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Thursday
Aug272009

A MIGHTY HOT WIND FOR SOLAR.

by Michael Kanellos

It's like a jet engine powered by the sun.

HelioFocus, which grew out of research at Israel's Weizmann Institute, has created an unusual solar thermal device that it claims that will take up far less real estate than the parabolic dishes and heliostats already being deployed to convert heat into electricity.

The details, though, take a little getting used to. The final version will be a six-story high parabolic dish that will concentrate the sun's energy onto an optical receiver at its center. The proprietary receiver in turn converts the light into a stream of hot air that can reach 1,000 degrees Celsius. The hot air then gets funneled through a gas turbine rejiggered for solar power.

"We take a gas generator and solarize it," said Sass Somekh, one of the founders and a partner at Musea Ventures. Musea and Israel Green Corp invested $20 million in the company in 2008.

An early prototype, which has operated for about a year, has generated air streams hitting 850 degrees Celsius, or about twice the temperature of the surface of Mercury at high noon (425 to 450 degrees Celsius.).

Ideally, a 50-megawatt power plant made with these would take up about half of the real estate of a traditional parabolic trough system. Like with heliostats, acres of land would not have to be graded flat because the towers would be independent and modular.

The closest analogy in the thermal world is to Stirling engine solar systems. In Stirling systems, heat from the sun drives a piston in the center of the parabolic dish. The HelioFocus device gets its power from a Brayton cycle engine: a constant stream of compressed gas is the source of power. George Brayton filed a patent for his Ready Engine in 1872. The Brayton cycle became the basis of gas turbines and jet engines.

One of HelioFocus' towers, though, can connect directly to a more conventional turbine because the temperature of the air stream is far higher, he said. Currently, the company retrofits micro turbines from Capstone for that job.

"The cost of the dish is higher [than the dish in Stirling systems], but we hope the cost of electricity is the same," he said.

The efficiency of the two is arguably comparable. HelioFocus says it will have efficiencies of over 20 percent. Stirling Energy Systems' 25 kilowatt SunCatcher dishes exhibit a 25 percent efficiency on average and once hit 31 percent, a record for solar.

HelioFocus will also sell the dishes to hybrid power plants, which get electricity from gas and solar power. It's one turbine after all. Various utilities have already mapped out hybrid solar thermal plants in Israel, Florida, and North Africa. And, like Ausra and eSolar, HelioFocus will sell its equipment to hospitals, mines, food producers and others that need large amounts of steam.

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