Everything Under The Sun

Every twist on solar energy imaginable has been proposed for the Mojave Desert, from technology invented during Thomas Jefferson's lifetime to 21st-century thin-film solar cells that rival fossil fuels for the low cost of the electricity they generate.

Here are some of the principle approaches now on the table.

Solar Thermal Parabolic Trough System: Rows of mirrors shaped like half-pipes reflect the sun's rays, heating synthetic oil that runs through a central tube up to 750 degrees Fahrenheit. The hot oil then is used to turn water into steam to drive the turbines of an electrical generator. Nine plants occupying some 1,600 acres in the Mojave Desert have been using this technology since the 1980s.

Solar Power Tower: Fields of sun-tracking mirrors called heliostats, which may cover thousands of acres, follow the sun throughout the day and beam concentrated light at a tower hundreds of feet high. Water or molten salt is superheated in a holding tank atop the tower to make steam for generating electricity. The experimental plants built in the Mojave in the 1980s called Solar One and Solar Two used this technology, and many of the new solar plants that power companies are now proposing to build in the desert would use this approach.

Scalable Power Tower: A Pasadena-based company called eSolar uses computerized tracking equipment and mirrors mounted on modular units to generate the same amount of power as the solar power tower technology described above, but on less land area. The smaller footprint makes these plants easier to install near cities and on abandoned farmland. ESolar has a small test facility in Antelope Valley, near Lancaster, and has contracted with Southern California Edison to build a 245-megawatt plant nearby. (One megawatt powers about 800 homes per year.)

Stirling Engine: The Stirling engine?also called the heat engine?has been around since 1816, an Industrial Age alternative to steam power. When attached to a computerized, sunlight-concentrating dish system called a SunCatcher, the device can generate clean electricity without steam.

Crystalline Silicon Solar Panels: Solar cells generate electricity directly from the sun's photons, which create a stream of electrons in certain light-sensitive substances. The first commercially viable solar cells utilized silicon to achieve this photovoltaic effect, and they remain the most commonly used cells for residential-rooftop and utility-scale installations. This is one of the most efficient technologies available for converting sunlight into electricity, but in both labor and materials it is also the most costly. Silicon panels are used at the largest photovoltaic generation plant in North America: the 140-acre, 14-megawatt plant at Nellis Air Force Base in Nevada.

Thin-Film Solar Arrays: New techniques for producing solar cells as a spray-on or electro-plated film have drastically reduced production costs for photovoltaics. Even though thin film is half as efficient as silicon crystal panels, its low cost makes it a viable alternative for large-scale photovoltaic plants around the world. -EH

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