PV is a spectacular technology for distributed generation and, in coming years, will adorn rooftops and eventually walls and windows around the world. But its future in utility-scale power generation is less certain, for reasons already discussed. To replace coal and natural gas in utilities' portfolios, something else is needed. And that something might be solar thermal. Whereas PV coverts sunlight directly into electricity, solar thermal converts sunlight to heat and then uses that heat to generate electricity. Its current versions are cheaper than PV, and some new designs appear to put it within range of coal. The main solar thermal designs include the following:
Parabolic trough, which uses curved mirrors to reflect sunlight onto a hollow tube running along above the trough. "Thermal oil" passes through the tube and is heated by the concentrated sunlight. The oil then passes through a heat exchanger, turning water into steam, which runs a turbine.
Solar tower, in which mirrors track the sun and reflect its rays onto water pipes at the top of a central tower. The water boils, generating steam that drives a turbine. The first commercial solar tower, with a capacity of 11 megawatts, was completed in 2005 near Seville in Spain. A second tower, capable of generating 20 megawatts, is scheduled for 2008.
Stirling engine, which uses dish-shaped mirrors to direct solar energy at an "external combustion" engine in which heat at one end causes a gas to expand, driving an internal piston. The heat dissipates at the other end, causing the gas to contract, and sending the piston back for another go-round. This process converts thermal energy (i.e., concentrated sunlight) into mechanical power and thus electricity-apparently very efficiently. Tests at Sandia National Laboratories were promising enough to lead Southern California Edison and San Diego Gas & Electric to put in big orders for Stirling engines.
Solar Thermal's story is similar to PV's: a hot idea during the first energy crisis that turned out to be too expensive to compete with coal and natural gas and was largely abandoned in the 1990s. But a few diehard fans kept plugging away, refining their designs and proselytizing to anyone who would listen. And now a lot of people are listening. Because it's easier to generate heat than electricity from sunlight, modern solar thermal is already far cheaper than PV. Existing solar thermal plants generate power for about $0.15 per kilowatt-hour (kWh), tantalizingly close to the $0.10 that's widely seen as the magic number for head-to-head competition with coal and gas.
And as with PV, there's a solar thermal start-up that claims a radical breakthrough and has attracted big bucks from venture capitalists: Silicon Valley-based Asura Whereas the standard solar thermal design uses curved mirrors, Asura uses relatively cheap, mass-produced flat mirrors. And instead of heating oil, it runs water through high-strength tubes. The water turns directly into steam, which runs a turbine. Asura claims that it's already at $0.11 per kWh, and in early 2008, it secured $40 million to build a square-mile, 175-megawatt plant in California.