Hydroelectric Energy

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Hydroelectric energy—electric power created by the kinetic energy of moving water—plays an important role in supplying the world's electricity. In 1996, nearly 13 trillion kilowatt-hours of electricity were generated worldwide; almost one-fifth of this electricity was produced with hydroelectricity. On average hydropower provides about ten percent of the U.S. electricity supply, although the annual amount of electricity generated by hydroelectric resources varies due to fluctuations in precipitation. In many parts of the world, reliance on hydropower is much higher than in the United States. This is particularly true for countries in South America where abundant hydroelectric resources exist. In Brazil, for example, 92 percent of the 287 billion kilowatt-hours of electricity generated in 1996 were generated by hydroelectricity.

There are many benefits for using hydro resources to produce electricity. First, hydropower is a renewable resource; oil, natural gas, and coal reserves may be depleted over time. Second, hydro resources are indigenous. A country that has developed its hydroelectric resources does not have to depend on other nations for its electricity; hydroelectricity secures a country's access to energy supplies. Third, hydroelectricity is environmentally friendly. It does not emit greenhouse gases, and hydroelectric dams can be used to control floods, divert water for irrigation purposes, and improve navigation on a river.

There are, however, disadvantages to developing hydroelectric power. Hydroelectric dams typically require a great deal of land resources. In conventional hydroelectric projects, a dam typically is built to create a reservoir that will hold the large amounts of water needed to produce power. Further, constructing a hydroelectric dam may harm the ecosystem and affect the population surrounding a hydro project. Environmentalists often are concerned about the adverse impact of disrupting the flow of a river for fish populations and other animal and plant species. People often must be relocated so that a dam's reservoir may be created. Large-scale dams cause the greatest environmental changes and can be very controversial. China's 18.2 gigawatt Three Gorges Dam project—the world's largest hydroelectric—will require the relocation of an estimated 1.2 million people so that a 412-mile reservoir can be built to serve the dam.

Another potential problem for hydroelectricity is the possibility of electricity supply disruptions. A severe drought can mean that there will not be enough water to operate a hydroelectric facility. Communities with very high dependence on the hydroelectric resources may find themselves struggling with electricity shortages in the form of brown-outs and black-outs.

This article begins with a description of how hydroelectricity works, from the beginning of the hydrological cycle to the point at which electricity is transmitted to homes and businesses. The history of the dam is outlined and how dams evolved from structures used for providing a fresh water supply to irrigation and finally to providing electricity. The history of hydropower is considered and the different hydroelectric systems (i.e., conventional, run-of-river, and pumped storage hydroelectricity) currently in use are discussed.


Hydroelectricity depends on nature's hydrologic cycle (Figure 1). Water is provided in the form of rain, which fills the reservoirs that fuel the hydroelectric plant. Most of this water comes from oceans, but rivers and lakes and other, smaller bodies of water also contribute. The heat of the sun causes the water from these sources to evaporate (that is, to change the water from its liquid state into a gaseous one). The water remains in the air as an invisible vapor until it condenses and changes first into clouds and eventually into rain. Condensation is the opposite of evaporation. It occurs when the water vapor changes from its gaseous state back into its liquid state.

Condensation occurs when air temperatures cool. The cooling occurs in one of two ways. Either the air vapor cools as it rises and expands or as it comes into contact with a cool object such as a cold landmass or an ice-covered area. Air rises for several reasons. It can be forced up as it encounters a cooler, denser body of air, or when it meets mountains or other raised land masses. It can rise as it meets a very warm surface, like a desert, and become more buoyant than the surrounding air. Air also can be forced to rise by storms—during tornadoes particles of air circling to the center of a cyclone collide and are forced up. When the water vapor collides with a cold object, it can become fog, dew, or frost as it condenses. The vapor cools as it rises into the atmosphere and condenses to form clouds and, sometimes, rain.

In order for rain to form, there must be particles in the air (i.e., dust or salt) around which the raindrop can form and which are at temperatures above freezing. When the particles are cooled to temperatures below the freezing point water condenses around them in layers. The particles grow heavy enough that they eventually fall through the clouds in the form of raindrops or—if the air temperature is below the freezing point all the way to the ground — as snow, sleet, or hail.

Much of the rain that reaches the ground runs off the surface of the land and flows into streams, rivers, ponds and lakes. Small streams lead to bigger ones, then to rivers, and eventually back to the oceans where the evaporation process begins all over again. Although water continuously changes from solid to liquid to gas, the amount of water on the earth remains the same; there is as much water today as there was hundreds of millions of years ago.

