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Hydroelectricity is electricity produced by hydropower. Hydroelectricity now supplies about 715,000 MWe or 19% of world electricity (16% in 2003). It is also the world's leading form of renewable energy, accounting for over 63% of the total in 2005. Although large hydroelectric installations generate most of the world's hydroelectricity, small hydro schemes are particularly popular in China.

Most hydroelectric power comes from the potential energy of dammed water driving a water turbine and generator. The energy extracted from water depends on the volume and on the difference in height between the source and the water's outflow. Pumped storage hydroelectricity schemes produce electricity to supply high peak demands by moving water between reservoirs at different elevations. They currently provide the only commercially important means of grid energy storage. At times of low electrical demand, excess generation capacity is used to pump water back into the higher reservoir, from where it can be released through the turbines at short notice. Less common types of hydroelectricity include run-of-the-river, waterwheels, and tidal power schemes.

A major advantage of hydroelectricity is the elimination of fuel costs and the associated carbon emissions although, in tropical regions, decaying plant material behind the dam can sometimes result in greater greenhouse gas emissions than a conventional power station. Other issues include the need to relocate people from areas to be flooded and disruption caused to aquatic ecosystems. Read more...