Process — Electricity from a pumped-storage hydroelectric station

The diagram shows how a pumped-storage hydroelectric power station generates electricity, and how its operation is reversed between day and night.

Overall, the station consists of a high-level reservoir held back by a dam, a steep pipe called a penstock, a power house containing a reversible turbine, and a low-level reservoir. Its defining feature is that water is released downhill to generate power during the day, and pumped back uphill at night using cheap electricity, so the same water is used over and over.

During the day, when demand for electricity is high, water from the high reservoir passes through an intake in the dam and rushes down the penstock. The force of this water spins the reversible turbine, which drives a generator, and the electricity produced is carried by pylons to the national grid. The used water then flows out into the low-level reservoir.

At night the process runs in reverse. Because demand is low, electricity from the grid is cheap, and it is used to run the turbine backwards as a pump. Water is therefore pushed from the lower reservoir back up to the high one, refilling it so that the daytime cycle can begin again the next morning.