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Hydroelectric systems can be connected to the main electricity grid, or can be 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 to the user or via a battery bank.

There are three main types of hydroelectric schemes:

· Storage schemes: In storage schemes, a dam impounds water in a reservoir that feeds the turbine and generator, usually located within the dam itself.

· Run-of-river schemes: Run-of-river schemes utilize the natural flow of a river, where the continuity of flow can be enhanced by a weir. Both storage and run-of-river schemes can be diversion schemes where water is channeled from a river, lake or dammed reservoir 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 or to another watercourse.

· Pumped storage: Pumped storage incorporates two reservoirs. At times of low demand, generally at night, electricity is used to pump water from the lower to the upper basin. This water is then released to create power at a time when demand, and therefore price, is high. Pumped storage is not a renewable application as it is reliant upon an electricity supply and energy losses are always involved when pumping the water. However, by providing a rapid supply of electricity in response to sudden changes in demand, it does have value in aiding the overall efficiency of the generation infrastructure.

Tidal barrage systems can, like run-of-river schemes, use the incoming and outgoing tidal flow, or, like pumped storage schemes, store the incoming tidal flow in a reservoir to be released at low tide.

A typical illustration of a hydroelectric turbine generator. As the water flows through the lower portion of the system it turns a turbine, which in turn causes the generator to rotate by kinetic energy transferred through a central shaft. As the generated rotates; positive and negative magnetic brushes attached to the outside and inside of the generate produces alternating electrical current.

Hydroelectric dam

Hydroelectric dam spill way.

Electrical turbine generators producing electrical current as the flow of rushing water turns the blades of the turbine.