Pumped hydro: The oldest and most common type of storage. It consists of two reservoirs at different altitudes. Water is pumped from the lower reservoir to the higher one when electricity prices are low, then run through turbines to generate electricity at times of high demand. It provides 15% of the National Grid’s balancing services.
Compressed air: The second largest form of storage. Electricity from a gas-fired plant is used to compress air which is stored in salt caverns. This is then used as combustion air in the gas plant, improving its efficiency and reducing gas use by 60%. There are plants in Germany and the US.