"The issue of the linear Fresnel concept is proof of performance of a large system, not just a prototype system in the field," says Mark Mehos, concentrating solar power program manager at the National Renewable Energy Laboratory (NREL) in Golden, Colo. Ausra and other companies that employ the same technology, such as New York City–based SkyFuel and Solar Power Group in Munich, Germany, "are making large claims," he says, "without testing in the field."
If those claims stand up, however, solar-thermal plants could provide a significant chunk of the Southwest's—and potentially the nation's—electricity. "The maximum you can get into the grid is about 25 percent from solar," including photovoltaics, Mills says. But "once you have storage, it changes from this niche thing to something that could be the big gorilla on the grid equivalent to coal."
Ausra claims to have solved the storage problem without using molten salts or other expensive means of conserving heat. In fact, the company estimates that the price of its electricity will drop to roughly 8¢ per kilowatt hour if it can store heat for 16 hours. "Thermal storage is generally considered to be quite a bit cheaper than electrical storage," says Nate Blair, a senior analyst at NREL. "There isn't a lot of power generation combined with storage systems that can take advantage of that. [Concentrated solar power] has a leg up on storage in the grid or flow batteries or even ultracapacitors."
The system will employ pressure and a steam accumulator to accomplish the trick. "You allow some of the steam to recondense," O'Donnell explains. "It flashes back to steam when you reduce the pressure just by opening the valve to the turbine."
Such long-term steam storage, however, is unproved. "Steam storage is currently feasible at small levels, for example, one hour or so," NREL's Mehos notes. "Due to large volumes and high pressures involved with steam storage, scaling up steam storage to baseload applications is very high risk."
Assuming that their storage system works, Mills and his colleagues calculated in a paper presented today at the Solar Energy Society World Congress in Beijing that such solar-thermal power plants could match the electricity needs of both California and Texas. And, by combining a system that would meet the needs of California and Texas, solar-thermal plants could supply 96 percent of the national electricity demand. "The entire energy use of 2006, the current technology including storage would use a patch of land 92 miles by 92 miles," O'Donnell says. "Ten percent of the [Bureau of Land Management] land in Nevada is enough."
Even adding a transition to electric-powered vehicles did not alter the sunny picture. "You have to generate more electricity," Mills says. But "it doesn't destroy the correlation" between solar output and electricity demand for things like air conditioning.
Such a solar-dominated grid could also tolerate intermittent resources like wind energy, as long as storage systems worked. "A lot of the [winter] heating load correlates with wind [resources]," Mills adds, and the fickle supply of wind generation can be smoothed with hydropower and solar, he argues.
Such a solar solution to the nation's energy needs would require a host of other investments, including high-energy, long-distance, direct current transmission lines from areas like the Southwest or Southeast with fewer clouds to areas like the Northwest and Northeast with too many. "To do it in the East would drive up the cost because the solar resource isn't as good," NREL's Blair says. "Or you could build some kind of massive transmission system to try and get that power up to the East."