Technology : Solar Power Turns on the Heat

Many modern power stations use hot gas to drive the turbines that produce electricity. The gas must be between 1200 C and 1350 C and must reach pressures of between 10 and 30 bar.

Existing solar collectors, which focus the Sun's rays on tubes of air, cannot do this. The best they can achieve is about 700 C at normal atmospheric pressure.

The new device focuses the Sun's rays through a quartz window to heat ceramic pins around which air flows. The array of pins-which researchers Jacob Karni, Abraham Kribus and Rahamim Rubin have nicknamed the "porcupine"-absorbs solar energy and transfers it to the air. Because the ceramic pins have a large surface area they transfer heat to the surrounding air very efficiently.

A funnel-shaped device concentrates the energy using internal mirrors that channel the rays. This can concentrate the energy to 10 000 kilowatts a square metre. "That makes it the hottest thing in the solar system other than the Sun," says Karni.

This concentration of solar rays then passes through the quartz window.

Quartz is used because it is transparent and is strong enough to withstand the pressure of the gas inside. The pins absorb the Sun's energy and reach 1800 C. Air flow around the pins is carefully controlled to prevent them overheating.

In principle, this technology could be used in any size of power station, from small industrial generators to large stations feeding a national grid.

Karni estimates that Israel's peak-hour electricity demand of 6000 megawatts could be provided by solar power stations collecting sunlight from an area of 2000 hectares. This is considerably less than the area currently taken up by Israel's fossil-fuel power stations.

A solar plant in Israel's southern desert could "turn Israel into a net exporter of electricity", says Karni. The technology could also be used to convert solar energy into chemical fuel, he adds.

Uri Fischer, director of development at Ormat, an Israeli company that produces gas turbines, geothermal plants, and other energy devices, is cautiously optimistic. His company has invested in the Weizmann Institute project. He says the technology could produce solar-generated electricity at competitive prices: "We estimate seven cents per kilowatt, and that's before improvements that we expect will be made." Electricity produced by fossil fuel power stations normally costs between 5 and 6 cents a kilowatt-although in many places the cost can be far higher.

However, Fischer says the researchers still have to show that a full-scale version can run efficiently. Ormat is continuing to support the research.

Fischer says there is a market for small industrial uses of the technology. He believes that industrial-sized solar power generators should be on the market in three or four years.