Images: Supersize solar power

A number of companies are developing alternatives. BrightSource Energy signed a deal with Pacific Gas & Electric to supply 900 megawatts of electricity using a "power tower." The idea is to use heliostats, mirrors that track the sun, to reflect light onto a boiler that makes steam to turn a turbine.

BrightSource Energy argues that power towers will start to displace solar troughs because they require less capital and are more efficient at making steam.

Demand for solar power plants is coming from government mandates. There are requirements by 26 U.S. states for utilities to provide renewable energy alternatives to coal and natural gas. California demands that utilities by 2030 deliver one-third of energy via less-polluting solar, wind, and other sources.