solar energy
Solar energy is a term used to identify the electromagnetic radiation emitted by the Sun and intercepted by
Earth. It is the world's most abundant source of energy and, unlike fuel sources such as oil, one that will never
run out. The amount of solar energy intercepted by Earth is about 170 trillion kW, an amount 5,000 times
greater than the total of all other energy inputs such as terrestrial nuclear energy, geothermal energy, and
gravitational energy. About 30% of solar radiation is reflected into space, 47% is converted to heat on Earth
and reradiated to space, and 23% powers the evaporation-precipitation cycle of the biosphere. The amount of
the Sun's energy intercepted by Earth is only one-thousandth of one-millionth of the total released by the
conversion of 4 million tons of hydrogen per second to helium in the Sun.
The practical uses of solar energy on Earth include heating for residential and other buildings, power for
industries, and electricity production.
Solar Resources
Although it is abundant, solar energy reaching Earth is quite dilute—approximately 430 Btu/hr per ft2. (One Btu,
or British thermal unit, is the amount of heat needed to raise the temperature of 1 lb of water 1 F degree; a
typical home in Denver, Colo., will consume between 50 and 100 million Btu for heating in one year, for
example.) As solar radiation crosses the Earth's atmosphere, its intensity is reduced further by absorption,
clouds, and air pollution. Therefore, on the average, the radiation striking the Earth's surface averages only
150–200 Btu/hr per ft2.
Because of the dilute nature of solar energy, relatively large areas are needed to collect it, and in northern
climates solar systems for heating or electricity production may cost more than systems using fossil or nuclear
fuels. When using properly designed and constructed systems, however, solar energy is competitive with other
energy sources in many parts of the world. For example, solar heating of both buildings and domestic hot water
is cheaper than electric heating in many parts of the United States. Solar electricity is cheaper than power from
large central power plants in most of the tropics.
Most applications of solar energy rely on systems consisting of collectors, storage, and controls. Storage is often
needed because solar energy is available only during daylight hours, but the demand for energy may be
continuous. Controls are needed to ensure that the collection and storage systems operate safely and
efficiently.
The availability of solar energy is determined by three characteristics of a particular site: (1) location as
measured by latitude, longitude, and altitude; (2) time of year; and (3) local weather conditions. Solar energy
levels are lower the farther north the site. The U.S. Northeast and Pacific Northwest have particularly low levels
on average, whereas the Southwest is blessed with abundant sunshine, almost twice as much as in cloudy
regions of the United States. The sunny areas are where solar conversion systems are likely to flourish when
energy demand rises. A solar home-heating system is not needed in sunny Phoenix, Ariz., but a photovoltaic
system for producing electricity could be quite useful there.
In addition to geography, season is an important determinant of solar energy levels, because the position of the
Sun and the weather vary greatly from summer to winter. Solar-systems designers tilt solar-collection surfaces
to favor the position of the Sun during the part of the year when energy is most in demand. For example, a
solar space-heating collector would be tilted at a relatively large angle because the Sun is low in the sky in
midwinter. In winter the amount of solar energy on a vertical surface (90° tilt) is almost the same as on a 60°
surface. This is one of the reasons that many solar heating systems use vertical collection surfaces.
Simple Applications of Solar Energy
The Sun's rays have long been used as the heat source for evaporating and distilling water. Solar evaporation
has always been an important salt-production process. Salt water is pumped into shallow ponds that are open
to the Sun, and as the water evaporates, its salts form crystals that settle at the bottom and are eventually
collected. Producing drinkable water from brine is accomplished in a solar still, where salty water is evaporated.
The salt becomes concentrated in the bottom of the still basin, while the water vapor rises, condenses on the
still cover as fresh water, and is drawn off.
The solar cooking stove has become an important device in tropical countries where firewood is in short supply.
The stove may be simply a hot box, an insulated container, perhaps with a mirrored cover that intensifies the
Sun's heat. More complex stoves have reflectors that focus sunlight directly on the cooking area, or they use
separate heat-collector devices that transfer heat to water inside a steam cooker.
Solar heat is also used throughout the world to dry agricultural crops, fruits, and vegetables.
Solar Energy Use in Buildings
Both water and space heating are among the most successful small-scale applications of solar energy.
Thousands of systems of these types have been installed throughout the world.
Solar Heat Collectors.
Solar collectors are devices that absorb solar energy and produce heat. They are mounted on the roofs of
buildings or in other areas that are open to direct sunlight, and they are used for heating and cooling living or
work spaces and for heating water.
The flat-plate collector is made of a copper or steel heat-absorber plate, the surface of which is blackened to
make the plate a more-efficient solar heat trap. A heat-transfer liquid—usually a water-and-antifreeze solution—
is warmed as it circulates through a set of tubes and removes heat from the plate. The tubes may be attached
to the plate. To minimize heat loss from the absorber plate to the surrounding air, a plate of glass or
transparent plastic forms an insulating air space above the plate. The cover also minimizes reradiation from the
collector. To reduce heat loss further, the back and sides of the collector are heavily insulated, as are all pipes
leading to and from the building heating system.
