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East of the Sun, West of the Moon

Wednesday, May 05, 2004

A Stirling Idea

Solar Power for the futureStirling engines have been around for a long time. Almost two hundred years, in fact. Unlike internal combustion engines, Stirling engines are a closed cycle with no exhaust gases. The pistons are driven by gas that cycles between two cylinders as it is heated and then cools. Yet Stirling engines have never really caught on because of the power and low cost of internal combustion engines.

Now a Phoenix based firm is developing a solar power system that relies on a Stirling engine to produce electricity. Unlike photovoltaic cells which directly convert sunlight into electricity, Stirling Energy Systems' Solar Dish Stirling concentrates sunlight across a 37 foot dish onto a single focal point, a specially designed Stirling engine. The focused sunlight provides the heat to power the gas exchange cylinders in the Stirling engine. A single dish is capable of producing 25 kilowatts. The company estimates that an 11 square mile solar farm in the American Southwest could produce as much electricity as Hoover Dam. A 100 square mile installation could solve the energy needs of the entire country. Wow.

This really caught my interest. The technology surrounding this system is decades old. Originally developed by McDonnell-Douglas, the concept stagnated in the 1980's when oil prices were low. The idea languished for nearly a decade until Stirling Energy Systems started up and purchased the technology from Southern California Edison for $180,000.
Stirling was founded by David Slawson in 1996 to acquire and develop the technology behind the solar generator. So far the company has raised about $15 million from private investors and has received additional funding from the U.S. Department of Energy.

The array of mirrors that covers the dish concentrates sunlight on an 8-inch opening. There, heat from the sun's rays runs a four-cycle engine that powers an electric generator. Stirling has acquired a license to manufacture the engines in the United States from Sweden's Kockums AB. Kockums initially developed the engine for use in Swedish submarines.

McDonnell Douglas Corp., Kockums and the Department of Energy developed the technology for the solar generator in the mid-1980s. McDonnell Douglas eventually sold the technology to Southern California Edison, which sold it, along with six prototype dishes, to Stirling.

Robert Liden, Stirling's chief financial and administrative officer, said the California utility lost interest in the dishes when the state limited the amount of research and development costs it could recover from ratepayers.

Liden noted that the technology was heralded in the 1980s as one of the most-efficient means to harness the sun's energy. Unlike photovoltaic cells that convert sunlight into electricity through a chemical process, the dishes do not become less efficient in hot weather.

And the Stirling units use no water, in contrast to so-called solar trough generators that use the sun's energy to produce steam to turn turbine generators.

But at a cost of $300,000 per unit, the Stirling generators are hardly cost-effective. They can produce electricity for about $1 per kilowatt, or 50 times more than power generated by the Palo Verde Nuclear Generating Station, west of Phoenix, and 20 times more than that produced by a natural-gas generator.

But Liden notes that the first units, including the engines, are being built by hand. Stirling believes it can bring the cost down to about $25,000 per unit if they are mass-produced. That would make them competitive with conventional power plants, Liden said.

-From 2 Phoenix firms to build generators, The Arizona Republic, April 28 2004
I certainly hope this idea pans out. Renewable energy sources need to be developed if we ever want to cut fossil fuel emissions and stem global warming.