Conventional pumped hydro uses two vertically-separated water reservoirs. During off peak hours water is pumped from the lower reservoir to the upper reservoir. When required, the water flow is reversed to generate electricity. Some high-dam hydro generating plants have a storage capability and can be dispatched as a pumped hydro. Underground pumped storage, using flooded mine shafts or other cavities, is also technically possible. The sea can also be used as the lower reservoir. A 30 MW seawater pumped hydro plant was built near Yanbaru, Japan, in 1999.
Pumped hydro was first used in Italy and Switzerland in the 1890s. By 1933 reversible pump-turbines with motor-generators were available. Adjustable speed machines are now being used to improve efficiency. Pumped-hydro storage is available at almost any scale with discharge times ranging from several hours to a few days. Its efficiency is in the 70% to 85% range.
Pumped storage is the most widespread energy storage system in use on power networks. Its main applications are for providing energy management, frequency control, and reserve capacity. Over 90 GW of pumped-hydro storage, representing approximately 3% of global generation capacity, are currently installed worldwide. Pumped-hydro storage plants are characterized by long construction times and high capital expenditure.
A recent Bonneville Power Administration report discusses pumped storage's role in integrating variable energy resources into the Federal Columbia River Power System and expands on pumped storage's current and future role in the Pacific Northwest. The report's appendices can be found here.
Updated February 2012
HDR Engineering, Inc.