficulty. After 200
generation-absorption cycles with this granulate no
decrease in the absorption capacity was noticed. The
addition of cement does not reduce the yield in
similar
comparison with pure calcium chloride.
strontium chloride granulate was produced, but it
showed no advantages over calcium chloride.
Simulation experiments have been conducted in
Copenhagen on a plant to be placed in Khartoum, the
Sudan. The greatest efficiency was achieved when the
degree of generation was kept at about 70% of the
maximum that could be obtained on clear days.
Under these conditions maximum collector temperature was 1IO”-1 15°C. The simulation experiments
A
indicate an optimum generator pipe diameter of
about 40 mm and a distance between the pipes of
about 100 mm.
The heat radiated away to the surroundings during
the night is insufficient to secure full reabsorption of
the ammonia. Extra cooling, for example, by means
of arranging for a slight distance between the
collector and the rear insulation is likely to prove
insufficient, and will increase the heat loss during the
generation cycle. The best solution appears to be a
hinged rear insulation, which can be swung open at
nightfall.
A pilot plant for the production of refrigerated
drinking water is being constructed at the Technical
University at Copenhagen. It has a collector area of
rn’, and the calculated yield IS about 7 500 kJ
every 24 hours. The plant may later be tested at the
Institute of Solar Energy and Related Environmental
Research, University of Khartoum. Both the experimental work and the revised calculations in
connection with the design of this demonstration
plant confirm that an intermittent calcium-chloride/
ammonia solar absorption refrigeration plant can be
used to produce ice. Daily production per unit of
collector area is likely to be 4 kg/m2.
The design of an intermittent
absorption
refrigeration plant can be very simple; no advanced
technology is required for building-one.
Calcium chloride/ammonia appears to be the best
all-round absorbent/refrigerant combination.
4
Refrigeration chains
In many developing countries “refrigeration
chains” are needed for the cold storage of perishable
products while they are being transported and
distributed. In Mexico, for example, such chains are
needed for fruit because the present facilities are
barely sufficient to cover the cold-storage requirement of 10% of production.
Since 1971, the Material Research Center of the
University of Mexico has been working to establish a
refrigeration chain for the distribution of perishable
foods. From the studies undertaken so far it has
concluded that an effort should be made to develop
in rural areas in Mexico:
(a) Large,
5 000-10 000 t)
stored before
exported;
(b) Small,
20-200 m3) for
regional cold stores (capacity
where products can be processed and
distribution
to urban centres or
multipurpose cold stores (capacity
rural communities.
The National Council for Science and Technology has approved the development of small cold
stores.
Solar refrigeration plants with solid ,osorbents
may prove useful for refrigeration chains in Mexico
and other developing countries.
.:
‘_,
‘I/
,’
0 , ,, 3 I
’
Technology for Solar Enerw
-. Utili-ali, ,)I
BIBLIOGRAPHY
A case for a solar ice maker. Solar energy (Elmsford.
N.Y.) 7: 1: 1-2, 1963.
Chinappa, J. C. V. Experimental study of the intermittent vapour absorption refrigeration cycle
employing the refrigerant-absorbent systems of
water ammonia lithium nitrate. Solar energy
(Elmsford, N.Y.) 5: 1: 1-18, 1961.
Performance of an intermittent refrigerator
operated by a flat-plate collector. Solar energy
(Elmsford, N.Y.) 6:4: 143-150, 1962.
Chung, R. and J. A. Duffie. Cooling with solar
energy. Proceedings of the United Nations
Conference on New Sources of Energy, Rome,
21-31 August 1961. (Solar energy: III, v. 6, p. 20
(S/WI
Sales no.: 63.1.40.
Factors affecting the use of solar energy for cooling.
By K. H. Khalil and others. Proceedings of the
International Conference on Heliotechnique and
Development, Dhahran, 1975. (V. 2, p. 125-132)
Niebergall, W. Arbeitsstoffpaare fiir Absorptionskalteanlagen und Absorptionkiihlschrinke.
Miihlhausen, Federal Republic of Germany, Verlag fiir
Fachliterahrr, 1954.
In German.
Possibilities for solar ice-makers. By F. Ba Hli and
others. Proceedings of the International Solar
Energy Society Conference, Melbourne, 1970.
(6/59)
Potential use of solar-powered refrigeration by an
intermittent
solid absorption system. By
A. Eggers-Lura and others. Proceedings of the
International Conference on Heliotechnique and
Development, Dhahran, 1975. (V.2, p. 83-104)
Stubkier, H. B. und B. Bechtoft Nielson. Solar refrigeration plants. M. SC. thesis. Technical University
of Denmark, Refrigerarion Laboratory, Copenhagen, 1974.
In Danish.
solar-powered
refrigeration.
f3y
survey
of
R. K. Swartman and others. American Society of
Mechanical
Engineers.
Quwterly
transactions.
(New York) 73-WA/Sol-6, ! 973.
Swartman, R. K. Survey of absorption refrigeration
systems. Proceedings of the International Conference on Heliotechnique and Development,
Dhahran, 1975. (V.2, p. 71-82)
Trombe, F. and M. Foex. Economic balance sheet of
ice manufacture with an absorption machine,
utilizing the sun as the heat source. Proceedings
of the United Nations Conference on New
Sources of Energy, Rome, 2 l-3 1 August 196 1.
(Solar energy: III, v. 6, p. 56 (S/1091)
Sales no.: 63.1.40.
Solar distillation:
The st;;lte ;I$!?
B. W. Tleimat
Sea Water Conversion Laboratory, Richmond, California, United States of America
Background
The modem use of solar energy for the
distiilation of saline waters began in 1872 with the
installation in northern Chile of a large basin-type
solar still which served the needs of a mining
community for many years. The original still’
consisted of parallel bays 1.14 m wide by 61 m long,
with a total area of 4 700 m*. The still basin was
constructed of wood blackened with logwood dye
and alum to absorb the sunlight. Other means of
obtaining potable water having come into use, the
only remains of this still in 1965 were reported to be
some of the ;::Imdation and numerous pieces of glass.
Figure 1 is a simplified diagram of a solar still. It
consists of a basin containing saline water with a
black bottom to absorb the sunlight, covered with
transparent glass sheets that form an airtight
enclosure. The transparent covers slope toward a
collection trough. In operation, solar energy passes
through the transparent cover and is absorbed by the
water and basin liner. The absorbed energy warms the
saline water in the basin, causing the evaporation of
some of the saline water and increasing the humidity
close to the water surface, thus producing convection
currents within the still enclosure. The warmer humid
air rises to the cooler glass, where part of the water
’ Josiah Harding, “Apparatus for solar distillation”,
L’roceedings of Institution of Civil Engineers, No. 73 (1883),
pp. 28428R
vapour condenses on the surface, slides dowr! drld
drips into the collection trough, and is withdrawn
from the ends as fresh water. In order to prevent
precipitation of salts in the basin, salt water could be
added either continuously or on a batch basis. The rate
of adding feed water should be at least twice the rate
of production of fresh water.
Interest in solar distillation began again during
the Second World War with the development of
plastic stills for use in emergency life rafts. These
stills consisted of an inflatable transparent plastic
container with a felt pad on the bottom and a
distillate collector bottle attached to the plastic
container. During use, the container was inflated, the
felt pad was saturated with sea water, and the
assembly floated alongside the raft. In operation,
solar energy, passing through the transparent plastic
and being absorbed by the felt pad, produces water
vapour which then condenses on the inside of the
piastic and is collected in the borrle at the bottom of
the assembly.
During the decades following the Second World
War, sustained drought zgnditions in many parts of
the world brought water supply problems into sharp
focus. The use of desalination to produce potable
water seemed to give a promise of relief from the
drought. All over the world, programmes for the
development of methods for desalting saline water
were begun. Solar distillation was one of the many
methods investigated. However, research on desalting
\
Insulation
Black basin liner
Brine
Figure 1. Solar still: simplitkd cross-sectional diagram
113
114
Technology for Solar Energy Utilization
-
methods using electrical and fossil-fuel energy seemed
likely to reach commercial application sooner than
solar distillation, which received a relatively small
part of available financial support.
The Sea Water Conversion Laboratory at the
University of California started its investigation of
desalination methods in 1951. During the initial
phases of its investigation, it concluded that processes
using non-fuel sources of energy should be given close
consideration. This conclusion was based on the
calclllation of the energy required to desalt sea water
by the then avai!able desalination methods. For
example, if sea water were to be distilled to supply all
the needs of a city the size of Los Angeles, the energy
required by the then-in-use multipleeffect
type
distiller M’ .i’\: hc lncrp than all the oil fuel produced
in Southern i‘a’;?:;:nid. Based on this conclusion, the
University built a s.>lar distillation station at the
En@:,cering Field Station in Richmond, California,
and continued these investigations for the next
10 years at a vigorc>t,. “’ L’~rious configurations for
2nd tc~r~~,l 10 reduce
simple solar stills wc~t’
construction costs and improve efficiency. The
United States Government, through its Office of
Saline Water, also instituted a solar distillation
programme at about the same time and financed the
building and testing of varieties of solar stills at its
testing station at Daytona Beach, Florida.
The investigators at the University of California
and other research centres eventually concluded that
the basin-type solar still in any of the forms
considered could not compete economically with
iarge piants -using other desalting processesto produce
bulk:
large quantities of water. Since about 1961, the work
on solar distillation at the University of California has
been shifted toward the development and improvement of small plants to supply drinking water to
small communities. A summary of the work at the
University of California has been summarized by
Howe and Tleimat.*
Although work on the development and
construction of large-scale stills was discontinued in
most parts of the world, medium-scale solar-still
development has continued in Australia under the
auspices of the Commonwealth Scientific and
Industrial Research Organization (CSIRO). The most
up-to-date review of work on solar distillation has
been summarized for the Office of Saline Water by
Talbert et al. 3
The cost of water produced from desalination
plants is the sum of the cost of energy, capital and
labour. Because the solar energy is free, the cost of
water produced by simple basin-type solar stills is a
function of the initial cost of the still. Efforts to
reduce the water cost have focused on the lowering of
+he irlitial capital cost without unduly affecting
p ,fi~mance. The bulk of these efforts has involved
dillrent
geometric configurations. Some of those
tried out at the University of California are shown in
figure 2. The most promising design in terms of initial
‘E. D. Xowe and B. W. Tleimat, “Twenty years of work
on solar distillation
at the University of California”, Solar
Energy, No. 16 (1974), pp. 97-105.
3S. C. Talbert, J. A. Eibling and G. 0. G. Liif, Manual
on Solar Distillation
of Saline Water, Research and
Development Report No. 546 (Washington, United States
Department of the Interior, Office of Saline Water, 1970).
,’
Fibrocamant
Alumonium
pm
SOLAR
STILL
No. 1
SOLAR
Conical
bin
BIA
paintuf
tray
marina
SOLAR
plywood
basin
STILL
No.16
plastic
STILL
No. 20
film cow
liner
smd
SOLAR
STlLl
4x42
Figure 2. Cross-section of some solar stills at the University of California, Richmond
Solar distillation:
I-
115
The state of the art
I ommnl ------I
Figure 3. Simplified cross-sectional diagrams of some large
stills in various places
cost and relative efficiency was, however, that of
figure 1. It has been installed at several locations4 in
the South Pacific. The glass-covered basin has an area
of 5 m2. This design was also used in modular form
with additional modules of 2.4 m2 each, giving
flexibility of size and production.
Cross-sections of some of the large solar stills
constructed throughout the world are shown in
figure 3. Cross-section A consists of concrete side
members which contain grooves to support the glass
cover and form the product trough. A black basin
liner is laid over the insulated bottom, formed into
the product trough and sealed against the glass edges.
The top ridge, between the two sheets of glass. is
formed by using silicone rubber sealant which, after
drying, acts as a hinge for the two glass panes. This
design has been successful and has been used in
several Australian installations.
Cross-section B in figure 3 is similar to a large
installation on the Greek island of Patmos. It is
glass-covered with aluminium distillate troughs and
glass supports. Cross-section C is similar to the one
used by the Brace Research Institute of McGill
University at the installation in La Gonave, Haiti.
Cross-section D is similar to the still used at Las
Marinas, Spain. It was modelled on the deep-basin
‘E. D. Howe and B. W. Tleimat, “Solar distillers for use
NO. 2 (19671, pp. 109-l 15.
on coral islands”, Desalination,
concept tried at Daytona Beach, Florida. The
deep-basin concept is based on the argument that this
construction does not involve accurate levelling of the
ground and would prevent the occurrence of dry
spots when evaporation took place. Also, because of
the large mass of water in the basin, evaporation
continued into the night because of the heat stored in
the saline water. However, after considerable
experimentation and comparison with shallow-basin
stills, it was concluded that for better performance
the depth of water should be as small as practicable.
Other designs of solar stills that have been tested
at different stations include the tilted-tray still and
the wick-type stills, shown schematically in figure 4.
The tilted-tray solar still is a unit in which the basin is
broken up into narrow parallel strips, with each strip
at a different height. Becauseof this arrangement, the
water surface is brought closer to the glass surface.
This type of construction resulted in higher efficiency
than the simple basin-type: howese:, the additional
cost of construction could not compensate for the
increase in performance. The wick-:ypr still is a
glass-covered type and consists of a cioth wicking
material laid on an inclined surface. Sa!inc water is
distributed along the upper edge of the inclined
surface and flows down !hrough the absorbent
material, maintaining it in a wet condition. Because
of the small thermal capacny of this type of still, it
can operate at high efficiency: however, difficulties in
keeping the wick wet and cie;:n and the cost of
construction prevent its use on a Largescale.
The most recent entry in the field of solar
distillation is taking place in Mexico under the
auspices of the Comision para el aprovechamiento de
Figure 4. Tilted-tray and wick-type solar stills
‘_;
116
~_------‘.
..-
aguas salinas in Tecamachalco, Mexico. It involves
three o erating installations with rated outputs of
l-l .5 m P/d. These installations consist of individual
stills laid on the ground and connected together with
PVC piping. These individual stills are similar to the
fibro-cement tray stills constructed by Gomella and
consist of a fibreglass tray, 0.93 m X 1.063 m, with a
black-painted bottom and covered with glass sloping
at 30” to the horizontal. The reported average cost
per still in Mexico is about $25 and the cost per unit
of product water is 34 %/m3, including capital cost,
labour and maintenance.
Performance of solar stills
The performance of a solar still is usually
expressed as the quantity of water produced per unit
of area per day. This quantity varies with the
geometry of the still,the insolation and the surrounding
conditions. Once a solar still is built and installed, its
geometry is fixed, and thus the performance of the
still would be a function of the insolation and
ambient conditions only. Figure 5 shows the
productivity of the stills shown in figures 1 and 2 as a
function of the insolation on a horizontal surface.
They were built and tested at the University of
California. Solar still No. 1 was the first built at the
University. It was kept in good working condition
and used as a model for comparison with other stills.
Except for still No. 41, which was of the tilted-tray
type, all the other stills were of the basin-type.
Although figure 5 shows the productivity only of
solar stills built and tested at the University of
----
Techrlology for Solar Energy Utilization
California, a similar trend was observed in other stills
constructed elsewhere, with productivities somewhat
lower than that of the tilted tray.
The presence of these features in a still seemsto
lead to high efficiency:
Low heat capacity
Low water content
Low air content
Vapourtight cover
Watertight basin
Good insulation around basin
The work done to date at various laboratories has
also yielded useful information on construction
materials. The transparent cover can be either glassor
plastic. Except for vulnerability
to mechanical
damage, glass possesses all the desirable properties,
such as high transmissivity for solar radiation, low
transmissivity for low-temperature radiation, wettability with water, availability and high stability of
properties over extended periods of exposure.
Transparent plastic filmcondensing covers used for
solar stills have shown relatively high transmissivity
for solar radiation and low-temperature radiation, but
also degradation due to ultraviolet radiation. They
must be mechanically treated to be wettable with
water, and are usually manufactured only in highly
developed countries. Moreover, although plastic films
are less vulnerable to mechanical damage, they have
shown failures because of wind conditions and have
not yet been developed ‘to be sufficiently long-lasting
under conditions encountered in solar distillation.
Figure 6 shows the productivity of solar stills,
No. 38-1 and No. 38-2. These two stills were of the
0
Figure 5. Productivity of some solar stills tested at the
University of California, Richmond
Figure 6. Comparison of productivities of University of
California solar stills Nos. 38-l and 38-2
Solar distillation:
The state of the art
tilted-tray type and, except for the transparent
covers, of identical construction. Still No. 38-1 had a
double-strength glass cover, 3 mm thick, while still
No. 38-2 had a cover of mechanically-tr-eated PVF
film 0.05 mm thick. The two stills were installed next
to each other and tested under identical conditions
for two years. It is evident from figure 6 that the
average producitivity of the plastic-covered still was
about 20% lower than that of the glass-covered still.
Since vapourtightness strongly affects the performance of solar stills, the sealing of the transparent
cover to prevent vapour leakage from the still is very
important. The most effective and longest-lasting
sealant used at the University of California was
silicone rubber. Other sealants, such as ordinary
putty, tar and plastic adhesive tape, deteriorated with
time and formed cracks that permitted leakage of
condensate and water vapour.
