Hydroelectric Power Plants

Summary

Hydroelectric power plants produce electricity using water, a renewable resource. The power plants convert the energy in flowing water into electricity that can supply the needs of an entire city or supplement the power available for a region or other area on the power grid. To produce the electricity, water collects behind a dam before flowing through a turbine. As the water flows through the turbine, a generator uses magnets to create an electromagnetic field and then electricity. There is little pollution created in the process, but there is an impact on the ecosystem at the site of the plant.

Definition and Basic Principles

Hydroelectric power plants take the stored energy of water in a reservoir and convert it into hydroelectricity. Swiftly moving water is brought into the powerhouse and then moved through the turbine engine. As the water passes by the turbine, it causes the blades of the turbine to spin. These blades in turn cause a series of rotors with magnets mounted on rotors inside the generator to rotate past copper coils. The magnets and coiled copper wire act as giant electromagnets and produce an alternating current. The alternating current, or AC, is then converted into a higher voltage of current before exiting the powerhouse by way of the power lines that carry the power to the electric grid.

The amount of electricity generated depends upon the volume of water flow from the reservoir. The larger the reservoir, the greater the volume flow and the greater the amount of electricity produced, as the greater the volume flow, the more quickly the magnets will spin within the coil.

For a hydroelectric power plant to produce a steady flow of electricity, a steady source of water must flow into the reservoir. To ensure this, some power plants have an upper reservoir and a lower reservoir. Water from the upper reservoir is used to produce electricity and then channeled into a lower reservoir, rather than back into the river downstream of the dam. During off-peak hours, water from the lower reservoir is pumped back up to the upper reservoir to be used again as a source of flowing fluid for the turbines. In the absence of a lower reservoir, water is directed into the river downstream of the dam.

Current technology calls for large quantities of water moving at a high rate of speed. Newer technologies that use the kinetic properties of water without building dams, like the waterwheels of years ago, are also being explored.

Background and History

Civilizations have been using water as a power source for thousands of years. From the ancient Greeks, who used water wheels to replace manual labor in the grinding of wheat into flower, to the Romans, who created floating mills when under siege by the Goths in 537 c.e., water has been used to get work done. Because they had fewer slave workers than the Romans, the ancient Chinese used waterpower to their advantage throughout the empire. Water, for example, was used to power the bellows used in iron casting.

During the medieval period in Europe, waterpower grew in prominence. Because of the Black Death (the plague) and the shortage of labor that resulted, it was critical to find an inexpensive way to grind wheat to sustain the surviving population. The number of mills increased dramatically, with some of the mills built to take advantage of tidal changes.

Another important development was the use of waterpower by a religious order. The Cistercian monastic order lived in rural areas, where they perfected the use of hydropower for milling, woodcutting, and olive crushing. They did not use dams, but rather placed their waterwheels in swiftly moving water without impeding the flow of that water. They also used the water for washing and for sewage disposal. As the order moved through Europe, the technology traveled with them, until waterpower next came to prominence during the Industrial Revolution. At that time, swiftly running streams were used to provide power for a variety of manufacturing processes before the widespread use of fossil fuel.

Hydroelectric power plants in the United States have often been constructed to take advantage of the force of moving water while also solving flooding or other water-related problems. Typically the dams have been among the largest concrete structures built in the area. They also have altered the ecosystem by controlling the flow of water downstream while flooding the area upstream of the dam. Because of this, a great deal of opposition to new construction accompanies any dam proposal; people debate issues of water rights and the environmental impact of dams in general.

How It Works

The process of converting the energy from a flowing liquid, such as water, into electricity that can be used to supply the power needs of a city or to increase the amount of electricity available on a local or regional grid is actually simple, as long as the conditions are right.

Swift Water Flow. These conditions include the existence of a steady supply of swiftly moving water. The water can flow swiftly as a result of the pressure on the water from the reservoir as the water enters the intake, or penstock, area of the powerhouse located near the base of the dam. Alternately, it can flow swiftly as a result of being released over the top of the dam to spill into the penstock at a high speed. Either way, it is essential the water moves rapidly when it flows through the turbine, although it is not necessary that the water first be held in a reservoir created by a dam.

The turbine has blades that are turned by the flowing water. The more rapidly the water flows, the more rapidly the blades spin. The spinning motion of the blades in turn causes the magnets in the generator, mounted on rotors, to spin inside coils of copper wire. This spinning creates an electromagnetic field that produces alternating current that is then converted to a higher voltage current in the transformer. The higher voltage current can be stored but is typically transmitted over power lines to become part of the power grid serving a city or region.

