Sun catchers tuned to crank out the juice
R. Colin Johnson
EE Times (11/22/2004 10:00 AM EST)
PORTLAND, Ore. — EEs are turning a 19th-century invention into a 21st-century alternative-energy source.
The last leg of a two-decades-long effort by the U.S. Energy Deaprtment to unleash superefficient solar power by 2011 is homing in on the so-called Stirling engine, which is being used to drive solar generators. DOE test site measurements suggest the setup could bring the cost of solar power on a par with traditional fossil fuels and hydroelectric sources — assuming the project engineers can balance the separate power feeds from farms of thousands of simultaneously online 25-kilowatt Stirling solar dishes.
The heart of the design, the engine itself, was invented by the Scottish minister Robert Stirling in 1816.
"The Stirling engine makes solar power so much more efficiently than photovoltaic solar cells can," said Robert Liden, chief administrative officer at Stirling Energy Systems Inc. (Phoenix). "That's because the Stirling solar dish directly converts solar heat into mechanical energy, which turns an ac electrical generator." The bottom line, he said, "is that large farms of Stirling solar dishes — say, 20,000-dish farms — could deliver cheap solar electricity that rivals what we pay for electricity today."
Under a multiyear Energy Department contract that started in 2004, Stirling Energy Systems will supply Sandia National Laboratories with solar dishes for integration into full-fledged power-generation substations capable of direct connections to the existing U.S. power grid. Right now about 20 EEs, including more than a dozen from Stirling Energy Systems, are working full time at Sandia to create the electrical-control systems to manage these sunshine stations.
By the end of 2005, they plan to have six dishes connected into a miniature power station capable of supplying enough 480-volt three-phase electricity to power about 40 homes (150 kW). The next step, in 2006, is a 40-dish power plant that will transform the combined output of the farm from 480 to 13,000 V, for distribution of industrial-level power to an existing substation. From 2007 to 2010, the program proposes mass-producing dishes to create a 20,000-dish farm supplying 230,000 V of long-haul power from its own substation directly connected to the grid.
If the project succeeds, the DOE predicts that by 2011, Stirling solar-dish farms could be delivering electricity to the grid at costs comparable to traditional electricity sources, thereby reducing the U.S. need for foreign sources of fossil fuels.
Eventually, according to DOE estimates, an 11-square-mile farm of Stirling solar dishes could generate as much electricity as the Hoover Dam, and a 100 x 100-mile farm could supply all the daytime needs for electricity in the United States. By storing the energy in hydrogen fuel cells during the day, Stirling solar-dish farms could supply U.S. electrical-energy needs at night too, as well as enough juice for future fuel-cell-powered automobiles, the DOE believes.
Power today costs from about 3 cents to 12 cents per kilowatt-hour, depending upon the customer's location and the time of day. The average is 6.6 cents/kW-hr for the industrial sector in 2004, according to DOE. In contrast, the Stirling solar-powered substations operate only during peak hours (daytime) but at potentially the same or less than the peak rates paid today — or "about 6.5 cents per kilowatt-hour during peak periods," said Liden of Stirling Energy Systems.
Prior DOE tests settled on the Stirling solar dishes by comparing traditional solar power with three kinds of "focused thermal" solar energy — all of which operate in a manner similar to the solar-power generator in the James Bond movie The Man with a Golden Gun. There, Roger Moore narrowly escapes being fried by the concentrated beam from a focused solar mirror that uses the same principle whereby leaves are set on fire with the focused sunlight from a magnifying glass.
The DOE compared the Stirling solar dish, parabolic troughs, power towers and concentrated photovoltaics. The study, conducted at Sandia National Laboratories' Solar Thermal Test Facility, concluded that Stirling dishes outperformed all other sources of solar power.
Today Stirling-powered solar dishes at the Sandia test facility operate at 30 percent efficiency while delivering grid-ready alternating current. In contrast, 30-percent-efficient solar cells are direct current and drop to 16 percent efficiency by the time they generate grid-ready ac. And that's on a hot day. Efficiency can drop as low as 10 percent on a cool day.
"Tests have already shown that the Stirling engine can be made into a very efficient power generator," said Chuck Andraka, project leader at Sandia's Solar Technology Department. "Now what we need to show is that many small Stirling engines can be coordinated in farms that together rival traditional power sources."
Time for a change
Historically, the Stirling engine could never compete with the bigger bang per cubic inch of a gas-guzzling internal-combustion engine. However, dependence on foreign oil, increasing pollution and America's seemingly unquenchable thirst for more energy hint that it might be time for a turnaround.
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