Thu, Jul 19, 2007 - Page 9 News List

Harnessing power by capturing heat, not sunlight

Researchers around the world are making progress in the search for more affordable and sustainable forms of energy production


Scientists have struggled for years to make electricity from the sun's light at a cost competitive with power from coal or natural gas.

The challenges are formidable. But now they are close, using the sun's heat instead.

Last month, Acciona Energy, a Spanish company, opened a solar thermal installation spread across 160 hectares of desert outside Boulder City, Nevada, 40km southwest of Las Vegas. Called Nevada Solar One, it has 75km of trough-shaped mirrors, lined up in rows. Producing 64 megawatts, it is many times larger than the largest photovoltaic installations, which use the cells that are found in everything from rooftop panels to pocket calculators.

Acciona will not disclose the production costs at the thermal plant, which was subsidized by the Energy Department.

But according to the Solar Energy Industries Association, representing manufacturers of both photovoltaic and solar thermal systems, power from solar thermal electricity costs US$0.12 to US$0.14 a kilowatt-hour to produce, while power from solar cells costs US$0.18 to US$0.40 a kilowatt-hour.

The national average retail price of electricity is about US$0.11 a kilowatt-hour.

Photovoltaic cells, first made practical for the space program, are falling in price, but so are the thermal systems, which focus sunlight on a fluid-filled pipe to collect the heat.

"There's really this renaissance of solar thermal technology, and people are working very hard on how to reduce costs," said Martin Heming, the executive in charge of solar technology at Schott, the German company that made the collector pipe for Nevada Solar One.

Schott is developing a system that will use molten salt, rather than a liquid, to fill the pipes. Salt could absorb the same amount of heat or more without boiling, and it would stay at atmospheric pressure. The current system heats the pipe to 400oC, helped by small electric motors that change the angle of the mirrors during the day to face the sun.

The heated fluid runs through the pipes to a central power block, where it flows through a network of pipes bathed in ordinary water. The water is boiled and the steam drives a turbine, as in coal plants.

But a typical coal-burning plant produces about 600 megawatts, roughly 10 times more than Nevada Solar One does.

"The optimum economic size is probably even larger than Nevada Solar One," said Thomas Fair, executive for renewable energy at the two companies that are buying the solar plant's output, the Nevada Power Co and the Sierra Pacific Power Co. There is a lot of open desert land, he said.

A new generation of solar thermal plants is likely to use more efficient technology. Heming said that switching to sodium, for example, would allow for higher temperatures.

Less energy would leak out of the pipe, and electric output would rise.

Solar thermal power is also being applied in two other ways.

One is as a supplement to a combined-cycle gas plant. In those plants, natural gas is burned in a jet engine, turning a turbine shaft to make electricity.

The exhaust is used to boil water, and that steam is used to turn a second turbine, making more electricity.

In the solar thermal variant, heat from the sun is used to preheat the water that the exhaust gases will boil. Proponents say this system could get about one-sixth more work out of the natural gas as it operates at about 70 percent efficiency.

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