Cadmium telluride CdTe and copper indium diselenide CIS

These cells are a further development of thin film technology, having efficiencies of about 7 per cent and 9 per cent respectively. At present prices are comparatively high but will reduce as volume of sales increases.

In summary, costs range between £2 and £4 per Wp. However, unit cost is not necessarily the only criterion. Different cells have varying optimum conditions which has been highlighted by a research programme recently completed by the Oxford University Environmental Change Institute. This showed that the amount of electricity generated by a PV array rated at 1 kWp in one year varies considerably between different technologies. For example, CIS (Seimens ST 40) gave the best returns at over 1000 kWh per kWp per year in the UK. Double junction amorphous silicon cells were close behind. This is because these cells are more effective in the cloudy conditions so prevalent in the UK. Single junction amorphous silicon cells were the poorest performers. The best performing modules produced nearly twice as much power as the lowest yielding cells, so it is very much a case of 'buyer beware'.

A sloping roof facing a southerly direction is the ideal situation, provided it is not overshadowed by trees or other buildings. However, east and west orientations can produce significant amounts of electricity. The optimum angle of tilt depends on latitude. In London it is 35°. As a rough guide, in London 1 square metre of monocrystalline PVs could produce 111 kWh of electricity per year. On low pitch or flat roofs it is advisable to mount the cells on tilt structures at the correct orientation. However, in the UK climate, a flat roof can still deliver 90 per cent of the optimum output.

Standard PV modules can easily be fixed to an existing roof. However, if a roof covering needs to be replaced, it then could become a cost-effective option to use solar slates, tiles or shingles to maintain a traditional appearance.

A housing project which integrates PVs into its elevations and roofs is the Beddington Zero Energy Development (BedZED) in the London Borough of Sutton, designed by Bill Dunster with Arup as the services engineers. Originally the intention was to use their power to meet the needs of the buildings. The problem was the extent of the expected payback time at current low energy prices. Their purpose is now to provide battery charging for a pool of electric vehicles for the residents which has the advantage of avoiding conversion to AC current (Figure 7.4).

It is important to ventilate PV cells since their efficiency falls as temperature increases. This requirement has been put to good use in a restaurant

Figure 7.4

Southern elevation, BedZED housing development, South London

Figure 7.4

Southern elevation, BedZED housing development, South London

in North Carolina, USA. Its integrated roof system has 32 amorphous PV modules serving a 20 kWh battery facility. This supplements demand at peak times and also bridges interruptions in the grid supply.

What makes this system special is the fact that warmth that builds up under the cells is harnessed to heat water which supplements space heating. A fan circulates air through a series of passages beneath the modules. As solar heat builds up, the fan cuts in automatically to circulate heat away from the PVs and direct it in a closed loop to a heat exchanger. This technology will save the restaurant about $3000 per year in utility and hot water costs, at the same time avoiding 22 680 kg of CO2 emissions (Figure 7.5). See also Figure 18.16, p. 234.

Figure 7.5

Diagrams of PV heat recovery system (courtesy of CADDET)

Figure 7.5

Diagrams of PV heat recovery system (courtesy of CADDET)

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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