Correction: see below.

Researchers and startups might tout flow or sodium batteries, but caves are going to be tough to beat when it comes to energy storage, says Robert Schainker, a senior technical executive at EPRI.

Compressed air storage – i.e., compressing air and storing it in caves, underground aquifers or abandoned mines until the air is needed to turn a turbine – will beat out other mass storage technologies in terms of cost largely because of the relative technical simplicity and the potential volumes for storage.

Compressed air right now costs about $700 a kilowatt/hour, he said. By comparison, Deeya Energy just released its first flow batteries. A 2-kilowatt device costs about $4,000 a kilowatt/hour, says Izak Bencuya. With volume manufacturing, Deeya hopes to lower that to $1,000 a kilowatt/hour.

That is still higher than compressed air. The advantages of air, however, kick in when utilities want to expand their storage. The volume of salt caverns and other underground storage areas can be increased through solution mining techniques.  (You pump liquid solutions underground, it carves out new space, and you suck out the fluid.) An incremental cost of adding another kilowatt/hour comes to around $1 to $2. Electrical devices won't be able to match that, says Schainker.

Compressed air can also be stored at depleted oil fields. If the underground cistern cannot be expanded, the air can be stored at a higher pressure, which will still be cheaper than large-scale batteries.

Finding caves isn't a problem either. There are billions of cubic meters worth of underground salt caverns available, Schainker said. Leaking is more of a perceived than an actual problem, he argued.

Compressed air, of course, has been an idea that has been ready for takeoff for years. Prototypes in Alabama and Germany have operated for years. The Department of Energy, Sandia National Labs, and a host of municipal utilities in the Midwest are in the midst of designing a compressed air generation plant in Iowa. The plant is expected to be operational by 2012 and produce 268 megawatts of power or 50 hours of power storage. That could enough to save a municipal utility $5 million a year.

PSEG Global is also investing $20 million into startup Compressed Energy & Storage.

Correction: we inadvertently used kilowatt instead of kilowatt/hour.

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