Evacuated-tube collectors are more efficient than flat collectors and consist of two glass tubes, one within the
other, with a vacuum between them to minimize heat loss. Because the tubes are round and are backed with
reflecting material, this type of collector can absorb more sunlight and has a significantly higher overall
efficiency than the flat-plate collector. Evacuated-tube collectors are used in northern climates where light
intensities are low.
Concentrating, or focusing, solar collectors focus the Sun's rays on a tube (trough type), a point (dish type), or
a concave mirror to provide higher temperatures for special purposes, such as industrial-process heat. Such
collectors must be able to move both vertically and horizontally in order to track the Sun across the sky in all
seasons.
Solar Water Heating.
Solar water heating is an old and simple application of solar heat and an inexpensive system for many buildings.
The most common system consists of a collector located outside the building and tilted at an angle that favors
uniform yearlong solar input. (The tilt angle is approximately equal to the local latitude.) In addition to the
collector, there is a small fractional-horsepower pump for water circulation and a tank to store the heated water
for later use. A simple controller that compares tank and collector temperature operates the pump. Whenever
the Sun warms the collector to a temperature greater than that of the tank, the pump is turned on by the
automatic controller.
The size of collector needed can be approximately determined by the rule of thumb that states that the
collection area, in ft2, should be the same as the number of gallons of hot water needed per day. In the United
States each resident in a home uses between 15 and 20 gal (57 and 76 liters) of hot water per day. A fourperson family would therefore need 60 to 80 ft2 (5.6 to 7.4 m2) of solar collector.
If poor weather reduces the amount of available sunlight, the solar system will produce no hot water. Then the
conventional water-heating system will take over the task of providing domestic hot water.
An alternative to the pumped system is the "thermosiphon" system, where fluid circulation is produced by a
density difference between hot fluid in the collector and cold fluid located above the collector in a tank. The
lighter, warm fluid will tend to rise, causing cold fluid to replace it in the collector. The performance of these
systems is good; the only problem is the need for the tank to be located in a position above the collector.
Active Solar Space Heating.
Most of the heating energy used in residences is for space heating, that is, for providing the heat needed to
maintain comfort within a building. Solar energy is a good match for this heating task because it is able to
produce heat at temperatures close to those needed for heating buildings. In addition, the amount of solar
energy falling on the roof of a properly oriented residence is roughly equivalent to what is needed to provide
space heat.
A typical space-heating system consists of a roof-mounted collector array whose tilt angle is equal to the local
latitude plus 15°, a heat-storage tank or bin, pumps or a fan, and a network of pipes or ducts through which
the heat is conveyed from the collector array to the building heat system.
An auxiliary heat source is used in periods when solar heat is not available. A control system operates the
pumps or fans and the auxiliary heat source. Measures for reducing the need for conventional energy should
include insulating and tightening a building before installing a solar system.
Passive Solar Space Heating.
Passive systems avoid the use of mechanical components. The simplest passive system, the direct-gain system,
involves larger-than-normal south-facing windows, with a massive floor slab that serves as the heat storage.
This system is particularly effective in bringing up the temperature of a house quickly in the morning, but it can
cause overheating problems in sunny climates if sufficient floor or wall moss for heat storage is not available.
The thermal storage wall avoids some of the shortcomings of the direct-gain system by interposing a thick
concrete wall between the windows and the rooms to be heated. The wall stores heat for night heating, while
natural air circulation (without fans) transports heat from the windows during the day.
In climates where heating is necessary, good architectural practice always includes some measure of passive
solar heating, often with features of both of the approaches described here.
Solar Energy in Industry
Temperatures sufficiently high to be of use in industry can be achieved using solar heat. In addition, the
technology for converting solar radiation directly into electricity is proving more practical every year.
Solar Process Heat.
In order to produce high temperatures, sunlight must be focused or concentrated via an optical focusing
system. In one such system a parabolic "dish" focuses solar radiation onto an absorber, concentrating sunlight
by a factor of over 100 to produce temperatures exceeding 538° C (1,000° F). If such high temperatures are
not needed, smaller degrees of concentration will siffice. Although not widely used today, solar industrialprocess heat systems have been effective in food processing and other industries with moderate temperature
needs.
Solar-Thermal Electric Power Production.
The high temperatures produced by concentrating solar collectors can be used to produce steam, which in turn
can drive a turbine to produce electric power. A number of solar power plants have been built, although they
are quite small relative to the normal fossil-fuel or nuclear power plant. The technology has been shown to be
reliable but more expensive, in most cases, than conventional technology.
A successful and economical 300-MW solar power plant has been constructed in the desert of southern
California, however. Its collectors are large, curved, parabolic mirrors; each is about 1.8 m (6 ft) high, mounted
above the desert floor and motorized to track the Sun. Steel pipes circulate a special heat-transfer fluid that can
reach temperatures of 391° C (735° F). The hot fluid is used to boil water, and it is this steam that operates a
conventional turbine. (See energy sources; power, generation and transmission of.)
