Flat Mirrors at Center of Solar Thermal Smackdown

Flat means flat, right? In most context yet, but the definition of the word was a bit of askirmish this week in the world of solar thermal and it opened up aportal onto an issue that even many people steeped in solar didn’t knowexisted.

On Wednesday, eSolar unveiled its 5 megawatt solar thermal plant inSouthern California. The system consists of 24,000 mirrors orheliostats focusing thermal energy onto two water-filled towers. Thewater turns to steam. The steam turns a turbine, and SouthernCalifornia Edison gets some power for homes.

Bill Gross, eSolar’s CEO, says one of eSolar’s advantages lay incost. It is the only company in the world to use flat mirrors to focusheat, he claimed. Historically, solar thermal systems have used curved,parabolic mirrors.

"Our breakthrough is that we are the ONLY ones who haveinvented/patented/perfected using tens of thousands of flat mirrors,"he wrote me in an email. "That is what gives us the cost advantage. Wemake the parabola is software."

But doesn’t Abengoa use "flat" mirrors in its Solucar solar thermalplant in Spain? The white paper says so and the mirrors in all of itspictures of the two year old project look flat. And so do the mirrorsin the 6 megawatt solar thermal field erected by competitorBrightSource Energy in Israel unveiled in June 2008. (In case you’rekeeping score, that means that eSolar is third to market with a heliostat field among these three but first in the U.S.)

The mirrors in the BrightSource field are flat, says Keely Wachs, aBrightSource spokesman. They come from conventional mirrormanufacturers. When BrightSource mounts them onto the tripods so thecan focus heat on the sun, the mirrors curve a little bit. That slightcurve helps focus energy onto the water tower, but they are stillconventional mirrors, not specialty items "curved by design."

Thus, the mirrors probably cost as much as the eSolar ones. Does themounting device that BrightSource have that curves it a tiny bit costmore than the eSolar one. Hard to say. Gross says yes. Wachs madesomething approximating a sound effect and common sense says themounting things look really similar.

So there you have it. Both are right (although I haven’t checkedwith Abengoa.) eSolar likely is the first with perfectly flat mirrors.And the cost issue may be within a rounding error although it can’tfully be determined.

It is going to be an interesting space to watch. Solar thermalprojects consist of an astronomical number of variables. Which companyhas the best system will likely be decided by the banks and utilties:whatever they decide to finance by default will become the best.



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