Several megawatts of electrical power are produced in the United States by pumped-storage hydroelectric systems. This type of system operates by pumping water to a higher elevation and storing it in a reservoir until it is released to drop to a lower elevation to drive the hydraulic turbines of a hydroelectric power-generating plant.
The variable nature of the electrical load demand makes pumped-storage systems desirable systems to operate. During low-load periods, the hydraulic turbines may be used as pumps to pump water to a storage reservoir of a higher elevation, from a water source of a lower elevation. The water in the upper reservoir can be stored for long periods of time, if necessary. When the electrical load demand on the power system increases, the water in the upper reservoir can be allowed to flow (by gravity feed) through the hydraulic turbines, which will then rotate the three-phase generators in the power plant.
Thus, electrical power can be generated without any appreciable consumption of fuel. The pump-turbine and motor-generator units are constructed so that they will operate in two ways: (1) as a pump and motor, and (2) as a turbine and generator. In both cases, the two machines are connected by a common shaft and operate together. However, the multiple use of these machines, although economically very attractive, limits the amount of time that a pumped-storage system can generate electrical power.