Sunrise for solar heat power

Currently, CSP costs about 14 cents per kilowatt-hour (kWh), within striking range of current combined-cycle natural-gas plants, in which a gas turbine generator generates electricity and a steam turbine uses the waste heat to generate more. A combined-cycle natural-gas plant produces electricity for about 12 cents per kWh.

Pulverized coal plants, on the other hand, generate electricity for 6 cents per kWh – less than half CSP’s cost. But, says Mr. Mehos, if you assume that future coal-fired plants will require carbon sequestration, then that cost moves up to about 10 cents per kWh. That means CSP prices still need to drop by nearly one-third to be competitive with future coal plants.

A plethora of CSP companies are racing to innovate and reduce costs. At this point, CSP technology comes in four general “flavors,” each with different perceived strengths and weaknesses.

Parabolic-trough systems focus the sun’s energy onto a tube running their length. Temperatures in the tube can reach 750 degrees F. A medium in the tube – sometimes synthetic oil that transfers its heat to water, sometimes water itself – collects heat to drive turbines. A second troughlike system, called a Compact Linear Fresnel Reflector, uses several mirrors to focus the sun’s rays on a single receiver tube above.

Trough technology benefits from being proven. That was Shuman’s design, and it’s the one the Mojave plants installed in the 1980s and ’90s. A planned 280-MW CSP plant near Phoenix will use trough technology, and store heat for electrical generation in molten salts.

‘Power towers’ more efficient
But experts say that, although it’s a known quantity, trough technology may be less efficient than newer, albeit less-tested, approaches. One reason: Newer systems achieve higher temperatures, which greatly increase efficiency.

So-called “power towers” – thousands of mirrors, or heliostats, directing the sun’s rays at a central tower – can achieve 1,300 degrees F. They also benefit from not having to pump the receiver fluid through tubing, an energy loss.

The first commercial scale “power tower” plant began operating near Seville, Spain, in 2007. The 11-MW plant resembles a gigantic, silver-petaled flower reflecting rays of light toward a central stamen. (Others compare it to the lidless Eye of Sauron in the “Lord of the Rings” movies.)

In the US, eSolar recently brought a 5-MW “power tower” demonstration plant on line near Lancaster, Calif. It includes a number of innovations, says Jim Shandalov, eSolar’s vice president of business development. Its relatively small mirrors – thousands of them about a yard square – rise no higher than four feet. Compared with troughs, which can be up to 10 feet high, or the Spanish plant, which has mirrors mounted on frames more than 120 square meters in area, the four-foot height keeps the wind profile down. A smaller profile also means fewer building materials.

Small, flat mirrors are also cheaper to manufacture, transport, and install, he says. An automated robot on a track can clean and maintain the mirrors, further cutting costs.
Finally, new software calibrates each mirror individually, an improvement over the “two walkie-talkie men” method of yore, says Mr. Shandalov, “We’ve made it more efficient.” (He won’t say by how much – that’s a trade secret.)

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