This water cycle—the process of moving water from oceans to streams and back again—is essential to the generation of hydroelectricity. Moving water can be used to perform work and, in particular, hydroelectric power plants employ water to produce electricity. The combination of abundant rainfall and the right geographical conditions is essential for hydroelectric generation.

Hydroelectric power is generated by flowing water driving a turbine connected to an electric generator. The two basic types of hydroelectric systems are those based on falling water and those based on natural river current, both of which rely on gravitational energy. Gravitational forces pull the water down either from a height or through the natural current of a river. The gravitational energy is converted to kinetic energy. Some of this kinetic energy is converted to mechanical (or rotational) energy by propelling turbine blades that activate a generator and create electricity as they spin.

The amount of energy created by a hydroelectric project depends largely upon two factors: the pressure of the water acting on the turbine and the volume of water available. Water that falls 1,000 feet generates about twice as much electric power as the same volume of water falling only 500 feet. In addition, if the amount of water available doubles, so does the amount of energy.

The falling water hydrosystem is comprised of a dam, a reservoir, and a power generating unit (Figure 2). The dam is constructed so that a reservoir is created within which water accumulates and may be stored until it is needed. Water is released as required to meet electricity demands of customers. At the bottom of the dam wall is the water intake. The water intake controls when and how much water is moved into a steel called the "penstock." Gravity causes the water to fall through the penstock. This pipe delivers the running water to a turbine—a propeller-like machine with blades like a large fan. The water pushes against the turbine blades and the blades turn. Because the turbine is connected to an electric generator, as the turbine gains speed, it powers the generator and electricity is produced. The largest falling water facility in the United States is the Grand Coulee hydroelectric project on the Columbia River, in Washington State. Indeed, the largest power production facilities in North America are found at the Grand Coulee Dam project where an average 21 billion kilowatt-hours of electricity are produced each year.

The second type of hydroelectric plant is called a run-of-the-river system. In this case, the force of the river current applies pressure to the turbine blades to produce electricity. Run-of-the-river systems do not usually have reservoirs and cannot store substantial quantities of water. As a result, power production from this type of system depends on the river flow—the electricity supply is highly dependent upon seasonal fluctuations in output. Run-of-river projects are most successful when there are large flows in flat rivers or when a high natural geological drop is present, and when the required electricity output is below the maximum potential of the site.

Hydropower systems, as in all electricity-producing systems, require a generator to create the electricity. An electric generator is a device that converts mechanical energy into electric energy. The process is based on the relationship between magnetism and electricity. When a wire or any other electrically conductive material moves across a magnetic field, an electric current occurs in the wire. In a power plant, a strong electromagnet, called a rotor, is attached to the end of a shaft. The shaft is used to spin the rotor inside a cylindrical iron shell with slots—called a stator. Conducting wires are wound through the slots of the stator. When the rotor spins at a high rate of speed, an electric current flows through the conducting wires.

In a hydroelectric system, flowing water is used to propel a turbine that spins the shaft connected to the generator and creates the electric current. The kinetic energy of the moving water is changed into rotational energy and thereby causes the turbine blades to rotate. The turbine is attached to an electric generator and the rotational energy of the turbine is then converted to electric energy. The electricity then leaves the generator and is carried to the transformers where the electricity can travel through electric power lines and is supplied to residential, commercial, and industrial consumers. After the water has fallen through the turbine, it continues to flow downriver and to the ocean where the water cycle begins all over again.

Pumped storage hydroelectricity is an extended version of the falling water hydroelectric system. In a pumped storage system, two water sources are required—a reservoir located at the top of the dam structure and another water source at the bottom. Water released at one level is turned into kinetic energy by its discharge through high-pressure shafts that direct the downflow through the turbines connected to the generator. The water flows through the hydroelectric generating system and is collected in a lower reservoir. The water is pumped back to the upper reservoir once the initial generation process is complete. Generally this is done using reversible turbines—that is, turbines that can operate when the direction of spinning is reversed. The pump motors are powered by conventional electricity from the national grid. The pumping process usually occurs overnight when electricity demand is at its lowest. Although the pumped storage sites are not net energy producers—pumped storage sites use more energy pumping the water up to the higher reservoir than is recovered when it is released—they are still a valuable addition to electricity supply systems. They offer a valuable reserve of electricity when consumer demand rises unexpectedly or under exceptional weather conditions. Pumped storage systems are normally used as "peaking" units.