Basin-liner materials must be watertight and
capable of absorbing solar radiation. Also, because
the still may run dry, the basin liner must be capable
of withstanding relatively high temperatures without
being damaged. The most satisfactory material found
so far is butyl rubber. Other materials have been
tried, such as asphalt mats, used for lining stock tanks
and roofing buildings, and polyethylene. While the
asphalt mats were found satisfactory for the
deep-basin stills, they are too stiff for shallow-basin
stills with long narrow bays, and the polyethylene
degrades rather rapidly when exposed to ultraviolet
radiation.
The product trough to collect the condensate
and the piping must be made of materials that can
withstand distilled water. Plastic can be used for this
application. Steel was found to corrode very quickly,
and galvanized-steel sheets were found satisfactory
but showed corrosion at the points where joints and
bends were made. Aluminium, if protected galvanically, can be used successfully. Stainless-steel product
troughs were found to be most satisfactory and
showed little, if any, corrosion.
The use of simple solar stills has been limited to
the production of relatively small quantities of water.
This is because of the large area requirement, which
means capital investment and maintenance costs so
high that these stills cannot compete with other
methods of desalination.
In solar stills, the solar energy is applied only
once in the process, as contrasted with multipleeffect
distillation where the energy is applied several times.
Efforts to use solar energy more effectively have been
expanded by several investigators.’ Figure 7 is a
‘R. V. Dunkle, “Solar water distillation: the roof type
still and a multiple effect diffusion still”, American Society
of Mechanical Engineers. Quarterly
Transactions, 196 1,
pp. 895-902; M. K. Selcuk, “Design and performance evaluation of a multiple-effect tilted solar distillation unit”, Solar
Energy. No. 8 (1964), pp. 23-30; Maria Telkes, Research on
Methods for Solar Distillation,
Research and Development
Report No. 13 (Washington, United States Department of the
Interior, Office of Saline Water, 1956).
117
schematic representation of a multiple-effect still
investigated by one of them (Dunkle). It consists of a
series of vertical plates within a box, with the first
plate heated by solar-heated water and the last plate
cooled by saline water, inside tubes bonded to the
first and last plates. The saline-water feed, heated by
the condensing vapour on the left side of the last
plate, is introduced into an open trough and is
allowed to fall as a thin film on the right-hand side of
the first three plates. Vapour produced from the
right-hand side of the first plate condenses on the left
side of the second plate, and so on, until the last
plate. In this arrangement, the solar heat delivered to
the first plate by the solar-heated water passesto the
second plate and, in turn, then passes to the third
plate and then to the fourth plate to pre-heat the
saline water feed. Condensates forming on the left
sides of the second, third and fourth plates are
collected and used as fresh water. The number of
plates (effects) depends on the temperature difference between the solar-heated water and the cold
saline-water feed. It will be recognized that such a
still uses solar energy indirectly. If the left side of the
box were made of transparent material and the first
plate painted black with the tubes eliminated, the still
would then utilize the solar energy directly. This
arrangement has the advantage of producing when the
sun is not shining, if only the solar-heated water is
available. Unfortunately, the additional productivity
of such stills does not compensate for the increase in
construction costs. The cost of water from a simple
basin-type still is lower than that from the
multiple-effect still, and this seems to account for the
lack of interest in such construction.
Numerous methods to utilize solar energy for the
production of steam to drive conventional multipleeffect and multiple-stage distillation systems have
Technology for Solar Energy Utilization
118
been proposed and tried. 6 In these schemes, solar
collectors were used to capture solar energy for the
production of steam. This steam is then used as the
heating steam in the first effect of a multi-effect still
or in the brine water of a multi-stage flash still. Of all
these schemes, only that of Baum was built and
tested. He used a concentrating paraboloid collector
to generate steam at high temperature to drive a
three-effect still. Unfortunately, because of the low
heat-transfer coefficient in the still, only three effects
could be used, and the cost of steam from the solar
boiler was too high for economical operation.
The performance of a multipleeffect still is
defined as the amount of water produced per unit of
heat released by the condensing steam in the first
effect and is called the gained output ratio (GOR).
The GOR increases with the number of effects and
with increasing steam temperature. For a constant
steam temperature and constant cooling-water temperature, the GOR is a function only of the number of
effects; consequently, increasing the number of
effects decreases the amount of steam required to
produce a unit amount of water and results in a
smaller collector. However, increasing the number of
effects increases the cost of the evaporators in the
multi-effect still. This increase must be balanced
against the benefits resulting from decreasing the size
of the solar collector.
As an illustration, figure 8 shows a schematic
flow diagram adapted from a scheme proposed by
Tleimat and Howe.’ In this scheme, flat-plate solar
collectors are used in the steam-production (solar
boiler) section and rotating-disc evaporators in the
multieffect section. In operation, solar-heated water
is taken from one of the storage tanks and introduced
into the degasser to eliminate non-condensable gases.
After the degasser, the water is introduced into the
flash chamber, where a small part flashes into steam.
The other part is reheated in the solar collector
during sunny hours and stored in the second tank or
6V. A. Baum. “Solar distillers”, Paper prepared for the
United Nations Conference on New Sources of Energy,
Rome, 1961 (35/S/119); D. B. Brice, “Saline water conversion by flash evaporation utilizing solar energy”, Advance
in Chemistry Series, No. 38 (1963). pp. 99-116; J. A. Eibling,
R. E. Thomas and B. A. Landry. An Investigation
of
Multiple-Effect
Evaporation of &line Waters by Steam from
Solar Radiation, Research and Development Report No. 2
(Washington, United States Department of the Interior,
Office of Saliie Water, 1953); H. Weihe, “Fresh water from
sea water, distilling by solar energy”, Solar Energy, No. 13
(1972). pp. 439-444; B. W. TJe,nat and E. D. Howe, “Solarassisted distillation of sea wdter”,
Proceedings of the
international Conference on Heliotechnique and Development, Dhahran, 2-6 November 1975.
‘Op. cit., p. 439.
LIrmc
bl.awdMwl
Figure 8. Solar-assistedmulti-effect still
is sent directly to the second storage tank during the
night and the condensate from the first effect
returned to the s:-iar boiler.
The unique feature of this scheme is the use of
the Tleimat wiped-film, rotating-disc evaporator, an
extremely high-performance evaporator developed at
the Sea Water Conversion Laboratory, University of
California, Berkeley. Overall heat-transfer coefficients
of 2-5 W cm-20C-1 have been obtained experimentally. These values are almost 10 times higher than
values obtained in multi-stage flash stills now used in
sea-water desalination. Because of these high
heat-transfer coefficients, a large number of effects
can be used, which means small-size collector
systems. From data on the Tleimat evaporator,
Tleimat and Howe were able to show that, for 60°C
steam from the solar boiler and 20°C cooling
sea water, it is possible to have a 15effect still,
resulting in a GOR of 12.8. Also, because the steam
from the solar boiler is needed at 60°C, the collectors
operate at relatively low temperature and consequently higher collection efficiency. Such a scheme
could be used to increase the productivity of solar
stills far above that of simple basin-type solar stills.
Solar water distillation
Carlo Mustacchi
and Vincenzo
Cena
Rome, Italy
Solar water distillation is the solar technology
with the longest history. Installations were being built
for this purpose as early as 20 centuries ago.
However, except for particular environments, such as
islands, and for special requirements, such as
emergency equipment for seafaring, only in the last
20 years has there been a revival of interest in this
field as a result of population growth in arid regions
and rising standards of living.
Two widely different approaches are possible: a
low-technology integrated approach, whereby trapping solar energy and using it to evaporate water are
done in the same piece of equipment, with low yields
and low investment costs; and a high-cost, high-technology approach, whereby the two processes are
carried out separately by means of specialized
subsystems optimized for high yields.
Solar stills
The most elementary solar-still construction
requires a flexible plastic sheet (polyethylene) of
about one metre square, a small pan, a stone and a
hoe. The last item is used for digging a round conical
hole in the ground. This hole is covered with the
plastic sheet, which is kept in place by piling up soil
on its outer edges. The pan is placed at the centre of
the hole to catch the water dripping from the plastic
sheet, which is loaded at its centre with the stone. No
feed water is required at all; capillarity and the
temperature and humidity gradients in the soil are
sufficient to feed the system. With suitable
precautions, such as covering the bottom rf the hole
with well-trampled dark silt to increase absoy;i
and capillary action, yields of over 1 I m-‘d
been observed in desert locations as far as 500 km
from the sea, where daily humidity is of the order of
1%15%. The unit installation cost is around 1 S/m’,
including materials and manpower. This low cost does
not justify the building of large installations, which
would require an excessive amount of manpower for
supervision and operation (emptying the pans at
dawn).
The next step consists in covering the cavity with
a black impermeable layer (tar, butyl rubber, black
polyethylene) and sealing the joints between the
transparent roofing and the sides. The increase in
absorptivity entails a requirement of feeding the still
with any available source of water (desert plants,
brackish water, sea water etc.). The geometry of the
basin seems to have only a minor effect on yield
(typically
l-5 I m-‘d-l ). The advantage of also
catching rainfall leads to symmetrical or asymmetrical
shed-like constructions. The basin material can be
compacted earth, concrete, masonry, asbestosconcrete, wood or plastic. The roofing is generally of
glass or of PVF, polyester or other plastic film. The
support for the roofing can be a frame made of the
same material as the basin or of metallic frames
(aluminium, or galvanized or painted steel). The
sealant in most experimental stills is of silicone
rubber, but recent reports concerning the good
durability of asphalts or mixtures of asphalt and
polyesters suggest that these can be selected as
sealants, especially if suitable pigments are added to
the mixture to inhibit further polymerization and
embrittlement. Insulating the bottom or sides of the
still appears to be less important than conditioning
the soil by providing drainage and preventing the
growth of vegetation that can damage the stills.
Current cost estimates for different designs have
to be based on local materials, especially manpower.
They range from 8 to 20 $/m’, and very often a
higher investment is more than offset by lower
intervention, maintenance and operation costs. In
fact, large installations (20-50 m3/d) should be
provided with automatic filling and emptying
systems, ‘even though these operations are mostly
carried out at weekly intervals. Plant maintenance,
with renewal of the sealant every few years, is just
about the only chore to be performed by the
operation crews.
Observations show that convection and ground
losses and vapour leakage are. all of the same order of
magnitude, namely, 4%6%. No great advantage is
foreseen in trying to improve geometry or thermal
behaviour. Improved quality can rather be expected
from combined systems, such as those which feed the
basins with the cooling water discharge from a power
plant condenser, with the glass roofing being cooled
by pre-heating the feedwater or using condensation
119
120
Technology for Solar Erlergy Utilizatiorl
surfaces (other than the transparent roofing) capable
of providing increased radiation and convection.
With regard to construction materials other than
sealants, there is no great room for technological
improvements. However, more data could be found
on the reliability and failure of materials over
extended periods at different maintenance levels.
Altogether, an approximate cost of $2 for each cubic
metre of fresh water production seems to be a basic
value for full-scale installations (2-50 m3/d).
It must be stressed that basin stills are a poor
solution for communities of IO 000 or more
inhabitants with no other source of fresh water. The
lack of an effective economy of scale leads to
excessive investment levels and the collection area is
so large that piping and pumping systems become
major problems.
location with a drier climate between the latitudes of
10’ and 20°, it can provide about 7 X lo6 kJ/m’ per
year.
All the construction materials (but not the
assembled collector) are already proven for a lifetime
of 20-25 years. Mass production of such collectors is
expected to bring down the costs to 25-30 $/m*,
even though the retail price of most collectors in
Europe and the United States are, at present. in the
70-100 $/m* range.
No skilled manpower at all is required for
assembling the collectors, building the case and
glazing the top.
The operating principles of the main distillation
units which can be coupled to the solar collectors are
reviewed below.
Multi-stage flash process
Indirect collection
Indirect cotlection and transformation of solar
radiation for the separation of water and dissolved
salts involves the use of flat-plate collectors in a
conventional desalination plant of the multi-stage,
multi-effect or vapourcompression type.
The flat-plate collectors are the same as those
currently available on the commercial market for
providing !rot water to dwellings. No special
provisions have to be made to increase the operating
temperature, since 90°C is at present the upper limit
for the operating temperature of distillation plants.
Beyond this limit, sea water is highly corrosive for
most common construction materials, and precipitation of calcium salts occurs unless costly additions
of chemicals (sulphuric acid, polyphosphates) are
made. Even below the limit of 9O”C, the collector
material must be a nickel-copper or an ahrminium
alloy. It is, however, often convenient to circulate
fresh water in a closed loop to a standard carbon-steel
collector and construct a separate heat exchanger to
transfer the collected energy to the brine of the
distillation plant.
The leading types of collectors for this use are
made by encasing a double corrugated-steel plate
inside a flat rectangular case of fibreglass, galvanizedsteel and concrete. The metal collecting surface is
either painted with flat black paint or receives a
selective fmish (copper oxide). The box cover consists
of one or two layers of common window glass with a
few centimetres separating the glass and the collecting
surface. Below the metal collector, the box is lined
with a layer of thermal insulating material (rockwool, or polyurethane or polystyrene foam). A
typical collector in orte of our experimental loops is
1 m X 2 m X 15 cm, with a l-mm glass-resin box, a
S-cm urethane foam insulation, double glass and
silicone sealant. It costs 30-40 $/m* for very
small-scaleproduction and provides a heat output of
over 2.9 X lo6 kJ/m* per year in Rome. In a desert
In this process (figure 1) the sea water is
pre-heated by heat exchange ,vith the condensing
fresh-water vapours, then the warm brine receives the
solar input energy and is brought to a temperature of
70”-90°C. Its pressure is decreased gradually in a
series of enclosures which contain the tube bundle of
the pre-heater. Condensed fresh water drips from the
bundle and is collected and circulated along the chain
of vessels to recover its sensible heat. The highest and
lowest pressure stages are kept respectively at about
0.8 bar and 0.05-0.07 bar, and the number of stagesis
such that the overall temperature difference (between
the hot collector and the cold sea water) is
fractionated into temperature drops or “flashdowns”
of about 2°C per stage. Current commercial
installations are designed with lo-30 stages. Although
theoretically in each stage the latent heat of
evaporation is reused to evaporate as much water at a
lower pressure, and therefore with 30 stages the
theoretical maximum yield per unit of collector
surface should be over 120 1 m-*d-r , in actual
practice some losses are incurred, due to pressure
drops, the small size of the heat-transfer surfaces and
ebullioscopic increase, so that the actual yields are in
the range of 60-100 1 m-‘d-l .
Figure 1. Multi-stageflash
A number of parametric estimates of multi-flash
plants ‘were carried out, and the results’ were
correlated by the following equation:
C=256000+853~+4.5(10764)AL4tO.45Pt
+ 2O.OpO.at 562 P”v6 t 12 900Pe.3
p,ty,/,
”pI _* :“Y.’
8”
I,,*”
><2,
F&
‘2
‘.
1:~‘;_ ,;;“l;,”,,,, -3:
‘)
Solar water distillation
121
‘The above equation has the following variables:
P=
N=
fresh-water production (lb/h)
number of stages
A=
heat-transfer surface per stage (m*)
c = cost (rs)
The depreciation of this cost plus an estimated $200
per day for manpower and chemicals and the
depreciation of the flat-plate collectors in the range
of 50-70 S/m* lead to a decrease in cost per unit
volume of fresh water from 2 to 1 $/m’ in the range
of 50-500 m3/h. These estimates are considered
conservative and are based on present-day technology. No attempt is made to store solar energy for
use at night; a conventional fossil-fuel boiler is
provided for operating the plant when the sun is not
shining.
Typical optimized design data for two multistage flash plants are as follo&:
Nommal production
(lb/h)
Wh)
Seatemperature CF)
CO
Concentrated brine discharge
temperature $Fi
Flashdown
c F per stage)
CC per stage)
Maximum collector temperature
C’F,
i”C)
Number of stages
Brine discharge (10’ lb/h)
Brine recycle (10” lb/h)
Flow rate in collectors (10’ lb/h)
Overall heat transfer surface
IlO3 ft*)
(IO’ m’)
Inve &nent cost ( lo6 dollars)
Sol2 collector surface ( 1O3m’)
Freakwater cost (S/m’)
Daily fresh-water production per
unit of collector surfice (l/m’)
i
lo5
45 x 10’
17
25
10”
45 x lo5
37
25
82
21.8
4
2.2
82
21.8
4
2.2
180
82
24
780
75
950
200
93
29
6 300
590
I 900
60
5.5
3.48
6.1
1.98
560
52
16.3
66
0.94
74
b8.5
kulti-effect process
This process (figure 2) also involves a series of
enclosures with decreasingpressureand temperatures.
The warmest stage is fed directly by the solar
collectors, and the coldest is kept close to the
Figure 2. Multieffect
prevailing sea temperature. The steam output from
one stage is circulated in the tubes of a bundle which
heats the liquid of the next stage. Thus latent heat is
recovered and transferred stage by stage. The
temperature differential between two stages is of the
order of IO’-1 5°F (5.6”-8.3”(J), and therefore seldom
are more than 5-10 stagesused.
A parametric cost estimate based rIpon a number
of detailed designs led to the following equation for
the plant investment:
C=(2-
1.37N-‘)P+5561/PN+4.3P”.*+
+ 139 Po.6 + 7 !20 Po.3 + 422 000
In this equation P and li’ have the same meaning as in
the preceding equation. The multi-effect process,
from the point of view of convenience, appears to
have a slight advantage over the multi-stage flash in
the range of 10 to 80 m3/h, but loses this advantage
at higher design capacities. In the range of interest
(about 50 m3/h), a typical design wou!d represent a
fresh-water cost of 1.7-I .8 $/m3, including depreciation, manpower, collectors and chemicals.
