An artists' depiction of a Stirling Energy Systems solar power plant. View Slideshow The barren deserts of Southern California are known for relentless sunshine and miles of empty space – the perfect combination for the world's most ambitious solar-energy projects.

Two Southern California utility companies are planning to develop a pair of sun-powered power plants that they claim will dwarf existing solar facilities and could rival fossil-fuel-driven power plants.

Southern California Edison and San Diego Gas & Electric are working with Stirling Energy Systems, a Phoenix startup that has paired a large and efficient solar dish with a 200-year-old Stirling engine design.

Stirling Energy Systems is planning to build two separate solar farms, one with the capacity to generate 500 megawatts of electricity in the Mojave Desert near Victorville, California, for SoCal Edison, and a 300-megawatt plant in the Imperial Valley, near Calexico, California, for SDG&E. The utilities have signed 20-year deals to buy all the juice the farms can turn out, and have options to expand the plants if they are successful.

"Without question, this will be the largest solar project in the world," said Gil Alexander, a spokesman for SoCal Edison. "It will be bigger than all U.S. solar-energy projects combined."

Alexander said traditional coal or gas plants typically generate 500 to 1,000 megawatts, and that current solar farms are much smaller – generally in the 35- to 80-megawatt range. At the end of 2004, the United States had only 397 megawatts of solar-energy capacity, according to the U.S. Department of Energy's Energy Information Administration.

"There is a possibility with this project that solar energy could go commercial in a big way for the first time," said Alexander. "It's playing in the big leagues."

Instead of using panels of photovoltaic cells – solar power's mainstay technology for decades – Stirling Energy Systems uses 40-foot-tall curved dishes that focus the sun's energy onto Stirling engines.

Also called an external heat engine, the Stirling engine is a completely sealed system filled with hydrogen. Its design dates to 1816, and it's named for its inventor, a Scottish minister named Robert Stirling. The focused solar energy, which can reach 1,350 degrees Fahrenheit, heats the hydrogen, making it expand and drive the engine's four pistons.

Though Stirling engines have been around for almost two centuries, there have been few efforts in the past to harness the sun to run them, said Stirling Energy Systems CEO Bruce Osborn.

Osborn said the Stirling dishes are 30 percent efficient – 30 percent of the sun's energy is converted into electricity – which is two to three times as efficient as conventional photovoltaic cells.

"Solar panels are more common, and they have gotten more efficient, but they still have a long way to go," he said.

Osborn said his company's dishes are easy to maintain because the engine is a closed system that never needs to be refilled – an important factor for a large-scale facility in the middle of the desert. In fact, the only resource it consumes is "a little bit of water to wash the mirrors off every few weeks," he said.

The company is currently operating a six-dish test site at Sandia National Laboratories to showcase the concept, but the SoCal Edison and SDG&E plants are Stirling Energy Systems' first commercial contracts.

The first phase of the SoCal Edison project will be to build a 1-megawatt test site using 40 dishes, which should be complete by spring 2007. Construction on the full, 500-megawatt facility is expected to begin in mid-2008, and should take three to four years. Each dish can produce up to 25 kilowatts, and the site will eventually have 20,000 dishes stretching across 4,500 acres of desert.

Stirling plans to begin construction on SDG&E's 300-megawatt project in late 2008, and it should take about two years to install the 12,000 dishes covering about 2,000 acres.

None of the companies would give a price for building the solar sites or disclose the rates the utilities will pay for power, but both said the cost would be similar to traditional coal or gas.

But as oil prices go up, so could the cost of electricity from fossil fuels.

"Soon, solar may be less expensive," Osborn said.

Joel Makower, co-founder of market-research firm Clean Edge, said Stirling Energy Systems' solar-thermal power systems are impressive but unproven. One promising sign is the utility companies' level of commitment to the new technology.

"This is all on paper so far," he said. "They haven't delivered anything yet. And until they do, we can't say what it will cost."

Still, Makower said he was optimistic.

"Photovoltaic was the first-generation, utility-scale solar technology," he said. "The Stirling engine looks like it will be the second generation."

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