Hydroelectric energy is obtained when a water current turns turbines to produce electricity. About 7 percent of the world's electrical energy supply is hydroelectric.
Dams and Power Stations
Hydroelectric power stations capture energy from water's motion as it flows from a higher level to a lower level. Nearly all such facilities require the construction of dams, which hold back water in reservoirs (artificial lakes) upstream. Dams control the rush of water from the reservoirs through channels through the turbines and then downstream. The greater the amount of water flowing and the greater its level differential (change in height), the more electricity the station can generate. In a few locations, natural drop and flow make a dam unnecessary.
Dams at most of the world's power stations are tens of yards high. Dozens of them, sometimes called megadams, rise to a height of more than 492 feet (150 m). The world's tallest dam, the Nurek Dam in Tajikistan, towers more than 984 feet (300 m) high. Waters stored behind such large dams may create huge artificial lakes, changing Earth's natural landscape.
Advantages and Drawbacks of Hydroelectric Energy
Hydroelectric power has certain advantages over other energy sources. Water is a renewable resource, unlike Earth's limited supply of fossil fuels. Also, large hydroelectric power stations can produce huge amounts of electricity without polluting air or water. For instance, the Guri Dam on the Carana River in Venezuela, one of the world's largest power stations of any type, can generate as much electricity as ten large coalburning or nuclear-powered plants.
However, hydroelectric power stations do have a number of drawbacks, including effects on the environment. Tropical reservoirs can release large amounts of methane, a gas that helps cause global climate change. New lakes that flood areas behind dams may force people and animals to move elsewhere, and dams may keep fish from reaching their natural spawning areas. Dams interfere with the flow of rivers and trap silt, at the same time causing soil erosion downstream when engineers release large amounts of water. The buildup of sediment in the reservoir may also lower the water level differential, limiting the life of the power station. In addition, only a limited number of streams and rivers are suitable for damming.
A few large dams have collapsed, causing widespread loss of life and property. For all of these reasons, many people prefer small dams that produce modest amounts of electricity for local use. Large dams are also very expensive to build, and this expense can put a heavy burden on a developing nation. Hydroelectric stations can be unreliable during periods of little or no rainfall, when water levels drop in rivers and reservoirs. Electric utility companies in the United States often use other types of power stations along with hydroelectric plants to avoid this problem.
The United States, Canada, Brazil, Russia, and China together produce about half the world's hydroelectric energy. Among these countries, Canada produces the most power, while Brazilians depend most on hydroelectricity for their energy needs. Several nations, including Norway and Zaire, rely on water power for nearly all of their electricity.
The History of Water Power
Moving water was one of the earliest sources of power used by human societies. The ancient Greeks and Romans used wooden waterwheels to grind grain. During the Industrial Revolution of the 1700s and 1800s, waterwheels ran mills and machinery in many cities.
Steam engines replaced waterpower for a time in the 1800s, but a hydroelectric dam built in 1882 in Appleton, Wisconsin, reestablished waterpower's importance in the United States. In the late 1920s, however, steam-powered electric plants running on fossil fuels grew larger and more efficient. In order to compete, hydroelectric engineers began to increase the size of dams--with the help of government funding.
In 1930, Congress authorized construction of the Boulder Dam, later renamed Hoover Dam, on the Colorado River along the border between Arizona and Nevada. Standing 722 feet (220 m) high, it is the tallest concrete dam in the United States. During the 1930s, the Tennessee Valley Authority built a network of large dams to generate power, control flooding, and provide water for agriculture. The Columbia River's Grand Coulee Dam, completed in 1942, is the country's largest single source of water power.
Generation of hydroelectric power peaked in the United States in 1983. Dams now account for less than 10 percent of U.S. electrical power. Worldwide, however, the production of hydroelectricity continues to grow.
- machine that uses a moving fluid to turn a rotor, creating mechanical energy
- fossil fuel
- substance such as coal, oil, or natural gas, found underground in deposits formed from the remains of organisms that lived millions of years ago
- particles of soil and rock carried by rivers and streams
- wearing away of land by wind and water
- soils, rock particles, and other materials that are deposited over time and make up the ground, whether on dry land or at the bottom of a body of water