Hydroelectricity is a renewable energy source, since the water flowing in rivers comes from rain or snow. Worldwide, about 24 percent of the world’s electricity is produced by hydropower plants and more than 1 billion people are supplied with power from these plants. Today the largest hydroelectric project in the world is the Itaipu Dam on the border of Brazil and Paraguay, supplying approximately 25 percent of Brazil’s power and 78 percent of Paraguay’s power. However, China’s massive Three Gorges Dam will be the largest when it has been completed.     There are six main components to a hydroelectric power station. First, the dam. Most hydropower plants have a dam to contain water, creating a large reservoir of water behind the hydroelectric station. Dams are usually placed where water descends from a height, as the energy that is generated from water depends not only on the volume but also on the difference in height between the dam and the water outflow. This height difference is called the head. For this reason, dams are built as high as possible to produce the maximum electrical energy. However, some early hydroelectric systems used the flow of water over an existing waterfall, with no dam needed; for example, a large amount of electricity is generated by Niagara Falls.   Second, the intake where gates on the dam are opened and water is pulled through the penstock –a pipe that leads to the turbine. Water pressure is built up as it flows through this pipe. Third, and perhaps the most important component, is the turbine. The turbine has large blades, which are moved by the flow of water, so are a series of magnets inside the generator. These giant magnets move past copper coils, producing electricity. Fifth, the electric current inside the powerhouse is taken to the transformer and converted to a higher-voltage current. The current is then taken out of the hydroelectric station via power lines. Finally, the water that has been used to generate power is released through an outflow –the water is carried through pipelines and rejoins the river.     An alternative method of hydroelectric power generation is via a pumped storage plant. An example of this is Dinorwig in Wales. This requires two reservoirs: an upper reservoir where water is stored to provide power; and a lower reservoir where water enters from the upper reservoir after being used for power generation. Using a reversible turbine, water is pumped back up to the upper reservoir when the demand for electricity is not as great. The advantage of this kind of hydroelectric power station is that electricity can be generated at times of greatest demand.Taken from: Harrison, L. & Cushen, C. (2005). Achieve IELTS. Marshall Cavendish Ltd.The amount of electricity produced depends on the amount of water only. Answer 1 Question 1Hydroelectric power stations produce some pollution.Answer 2 Question 1All hydroelectric power stations release water back into the river. Answer 3 Question 1Early hydroelectric power stations release water back into the river. Answer 4 Question 1Hydroelectric power stations supply a large number of people with electricity.