A new technology is standing conventional wisdom about solar energy on its head - or, to be exact, on its roof.
The first house roofed with "solar shingles" was unveiled at the Atlanta Olympics by United Solar Systems of Troy, Mich.Solar shingles look just like asphalt tiles, but have embedded photovoltaic cells to convert sunlight into power.
Conventional solar panels are a bulky sandwich of silicon-crystal PV cells encased in glass. Shingles are 100 times thinner, made of layers of non-crystal silicon alloy bonded to stainless steel and coated to blend in with other roof tiles.
And, like ordinary tiles, they overlap to keep out ice and rain.
United Solar vice-president Subhendu Guha, an electronics engineer, says: "Glass panels on top of a roof are heavy and fragile and need a support structure, but the solar shingle is the roof."
Roofers simply nail the solar tiles alongside ordinary roof tiles; electricians wire the tiles to the electricity supply. The Atlanta house has one solar tile for every five ordinary tiles.
To keep down production costs, United Solar has developed a process similar to newsprint manufacturing, which allows foot-wide solar cells to roll off the production line in half-mile lengths that are cut to the finished size.
Guha points out that while solar tiles are a similar price to conventional glass panels, solar shingles could be produced in large volume for half that price, making solar-generated electricity - now between two and four times as expensive as electricity from fossil fuels - a serious competitor.
At the heart of the solar shingle is the "amorphous" silicon PV cell. The atoms in conventional solar panels are stratified; in amorphous cells they are disorderly, which allows light to be absorbed more quickly.
But the disorder also makes amorphous silicon less efficient in the next step of the process: pushing the absorbed sunlight - the electrons - out of the cell and on to the electrodes that create electric current. While traditional PV cells can achieve up to 17 percent conversion, the amorphous cells in solar shingles have achieved only 11.8 percent. The goal is to boost this to 15 percent by 2000.
The company is working on improving the materials that collect the electrons. But it's also attacking the efficiency problem by mimicking the way crystal PV cells capture the different wavelengths of sunlight.
The thin films of amorphous silicon alloy are layered so that the top cell captures blue light, while the middle and bottom cells - by mixing silicon with a germanium semiconductor - pick up green and red light. At 20-30 cents a kilowatt hour, the solar shingle can't compete with the base rate charged by U.S. utilities of 6-10 cents. But it is competitive with peak rates of 20 to 50 cents - charged on hot afternoons, when everyone has air conditioners on full throttle.
"There is a tremendous market in the U.S. for peak-load applications," says Guha.
United Solar is a joint venture between Canon, the Japanese camera and electronics company, and Energy Conversion Devices of the U.S.
The first big customers for the technology are likely to be in Japan, where beginning next year the government will provide subsidies for its citizens to buy solar power systems of their choice. Germany is also considering a subsidy to boost solar power.
In Japan, the solar cells will be embedded into iron, rather than the asphalt tiles favored by the American building trade. Solar cells in terracotta and corrugated iron could come next.
But the solar cell technology isn't confined to rooftops. Canon is developing the amorphous silicon alloy for applications such as photocopiers and image sensors.
(Distributed by Scripps Howard News Service.)