One frequent criticism you hear about solar power is that the sun only shines during the day, and the prospect of heavy cloud cover or rain makes solar too unpredictable to provide constant baseload power to the grid. But the thing is, that's increasingly very untrue. As David Biello recaps in a nice piece for Environment360, solar-thermal plants are increasingly using basic storage techniques to provide a steady power source at all hours, even after sunset. Here's the rundown:

In the high desert of southern Spain, not far from Granada, the Mediterranean sun bounces off large arrays of precisely curved mirrors that cover an area as large as 70 soccer fields. These parabolic troughs follow the arc of the sun as it moves across the sky, concentrating the sun’s rays onto pipes filled with a synthetic oil that can be heated to 750 degrees Fahrenheit. That super-heated oil is used to boil water to power steam turbines, or to pump excess heat into vats of salts, turning them a molten, lava-like consistency.

The salts are just fertilizers—a mix of sodium and potassium nitrate—but they represent a significant advance in the decades-old technology of solar thermal power production, which has traditionally used mirrors to heat water or oil to generate electricity-producing steam. Now, engineers can use the molten salts to store the heat from solar radiation many hours after the sun goes down and then release it at will to drive turbines. That means solar thermal power can be used to generate electricity nearly round-the-clock.

The Andasol plant in Spain is a decent size, too—when it's finished in 2011, it'll likely generate about 150 MW of electricity, enough to power 150,000 homes. And the cost is coming down. Standard solar thermal plants generate electricity for about 13 cents per kilowatt hour, compared with less than 10 cents per kilowatt-hour for conventional electricity in most parts of the United States. The salt storage makes solar thermal a bit more expensive, but those costs could tumble further as production ramps up. (The National Renewable Energy Laboratory expects solar thermal to compete with natural gas by 2015—and that's without any sort of carbon regulation.)

Biello notes that the idea of using curved mirrors heat up synthetic oil in pipes to power turbines isn't all that newfangled. It's been around since the 1980s, part of the initial push to develop alternative-energy sources after the OPEC oil crisis. When oil prices fell again, the Reagan administration phased out solar research and tax credits. Now there's renewed interest. In the past 18 months, three large plants have been announced for Arizona, and Biello estimates that the United States will see 1,800 MW worth of solar thermal by 2011, and again, that's before Congress passes a cap-and-trade regime making coal power more expensive. So this isn't just some lavish green fantasy; there's a massive amount of potential here for reliable, carbon-free power that's already bounding out the gate.

--Bradford Plumer