Flat means flat, right?

In most context yet, but the definition of the word was a bit of a skirmish this week in the world of solar thermal and it opened up a portal onto an issue that even many people steeped in solar didn't know existed.

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

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

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

But doesn't Abengoa use "flat" mirrors in its Solucar solar thermal plant in Spain? The white paper says so and the mirrors in all of its pictures of the two year old project look flat. And so do the mirrors in the 6-megawatt solar thermal field erected by competitor BrightSource Energy in Israel unveiled in June 2008. (In case you're keeping 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, a BrightSource spokesman. They come from conventional mirror manufacturers. When BrightSource mounts them onto the tripods so the can focus heat on the sun, the mirrors curve a little bit. That slight curve helps focus energy onto the water tower, but they are still conventional mirrors, not specialty items "curved by design."

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

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

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