Pumped Hydroelectric Energy Storage (PHES) is a form of energy storage technique that is based on the storage of a large amount of water at a high elevation. This is accomplished with two water reservoirs (artificial lake or sea) which are at two different elevations (pressure heads), by pumping water from the lower elevation to the higher elevation.
Water in the upper reservoir will be stored until it is needed, holding enough water for up to several days’ worth of electrical energy. When needed, the stored water is released from the upper reservoir and is used to drive hydro/water turbines, which intern drive electric generators to generate electricity.
The traditional application of PHES is arbitrage, where low-cost or off-peak electric power is used to run pumps, to pump the water from the lower reservoir to the upper reservoir and the stored water is released to produce electric power during periods of high electrical demand at which time the cost of electric energy is much higher.
Although the losses of the pumping process make the plant a net consumer of energy overall, the system increases revenue by selling more electricity during periods of peak demand, when electricity prices are highest. Pumped storage is the largest-capacity form of grid energy storage now available.
A more suitable application for this type of energy storage system is when it is used in conjunction with intermittent renewable energy resources such as wind and solar farms to optimize the availability of wind and solar power. See the article Grid level Energy Storage for more on large-scale energy storage systems.