How Do Dams Make Electricity Without Polluting the Water?

Dams convert the energy of flowing water into electricity.

Hydroelectric dams provided about 96 percent of the renewable energy used in the United States in 2005, according to the Department of the Interior. These dams use water and gravity to power turbines and generate electricity without polluting water. They do have effects on the environment, however, such as preventing the migration of fish and other animals and lowering the amount of sediment carried by the river.

Reservoir

Hydroelectric dams are built on large rivers where there is a drop in elevation, such as at Niagara Falls on the New York-Canada border. They convert the energy of flowing water into electrical energy that people can use. The dam stores the river water behind it, creating large reservoirs. This water is released to generate electricity, but it can also be released to be used for irrigation or to supply to downriver communities.

Intake

The dam creates the "head," the vertical distance the water flows. An intake valve built within the dam connects to a pipe, called a penstock. When the intake valve is opened, gravity causes water to fall from the head, through the penstock, and past the turbine. The head produces water pressure and the greater the head, the greater the water pressure flowing through the penstock. The amount of energy a hydroelectric dam can generate is determined by the volume of water held by the dam and by the head.

Turbine

A turbine is a propeller connected to a generator by shaft. When the blades of the propeller move, they turn the shaft, converting the energy of the moving water into mechanical energy. The turbines used in a coal-fired plant are very similar to those used in hydroelectric dams, but the in a coal-fired plant coal is burned to heat water and generate steam that turns the turbine.

Generator

The generator converts the mechanical energy of the turbine into electrical energy. It uses concepts first discovered by Michael Faraday in 1831. Faraday discovered that when magnets were moved passed conductors, such as copper wire, electricity began to flow. In a hydroelectric dam the turbine shaft rotates magnets inside the generator. Wires are wrapped around the perimeter of the generator. When the magnets turn inside of these wires, electricity is generated and collected at spots called output terminals. From there, the electricity can be transmitted along power lines.

Outflow River

Once the water has flowed passed the turbine and passed some of its energy to the turbine propeller, it flows out of the dam and continues on its downstream course, unpolluted. Although dams allow water to pass, they can make it difficult for fish, such as salmon, to migrate upstream or downstream to spawn. Many dams include "fish ladders," a series of relatively low steps that fish are able to swim and leap up. Since the water leaving a dam carries less sediment than the undammed river did, erosion can occur downstream.

References

• Bureau of Reclamation: Managing Water in the West: Hydroelectic Power (PDF)
• Water Science for Schools: Hydroelectric Power - How It Works