Pumped Storage, Explained
Pumped storage plants are basically hydroelectric generators built as a means to store electrical energy. While similar in design to hydroelectric plants, pumped storage plants do not produce any net electricity. They consume electricity from the power grid by pumping water uphill for storage, and then return a portion of that electricity to the grid at a later time by allowing that water to run back downhill through generators.
This process is illustrated below. Electricity supplied by power generating plants on the grid spins turbines in the powerhouse, pumping water from a lake, river, ocean or lower reservoir, into an upper reservoir where the power is stored. Later, the water is released from the upper reservoir back down into the lower reservoir. This spins the turbines in the opposite direction, driving the generators to produce electricity.
The idea is that when a power utility company has excess generating capacity, it uses electricity to pump water from a lower reservoir, uphill to the upper reservoir. Later when extra electricity is needed, the water can be released back down the pipes to turn the hydroelectric generator. The levels in both reservoirs can be expected to vary significantly over a 24 hour period as water is alternately drained and pumped back.
A pumped storage plant returns only about 75% of the energy it uses back to the power grid because of inefficiencies and losses in the process. For example, a pumped storage plant capable generating 300 megawatts would consume 400 megawatt pumping the water uphill and return only 300 megawatts of that, representing a net loss of 100 megawatts to the power grid capacity. For this reason, pumped storage plants are measured in negative number in the overall national energy plans.(e.g. -100 megawatts ).
Because nuclear and coal fired power plants run most efficiently at full power and produce very cheap power, they are an excellent match to pumped storage. Although the utility company does lose money constructing, operating and fueling the power needed to pump water up and down at a storage plant, those costs can be recovered by selling the stored electricity to consumers at higher price during times of peak demand.
Pumped storage is also being considered for use with wind generators, which cannot be reliably counted on to produce their power at time of peak electrical demand.
With an increased focus on energy efficiencies and more expensive generating technologies like wind and solar, it is becoming difficult to justify the environmental footprint and large amounts of energy wasted with pumped storage. Smart grid technologies are being developed which minimize or completely avoid physical energy storage through sophisticated large-scale grid usage monitoring, load balancing and energy forecasting models.
Short animation showing the water and electricity flows:
Penstocks Feeding Into a Lower Reservior
Impound Dams and Reservoir