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 onemillionth 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 waterand-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 industrialprocess 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 fractionalhorsepower 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 four-person 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 industrial-process 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 heattransfer 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 Sun-warmed 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 local-terrain 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 internalcombustion 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
oil-supplying 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
Further Reading:
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).
How to cite this article:
MLA (Modern Language Association) style:
Kreider, Jan F. "Solar Energy." Grolier Multimedia Encyclopedia. Grolier Online, 2014. Web. 13 Nov. 2014
Solar Energy
The sun radiates vast amounts of energy. This energy nourishes all life on Earth. It is also the driving
force behind the planet's weather patterns and other natural cycles. People have been using solar
energy for many thousands of years. Scientists continue to discover new and better ways of harnessing
and using it.
The Earth receives several thousand times more energy from the sun than is used by all the people in
the world at any moment. To take advantage of this vast and continuous energy supply, we have
learned to convert solar energy into other forms of energy. These other forms can be used for heating,
power, and transportation.
Using Solar Energy in Buildings
Solar energy systems developed for buildings can help meet the energy needs of the people who live or
work in them in many ways. Such systems are often described as either passive or active. Passive
systems rely on the design and materials of the building to distribute light and heat from the sun
without mechanical help. Active systems use pumps or fans.
Daylighting
Daylighting is a passive use of solar energy. It makes the most efficient use of sunlight because it does
not have to be converted to another form of energy. A daylit building uses specially designed windows
to let sunlight into interior spaces. This reduces the need for electrical lighting. Daylighting can save a
significant amount of electricity. It is most effective in office buildings, schools, and other large buildings
that are used mostly in the daytime. Well-designed daylighting also makes work spaces more attractive
and pleasant to be in.
Solar Heating and Cooling
In climates with cold winters, solar energy can provide between 35 and 75 percent of the heating needs
of a passive solar home. A good passive solar design includes glass, mass, and insulation. The building
should have south-facing windows to let in sunlight. This light can then provide heat when it strikes the
interior of the home. Objects such as furniture, walls, and floors store the heat in their mass during the
daytime. Then they slowly release it at night. Tile floors and masonry walls designed into the building
increase the mass. And greater mass means greater heat-storage capacity within the building. Insulating
walls and ceilings also help to hold heat in.
In warm climates, the goal of passive solar design is to keep the sun's heat out of a building. This is done
by carefully positioning the building and its window placements to reduce the amount of heat it absorbs
during daylight hours. Also, roofing overhangs are used to block the sun and encourage natural air
movement within the building. Passive cooling design also includes the planting of trees to provide
shade.
Solar Thermal Power Plants
In a solar thermal power plant, sunlight is converted to heat energy and used to generate electricity.
Special collectors concentrate sunlight in order to achieve the high temperatures needed for this type of
system. Advanced systems use rows of trough collectors or a central receiver surrounded by mirrors.
Trough Collector Systems
Trough systems contain curved metal reflectors arranged in long rows. With the help of small
computers, they track the sun during the day and reflect sunlight precisely onto heat-collecting, glasscovered steel pipes held several feet away. Oil is pumped through the pipes to absorb the heat, and it is
then used to produce the steam needed to drive a large electric generator.
Central Receiver Systems
These systems consist of a tall tower surrounded by thousands of mirrors called heliostats. The
heliostats track the sun and continuously reflect beams of sunlight to the top of the tower. There the
sunlight is converted to heat and absorbed by a heat-transfer fluid. The heat is then used to power a
steam-driven electric generator or it is stored for later use.
Photovoltaic Systems
Photovoltaic (PV) systems use solar cells to convert sunlight directly to electricity. A solar cell is a thin,
waferlike device. It is usually made of a semiconductor material such as silicon. This material produces
an electric current when sunlight strikes its surface. This current is then used to provide power. Solar
cells are generally interconnected in a sealed panel, or module. Modules can be connected to form an
array. Power from a PV array is either used directly or it is stored in low-voltage batteries for use when
the sun is not shining.
Stand-alone PV systems are most commonly used to provide electricity in locations not reached by a
power company's electrical distribution system. In countries with large rural areas not served by a
central utility, PV is a cost-effective and reliable source of electricity. PV has brought electricity to
homes, schools, and hospitals in thousands of villages around the world. Small PV systems are costeffective ways to satisfy small electrical needs, such as water pumping and highway signs.
Large-scale PV systems use large fields of PV arrays. These systems generate electricity for private or
government-owned power companies around the world. They usually lack battery storage. However,
they are connected directly to a power grid. Some utilities in the United States have installed gridconnected PV systems on the rooftops of residential and commercial buildings. These systems provide
extra power during periods of high electrical demand.
PV arrays are also being used in several places throughout the world to charge the batteries of electric
vehicles. Solar energy has also been used to provide the electricity needed to create hydrogen. The
hydrogen can then be used as a transportation fuel.
To promote the use of PV systems by businesses and homeowners, several nations support a policy
called net metering. If a PV system generates extra power and sends it to the grid, net metering
subtracts the value of this power from the customer's utility bill.
Benefits of Solar Energy
Much of the interest in solar energy is related to our environmental concerns about using fossil fuels as
well as our safety concerns over the use of nuclear power. Solar energy can be a reliable alternative to
energy sources that cause pollution or other environmental or safety hazards. Solar energy can also
stimulate the development of energy-producing industries that create jobs.
Burke Miller Thayer
American Solar Energy Society
How to cite this article:
MLA (Modern Language Association) style:
Thayer, Burke Miller. "Solar Energy." The New Book of Knowledge. Grolier Online, 2014. Web. 13 Nov.
2014.