Think of the energy and power in a giant waterfall. Now imagine being able to use that energy to make electricity. That is hydroelectric power. A hydroelectric plant uses the flow of water from a higher to a lower elevation to generate power. It accomplishes this by taking control of a river, usually through the construction of a dam.  Hydroelectric plants provide about 20% of the world’s electricity. Only oil, coal, and natural gas generate more electricity worldwide.

	Click for animation. Hydroelectric power A typical hydroelectric power plant uses a dam to block and control the flow of a river. The water behind the dam flows past turbines attached to a generator, which makes electricity.

Hydroelectric plants provide 650,000 megawatts of power worldwide. However, not all parts of the world are suited to produce power this way. To produce power this way, a region needs mountains and rapidly flowing rivers and streams, or heavy rainfall.

The major users of hydroelectric power include the United States, Canada, Russia, and Brazil. China will soon surpass Brazil in hydroelectric capacity with the completion of the huge Three Gorges project. Norway and Egypt also use hydroelectric power for a good portion of their electricity supply.

Hydroelectric plants vary in size. Large plants producing 30 megawatts or more supply electricity to large regions. Small plants making between 110 kilowatts and 30 megawatts provide enough electricity to power local developments. Micro plants, with ratings of 100 kilowatts or less, make power for small farms or singles homes.  

How It Works

Hydroelectric power works very simply. To use the natural power found in a river that flows swiftly from a higher elevation to lower ground, the flow must be controlled. This is accomplished by building a dam. The dam stops the natural flow of the river, creating a reservoir behind it. A power plant constructed within or alongside the dam houses two different engines: a turbine and a generator. The turbine consists of a series of angled blades mounted to a central shaft. The turbine turns around and around as a controlled flow of water passes through the power plant. The movement of the turbine drives the generator, which then makes electricity.

Grist Mill Photos courtesy of TalkingTree.com,

Two factors determine the amount of electricity generated by a hydroelectric plant. The first factor: the height from the turbines to the water surface, a distance known as the head. The second key factor is the flow volume of water moving through the turbine. As a general rule, 3.79 l (1 gal) of water per second falling 30.48 m (100 ft) can generate 1 kilowatt of electrical power. The head remains the same all the time, but the water flow can be increased or decreased, depending on the demand for electricity.  

History

People have harnessed the power of moving water for centuries. The ancient Greeks and Romans used the waterwheel, which operates on the same principle as a turbine, to turn machinery. The waterwheel was known in ancient China as well. In Europe, from medieval times on, flowing water drove waterwheels that ground corn or wheat into flour. Water mills powered the textile factories of England and New England in the early 1800s. Development of a turbine driven by steam made water power even more efficient.

Electricity and hydroelectric plants arose around the same time, in the late 1800s. In 1878, Cragside, the Northumberland home of British inventor Lord Armstrong, became the first house powered by a hydroelectric plant. Two years later, the city of Grand Rapids, Michigan, connected a water turbine to a Brush dynamo, an early type of electric generator developed by Charles F. Brush. This arrangement created enough power to light theaters and storefronts. In 1881, another Brush dynamo connected to a turbine in a flour mill provided street lighting at Niagara Falls, New York.

The first hydroelectric power plant opened in Appleton, Wisconsin, in 1882, on the Fox River. The owner of a local paper mill linked a water-driven turbine and a generator. This first plant produced only 12.5 kilowatts of electricity, just enough to power two paper mills and the home of the mill owner. By 1886 a bigger plant, with enough power for Appleton’s electric streetcar system, replaced the original plant.

Hydroelectric power grew rapidly after that. In 1886 there were 45 hydroelectric plants in the United States. By 1889, 200 plants were generating electricity by using water for some or all of the power.

At the same time, hydroelectric power plants opened around the world. Italy built its first hydroelectric plant in 1885 at Tivoli, in the mountains outside Rome. The plant initially powered lights in the nearby town. But by 1892 a second plant in the same location was providing power to Rome, the first long-distance power transmission in Italy.

Other countries with good conditions for hydroelectric power soon built plants. Canada, France, Japan, and Russia were among the first on board. During the period from 1900 to 1950 the use of hydroelectric power increased rapidly.

When the first hydroelectric power plants opened, all electricity was sent out as direct current. This limited the distance the electricity could be transmitted. Because of this, these plants provided power within about 2.6 sq km (1 sq mi) of the dam. Power from several separate plants would be combined to serve larger cities. Smaller towns fortunate enough to be able to build hydroelectric plants had their own electric systems. With the development of alternating current in the late 1880s, electricity could travel longer distances. The separate systems serving cities became one larger system. Plants in remote locations—for example, the Hoover Dam in the southwestern United States (built in 1936)—provided power to distant cities. Better hydraulic turbines contributed to this advance. However, after the 1940s, cheap fossil fuels became the prime source for electricity generation. Hydroelectric power continued to provide some of the world’s electricity, but use of oil, natural gas, and coal surpassed hydropower.

Current Status

Technological advances in the plants and in power transmission make it feasible to build hydroelectric plants in remote locations, far from where the power will be used. The Itaipu Dam on the Paraná River between Paraguay and Brazil can produce up to 12,600 megawatts of power. This plant, opened in 1982, supplies nearly all Paraguay’s electricity and one-quarter of that needed for Brazil. Tasik Kenyir (Lake Kenyir) Sultan Mahmud hydroelectric power station in Malaysia has a capacity of 400 megawatts of power.

