Wednesday, May 07, 2008

Focusing on Solar's Cost

Sunrgi claims that its concentrated photovoltaic system outshines the competition.

By Tyler Hamilton

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Focus, focus: Sunrgi’s concentrated photovoltaic module consists of lenses that focus sunlight on a high-efficiency solar cell, and a special heat sink designed to dissipate the extreme heat produced by such an intense concentration of the sun’s rays.
Credit: Sunrgi

A Hollywood-based solar startup says that it will soon be able to produce electricity from the sun at costs that are competitive with fossil-fuel generation. The key is the company's dramatic improvement in the performance of concentrated photovoltaic technology.

Sunrgi, which emerged out of stealth mode last week, has created a concentrated photovoltaic system that uses a lens to focus sunlight up to 2,000 times onto tiny solar cells that can convert 37.5 percent of the sun's energy into electricity. Stronger concentrations of sunlight allow engineers to use much smaller solar cells, making it more economical to use higher-efficiency--but higher-cost--cells. Sunrgi, for example, will use cells based on gallium arsenside and germanium substrates.

Paul Sidlo, one of seven founding partners of Sunrgi, says that the system uses four times less photovoltaic material than other approaches, which typically aim for 500 times sun concentration. This includes systems being developed by California rivals SolFocus and Soliant Energy.

"We've miniaturized everything," Sidlo says. "What this leads to is reduced cost, and the big breakthrough here is all about lower cost." The company has also designed its system to be produced on slightly modified computer assembly lines, enabling further savings through high-volume production. The higher efficiency also means that a solar park built with Sunrgi's modules could use one-sixteenth of the space needed with conventional thin-film solar cells, adds Sidlo. The result is lower real-estate costs for developers.

Sunrgi estimates that its system will be capable of producing electricity at a wholesale cost of five cents per kilowatt-hour. Prototypes have been built and tested both in the laboratory and in the field, and the company expects to start commercial production in 12 to 15 months. "It's quite an aggressive claim," says Daniel Friedman, a solar-energy researcher at the U.S. National Renewable Energy Laboratory (NREL). He says that most others in the space are still working toward seven or eight cents per kilowatt-hour. "I can't say Sunrgi won't achieve what it's claiming, but right now, it's just on paper, and costs like that are only going to be a reality at the large manufacturing level," he says. "Even then, the five-cent figure sounds really optimistic."

Arguably the biggest breakthrough for Sunrgi is in the area of heat management, which is essential to any concentrated photovoltaic system. The intense heat created by concentrating the sun so much can reduce both the efficiency and the life of the solar cell. At 2,000 times sun concentration, temperatures can exceed 1800 °C--similar to the heat from an acetylene torch, and hot enough to melt the solar cell.

Cells in such systems are usually cooled through a combination of heat conduction, air or liquid convection, and radiation; the goal is to remove as much of the heat as quickly as possible, says Sunrgi partner KRS Murthy, who has been labeled the "thermal wizard" by his colleagues. "At each stage of conduction, convection, and radiation, we've made an improvement over what others have done," he says.

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