Sunrise for solar heat power

A fourth CSP option uses a reflective parabolic dish. A Stirling engine, the basic design of which is 200 years old, sits at the dish’s focal point. About the size of a motorcycle engine, it contains hydrogen gas. When heated, the gas drives four pistons, generating electricity on the spot. Stirling Energy Systems and its partner company, Tessera Solar, use this approach in their SunCatcher. A 1.5-MW demonstration plant is slated to begin operating in January near Phoenix.

Most efficient method costs more

Some point out that this method’s large pedestals holding the glass parabolas, some 38 feet across, represent a cost that low-profile, flat-mirror approaches have successfully eliminated.

Nonetheless, in purely thermodynamic terms, the SunCatcher is the most efficient CSP approach so far. It converts 31 percent of the sun’s energy to grid-ready electricity. By comparison, photovoltaic ranges between 8 and 15 percent efficiency, and trough CSP between 15 and 19 percent.

The SunCatcher has one more advantage: The engines aren’t cooled with water. The hydrogen cools in an air radiator similar to that found in a car.

That’s important because, in the arid and semiarid regions of the desert Southwest, where CSP makes the most sense, water is already a hot-button issue. And it’s predicted to become more so.

“The Southwest is a great place to build solar, but it’s not a great place to build water-intensive solar,” says Peter Gleick, president of the Pacific Institute in Oakland, Calif., a nonprofit devoted to sustainability issues.

Air-cooling CSP is possible, but can decrease efficiency and – in some situations and regions – increase costs by 5 to 10 percent. Yet for some, it’s already a selling point. BrightSource Energy, a company developing a 400-MW “power tower” plant in the Mojave, touts the fact that its plant will be entirely air-cooled.

Other companies are developing new, cost-saving materials. SkyFuel in Albuquerque, N.M., is testing a reflective laminate material that, it says, is more durable than standard glass. Theoretically usable in any of the CSP approaches, it’s 10 to 20 percent cheaper than glass mirrors, says Andi Plocek, SkyFuel’s marketing director.

Many see cost-­effective heat storage as the biggest long-term hurdle for CSP. “The cost of storage is higher than we thought,” says Mehos.

Eventually, utilities will want to generate electricity from renewable sources through the night. To do that with CSP, they’ll need to store the sun’s energy.

Molten salts – used as fertilizer in agriculture – are one solution. But they’re also potentially difficult to handle. They freeze below a certain temperature, possibly clogging the system if allowed to cool. Another consideration with any storage is that with each transfer of heat between media, energy is lost and efficiency diminishes.

In other words, on the cusp of a CSP boom, which technology will dominate and how it will store heat is still very much an open question.

“It’s hard to predict who’s going to win in the end, if there is a winner,” says EPRI’s Ms. Libby: “We need to get more hardware on the ground and really test out all the ­options.”

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