Solar power plants are of particular interest to those utilities which have their maximum demand as a result of
air-conditioning loads drawn off by homes and office buildings. Solar plants produce maximum output during
sunny periods of the summer, just when this particular demand is greatest.
Photovoltaic Conversion.
Photovoltaic cells, or solar cells, convert sunlight directly into electricity. The solar cell is a specially
constructed semiconductor device fabricated from exceptionally pure silicon. Small portions of carefully selected
impurities are added to produce a region where light energy breaks electron bonds, creating free electrons.
These charges migrate, producing current. The amount of current depends on the amount of solar radiation and
the size of the cell. By connecting a number of cells in an array, any desired current and voltage level can be
produced. (See also photoelectric effect.)
The efficiency of solar cells is in the 12–30% range: that is, only 12–30% of the incident sunlight is actually
utilized. Even more-efficient cells have been tested in the laboratory and will soon be available on the market.
Single solar cells power everything from pocket calculators to large systems connected to power grids. Small
arrays keep batteries charged, power irrigation pumps, and keep refrigerators cold in tropical areas where there
is no commercial electricity.
Indirect Solar Energy
The solar-energy applications discussed above all make direct use of solar energy as it strikes the Earth. There
are also important indirect applications, where solar radiation is converted into other usable energy forms. For
example, the Sun's energy profoundly affects the world's wind patterns, causes ocean water to evaporate as
part of the hydrologic cycle, and is essential for plant growth. The hydrologic cycle makes hydroelectric
power possible. Vegetation can be burned directly—for instance, as wood in a stove—or made into other forms
of fuel in a process known as biomass conversion. The winds are used to turn windmills. Solar energy also
makes it possible to harness ocean thermal energy, which uses the temperature difference between Sunwarmed surface water and cold water from the ocean depths to produce power. Geothermal energy taps the
heat within the Earth—either directly, by using naturally heated groundwater, as in Iceland, or by injecting
water that is heated by hot interior rocks. The geothermal heat pump captures the solar heat retained by the
earth. Water-filled coils of pipe are buried 1 m (3 ft) underground, and the heated water is passed to the pump,
where the heat is compressed and distributed.
The utilization of the immense force of ocean tides to generate electricity is yet another form of solar energy
(see tidal energy). By installing turbines that are powered by the inflow and outflow of tidal waters—within a
narrow estuary, for example—this force can be made to produce large amounts of usable energy, at negligible
environmental cost.
Wind Energy.
The winds are produced by the motion of the Earth and by temperature differences within the atmosphere,
which in turn are the result of solar heating. Human beings have harnessed the wind for centuries, using
windmills to provide power for pumping water and for other mechanical energy needs. Recently, a new
generation of aerodynamically designed windmills, called aerogenerators, have been successful in producing
electricity from wind as cheaply as coal power plants can but without any environmental damage.
Like solar radiation, wind energy is a diffuse energy form. Hence, large areas for the moving aerogenerator
blades are needed. Modern aerogenerators may have diameters of 91 m (300 ft) or more and achieve
efficiencies of 30% or better.
Because wind energy is not limited to the daylight hours, it has an advantage over applications that use the
Sun's energy directly. Wind variation must still be accounted for in system designs, however, and the siting of
wind systems requires more-careful study than do other types of solar-energy systems. Seasonal and localterrain effects can be large and must be considered well to ensure maximum output.
Biomass Conversion.
The term biomass refers to the energy produced and stored in vegetation through photosynthesis. Biomass
conversion has always been used in its most direct form, the burning of plant matter to produce heat. Biomass
can also be converted into liquid fuels that are usable in internal-combustion engines for cars, trucks, and
buses. Agricultural crops and organic wastes—especially those with high starch or sugar content—are fermented
into alcohols that, when treated, become relatively clean-burning fuels (see, for example, gasohol).
Since the 1990s, U.S. interest in solar energy has rekindled as political uncertainties continue to plague oilsupplying parts of the world. Prices have dropped for photovoltaic and wind systems. In the developing world,
especially, alternative energy forms—all of them based on solar technologies—offer the hope of providing
inexpensive power while avoiding the pollution that accompanies the use of fossil fuels.
Jan F. Kreider
Bibliography:
Anderson, Bruce N., The Fuel Savers: A Kit of Solar Ideas for Your Home, Apartment, or Business, 2d ed.
(1991).
Balcomb, J. Douglas, Passive Solar Buildings (1992).
Behling, Sophia and Stefan, Solar Power: The Evolution of Solar Architecture (2000).
Bouquet, Frank L., Solar Energy Simplified, 5th ed. (1994).
Goswami, D. Yogi, et al., Principles of Solar Engineering, 2d ed. (2000).
Markvart, Thomas, Solar Electricity, 2d ed. (2000).
Norton, Brian, Solar Energy Technology (1991).
Radabaugh, Joseph, Heaven's Flame: A Guidebook to Solar Cookers, 2d ed. (1998).
Schaeffer, John, Solar Living Sourcebook: The Complete Guide to Renewable Energy Technologies and
Sustainable Living, 11th ed. (2001).
solar collectors in the Mojave Desert
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