Other processes
The vapour-compression process (figure 3) recovers heat by extracting steam from one stage,
raising its thermal level by means of a compressor
(steam ejector or centrifugal), and feeding it as
heating steam to the stage it came from or to another
stage. A study of parametric cost estimates and
process designs show that this type of plant is not
especially convenient unless it is combined with a
multi-stage flash section. Further, it appears that the
mechanical energy requirements have to be provided
with a primary drive such as a diesel engine, and
cooling the radiator of such an engine provides more
than enough heat for the thermal requirements of the
process, so that solar collectors are redundant and
cannot be conveniently connected to the system.
The humidification and dehumidification system
warms the sea water in solar collectors, saturates the
ambient air in a packed, counter-current tower, then
recovers fresh water by cooling the wet air in a
condenser where heat is partly recovered by
pre-heating the sea-water feed to the solar collectors.
Figwe 3. Vapour compression
Technology
122
As such, this process yields no promise for the
purpose of providing fresh water alone, but can have
an application if the humid air by-product is needed
for other applications (greenhouse crops, airconditioning etc.).
Limitations and outlook
At first sight, it may appear that the basin-still
approach is the only one that can make full use of
local manpower and materials in developing countries. On the other hand, the so-called high
technology involved in the construction of, for
example,, a multi-stage flash is actually transferable
with ease and in some cases already available in most
arid countries, being the same technology required
for constructing or repairing a conventional chemical
processing unit. Except for purchasing a number of
special materials for the warmest stages (nickelcopper alloys) and possibly the vacuum equipment, in
addition, any crew of welders and pipe-fitters
currently ensuring the maintenance of an oil refinery
has all the skills necessary to manufacture the few
pressure vessels and heat exchangers which make up
the unit.
It must also be stressed that a number of arid
countries (for example Bahrain, Kuwait, Qatar and
Saudi Arabia) have built or are building large
desalination plants using conventional fossil fuels. A
constructive proposal is that a battery of flat-plate
collectors could be added to these plants to save fuel
without interfering with operations or requiring
design changes. With a limited investment, one half of
the fuel could be saved, or rather “frozen” and
turned into collectors which, after a period of about
10 years, would go on to supply virtualiy free energy
for the process.
This proposal circumvents one of the main
difficulties of solar-fuelled desalination plants: the
fluctuating energy input. Large desalination plants are
mechanically designed so that the pressure drops
between stages or effects are determined by liquid
levels in the different enclosures. Raising or lowering
the heat input will vary the flow rates and unbalance
the operation. Thus, unless specific new designs are
drawn up for solar desalination, some provision must
be made for keeping temperature fluctuations within
the range 2’-5°C at the heat source. This may be
done by one of the following means:
(a) Installing a compensating fossil-fuel boiler;
(b) Shutting down the plant at night;
(c) Providing a reservoir to store the solar heat.
Solution (a) is the method upon which the above
estimates were based and coincides with the proposal
to add solar collectors to existing desalination plants.
Solution (b,J is just beginning to be possible: since the
latest designs of desalination plants involve start-up
for Solar Energy Utilization
transients as short as 15 minutes, thus entailing
limited heat waste. Solution (c) requires more
development work to make it economically convenient but holds the greatest potential.
A recent survey of the main procedures for
storing the huge thermal energy requirements of a
sizeable desalination plant at night led to the
following conclusions. The idea of storing warm
water should be discarded since it would add about
2 $/m3 (depreciation cost of the reser*Dirs) to the
cost of the fresh water, thus doubling costs. Chemical
storage, frequently advocated in many proposals, is
not yet convenient. For instance, releasing ammonia
from an absorbent such as calcium chloride during
the day and allowing reabsorption at night to yield
the heat of reaction, requires compression and
liquefaction of the ammonia and would waste almost
as much heat as that to be stored. Another possibility
involves the storage of heat by melting a compound
and recovering the latent heat of melting when the
compound refreezes at night. The four studies that
have been conducted suggest that this solution is
feasible with paraffin (melting-point 70°-8O’C) kept
in polyethylene bags in the warm-water tanks. This
would save 50%80% of the tank volume for an equal
thermal energy storage. Further work is being carried
out along these lines.
Future trends
Several projections of world market trends in the
field of fossil and nuclear fuels indicate that energy
costs will increase by 50% every 10 years. At
present-day costs, of the l-2 $/m3 of the cost of fresh
water produced by a sizeable distillation plant
(50-500 m3/h), 0.75 $/m3 is accounted for by fuel
costs. Looking 20 years ahead, the 0.75 $/m3 will rise
to about 1.7 $/m3, whereas with solar collectors that
will be fully depreciated by then, the cost will be nil.
The overall saving would therefore be of this order of
magnitude. On the other hand, the usual difficulty
for most developing countries is that of securing
sufficient capital to invest in collectors. It must be
pointed out that in the mentioned range, the solar
collectors represent 5%25% of the overall investment
cost. The decision can be simplified by reducing it to
the acceptance of this additional lumpsum investment today in order to reduce by approximately one
half the costs of fresh water 15 to 20 years in the
future. However, if venture capital is unavailable, the
increased use of solar stills represents an alternative.
The modular character of such a construction allows
the establishment of a small number of units each
year, or whenever the few materials required (mainly
glass) can be afforded.
This solution is expected to be the best one, at
least when the population to be served by the
installation is less than 5 000-6 000 people, as is the
case for most villages in arid parts of the world.
123
Solar water distillation
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solar stills by the surface treatment of the glass.
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(Weinheim, Federal Republic of
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Some aspects of solar distillation for water
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14: 387, 1973.
Hay, H. R. Plastic solar stills: past, present and
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and industrial
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(New
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prepared for the United Nations Solar Distillation Panel Meeting, October 1968.
..
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‘,,
124
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the engineering development of a solar still. Solar
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Operating experiences with solar stills for water
supply in Australia. Paper presented at the Solar
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California, October 1968.
Proctor, D. The use of waste heat in a solar heater.
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Satcunanathan, S. and H. P. Hansen. An investigation
of some of the parameters involved in solar
distillation.
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14: 353, 1973.
Scope and development of solar stills for water
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(Amsterdam) 5: 1: 64-74,
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Solar energy (Elmsford, N.Y.) 13:403, 1972.
Szulmayer, W. Solar stills low thermal inertia. Solar
energy (Elmsford, N.Y.) 14:415, 1973.
Telkes, Maria. Fresh water .from sea water by solar
distillation. Industrial and engineering chemistry
(Washington, D.C.) 45:5: 1108, May 1953.
Tleimat, B. W. and E. D. Howe. Comparison of plastic
and glass condensing covers for solar distillers.
Proceedings of the Solar Energy Society
Conference, Phoenix, Arizona, March 1967.
Weihe, H. Fresh water from sea water: distilling by
solar energy. Solar energy (Elmsford, N.Y.)
13:439, 1972.
Technology for Solar Energy Utilization-
No. 60 Development of an improved solar still (by
W. N. Grune and others). March 1965.
No. 147 Second two years’ progress on study and
field evaluation of solar-water stills (by the
Battelle Memorial Institute). July 1965.
No. 190 Final three years’ progress on study and field
evaluation of solar sea-water stills (by the
Battelle Memorial Institute). May 1966.
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gallon per day msf desalination plant and
test module. November 1966.
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flash progress. March 1967.
No. 276 Preliminary design of a diesel-powered
vapour-compression plant for evaporation of
sea-water. August 1967.
No. 355 Analysis and optimization of a multieffect
multi-stage flash distillation system. May
1968.
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modification. April 1969.
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test-bed plant, Freeport, Texas. Annual
report. September 1969. (FY 1968)
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2.5 MGD horizontal-tube
multiple-effect
plant. October 1969.
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April 1970.
of saline water.
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of the Interior, Office of Saline Water, Washington, D. C.
No. 635 Seawater desalting with chemical recovery.
January 1971.
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No. 638 Economic analysis of the membrane water
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13 Research on methods for solar distillation
(by M. Telkes). December 1965.
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September 196 1.
No. 641 Study of unconventional methods for brine
concentration. April 197 1.
The potential
in developing
of solar agricultural
areas
driers
T. A. Lawand
Brace Research Institute, Macdonald College of McGill University, Quebec, Canada
Introduction
Since the dawn of civilization solar energy has
been used to dry and preserve agricultural surpluses.
The methods used are simple ar!d often crude, but
reasonably effective. Basically, crops are spread on
the ground or on platforms, often with no
pre-treatment, and are turned regularly until sufficiently dried so that they can be stored for later
consumption. The process is labour intensive, and
little capital is required for equipmeni:.
Diverse products such as fruit, vegetables, cereals,
grains, skins, hides, meat, fish and tobacco are dried
using these simple techniques. There is probably no
accurate estimate of the vast amounts of material
dried in this way. Sun drying is a wide-spread
technology practised in almost every country of the
globe and at nearly every latitude.
Since sun drying originated in many of the
developing countries there is no major social problem
in persuading local populations to eat dehydrated
foods. The process presents several technical
problems, however:
(a) It is intermittent, being affected by cloudiness and rain;
(6) It
pollution;
is affected by dust and atmospheric
(c,! It is not safe from intrusion by people and
animals;
(dl The products being dried are subject to
infestation by insects.
In the more advanced segment of the society,
whether in developing areas or in industrialized
regions, artificial drying has often supplanted
traditional sun drying in order to achieve better
quality control, reduce spoilage and cut down on the
losses and inefficiencies engendered by the abovementioned dii’ffculties.
The high cost of labour in most industrialized
areas and, until recently, the low cost of fossil fuel
permitted artificial, large-scale drying processes to be
developed. The cost of dehydration was added to the
cost of selling the process materials. The advent of
higher charges for fossil fuels and the prospect of
depletion and scarcity of t!?ese fuels !?ave stimulated
renewed interest in solar agricultural driers.
The amount of agricultural produce dehydrated
in 1968 using solar energy has been estimated at
255 X lo6 t. In that year alone Australia exported
over 72 X lo3 t of sun-dried foods worth over
1627million. Over the past three decades, interest in
deve!oping solar sgricu!turs! driers th2.t m,ake use of
known principles of heliotechnology to combat some
of the principal disadvantages of traditional sun
drying has been increasing.
In evaluating technologies that might be suitable
for developing areas, one should distinguish between
small- and large-scale operations. Small-scale systems
would be used where land holdings are not large, and
individual farrners, fishermen and herdsmen produce
only modest surpluses. The objective is to dehydrate
these surpluses for use only by the family of the
producer or for sale in the local market in the
immediate vicinity. At times, small-scale surpluses of
certain products such as peanuts or rice are delivered
to central facilities for processing, dehydration and
eventual marketing. These systems are generally well
established and require a certain degree of organization in the industry. Such handling facilities,
however, often do not exist. Therefore, in choosing
technologies, one must differentiate between the
existence of commercial and physical infrastructures
within a given !ocality.
Large-scale systems invariably require an external
power source. Where conventional electric power is
available, reliable and not excessive in cost, it is
logical to use it for operating fans and blowers, vents
and duct baffles to increase the efficiency and
operating performance of a solar agricultural system.
Some driers are of the portable, powered type
wherein solar air-heater collectors are fitted with
electrically powered fans (gasoline or diesel engines
could as well be used) and are taken directly to the
areas of production for in situ drying. Traditionally
this process was used with fossil fuel, often butane or
12.5
-’
:
”
126
Technology for Solar Energy Utilization
propane gas, as the energy source. As the price of
these systems increases, the tendency is to develop
systems of this nature that rely on solar energy to
provide the bulk of the energy required for
dehydration. In fact, fossil fuels are sometimes used
to supplement these solar collectors in order to
maintain optimum operating conditions in a system
partially operated by solar energy.
The other major system applicable for dehydration in the industrialized sectors of developed and
developing countries is lo use the rouf atea of
buildings as the solar collector, fitting the buildings
with suitable blowers, ducts, collectors and often
sturage mechanisms. In the United States, California
Polytechnic University and Thompson Ramo Wooldridge Systems have undertaken a project, funded by
the Government, in which solar energy is to be used
as a substitute for natural gas in dehydration. The
State of California alone produces annually fruit and
vegetables valued at over $450 million. The system
will no doubt become increasingly economic as the
cost of fossil fuel continues to escalate.
Another system receiving increasing interest in
this field, both in developed and developing regions,
is the use of greenhouses to dehydrate surplus
produce. The combination of drying and greenhouse
operations has many advantages and should be
examined for each set of circumstances. Finally, an
older, but certainly no less valid, system has been the
use of heat extracted from the underside of roofs.
This system, one of the oldest applications in solar
agriculture drying, has proved quite satisfactory in a
number of applications.
Basic principles of solar drying
The two principal phases of the process used in
solar agricultural driers are the solar hsating of the
working fluid (generally air) and the drying itself,
wherein the heated working fluid extracts moisture
from the material to be dried.
The first of these phases can be accomplished in
two ways: (I ) Indirectly, by separate solar air-heater
collectors using natural or forced convection to
pre-heat the ambient air and reduce its relative
humidity; or (2) Directly, by in situ heating of the
air, which in turn directly dehydrates the produce.
A discussion of drying theory is beyond the
scope of this paper, but a few principles applicable to
direct radiation drying may be outlined here, since
the principles involved in the drying of materials in
opaque enclosures by means of hot air, whether from
a solar heater or some other type of heating unit,
have been examined elsewhere.
The first requirement is a transfer of heat to the
surface of the moist material by conduction from
heated surfaces in contact with the material, by
conduction and convection from adjacent air at
temperatures substantially above that of the material
being dried, or by radiation from surrounding hot
surfaces or the sun. Absorption of heat by the
material supplies the energy necessary for vaporization of water from it, some 2.5 kJ (590 cal) per gram
of water evaporated. Water starts to vaporize from
the surface of the moist material when the absorbed
energy has increased the temperature enough for the
water vapour pressure to exceed the partial pressure in
the surrounding air. Steady state is achieved when the
heat required for vaporization becomes equal to the
rate of heat absorption florn the surroundings.
To replenish the moisture removed from the
surface, diffusion of water from the centre to the
surface of the drying material must take place. This
process may be rapid or slow depending upon the
nature of the material being dried and upon its
moisture content at any time. It may thus be the
limiting rate in the drying operation; or, if moisture
diffusion is rapid, the rate of heat absorption on the
suiface or the rate of vaporization may be the
controlling factor. In some very porous materials,
vaporization may take place even below the apparent
surface of the material, vapour then diffusing through
pores in the so!id.
In direct radiation drying, part of the radiation
may penetrate the material and be absorbed within
the solid itself. Under such conditions heat is
generated inside the material and at the surface, and
thermal transfer in the solid is facilitated.
For economic reasons, maximum drying rates are
usually desired. Product qualily must be considered,
however, and excessive temperatures must be avoided
in many materials. Jn addition, because drying occurs
at the surface, materials that have a tendency to form
hard, dry surfaces relatively impervious to liquid and
vapour transfer must be dried at a rate sufficiently
low to avoid this crust formation. The heat transfer
and vaporization rates must be closely controlled,
either by limiting the heat supply or by regulating the
humidity of the surrounding air.
The drying of a product simply by permitting
dry air to circulate around it, without the use of any
direct or indirect heat source, is known as adiabatic
drying. The heat required for vaporizing the moisture
is supplied by the air to the solid material, thereby
reducing the air temperature while increasing its
absolute and relative humidity. Because the heat
capacity of air is low in comparison with the high
latent heat of vaporization of water, large vo!umes of
air at reasonably low relative humidity must be used
in this type of drying. Air leaving the drier is nearly
saturated with water at the wet-bulb temperature.
The moist solids in contact with this air approach the
same temperature.
The foregoing generalization must be somewhat
modified if the materials being dried are soluble, even
to a small extent, in the water present. Fruits and
other agricultural products contain salts and sugars
that cause a lowering of the vapour pressure. The
surface temperatures of these materials must
The potential
of solar agricultural driers in developing.-. areas
therefore be higher than the wet-bulb temperature of
the air to permit vaporization to take place. This
means that the adiabatic drying of these solids
requires air at lower relative humidities than do the
materials having no solutes in the aqueous phase.
An important property of materials processed by
direct radiation drying is their absorptivity of
radiation. Fortunately, most solids have relatively
high absorptivities, but they may change as drying
proceeds, the surfaces of the materials becoming less,
sometimes more, “black” during the process. Also,
there may be changes in opacity of the surface of
materials that are partially transparent at certain wave
lengths represented in the spectrum of the radiant
source.
The thermal conductivity of the material is also
an important property, particularly if it is dried in a
layer of sufficient depth to require conduction of
heat from particle to particle. If the thermal
conductivity is poor, circulation of heated air through
and between the particles of moist solid would permit
better heat transfer than direct radiation on the
surface of a deep bed of particles.
In large-scale dehydration systems, forced convection, generally powered with an external, nonrenewable power source, increases the diffusion
transfer of moisture and, if properly applied,
increases the rate of dehydration and improves the
quality of the produce.
Solar agricultural driers mainly used in developing countries for small-scale applications are described
briefly below.
Classification of driers
Solar driers are classified according to their
heating modes, or the manner in which the heat
derived from the solar radiation is used. In this
regard, several general categories have been set up,
which are defined below. In general, a drier has been
classified according to its principal operating mode.
Some of the direct and mixed-mode driers also use
circulating fans and are not, strictiy speaking, totally
passive systems. Driers using only solar or wind
energy for their operations are classified as passive
systems.
