page_title: Pumped-storage hydroelectricity
section_title: Basic principle
hierarchical_section_title: Pumped-storage hydroelectricity / Overview / Basic principle
context_page_description: Pumped-storage hydroelectricity, or pumped hydroelectric energy storage, is a type of hydroelectric energy storage used by electric power systems for load balancing. The method stores energy in the form of gravitational potential energy of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost surplus off-peak electric power is typically used to run the pumps. During periods of high electrical demand, the stored water is released through turbines to produce electric power. 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. If the upper lake collects significant rainfall or is fed by a river then the plant may be a net energy producer in the manner of a traditional hydroelectric plant.
  Pumped-storage hydroelectricity allows energy from intermittent sources and other renewables, or excess electricity from continuous base-load sources to be saved for periods of higher demand.
context_section_description: At times of low electrical demand, excess generation capacity is used to pump water into the upper reservoir. When there is higher demand, water is released back into the lower reservoir through a turbine, generating electricity. Reversible turbine/generator assemblies act as a combined pump and turbine generator unit (usually a Francis turbine design). Variable speed  operation further optimize the round trip efficiency in pumped hydro storage plants.
  In micro-PSH applications, a group of pumps and Pump As Turbine (PAT) could be implemented respectively for pumping and generating phases. The same pump could be used in both modes by changing rotational direction and speed: the operation point in pumping usually differs by operation point in PAT mode.
caption: Power distribution, over a day, of a pumped-storage hydroelectricity facility. Green represents power consumed in pumping; red is power generated.
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