Text 3. Hydroelectric power
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.
Hydroelectric power stations can produce a great deal of power very cheaply.
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.
Although there are many suitable sites around the world, hydroelectric 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.
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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, hydroelectric 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.
When it rains in hills and mountains, the water becomes streams and rivers that run down to the ocean. The moving or falling water can be used to do work. Energy, you'll remember is the ability to do work. So moving water, which has kinetic energy, can be used to make electricity.
For hundreds of years, moving water was used to turn wooden wheels that were attached to grinding wheels to grind (or mill) flour or corn. These were called grist mills or water mills.
In the year 1086, the Domesday Book was written. The multivolume books are very large. Hand-written on the pages of the books are lists of all properties, homes, stores and other things in England. The Domesday Book listed 5,624 water wheel-driven mills in England south of the Trent River. That was about one mill for each 400 people.
Water can go over the top of the wheel or the wheel can be placed in the moving river. The flow of the river then turns the wheel at the bottom.
Today, moving water can also be used to make electricity.
Hydro means water. Hydroelectric means making electricity from water power.
Hydroelectric power uses the kinetic energy of moving water to make electricity. Dams can be built to stop the flow of a river. Water behind a dam often forms a reservoir. Dams are also built across larger rivers but no reservoir is made. The river is simply sent through a hydroelectric power plant or powerhouse.
Hydro is one of the largest producers of electricity in the United States. Water power supplies about 10 percent of the entire electricity that we use. In states with high mountains and lots of rivers, even more electricity if made by hydro power. In California, for example, about 15 percent of all the electricity comes from hydroelectric.
The state of Washington leads the nation in hydroelectricity. The Grand Coulee, Chief Joseph and John Day dams are three of six major dams on the Columbia River. About 87 percent of the electricity made in Washington state is produced by hydroelectric facilities. Some of that electricity is exported from the state and used in other states (source: http://www.energyquest.ca.gov/story/chapter12.html).