Using the Sun’s Heat, Not Light

New York Times
Matthew Wald
July 16, 2007

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 very close, using the sun's heat instead.

Last month, Acciona Energy, a Spanish firm, opened a solar thermal installation spread across 400 acres of desert outside of Boulder City, Nev., 25 miles southwest of Las Vegas. Called Nevada Solar One, it has 47 miles 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 on 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 costs 12 to 14 cents a kilowatt-hour while power from solar cells costs 18 to 40 cents per kilowatt-hour. The national average retail price of electricity is about 10.5 cents a kilowatt-hour.

Photovoltaic cells, first made practical for the space program, are falling in price, but so are the thermal systems, which focus the sun's light 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 firm that made the collector pipe for Nevada Solar One.

Schott is developing a new 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 would stay at atmospheric pressure. The current system heats the pipe to 750 degrees, helped by small electric motors that change their angle all 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. But Thomas R. Fair, executive for renewable energy at the two companies that are buying the solar plant's output, Nevada Power and Sierra Pacific, said "the optimum economic size is probably even larger than Nevada Solar One." Lots of open desert land is available, he pointed out.

A new generation of solar thermal plants is likely to use new technology. Mr. Heming said that switching to sodium, for example, would create higher temperatures. And less energy would leak out of the pipe.

Solar thermal is also moving in two other directions. One is 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, and the hot exhaust is then 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 into steam. Proponents say that such a system could get about one-sixth more work out of the natural gas by operating at 70 percent efficiency. Most current plants operate with efficiencies in the range of 50 to 60 percent.

The World Bank is considering financial help for projects in Egypt and Morocco that would create such a hybrid. In the United States, such systems are not practical because they would lose the tax benefits that the federal government gives solar projects.

But not all solar thermal projects are large and high-tech. An American start-up company, the Solar Turbine Group, founded by engineers at the Massachusetts Institute of Technology, got $130,000 from the World Bank and is testing two prototypes in Lesotho, in southern Africa, that use simple components, including old car parts.

Four parabolic dishes, each 10 feet high and 4 feet across, focus heat on a black collector, to boil water. The steam can be run through an old car air conditioning unit, using the compressor backwards, to convert heat energy to mechanical power. (Under the hood, the compressor takes mechanical power from the engine and uses it to squeeze the refrigerant down to a liquid, so it can give off its heat.)

In the solar turbine design, the mechanical power turns a generator, which can also be scavenged from a car. Or the mechanical power can be used to drive a refrigeration unit.
In production, the units will sell for $5,000 and produce about 600 watts, said Sam White, a founder. The price is about the same as for solar cells, but this system also creates steam for hot water.

Solar thermal may have another advantage: it may be easier to store energy by storing it as heat. That is important because solar production is strongest in the afternoon but ends long before the peak demand arrives. High temperatures persist when the sun is very low in the sky or below the horizon.

One possibility is to store large amounts of hot water, or molten sodium, to allow electricity production into the early evening hours. If utilities built solar plants instead of natural gas plants to meet peak demand, the value of the solar output would increase.