SOLAR ENGINE: Stirling engines will be employed to harvest the sun's heat commercially for the first time early next year in Arizona.
COURTESY OF SES

Nearly 200 years after their invention, and decades after first being proposed as a method of harnessing solar energy, 60 sun-powered Stirling engines are about to begin generating electricity outside Phoenix, Ariz., for the first time. Such engines, which harness heat to expand a gas and drive pistons, are not used widely today other than in pacemakers and long-distance robotic spacecraft.

The 1.5 megawatt (MW) demonstration site, known as Maricopa Solar, is set to begin operations early January 2010, with units provided by the Arizona-based Stirling Energy Systems (SES). While 1.5 MW is only a fraction of the power that may be generated at sites SES has contracted to develop in California and Texas, spokesperson Janette Coates says this is a necessary first step in the technology’s commercialization. “It’s important for our industry to see—and our partners and investors—that we can take a small-scale plant and get it operational before we break ground on larger ones,” she says.

That's because Stirling heat engines have a reputation for being a bit impractical. First invented by Robert Stirling in 1816, the engines use a heat source to warm gas, which expands and is pushed into another chamber. When the gas cools and contracts, it flows back. The expansion and contraction pushes a piston, which in turn produces electricity.

In 1996, SES bought solar Stirling design and engineering patents from companies such as McDonnell-Douglas and Boeing. SES then partnered with Sandia National Laboratories, and over the next decade tweaked and refined the technology. In the SES SunCatcher, a circle of curved mirrors, resembling an upturned satellite dish, tracks the sun on two axes and reflects the sun’s heat onto a single focus point, the power conversion unit (PCU). The PCU contains four cylinders, in which hydrogen gas expands and contracts to move pistons.

Stirling engines are significantly more efficient at converting sunlight into energy than most photovoltaic panels or concentrating solar power plants, whether parabolic trough or tower designs. The test units have reached 31 percent efficiency, compared to 16 percent for parabolic troughs and about 14-18 percent for PV panels in use today (though newer designs not yet on the market range from 24 to as high as 41 percent). The high efficiency numbers alone, however, have not made Stirling an easy sell. The systems have been criticized as being too expensive, unreliable and requiring extensive maintenance thanks to many moving parts. Also, ground has not yet been broken on either California site for which SES signed purchase power agreements in 2005, adding to skepticism that these systems will ever become commercially viable.

“At these high temperatures, with this many moving parts, people doubted whether SES could really pull it off,” says Reese Tisdale, research director for solar power at Cambridge, Mass.-based Emerging Energy Research. The relatively small Arizona plant is intended to allay those concerns.

Proponents of the technology point to the advantages it has over other forms of solar power, particularly concentrating solar power (CSP), which also captures the sun’s heat. Most CSP systems require significant amounts of water, which has proven to be a challenge in desert regions of the U.S. where solar power is most attractive, while Stirling engines require none other than small amounts for cleaning the mirrors. In addition, if one engine goes down, it has minimal impact on overall production.