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different grains, color and module size. As shown above, before such decisions are
made, it is however necessary to evaluate the energy need of the building, the energy
supplied from either system, and the envelope area that either system will occupy in
order to perform.
One possible discussion arises from these considerations: efficient building by a
compact structure (low surface to volume ratio) or more solar usable surface area?
[Musall 2012c]
Obviously the decision on the envelope design approach has to take into account the
local climate and the right solutions for that climate. For example, in South Europe,
where an important design issue is protecting the building from the sunshine in warm
and hot seasons, sun-shading systems (which extend the building beyond its physical
boundary) are very diffuse. Moreover, very often the building is designed so to have
some intermediate buffer spaces between inside and outside (a typical example are the
traditional courtyard houses in Italy). In such cases it is not surprising that PV wings can
stick out of the building’s envelope to catch the sunshine (figure 6). [Scognamiglio
2012b]
4 (Nearly) Net ZEBs scenario, PV and architectural issues
4.1 PV for Net ZEBs as it results from the IEA Task 40-Annex 52 investigation
As shown above (paragraph 1), it seems not possible or at least useful to create a Net
ZEB without use of PV systems. This influences the buildings shape or the use of the
site if the systems are installed outside of the buildings footprint (e.g. as tracking
systems or additional open field structures). Because of that it seems to be important for
the design process to know how much PV must be installed and how much surface or
site area must be available. Beside the question if the building is solely supplied by PV
this depends on the climate (radiation), the site situation (shading, orientation), the
chosen PV technology (mono or polycrystalline cells), and above all on the energy
demand of the building and hence also on the balance approach (see above). Herein the
accounting system classifies the energy demands sectors which are included in the
balance.
Within the framework of the IEA research, data were acquired and analysed from more
than 300 worldwide known Net ZEB projects around the world. [IEA 2008] All types of
building categories and sizes are represented. Most of the buildings are located in
heating dominated climates in Europe and North America (figure 7). Some examples of
existing buildings that were renovated to meet the net zero-energy building criteria are
also included. The database gives the chance to point out how much PV is normally
used in built examples depending on the points mentioned above.
The study shows that an average small residential building use almost 22 W
p
/m²
NFA
(0,18
m
2
/m
2
NFA
at a PV power density of 120 W/m
2
) to balance the energy consumption
sectors according to EPBD (space heating, domestic hot water, cooling, air conditioning,
and lighting for non-residential buildings). If the complete primary energy consumption is
considered (including plug loads, etc.), this figure is doubled (almost 40 W
p
/m²
NFA
or 0,36
m
2
/m
2
NFA
at a PV power density of 120 W/m
2
).
In larger residential buildings or housing estates the PV systems are, in relation to the
living area, in average about half the size of those in single family houses. This is due to
the fact that here heating systems are used which lower the overall primary energy
demand (e.g. by a higher efficiency or use of biomass) or CHP systems, which also
generate electricity locally.
