Pressurized Air Tops Batteries In Intermittent Energy Storage

by Stuart Fox

Like comedy, the secret to intermittent energy sources like wind and solar comes down to timing. For the most part, when the wind blows and the sun shines does not coincide with the highest demand for electricity. To compete with power from natural gas or coal plants, solar and wind energy producers need a method to store power during the time between when plants produce it and consumers want it.

For Jared Garrison and Michael Webber, both in the Department of Mechanical Engineering at the University of Texas, Austin, compressed air is that storage system. Unlike batteries, which become prohibitively expensive at large scales, compressed air coupled with wind and solar power can provide electricity to an entire grid when people need it, and inexpensively store that energy in mechanical form when demand drops.

"Only compressed air can really give you the size and the scale, at a cost that's reasonable, for load leveling," said Garrison, a PhD student at UT Austin. "The other ones, batteries, could potentially do it. But you'd need so many batteries to do it, that it would be way too expensive."

Named "Dispatchable Solar and Wind Storage System" (DSWiSS) by Garrison and Webber¹, the storage method basically uses compressed air as a spring to story energy. During the time when the wind blows the hardest, turbines special designed to work with the DSWiSS assembly power compressors that force air into an underground cavern. When users need electricity, that compressed air bursts out of the cavern, gets heated by stored up solar power, and spins turbines to produce energy.

For every unit of energy put into the process, DSWiSS puts out one half unit of energy. However, because the system stores power that would have otherwise been lost, providing profit to the entire grid, the output of the system in electrical energy is actually one unit in to one unit out.

Two plants of coupled solar, wind, and DSWiSS storage already exist in Macintosh, Alabama, and Huntorf, Germany. To turn a profit, those plants generate and store energy during off-peak hours, and then sell the energy back to the grid during peak hours where each kilowatt demands a higher price.

The DSWISS system is not without complications, as it requires a specific set of geological formations in which to store the compressed air, and requires dedicated wind turbines and solar panels (reducing its compatibility with wind and solar generators already online). However, Garrison and Webber believe that when properly implemented, the DSWiSS system can provide electricity on economic par with coal or natural gas.

"The term is dispatchability. You can turn it on or off," Garrison said. "The stuff we use today, coal, nuclear, natural gas especially, we can turn on and off. With wind and solar we don't have that ability. That's the purpose of this kind of load leveling storage. We can gain that control and use it to our advantage."

¹"An integrated energy storage scheme for a dispatchable solar and wind powered energy system," by Jared B. Garrison and Michael E. Webber, was published in the July 2011 issue of Journal of Renewable and Sustainable Energy.