Friday, November 10, 2006

Cheap, Superefficient Solar

Technologies collectively known as concentrating photovoltaics are starting to enjoy their day in the sun, thanks to advances in solar cells, which absorb light and convert it into electricity, and the mirror- or lens-based concentrator systems that focus light on them. The technology could soon make solar power as cheap as electricity from the grid.

The idea of concentrating sunlight to reduce the size of solar cells--and therefore to cut costs--has been around for decades. But interest in the technology has picked up in the past year. Last month, Japanese electronics giant Sharp Corporation showed off its new system for focusing sunlight with a fresnel lens (like the one used in lighthouses) onto superefficient solar cells, which are about twice as efficient as conventional silicon cells. Other companies, such as SolFocus, based in Palo Alto, CA, and Energy Innovations, based in Pasadena, CA, are rolling out new concentrators. And the company that supplied the long-lived photovoltaic cells for the Mars rovers, Boeing subsidiary Spectrolab, based in Sylmar, CA, is supplying more than a million cells for concentrator projects, including one in Australia that will generate enough power for 3,500 homes.

The thinking behind concentrated solar power is simple. Because energy from the sun, although abundant, is diffuse, generating one gigawatt of power (the size of a typical utility-scale plant) using traditional photovoltaics requires a four-square-mile area of silicon, says Jerry Olson, a research scientist at the National Renewable Energy Laboratory, in Golden, CO. A concentrator system, he says, would replace most of the silicon with plastic or glass lenses or metal reflectors, requiring only as much semiconductor material as it would take to cover an area the size of a typical backyard. And because decreasing the amount of semiconductor needed makes it affordable to use much more efficient types of solar cells, the total footprint of the plant, including the reflectors or lenses, would be only two to two-and-a-half square miles. (This approach is distinct from concentrated thermal solar power, which concentrates the heat from the sun to power turbines or sterling engines.)

"I'd much rather make a few square miles of plastic lenses--it would cost me less--than a few square miles of silicon solar cells," Olson says. Today solar power is still more expensive than electricity from the grid, but concentrator technology has the potential to change this. Indeed, if manufacturers can meet the challenges of ramping up production and selling, distributing, and installing the systems, their prices could easily meet prices for electricity from the grid, says solar-industry analyst Michael Rogol, managing director of Photon Consulting, in Aachen, Germany.

But the approach has been difficult to implement. "It has not delivered on the promise, mostly because of the complexity of the systems," Rogol says. The goal is to engineer a concentrating system that focuses sunlight, that tracks the movement of the sun to keep the light on the small solar cell, and that can accommodate the high heat caused by concentrating the sun's power by 500 to700 times--and to make such a system easy to manufacture.

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