Pumped Hydroelectric Energy Storage (PHES) is a form of energy storage technique that is based on the storage of 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 store water is released from the upper reservoir and is used to drive a turbine, which intern drive an electric generator to generate electricity.
In the traditional application of PHES, low-cost or off-peak electric power is used to run a pump which pumps 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 makes 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 where it is used in conjunction with intermittent renewable energy resource such as wind farms to optimize the availability of wind power. See the article Grid level Energy Storage for more on large scale energy storage system.