A typical powerhouse will have water entering through multiple penstocks to flow through one of several turbines mounted under generators. Once the water has flowed past the turbine blades, it will reenter the flow of the river downstream of the dam. The water can also be channeled into a lower reservoir to be pumped back to the upper reservoir for reuse in generating additional electricity. Whether or not this will be done depends on the amount of water available in the reservoir and the expediency of pumping the water back to the higher reservoir.

The placement of the hydroelectric power plant is vitally important. Because a steady flow of water must be moving at a high speed, the dam needs to be located on a river with a reliable supply of water. Gravity also plays an important part in the generation of hydroelectric power because water picks up speed as it moves from a higher level to a lower level. Thus, dams are often built in areas where there is a natural downward flow.

Reservoirs. The construction of a dam often also includes building a reservoir that covers hundreds of acres of land. When that land is flooded, wildlife will lose their habitat and plant life will be ruined. Reservoir construction may also require the relocation of a significant number of people. Because of this, a study of the impact of the dam is often an important part of the planning process, and not all dams are built at the optimal site.

Land Integrity. Finally, the dam site must be one that can bear the weight of the dam and the water that will accumulate. It must also be a site without significant seismic activity or the likelihood of such activity. To ensure this, a thorough geological study of the site is necessary before construction begins.

Applications and Products

Hydroelectric power plants supply power that is used for many purposes. Most plants supply power to an existing power grid. Once part of the grid, the power is allocated to the area of greatest need (along with power from other sources). It is possible, though not common, to have a dedicated hydroelectric power plant, one that is created specifically to meet the needs of an individual or factory.

The main application of hydroelectric power plants is to ensure a steady supply of electricity through a process that uses a renewable resource with little pollution. Hydroelectric power plants do have an environmental impact, however, and that needs to be taken into consideration.

Hydroelectric power plants have had a significant impact on industry through the ages. Originally a simple replacement for slave labor, the use of hydroelectric power peaked before the widespread use of fossil fuels as power sources. It is possible for a dedicated waterwheel to power a simple manufacturing process such as milling, on a small scale.

The largest hydroelectric power plant in the world is the Three Gorges Dam in the People's Republic of China. It has a capacity of 22,500 MW (megawatts). The largest hydroelectric power installation in the United States, and the fifth largest in the world, is the Grand Coulee Dam. Located in Washington State, the power plant produces electricity and is also used to irrigate the land around it. One of the largest concrete structures in the world, the Grand Coulee Dam has a capacity of 6,809 MW. In 2021, the United States generated 60 billion kilowatt hours (kWh) of power through hydroelectric plants. That was about 6.5 percent of total electricity generation. Brazil, Venezuela, Russia, and Canada are also home to hydroelectric power plants with significant capacities.

Social Context and Future Prospects

Hydroelectric power plants are important sources of renewable energy. Producing the electricity generated by these plants does not result in significant levels of pollution. It also does not consume resources that take centuries to replenish, does not require labor-intensive or costly processes, and does not damage the environment.

The dam site and its reservoir, however, do affect the local ecosystem. The land lost to the reservoir through flooding is likely already home to many species of animals and plants and may include towns or villages, all of which will be displaced or destroyed by the dam project. It is possible the reservoir will form a wetlands area at the shoreline. It also is possible that this will not occur, resulting instead in areas of stagnant water that are not hospitable to wildlife.

Furthermore, the dams built for hydroelectric power plants, for example, cut off access to the spawning grounds of anadromous, migratory salmon or the American shad. These fish must return upriver to lay their eggs. To facilitate the return journey, fish lifts, ladders, or elevators are in place at many dams. These structures, which help fish move upstream, also help to avoid the disruption of the local habitat.

As the importance of renewable energy sources becomes incontrovertible, greater demand for hydroelectric power can be anticipated. As the cost in terms of loss of habitat gains greater appreciation, the call to protect existing wildlife and vegetation can also be expected. With existing technology, these goals are not easily met simultaneously.

The challenge for the next generation of hydroelectric power professionals will be to modify this existing technology or explore the use of alternatives that do not require such a large footprint. Using water as it moves, without impeding its flow, is one possible way that hydroelectric power plants can better coexist with the populations they serve.

Bibliography

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Hicks, Tyler. Handbook of Energy Engineering Calculations. New York: McGraw, 2011.

"Hydropower Explained." U.S. Energy Information Administration, 16 Mar. 2022, www.eia.gov/energyexplained/hydropower/where-hydropower-is-generated.php. Accessed 10 June 2022.

Nag, P. K. Power Plant Engineering. New Delhi: Tata, 2008.