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In the smallest hydroelectric schemes, the head of water can be a few metres, in the larger schemes the power station which houses the turbines is often hundreds of metres below the reservoir.
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Hydroelectric power requires the source either to be relatively close to the site of power usage, or to a suitable grid connection. Hydroelectric systems can be connected to the main electricity grid, or as part of a stand-alone power system. In a grid-connected system, any electricity generated in excess of consumption on site can be 'sold' to electricity companies. In an off-grid hydroelectric system, electricity can be supplied directly, or via a battery bank.
There are three main types of hydroelectric schemes. In run of the river schemes, the turbine and generator are located either in the dam or are found alongside it. The dam uses the flow of the river to create the hydrostatic head. This method can also be applied to tidal barrage systems. A diversion scheme relies on the channelling of water taken from a dammed river or lake to a remote powerhouse containing the turbine and generator. A canal or low pressure tunnel transports the water to this end point and then back to the river to continue its course. Pumped storage incorporates two reservoirs. At times of low demand, generally at night, electricity is bought to pump water from the lower to the upper basin. This extra water can then be released to create power at a time when demand and therefore prices, are high. Pumped storage is not a renewable application as it is reliant upon an electricity supply, however this supply could, eventually, be sourced from renewable technologies.
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