Visualizing Environmental Science

Power plant

Generator

Turbine

River or

stream

Penstock

Discharge pipe

Reservoir

Generator

Water supply

flows down

penstock from

reservoir.

Turbine

(Flowing water turns

turbine, providing the

energy to generate

electricity.)

Water is discharged

into river or

stream.

© TPG/Top Photo/Corbis

a. Water generates electricity as it moves through the Three Gorges

Dam, which spans the Yangtse River in China.

b. A controlled flow of water

released down the penstock

turns a turbine, which generates

electricity.

Hydropower

The sun’s energy drives the hydrologic cycle,

which includes precipitation, evaporation,

transpiration, and drainage and runoff (see

Figure 5.9). As water flows from higher eleva-

tions back to sea level through rivers and streams, dams

can harness and make use of its energy. The potential en-

ergy of water held back by a dam is converted to kinetic

energy as the water turns turbines to generate electric-

ity (Figure 18.10). Hydropower, which is more concen-

trated than solar energy, is more efficient than any other

energy source for producing electricity; about 90 percent

of available hydropower energy is converted into con-

sumable electricity.

Hydropower generates approximately 19 percent of

the world’s electricity, making it the form of solar energy in

greatest use. China is the world’s largest producer of hydro-

electric power, generating over 200 GW. Besides China, the

nine countries with the greatest hydroelectric

production are, in decreasing order, Canada,

Brazil, the United States, Russia, Norway, India,

Venezuela, Japan, and Sweden.

In the United States, approximately 2200

hydropower plants produce about 7 percent of the

country’s electricity, the most of any renewable energy

source. Highly developed countries have already built

dams at most of their potential sites. In many developing

nations—particularly in undeveloped, unexploited parts

of Africa and South America—hydropower represents a

great potential source of electricity.

Traditional hydropower technology is only suited for

large dams with rapidly flowing water, such as those found

on the Columbia River and its tributaries in the Pacific

Northwest and British Columbia. Therefore, only about 3

percent of existing U.S. dams generate electricity. New de-

signs allow modern turbines to harness electricity from large,

hydropower

A form of renewable

energy that relies on

flowing or falling water

to generate electricity.

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454 CHAPTER 18 Renewable Energy Resources