What Is Solar Energy?
Solar energy is energy
that comes from the sun. Every day the sun radiates, or sends out, an enormous
amount of energy. The sun radiates more energy in one second than people have
used since the beginning of time!
Where does all this
energy come from? It comes from within the sun itself. Like other stars, the
sun is a big gas ball made up mostly of hydrogen and helium. The sun generates
energy in its core in a process called nuclear fusion. During nuclear
fusion, the sun's extremely high pressure and hot temperature cause hydrogen
atoms to come apart and their nuclei (the central cores of the atoms) to fuse
or combine. Four hydrogen nuclei fuse to become one helium atom. But the
helium atom weighs less than the four nuclei that combined to form it. Some
matter is lost during nuclear fusion. The lost matter is emitted into space as
It takes millions of
years for the energy in the sun's core to make its way to the solar surface,
and then just a little over eight minutes to travel the 93 million miles to
earth. The solar energy travels to the earth at a speed of 186,000 miles per
second, the speed of light.
Only a small portion of
the energy radiated by the sun into space strikes the earth, one part in two
billion. Yet this amount of energy is enormous. Every day enough energy strikes
the United States to supply the nation's energy needs for one and a half years!
Where does all this
energy go? About 15 percent of the sun's energy that hits the earth is
reflected back into space. Another 30 percent is used to evaporate water,
which, lifted into the atmosphere, produce's rain-fall. Solar energy also is
absorbed by plants, the land, and the oceans. The rest could be used to supply
our energy needs.
History of Solar Energy
People have harnessed
solar energy for centuries. As early as the 7th century B.C., people used
simple magnifying glasses to concentrate the light of the sun into beams so hot
they would cause wood to catch fire. Over 100 years ago in France, a scientist
used heat from a solar collector to make steam to drive a steam engine.
In the beginning of this
century, scientists and engineers began researching ways to use solar energy in
earnest. One important development was a remarkably efficient solar boiler
invented by Charles Greeley Abbott, an American astrophysicist, in 1936.
The solar water heater
gained popularity at this time in Florida, California, and the Southwest. The
industry started in the early 1920s and was in full swing just before World War
11. This growth lasted until the mid- 1950s when low-cost natural gas became
the primary fuel for heating American homes. The public and world governments
remained largely indifferent to the possibilities of solar energy until the oil
shortages of the 1970s. Today people use solar energy to heat buildings and
water and to generate electricity.
Solar Collectors and Solar Space Heating
Heating with solar energy
is not as easy as you might think. Capturing sunlight and putting it to work is
difficult because the solar energy that reaches the earth is spread out over a
large area. The sun does not deliver that much energy to any one place at any
one time . How much solar energy a place receives depends on several
conditions. These include the time of day, the season of the year, the latitude
of the area, and the clearness or cloudiness of the sky.
A solar collector is one
way to collect heat from the sun. A closed car on a sunny day is like a solar
collector. As sunlight passes through the car's glass windows, it is absorbed
by the seat covers, walls, and floor of the car. The light that is absorbed
changes into heat. The car's glass windows let light in, but don't let all the
heat out. (This is also why greenhouses work so well and stay warm year-round.)
So, a solar collector
does three things:
it allows sunlight inside the glass (or plastic);
it absorbs the sunlight and changes it into heat;
it traps most of the heat inside.
Solar Space Heating
Space heating means
heating the space inside a building. Today many homes use solar energy for
space heating. There are two general types of solar space heating systems:
passive and active. A "hybrid" system is a mixture of the passive and
Passive Solar Homes
In a passive solar
home, the whole house operates as a solar collector. A passive house does not
use any special mechanical equipment such as pipes, ducts, fans, or pumps to
transfer the heat that the house collects on sunny days. Instead, a passive
solar home relies on properly oriented windows. Since the sun shines from the
south in North America, passive solar homes are built so that most of the
windows face south. They have very few or no windows on the north side.
A passive solar home
converts solar energy into heat just as a closed car does. Sunlight passes
through a home's windows and is absorbed in the walls and floors.
To control the amount of
heat in a passive solar house, the doors and windows are closed or opened to
keep heated air in or to let it out. At night, special heavy curtains or shades
are pulled over the windows to keep the daytime beat inside the house. In
the summer, awnings or roof overhangs help to cool the house by shading the
windows from the high summer sun.
