Along with cost per watt, solar thermal's biggest selling point is its ability to store energy and deliver electricity to consumers during periods of peak power demand. Experts at a solar conference here this week said "concentrating" solar thermal power could allow utilities and other emerging operators to store steam energy for up to six hours. Super-heated steam is used to drive turbines that generate electricity.

Concentrating, or sun-tracking, photovoltaics and solar thermal power collectors like parabolic troughs follow the sun across the sky at one of more axis points, focusing sunlight to improve the efficiency of solar panels.

Experts here noted that solar thermal's so-called "dispatchability" means stored power could be used to generate electricity that could then be sold to utilities during load peaks on electric grids, usually after 5 p.m. The approach would make solar thermal power far more valuable for plant operators than, say, photovoltaic energy that must be used immediately.

"Thermal energy storage is the killer app of concentrating solar power technology," Andrew McMahan, vice president for technology and projects at SkyFuel (Albuquerque, N.M.), told the solar technology conference held Monday (July 14) in conjunction with Semicon West. Solar thermal collector technologies like parabolic troughs have a good track record after more than 20 years of use, McMahan added. "The technology has steadily improved and is being demanded by utilities" when negotiating power supply agreements with solar operators.

Industry analysts like Jim Hines, Gartner Inc. research director for semiconductors and solar, agree that solar thermal appears best suited to large power projects aimed at supplying electricity to utilities. Other technologies like traditional photovoltaics and concentrating PV systems work best in residential and commercial applications, Hines said.

Among the solar thermal projects discussed here were several "power tower" projects that use concentrating solar collectors to refocus sunlight on "solar boilers." For example, solar developer Brightsource Energy (Oakland, Calif.) is building a 400-megawatt solar thermal complex in California's Mojave Desert, a prime location for a number of planned solar thermal projects. Along with other industry executives here, Brightsource CEO John Woolard noted that the primary challenge for solar thermal is efficiently transmitting power from remote desert locations to population centers.

Still, experts here agreed that for large alternative energy projects, solar thermal for now appears to be the best approach. According to estimates compiled by the Prometheus Institute for Sustainable Development, solar thermal power-generating costs could drop from about $4.25 per watt in 2008 to $2.5 per watt by 2020.

Solar thermal "is an extremely cost-effective technology compared to other [solar] technologies," said Travis Bradford, founder of the Prometheus Institute, although costs may not drop as fast as competing technologies like traditional and concentrating photovoltaics.