Dams have existed for thousands of years. The oldest known dam, the Sadd el-Kafara (Arabic for "Dam of the Pagans"), was constructed over 4,500 years ago twenty miles south of Cairo, Egypt. The 348-feet wide, 37-feet high dam was constructed to create a reservoir in the Wadi el-Garawi. The dam was built with limestone blocks, set in rows of steps about eleven inches high. It appears that the dam was supposed to be used to create a reservoir that would supply drinking water for people and animals working in a nearby quarry. Scientists and archaeologists believe that the Sadd el-Kafara failed after only a few years of use because there is no evidence of siltation at the remains of the dam. When water flows into the reservoir created by a dam, the silt—sand and other debris carried with the stream—is allowed to settle, rather than be borne further downstream by the force of the flow of a stream or river. In the still water behind a dam this sediment is deposited on the bottom of the reservoir.

The earliest dams were built for utilitarian purposes, to create reservoirs for drinking supplies—as in the Sadd el-Kafara—or to prevent flooding or for irrigation purposes. Early dams were even used to create lakes for recreational purposes. In the middle of the first century, the Roman emperor, Nero, constructed three dams to create three lakes to add to the aesthetic beauty of his villa.

Today a variety of dam structures are utilized. These can be classified as either embankment dams or concrete dams. Embankment dams are constructed with locally available natural resources. There are several different types of embankment dams, including earth dams—which are constructed primarily of compacted earth; tailings dams—constructed from mine wastes; and rockfill dams—which are constructed with dumped or compacted rock. The shape of these dams is usually dependent on the natural settling angle of the materials used to build them. Concrete, bitumen, or clay is often used to prevent water from seeping through the dam. This can be in the form of a thin layer of concrete or bitumen facing which acts as a seal, or in the form of a central core wall of clay or other fine materials constructed within the dam, allowing water to penetrate the upstream side of the structure, but preventing it from moving beyond the clay core.

Concrete dams are more permanent structures than embankment dams. Concrete dams can be categorized as gravity, arch, or buttress. Because concrete is a fairly expensive material, different construction techniques were developed to reduce the quantity of concrete needed. This is highly dependent upon geological considerations. In particular, the rock foundations must be able to support the forces imposed by the dam, and the seismic effects of potential earthquakes. Gravity dams work by holding water back by way of their own weight. A gravity dam can be described as a long series of heavy vertical, trapezoidal structural elements firmly anchored at the base. It is generally a straight wall of masonry that resists the applied water-pressure by its sheer weight. The strength of a gravity dam ultimately depends on its weight and the strength of its base.

An arch dam, on the other hand, relies on its shape to withstand the pressure of the water behind it. The arch curves back upstream and the force exerted by the water is transferred through the dam into the river valley walls and to the river floor. They are normally constructed in deep gorges where the geological foundations are very sound. The United States's Hoover Dam is an example of a concrete arch dam.

The buttress dam uses much less concrete than the gravity dam, and also relies on its shape to transfer the water load. Buttress dams were developed in areas where materials were scarce or expensive, but labor was available and cheap. They have been built primarily for purposes of irrigation. The dams are particularly suited for wide valleys. They have a thin facing supported at an incline by a series of buttresses. The buttresses themselves come in a variety of shapes, including the multiple arch and the simple slab deck (see Figure 3). The weight of the concrete is transferred to bedrock through the downstream legs or "buttresses" of the structure. The Coolidge Dam near Globe, Arizona is an example of a buttress dam, constructed of three huge domes of reinforced concrete.

Using water as a source of power actually dates back more than 2,000 years when the Greeks used water to turn wheels to grind wheat into flour. The first recorded use of water power was a clock, built around 250 B.C.E. and since that time, falling water has provided power to grind corn, wheat, and sugar cane, as well as to saw mills. In the tenth century, water wheels were used extensively in the Middle East for milling and irrigation. One of these dams, built at Dizful—in what is now Iran—raised the water 190 feet and supplied the residents with water to grind corn and sugar cane.

