Compressing air is a way to store energy, and combining this with a generator yields a way of storing electricity. Systems have been in use in applications such as starting large engines or in the propulsion of mine locomotives. At a larger scale, the technology is used to shift the energy demand for smaller and larger electricity grids, providing storage. Air heats up when compressed, and cools down when decompressed, generating a natural barrier for efficiency. To counter this, some installations use an external source of heat, like natural gas. Efficiencies up to 45% are attainable. Newer developments use water or other materials to achieve higher efficiencies. This technology is listed as Advanced Compressed Air Energy Storage (ACEAS).
Applications: long term-storage, short-term storage, arbitrage, distributed/off-grid storage, frequency or voltage regulation, reserve grid capacity