The largest series of solar installations in history, more than 1,300 megawatts, is planned for the desert outside Los Angeles, according to a new deal between the utility Southern California Edison and solar power plant maker, BrightSource.

The momentous deal will deliver more electricity than even the largest nuclear plant, spread out among seven facilities, the first of which will start up in 2013. When fully operational, the companies say the facility will provide enough electricity to power 845,000 homes — more than exist in San Francisco — though estimates like that are notoriously squirrely.

The technology isn't the familiar photovoltaics — the direct conversion of sunlight into electricity — but solar thermal power, which concentrates the sun's rays to create steam in a boiler and spin a turbine.

"We do see solar as the large untapped resource, particularly in Southern California," said Stuart Hemphill, vice president of renewable energy and power at Southern California Edison. "It's barely tapped and we're eager to see it expand in our portfolio."

BrightSource is the reincarnation of Luz International, which built the only currently operating solar thermal facility during the 1980s in the Mojave Desert. After natural gas and energy prices plunged in 1985, that operation became unprofitable. The group's engineers and founders moved the business to Israel, where they continued to work on their technology.

The new deal breaks the company's own record for the largest ever solar deal. The new installations, when completed, will produce 3.7 billion kilowatt hours of electricity per year. Previously, they'd cut a deal to deliver 900 megawatts of power to the Northern California utility, PG&E.

"Coupled with our earlier partnership with PG&E, this agreement proves that the energy industry recognizes the important role that solar thermal will play in the energy future," John Woolard, CEO of BrightSource, said in a press conference with reporters.

While Brightsource is a leader in the field, a variety of other companies compete in the solar thermal space. Google.org and other investors have backed eSolar's with $130 million funding. Abu Dhabi's clean-tech fund, Masdar, has funded a $1.2 billion solar thermal company called Torresol. Yet another player, Abengoa, recently signed a $4 billion deal with Arizona Public Utilities, and Stirling Energy Systems, a company that has adapted the Stirling Engine, a 200-year-old invention, for concentrated solar power, even pulled in a $100 million investment.

The first of the seven installations will be in Ivanpah,

California and will be rated at 100 megawatts of peak power. The companies expect it to produce 286,000 megawatt hours of electricity per year. When all the installations are finished, they'll stretch over

10,500 acres of land.

Southern California Edison's Hemphill said that the new plants would provide a valuable hedge against volatile natural gas prices, noting that his company had seen natural gas prices as low as $4 per thousand million cubic feet (a standard industry measure) and as high as $16. Given the variability of natural gas pricing, Hemphill said that his company did not expect the solar thermal electricity to exceed the market cost of electricity in California.

The 1980s-era solar thermal plants use the oldest solar thermal technology around, known as a parabolic trough.

Mirrors shaped like a paper-towel roll cut in half concentrate the sun's rays on a liquid. That heat can be transformed into various types of energy. The Luz fields made electricity, but Frank Shuman built a plant based on this principle to pump water in Egypt in the first decade of the 20th century.

The new design sounds more exciting. Mirrors that track the sun —

heliostats — sit in a massive field around a tower with a boiler. All those mirrors concentrate the sun's heat on the boiler, which makes steam and drives a turbine.

Solar thermal is seen as a promising source of energy for city-scale power because it works on very well established principles.

Photovoltaics have come down in price — and thin-film plastic solar cells could get even cheaper — but the conversion of sunlight to electricity remains a novel source of energy. The first working cells were only built half a century ago, and they were truly something new in the world.

Steam-driven turbines, on the other hand, make almost 90

percent of the world's electricity and their ancestry stretches back to the start of the Industrial Revolution. Solar thermal engineers, then, can use the knowledge gained from more than a century of tinkering at coal, natural gas, and nuclear fission plants.

See Also:

WiSci 2.0: Alexis Madrigal's Twitter , Google Reader feed, and project site, Inventing Green: the lost history of American clean tech; Wired Science on Facebook.