Energy world

Energy sources: Hydro

Status: Renewable

What is it?

Hydroelectric energy is generated by the flow of water and like most other renewable energy sources, it is actually indirect solar power which drives the Earth's water cycle.

The Earth receives 1.5 billion TWh (terawatt hours) of solar energy every year. Nearly a quarter of this is consumed in the evaporation of water from the surface of oceans, lakes and rivers. This means that water vapour in the atmosphere is actually a massive store of solar energy. When the vapour condenses, most of this energy is released as heat. However, a tiny fraction, about 0.06%, is retained as gravitational potential energy by the rain that falls on mountains and hills. As this water flows downstream, its gravitational potential energy is transformed into kinetic energy.

How is it used to generate electricity?

The kinetic energy of flowing water spins a turbine which is connected to a generator which creates electricity.

Where can you find it?

Hydroelectric power stations need to be near large lakes or reservoirs high above sea level, or where a large flow of water can be dammed.

Advantages and disadvantages of using hydroelectric energy to generate electricity


Once the plant is built, operating costs are very low so the energy produced is virtually free

It does not produce any carbon dioxide, which can lead to global warming which makes countries hotter and drier

It is sustainable – the rain that fills the reservoir never runs out

Dams store water so we can control when electricity is made

Power output can be increased very quickly to meet sudden demand

Electricity can be generated constantly as long as there is enough water


Dams are very expensive to build

Valuable land is flooded when a reservoir is made: homes and wildlife habitats can be lost

A good site for a hydroelectric power plant, such as a mountainous region, is not always near towns where energy is needed

There are not many suitable sites in the UK for new hydroelectric schemes

How many power schemes in the UK?

7 (50MW schemes)

How much does it cost to produce?

Between 2p and 7p p/kWh

What is the carbon cost?

Generating hydroelectric energy does not produce any carbon dioxide.

The future?

It is unlikely that there will be a huge increase in hydroelectric power as all large-scale site are now being used, but some small-scale sites are being developed.