The discoveries associated with electromagnetism in the early nineteenth century had a major impact on the development of hydropower. The development of electric power generators and the fact that electric power was the only form of energy in a "ready to use" state which can be transmitted over long distances has been particularly significant to hydroelectricity. Dams located away from population centers could be useful generators if there was a way to supply the consumers. Water turbines were also developed during the nineteenth century as a natural successor to the water wheel. The high performance and small size of the turbine relative to the water wheel were important advancements. Combining the technology of electric generators, turbines, and dams resulted in the development of hydroelectric power.

Water was first used to generate electricity in 1880 in Grand Rapids, Michigan when a water turbine was used to provide storefront lighting to the city. In 1882—only two years after Thomas Edison demonstrated the incandescent light bulb—the first hydroelectric station to use Edison's system was installed on the Fox River at Appleton, Wisconsin. In 1881, construction began on the first hydroelectric generating station on the Niagara River in Niagara Falls on the New York-Canadian border. For twenty years, this project met the small electricity needs of the city. Water from the upper Niagara River fell 86 feet down a flume (a narrow gorge with a stream running through it) onto spinning water wheels on the lower river to generate electricity. The electricity was used to run the equipment of a paper company, other small factories, and sixteen open arc-lights on village streets. By 1896, the first long-distance transmission of electricity allowed the Niagara Falls project to provide electricity to Buffalo, New York—some 26 miles away.

By the early twentieth century, hydroelectric power was providing more than 40 percent of electricity generation in the United States. In 1940, hydropower supplied about three-fourths of all the electricity consumed in the West and Pacific Northwest, and still supplied about one-third of the total U.S. electricity supply. Although hydroelectricity's share of total electricity generation has since fallen to about 10 percent in the United States, hydroelectricity provides almost one-fifth of the world's total electricity generation today.

Hydroengineering has evolved over the past century so that it has become possible to build larger and larger hydroelectric projects. In 2000, the largest hydroelectric project in the United States was the Grand Coulee power plant on the Columbia river in Washington State. Grand Coulee is also the third largest currently-operating hydroelectric project in the world. The Grand Coulee project began operating in 1941 with 20 megawatts of installed capacity. The project has been expanded so that in 2000 it operated with an installed capacity of 6,180 megawatts.

The world's largest hydroelectric plant is the Itaipú power plant, located on the Paraná river separating Brazil and Paraguay. This 12,600-megawatt hydroelectric project is jointly-owned by Brazil and Paraguay. It is comprised of eighteen generating units, each with an installed capacity of 700 megawatts. The plant produces an estimated 75 billion kilowatt-hours each year. In 1994, Itaipú supplied 28 percent of all the electricity consumed in Brazil's south, southeast, and central-west regions, and 72 percent of Paraguay's total energy. Construction on this mammoth project began in 1975 and was not completed until 1991 at a cost of about $18 billion (U.S.). It required fifteen times more concrete than the Channel Tunnel that connects France and the United Kingdom; and the amount of steel and iron used in its construction would have built 380 Eiffel towers. The main dam of Itaipú is a hollow gravity type dam, but a concrete buttress dam and two embankment dams were also incorporated into the system design. At its highest point, the main dam is 643 feet—more than twice as high as the Statue of Liberty; it extends nearly five miles across the Paraná River.

The large-scale disruptions to the environment of projects like the Itaipú hydroelectric system are profound and this is one of the major disadvantages to constructing large-scale hydroelectric facilities. As a result of construction on Itaipú, over 270 square miles of forest land has been negatively impacted, mostly on the Paraguayan side of the Paraná River where an estimated 85 percent of the forest was destroyed during the early years of construction. Despite programs to minimize the environmental damage through migration to reserves of the wildlife and plants facing extinction, several plant species became extinct mostly because they did not survive the transplant process.

Itaipú no longer represents the upper limit of large-scale hydroelectric expansion. At the end of 1994, the Chinese government announced the official launching of construction on the Three Gorges Dam hydroelectric project. When completed in 2009, this 18,200-megawatt project will provide the same amount of electricity as thirty 600-megawatt coal-fired plants. Project advocates expect the dam to produce as much as 85 billion kilowatt-hours of electricity per year, which is still only about 10 percent of the 881 billion kilowatt-hours of electricity consumed by China in 1995. It will also be used to control flooding along the Yangtze River and will improve navigational capacity, allowing vessels as large as 10,000 tons to sail upstream on the Yangtze as far as Chongqing, 1,500 miles inland from Shanghai. It is a very controversial project that will require the relocation of an estimated 1.2 million people, and that will submerge thirteen cities, 140 towns, 1,352 villages, and some 650 factories in its 412-mile reservoir that is to be created to support the dam. Disrupting the flow of the river will place several rare plant and animals at risk, including the endangered Yangtze River dolphin. Some environmentalists believe the reservoir created for the Three Gorges Dam project will become a huge pollution problem by slowing the flow of the Yangtze River and allowing silt to build up, possibly clogging the planned harbor at Chongqing within a few decades. The World Bank and the U.S. Export-Import Bank have refused to help finance the project, primarily because of the adverse environmental effects this massive hydroelectric project might have. Opponents to the Three Gorges Dam project have stated that smaller dams built on Yangtze River tributaries could produce the same amount of electricity and control flooding along the river, without adversely impacting the environment and at a substantially lower price than this large-scale project.

