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Revolution waits in the wings

technical

According to one estimate, 75 per cent of the uk's total electricity demand could be met if all suitable south-facing walls and roofs were clad with photovoltaics (pv). Though not a realistic goal, it shows there is plenty of potential.

The hold-up in deploying pv is the cost. Currently, use can be justified only in remote locations where the alternative of installing mains supply cabling is very costly. In the sunniest climes though, there are a lot more opportunities. The few recent uk installations, such as at Doxford business park (aj 19.6.97), make no claim to cost-effectiveness but they do have the useful function of field-testing what has already emerged as a reasonably robust technology.

One potential route to cost reduction is to integrate pv into new or replacement cladding, rather than mounting it separately on its own support structure. This can, sometimes, be cheaper.

According to speakers at a recent seminar organised by Halcrow Gilbert for the European Commission, pv prices have halved over the last decade, but are still around five times too expensive. Halcrow Gilbert's Paul Ruysevelt suggested the current installed cost of cladding with integral pv is £500-1000/m2. This compares with current building costs of, say, £50-100/m2 for domestic roofing, £150-800/m2 for curtain walling or £1000/m2 for marble and other polished stones. The amorphous pv panels on the recently completed offices at bre (Feilden Clegg, Max Fordham) cost around £120/m2 more than straightforward rainscreen cladding would have done.

Currently, pv faces the classic chicken and egg problem, typical of new technologies, from cars to computers - that a volume market is needed to bring down unit prices and fund further development. For the uk pv industry there is more immediate prospect in the export market. Some speakers suggested that in 10 years or so there will be a greater volume, though not a mass market, in the uk. Meanwhile, the world pv market generally has been growing recently by around 25 per cent a year.

What you get

As an energy technology, integrated pv has several characteristics to recommend it.

no emissions in use

silent (except if there are very large transformers)

no moving parts

cladding technology is already well developed

the potential to deliver electricity where it is needed

pv-generated electricity can be a substitute for mains electricity, a fuel with one of the highest atmospheric co2 overheads

arrays of pv cells can be spaced out with a grid of clear glazing in- between, providing both translucent shading and electricity generation (see illustrations)

good for remote locations with high electricity supply infrastructure costs

pv can be set up in the open as power stations (unlikely in the uk).

But, price apart, there are some disadvantages too:

sunlight conversion efficiencies are currently only five to 18 per cent

existing roof structures may not be able to carry the extra load

pv does not necessarily deliver power when it is needed. The high cost of electricity storage is normally avoided by connection to the mains. If the pv installation generates more electricity than is needed in the building, the surplus can be exported to the grid: the electricity meter effectively runs backwards. Just how many pv installations doing this the grid could cope with is not clear

pv electricity is dc, not ac. In a future of widespread pv, some dc equipment may come to be used. Normally pv arrays are wired to central invertor(s) to convert dc to ac. (A recent development is known as ac modules - see 'New technologies', below)

invertors can have low efficiency when sunlight is limited

lack of client confidence

lack of skilled installers

opposition/indifference from some of the regional electricity companies and licensing authorities

if pv cells get hot, electricity-generation efficiency falls significantly. Integrated pv arrays need ventilation to cool them at the back, typically natural draught. However, this stack effect of rising warm air may also become a useful part of a whole-building ventilation strategy, as at Doxford.

New technologies

ac modules are panels of pv cells in which each modular panel has its own invertor. This modularity, along with the ac output from each module, provides several advantages:

with a modular arrangement, failure of one module does not affect others

modularity provides flexibility for expanding systems

modules are simply chained together with cable. The ac cabling is simpler and more familiar than for dc

with this simplicity, smaller minimum installation sizes are practicable. This may encourage wider application of pv.

In the future, the new generation of invertors being developed should allow bigger modules and a fall in price.

Another development is thin-film pv, which uses vapour deposition on glass, giving an appearance like tinted glass. Typical panels are up to 900 x 300mm, and can be linked together. Thin laser lines for interconnection can be opened out to reveal some clear glass for partial shading applications. Currently they are cheaper, but less efficient than normal pv (crystalline and amorphous) and so are comparable in capital cost per kWh delivered. There have been some teething problems with durability and weathertightness of panels. Intersolar, the UK's only maker to date, claims to have great confidence in thin film's future.

Getting the most from pv

Integrating pv on to buildings needs southerly orientation, panels ideally inclined at about 10degrees less than the latitude, and with minimal overshading. Mounting pv on a vertical wall reduces its peak output by about 30 per cent.

At the seminar, speaker Nicola Pearsall pointed out some of the options for optimising pv performance:

output is much the same for orientations between south-east and south- west

shifting panel orientation changes the daily pattern of output. For example, for a coastal site with misty mornings, a south-west orientation would be preferable

a steeper angle of inclination will shift a little of the output from summer to winter

glass cleaning does not need to be more frequent than a good general building maintenance schedule.

Integration of pv into wall or roof cladding is not an ideal partnership. The effect of higher temperatures in reducing efficiency has already been noted and active cooling is not cost-effective. There also needs to be access to the backs of the panels for occasional maintenance. Alternatively, panels set on racks on flat roofs will need to be removable for routine roof maintenance. As one cynic remarked, you could always use pv arrays to replace broken-down solar water collectors.

More creatively, a range of architectural options is being explored for walling, cladding and for shading such as screens and louvres. Tjerk Reijenga, of bear Architecten, described the use of pv for roofing a street between buildings of an education centre in the Netherlands. The pv cells are spaced on a grid to provide 70 per cent shading, avoiding the very dark soffit that solid pv would provide. As the illustration shows, the street roof is set as a monopitch, with automated opening roof lights for ventilation at the higher edge. Built two years ago, indoor temperatures during summer have not risen above outdoor ones.

At this early stage, pv is often an obtrusive feature of buildings: deliberately so as to promote the technology or the client's green credentials. As the technology matures, the opportunity for a more subtle architectural treatment evidently exists. 'Installing the Solar Solution'. 22/23.1.98. Organised by Halcrow Gilbert for the European Commission

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