Those of us who have tracked solar energy innovations for several decades will likely have heard about Stirling engines. This sealed "heat engine" technology has been around a long time--since 1816, in fact--when it was invented by the Scottish clergyman and inventor Robert Stirling. For decades, while there were cool models to demonstrate the principle in classrooms, actual full-scale solar applications of the Stirling engine seemed always to be just around the corner.

Well, it appears that the wait is over.* (see below)Stirling Energy Systems is now manufacturing the SunCatcher, a large, tracking, concentrating solar power (CSP) dish collector that generates 25 kilowatts (kW) of electricity in full sun. Each of the 38-foot-diameter collectors contains over 300 curved mirrors (heliostats) that focus sunlight onto a power conversion unit, which contains the Stirling engine. The dish uses dual-axis tracking to follow the sun precisely as it moves across the sky. To understand why I'm excited about the Stirling SunCatcher you have to understand how a Stirling engine differs from steam engines and steam turbines. Stirling engines are closed-cycle engines (sometimes called "external-combustion" engines) in which an external heat source heats air or another gas, such as helium or hydrogen, in one of the cylinders. The expanding gas pushes a piston, which compresses gas in the cooler portion of the engine. The SunCatcher uses hydrogen in an advanced, four-cylinder design. Concentrated heat from the sun is the external heat source to drive the process. No water or steam is involved--which has very significant implications, since places with a lot of sunlight where CSP applications are most cost effective are also usually very dry.

In January, 2010, four months after breaking ground, Stirling Energy partner company Tessara Solar completed the 1.5 MW Maricopa Solar power plant in Peoria, Arizona, just outside Phoenix. The power plant is comprised of 60 SES SunCatchers. The Maricopa plant is considered a demonstration plant; Tessara hopes to break ground on much larger plants in California and Texas, totaling 1,500 MW of capacity, later this year.

The Stirling engine SunCatcher isn't the only solar-thermal power-generation system that is ramping up in a big way across the desert Southwest these days. A different dual-axis-tracking technology, from BrightSource Energy, is going to be used in a 392-MW Ivanpah solar power plant in the Majave Desert, with groundbreaking planned for later this year. The U.S. Department of Energy recently announced a $1.37 billion load guarantee to support this project. BrightSource has contracts with Southern California Edison and Pacific Gas & Electric to supply a total of 2,600 MW of electric capacity over the next several years, though all of these projects are pending final permits.

The Stirling SunCatcher differs from BrightSource technology--and that of a somewhat different technology that is used in the Solar Energy Generating Stations (SEGS) plants that have been operating since the early 1980s--in how the heat is converted into electricity. Most CSP systems, including BrightSource, uses the heat to boil water, creating high-pressure steam that then spins a steam turbine. Thus, these CSP systems require water to operate. The SunCatcher does not consume water, since the Stirling engine uses sealed, hydrogen-filled cylinders to convert thermal energy into electricity.

One thing is for sure: the next year will be an exciting period for large-scale solar thermal plants. Keep watching!

For more information:

Stirling Energy Systems

Scottsdale, Arizona

602-957-1818

www.stirlingenergy.comTessara Solar

Houston, Texas

713-554-8484

www.tessarasolar.com

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Alex Wilson is the executive editor of Environmental Building News and founder of BuildingGreen, LLC. To keep up with his latest articles and musings, you can sign up for his Twitter feeds.

Photos of SunCatcher dish collectors at the Maricopa solar power plant. Source: Stirling Energy Systems.

See more on this product in the GreenSpec Guide* Well, maybe not. We just learned (12/27/11) that Stirling Energy Systems is another victim of the fallout that's happening in the solar electric industry. Low-cost photovoltaic modules from China are making it hard for innovators in solar electric generation to compete economically. The company filed for bankruptcy in late-September, 2011.

It's still a neat technology, though. Someday, some company will make a viable Stirling engine generator!

-Alex