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F. Garde1, A. Lenoir1, A. Scognamiglio2, D. Aelenei3, D. Waldren4, H. N. Rostvik5, J. Ayoub6, L. Aelenei7, M.
Donn8, M. Tardif6, S. Cory8.
1: Faculty of Engineering ESIROI, University of Reunion, France
2: ENEA (Italian National Institute for New Technologies, Energy and Sustainable Economic Development),
Photovoltaic Technologies Area, Portici Research Centre, largo E. Fermi 1, Portici, Italy
3: Universidade Nova de Lisbon, Faculty of Science and Technology, Caparica, Portugal
4:Grocon Group, Melbourne, Australia
5: Bergen School of Architecture, Bergen, Norway
6: Canmet ENERGY Natural Resources Canada/Government of Canada
7: National Laboratory of Energy and Geology, Lisbon, Portugal
8: Victoria University of Wellington School of Architecture, Wellington, New Zealand
The International Energy Agency (IEA), through the Solar Heating and Cooling programme (SHC) Task 40
and the Energy Conservation in Buildings and Community Systems programme (ECBCS, now named ECB)
Annex 52, works towards developing a common understanding and setting up the basis for an international
definition framework for Net Zero Energy Buildings (Net ZEBs). One of the subtasks of this programme –
SubTaskC focuses benchmarking the Net ZEBs around the world to identify the innovative solutions sets that
makes up this new type of building. This paper presents an overview of the work conducted by the
participants of Subtask C and of Zero Energy Building projects that have been identified.
Keywords: Net-ZEBs, Case studies, solution sets, design strategies
By 2005 the number of Net Zero Energy Buildings (Net ZEBs) had started to reach significant numbers and
the number of projects and publications about this kind of building has grown constantly since. In 2002, the
EU adopted the Energy Performance of Buildings Directive [1], which set minimum efficiency standards for
both residential and commercial buildings. The Commission then proposed a recast of the directive as part of
its Second Strategic Energy Review in November 2008. The second article of the directive of the 14th of
April 2010 [2] gives a definition of a “nearly zero-energy building”. The nearly zero or very low amount of
energy required should be covered to a very significant extent by energy from renewable sources produced
on-site or nearby. According to article 9, “by 2020, all new buildings are nearly zero-energy buildings”.
In 2005/2006, the Net-ZEB concept was still generic and in 2006 there was no harmonized understanding
about what was really a Net Zero Energy Building [3]. This was one of the principal motivations for an
international collaborative research project that started in 2008 within the framework of the International
Energy Agency “Towards Net Zero Solar Energy Buildings” [4]. The objective of this work is to study
current net-zero, near net-zero and very low energy buildings and to develop a common understanding, a
harmonized international definitions framework, new design tools, innovative solutions sets and industry
guidelines. Since then, the Task participants have produced a range of deliverables including journal papers
and books providing clarification about definitions [5], [6] and presenting interesting case studies from
around the world [7].
In simple terms, a Net ZEB is a very low energy building that balances its low annual energy consumption
by the use of renewable energy on site.
To reach the Net-ZEB goal, two important analyses must be conducted at the design stage :
- to reduce building energy demand using passive solutions and energy efficient systems;
- to generate sufficient electricity by renewable energy systems to achieve the desired energy balance.
Passive approaches play a crucial role in the design of Net ZEBs as they directly affect the heating, cooling,
ventilation and lighting loads put on the building’s mechanical and electrical systems, and indirectly reduce
Figure 1 : The Net Zero Energy Building Challenge [8]
One of the subtasks of IEA Task40/Annex52 – SubTask C (STC) – focuses on a benchmark set of NZEBs
from around the world in order to identify the innovative solutions sets that make up this new type of
building. A list of 30 zero energy buildings and projects was established matching a set of mandatory criteria
intended to ensure high quality data about the buildings and their measured performance. The locations of
those 30 Net-ZEBs are plotted on a world map in Figure 2. The colour coding describes the principal thermal
environment design challenge faced by each design team and the size of icon the type of building.
Figure 2 : World map of the 30 Net ZEBs by climate challenge and by building type. The three
pictures are the Net ZEBs in tropical climates : Enerpos (left) and Ilet du Centre (middle) in Reunion island
and BCA Academy in Singapore (Right)
In every new building the various challenges faced by the design team are met in a unique response which
combines particular building design approaches and strategies to lower the energy impact of the space
conditioning and building loads. A solution set is a set of passive design solutions, energy efficiency
solutions and/or renewable energy solutions that are used in a building to mitigate or lessen the various
building challenges and achieve a building design goal. The STC database has been analysed in terms of
these solution sets. Two kinds of solution set are reported: Whole Building Solution Set – The set of
solutions used to lower the energy consumption of the whole building; Building Challenge Solution Set – the
set of solutions used to lower the energy needed by a particular building challenge (e.g. heating, cooling,
lighting, plug loads etc...).
