BrightSource Energy, one of the biggest and most promising companies building solar thermal power plants, has just raised $150 million in a fourth round of capital — boosting its total over the $300 million mark — to establish its dominance in the southwest U.S. and a foothold overseas.

Departing from traditional photovoltaics, BrightSource develops fields of mirrors that concentrate sunlight on a single focal point. There, the heat is converted into steam, which turns turbines and generates electricity. Because it doesn’t require pricey silicon and other solar module materials (and mirrors are relatively pretty cheap), solar thermal technology has become a viable competitor with even the top solar developers, including First Solar.

Based in Oakland, Calif., BrightSource already has a significant project pipeline. It’s committed to building 2.6 gigawatts-worth of solar arrays in the U.S. — having landed lucrative contracts with both the Pacific Gas & Electric Company and Southern California Edison.

The company predicts that it will be building 14 power plants in the American Southwest by 2016 — including the somewhat controversial Ivanpah Solar Energy Generating Station in California’s Mojave Desert. Plans for that plant had to be scaled down in February when too many objections were raised over the displacement of endangered desert tortoises.

Despite this hang-up, the government is clearly on the company’s side. The U.S. Department of Energy awarded BrightSource a $1.37 million loan guarantee, shortly after the turtle issue was raised, to apply toward the Ivanpah project. But it still needs to earn the requisite permits and approvals.

But even as its U.S. plans inch along, the company is already turning its attention to the outside world. Its top competitor in the solar thermal arket, a company called eSolar, is already licensing its technology to solar plant builders in China, South Africa and India. China, in particular, has yet to produce a solar thermal winner capable of competing with its photovoltaic giants SunTech and Jinko Solar. Greentech Media predicts that the country might be BrightSource’s next frontier.

The recent round of funding — the second largest amount taken by any cleantech company so far this year, behind the $350 million bagged by Better Place — was provided by existing backers VantagePoint Venture Partners, Draper Fisher Jurvetson and Morgan Stanley. But it also included about $55 million from French energy engineering firm Alstom, which will be helping BrightSource with product development.