Energy from solar cells: clean, quiet, but not cheap - yet
Solar cells are the high-tech alternative energy source which almost everyone loves. Close cousin to the transistor and integrated circuit, these cooky-sized wafers, which transform light directly into electricity, may play an increasing role in the world energy picture.Skip to next paragraph
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Engineers admire them for the sophisticated science designed into their atomic structure. Alternative energy advocates and environmentalists favor them because they promise quiet, clean, economical, and decentralized electrical power. Major oil companies invest in them as potentially profitable new products. And energy analysts like to speculate how they could revolutionize the way energy is generated, distributed, and used if their costs can be brought low enough.
The cost of a peak watt (a watt produced in full sunlight) from a solar cell has fallen 200-fold to $7 to $10. As a result, the domestic US market for photovoltaics has grown from almost zero to $25 million last year. US firms, which currently dominate the world market, sold another $25 million worth of solar cells overseas. And this year's market is expected to be 50 percent better. Photovoltaic or PV devices, as the solar cells are called, now are economically competitive with diesel-electric generators.
Today, largely due to an ambitious federal research and development effort, US companies are at the forefront of photovoltaics technology. Cutbacks in US government support, coupled with increased spending by Japan and European nations, raise the question of whether the US will retain this lead. However, US manufacturers generally believe that, with the market expanding as fast as it is , they can retain the lion's share of sales for at least the immediate future.
Still, to realize its full potential, the costs of solar-cell electricity must fall another factor of 10. According to researchers in the field, this is not a trivial challenge. However, recent developments have increased their confidence that the federal goal of producing 50 cent per peak watt electricity will be achieved by the end of the decade.
The Solar Energy Research Institute (SERI) is the lead federal agency in photovoltaic research. ''In the past few years, we have made enormous progress, '' says Satyen Deb, head of SERI's photovoltaics program. Raising the efficiency , increasing the stability, reducing the cost of the materials and the cost of manufacturing PV devices are the areas which are actively being pursued.
To generate 50 cent per watt solar electricity from cells of silicon (the traditional material), the Department of Energy has determined that the cost of solar cell grade silicon must be less than $14 per kilogram. Bulk silicon, made from sand, is both cheap and abundant. But to purify it to the point needed for efficient solar cells takes a number of steps. As a result, the cost of the highly purified material needed for traditional single crystal solar cells runs about $370 per kilogram, although fairly efficient cells have been made in the laboratory from $80 per kilogram silicon.
In recent laboratory work, however, SERI researcher Jerry Olson has developed and bench-tested a purification technique that may eventually produce PV-grade silicon for $3 to $10 per kilogram.
The traditional way to make a solar cell involves growing a large ingot or single crystal of silicon from a melt and slicing this into a series of wafers. These are called Czochralski cells, and they convert as much as 20 percent of incident sunlight into electricity. To make them, however, wastes about half of the high-cost material and the wafers do not pack efficiently into panels. To avoid these problems, a number of laboratories are working on ways to grow ribbons of silvery silicon, rather than ingots.