In terms of installed capacity, compressed air energy storage (CAES) follows next, but some way behind pumped storage. In the, US CAES capacity is around 100 MW, but more is being planned. As the title implies, compressed air is pumped into caverns and then released, usually into gas turbines. Research into improving CAES technology is also in progress in Germany, where a facility has been operating for some time, and elsewhere.
Other storage technologies include batteries, flywheels and superconducting magnets. The latter have relatively low energy storage capabilities and high costs but "flow batteries" -- where chemical reactions in electrolytes take the place of the water in a pumped storage system -- show some promise. The Regenesys technology based on this technique, developed by the UK's National Power (Windpower Monthly, October 2000) fell into this category. Although its new owner, German utility RWE, discontinued the work -- apparently confirming the poor options for making money on storage -- other, similar, systems are being developed in Canada, Japan and elsewhere.