Sun or natural driers. These driers make use of
the natural action of ambient solar radiation, and of
the ambient temperature, humidity and motion of
the air to achieve drying.
Solar driers-direct. In these units, the material
to be dried is placed in an enclosure with a
transparent cover or side panels. Heat is generated by
absorption of solar radiation on the product itself as
well as on the internal surfaces of the drying
chamber. This heat evaporates the moisture from the
127
drying product. In addition, it serves to expand the
air in the enclosure, causing the removal of this
moisture by the circulation of air.
Solar driers- mixed-mode (direct arld indirect).
In these driers, the combined action of the solar
radiation incident directly on the material to be dried
and pre-heated in a solar air heater furnishes the heat
required to complete the drying.
Solar driers-indirect.
In these driers, the solar
radiation is not directly incident on the material to be
dried. Air is heated in a solar collector and then
ducted to the drying chamber to dehydrate the
product.
Solar timber driers. These driers have been put
in a special category, since they constitute an
important application of this technology. In most
cases forced ventilation is used, since proper
circulation of air helps control the drying rate so as to
avoid case-hardening.
A chamber drier is a drier in which the material
to be dried is dried in an enclosure.
A rack, or tray, drier is one in which the material
to be dried is placed on wire-mesh or similar holding
racks.
A hybrid drier is a drier in which another source
of energy, such as a fuel or electricity, is used to
provide supplementary heat or ventilation.
The characteristics of one or two solar driers
belong to each category defined above are described
in the next section.
Characteristics of specific driers
Drying grapes on racks (Australia)’
In Australia, grapes have been sun-dried on racks
for quite some time. In 1972, about 100 000 t of
fresh grapes were rack-dried in 8-14 days.
The drying rack consists of 8-12 tiers of
galvanized-wire netting. The wire netting is reinforced
lengthwise along both edges with fencing wire. At
each end of the rack, the load is taken by two heavy
posts embedded in the ground and sloped and stayed
against the strain. One 50-m drying unit is considered
to provide enough rack space to dry the fruits from
three acres (1.2 ha) of vines. At 3-m intervals along
the racks, intermediate posts carry cross-supports for
the tiers.
The rack can be covered by a roof to protect the
drying grapes against rain or excessive sun, thus
’ Principal investigator: J. V. Possingham,CSIRO Division of Horticulture Research, Adelaide, Australia.
/.
”
Technology
128
leading to a product of a better quality. The roof is
constructed of corrugated-iron sheets fixed crosswise.
Equal overhangs on both sides of the rack keep
dripping rain water away from the racks. There is no
pitch to the roof, so that when wind from any
direction accompanies rain, it will blow the water on
the roof away from the fruit. Certain raisin species
obtain a superior quality when shade-dried. To
provide the shade, burlap side curtains are often
placed on the rack. (The use of such curtains should
be avoided iu wet c:liinaies, sini;e SX~SSIW
humidity
favours mould development .)
Natural vertical drier (Coiombia)2
In Colombia, a vertical drying system designed
for drying cassavaparticles has been tested. It consists
of two wire-mesh panels pinned on two wooden
uprights, which are set in the ground and act
simultaneously as supports and end walls. Top and
bottom openings make the drier easy to load and
unload. The distance between the wire-screen walls
was variabie in the experimental prototype so as to
allow the drying space tn be charged with different
loading densities of cassava on the exposed surface.
The unit is covered with a wooden roof to protect the
product from rain and to allow the drying to
continue overnight. The drying method gives a
=nva that is easy to handle and store.
high-quality CL~
Solar cabinet drier (Syrian Arab Republic)3
A cabinet drier designed for direct solar drying
has been used in the Syrian Arab Republic to dry a
wide variety of agricultural products on a small r .ale.
It is easy to build from almost any kind of available
building materials and simple to operate, maintain
and control. Tested and used in many countries with
different climates, this drier has proved to be very
effective for its purpose.
The drier is essentially a solar hot box in which
fruit, vegetables or other matter can be dehydrated
on a small scale. It consists of a rectangular container
insulated at its base and sides and covered with a
double-layered transparent roof. Solar radiation is
transmitted through the roof and absorbed on the
blackened interior surfaces. Owing to the insulation,
the internal temperature rises. Holes drilled through
the base and outlet ports located on the upper parts
of the cabinet side and rear panels provide
ventilation. As the temperature increases, warm air
passes out of the upper apertures by natural
‘I Principal investigator: Gonzola Roa, Department de
IngenieraAgricola, Umrxsidad del Valle, Cali, Colombia.
“Principal investigator: T. A. Lawand. Brace Research
Institute, MacdonaldCollegeof McGill University, Ste. Anne
de Bellevue, Quebec, Canada.
for Solar Energy Utilization
convection, fresh air being drawn up through the
base. As a result, there is a constant perceptible flow
of air over the drying matter, which is placed inside
on perforated trays.
See-sawdrier (Ivory Coasf)4
The see-saw drier was originally developed in the
Ivory Coast for drying coffee and cocoa beans. Its
clesigrl was further refined under work sponsored by
the Government of Ghana and FAO. This simple drier
is suitable for small-scale drying and can be easily
operated. Its use is envisaged for tropical regions. The
see-saw operation permits the drying material to
receive more direct sunshine during the day,
increasing output and leading to a more evenly dried
product.
The drier consists of a rectangular tray framed in
wood and divided lengthwise into parallel channels of
equal width. Retaining bars are placed crosswise. The
bottom of the tray, on which the material to be dried
is placed directly, is made of bamboo matting. The
cover is made of a fi!m of transparent PVC, which
provides a substantial screening effect against
ultraviolet light, thus reducing photodegradation of
the product being dried. All the internal parts of the
drier are coated with flat-black paint.
The drying frame is mounted on a north-south
trestle whose height is equal to one fourth of the
frame length. The see-saw motion is thus in the
east-west vertical plane, so that the drying frame can
then be tilted to face east during the morning and
west during the afternoon.
The effective area of the drier is limited by two
transverse retaining bars futed at 200 mm from each
end of the drying frame and two others set 300 mm
apart in the centre of the crying frame. The three
small black bands thus delimited by the retaining bars
are left free from the product being dried; their
purpose is to convert the radiant energy from the sun
into heat. The heated air is circulated by natural
convection from the lower to the upper end of the
frame by means of gaps provided at each end.
Additional air is also drawn through the matting base
by the natural convective effect. The produce to be
dried is loaded, with the frame horizontal, up to the
level of the crosswise retaining bars, allowing
clearance under the cover to allow free flow of air.
Glass-roof solar drier (Brazil) ’
A glass-roof solar drier is similar to a greenhouse,
except that it has a special ridge cap made of folded
zinc sheet that allows the heated air charged with the
M.
Vincent,
‘Principal investigator: M. Richard and
Institut Franqais du Cafk et du Cacao, Paris, France.
‘Principal investigator: Biswa Nath Ghosh, Centro de
Pesquises do Cacav (CEPEC), Itabuna, B. A., Brazil.
.~‘_(’
,‘j~I
“’
,-
.,
/
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: :-,
The potential
of solar agricultural
129
driers in developing areas
Translucent
f,breg,au
top
Sunhght
Wood
accerrdoor
O?N
Figure 1. Air circulation in the glass-roof solar drier
moisture removed from the product to escape and
fresh air to enter through side shutters. Inside the
drier are rows of drying platforms along the long sides
and a central passagefor an operator. The long sides
are oriented north-south and all inside surfaces are
painted black to facilitate the absorption of solar
radiation (see figure 1).
The drying surface is made of galvanizediron wire mesh laid over wooden beams fixed across
the platform. Strong metal wires stretched perpendicularly over the wood beams and under the wire
mesh provide additional support to the loaded wire
mesh during the drying operation. This permits
heated air to pass easily through the wire mesh on
which the product is being dried.
In the prototype drier, gas heaters are situated
underneath one of the two rows of drying platforms
and are used only during rainy periods or at night to
shorten the drying time.
A free vertical space between the lower edge of
the glass roof and the outer edge of the platform
permits the proper ventilation of the drier. Six
wooden shutters on hinges are located in this space
along the length of the drier on each side, and they
can be opened or closed independently to regulate
the air flow inside during the solar drying hours.
A solar drier in use in the Syrian Arab Republic
has a unique air-circulation system. Air is drawn
through the drier by a wind-powered rotary vane
located on the top of a chimney. The temperature
and air&w
rate are controlled by a damper. The
drier is described in detail in the article by
Assad Takla, on page 7.
Solar fruit and vegetable drier (United States)6
Solar supplementary heat drying bin (United States)’
This simple chamber drier, operating in both the
direct and indirect modes, was designed to dry food
for domestic needs and for small restaurants. It has
been successfully used for drying a wide variety of
food products, ranging from fruits and vegetables to
herbs and meat. Figure 2 gives a section view of this
drier.
Air, pre-heated in a solar air heater located at the
base of the drier, is admitted at the bottom of the
drying enclosure. From there it rises through the
drying racks, dehydrating the product laid on them,
and is then exhausted, with its moisture content,
through openings located at the top rear wall of the
chamber by natural convection. Drying is also carried
out with the help of direct sunlight reaching the
product through the transparent sides, front and top
panels. The drier faces south.
A conventional bin drier can be transformed into
an indirect solar bin drier using the original structure
of the drying bin. Considerable savings in fuel
consumption will result.
The drying bin is oriented east-west, the side of
the roof facing south being used as the solar heat
collector (figure 3). This roof collector is sloped
about 30” from the horizontal and is designed to
produce an optimal temperature rise of 5”.12°C over
the outside air. The bin structure provides about
1 m2 of collector area for each 2 m3 of grain. This
ratio provides an acceptable drying rate for shelled
maize The roof surface is painted black to absorb the
-- -
‘Principal investigator: PeterVan Dresser, El Rito, New
Mexico, United States.
Rack
ClearPl.Sl%front
Eel---G---
ra------Warm au
Black sheet-metal
panel
i
e
i
Air in%-+
Figure 2. Section view of the fruit and vegetable drier
Solar wind-ventilated drier (Syriarr Arab Republic)’
‘Principal investigator: T. A. Lawand, Brace Research
Institute, Macdonald College of McGill University, Ste. Anne
de Bellevue, Quebec, Canada.
‘Principal
investigator: W. M. Peterson, South Dakota
State University, Depxtment
of Agricultural
Engineering,
Brookings, South Dakora, United States.
I,“,;
).^_
‘;,-
‘_.
,
~-
_-
I
,.,-
Technology for Solar Energy Utilization
130
Plastic suspended 7.6cm above roof
/
,.“
Solar heat collector
,
kide
Outside
air duct
Air
flow
-‘. act
Fan hole
(b Jcron-section
Figure 3. Solar supplementary heat drying bin
solar energy. A transparent plastic film is supported
about 8 cm above the roof by stretching it over the
framing members set edgewise. Air, drawn by a fan,
enters the opening along the roof ridge and moves
through the collector and down the south wall into
the outside air duct. From there the fan pushes the
warmed air into the inside air duct and through the
grain by way of a perforated floor. The bin is
designed to dry half its depth of shelled maize at one
time (1.22 m). The fan should be able to deliver air at
the rate of about 2 m3/min for each cubic metre of
corn to be dried, assuming the bin is full.
Large-scale solar agricultural drier (Barbados)’
The large-scale agricultural drier in Barbados was
designed to dry feed for livestock, primarily corn. It
operates m two stages handling 0.9 m3 (770 kg) of
freshly shelled corn per d.ay. The first stage utilizes a
mobile solar air-heated drying cart in which the
PPrincipal investigator:
Institute,
T. A. Lawand, Brace Research
Macdonald CoIlege of McGill University, Ste. Anne
de Beilevue, Quebec, Canada.
moisture content of the freshly harvested corn is
supposed to be reduced from 30% to 18% in the first
day of operation. The corn is then transferred to an
18-m3 capacity solar air-heated storage bin, where the
moisture content is gradually reduced to the
equilibrium moisture content of 13%. Particular
attention was paid to the air-flow design so as to
cause minimum pressure drop through all parts of the
air heaters and drying chambers.
A centrifugal fan blows air through a diffuser
duct into the solar field air-heated collector, which is
29 m long. The collector is fabricated from three
plastic sheets-the top transparent, the centre a black
mesh with 50% openings, and the bottom a layer of
insulation sandwiched between two films, the upper
one coloured black and the lower one, aluminium. The
sheets are sealed along the long edges, and supported
by tension straps every metre. The centre mesh rests
on a rigid screen stretched between posts in the field.
The collector is inflated on both sides of the
tensioned layer and heats the air blown longitudinally
through it. The end of the collector is connected to
the mobile drying cart. The latter is insulated to
reduce heat losses and fitted wit.h air-flow dividers
supporting a perforated drying floor. The fresh corn
is loaded into the cart, which is covered by a sloping
double-layered plastic roof.
When the moisture content has been reduced to
the required level, the corn is fed into a blower and
transferred to the storage-bin drier, where its
moisture is gradually reduced to 13%. This drier is
part of a farm building, one of the rooms being
converted into a drying chamber, with a plenum
chamber and perforated drying floor. The roof is used
as a solar air-heater collector.
Solar timber-seasoning kiln (India)’ ’
A solar timber drier was designed to increase the
drying rate of timber as compared with the
traditional air-drying method. This particular design
makes use of large quantities of warmed air (up to
60°C) and permits rapid drying without undue
degradation of the timber (cracks, warps).
The wood-frame structure of the kiln is oriented
lengthwise on an east-west axis, the higher wall facing
north. Except for the north wall, the whole structure
is covered with two layers of transparent polyethylene film separated by an air gap; the north wall is
sheathed with plywood. The roof faces south and is
tilted at an angle of 0.9 times the latitude above
horizontai (27”). The drying space in the kiln has
room for about 3.5 m3 of 25mm planking. Inside the
kiln, a false ceiling is installed above the wood stacks.
A false north wall running the entire length of the
kiln exten& from the floor to the false ceiling and is
’ ‘Principal investigator: S. N. Sharma and Prem Nath,
Wood Seasoning Branch,
Dun, U. P., India.
Forest Research Institute,
Dehra
:
~
;
.I
,;.
131
The potential of solar agricultural driers in developing areas
(a)
Plenum chamber
closed
(b)
Plenum chamber
opened
IL
Back fan cover
Vents open ------~-
Figure 4.
.- .- -.
1
Solar timber-seasoning kiln working as (a) a single-passforced-air drier; (b) recirculating air
drier with partial venting
provided with a hole in its centre for housing the fan.
The interior surfaces of the structural parts of the
kiln (studs, pillars, false ceiling and wall, north wall,
baffles and concrete floor) are painted black for
maximum absorption of heat. The fan is driven b.,
0.75 kW reversible electric motor and is used i 1
forced air circulation. The use of plywood baffles anti
movable partitions permits the drier to be used in
either the single-pass or the recirculating mode (see
figure 4).
BIBLIOGRAPHY
Davis, C. and R. I. Lipper. Solar energy utilization for
crop drying. Paper prepared for the United
Nations Conference on New Sources of Energy,
Rome, 21-31 August 1961. (Solar energy: II,
v. 5, p. 273 (S/53))
Sales no.: 63.1.39.
Dried Fruits Processing Committee. Grape drying in
Australia. Melbourne, 1973 (CSIRO, ISBN-O643-00054-4)
Ghosh, B. N. A new glass roof dryer for cocoa beans
and other crops. Proceedings of the International
Congress: The Sun in the Service of Mankind,
Paris, 2-6 July 1973. p. 17.
Green, L.
Solar heat
Electricity
M. and M. K. Selcuk. A solar dryer supplemented with auxiliary heating systems for
continuous operation. Solar energy (Elmsford,
N.Y.) 6:4, 1962.
Akyurt, M., M. K. Selcuk and S. Akyurt. A continuous solar dryer. Paper prepared for the
Conference of the Coop6ration mCditera&enne
pour l’bnergie solaire, Athens, October 1971. 4 p.
A two-pass solar air heater. By A. S. Satcunanathan
and others. Solar energy (Elmsford, N.Y .) 15: 1,
1973.
Bevill, V. D. and H. Brandt. A solar energy collector
for heating air. Solar energy (Elmsford, N.Y.)
12: 19-29, 1968.
Buchberg, H., D. K. Edwards and 0. Lalude. Performance characteristics of rectangular honeycomb solar-thermal converters. Solar energy
(Elmsford, N-Y.) 13: 95, 197 1.
Buelow, F. H. Drying crops with solar heated air.
Paper prepared for the United Nations Conference on New Sources of Energy, Rome, 2 l-3 1
August 1961. (Solar energy: II, v. 5, p. 267
(S/17))
Sales no.: 63.1.39.
Close, D. J. Solar air heaters for low and moderate
Solar
energy
temperature
applications.
(Elmsford, N.Y.) 7:3, 1963.
Akyurt,
-~ ------.-
for
on the farm
drying
magazine
shelled corn.
(New York)
47:4: 14-15, April 1974.
Gueneau, P. Une experience de sBchage solaire du
bois. Bois et for&s des tropiques (No;rent sur
Marne, France) 131:69-78, 1970.
Gupta, C. L. Draft for action plan for solar drying.
Pondicherry, India, Auroville Centre for Environmental Studies, 1973. (Occasional Paper No. 5)
Gupta, C. L. and H. P. Garg. Performance studies of
solar air heaters. Solar energy (Elmsford, N.Y.)