One of the largest hydroelectric projects in the world is China’s Three Gorges project on the Yangtze River. The dam, designed to control the devastating floods on the river, includes a large hydroelectric plant. With a capacity of 18,200 megawatts, Three Gorges is projected to be able to provide up to one-ninth of China’s electricity needs. China has also built many small hydroelectric plants for local use around the country.

Types of Facilities

There are three main types of hydroelectric power facilities. The most common hydroelectric plant is an impoundment plant. This type of plant uses a dam to impound, or store, water in a large reservoir for later use. To generate electricity, water released from the reservoir flows through a channel called a penstock to the turbines. The falling water spins the turbines, which then power the generator. An impoundment plant releases water as needed to meet energy requirements, increasing or decreasing flow with power demand. Impoundment plants can be of any size, depending on the river, the dam, and the head.

The dam associated with impoundment facilities serves two functions. It raises the water height of the stream or river to a different elevation, creating water pressure, or head. In addition, it provides a way to control flooding on a river. The water can be released or held back to prevent dramatic changes in the flow of the river below the dam. One drawback of impoundment plants is that they depend on the precipitation in a region. During a drought, the head may be affected if the reservoir level drops too much. This can limit the amount of electricity produced by an impoundment plant.

Drawing courtesy of U.S. Department of Energy,

A diversion plant, sometimes called a run-of-river facility, in most cases does not use a dam. The plant diverts some of the river water through a canal or penstock, where the flow powers a turbine. These plants rely entirely on the flow of the river to produce electricity. There is no dam to artificially raise the height of the water. A diversion plant depends solely on the landscape to create the head.

Because of this, most diversion plants produce a limited amount of power. These plants rely entirely on precipitation for water flow. A drought can have a tremendous impact on electricity production. There are exceptions when a diversion plant is built where there is a tremendous flow of water. For example, the Canadian and American hydroelectric plants on Niagara Falls each produce more than 2 million kilowatts.

Like impoundment plants, pumped-storage plants use dams to store water. However, the pumped-storage facility has two reservoirs. One, located above the dam, has a much higher elevation, while the other, below the dam, has a lower elevation. In times of high energy demand, the pumped-storage plant releases water from the upper reservoir to the lower reservoir, turning the turbines along the way. When the energy demand decreases,  the plant pumps water from the lower reservoir back to the upper reservoir. Some of the plant’s own power is used up during pumping. Droughts do not affect pumped-storage plants, because water can continually be pumped into the upper reservoir to keep the electricity production steady. However, these plants cost a lot of money to build. In addition, it is difficult to find the right location to accommodate two reservoirs.

Drawing courtesy of Tennessee Valley Authority,

Advantages

Hydroelectric power is long-established as a clean, safe method of making electricity. It does not add carbon dioxide (CO2) to the environment, and it uses a renewable power source, water.

Hydroelectric power has other benefits as well. Dams and hydroelectric plants last a long time. This means that once the costs of building are paid off, a hydroelectric plant becomes a relatively inexpensive source of electricity.

Hydroelectric dams built on flood-prone rivers help control flooding. And, the reservoirs created behind the dams can be used for recreation. Lake Mead in Arizona and Nevada attracts boaters and campers. Malaysia’s Tasik Kenyir draws tourists to its waterfalls, caves, and tropical islands.

Issues

Over the years, serious concerns have surfaced about the environmental impact of hydroelectric plants. For starters, a hydroelectric plant changes the flow of a river both in front of and behind the dam. This significantly alters the river ecosystem. The dam blocks migrating fish from moving upstream. The turbines injure or kill some of the fish migrating downstream. Because of the change in flow, water oxygen levels drop. This affects both plant and animal life in the river and along its banks. The water level rises and drops with power use, forcing aquatic and shoreline plants to cope with frequent changes in water level.

Hydroelectric providers have taken steps to address many of these issues. Some have built fish ladders—a series of shallow, water-covered steps for the fish to swim and leap up—or fish elevators to help fish migrate upstream to spawning grounds. Some dams have special diversion channels to guide fish migrating downstream, while others decrease turbine flow during peak migration times. Many facilities use techniques to add oxygen to the water to improve water quality for plant and animal life.

The scope of some newer projects has raised a number of concerns for the people in those regions. The huge Three Gorges project displaced 1 million people. Historic sites and antiquities have been submerged as the reservoir fills. The dams are in an earthquake-prone area, and dam failures would be disastrous.

Recent research shows that hydroelectric facilities in tropical regions may produce as much methane, a greenhouse gas, as facilities that make power from fossil fuels. The cause: the decomposition of plant matter. Some of the plant matter comes from the vegetation destroyed when the reservoir initially fills. Later, plants grow and decay as the water level rises and drops seasonally. This issue remains under study.

Down the Road

In developing countries, the trend is to build smaller hydroelectric plants to meet local needs. Countries such as the United States and Canada have many small hydroelectric plants that are no longer working. Theoretically these could be renovated and put to use. However, environmentalists favor taking down unsound dams to let rivers run free again. These older dams may be removed rather than repaired.

Hydroelectric providers continue to improve their technology. For example, many are now making more-efficient turbines.

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