24-Hour Solar Energy: Molten Salt Makes It Possible, and Prices Are Falling Fast

Molten salt storage in concentrated solar power plants could meet the electricity-on-demand role of coal and gas, allowing more old, fossil fuel plants to retire.

The first thing you see of the Crescent Dunes Solar Energy Facility, and you can be miles away, is a light so bright you can't look directly at it.  This sits atop a 640-foot cement tower, rising from the flat, empty Nevada desert around the halfway point on the highway from Reno to Las Vegas.  The tower's surrounded by a nearly two-mile-wide field of mirrors that send shimmering beams of light into the sky.
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What people are actually seeing is a 110-megawatt concentrated solar power (CSP) plant, built and operated by SolarReserve of Santa Monica, California.  It's not from outer space, but there's not yet anything quite like it of this size anywhere else on the planet. 

SolarReserve is trying to prove that the technology that drives Crescent Dunes can make solar power an affordable, carbon-free, day-and-night energy source, dispatched on the electric grid like any fossil fuel plant.  Here, concentrated sunlight heats molten salt to 1,050 degrees Fahrenheit in that shimmering tower; then the salt gets stored in a giant insulated tank and can be tapped to make steam to run a turbine.

If this plant and several similar facilities under construction, or soon to be, prove reliable, the technology is poised to take off.  Solar photovoltaic (PV) panels can displace fossil fuels during the day, and wind turbines can do the same as long as it's windy.  But molten salt towers may be able to meet the challenge of electricity on demand, and push more older, dirtier fossil-fuel plants into retirement.
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The Next Big Thing?  It's the Storage
Power generation at Crescent Dunes starts with 10,347 mirrors, a total of 13 million square feet of glass—enough to completely cover the National Mall in Washington from the steps of the Capitol to the Washington Monument.  The mirrors are called heliostats because each one can tilt and turn to precisely point its beam of light.  Arranged in concentric circles, they focus sunlight on the "receiver" at the top of the central tower.  Tourists' assumptions aside, this is not actually a light.  The receiver, matte black when there's no sunlight on it, absorbs energy to heat the molten salt flowing through a series of pipes.  Hot salt then flows down to a 3.6 million gallon stainless steel storage tank.

The salt, which at these temperatures looks and flows pretty much like water, runs through a heat exchanger to make steam to run a standard turbine generator.  The tank holds enough molten salt to run the generator for 10 hours; that represents 1,100 megawatt hours of storage, or nearly 10 times more than the largest lithium-ion battery systems that have been installed to store renewable power.

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