11:25, 1967.
Ivory Coast. Ministry of Planning. Institute for
Tropical Agricultural Products. See-sawsun dryer
perfected by ITIPAT. Abidjan, 1966. 20 p.
Jordon, R. C. Low temperature applications of solar
energy. New York, American Society of Heating,
Refrigeration and Air-Conditioning
Engineers,
1967.
Khan, E. U. The utilization of solar eaergy. .YoZar
energy (Elmsford, N.Y.) 8: 1: 17-22, 1964.
Khanna, M. ‘L. and N. M. Singh. Industrial solar
drying. Solar energy (Elmsford, N.Y.) 11:2,
1967.
Lalude, 0. and H. Buchberg. Design and application
of honeycomb porous-bed solar-air heaters. Solar
energy (Elmsford, N.Y.) 13: 2, 197 1.
<<>‘,~
..;.;
,,
<
,,
P.~“,~
.^_
I.
i-.,
^/
.:;-.
Technology for Solar Energy Utilization
132
Lawand, T. A. A description of two simple solar
agricultural dryers. Coop&ration mediterrantenne
pour 1’Cnergie soiaire, December 1965. (Bulletin
No. 9)
A solar-cabinet dryer. Solur energy (Elmsford, N.Y.) 10:4: 158-164, October 1966.
Laf, G. 0. G. Solar energy for the drying of solids.
Solor energy (Elmsford, N.Y.) 6:4, 1962.
Lukes, T. Research on the application of solar energy
to the food drying industry. San Luis Obispo,
Calif., California Polytechnic State University,
October
1974.
(NSF/RANN/SE/GI
42944/PR/74/3)
Maldonado, E. D. and E. C. Peck. Drying lumber by
solar radiation in Puerto Rico. Forest products
journal (Madison, Wisc.1 October 1962, 487-488.
McLean, K. Principles of sun drying with special
references to a see-saw dryer. Rome, Food and
Agricultural Organization, 1972. 40 p. (Report
AGS, SF/GHA7)
Peterson, W. H. Harness the sun to dry grain. World
farming (Kansas City, MO.) 1968, 36-37.
Solar heat for drying shelled corn. ASAE
(St. Joseph, Mich.) 73:302, 1973.
transactions
Phillips, A. L. A solar energy method for reducing
coffee drying costs. Journal of agriculture
(University of Puerto Rico) 17:4:226-235,
October 1963.
Drying coffee with solar heated air. Solar
energy (Elmsford, N.Y.) 9:4:213-216, 1965.
Evaluation of a “mylar” plastic film as a
surface for a solar heat collector. Journal of
of Puerto Rico)
agriculture
(University
49:4:484-486, October 1965.
Further observations on the use of solar
energy for reducing coffee drying costs. Journal
of agriculture
(University of Puerto Rico)
49121272, April 1965.
Read, W. R., A. Choda and P. 1. Cooper. A solar
timber kiln. Soiar energy (Elmsford, N.Y.) 15:4,
1974.
Roa, G. Natural drying of cassava. Ph.D. thesis. East
Lansing, Michigan State University, Agricultural
Engineering Department.
Roa, G. and H. J. Cock. Natural drying of cassava.
Proceedings of the Third Symposium of Tropical
Root Crops. Ibadan, Nigeria, December 1973.
Santa Barbara, CaIif. Community Environmental
Council. How to build a solar herb dryer. By
I. E. Thomas and M. ZiIIes. 10 p.
Selcuk, M. K., 0. Ersay and M. Akyurt. Development,
theoretical analysis and performance evaluation
of shelf type solar dryers. Solar energy
(Elmsford, N.Y.) 2: 81-88, 1974.
Shove, G. Low temperature drying of shelled corn.
Electricity
on the farm magazine (New York)
47:4: 16-17, April 1974.
Solar Energy Products Co. Soloron tir heater. Avon
Lake, Ohio, 1974. (Brochure distributed at the
International
Solar Energy Society Meeting,
1974)
Un nouveau type de &choir experiment&e en C6t.e
d’Ivoire. Cafp, cucuo, thP (Paris) 13: 1:57-64,
1969.
Whillier, A. Black painted solar air heaters of
conventianal design. Solar energy (Elmsford,
N.Y.) 8: 1, 1963-64.
Wilson, B. W. The role of solar energy in the drying of
vine fruit. Paper prepared for the United Nations
Conference on New Sources of Energy, Rome
21-31 August 1961. (Solar energy: II, v. 5,
P. 296 (S/4))
Sales no.: 63.1.39.
Publications
of the Brace Research Institute,
Macdonald College of McGill University, Ste. Anne de
Bellevue, Quebec, Canada
A survey of solar agricultural dryers. December 1975.
151 p.
Goldstein, 0. Production drawing for solar cabinet
dryer. June 1973.
Lawand, T. A. Description and construction cost
analysis of a solar agricultural maize drying
system in Barbados. December 1966. 65 p.
How to make a solar cabinet dryer for
agricultural produce. March 1966. 9 p.
Solar driers for farm produce. Progress
report. April 1963. 59 p
The drying of maize in a solar cabinet dryer.
January 1966. 8 p.
The evaluation of solar agricultural dryers
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The operation of a large-scale solar agricultural dryer. February 1967. 11 p.
Nahlawi, N. The drying of yams with solar energy.
July 1966. 16 p.
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and storage trials in Barbados. September 1965.
The potential
in developing
of solar cooking
areas
T. A. Lawand
Brace Research Institute, Macdonald CoSiegeof McGill University, Quebec, Canada
Several traditional fuels have been used in
developing areas for cooking. These include:
Approximate
Dried animal dung
Firewood
Charcoal
Coke
Kerosene. oil
calorijk
value
IMJIW
(Mcallkgl
9
20
33
30
42
22
48
80
12
10
Each of the fuels has a varying degree of cooking
efficiency. It has been estimated in India that the
effectivenes: of dried animal dung as a cooking fuel is
very low, about 1.4 MJ/kg (0.34 Meal/kg). Not all
fuels are as ineffective as this. In Senegal for example,
it has been estimated that 1 kg of butane gas has the
same heating effect as 24 kg of charcoal.
Another factor should be taken into account.
The traditional method of preparing food, which is
boiling it in a large pot on an open fire, a gas stove or
an electric hotplate is in itself very inefficient.
Because of their high water content, most foods
have a high specific heat, close to that of water
(4 kJ kg-‘“C-r).
The higher the rate of heating the
faster the food will he heated to the cooking (boiling)
temperature. Then, except when water vaporization is
essential to the cooking process, as in baking bread,
the speed of cooking is practically independent of the
heating rate. as long as the temperature is maintained
by a heating rate equal to the rate of thermal loss.
Thus, the differences in the times required for
cooking equal quantities of food on cookers having
various capacities for heating are due mainly to the
different lengths of the heating-up periods. Thus
cookers with high heating rates may require almost
the same amount of time for cooking foods that must
be cooked for several hours as cookers with low
heating rates.
The large& of the heat losses in cooking is
usually the heat consumed in vaporizing water
present in the food or added for cooking, nearly
2.5 MJ/kg. Next in importance are the convection
losses from utensils and oven walls. These losses can
be cut down by the use of covers on the utensils, and
of insulation on the oven walls. Estimating an hourly
convection loss (outdoors), at boiling-water temperature, of about 6.8 MJ per square metre of utensil, and
a surface area of 0.1 m* per kilogram of container
contents, the enera input for 1 h of food boiling, if
one fourth of the water present is vaporized, would
be distributed roughly as follows (%):
Heating food to boiling temperature
Convection losses from vessel
Vaporization of water
20
45
35
Although variation in the assumed conditions would
materially alter this distribution, the figures would
still show that most of the heat supplied in
long-duration cooking is dissipated.
The whole question of the transference of energy
from the heat source to the cooking pot needs to be
reexamined in order to improve the efficiency of this
operation. Until boiling occurs, the coefficient of
heat transfer into the cooking pot is low because the
liquid is relatively stagnant. Food often bums, hence
the necessity of stirring not only to increase the
transfer of heat but to avoid localized concentration
of heat. The mechanism of heat transfer into the
cooking pot, even the shape of the cooking pot, has a
profound effect on the design of solar cooking
devices. In developing solar cooking technologies, the
classical inefficiencies found in existing systems
should not be repeated, especially in developing
countries, where cooking may account for as much as
80% of all energy used.
Solar cookers similar in capacity and size to
classical cooking systems are needed. The rate of
energy supply in conventional cooking systems, is
fairly high, in spite of their inefficiency. Where
electric or gas cooking is used, the normal burner
supplies energy at the rate of approximately 1 kW
and is capable of bringing 2 1 of water to boil in about
10 min. Automatic ovens for roasting or baking may
have an installed capacity of approximately double
that. Therefore, a solar unit would have to have an
energy delivery rate of roughly 1 kW to be
comparable with existing systems. The alternative
would be accepting longer cooking periods and
133
Technology for Solar Energy Utilization
134
possibly cooking smaller amounts of food at one
time. A solar collector area of about 2 m2 would be
necessary (at 50% collection efficiency) to give
comparable normal cooking rates.
Several basic types of solar cookers have been
developed to date. A brief review of the bibliography
will reveal the degree to which researchers in
developing areas have devoted themselves to this
subject. In developed countries, except for groups
interested in appropriate technological systems, most
attention has been paid to small production units for
use by campers or weekend travrllers. These units,
either collapsible umbrella reflectors or hot boxes, are
designed to be portable and compact.
Three types of solar collectors exist:
(a) Solar hot box, an insulated solar cooker with
double glazing, generally in a form of a box set out in
the sun and oriented manually (figure 1). To increase
efficiency, reflectors are often added that make it
possible to obtain higher temperatures in the interior
cooking chamber. One of the inherent disadvantages
of this type of cooker is that cooking must be done in
the open, a feature that is often socially unacceptable, particularly during periods of warmer weather;
(b) Parabolic reflector solar cooker that concentrates cooking rays on a focal point or area on which
a cooking pot or frying pan is placed (figure 3). Again
this process requires cooking outside. There is an even
greater heat loss than with the hot-box cookers owing
to convection from the wind. The reflectors can
effectively concentrate only direct radiation, with the
result that these units are less efficient in areas having
high percentages of diffuse radiation;
(c) Detached solar collection and cooking
chamber unit. In these units the heat-transfer fluid,
whether water converted to steam or heat-transfer oil,
is heated in a separate collel:tor, whether of the
flat-plate type, as is t1r.z)case with the solar steam
cooker, or the concentrating type, as is the case with
Figure 1. Solarhot box asa low-costoven
Figure 2. Parabolic reflector solar cooker, adjustable umbrella type
some of the hot-oil heaters. The heated fluid is
transferred to a separate, insulated cooking chamber,
which can be located on the inside of the house
where the cooking is done. In this way it can reduce
the social inconvenience of external cooking. (See
figure 3.)
These systems have not been extensively field
tested to date. Often the limiting technical factor is
the transfer of heat into the cooking pot, though this
factor is weaker in reflector cooking owing to the
higher temperatures generated.
Figure 3. Solarsteamcwkex
.i
., ,
I;;
135
The potential of solar cooking in developing areas .*-. .
COMPARATIVE COSTS OF COOKING WITH VARIOUS ENERGY SOURCES IN SENEGAL
Bu tune
(rechargeable
reservoirs
with
gas burners)
Electricity
(urban
arear)
19
15
15
15
2.09
0.32b
2 700C
865
867
Capital investment ($1
Estimated life of equipment (y)
Annual capital recovery charge’ ($1
Price of fuel (S/kg)
Estimated annual fuel consumption (kg)
Total annual fuel bill ($1
Annual maintenance cost ($1
Total annual cost ($1.
Total annual cost if fuel price doubies ($1
Total annual cost if fuel price triples ($1
Note: All costs are calculated on an equivalent
“Assuming 11% interest.
2.64
0.44
220
97.80
1.90
101
198
295
annual
Charcoal
(urban
areas)
10
15
1.39
0.10
1 750
175
176
Charcoal
(rural
areas)
Solar radia tiorr (solar
cooker with auxiliary
butane burner)
Model I
Model 2
10
150
200
15
15
1.39
0.06
1 750
105
106
1 732
351
211
2 597
526
316
20.86
0.44
44
19.40
22.88
66
85
105
15
27.81
0.44
44
19.40
30.38
81
100
119
cost basis.
bin dollars per kilowatt-hour.
Cln kilowatt-hours.
No comprehensive study of solar cooking
technology has yet been made. It would be useful to
coIlate all the existing information on cooking needs,
available energies and similar types of problems so
that it might be possible to determine what role solar
cooking can play in meeting the energy needs of
fuel-scarce societies. So many techniques have been
developed that they need to be put into a
compendium that can serve to assist societies in
developing areas to either make use of these technologies or modify and adapt them to existing situations.
Programmes such as biogas conversion, which is
actively being pursued in countries such as India,
should be coupled with an investment in solar
cooking technology, since the generated biogas has
too high a potential to be wasted in cooking. It would
be best used to generate mechanical shaft power or
electricity, as has been demonstrated in India..
Currently it must be used for cooking, since there are:
few alternatives.
Finally, with regard to economics: It was showl,
in Senegal that solar cooking with a smokeless woo,1
stove as a back up (for the 10%15% of the time whedl
climatic conditions did not permit using solar energy)
was far less expensive than using butane gas or
charcoal. An example for Senegal’is given in the table
above, using two models of a solar cooker with an
auxiliary butane burner. It is seen that making a
capital investment in these units would in the long
run be beneficial to the users.
BIBLIOGRAPHY
Abou-Hussein, M. Temperature-decay curves in the
box-type solar cooker. Paper prepared for the
United Nations Conference on New Sources of
Energy, Rome 21-31 August 1961. (Solar
energy: II, v. 5, p. 335 (S/75))
Sales no.: 63.1.39.
Alward, R., T. A. Lawand and P. Hopley. Description
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1973. P. 10.
Une cuisinibre solaire en Hai’ti. Architecture
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30:4, 1963.
Beason, R. G. Solar cooking turns practical. Mechanix
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Duffie, J. A. Reflective solar cooker designs. University of Wisconsin, Engineering Experiment
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Duffie, J. A., G. 0. G. Lijf and B. Beck. Laboratory
and field studies of plastic reflector solar
cookers. Solar energy (Elmsford, N.-Y.) 6:94-98,
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P. 9-13.
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Station.
Engineering
(Reprint 327)
Plastics in solar stoves. Modern plastics
(New York) November 1957.
Food and Agricultural
Organization, Nutrition
Division. Report on tests conducted using the
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and industrial
research (New Delhi) 12A:4: 165-175, 1953.
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Technology for Solar Etrerg?~ Utilizatiorl
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Ghai. M. L. and
reflector-type
B. S. Phandher. Manufacture of
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scientific
and industrial research (New Delhi)
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Ghosh, M. K. Sun cookers for villages. Paper presented for the All India Solar Energy Working
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Gupta, J. P. Studies on solar hot box. Paper prepared
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Madras, November 1973.
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1960.
Khan, E. U. The utilization of solar energy. Solar
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G. 0. G. Recent investigations in the use of solar
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1976. 12 p. (764021)
Performance of reflector-type direct solar cookers. By
M. L. Ghai and others. Journal of scientific and
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1953.
Rata, Salgado. A cylindro-parabolic solar cooker.
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(S/l 10))
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Swet, C. J. A universal solar kitchen. Baltimore,
Johns Hopkins University, Applied Physics
Laboratory, July 1972. p. 23.
Tabor, H. Solar cooker for developing countries.
Paper prepared at the annual meeting of the
Solar Energy Society, Boston, Mass., March
1966.
Telkes, M. The solar cooking oven. New York
University, College of Engineering, Research
Division, January 1958.
Telkes, Maria und S. Andrassy. Practical solar cooking
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Conference on New Sources of Energy, Rome,
21-31 August 1961. (Solar energy: II, v. 5,
p. 394 (S/101))
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University of Florida. Solar cooking turns practical.
Mechanix illustrared (New York) July 1976.
Vita Union College Campus. Solar cooker construction manual. Schenectady, N.Y., June 1967.
Whillier, A. A stove for boiling foods using solar
energy. Sun at Work. 10: 1:9-l 2, January 1965.
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of the Brace Research Institute,
Macdonald College of McGill University, Ste. Anne de
Beilevue, Quebec, Canada
AlwLrd, R. and 0. Goldstein. Assembly drawings for
the construction
of solar steam cookers:
3 drawings. February 1972.
A study of the feasibility of establishing a rural
energy centre for demonstration purposes in
Senegal. August 1976. 380 p.
Cheng, K., H. Wong and 0. Tanaka. Experimental
study of a solar steam cooker. Project for the
Department of Mechanical Engineering, Course
No. 305-463A-464B, May 1973,47 p.
Greenwood, C. Effect of heat input from solar
cookers on the ascorbic acid content of peas.
Final year project for the Department of Food
Science, Course No. 344490D. March 1974.
56 p.
Pons, M. F. and G. T. Ward. La cocina solar construide en Chucuito, Puno, Peru. Diciembre
1963. 22 p.
Sinson, D. A. Design and performance evaluation of a
6 ft X 4 ft parabolic solar steam generator and its
application to pressure cooking. April 1964.
67 p.
Vickery, S. The effect of the sunbroiler on the
ascorbic acid content of sweet peas. Final year
project for the Department of Food Science,
Course No. 377-490D. March 1974. 57 p.
