Pumped Hydro
From Open Energy Information
Introduction
Pumped Hydro is an energy storage technique where water is used as a medium in order to store energy. During periods where excess electricity is available on the grid, water is pumped to a higher elevation. During periods of peak demand, this water is released through a turbine in order to generate electricity. There are currently over 150 plants with 22,000 MWs of storage in the United States.[1]