Can Stirling Engines and a Record-Holding Company Deliver Massive Solar Power Gains?

Photo: Stirling Energy Systems

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Phoenix-based Stirling Energy Systems plans to begin construction in 2009 on two $1 billion solar power farms on federal land in California's Mojave Desert northeast of Los Angeles and in the Imperial Valley east of San Diego, reports USA Today. When finished the farms will be among the world's largest solar energy deployments.

The plants would nearly double the amount of solar energy produced in the U.S., would power 1 million Southern California homes, and would be around the equivalent of two dirty coal plants.

State and federal regulators still have to approve the plans. But, in 2005, Southern California Edison and San Diego Gas & Electric, two of the state's biggest utilities, signed contracts enabling them to buy all of Stirling's solar power for 20 years. The utilities have pledged to buy 800 megawatts annually with the possible addition of 950 more. Although the contract prices haven't been revealed, Stirling officials say they are competitive with other sources for peak power. Observers say that's likely to be more expensive than coal energy, but less than many current forms of solar.

So how does Stirling plan to harvest the sun's rays? The company already has a 40-foot test dish in place. It uses a mirror to track the sun like a sunflower, and focuses the heat to drive an engine, which makes electricity. This makes it a solar thermal process, as opposed to solar electric, in which solar panels convert energy directly into electricity. Stirling hopes to deploy 70,000 similar solar thermal dishes on the two sites.

Experts point out that solar thermal is likely to be the next high-growth renewable power. Nearly 5,000 megawatts may go online by 2020, according to Cambridge Energy Research Associates, more than 10 times the amount produced today.

Stirling's system is different than traditional solar thermal arrays, however, because most so far have used many mirrors to focus energy on one point, where a boiler and engine make the conversion to electricity. In contrast, each of Stirling's mirrors come with their own high efficiency Stirling engine.

Named after a Scottish amateur inventor who first applied for the patent in 1816, Stirling engines boast high efficiency and quiet operation, and have been in use in specialized applications for many years, from submarines to auxiliary power generators. They can run off any heat source, and are powered by the expansion of a sealed gas when heated, followed by the compression of the gas when cooled. They have so far not been cost competitive with internal combustion engines for many applications, such as transportation, but their use is certainly not unprecedented.

Skeptics, including a former founder of the company, point out that Stirling Energy Systems has a way to go to get the technology ready for prime time. Some are concerned costs and reliability will always be big hurdles, and are doubtful solar thermal will ever be more than a niche player, or supplemental source. But others think the development points the way to an exciting, bright energy future.

For its part, Stirling Energy points out that its solar dish Stirling equipment has held the world's efficiency record for converting solar energy into grid-quality electricity since 1984.

Update on 6/9/08 Also check out Ausra's prototype system, which will reportedly become the largest solar thermal array in the U.S. in the near future.

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