Hydropower is the renewable energy contained in flowing water. Electricity is generated using hydropower known as hydroelectricity and is generally considered to be reliable.

Generating hydro power

A hydroelectric power station converts the kinetic energy of flowing or falling water into electrical energy that can be used in homes and businesses. Hydroelectric power can be generated on a small scale with a ‘run-of-river’ installation, which uses naturally flowing river water to turn one or more turbines, or on a large scale with a hydroelectric dam.

A hydroelectric dam straddles a river, blocking its progress downstream. Water collects on the upstream side of the dam, forming an artificial lake known as a reservoir (1). Damming the river converts its kinetic energy into potential energy – the reservoir becomes a sort of battery, storing energy that can be released a little at a time. Some reservoirs are also used as boating lakes or for drinking water.

The reservoir’s potential energy is converted back into kinetic energy by opening underwater gates or intakes (2) in the dam. When an intake opens, the immense weight of the reservoir forces water through a channel called the penstock (3) towards a turbine. The water rushes past the turbine, hitting its blades and causing it to spin, converting some of the water’s kinetic energy into mechanical energy. The water then finally flows out of the dam and continues its journey downstream.

A shaft connects the turbine to a generator (4) – when the turbine spins, so does the generator. The generator uses an electromagnetic field to convert this mechanical energy into electrical energy.

As long as there is plenty of water in the reservoir, a hydroelectric dam can respond quickly to changes in demand for electricity. Opening and closing the intakes directly controls the amount of water flowing through the penstock, which determines the amount of electricity the dam is generating.

The turbine and generator are located in the dam’s powerhouse (5), which also houses a transformer. The transformer converts the electrical energy from the generator to a high voltage. The national grid uses high voltages to transmit electricity efficiently through the power lines (6) to the homes and businesses that need it (7). Here other transformers reduce the voltage to a usable level.

Our approach to hydro

EDF Group is the European Union's leading supplier of hydroelectricity. In France alone, EDF Group operates 436 hydroelectric plants, with a total capacity of around 20GW. EDF Energy does not currently operate any hydroelectric power stations in the UK.

Flowing water is a renewable energy source, and modern hydroelectric power stations can convert at least 90% of this energy into electricity. But sites in the UK where water power can be harnessed on a large scale are very limited. The cost-effectiveness of a hydroelectric power station is affected by the geography of the site it is built on. Most suitable sites in the UK are already home to hydroelectric power stations, so there is very little scope to increase hydroelectricity's small (about 2.1%) share of the UK electricity generation mix in the future. Hydroelectricity is generally considered to be a reliable method of electricity generation.

The working life of a large-scale hydroelectric power station can exceed 100 years – so when we calculate the average lifetime emissions of a hydroelectric power station, initial CO2 emissions will be offset by subsequent decades of zero-carbon electricity generation.

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