Whillier, A. A stove for boiling of foods using solar
energy. April 1964. 13 p.
How to make a solar steam cooker. Rev.
February 1973. 2 p.
Preliminary report on solar stove for
cooking by boiling. September 1963. 10 p.
Assessment’ of solar applications
f&r technology transfer
Jyoti K. Parikh
International Institute for Applied Systems Analysis (IIASA), L.axenburg, Austria
Introduction
As the intensity of solar radiation is low, a
considerable amount of land is required for utiiizing
solar energy. Solar applications would therefore be
more suitable for a rural environment where land is
easily available rather than for urban areas.
Conventional centralized energy systems have not yet
reached the large rural population in many developing
countries. Solar applications which contribute to the
development of a decentralized energy system could
lead to significant improvements in the economic
productivity of rural areas.
According to United Nations’ estimates, in 1970
the rural population of the world was 2.26 billion,
1.89 billion of whom were in the developing
‘United Nations, Department of Economic and Social
1975
Affairs, “Selected world demographic indicators”,
(ESA/P/WP.SS).
countries. The percentage of persons living in the
rural areas of the developing countries is expected to
decrease from 75% in 1970 to 59.2% in 2000.
However, their absolute number will still be
2.92 billion, a substantial increase over the present
number. Figure ! shows regional rural population
growth trends, as projected by the IJnited Nations.’
The energy requirements of rural peopie,
although extremely low, are largely met at present by
locally-available non-commercial resources such as
firewood, agricultural waste and dung. Nevertheless,
government energy planners in most of the
developing countries are concerned primarily with the
development of large energy systems, appropriate for
urban and industrial purposes. Although efforts are
being made in most of the developing countries to
expand rural electrification, its progress is slow
because it is capital intensive, especially when it
‘/bid
2ooo
c
Asia
lCentrally planned
economws)
Africa
-
Latin America
1950
1960
1970
1960
1990
2000
Figure 1. Past and projected rural population of developing regions
137
Technologyfor Solar E1lerg.yUtilizario,l
138
involves connecting remote villages to the network.
Thus, then is a need for developing decentralized
energy systems for rural areas.
Scientists and technologists do come up occasionally with solutions for decentralized energy
systems. When these are not adopted they generally
complain about the difficulties of technology
transfer, resistance from established interests etc.
Though those obstacles are not to be underestimated,
their claims about the relevance of their researchand
development are many times found not to be valid
for actual adaptations in field conditions when all the
facts are put together. Therefore, a careful appraisal
of the diffkuhies of the transfer of technology is
essential.
In this paper we consider first the issuesthat are
important in assessingtechnology. Then some of the
solar [email protected] are evaluated keeping these issues
in mind. Since photovoltaic cells are very expensiveat
present, we have considered for this analysis only
decentralized low-power thermal devices In particular, solar pumping is evaluated in detail as a case
study.
The algebraic expressionsintroduced here can be
used for qplication to any country; numerical results
are given for the spetic caseof India.
Thus, the cost-benefit analysis should be done
also from the user’s point of view together with the
analysis from the social viewpoint. One then
identifies the loss, if any, that the user would have to
incur and to what extent society might subsidize him
in view of the indirect costs it would have to bear if
the new technology were not promoted.
Comparison with other alternatives
The economic benefits to the user should be
calculated keeping in mind the best possible
alternatives open to him. For example, if the
advantagesof biogas plants are calculated by taking
petrol or even coal as the alternative, they would
appear substantial. But the comparison actually has
to be made with the cheapest possible alternative, i.e.
burning dung and purchasing fertilizer, if needed. The
positive and negative aspects of both alternatives
should of course be carefully weighed. Only then can
one understand why certain innovations are not
catching on. In addition, possible future developments of the existing alternatives should also be
considered.
Scale of technolog)l
ti
invoIvuI in the techndogy transfer
Here “techndogy transfer”
. ..,_..-Imeansthe transfer of
invention jmg.a labo%ory @ tfie ,fj&%-?‘.iGi&i&i~ that 3ij Usersc~nnqt~~~ an [email protected];l
-_xIenergy.gyft&m, Due weight has to be given to the
-$zection of the invention and the development of
required institutions, such as establishments to look
after the problems of the user. The user’s viewpoint
could be dassifiid into two categories: technoeconomic and social, with the latter referring to the
operating environment in which the technology has
to be used. In geasral, the following points need to be
considered.
Riwte and social benefits
The benefita, savingsetc. are often calculated on
national, state or village levels and not for the
consumer who is ping to use the technology. Though
benefits at the national level, such as the saving of
foreign exchange, curbing environmental degradation.
overaIl h&-h effect6 etc. are important, they ‘ire
meanin@ anty If the nv technology is acceptable
to the wer. If the user does not benefit when an
invention needs to be promoted for national or social
purpcue~, he IIS to be compensated if he is to be
induced to uoc it. ‘I%& means that a national policy
invalving aukdies, fuuncing facilities, tax rebates
etc. has to be introduced to promote better
technology.
Some technologies may turn out to be unsuitable
in economic and managerial terms if the proper scale
is not chosen. For example, in some situations many
small solar pumps may be more expensive than a large
pumping station. Yet the small pumps may be
preferable when the managementproblems associated
with the different scalesare considered. Again, giving
an example of biogas technology, an earlier analysis3
shows that a community biogas plant may be more
economical and socially desirable than family biogas
plants.
Introduction
of technology
The manner in which a technology is introduced
determines its success. For example, groups which
benefit less or are adversely affected may offer
resistance. Besides, at the planning stage itself,
problems of co-operation, maintenance and repair
would have to be dealt with.
Compatibility
with the environment
If an invention requires a change in life-style or is
in conflict with the surroundings, it will face
difficulties in its adoption. In such a case, the
‘J. K. Parikh and K. S. Parikh, ‘The potential of bio-gas
plants and how to realize it”, Proceedings of UNITAR
Symposium on Microbial Energy Conversion, Gijttingen,
Federal Republic of Germany, 1976.
.
Assessmenrof solar applications for technology transfir
139
practical considerations. An attempt will therefore be
made to analyse the difficulties of technology
transfer for one of the applications of solar energy
within the context of the above-mentioned criteria.
Although the general framework of the analysis is
applicable to any country, a case study of India is
carried out.
strength of the existing establishment of older
technology should be carefully assessed, and the
question whether society is ready for the change
should be considered.
Acceptance of technology
An invention has to be appropriate for the kind
of use for which it is meant. For Lxample, as will be
demonstrated later in this paper, there is a need to
consider the manner in which pumps are currently
used when designing a solar pump for agricultural
purposes.
It is therefore necessary that a Government with
limited resources should evaluate new technologies
carefully so that only appropriate ideas are
encouraged. The development of inappropriate
inventions may waste precious scientific manpower
and limited research funds and also cause a loss in the
credibility of new technologies in general. Even
though they made good sense, many new technologies have failed because of the neglect of simple,
TABLE
-
Solar pumps
Solar pumps for irrigation purposes would be a
significant application of solar energy for the
developing countries, where 40% to 50% of GNP
originates in the agricultural sector, for which water is
an essential input, Table 1 provides data on energized
pump-sets and their electrical energy consumption in
India. About 9% of the total electricity consumption
in India is accounted for by energized pumpsets
alone, despite the fact that hardly 20% of the villages
were electrified in 1967, as shown in table 2. The
number of pumps required in the next two to three
decades may be more than 10 million. Table 2 also
1. PUMP-SETS AND ELECTRICAL ENERGY CONSIJMP’TION
Past and projected trends
IN INDIA
Consumption
By pump-sets
(10’ kWh)
Total
(10’ kWh)
Per pump-set
fkwhl
Per unit of
connected
Iwd (kWh
per kWJ
2.107
2.585
3.466
3.770
4.110
33.26
36.76
41.46
45.02
48.65
3 245
3 050
3 183
2 809
2 503
842
814
834
738
657
Year
Number of ,“ets
in opemrion
Ze
1966167
1967168
1968/69
1969/70
1970/71
31 March 1974
31 March 1979
649
847
1088
1 342
1 642
2 444
4 022
182
357
774
006
006
599
790’
Projections b
1983
1990
2000
6.5 X lo6
12.0 x 106
20.0 x 10’
Sources: India, ‘hiinisiry of irrigaiioti ai1d Po~ei, ,‘<iiir;i Aixi&
?~:ci-r Commirree
Second India Studies: Energy (New Delhi. McMillan Press, 1976), p. 5 5.
‘Target
bTaking
potential.
figure.
into account
population
TABLE
Village
pop&a tion mnge
(I 961 census)
2.
growth
and the r+d
ELECTRIFICATION
for additioaal
OF VILLAGES
food
Report. New Delhi,
production
1% INDIA,
conristcnt
with
1972; K. S. Parikh,
the ground-water
1961-1973
Number of villages electrifled
on 31 March
Number of
villages
1961
1966
1971
1972
1973
2 000-4 999
5 000-9 999
> 10 000
351 653
119086
65 377
26 565
3 421
776
3 986
4306
5 918
5 458
1 319
560
10 265
9 787
il 567
9441
1 963
647
31 518
26 436
25 715
17 036
2 674
702
39 730
32 602
27 971
18 326
2 753
712
46 665
37 880
31 S86
19 922
2913
729
Total
566 878
43 670
104 081
122 094
139 695
G 499
501F999
1000-l
999
Source:
Central
Electric
Authority,
21547
New Delhi,
1974.
140
Technology
shows that the rate of electrification for small villages
of 500 inhabitants is much slower than that for the
large towns. In view of the slow electrification of the
rural areas, the importance of the agricultural sector,
and the high projected pumping needs, solar pumps
could play an extremely useful role.
different availabilities at night of the two alternatives
being compared. Before this problem is considered.
an exact formula for the discounted ccst instead of
the approximate form ( 1) will be given.
Discounted cost for a giver7period of service
The discounted cost of installing capacity in the
initial period for any option would be as follows:
Techno-economic comideratio?s
In order for solar pumps to be acceptable, they
should provide adequate pumped water and be
cheaper than the existing alternatives or sufficiently
convenient for farmers to be willing to pay a higher
price.
A general framework for such a techno-economic
comparison between any two alternatives is developed below. The symbols used in the calculations are
these:
Capital cost
Discount rate
Discounted cost
Lifetime of pump
Number of pumps requited for a period
T of service
Annual maintenance cost
Price of fuel
Distance
Time
Amount of fuel used annually
Annual operating cost (= m + pq)
Efficiency
Operating time (hours per day)
Installed capacity
Collector area
Average daily solar radiation per unit area
Work done
for Solar Energy Utilization
K
d
c
1
n
m
P
s
t
4
0
C=K+
i
A!!--
i=l (1 +d)’
In order to compare solar and diesel engines,
the> must provide service over the same period T,
since the lifetimes of the options may be different.
The period T is chosen so that
ns I, = nd Id = T
Thus, if the lifetimes of the solar and diesel
engines are respectively 20 y and 5 y one would
Ipqwire four diesel enginesto provide the sameservice
2s tine solar engine, with a new investment every 5 y
w&h would have to be discounted on the initial
period:
h”
2
S
W
When necessay, the subscripts s, e and d will be
attached to the symbols to refer specifically to solar,
electric and diesel pumps.
There are many ways in which the costs could be
worked out, some of which are illustrated below. If
electricity is available, the solar pump would have to
compete with electrical pumps; if not, with diesel
pumps.
Average annual cost per unit of installed capacity
It is assumed that the annual cost of the loans
made to finance the installation would be equal to
half the rate of the interest d (discount) plus the
operating costs. Neglecting inflation, but taking
depreciation into account, the following equation is
obtained:
A similar expression has been derived by Takla.3
Mowever, such an expression does not consider the
’ Seearticleby Assad Takla.p. 7.
I
4
.
2oy ~
I
A general formula for discounting an investment
every 1years over the period T = nl is
czj;,
1
T/l
( K+;Z1 (1
Oi
>
1
(1+6)0‘-I)[
(4)
On the other hand, without storage capacity the
solar pump cannot be operated for the samelength of
time each day as the diesel pump. In using the
formula, the work that would be done by both these
pumps in a day has to be taken into account. The
following two casesare considered.
Slow rate of pumping. In some areas the rate at
which water recharges may be slow, and hence there
may be an effective limit to the rate at which water
can be pumped. In this case,we have to compare two
pumps of the same capacity. Since the solar pump
can be operated for only about 6 hours a day and the
diesel pump for 18-20 hours it means that the two
pumps cannot be compared. In fact, the solar pump
without adequate storage may npt be considered a
feasibleoption in this case.
Comparison of equivalent work. A solar pump
works with an average efficiency of qs for h,
equivalent hours of full capacity (see figure 2) where
,.I
I
Assessment of solar applications for technology
!
I
Inaolatlon/
141
transfer
.H .-‘W.
\
*.
Solar engine
/‘
Equivalent output
of solar engine at full capacltv
g =I
P
% /
$
2
Cd
-
engine
c
OL
2400
12bo
07bO
Time of day
Figure 2. Diurnal variation of insolation and of the output of solar and diesel engines
C, for equivalent work and the required collector
area.
an average daily solar radiation per unit collector area
of S is available. The collector area required for the
installed capacity c, is A. The daily work done is
IV, = rj SA
Numerical comparison between the alternatives
(5)
Having developed a general framework for a
techno-economic comparison, the alternatives open
to a user will be compared. In doing so, various
elements of uncertainty should be considered, such as
possible efficiency improvements, the cost of solar
pumps, the escalation of diesel prices etc.
The diesel pump,& the other hand, can operate
for a much longer time, say hd hours. This may mean
higher consumption of fuel but better utilization of
the installed capacity, which is denoted by cd. The
daily work done in this case is
3
W, = h, c,,
!6!
Stipulating that
Iv, = w,
(7)
we have
9, S-4 = h, c, = h, cd
i@
Present design and feasible recimicai improvements
Low-temperature pumps operating only on
temperature differences will not be considered
because the technology is not yet developed enough
to give the required output, and because the main
concern is a pump for agricultural needs. Instead,
consideration will be given to an engine-driven pump,
such as the one shown in figure 3.
Here vs = wp, where qC and vp are the efficiencies
of the collector and the pump respectively. Equation
(8) makes it possible to determine both the value of
Boiler
\\
\,,<
\ \ “‘1
41
Cmxlatlng
(3
rnp water
L
,
rI
~aOlttiw
fluidi
Cunden*i
Irrigation
Figure 3. Schematic diagram of a solar pump
pump
“”
‘6%
a;@:.’
_-.I
t;,,,, *.I,’*../ : I‘), “/_ I(>,..;,” ,‘_(
“, ,),.k :.: . .;.. :_
~‘4+~I ,, \ ‘. i ,~.b7,.,., ” I.2
.
Technology
142
If a manual tracking system operating with
concentrators utilizing Fresnel lenses is developed-a
realizable goai-and other improvements in the design
of solar engines and collectors are made, we can
expect an efficiency of 10% in the near future.
Solar Energy Utilization
that a reduction in price by a factor of 25 to 70 is
required before a solar engine would be economically
acceptable for driving an irrigation pump. If the
pump is only to be used for obtaining drinking water,
then it can be of the same capacity as the diesel pump
and may run only 4-6 hours per day. From the table,
it is seen that a price reduction by only a factor of
IO-20 is required in this case.
Numerical values
-_
for
To the advantage of the solar pump, it will be
assumed that electricity is not available in the region,
and that the alternative is to use a diesel pump.
Considering the data in table 1, it seems that on the
average, a farmer’s requirements are met by a pump
of cd = 4 kW running 1 000 hours a year. A solar
engine and a diesel engine required to drive such a
pump will be compared. The lifetimes I, and Id are
20 y and 5 y respectively. The capital cost Kd is
about 6 000 rupees ($600) for a 4-kW diesel engine
(K&k,, = 150 $/kW). Further
assumptions are:
0, = m, = $50; md = $50 plus cost of lubricant
($20) = $70, and p&d = $150, giving Od = $220.
Operating conditions in the field are such that
pumps have to run 18-20 or even 24 hours a day. A
diesel engine can be run round the clock, whereas a
solar engine without storage may be run for only 6-9
hours a day. Using equation (7), we find that the
capacity of the solar engine would therefore have to
be 2-3 times that of the diesel engine. These two
possible capacities, with three scenarios of oil-price
escalation-annual increases of 0%, 5% and 10%-were
therefore considered.
Equation (4) with d = lo%, was used to
calculate discounted costs at constant (current)
dollars; only price increases over and above inflation
were taken into account. The results of the analysis
are summarized in table 3.
The market price per unit capacity, of a solar
pump is 15 000-20 000 $/kW. From table 3 it is seen
Validity of assumptions
Most of the assumptions made in the above
analysis are quite generous to the solar engine, as can
be seen from the following considerations.
Technical assumptions. A solar engine with a
lifetime 1, = 20 y is not yet available. Moreover, a
solar engine with twice the capacity of the diesel
engine would also require a hydraulic pump, driven
by the solar engine, of twice the capacity of that used
by the diesel engine. This additional cost of the
hydraulic pump for the solar engine is not considered
in the calculation.
The present analysis assumes an operating time
h, = 6-8 h without storage. So far, no solar engine has
achieved h, = 8 h even with storage. The engine
designed by the National Physical Laboratory (India)
has h, = 4 h with storage. Storage requires additional
collector area as shown in equation (5), the costs of
which should also be included.
