Underground compressed air
How compressed air energy storage systems work
A 290-MW power plant in Lower Saxony has been converting base load electricity into peak load electricity since 1978. This power plant still provides minutes reserve power with high availability, although with relatively low overall efficiency. This is essentially because the air that is heated upon compression must be cooled down before it is stored in the cavern. Conversely, the cooling that occurs during expansion in the turbine must be compensated for by using natural gas.