Hydroelectric projects can have an enormous impact on international relations between surrounding countries. For instance, the controversy between Hungary and Slovakia over the Gabcĺkovo dam began at the dam's opening in 1992 and as of 2000 had not been completely resolved, despite a ruling by the International Court of Justice at The Hague in 1997. In its first major environmental case, the Court ruled that both countries were in breach of the 1977 treaty to construct two hydroelectric dams on the Danube River—the Slovakian Gabcĺkovo and, 80 miles to the south, the Hungarian Nagymaros. Hungary suspended work on its portion in 1989 due to protests from the populace and international environmental groups. In 1992, Czechoslovakia decided to complete Gabcĺkovo without Hungarian cooperation. Today, the $500 million, 180-megawatt project supplies an estimated 12 percent of the electricity consumed in Slovakia.

The Court stated that Hungary was wrong to withdraw from the treaty, but that Slovakia also acted unlawfully by completing its part of the project on its own. After almost a year of talks to resolve the differences between the two countries, Hungary and Slovakia agreed to construct a dam either at Nagymaros—the original site of the Hungarian portion of the project—or at Pilismarot. Unfortunately, soon after the agreement was signed, environmental protests began anew and little progress has been made to resolve the situation to everyone's satisfaction.

Another illustration of the political ramifications often associated with constructing dam projects concerns Turkey's plans to construct the large-scale Southeast Anatolia (the so-called GAP) hydroelectric and irrigation project. When completed, GAP will include twenty-one dams, nineteen hydroelectric plants (generating 27 billion kilowatthours of electricity), and a network of tunnels and irrigation canals. Neighboring countries, Syria and Iraq, have voiced concerns about the large scale of the hydroelectric scheme. Both countries argue that they consider the flow of the historic rivers that are to be affected by GAP to be sacrosanct. Scarce water resources in many countries of the Middle East make the disputes over the resource and the potential impact one country may have on the water supplies of another country particularly sensitive.

In the United States, in 2000 there were still over 5,600 undeveloped hydropower sites with a potential combined capacity of around 30,000 megawatts, according to estimates by the U.S. Department of Energy. It is, however, unlikely that a substantial amount of this capacity will ever be developed. Indeed, there is presently a stronger movement in this country to dismantle dams and to restore the natural flow of the rivers in the hopes that ecosystems damaged by the dams (such as fish populations that declined when dams obstructed migratory patterns) may be repaired. The U.S. Department of Interior (DOI) has been working to decommission many hydroelectric dams in the country and to restore rivers to their pre-dam states. In 1997, the Federal Energy Regulatory Commission (FERC) ordered the removal of the 160-year old Edwards Dam on the Kennebec River in Augusta, Maine and the demolition of the dam began in July 1999. Although Edwards Dam has only a 3.5-megawatt installed generating capacity and provided less than one-tenth of one percent of Maine's annual energy consumption, the event is significant in that it represented the first time that FERC had used its dam-removal authority and imposed an involuntary removal order.

In 1998, DOI announced an agreement to remove the 12-megawatt Elwha Dam near Port Angeles, Washington, but removal has been delayed indefinitely because Congress withheld the funds needed to finance the project. Some small dams in the United States have been successfully removed—such as the 8-foot Jackson Street Dam used to divert water for irrigation in Medford, Oregon and Roy's Dam on the San Geronimo Creek outside San Francisco, California. However, efforts to dismantle many of the larger dams slated for removal in the United States have been delayed by Congressional action, including four Lower Snake River dams and a partially built Elk Creek Dam in Oregon.

Linda E. Doman

See also:Electric Power, Generation of; Electric Power Transmission and Distribution Systems; Magnetohydrodynamics.


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