The U.S. Department of Energy has awarded a pair of loan guarantees to two solar thermal power companies that amounts to nearly $2 billion.

Abengoa Solar picked up a conditional commitment for a $1.2 billion loan guarantee for its Mojave Solar Project, which will generate 250 megawatts of power. NextEra Energy Resources received a conditional commitment for a $682 million loan guarantee for its Genesis Solar Project, another 250-megawatt project under construction in southern California.

Solar thermal projects use a huge array of mirrors to focus heat on a point. The heat from the focused sunlight is used to boil water, creating steam that moves conventional turbines to generate electricity. It’s an alternative to traditional solar power projects that use large arrays of photovoltaic cells to capture sunlight and convert it to electricity. The idea is reminiscent of the Archimedes Death Ray, an oft-used trope in popular culture.

Brightsource, a solar power tower developer, recently filed to go public and raise up to $250 million. The company began building the massive Ivanpah solar power tower project in October last year and expects to finish it by 2013. The Ivanpah project is funded by Google Ventures, which invested $168 million, and NRG Energy among others. One of the largest risk factors cited was whether the company could effectively wash and clean those mirrors every two weeks. The company lists the mirror cleaning technology as “largely unproven,” and said it may perform well below expectations.

SolarReserve was also able to secure a $737 million loan guarantee from the U.S. Department of Energy. A loan guarantee helps companies attract buyers and investors for new renewable energy projects. Basically, it means the government will foot the bill if the project does not take off or is unable to get some kind of return for investors. It’s one of the ways the U.S. government is promoting renewable energy sources.

While solar power towers are unproven sources of renewable energy, a few proof-of-concept projects exist. Two solar power towers deployed in California already show that the technology worked. The two towers generated 38,000 megawatt hours of electricity while they were active between 1982 and 1988. The average home in the U.S. uses around 920 kilowatt-hours of electricity each month. There are also a few active solar power towers in Spain that generate around 50 megawatts.