If adequate storage were to be provided so that
the solar engine could be run with h, = 18 h, the
capacity of the solar engine need not be larger than
that of the diesel engine. The break-even cost per unit
capacity of such a solar engine, including the costs of
collectors and storage, can be as high as 1 180 S/kW.
Economic assumptions. Although an electrical
pump provides a cheaper alternative, the cost
TABLE 3. COST COXPARISON OF SOLAR AND DIESEL ENGINES IN PUMPING SERVICE
Annual increase in price of diesel
Item
0%
5%
oil
10%
Discounted costs of diesel engine. T = 20 y
Capital cost
Operating cost
i 348
i 872
i 348
2 543
I 348
3 a25
Total Cd
3 220
3891
5 173
(425)
2 795
(425)
3 466
(425)
4748
349
233
(S/kW)
433
289
593
395
698
866
Less discounted operating cost of solar engine
Total breakcven discounted capital cost of solar engine
Break-even cost per unit capacity
In irrigation service
Cs=2Cd=8kW
Cs=3Cd=12kW
In d:ir&iig gater service oniy
c,=Cd=4kW
w-
1 186
Assessmentof solar applications for technology transfer
143
comparison has been made with a diesel pump. Since
the analysis is concerned with a solution which could
be adopted nation-wide, the question of the
unavailability of diesel fuel in individual remote areas
has not been considered. These areas might find the
solar pump useful in the near future, especially for
drinking water, as it may be the only feasible
technology. However, we do consider the case of an
eightfold increase in diesel fuel prices (10% annual
increase) over 20 years relative to other prices, which
are kept constant in the analysis.
However, if the solar pumps are manufactured in
the developing countries, they could be cheaper than
current quotations. For example, the pump developed in the laboratory in India4 has a material cost
of 1 200 $/kW. However, much progress is to be
expected; and it remains to be seen what the costs of
a commercial solar pump would be in the developing
countries.
solar radiation, utilization efficiency and utilizable
solar energy are given for two places, namely Nagpur,
Madhya Pradesh (central India), and Jodhpur, which
is in the western region near Rajasthan Desert. The
table shows that in Nagpur the utilizable energy drops
by a factor of 5 between the months of May and
August. In fact, these are the months when water is
required for cultivation. The reason why the solar
radiation drops is that it rains in this period. In case
the rains are delayed and it is nevertheless cloudy, the
installed solar pump may not be useful, unless the
collectors also collect diffuse radiation and the water
requirements are met.
Atmping pattern
In hot regions, some of the farmers may prefer to
pump during the evening or night-time so as to save
loss of water due to evaporation. In such cases,
storage may be essential.
Operational problems of solar pumps
Given a solar pump which is of comparable cost
to the other alternatives, it may be asked what are the
other factors that need to be considered. Among
those that have been ignored in the analysis above are
climatic variations, the desired pumping pattern and
the availability of land for collector installations.
Availability
of land for solar collectors
The collector area required for a pump of certain
capacity working a given number of hours a day is
obtained from equation (8):
A = h, drl, S
This could mean a collector area of 100 m2 for a
4kW pump.
Climatic and local vat-&ions
The intensity of solar radiation changes from
month to month. The efficiency of utilization
depends on the radiation intensity, the temperature,
the cloud cover etc. In table 4, monthly variations of
’ See article by V. C. Bhide, p. 55.
In the developing countries, farms are small in
size and an average farmer may not be willing to
allocate even a small portion of extra agricultural land
for the collectors when the area involved exceeds that
required for alternative pumps. If the collectors are
placed in such a way that sunlight for crops is
TABLE 4. ANNUAL SOLAR RADIATION VARIATION AND TYPICAL EFFICIENCIES IN TWO INDIAN CITIES
Nagpur
Number
of
Month
days
January
February
March
April
May
June
July
August
September
October
November
December
TOTAL
source:
hltiribuffon
Jodhpur
To tacrl
A vemge
daiiy
mdia tion
efficiency
availability
(MJim‘)
/%)
/MJlm’/
31
28
3:
31
30
31
31
30
31
30
31
19.3
21.3
23.9
25.5
26.4
20.1
16.7
15.9
20.1
20.9
20.1
17.6
88
a3
76
72
70
40
23
23
51
69
87
84
energy
530
500
560
550
570
240
120
110
310
450
520
460
A verage
daily
mdia tion
IMJ/m ‘)
UHliza tion
efficiency
1%)
17.2
20.1
23.4
26.4
28.5
28.5
22.2
20.1
22.6
21.3
la.4
lS.9
4 920
365
7he data on radiation
Utilira tion
and utih3tion
efficiency
are adapted from G. 0. G. Liif, J. A. Duffie
of Wisconsin, Solar Energy Laborntory,
July 1966).
of So&r Radiation (University
84
84
77
78
83
73
48
43
54
a4
91
88
Total
energy
availabiliry
(MJlm’ I
450
470
560
620
730
620
330
270
370
550
500
430
s 000
and C. 0. Smith,
World
obstructed, then it may not be a preferred alternative
unless the farmer is willing to grow certain types of
vegetables which can be grown in the shade under the
collector and other crops on the remaining land.
Otkr factors
Some other factors to be considered when
developing a solar pump are these:
(a) Compatibility of possible peak load with the
quantity of water required, i.e. the amount of water
pumped in comparison with its requirement over a
day;
(b) The availability of spare parts and necessary
services and the availability of skills for repairs;
{c) Compatibility
of water-table with the
possible capacity of the presently available pump
(however, if the rate of water recharge is small, the
pump would have to run at low speed and
continuously).
Summary
For the large and increasmg rural population in
the developing countries, decentralized solar energy
applications would be quite relevant. An especially
important application could involve the solar pump,
in view of the additional food required to support
growing populations. In India, the number of
energized pump-sets may in the coming decades
increase from 2 million to 20 million.
Crop yields depend primarily on the availability
of water at certain periods of the year. A solar pump
would therefore have to be designed to meet
irrigation requirements under all possible field
conditions. That means that a solar pump must have a
higher capacity to do the same amount of work than
a diesel pump. The foregoing analysis, which takes
into account fuel price escalation, shows that the
break-even cost of a solar pump is in the range
250-600 $/kW. The current cost of a solar engine is
higher by a factor of more than 20. However, this
cost could come down if the engines were
manufactured by developing countries. If the engine
is installed for obtaining drinking water, then six to
eight hours of running time per day may be
sufficient, and hence it could therefore be of the
same capacity as the diesel engine. In this case, the
break-even cost could be 1 200 $/kW. (Of course, if
neither diesel nor electricity is available in some
remote area, a solar pump might be the only
solution.)
Moreover, even when economic solar pumps are
developed, other factors based on climate, geography
and the local, social and institutional environment
must also be considered. For a successful transfer of
technology, what is developed must be appropriate
for the intended purpose.
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Annex I. Recommendations of the Expert Group Meeting
147
I. REdOMMENDATIONSOFTHEEXPERTGROUPMEETINGONTHEEXISTINGSOLARTECHNOLOGY
AND THE POSSIBILITIES OF MANUFACTURING SOLAR EQUIPMENT IN DEVELOPING COUNTRIES*
A plan of action by UNDO
1. Preamble: The Expert Group recognized that
solar technology is a multidisciplinary area of activity
with continued potential for successful economic
applications in both industrialized and developing
countries. Therefore, with due regard to the activities
of UNIDO in the encouragement of applied R and D,
manufacturing promotion and technology transfer in
the field of solar technology is applicable to the needs
of the developing countries, the Expert Group also
recognized the fast growing technological advances in
this field in industrialized countries and some
developing countries, and the Expert Group places a
great importance on UNIDO’s capabilities of assisting
developing countries through continued up-to-date
sources of knowledge and effective assessmentof fast
growing technology.
Recommendation
I: Therefore, the Expert
Group recommends that UNIDO establish an
“Advisory Group on Solar Technology” on an
honorary basis. Such an Honorary Advisory Group
shall consist of selected eminent technical personnel
in solar technology, both from industrialized and
developing countries, and shall act as the “focal point
of reference and technical contact” for UNIDO. Each
member of the Advisory Group will be requested to
advise UNIDO in his individual and honorary capacity
in a technological information dissemination system.
The Advisory Group at the request of UNIDO, will
meet on an ad hoc basis to discuss specific technical
problems, assist the UNIDO secretariat and also be
available to render short term Expert Consultancy
Services in the field when requested by UNIDO. Such
field services shall be based on a non-conventional
financial basis, with a token recognition fee only.
II. Preamble: The Expert Group is of the strong
opinion that solar technology, in terms of scientific
and theoretical principles, is well established.
However, it is the transformation of scientific
principles into technological hardware which is still
under intensive work in industrialized and a few
developing countries. For example: (a) Only collectors (flat plate and small concentrated type) and
water heaters are commercially available; (b) the
simple water distiller stills, simple dryers and cookers
have been successfully manufactured but have not yet
been taken up on an industrial production basis
primarily due to need for market development and
extension; (c) the pump system, space heaters
(household, industrial, recreational, etc.), driers,
compact cookers, solar generators, multi-stage distillers and intermittent low temperature absorption
refrigeration systems have been successfully designed
*As they appear in the report of the Meeting
(UNIDO/IOD.73), except for correction of obviouserrors in
spellingand grammar;the report wasnot formally edited.
and fabricated by a few manufacturers and
Institutions (any judgement on large scale manufacturing possibilities/investment promotion could be
given only after field trials with emphasis on
and performance analysis); (d) the
reliability
system (solid absorbers: calciumrefrigeration
chloride and others) and air-conditioners are still in
the prototype stage and require further R and D work
and field testing of prototypes; (e! the central power
station system is in the conceptual stage and has a
great future potential; (f) solar cells for space
application have been successfully designed, manufactured and used in selected industrialized countries.
However, transformation of the same to large scale
terrestrial usagerequires further technological work to
reduce costs. Solar cells, however, hold a great
promise to all countries.
Therefore the work of UNIDO in the promotion
of solar technology and eventual manufacturing
programmes should be geared up to different levels of
“State of the Art” and future potential.
Recommendation
2: In order to promote the
concept of solar technology and its potential to the
developing countries, UNIDO should collect available
information and make it available to all developing
countries and Institutions in industrialized countries.
In addition, UNIDO should launch a programme for
preparation of the specific technical manuals (sources
of information, State of the Art, etc.) and also
organize workshops and initiate fellowships (training)
for the benefit of developing countries.
Recommendation 3: UNIDO should assist developing countries, at their request, in negotiating
with foreign manufacturing firms for local manufacture of well-established commercial products, with
emphasis on assessmentof technology, suitability for
local application, performance evaluation through
local testing and commercial/techno-economic negotiations in manufacturing. It is recommended that at
the request of the developing country, UNID(,
become the active adviser in all such negotiations
with a view towards achieving a most satisfactory
agreement with due consideration to the interests of
the developing countries.
Recommendation
4: UNIDO should actively
associate with the programmes of development of
selected R and D Institutions of industrialized
countries and also of selected developing countries,
with reference to products that have been fabricated
but require further testing. In this connection,
UNIDO shall act as an active partner in twinning of
Institutions in industrialized and developing countries
and assist in installation of such prototypes in
developing countries through testing and technoeconomical evaluation. Assistance in transformation
of the test prototypes to “commercial manufacturing
prototypes” is also necessary. UNIDO shall also assist
the Institutions of developing countries in securing
148
such a technology and in manufacturing promotion.
In this connection, UNIDO at the request of
developing countries, should depute appropriate
experts to assist the Governments in formulating such
an integrated programme and also assist in the
effective realization of the same.
Cecommendution 5: UNIDO,
through active
association with selected R and D Institutions of
industrialized
countries shall assist R and D
Institutions
of developing countries in keeping
informed about the activities in the conceptual stage
and of activities which have a great potential future.
III. Preamble: The Expert Group-taking into
account the required level of technological infrastructure, the continuous technological changes that
are taking place and the need for adaptation of
technologies and prototypes through field testing, as
well as laboratory analysis, adaptation, negotiation
for local manufacture and local entrepreneurship
development-attaches a great importance on technology transfer from industrialized countries as well as
co-operation among developing countries.
Recommendation
6: The Expert Group recommends that LJNIDO assist all interested developing countries at their request to establish or
strengthen appropriate national Institutions with
emphasis on applied R and D, evaluation of
technologies, negotiations on manufacture and local
entrepreneurship
development. The work programmes should take into consideration the local
needs, level of technology, product priority and
potential, and operate on the basis of technology
transfer from Institutions of industrialized countries,
with emphasis on cooperation among developing
countries, with a view towards developing local
capabilities. In this connection, UNIDO shall assist the
developing countries in making judicious decisions
with respect to programmes on solar technology
development: R and D, institutional, investment
promotion and manufacturing promotion.
Recommendation
7: In order to (a) promote
cooperation among developing countries, (b) mobilize technological and financial resources, (c)
promote the concept of self-reliance, and (d) utilize
effectively the technological accomplishments and
capabilities of selected developing countries for the
benefit of other developing countries, it is recommended that UNIDO strengthen anpropriate existing
Institutions in selected developing c&rntries (which
have technological, industrial and infrastructural
capabilities) and transform the same into “Solar
Energy Centres of Excellence’* with the objective of
developing an applied R and D programme for the
benefit of other interested developing countries.
Therefore, the Expert Group recommends that
UNIDO ascertain the interest of selected potential
developing countries in being associated in this
programme, to become one in the nucleus of the
Technology for Solar Energy Utiliza tiorl
activities within the framework of co-operation
among developing countries, and assist the local
Institutions to develop an integrated work programme of applied R and D, assessment of
technologies, and evaluation of prototypes and
products, disseminate technological information and
techniques, train technical personnel from other
developing countries and promote entrepreneurship
development. Such “Centres of Excellence” should
also make available prototypes and designs and
technologies to other developing countries, engagein
co-operative evaluation and develop a programme for
eventual local manufacture of appropriate products
with due emphasis on relevant technology. In this
connection, UNIDO should also develop a cooperative programme between two such “Centres of
Excellence”. It is also strongly recommended that
UNIDO activities promote co-operation and transfer
of technology to such “Centres of Excellence” by
Institutions and manufacturers from industrialized
countries.
IV. Preamble: The Expert Group is of the
strong opinion that practical use of solar energy has
become a reality and that the world today has
entered this threshold. It is a technology with great
potential and it is of great benefit to the needs of
developing countries. The work will require applied R
and D, technology assessment, prototype field
analysis, assessment of reliability
of products,
including techno-economic and cost analysis, development of manufacturing technology, entrepreneurship
promotion and eventual local manufacture. Therefore, the Expert Group is of the opinion that UI’!IDO
should take leadership and initiative and develop an
integrated programme of action.
Recommendation
8: It is recommended that
UNIDO develop an integrated short term and medium
term action programme for the benefit of developing
countries. In this connection, it is recommended that
UNIDO develop a co-operative programme with
Institutions
and manufacturers in industrialized
countries as well as in developing countries. Special
emphasis is to be paid to cooperation from other
United Nations organizations and agencies, as well as
appropriate intergovernmental and non-governmental
organizations.
Recommendation
9: It is recommended that the
Governments of developing countries give sufficient
priority on initiation of a programme of action in the
field of solar technology. This may require allocation
of a solar technology programme to an existing
appropriate Institution and provision of relevant
finances and technical manpower and development of
a practical work plan. In addition, it is also
recommended that the Governments, through
existing meteorological Institutions,
initiate the
necessary steps for collection of appropriate data,
with a view towards assisting and guiding a solar
technology programme.
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Annex I. Recommendationsof the Expert Group Meeting
Recommendation 10: It is recommended that
UNIDO take an active role with the Governments of
industrialized countries on the concept of initiation
of appropriate integrated activities through UNIDO
for the benefit of developing countries. In this
connection, the Expert Group strongly urges that
UNIDO initiate a programme to secure financial
contributions
from industrialized
countries for
integrated solar energy projects and initiate meaningful and effective plans of action.
149
Recommendation
I I: As finances are most
important for initiation of any solar technology
programme, it is recommended that Governments of
developing countries earmark appropriate finances for
such an activity. In addition, it is also recommended
that UNIDO initiate a programme to secure
appropriate contributions (financial and in-kind
(physical facilities, technology, etc.)) within the
framework of co-operation among developing
countries.
Technology for Solar Energy Utilization
150
II.
CURRENT IEA PROJECTS IN SOLAR ENERGY
In November 1974, the International Energy
Agency (IEA) was established. Its members are
Austria, Belgium, Canada, Denmark, Germany,
Federal Republic of, Ireland, Italy, Japan, Luxemburg,
Netherlands, New Zealand, Spain, Sweden, Switzerland, Turkey, United Kingdom OBGreat Britain and
Northern Ireland, and the United States of America.
Norway and the Commission of the European
Communities participate as observers.
IEA consists of several committees, one of which
is the Committee on Energy Research and Development. The Solar Energy Expert Group within this
Committee has developed five co-operative projects in
solar heating and air cooling of buildings and solar
radiation measurement and analysis. In November
1975, the IEA Governing Board approved projects in
seven new technological areas, including solar power
systems, wind energy, ocean thermal energy and
biomass conversion. Five of the projects undertaken
are described below.
Project 1. Development
of solar heating, cooling and
hot-water supply systems
The objective of project 1 is to organize
co-operation among the participating countries in two
areas: The first area covers modelling and simulation
of solar heating, cooling and hot-water supply
systems in order to calculate their thermal perfor!;:ance. The second covers measuring and reporting
the thermal performance of the systems and reporting
on their durability and cost. This work is expected to
provide a basis for the cost-benefit optimization of
such systems.
