The most widespread large-scale electricity storage technology is pumped storage hydropower. This is also the oldest storage technology in use, with the first plant built at the beginning of the twentieth century. By the beginning of the twenty-first century there was probably 140,000 MW of pumped storage capacity in operation.
A pumped storage plant is like a conventional hydropower plant with a dam and reservoir but in this case there are two reservoirs rather than one. These two reservoirs must be separated vertically; one must be higher than the other. The difference in height provides the head of water to drive the station's turbines.
In order to generate power, water runs from the top reservoir through a high-pressure channel to turbines at the bottom of the drop. The turbines extract the potential energy from the water and then discharge it into the bottom reservoir where it is saved. When energy is to be stored, the turbines are reversed and act as pumps, pumping water from the lower reservoir
into the upper. The turbine pumps are driven using off-peak electricity so storage will normally take place at night. Once water has been pumped into the upper reservoir it is available again for power generation.
This type of plant is extremely robust and though round trip efficiency is lower than for some other technologies, long-term energy losses are low. Leakage and evaporation are the main sources of loss and if these are managed well, water loss can be kept small.
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