Solar house types energy source that generates neither pollution nor hazardous waste. Solar-powered cars have already competed in long-distance races, and solar energy has been used routinely for many years to power spacecraft. Although many people consider solar energy a product of the space age, the Massachusetts Institute of Technology built the first solar-powered house in 1939.
Solar radiation is nearly constant outside Earth's atmosphere, but the amount of solar energy, or insolation, reaching any point on Earth varies with changing atmospheric conditions such as clouds and dust, and the changing position of Earth relative to the sun. In the United States insolation is greatest in the West and Southwest. Nevertheless, almost all U.S. regions could use solar energy.
Passive and Active Solar Energy Collection Systems
Passive solar systems such as greenhouses, or windows with a southern exposure, use heat flow, evaporation, or other natural processes to collect and transfer heat. They are considered the least costly and least difficult systems to implement. (See Figure 10.12.)
Active solar systems use mechanical methods to control the energy process. They require collectors and storage devices as well as motors, pumps, and valves to operate the systems that transfer heat. Collectors consist of an absorbing plate (solar panel or collector) that transfers the sun's heat to a working fluid (liquid or gas), a translucent cover plate that prevents the heat from radiating back into the atmosphere, and insulation on the back of the collector panel to further reduce heat loss. (See Figure 10.12.) Excess solar energy is transferred to a storage facility so it may be used to provide power on cloudy days. In both active and passive systems the conversion of solar energy into a form of power is made at the site where it is used. The most common and least expensive active solar systems are used for heating water.
Solar Thermal Energy Systems
A solar thermal energy system uses intensified sunlight to heat water or other fluids to more than 750 degrees Fahrenheit. Mirrors or lenses constantly track the sun's position and focus its rays onto solar receivers that contain fluid. Solar heat (energy) is transferred to the water that, in turn, powers a steam-driven electric generator. In a distributed solar thermal system, the collected energy powers irrigation pumps, provides electricity for small communities, or captures normally wasted heat from the sun in industrial areas. In a central solar thermal system, the energy is collected at a central location and used by utility networks for a large number of customers.
Other solar thermal energy systems include solar ponds and trough systems. Solar ponds are lined ponds filled with water and salt. Because salt water is denser than fresh water, the salt water migrates to the bottom and absorbs the heat, while the fresh water on top keeps the salt water contained and traps the heat. Trough systems use U-shaped mirrors to concentrate the sunshine on water or oil-filled tubes.
Photovoltaic Conversion Systems
The photovoltaic (PV) cell solar energy system converts sunlight directly into electricity without the use of mechanical generators. PV cells have no moving parts, are easy to install, require little maintenance, do not pollute the air, and usually last up to 20 years. PV cells are commonly used to power small devices such as watches or calculators. They are also used on a larger scale to provide electricity for rural households, recreational vehicles, and businesses. Solar panels using PV cells have generated electricity for space stations and satellites for many years. Solar panels have also provided electricity for a few major buildings in the United States.
Since PV systems produce electricity only when the sun is shining, a backup energy supply is needed. PV cells produce the most power around noon when sunlight is the most intense. A PV system typically includes storage batteries that provide electricity during cloudy days and at night.
The use of PV technology is expanding both in the United States and abroad. Although PV systems have a higher initial cost than conventional power plants, they have a much lower operating cost.
Using Solar Energy
Because it is difficult to measure solar energy directly, other measures are often used as an indicator. According to the DOE there were 84 low-temperature collector manufacturers in 1979. That number dropped to ten by 2001. (See Figure 10.13.) Total shipments of solar thermal collectors peaked in 1981 at more than 21 million square feet and declined sharply after that. According to the DOE's Annual Energy Review 2002 (Washington, DC, 2003), since 1991 there has been a steady increase in shipments, including a sharp spike in 2001 up to 11 million square feet.
By 1999 the market for solar thermal collectors for space heating had virtually disappeared. However, solar energy became increasingly popular as a means for heating swimming pools and water used in the home. Most of the solar thermal collector market is driven by this residential demand (primarily in the Sunbelt states), with only a small proportion for commercial purposes. Some state and municipal power companies have added solar systems as adjuncts to their regular power sources during peak hours.
Advantages and Disadvantages of Solar Energy
The primary advantage of solar energy is its inexhaustible supply, while its primary disadvantage is its reliance on a consistently sunny climate to provide continuous electrical power. Very few areas of the United States have enough constant sunshine to make this system an efficient replacement for conventional methods. In addition, a large amount of land area is necessary for the most efficient collection of solar energy for solar thermal units.
PV cell solar energy systems are probably the most attractive form of solar energy production. A PV cell system is nonpolluting, silent, and can be operated by computer. In addition, it is less expensive to operate because there are no turbines or other moving parts, which makes maintenance minimal. Above all the fuel source (sunshine) is free and plentiful. The disadvantage of a PV cell energy system is the initial cost. As shown in Figure 10.14 total shipments by PV cells increased dramatically over the last decade, reaching 98 thousand peak kilowatts during 2001.
Future Development Trends
Interest in PV cell solar energy systems is particularly high in rural and remote areas where it is impractical to extend traditional electrical power lines. In some remote areas PV cells are used as independent power sources for communications or for the operation of water pumps or refrigerators. This use will most likely increase where the traditional use of an electrical cord is a problem.
Although solar power still costs more than fossil fuel energy, utilities could turn to solar energy to provide extra power on extremely hot or cold days. Some people believe
FIGURE 10.13
Number of manufacturers of solar thermal collectors, 1974–1984 and 1986–2001
that building solar energy systems to provide peak power capacity would be cheaper in the long run than building new and expensive diesel fuel generators, utility regulators may decide that the price of fossil fuel power must include the hidden cost of fossil fuel damage to the environment caused by acid rain and the greenhouse effect.
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