SAN FRANCISCO (Reuters) - Google Inc has developed a prototype for a new mirror technology that could cut by half the cost of building a solar thermal plant, the company’s green energy czar said on Friday.

Bill Weihl said that if development and testing go well, he could see the product being ready in one to three years.

“Things have progressed,” Weihl said in an interview. “We have an internal prototype.”

Google has been looking at unusual materials for the mirror’s reflective surface and the substrate on which the mirror is mounted.

In solar thermal technology, the sun’s energy is used to heat a substance that produces steam to run a turbine. Mirrors focus the sun’s rays on the heated substance.

The Internet search engine company, which has been investing in companies and doing research of its own to produce affordable renewable energy, wants to cut the cost of making heliostats, the fields of mirrors that track the sun.

“There is a decent chance that in a small number of years, we could have a 2-X reduction in cost,” he said.

Global companies are increasingly investing in green technology as the world grapples with global warming and governments strive to implement regulations that could limit greenhouse gas emissions.

Google has invested in two solar thermal companies, eSolar and BrightSource, with which it has discussed the new mirror technology, Weihl said.

He said the technology was not at a stage where it could be tested externally, but he added that both eSolar and BrightSource were interested in it.

“If it works, it would absolutely be something they would use,” he said.