iowa_compressed_air_plantWind power has made incredible inroads into the U.S. energy system thanks to big, efficient machines standing hundreds of feet tall. But the future of wind power may be underground.

In the abandoned mines and sandstones of the Midwest, compressed-air storage ventures are trying to convert the intermittent motions of the air into the kind of steady power that could displace coal.

Compressed-air energy storage plants use compressors to store electricity generated when it’s not needed. The air, pumped into large underground formations, is like a spring that’s been squeezed and when it’s needed, it can deliver a large percentage of the energy that it received.

The first and only such plant in the United States went online in 1991, and though the technology didn’t take off, it did prove that it worked. And now, combining cheap wind energy and compressed-air storage could create a potent new force in the electricity markets.

“This is the first nonhydro renewables technology that can replace coal in the dispatch order,” said David Marcus, co-founder of General Compression, a new company that received $16 million in funding from investors including the utility Duke Energy to build a full-scale prototype of their energy storage system, which would be deployed with arrays of wind turbines.

The dispatch order is how grid operators decide which power plants to switch on. They have to balance the amount of generation and consumption or they risk the grid’s stability. The amount of power people use goes up and down, but it stays above a certain level all the time. To meet that need, utilities buy consistent always-on power from the large, cheap coal and nuclear power plants that are the backbone of the electric grid.

The electricity they need to meet the peaks in energy demand is generated by what are known as peaking plants, usually powered by natural gas. When the wind is blowing, it is usually the cheapest peaking power available, so it keeps the natural gas plants shut off. If they want to replace coal plants in the pecking order, though, they’ll have to work all the time.

And to do that, they’ll need a way to unlink themselves from the on-again, off-again nature of the wind.

“It’s a fractal problem,” said Marcus. “You have intermittency problems on every time scale.”

That problem has brought compressed-air energy storage roaring back. Marcus’ company has a long way to go before they can turn their prototype system into the kind of technology that can be deployed at the nation’s vast wind farms. But compressed air storage of one type or another is on the verge of becoming a mainstream power technology.

The nation’s largest energy storage option right now is pumped hydroelectricity. When excess electricity is present in a system, it can be used to pump water up to a reservoir. Then, when that power is needed, the water is sent through a turbine to generate electricity. The U.S. electric system has 2.5 gigawatts of pumped hydro storage capacity, but most of the good, cheap sites are already occupied, and creating new reservoirs is not environmentally benign.

While wind farmers say storage isn’t technically necessary until the amount of wind power on the grid exceeds 20 or 30 percent of the electrical load, private analysts, the Electric Power Research Institute, and the Department of Energy have identified grid-scale storage as a key need for the rapidly diversifying electricity system.

And going forward, compressed-air energy storage looks like the cheapest option available. Independent analysts have come to similar conclusions.

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