Pumped Storage

Pumped storage hydroelectricity is a type of hydroelectric power generation used by some power plants for load balancing. The method stores energy in the form of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost off-peak electric power is used to run the pumps. During periods of high electrical demand, the stored water is released through turbines. 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.

The off-peak electrical energy used to pump the water up hill can be stored indefinitely as gravitational energy in the upper reservoir. Thus, two reservoirs in combination can be used to store electrical energy for a long period of time, and in large quantities.

Switching large coal and nuclear power stations on and off frequently is an inefficient way to run them. Pumped hydro energy storage can be used to smooth out the demand for baseload generation from such plants. It can also be used to provide emergency power injection to the grid when a plant goes offline unexpectedly.

The upper reservoir (Llyn Stwlan) and dam of the Ffestiniog Pumped Storage Scheme in north Wales. The lower power station has four water turbines which generate 360 MW of electricity within 60 seconds of the need arising. The size of the dam can be judged from the car parked below.