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As soon as electricity is produced, the electrons flow through power lines to homes, businesses, schools, and hospitals—wherever it is demanded. More electricity is made than grid operators expect you to consume, so that when you flip a switch, a light turns on.

Regardless of whether you actually turn on the lights, power plants keep their turbines spinning, ready to send power to the grid at a moment's notice.

This problem of excess power-generating capacity is worse at night when demand is very low, and the disparity between the amount of power needed and available is even greater. This discourages the use of some renewables, specifically wind power, which runs mostly at night when winds are strongest (and when people are using less electricity). In short, a lot of electricity, and importantly, clean electricity, is produced at the wrong time.

That’s where energy storage comes in. Storing energy when it's made and releasing it when it's needed helps keep the grid reliable and paves the way for introducing intermittent renewables like wind and solar to the mix.

But one Alabama power company has found a different place to put large amounts of excess energy – in salt caverns. Half a mile underground, a salt cavern that could fit the Statue of Liberty holds Power South Energy Cooperative’s most useful resource: air.

For 25 years, the cooperative has mastered the art and science of compressing air, storing it in a salt cavern beneath the earth, and using it to produce electricity. The McIntosh Power Plant in McIntosh, Alabama, is the only utility-scale Compressed Air Energy Storage (CAES) facility in the United States, and one of just a handful in the world.

At night, while you sleep and your appliances rest, the plant uses excess electricity from the grid to compress air and pump it underground into an airtight salt cavern.

This is because the caverns provide large, impermeable spaces. The compressed air stays compressed, and the oxygen in the air does not react with the salt.

There the air is stored at pressures between 650 and 1058 PSI, about one tenth of the pressure of a high-pressure oil well.

When people demand energy during the day, the air is released to an on-site natural gas unit, where it is heated with natural gas, combusts and produces an even hotter gas stream that turns a turbine and produces electricity.

From above, the McIntosh plant looks like a standard natural gas power plant, but directly half a mile below the surface lies a unique energy storage mechanism.
(Courtesy Power South Energy Cooperative)

Compressed air is a key ingredient in producing electricity at all gas plants; it is the critical component that makes the gas combust, producing the gas stream that spins the turbine. At typical natural gas plants, making compressed air is the most energy-intensive part of running the plant—more than half of the energy generated by the turbine is simply fed back into the system to compress the air.

But the McIntosh CAES plant is able to compress the air independently of the power production process, when it is most economical to do so, because it has a place to store it – the salt mine. The plant has enough stored air to help a 110-megawatt turbine generate power for 26 hours, powering up to 110,000 homes.

By compressing the air at night when electricity is cheap and plentiful, instead of using the turbine’s output for compression, the plant uses less natural gas to make the same amount of electricity. It also allows clean sources of electricity to be used for compression.

While the McIntosh plant does not currently work directly in tandem with any renewable plants, it certainly could, and that’s the key idea. The electricity used to compress the air could be produced by wind turbines, as is planned for The Bethel Energy Center in West Texas. Apex CAES LLC has raised $100 million for the Center, which, if developed, would be a salt CAES facility coupled with wind. The Center plans to use the electricity produced by nearby wind turbines at night to compress air, and feed the air into a gas turbine during the day.

However, compressed air is not a panacea. While natural gas plants are relatively cheap to build and operate, the low price of natural gas also means there is less incentive to explore alternatives to the plants, or install options that save gas.

With the depressed cost of natural gas and the risk involved in deploying a relatively new technology, which has few projects to its name, it is hard to make the numbers pencil out for CAES. The Bethel Energy Center project has been under development since 2011, and has yet to raise the additional $400 million in capital needed.

"Something new and different is not easy to get off the ground," says Apex CAES COO Stephen Naeve.

Other test projects have failed because of the high cost of development—everything from the cost of disposing the salt brine created during the mining process to the risk of exploring locations that may turn out to be geologically unsuitable. As for the storage competition, batteries are in many ways more flexible because they can be sited closer to where the demand for power is, although according to Apex, batteries are (at least for now) significantly more expensive in the long run.

But Power South Energy Cooperative would still enjoy talking salt with some other CAES allies. Indeed, the McIntosh facility has drawn visitors from California, Utah, New York, and Idaho.

If salt is a way to make gas plants more efficient and utilize renewables, then pass the salt, please.

Editor's note: An earlier version of this article originally misstated that more power is sent to the grid than is demanded, which is where it is wasted. In fact, excess energy is wasted before it is sent to the grid through a number of processes. Smithsonian.com regrets the error.

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About Molly A. Seltzer

Molly A. Seltzer is a writer and photographer based in the New York area. She is passionate about telling stories of the American energy sector and the environment. Her series Electric America documents the people and places powering the U.S. You can follow the project at www.electricamerica.org and more of her work at www.seltzershots.com.