Google’s tastes in alternative energy run the gamut from proven technologies to ideas that haven’t left the lab. Two ideas the company already is exploring:

Solar thermal

One of the companies Google touted Tuesday, eSolar of Pasadena, develops solar power plants large enough to light a small city.

The basic idea is simple enough. An array of mirrors focuses sunlight on a tower, generating intense heat. That heat, carried by a liquid flowing through the tower, is used to generate steam. The steam turns a turbine and produces electricity.

The technology is called solar thermal, and it has been around for decades. Southern California’s Mojave Desert has the world’s largest solar thermal installation, which opened in 1987 near Edwards Air Force Base. But interest in the technology languished for years when electricity prices were low.

Now utilities such as Pacific Gas and Electric Co. are signing contracts with companies eager to build the next generation of solar thermal plants. The reason: Unlike photovoltaic solar panels that sit on your roof, solar thermal facilities can generate as much electricity as a mid-sized power plant burning natural gas.

The challenge is to lower the cost. With eSolar’s technology, the towers are mass-produced and shipped to construction sites largely assembled, and the mirror arrays follow a standard design. That should cut installation time and cost. The company boasts that it can design plants ranging from 25 megawatts to 500 megawatts, enough to power 375,000 homes.

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