Pumped storage, and how it works
Pumped storage saves power from electricity from any source for later use. Unlike an ordinary battery, which stores the power chemically, a pumped storage facility depends on gravity.
At low use hours, when supply exceeds demand, water is pumped to a reservoir at the top of a mountain. Then, when the demand is greater than the supply, water is run back down to the foot of the mountain to generate electricity. The relationship between supply and demand makes it possible to make money in the process. The graph above shows the effect. [Today in Energy, May 21, 2012]
Historically, the water was pumped up during the night after most people went to bed, and back down during the day when most people were working. Now, with the advent of renewable power that depends on natural sources, the process is a bit more complicated. Solar power is greatest at around noon, which also happens to be a peak demand time. Wind power is often greatest during the night. Pumped storage is one of several ways to use renewable energy to supply baseload power.