Technology : Heat on a hot, thin roof

2019-02-27 07:17:06

By Elizabeth Manning Washington DC SOLAR panels that generate electricity are bulky and expensive. But now an American company says it can provide unobtrusive photovoltaic panels at half the cost of traditional versions. The new technology will be on show at this month’s Olympic Games in Atlanta, Georgia, where the US Department of Energy is building a solar-powered house. On the roof are 110 solar shingles supplied by Energy Conversion Devices (ECD) of Detroit, Michigan. “From the ground, it’s hard to tell the difference between the asphalt shingles and the solar ones,” says company chairman Robert Stempel. Traditional photovoltaic cells are built by growing silicon crystals, slicing them into wafers 100 micrometres thick, and encasing them in glass. This process is now about as cheap as it is going to get, and assuming a cell lifetime of around twenty years, the electricity from crystalline cells is still roughly two or three times as expensive as electricity from fossil fuels. The cells of solar shingles, on the other hand, use noncrystalline silicon and are one hundred times thinner. Mass production will probably reduce the cost to half that of conventional cells. They should also be cheap and easy to install. Roofers nail the shingles to the roof base as usual, and electricians wire them together from inside the house. ECD’s manufacturing arm, United Solar Systems Corporation, begins with a roll of stainless steel sheet 800 metres long and 35 centimetres wide. The steel is fed through a series of rolling machines, “much like newspaper production”, according to company vice-president and electronics engineer Subhendu Guha. The machines deposit nine layers of amorphous silicon on the steel, followed by a protective layer of transparent polymer. Finally, a coloured top coating ensures that the shingle blends in with surrounding roof tiles. The shingles work in the same way as conventional cells. Photons knock electrons from the valence band into an energy level called the conduction band, where they form an electric current. According to Guha, 400 square metres of photovoltaic shingles—one-third of an average suburban American roof—exposed to six hours of sunlight per day can power the average home. At the moment, the company’s thin-layer technology is not as efficient as the crystalline silicon cells at converting sunlight. Some traditional cells achieve up to 17 per cent conversion, while the new cells reach a little over 10 per cent. But the efficiency of the new devices is improving all the time. A combination of higher utility costs and government subsidies means that Japanese and German householders are likely to be the first major customers. From next year, thousands of Japanese residents will be eligible for a government subsidy that pays for half the cost of installing the photovoltaic system of their choice. Germany is considering a similar programme. Japan plans to eliminate the subsidy gradually as economies of scale help solar energy to compete with fossil fuels. ECD already has a foot in the door of the Japanese market through a partnership with Canon. “I find the concept very interesting,” says Ajeet Rohatgi,