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Hydroelectric Energy: Background

What is it?

Hydroelectric energy, also referred to as hydroelectricity or hydropower, is the use of moving water to produce electricity. This power is generated by converting the kinetic energy of flowing water into mechanical and then electrical energy. This process involves the use of turbines, which consist of a set of blades. The moving water flows over the blades, causing them to turn. The blades are connected to a rotating drive shaft. The mechanical energy of the drive shaft is transferred to a generator through a gearbox. The generator uses the mechanical energy of the drive shaft to move an electric conductor (typically coils of copper wire) through a magnetic field, which causes electrons to flow through the conductor, generating electricity. The electricity is then fed into the electrical grid to be used in homes, businesses, and by industry.

Hydroelectric turbine
Turbine blades are used to produce hydroelectricity.
Source: Left image USGS | Right image Library of Congress

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Harnessing Energy from Water

There are three main sources of hydroelectric power: waves, tides, and rivers.

Waves are formed by wind blowing over the surface of the ocean. Waves contain an enormous amount of energy because of their frequency. At this point, however, no technology exists that is able to use waves as a source of power on a large scale. New technologies to capture wave energy are currently under development. These include channeling waves into catch basins and using underwater devices that are and anchored to the ocean floor.

Tides are another source of moving water that can be used to generate electricity. Tidal energy comes from the Earth's rotation and from the gravitational interactions between the Earth and the Moon. This gravitational pull raises tidal bulges in the ocean, one on either side of the Earth. As the Earth rotates, each coastline runs into these bulges, experiencing two high tides and two low tides every day. Tidal systems that generate electricity involve a barrier that is built across the mouth of a bay. Water is trapped behind the barrier at high tide. The water is then released at low tide and used to drive a turbine. Currently, there are several large scale tidal power plants in operation in other parts of the world, such as France and South Korea, but none in the United States. Overall, tidal power will never be able to provide more than a small fraction of the total need for energy.

Streams and rivers are another source of moving water that can be used for producing electricity. Hydroelectric power plants harness the power of flowing water in streams and rivers to run turbines and generators that create electricity. Today, there are hundreds of hydroelectric power plants across the United States. In 2013, 6% of total U.S. electricity was generated from stream-fed hydroelectric power plants. Most states make some use of hydroelectric power plants. Some states, such as Florida and Kansas use very little. Other states, such as Washington, Idaho and Oregon use hydroelectric power plants as their main power source. The state that produces the most hydroelectricity is Washington State, generating 29% of the total U.S. hydroelectricity.

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Types of Hydroelectric Systems

Penstock at Hoover Dam
Water is diverted into a
Source: Wikipedia

There are two basic types of systems used in hydroelectric power plants to produce electricity. One method utilizes the natural flows of streams and rivers. The other is based on falling water.

The first system is called a run-of-the-river system. In it, water is diverted from a river and into a canal or a large pipe, called a penstock. In these systems, the force of the river current running through the penstock applies pressure to the turbine blades to produce electricity. These systems are subject to seasonal river flows. During dry weather, the flow of a river may decrease, reducing the force of moving water applying pressure to the turbine.

The second system is called a falling-water system and is the type most commonly used to produce electricity. In these systems, a dam is built across a large river. Flowing water from the river builds up in a reservoir behind the dam. The water level behind the dam is higher than the river below. Under the force of gravity, the water falls through a penstock and applies pressure against the turbine blades, causing them to spin, which drives the generator to produce electricity. The vertical height of the water in the reservoir above the turbine is known as the head. The more head, the greater the potential energy of the falling water and the more power that is exerted on the turbine by the force of gravity.

Left image: Profile of a hydroelectric power plant that uses a falling-water system to generate electricity.
Source: U.S. Department of the Interior
Right image: Hoover Dam, built into a canyon of the Colorado River. Source: U.S. Department of the Interior

Hydroelectric power plants that use falling-water systems require the right combination of water resources and landforms. The best sites for these plants are fast-moving rivers or streams and areas with consistent rainfall. This ensures that the flow of water is sufficient enough to fill the dam. In addition, a strong flow of water compensates for water that is lost from the reservoir due to evaporation.

Falling-water systems also require landscapes that are hilly or mountainous with valleys and depressions in order to build a dam and form a reservoir behind it. A large volume of water is stored in a reservoir. This water exerts enormous pressure on the walls of the dam. If the walls are not strong enough to sustain this force, they will break and water will spread to the surrounding areas, producing devastating floods. The best location along a river is where a constriction such as a canyon, gorge or natural narrowing occurs. In addition, the bedrock on which a dam is to be built must be stable and strong enough to sustain the weight of the dam and the reservoir, and the pressure of the water stored in the reservoir.

hydroelectric power plant map
This map shows the locations of hydroelectric power plants in yellow. It also shows, in orange, sites at which the conditions are sufficient to support a hydroelectric plant. Source:

Hydroelectric power plants range in size from small systems for a home, farm, ranch, or small community to large plants that supply electricity to many consumers. The amount of electricity a hydroelectric power plant can produce is directly related to the volume of water available to flow through the dam and the rate at which it flows. The greater the volume of water in the reservoir and the faster it flows through the dam, the more electricity the plant can produce. The largest dam in the World is the Three Gorges Dam in China. It produces more than 20,000 megawatts of electricity. The Grand Coulee Dam is the Nation's largest hydroelectric power plant and is located in Washington State. It produces close to 7,000 megawatts of electricity.

diagram of hydroelectric system and grand coulee dam
Left image: A small hydroelectric power system can produce enough electricity for a home, farm, ranch, or small community.
Source: U.S. Department of Energy
Right image: Grand Coulee Dam in Washington State.
Source: U.S. Department of the Interior

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River Systems and the Water Cycle

Streams and rivers are part of a vast global cycling of water. Water is the only common substance that exists at the Earth's surface as a solid, a liquid, and a gas. This is because of the temperature range of Earth's atmosphere and the freezing temperature and boiling point of water. For these reasons, water is present at or near the surface everywhere on Earth. Water is also in a constant state of change and continuously moving from one place to another. The combination of all of these different movements is called the water cycle, also referred to as the hydrologic cycle.

Diagram of the Water cycle
The water cycle describes the continuous movement of water on, above, and below the surface of the Earth.
Source: U.S. Geological Survey

Evaporation and precipitation are the major processes in the water cycle. Evaporation is the process by which water molecules in liquid water escape into the air as water vapor. Precipitation is water in liquid form (rain or drizzle) or solid form (snow, sleet, or hail) that falls to the Earth's surface from clouds. The balance between these processes varies from place to place and time to time. There is more evaporation than precipitation over Earth's oceans. On the other hand, there is more precipitation than evaporation over Earth's continents. As a result, there is a net movement of water vapor from the oceans to the continents and a net movement of liquid (and solid) water from the continents to the oceans. Each year, about 37,000 km3 of water flows from the surface of the continents into the oceans. That is how much more precipitation there is than evaporation on the continents.

The oceans cover about three quarters of Earth. Ocean water is constantly evaporating into the atmosphere. If enough water vapor is present in the air, and if the air is cooled sufficiently, the water vapor condenses to form tiny droplets of liquid water. If they form at higher altitudes, by rising air currents, they form clouds.

When rain falls on Earth's surface, or snow melts, several things can happen to the water. Some evaporates back into the atmosphere. Some precipitation soaks into the ground. Under the pull of gravity, the water moves slowly downward. It percolates through the open pore spaces of porous soil and rock material. Eventually, the water reaches a zone where all of the pore spaces are filled with water. This water is called groundwater. Some water remains behind in the surface layer of soil as soil moisture.

Water that does not evaporate or soak into the ground flows downhill. This flowing water is called surface runoff. This water flows under the influence of gravity. Most landscapes are not perfectly flat: they slope in some direction. Upon reaching the surface, water flows initially as overland flow in the form of thin, slow-moving sheets. At high elevations, this water flows down slope into creeks or small streams. As small streams flow downhill, they join together to form larger streams. A stream that flows into another stream is called a tributary. Tributaries merge into large rivers. Most large rivers reach the coasts, where their water flows into the oceans. This input of freshwater helps maintain ocean levels, ocean chemistry, and keeps the Earth's water cycle in balance.

Copper River delta
Water flows into small streams that join together to form larger streams
© Bruce Molnia, Terra Photographics

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Advantages and Disadvantages of Hydroelectricity

Using moving water to generate electricity offers a number of advantages over other energy sources. Hydroelectric power plants produce little air pollution compared to power plants that burn fossil fuels, such as coal or natural gas. Also, there is limited thermal pollution compared to nuclear plants. Hydroelectric power plants can start generating electricity quickly. They do not need to wait for water to be heated into steam. Also, the flow of water through the turbines can be adjusted to make quick changes in power output during peak demands for electricity. Additionally, reservoirs created by hydroelectric power plants offer a variety of recreational opportunities such as fishing, swimming and boating. Other benefits may include drinking water supply, flood control, and water for irrigation.

Hydroelectricity has some negatives impacts as well. Hydroelectric power plants that require the use of dams can greatly affect the flow of rivers, altering ecosystems and affecting the wildlife and people who depend on those waters. For example, the reservoirs created by dams may flood agricultural land, archeological sites, and cause the relocation of people. In addition, some dams withhold water and then release it all at once. This causes flooding downstream which affects the habitats of local plants and animals. Hydroelectric power plants affect fish populations as well. Certain fish travel upstream to reproduce. Dams can obstruct migration of fish to their spawning areas. Another problem is that the sediments carried by rivers and streams accumulate in reservoirs behind dams. Eventually, a reservoir will fill up with sediment rendering the hydroelectric power plant inoperable.

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