Worlds of David Darling
Encyclopedia of Science
   
Home > Encyclopedia of Alternative Energy

hydroelectric power





Hoover Dam
Hoover Dam
Hydro means "water". So, hydropower is "water power" and hydroelectric power is electricity generated using water power. Potential energy (or the "stored" energy in a reservoir) becomes kinetic (or moving energy). This is changed to mechanical energy in a power plant, which is then turned into electrical energy. Hydroelectric power is a renewable resource.

In an impoundment facility (see below), water is stored behind a dam in a reservoir. In the dam is a water intake. This is a narrow opening to a tunnel called a penstock.

Water pressure (from the weight of the water and gravity) forces the water through the penstock and onto the blades of a turbine. A turbine is similar to the blades of a child's pinwheel. But instead of breath making the pinwheel turn, the moving water pushes the blades and turns the turbine.

The turbine spins because of the force of the water. The turbine is connected to an electrical generator inside the powerhouse. The generator produces electricity that travels over long-distance power lines to homes and businesses. The entire process is called hydroelectricity.


hydroelectric power plant


Types of hydropower plants

There are three types of hydropower facilities: impoundment, diversion, and pumped storage. Some hydropower plants use dams and some do not. The images below show both types of hydropower plants.

Many dams were built for other purposes and hydropower was added later. In the United States, there are about 80,000 dams of which only 2,400 produce power. The other dams are for recreation, stock/farm ponds, flood control, water supply, and irrigation.

Hydropower plants range in size from small systems for a home or village to large projects producing electricity for utilities.


Impoundment

The most common type of hydroelectric power plant is an impoundment facility. An impoundment facility, typically a large hydropower system, uses a dam to store river water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity. The water may be released either to meet changing electricity needs or to maintain a constant reservoir level.


impoundment facility
An impoundment hydropower plant dams water in a reservoir

Diversion

A diversion, sometimes called run-of-river, facility channels a portion of a river through a canal or penstock. It may not require the use of a dam.


diversion facility
The Tazimina project in Alaska is an example of a diversion hydropower plant. No dam was required.

Pumped storage

When the demand for electricity is low, a pumped storage facility stores energy by pumping water from a lower reservoir to an upper reservoir. During periods of high electrical demand, the water is released back to the lower reservoir to generate electricity.





Sizes of hydroelectric power plants

Facilities range in size from large power plants that supply many consumers with electricity to small and micro plants that individuals operate for their own energy needs or to sell power to utilities.


Large hydropower

Although definitions vary, the U.S. Department of Energy defines large hydropower as facilities that have a capacity of more than 30 megawatts.


Small hydropower

Although definitions vary, DOE defines small hydropower as facilities that have a capacity of 100 kilowatts to 30 megawatts.


Microhydropower

A microhydropower plant has a capacity of up to 100 kilowatts. A small or microhydroelectric power system can produce enough electricity for a home, farm, ranch, or village.


Advantages and disadvantages of hydropower

Hydropower offers advantages over other energy sources but faces unique environmental challenges.


Advantages

Hydropower is a fueled by water, so it's a clean fuel source. Hydropower doesn't pollute the air like power plants that burn fossil fuels, such as coal or natural gas.

Hydropower is a domestic source of energy.

Hydropower relies on the water cycle, which is driven by the sun, thus it's a renewable power source.

Hydropower is generally available as needed; engineers can control the flow of water through the turbines to produce electricity on demand.

Hydropower plants provide benefits in addition to clean electricity. Impoundment hydropower creates reservoirs that offer a variety of recreational opportunities, notably fishing, swimming, and boating. Most hydropower installations are required to provide some public access to the reservoir to allow the public to take advantage of these opportunities. Other benefits may include water supply and flood control.


Disadvantages

fish ladder
This fish ladder on the Ice Harbor Dam on the lower Snake River provides safe passage for migrating fish.
Fish populations can be impacted if fish cannot migrate upstream past impoundment dams to spawning grounds or if they cannot migrate downstream to the ocean. Upstream fish passage can be aided using fish ladders or elevators, or by trapping and hauling the fish upstream by truck. Downstream fish passage is aided by diverting fish from turbine intakes using screens or racks or even underwater lights and sounds, and by maintaining a minimum spill flow past the turbine.

Hydropower can impact water quality and flow. Hydropower plants can cause low dissolved oxygen levels in the water, a problem that is harmful to riparian (riverbank) habitats and is addressed using various aeration techniques, which oxygenate the water. Maintaining minimum flows of water downstream of a hydropower installation is also critical for the survival of riparian habitats.

Hydropower plants can be impacted by drought. When water is not available, the hydropower plants can't produce electricity.

New hydropower facilities impact the local environment and may compete with other uses for the land. Those alternative uses may be more highly valued than electricity generation. Humans, flora, and fauna may lose their natural habitat. Local cultures and historical sites may be impinged upon. Some older hydropower facilities may have historic value, so renovations of these facilities must also be sensitive to such preservation concerns and to impacts on plant and animal life.


History of hydropower

Humans have been harnessing water to perform work for thousands of years. The Greeks used water wheels for grinding wheat into flour more than 2,000 years ago. Besides grinding flour, the power of the water was used to saw wood and power textile mills and manufacturing plants.

For more than a century, the technology for using falling water to create hydroelectricity has existed. The evolution of the modern hydropower turbine began in the mid-1700s when a French hydraulic and military engineer, Bernard Forest de BÚlidor wrote Architecture Hydraulique. In this four volume work, he described using a vertical-axis versus a horizontal-axis machine.

During the 1700s and 1800s, water turbine development continued. In 1880, a brush arc light dynamo driven by a water turbine was used to provide theatre and storefront lighting in Grand Rapids, Michigan; and in 1881, a brush dynamo connected to a turbine in a flour mill provided street lighting at Niagara Falls, New York. These two projects used direct-current technology.

Alternating current is used today. That breakthrough came when the electric generator was coupled to the turbine, which resulted in the world's, and the United States', first hydroelectric plant located in Appleton, Wisconsin, in 1882.


Related category

   • WATER POWER