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estimated that by using inexpensive, energy-efficient measures, SOLAR TECHNOLOGIES
the average energy bills of a single home could be reduced by
The term solar energy is used to describe a number of technolo-
10 percent to 50 percent, and the emissions of carbon dioxide
gies that directly or indirectly utilize sunlight as an alternative
into the atmosphere could be cut.
energy source (Figure 3.20). There are eight main categories of
Many conservation techniques are relatively simple and
these solar technologies:
highly cost-effective. More efficient and less energy-intensive

industry and domestic practices could save large amounts of 1. Solar cells. A solar cell is a thin crystal of silicon, gallium,
energy. Improved automobile efficiency, better mass transit, or some polycrystalline compound that generates
and increased railroad use for passenger and freight traffic are electricity when exposed to light. Also called photovoltaic
simple and readily available means of conserving transportation devices, solar cells have no moving parts and produce
energy. In response to the oil price shocks of the 1970s, automo- electricity directly, without the need for hot fl uids or
bile mileage averages in the United States more than doubled, intermediate conversion states. Solar cells have been
from 5.55 km/L (13 mpg) in 1975 to 12.3 km/L (28.8 mpg) in used extensively in space vehicles and satellites. Here on
1988. Unfortunately, the oil glut and falling fuel prices of the Earth, however, use has been limited to demonstration
late 1980s discouraged further conservation. Between 1990 projects, remote site applications, and consumer specialty
and 1997, the average slipped to only 11.8 km/L (27.6 mpg). items such as solar-powered watches and calculators. Th e
It remains to be seen if the sharp increase of gasoline prices in problem with solar cells today is that the manufacturing
the early years of the twenty-first century will translate into in- cost is too high (they are essentially handmade). Research
creased miles per gallon in new car design. is continuing on the development of highly effi cient,

Several technologies that reduce energy consumption are affordable solar cells that could someday produce

now available. Highly efficient fluorescent lightbulbs that can electricity for the home. See page 167 to find out how a
be used in regular incandescent fixtures give the same amount solar cell is able to create a current.

of light for 25 percent of the energy, and they produce less heat. 2. Power tower. This is another solar technology designed to
Since lighting and air conditioning (which removes the heat generate electricity. One type of planned power tower will

from inefficient incandescent lighting) account for 25 percent have a 171 m (560 ft) tower surrounded by some 9,000 special

of U.S. electricity consumption, widespread use of these lights mirrors called heliostats. The heliostats will focus sunlight on a
could significantly reduce energy consumption. Low-emissive boiler at the top of the tower where salt (a mixture of sodium
glass for windows can reduce the amount of heat entering a nitrate and potassium nitrate) will be heated to about 566°C

building while allowing light to enter. The use of this type of (about 1,050°F). This molten salt will be pumped to a steam
glass in new construction and replacement windows could have generator, and the steam will be used to drive a generator, just
a major impact on the energy picture. Many other technologies, as in other power plants. Water could be heated directly in
such as automatic dimming devices or automatic light-shutoff the power tower boiler. Molten salt is used because it can be
devices, are being used in new construction. stored in an insulated storage tank for use when the Sun is not
The shift to more efficient use of energy needs encourage- shining, perhaps for up to 20 hours.
ment. Often, poorly designed, energy-inefficient buildings and 3. Passive application. In passive applications, energy fl ows by
machines can be produced inexpensively. The short-term cost natural means, without mechanical devices such as motors,
is low, but the long-term cost is high. The public needs to be pumps, and so forth. A passive solar house would include
educated to look at the long-term economic and energy costs of such considerations as the orientation of a house to the Sun,
purchasing poorly designed buildings and appliances. the size and positioning of windows, and a roof overhang
Electric utilities have recently become part of the energy that lets sunlight in during the winter but keeps it out during

conservation picture. In some states, they have been allowed the summer. There are different design plans to capture,
to make money on conservation efforts; previously, they could store, and distribute solar energy throughout a house, and
make money only by building more power plants. This encour- some of these designs are described on page 101 .
ages them to become involved in energy conservation educa- 4. Active application. An active solar application requires
tion, because teaching their customers how to use energy more a solar collector in which sunlight heats air, water, or
efficiently allows them to serve more people without building some liquid. The liquid or air is pumped through pipes

new power plants. in a house to generate electricity, or it is used directly
for hot water. Solar water heating makes more economic
sense today than the other applications.

3.5 ENERGY SOURCES TOMORROW 5. Wind energy. The wind has been used for centuries to
move ships, grind grain into flour, and pump water. Th e

An alternative source of energy is one that is different from the typi- wind blows, however, because radiant energy from the Sun
cal sources used today. The sources used today are the fossil fuels heats some parts of Earth’s surface more than other parts.

(coal, petroleum, and natural gas), nuclear, and falling water. Alter- Th is differential heating results in pressure diff erences
native sources could be solar, geothermal, hydrogen gas, fusion, or and the horizontal movement of air, which is called wind.

any other energy source that a new technology could utilize. Thus, wind is another form of solar energy. Wind turbines

78 CHAPTER 3 Energy 3-18