Photovoltaics in Buildings A Design Guide - DTI Home Page

If monocrystalline silicon cells were used this might require an area of
approximately 235m 2 and might entail an installed cost of about £150,000
(Chapter 4).
At this point the design team knew the area required was available but did
not know if the client had the money. In the meantime, the approximate cost
of the building was estimated at £900/m 2 or £3,550,000 without PVs, so
the PV cost would represent very roughly (allowing for partial replacement of
the metal roofing system that had been costed) about 5% of the building cost.
The design team then met the client to present its findings. There was some
disappointment at the cost and the fact that the entire roof would not be PV
modules. There was discussion about whether the electricity supplier would
pay for energy exported and, if so, how much. As the answers were not
known, it was decided to err on the side of caution and discount any
contribution for the time being. In the end, the additional cost was judged to
be acceptable and worthwhile on the basis of the percentage of the building
cost, the environmental benefits and the educational value.
In order to make the maximum public statement about the installation, the
client requested that the PV modules be at the southernmost part of the
bowling green roof and on its south facade. He also asked that the remaining
bowling green roof be designed so that it could be used for PVs in the future
when costs had fallen. The design team returned to their drawing boards (or
more precisely their computer screens).
6.3 Design development
The following aspects of the design were then developed:
• Optimisation of the daylighting and PV capability of the roof.
• PVs and the natural ventilation strategy.
• Choice of module.
• Sizing and costing.
• System considerations.
• The PV installation inside the building.
Daylighting and PVs
The initial assumption of a daylight factor of 6-8% was maintained and a
uniformity ratio of about 0.5 specified. Numerous configurations were
studied using computer modelling - Figure 6.7 shows three of them.
The main conclusions of the exercise (which looked at electrical energy but
did not go into energy for space heating which was thought to be a lesser
consideration) were:
• The optimum angle for a stand-alone array in Cambridge is about 31 0 ;
this is consistent with the rule-of-thumb of latitude less 20 0 . A uniform
roof based on this has a comparatively low daylight factor, thus
higher electrical costs.
• By increasing the angle to 45 0 daylight is improved; the PV output,
however, drops because the angle is not optimal and self-shading
losses are higher.
• If the 45 0 tilt is maintained, increasing the height of successive ridges
reduces self-shading and increases the PV output. The daylight factor,
however, drops slightly.
Figure 6.7(c) suited the initial ‘wave’ concept, provided good daylighting and
offered significant PV potential. There was a great deal of discussion about
the increased structural complexity and costs resulting from non-uniformity
and the appearance of the roof from the inside but, in the end, it was agreed
to take forward this design.
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