Mills will not say what material his company’s system will heat, although several recent solar-thermal plants by Ausra competitors–including one in Nevada that started up this summer and two under construction in Spain near Granada–plan to use molten-salt storage. Molten salts are inexpensive salt solutions that absorb considerable energy when they melt and give up that energy when they freeze.
What Mills can say for certain is that Ausra’s storage system will lower its power-generation costs. That is a surprising statement since energy storage can as much as double the cost of electricity from photovoltaics or wind turbines.
Heat storage is more efficient than electricity storage: just 2 to 7 percent of the energy is lost when heat is banked in a storage system, compared with losses of at least 15 percent when energy is stored in a battery. More important, says Mills, is the fact that storage enables thermal plants to use cheaper turbines.
The bottom line is that Mills vows that adding storage plus savings from economies of scale and lower cost of capital (as banks become familiar with solar-thermal technology) will cut Ausra’s current 10 to 11 cents per kilowatt-hour cost of power in half. By 2010, he expects solar thermal to provide California with baseline power cheaper than natural gas, currently set by the state at 9.2 cents per kilowatt-hour.
Why has solar-thermal power received little attention from the energy-storage community despite such promise? John Boyes, manager of the Energy Storage & Distributed Energy Resources at Sandia National Laboratories, in Albuquerque, NM, says that solar thermal is viable but inflexible compared with other means of storing energy, such as, say, coupling wind farms to large batteries, flywheels, and supercapacitors that can be placed almost anywhere on a power grid. “You can store energy anywhere you have electricity and a little bit of floor space,” says Boyes.
The footprint of Ausra’s planned 175-megawatt plant will be, in contrast, about one square mile.