One of the most ambitious solar energy projects on the planet is in trouble. The $2.2 billion Ivanpah concentrated solar power facility in California has fallen well short of its expected power output and now has a year to get itself back on track, or it risks being forced to shut down.

The California Public Utilities Commission granted a reprieve to the plant on Thursday, preventing it from going into default on its contract with Pacific Gas & Electric and Southern California Edison.

Built with great fanfare by BrightSource Energy, NRG Energy, and Google, Ivanpah has been dogged by criticism from environmentalists since construction began. The plant uses thousands of mirrors to concentrate the sun’s energy and heat water to produce steam and generate electricity. But since it came online in 2014, the power it produces has been much more expensive than electricity from solar plants that get their energy from photovoltaic cells, to say nothing of power from natural gas.

Once considered a high-tech rival to solar photovoltaic, concentrated solar power has fallen from favor in recent years as the cost of photovoltaic panels has plummeted. Last year BrightSource canceled a planned concentrated solar power plant in Inyo County, California.

Concentrated solar has one key advantage over solar photovoltaic, however: coupled with energy storage systems, such as heated molten-salt tanks, it can provide power even when the sun’s not shining. A 2013 study from the National Renewable Energy Laboratory found that the value of concentrated solar plus thermal storage could be $32 to $40 per megawatt-hour higher than that of a photovoltaic plant in 2020. BrightSource rival SolarReserve is building a 110-megawatt concentrated solar plant in Nevada that includes molten-salt storage, and a similar plant in South Africa.

(Read more: Wall Street Journal, The Press-Enterprise, “Solar Thermal Power Heats Up Outside the U.S.,” "World’s Largest Solar Thermal Power Plant Delivers Power for the First Time," "In Pictures: The World’s Largest Solar Thermal Power Plant")