Jerusalem
A PROTOTYPE solar device that produces enough hot air from the Sun鈥檚 rays to
drive the turbines of a 50 kilowatt power station has brought the prospect of
cheap, solar-generated electricity a step closer. The solar energy collector has
been successfully tested by Israeli researchers at the Weizmann Institute of
Science.
Many modern power stations use hot gas to drive the turbines that produce
electricity. The gas must be between 1200 掳C and 1350 掳C and must reach
pressures of between 10 and 30 bar.
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Existing solar collectors, which focus the Sun鈥檚 rays on tubes of air, cannot
do this. The best they can achieve is about 700 掳C at normal atmospheric
pressure.
The new device focuses the Sun鈥檚 rays through a quartz window to heat ceramic
pins around which air flows. The array of pins鈥攚hich researchers Jacob
Karni, Abraham Kribus and Rahamim Rubin have nicknamed the
鈥減orcupine鈥濃�攁bsorbs 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. 鈥淭hat makes it the hottest thing in the solar system other than the Sun,鈥�
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鈥檚 energy and reach 1800
掳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鈥檚 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鈥檚 fossil-fuel power stations.
A solar plant in Israel鈥檚 southern desert could 鈥渢urn Israel into a net
exporter of electricity鈥�, 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: 鈥淲e estimate seven cents per kilowatt, and that鈥檚 before
improvements that we expect will be made.鈥�
Electricity produced by fossil fuel power stations normally costs between 5
and 6 cents a kilowatt鈥攁lthough 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.
