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Energy Resources:
Hydroelectric power


Hydroelectric power:

Energy from falling water

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We have used running water as an energy source for thousands of years, mainly to grind corn.

The first house in the world to be lit by hydroelectricity was Cragside House, in Northumberland, England, in 1878.

In 1882 on the Fox river, in the USA, hydroelectricity produced enough power to light two paper mills and a house.

Nowadays there are many hydro-electric power stations, providing around 20% of the world's electricity.

The name comes from "hydro", the Greek word for water.

How it works

A dam is built to trap water, usually in a valley where there is an existing lake.

Water is allowed to flow through tunnels in the dam, to turn turbines and thus drive generators.

Notice that the dam is much thicker at the bottom than at the top, because the pressure of the water increases with depth.

Hydro-electric power stations can produce a great deal of power very cheaply.

Video clip: Hydro power - how it works

a hydro-electric dam
Hoover Dam


When it was first built, the huge "Hoover Dam", on the Colorado river, supplied much of the electricity for the city of Las Vegas; however now Las Vegas has grown so much, the city gets most of its energy from other sources.

There's a good explanation of how hydro power works at

Although there are many suitable sites around the world, hydro-electric dams are very expensive to build.

However, once the station is built, the water comes free of charge, and there is no waste or pollution.



The Sun evaporates water from the sea and lakes, which forms clouds and falls as rain in the mountains, keeping the dam supplied with water. For free. the water cycle, driven by the Sun


Gravitational potential energy is stored in the water above the dam.

Because of the great height of the water, it will arrive at the turbines at high pressure, which means that we can extract a great deal of energy from it. The water then flows away downriver as normal.

In mountainous countries such as Switzerland and New Zealand, hydro-electric power provides more than half of the country's energy needs.

An alternative is to build the station next to a fast-flowing river. However with this arrangement the flow of the water cannot be controlled, and water cannot be stored for later use.

Video clip: Hoover Dam in 1 minute

  • Once the dam is built, the energy is virtually free.

  • No waste or pollution produced.

  • Much more reliable than wind, solar or wave power.

  • Water can be stored above the dam ready to cope with peaks in demand.

  • Hydro-electric power stations can increase to full power very quickly, unlike other power stations.

  • Electricity can be generated constantly.
 Generators in the Hoover Dam. From


  • The dams are very expensive to build.
    However, many dams are also used for flood control or irrigation, so building costs can be shared.

  • Building a large dam will flood a very large area upstream, causing problems for animals that used to live there.

  • Finding a suitable site can be difficult - the impact on residents and the environment may be unacceptable.

  • Water quality and quantity downstream can be affected, which can have an impact on plant life.

  • For more information about how economic factors might be pushing too hard to get dams built, see the BBC news site.

Is it renewable?

Hydro-electric power is renewable.
The Sun provides the water by evaporation from the sea, and will keep on doing so.



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