Pumped storage

Pumped hydro storage plants store energy using two water reservoirs, one at a higher altitude than the other. Water is pumped up from the lower reservoir to the higher reservoir using electricity, converting the electrical energy into gravitational potential energy. The stored energy can be converted back into electricity by letting the water fall from the higher reservoir to drive a turbine. Pumped hydro was used in Italy and Switzerland as early as the 1890s and is now a mature technology that is widely used on a commercial scale, although limited by the need for the right geographical conditions. A number of improvements are enhancing some of its basic characteristics, including reuse and retrofitting for increased flexibility to accommodate variable renewables, extending the reach of pumped hydro to seawater or underground, and smaller modular projects.