Figure 3 : An example of analysis of solution sets by type of climate. One can see for instance that 100% of
the buildings use PV as electricity generation or that 100% of Net-ZEBs in cooling dominated climate use
solar shading systems
The STC database and the 30 detailed case studies are the backbone of a source book project entitled “Net
Zero Energy Buildings Solution Sets: Lessons learned from international projects” that is the main
deliverable of Sub Task C. The book is planned to be released by the end of 2013. The audience is all the
people involved in the design process of Net-ZEBs. The main objectives of the book are:
• to provide recommendations about innovative solution sets that can be adopted by building type and
by type of climate challenge. A cross-case analysis is planned in order to facilitate identification of
the set of relevant design strategies. Close inspection of the strategies and indicators of the relative
performance of the projects has revealed interesting features of the combination of design challenges
with techniques and technologies responsible for delivering the Zero Energy performance [9], [10];
• to provide experimental feedback and recommendations based on the monitored data available. Few
books and journal papers deal with the monitored feedback of multiple low energy buildings. The
book will address some interesting comparisons between design data and monitored data;
• to raise the issues of the design process and the integration of RE from the architectural point of
• to provide feedback about lessons learned, users behaviour and post occupancy evaluation of some
of the Net ZEBs.
The overview of the Net ZEBs carried out in the framework of Subtask C of the IEA Task 40/Annex 52 has
led to the identification of new ways of design for this innovative type of building. The building of the future
should be bioclimatic with a passive design approach; it should be reduced in cross-section size compared to
conventional buildings in order to improve cross natural ventilation and daylighting. Its envelope should be
not only dedicated to thermal insulation but becoming multi-functional to protect from the outside
environment while drawing from free sources of energy such as wind, sun, soil etc. Summer comfort should
be studied because on the one hand it becomes an issue in air-tight low consumption buildings and on the
other hand the design of Net ZEBs must take into account the effect of global warming on the climate for the
upcoming decades. The systems used should be more energy efficient. Ceiling fans play a crucial role and
can be used whatever the climate in non-residential buildings for summer thermal comfort conditions.
Unexpectedly, lifts are often a large source of energy consumption representing 5 to 10% of the total energy
use. In Net ZEBs it is necessary to encourage the users to take the stairs at least to reach floors only one
storey above or below the current floor.
The sensitization of the users is also highly important in this new type of building to ensure they understand
the specific operating processes of the building. With this comes an understanding of the impact of their
behaviour on the building energy consumption. The best-designed building in the world can consume more
than a conventional building if users are not informed and supported in the use of the building. The aim is to
design passive buildings with active occupants rather than active buildings and passive occupants. For
example, the measured plug load energy consumption of the Elithis Tower (a Net-ZEB in France) compared
to the predicted values shows that the energy balance can only be reached thanks to the involvement of the
users of the building.
The comparison of the energy consumption during the design phase and during occupancy demonstrates that
it is always complicated to forecast the consumption of a building. One of the major problems with
specification of the energy consumption during the design stage is the definition of a timetable to evaluate
the occupancy of the future building. For office buildings it is a key element. This occupancy and use of
equipment scenario should be considered as a primary input to energy consumption calculations to attain the
optimum Energy Use Index (EUI – a measure of energy use per square metre) objective. In building
standards, the EUI is often defined as taking into account some specific energy end uses (e.g. for the some
national thermal regulations: heating, DHW, cooling, ventilation, lighting); and a detailed floor area.
However, they seldom include this occupancy scenario parameter. The impact on the EUI measured in
practice can be considerably modified by this parameter. Therefore this scenario must absolutely be
integrated as an input data of the project.
Finally, the design stage cannot neglect the importance of the brief and of the Integrated Design Process. At
the briefing stage it is important to document the expectations of the building owner in terms of EUI, energy
efficient systems, and performance of the building envelope. The brief provides the objectives for the design
team and must be as accurate as possible. In all cases where the design team was interviewed, an Integrated
Design Process (IDP) structure for the collaboration of the various design disciplines was viewed as a key
element in the successful delivery of a high performance building. Design of NZEBs is not “just” adding
extra specialists to deal with the design aspects of the energy generation and its storage.
EBPD.(2010). Directive of European Parliament and of the Council of the Energy Performance of Buildings (recast).
Adopted by the Council on 2010/04/14. Brussels 2008/0223
EPBD.(2002). Energy Performance of Buildings Directive.
P. Torcellini, S. Pless and M. Deru. (2006). Zero Energy Buildings: A Critical Look at the Definition. National
Renewable Energy Laboratory (NREL).
IEA SHC Task 40/ECB Annex 52. Towards Net Zero Energy Solar Buildings (NZEBs). [].
Sartori, A. Napolitano and K. Voss. (2012). Net zero energy buildings: A consistent definition framework. Energy and
Buildings. Volume (48) 2012 220–232.
A. Marszal et al. (2011). Zero Energy Building - A Review of definitions and calculation methodologies. Energy and
Buildings 43 (4) 971–979.
K. Voss and E. Musall. (2011). Net Zero Energy Buildings. International project of carbon neutrality in Buildings.
2011. Ed. Detail Green Books. ISBN 978-3-0346-0780-3
L. Aelenei et al. (2011) Passive Cooling Approaches in Net-Zero Energy Solar Buildings: Lessons Learned from
Demonstration Buildings. In: CISBAT Conference 2011, 14-16 September 2011, Lausanne, Switzerland.
L. Aelenei et al. (2012). Design issues for net-zero energy buildings. In: ZEMCH, Glasgow, 20 - 22nd August, 2012.
D. Aelenei et al. (2013). Design strategies for non-residential zero - energy buildings. Lessons learned from Task
40/Annex 52 “Towards Net Zero - Energy Solar Buildings”. In proceedings of CLIMA2013 Prague.

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