to generate electricity, water stored behind dams is guided through water turbines that are con-

nected to generators located within the plant. Approximately 31% of the total U.S. hydropower

is generated by the water stored in the state of Washington behind the Grand Coulee Dam, the

largest hydroelectric facility in the country.

Solar Energy

Because of the current energy and sustainability concerns, there has been a renewal of inter-

est in solar energy. Solar energy starts with the sun at an average distance of 93 million miles

from earth. The sun is a nuclear fusion reactor, with its surface temperature at approximately

10,000°F (5500°C). Solar energy that reaches the earth is in the form of electromagnetic radi-

ation consisting of a wide spectrum of wavelengths and energy intensities. Almost half of the

solar energy received on earth is in the band of visible light. The solar radiation could be

divided into three bands: the ultraviolet band, the visible band, and the infrared band. Many

of you have a firsthand experience with the ultraviolet band that causes sunburn. The visible

band comprises about 48% of useful radiation for heating, and the near infrared makes up the

rest. As you know, the earth’s orbit around the sun is elliptical. When the sun is nearer the

earth, the earth’s surface receives a little more solar energy. The earth is closer to the sun when

it’s summer in the southern hemisphere and winter in the northern hemisphere. Because the

distance from the earth to the sun changes during the year, the energy reaching the outer atmo-

sphere of the earth varies from 410 to 440 Btu /ft

2

hr. At the average earth – sun distance, out

in the space at the edge of earth’s atmosphere, the intensity of solar energy is 428 Btu /ft

2

hr

or 1350 W/m

2

. The amount of radiation available at a location on the surface of the earth,
depends on many factors including geographical position, season, local landscape and weather,
and time of day.
As solar energy passes through the earth’s atmosphere, some of it is absorbed, some of it is
scattered, and some of it is reflected by clouds, dust, pollutants, forest fires, volcanoes, or water
vapor in the atmosphere. The solar radiation that reaches the earth’s surface without being dif-
fused is called direct beam solar radiation. Atmospheric conditions can reduce direct beam radi-
ation by 10% on clear, dry days, and by 100% during thick, cloudy days. The direct and diffuse
radiation beams are depicted in Figure 13.21.

13.6 Energy Sources, Generation, Consumption 399

Earth’s surface

Solar radiation

Upper

atmosphere

Direct radiation

Diffuse radiation

Cloud

■Figure 13.21

The direct and diffuse radiation.

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