The successful completion of this project will
have the following results:
Information on an evaluation of existing
computer programs for calculating thermal
performance of systems
A standard international
procedure for
measuring thermal performance of systems
Information on thermal performance, durability and cost of existing and, especially,
new systems
A procedure for designing the most economical systems
Project 2: Development
heating,
systems
of
cooling
components
for solar
and hot-water
supply
The objective of project 2 is to accelerate the
development of the components of solar heating,
cooling and hot-water supply systems by reviewing
and exchanging information on continuing and new
development programmes in the participating
countries.
The viability of solar heating, cooling and
hot-water supply systems for commercial application
depends upon the effectiveness of the key com-
ponents. Major R and D efforts are, accordingly,
being undertaken in participating countries to
improve performance and durability and to reduce
the costs of the key components.
The key components of the solar heating and
cooling system that will be included in this project
are:
Solar collectors
Thermal energy storage
Solar air-conditioning
Other major components, as appropriate
Project 3: Performance testing of solar collectors
The objective of project 3 is to develop and use
standard test procedures to rate the performance of a
broad class of collectors for heating and cooling
applications.
The collector is a key component in a solar
energy system. Many collector designs with a broad
range of qualitative differences are known. Performance testing to rate the technical and economical
potential of the component collector is urgent.
Therefore, standardized methods must be used to
determine the efficiency or the energy output of a
colle,:tor and to predict its reliability and durability.
It i$ expected that performance rating can be
achisved by specifying a few characteristic qualities,
such as optical, thermal, and mechanical properties. A
perfi>rmance test procedure must allow one to
mea:sureor to state thzse properties. The interim test
procedure (NBSIR-74-365) proposed by the National
Bureau of Standards (NBS) of the United States is a
first step in this direction. This test procedure is
already used or under consideration in many member
countries as a basis for further development. The
difficu!tics inherent in the NBS procedure are due to
a restriction in environmental conditions that permits
tesring in climatic regions such as in central Europe
only during a few weeks of the year. These problems
may be solved by applying simulators, i.e., by
applying artificial suns and reproduceable climatic
conditions in climatic chambers. The applicability of
simulation is a main goal of this project.
The successful completion of all the tasks will (a)
yield reliable data for system design; (b) provide
engineering advice in collector design; and (c) provide
a basis for quality standards. Thus, the test
procedures must be scientifically correct, sufficiently
accurate and & simple as possible.
The goal of the research work is to reduce
performance testing to the evaluation of a set of
characteristic parameters that define the optical,
thermal and mechanical properties. To achieve this
goal, four tasks have been identified:
Development of recommended test procedures to determine outdoor thermal
performance
Development of recommended test procedures to determine indoor thermal performance
Development of recommended long-term
outdoor and accelerated indoor test procedu;cs to determine mechanical performance
Concise documentation of the results in a
collector reference book
Project 4: Development ofan insolation
an instrumentation
handbook and
package
The objectives of project 4 are (a) to compile and
distribute a handbook on insolation and related
weather measurements for solar energy applications
and jb) to design, buiid, test, evaluate and
recommend a portable, low-cost insolation and
related weather data instrumentation package to be
used for measurements at the site of a solar energy
system, both before and during its operation. This
instrumentation package will provide essential data in
a suitable format for many studies on solar energy
application and system designs. It will also be used to
gather data for system or subsystem performance
evaluations.
A significant and well-developed body of
knowledge and experience in insolation and related
weather measurements exists in national meteorological centres throughout the world. This knowledge,
however, needs to be summarized, and used in the
many developing national solar energy programmes.
The handbook to be developed as part of this project
is designed to meet that need.
Each country, anticipating the future significant
use of solar energy, should have the means to measure
solar radiation and related parameters in all of its
climatic regions, and in areas where population or
industrial growth is forecast or at potent:a! solar
energy utilization sites. In addition, the design and
evaluation of solar energy systems and components
requires certain data on solar radiation and weather.
The instrumentation package that is one of the
subjects of this project will be designed to provide
that data within the constraints of low cost and
portability.
The successful completion of this project will (a)
provide a valuable data resource-the Insolation and
Related Weather Measurements Handbook,
drawn
together by international experts and disseminated to
IEA member countries, (b) significantly facilitate the
understanding of the insolation and weather aspects
of systems tests and demonstrations in the participating countries and (cl provide for a more effectrve
on the
and usa?lc exchange of information
performance of these systems, which will ;---mit .ach
participating country to fill in the gaps in its own
national programme with the experience and results
of activities in other countries.
Project 5: Use of existing meteorological
information
for solar energy application
The objectives of project 5 are (a) to determine
the quantitative relationship between measurements
of solar radiation and other meteorological parameters and ,‘b) to develcp an internationally uniform
system of presentation of solar radiation data to
facilitate the calculations for utilizing solar energy.
The project will concentrate on two major tasks:
(a) to advise on methods of estimating the solar
radiation incident upon a horizontal or an inclined
surface by means of solar radiation measurements or
other meteorological data and (b) to improve the
quality of current and past records of solar radiation
and to make them available in standardized form.
Technology for Solar Etlergy Utilization
III.
SOLAR ENERGY INFORMATION
General
Ideally, an information system in the solar
energy field should provide for the following
functions:
Support of R and D, by ordered dissemination of information on past and current
research results
Technology transfer, by prcviding repackaged research and development results
in a form suitable for direct application
Planning data, for example on climatic
parameters, but also including cost and
economic data, standards, legislative and
regulatory aspects, etc.
The survey has so far concentrated on the
support-of-R-and-D role, as probably being the most
important at this early stage, when several countries
are still in the programme planning stage. Nevertheless, in the long term the other information system
functions are at least of equal importance. At present,
most of the countries for which data have been
obtained are not yet in a position to implement
information systems in technology transfer and
planning data in solar energy, although there are plans
for such activities and in or?e case (the USA) these
plans are in the early implementation stages.
The concept of a total information programme is
also being discussed, particularly in the USA; such a
system is designed to perform multiple functions,
ranging from the traditional R and D support role
through technology transfer to information aimed at
local authorities, public utility organizations and even
the general public, to promote the use of solar energy
as a means of conserving or replacing other energy
resources.
Even in the area of K and D support, however,
the results of the survey so far obtained show that in
most countries the situation has not yet crystallized
to the point at which the special needs of the
scientists and engineers active in the field have been
assessed and a database constructed to meet the
needs. The information resource in general is
scattered throughout a wide variety of journals and
laboratory reports, and except in the USA and
possibly in the Federal Republic of Germany, is not
yet being organized for easy access.
Scope and coverage-the information resource
Solar energy, from a scientific and technical
information point of view, is a sub-field of energy as a
whole, and like energy information generally is
i.e. the information required by a
“tramdisciplinary”,
project team engaged in an R and D project is to be
found in many of the subdisciplines of physics,
engineering and other sciences. For project planning,
*Excerpted from Small Solar Power Systems; (appendix
pp. 17s ff.).
SYSTEMS*
access to meteorological and possibly geological
information will be required, and for planning and
assessment work, resource data and economics will
also be necessary.
It follows that while subdivisions of the field
may be useful for the purpose of classifying
documents, the total stock of information cannot and
must not be divided into isolated blocks, one relating
to power systems, another to heating and cooling
applications, a third and fourth to solar thermal and
photovoltaic conversion methods, etc. A database
relevant to solar electric power generation cannot be
uniquely identified as a separate entity distinct from
that for any other area of research, development or
application. Indeed, in many analytical and assessment studies in solar energy using, for example,
systems analysis or technology forecasting techniques, access to other areas of energy information
might be required in addition.
The scope and coverage of scientific and
technical information
in solar energy is well
illustrated by the following solar energy category
definition in the current ERDA Energy Information
Database subject category listing.
“Information on conversion of solar radiation to
useful amounts of electric energy, the use of solar
energy for heating and cooling, or any other use of
salar energy that might contribute to the total energy
budget. Information relating to all technical aspects
of the design, research and development, manufacture, testing, and operation of solar cells and solar
collectors are included. Also included is information
on materials with indicated utility in solar cells or
solar converters.”
Size and characteristics
of the database
Taking the above definition
of scope and
coverage, there are probably some ten thousand
relevant articles, reports and monographs, mainly in
the English language, which constitute the basic
information resource. This probably does not include
much material from Eastern Europe and the USSR,
and may not include literature published in Japanese.
Also, there may be an (unknown) quantity of
material produced by individual firms but not
published or otherwise made available for dissemination: the estimate certainly includes a high
proportion of results from work done by industry
under government agency contract, in the case of
material originating in the USA. It can be expected
that the proportion of so-called “non-conventional”
literature (i.e. laboratory and contractor reports,
which do not appear as formal publications) will be
high, as is usually the case in mission- or
application-oriented R and D fields. For example, i2
atomic energy the ratio of non-conventional to
conventional literature is 1:5. The growth-rate of
scientific and technical literature relevant to solar
energy is increasing: a reliable estimate is that
additions will reach a figure of 200 per month in
1976.
.
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Annex /II.
Solor energy infbnnation
153
systems
R and D information in Europe
Sources available
The survey of scientific and technical information activities in the IEA European member states is
far from complete, but a source of information
frequently mentioned was that contained in the
computer files of the RECON (Remote Console)
interactive information system operated by the Space
Documentation Service of the European Space
Agew , which can be accessed through a data
network, ESANET. Nodes of this network are located
in Denmark, France, the Federal Republic of Germany,
Italy, the Netherlands, Spain, Sweden and the UK.
Access is, however, possible from other countries
using normal dialled telephone connections. Some
twelve computerized information
databases are
available for interrogation, of which the following
have been identified as having some solar energy
content:
NASA (aerospace and related technologies)
INSPEC (applied physics)
METADEX (metallurgy, etc.)
ENGINEERING
INDEX
(general engineering)
Government Research Abstracts (NTIS-technology)
All the above, except INSPEC, are US
products. The occasional useful reference could also
be found in other files available over the network,
e.g., World Aluminium Abstracts, Nuclear Science
Abstracts, and Chemical Abstracts Condensates.
Because this information resource is available to
users in most of the European IEA member states, it
is of some importance to estimate the probable
relative coverage of the subject in these files, as
compared with the total national solar energy
database.
While it could be argued that the total database
does not yet exist, the nearest approximation to it is
the content of the Solar Energy Bibliography
produced by ERDA, which now contains approximately 10 000 items. The total relevant content of the
ESANET databases probably amounts to something
over half this number, but no exact estimate can be
made without a detailed overlap check, which is
costly and time consuming. Nevertheless, the
existence of the ESANET resource and its general
availability within Western Europe is a valuable asset
to R and D work in that area. It does not, however,
contain information on current research. With regard
to future growth it may be that coverage of the
subject from these databases may tend to deteriorate
somewhat owing to the fact that as the US solar
energy programme grows, much of the resulting
material wiII appear on the ERDA database (ERDA
contractor report, etc.) in preference to the other
databases. While non-ERDA solar energy material,
where appropriate, wi.Il appear in the Engineering
Index database, and NTIS will presumably maintain
its present coverage, the problem would best be
solved if the new ERDA database were available for
interrogation over ESANET.
Turning to non-computerized sources, contacts
in the IEA member States were asked to indicate
what journals and other publications they found
useful. Replies so far received are insufficient to
permit conclusions to be drawn. The general
impression is, however, that useful information is
dispersed over so many publications that classical
literature searches yielding a satisfactory percentage
of the material relevant to a problem is a major
difficulty even for those users with access to major
libraries.
So far only two attempts to create a specialized
energy database with a significant solar energy
content have come to light during the survey, both in
the Federal Republic of Germany, one at Jiilich
and the other at Karlsruhe. The latter centre is
the German focus for input to INIS and is therefore
the centre of German information services in the
atomic energy field. It is understood that this activity
is being extended to cover the whole energy field.
The Jiilich solar energy databank is in the planning
stage.
R and D information in the USA
Introdurtio,l
The present position can best be understood in
relation to the development of the ERDA Energy
Information Database (EEDB). This is a computer
database composed of all items reported in ERDA
Research Abstracts, which covers all publications
ERDA-sponsored
reporting
research,
Energy
Abstracts for Policy Analysis (also an ERDA
publication), the ERD \ input to INIS in nuclear
science and engineering rind non-ERDA non-nuclear
Energy
material appearing in the publication
Abstracts. This latter is a sub-set of Engineering Index
but specializes in all engineering disciplines related to
energy. Thus, both ERDA and non-ERDA material in
the field of solar energy is included in EEDB. Material
of non-US origin is acquired by exchange agreements.
*It is understood that EEDB will replace Nuclear
Science Abstracts in July 1976. Solar energy
information originating before the creation of EEDB
arose from many sources apart from the published
literature (the NASA and NSF programmes for
example), but it is believed that all this material is
included in a Solar Energy Bibliography published by
ERDA Technical Information Center.
Organization
The implementation of the ERDA scientific and
technical information programme is the responsibility
of the Office of Public Affairs. The Technical
Information Center at Oak Ridge, Tennessee, is
responsible for all ERDA technical publications and
creation of the database. The ERDA database is
2-A:.
,”
:-‘_:,
)_,
/’
,^_‘I.“I,‘_‘.
.,;
TechrzologJ*for Solar Energy Utilization
154
available for interrogation within the USA over the
ERDA (formerly AEC) RECON network, serving
ERDA centres and main contractors. The central
node of the network is also at Oak Ridge in the Oak
Ridge National Laboratory.
Thermoelectric conversion
Photosynthetic conversion
Photovoltaic power plants
Solar thermal power plants
Orbital power plants
Drying and curing
Desalination
Water heating
Heat engines
Solar collectors and
concentrators
Solar energy content of EEDR
The subcategories of solar energy included in
EEDB are as follows:
Tower focus power
Resources and availability
plants
parabolic
Linear
Heat storage and rejection
power plants
Ocean thermal gradiSite geology and meteorology
ent power plants
Solar radiation utilEconomics
ization
Space heating and
Environmental aspects
air-conditioning
Cooking
Solar energy conversion
Furnaces
Photovoltaic conversion
Distillation
Thermionic conversion
.
Other databases
Comnuter databases containina relevant materials
are NASA, NTIS, and COMPENDEX (Engineering
Index). The solar energy category of EEDB will, it is
understood, contain material from these files.
Pre-1976 relevant information from these files has
been included in TIC’s solar energy bibliography.
A bibliography on solar thermal energy is
published by the Technology Applications Center at
the University of New Mexico. It contains about
4 000 items and is being updated. The extent to
which its contents are included in the TIC
Bibliography, which has wider scope, is not precisely
known, but this ought to be nearly lOO%, since
sources are much the same. There is no computer
tape service.
IV. INSTITUTIONS INVOLVED IN SOLAR ENERGY DEVELOPMENT
Greece
Australia
National Council for Energy
Athens
Nuclear Atomic Energy Commission
Athens
Physics Laboratory
University of Patras
Patras
Committee on Solar Energy and Research
Australian Academy of Science
P.O. Box 216, Civic Square
Canberra, ACT 2606
Austria
Austrian Solar and Space Agency
Gamisongasse 9
A- 1090 Vienna
India
Central Salt and Marine Chemicals Research
Institute
Gijubhai Badheka Marg
Bhavnagar 364 002
Birla Institute of Technology and Science
Pirlani
Rajasthan
Brazil
University of Campinas
Caixa Postal 1170
Campinas, Sio Paul0
Canada
Mali
Brace Research Institute
Macdonald College of McGill University
Ste. Anne de Bellevue HOA ICO, Quebec
Netherlands
France
Centre National de la Recherche Scientifique
15 quai Anatole France
Paris
Centre National de la Recherche Scientifique
Laboratoire de 1’Energie Solaire
B.P. 5
F-66 120 Odeillo-Font-Romeu
Ecole Nationale Superieure des Arts et Metiers
15 1 Boulevard de l’Hbpita1
paris i3e
Laboratoiries d’Electroniques
Appliquee
3 avenue Descartes
94 Lirneil-Brevannes
Laboratoire de 1’Energie Solaire du Mali
B.P. 134
Bamako
et de Physique
Laboratoire de Physique des Solides
1 Place Aristide Briand
F-92 190 Meudon-Bellevue
Germany, Federal Republic of
Deutsche Forschungs- und Versuchsanstalt fiir
Luft- und Raumfahrt
Pfaffenwaldring 38-40
D-7000 Stuttgart
Institute of Applied Physics
1 Stieltjesweg
Delft
Stichting TOOL
P.0. BOX 525
Eindhoven
University of Technology
Delft
Niger
L’Office de 1’Energie Solaire
B.P. 621
Niamey
United States of America
College of Engineering and Applied Sciences
Arizona State University
Tempe, AZ 8528 1
Solar Energy Application Laboratory
Colorado State University
Fort Collins, CO 80523
Solar Energy and Energy Conversion Laboratory
College of Engineering
University of Florida
Gainesville, FL 32611
Previous issues in the UNIDO Development
cover the following topics:
No.
No.
No.
No.
1
2
3
4
nnd Transfer of Technology
Series (DTT)
National Approaches to the Acquisition of Technology (ID/ 187)
UNIDO Abstracts on Technology Transfer (ID/ 189)
The Manufacture of Low-cost Vehicles in Developing Countries (ID/ 193)
Manual on Instrumentation and Quality Control in the Textile Industry
(ID/2001

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