Energy storage with compressed air (CAES) in geological caverns is being trialled, using gas-fired or electric compressors. When released (with preheating) it powers a turbine, up to 300 MWe (Megawatt electric), with overall about 70% efficiency.* CAES capacity can even out the production from a wind farm and make it partly dispatchable. Two CAES systems are in operation, in Alabama and Germany, and others trialled or developed elsewhere in the USA. CAES capacity in the USA was 450 MWe in 2010 and is expected to grow to 6000 MWe by 2020. Duke Energy and three other companies are developing a 1200 MWe, $1.5 billion project in Utah, ancillary to a 2100 MWe wind farm.
As described above in the solar thermal subsection, some CSP plants use molten salt to store energy overnight. Spain’s 20 MWe Gemasolar claims to be the world’s first near base-load CSP plant, with 63% capacity factor. Spain’s 200 MWe Andasol plant also uses molten salt heat storage, as does California’s 280 MWe Solana.
In Germany Siemens has commissioned a 6 MW hydrogen storage plant using proton exchange membrane (PEM) technology to convert excess wind power to hydrogen, for use in fuel cells or added to natural gas supply. The plant in Mainz is the largest PEM installation in the world.
Ontario’s ISO has contracted for a 2 MWe flywheel storage system from NRStor Inc.
Another form of energy storage is ice. Ice Energy has contracts from Southern California Edison to provide 25.6 MW of thermal energy storage using its Ice Bear system, attached to large air conditioning units. This makes ice at night when power demand is low, then uses it to provide cooling during the day instead of the aircon compressors, thus reducing peak demand.
(Source: Energy.gov. The US Department of Energy Global Energy Storage database has more information. )