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Delta, Utah — If you know anything about solar and wind farms, you know they’re good at generating electricity when the sun is shining or the wind is blowing, and not so good at other times.

Batteries can pick up the slack for a few hours. But they’re less useful when the sun and wind disappear for days at a time — a problem the Germans call “dunkelflaute,” meaning “dark doldrums.”

Those long stretches of still, cloudy days are one of the main obstacles standing in the way of renewable energy fully replacing fossil fuels.

For Los Angeles, salt may be a solution.

One hundred miles south of Salt Lake City, a giant mound of salt reaches thousands of feet into the earth. It’s thick, relatively pure and buried deep, making it one of the best resources of its kind in the American West.

Two companies want to tap the salt dome for compressed air energy storage, an old but rarely used technology that can store large amounts of power.

It would work like a giant battery. Hollow caverns carved out of the salt — each more than 1,000 feet from top to bottom and several hundred feet wide — would be pumped full of air at high pressure, using energy generated by solar panels or wind turbines during times when the power isn’t needed. Like storing wind (or sun) in a bottle.

When the power is eventually needed, the tightly packed air would be released from the caverns, turning turbines on the way out to generate electricity.

The electricity would be ferried to Southern California through a 488-mile transmission line, built in the 1980s to transmit energy from Intermountain Power Plant, which is now the last coal-fired generating station serving California. The coal plant is scheduled to shut down in 2025.

The salt dome’s proximity to Intermountain — they’re literally across the street from each other — is a lucky coincidence.

“It’s extraordinarily rare to have geology, transmission and a coal plant all sitting right next to each other,” said Jeff Meyer, president of Range Energy Storage Systems. “You ought to take advantage of this, because all the stars have lined up.”

It’s not all rosy: Compressed air storage technology requires the burning of natural gas, a planet-warming fossil fuel.

Compressed air energy storage has been used for decades, but only at two facilities in Germany and Alabama, built before solar and wind started creating challenges for power grid operators.

“This is a pretty simple concept,” said Bobby Bailie, director of business development for energy storage at the German industrial firm Siemens. “You’re pushing air into a cavern, storing that energy. And at times when you need it, you pull it back out.”

Two companies hope to tap the salt for energy storage.

One is Magnum Development, which is backed by the Houston-based private equity fund Haddington Ventures.

Magnum has already built several hollow caverns in the salt dome by drilling wells, pumping in water to dissolve the salt, and pumping out the resulting brine. The caverns are used to store butane and propane.

The other company is Range Energy Storage Systems. It’s a partnership between North Carolina-based electricity giant Duke Energy, Sammons Enterprises of Dallas and American Transmission Co., whose headquarters is in Wisconsin.

Compressed air would provide the most value on an electric grid dominated by solar, wind and hydropower.

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