Heating a house by
warming the walls or floors is more comfortable than heating the air inside a
house. It is not so drafty. And passive buildings are quiet, peaceful places to
live. A passive solar home can get 50 to 80 percent of the heat it needs from
Many homeowners install
equipment (such as fans to help circulate air) to get more out of their passive
solar homes. When special equipment is added to a passive solar home, the
result is called a hybrid system.
Active Solar Homes
Unlike a passive solar
home, an active solar home uses mechanical equipment, such as pumps and
blowers, and an outside source of energy to help heat the house when solar
energy is not enough.
Active systems use
special solar collectors that look like boxes covered with glass. Dark-colored
metal plates inside the boxes absorb the sunlight and change it into heat.
(Black absorbs sunlight more than any other color.)
Air or a liquid flows
through the collectors and is warmed by this heat. The warmed air or liquid is
then distributed to the rest of the house just as it would be with an ordinary
Solar collectors are
usually placed high on roofs where they can collect the most sunlight. They are
also put on the south side of the roof where no tall trees or tall buildings
will shade them.
Storing Solar Heat
The challenge confronting
any solar heating system--whether passive, active, or hybrid--is heat storage.
Solar heating systems must have some way to store the heat that is collected on
sunny days to keep people warm at night or on cloudy days.
In passive solar homes,
heat is stored by using dense interior materials that retain heat
well--masonry, adobe, concrete, stone, or water. These materials absorb surplus
heat and radiate it back into the room after dark. Some passive homes have
walls up to one foot thick.
In active solar homes, heat may be stored in
one of two ways--a large tank may store a hot liquid, or rock bins beneath a
house may store hot air.
Houses with active or
passive solar heating systems may also have furnaces, wood-burning stoves, or
another heat source to provide heat in case there is a long period of cold or
cloudy weather. This is called a backup system.
Solar Hot Water Heating
Solar energy is also used
to heat water. Water heating is usually the second leading home energy expense,
costing the average family over $400, a year. Depending on where you live, and
how much hot water your family uses, a solar water heater can pay for itself in
as little as five years. A well-maintained system can last 15-20 years, longer
than a conventional water heater.
A solar water heater
works in the same way as solar space heating. A solar collector is mounted on
the roof, or in an area of direct sunlight. It collects sunlight and converts
it to heat. When the collector becomes hot enough, a thermostat starts a pump.
The pump circulates a fluid, called a heat transfer fluid, through the
collector for heating. The heated fluid then goes to a storage tank where it
heats water. The hot water may then be piped to a faucet or showerhead. Most
solar water heaters that operate in winter use a heat transfer fluid, similar
to antifreeze, that will not freeze when the weather turns cold.
Today over 1.5 million
homes in the U.S. use solar heaters to heat water for their homes or swimming
Besides heating homes and
water, solar energy also can be used to produce electricity. Two ways to
generate electricity from solar energy are photovoltaics and solar thermal
Photovoltaic comes from the words photo
meaning "light" and volt, a measurement of electricity.
Sometimes photovoltaic cells are called PV cells or solar cells for
short. You are probably already familiar with solar cells. Solar-powered
calculators, toys, and telephone call boxes all, use solar cells to convert
light into electricity.
A photovoltaic cell is
made of two thin slices of silicon sandwiched together and attached to
metal wires. The top slice of silicon, called the N-layer, is very thin and has
a chemical added to it that provides the layer with an excess of free
electrons. The bottom slice, or P-layer, is much thicker and has a chemical
added to it so that it has very few free electrons.
When the two layers are
placed together, an interesting thing happens-an electric field is produced
that prevents the electrons from traveling from the top layer to the bottom
layer. This one-way junction with its electric field becomes the central part
of the PV cell.
When the PV cell is
exposed to sunlight, bundles of light energy known as photons can knock some of
the electrons from the bottom P-layer out of their orbits through the electric
field set up at the P-N junction and into the N-layer.
The N-layer, with its
abundance of electrons, develops an excess of negatively charged electrons.
This excess of electrons produces an electric force to push the additional
electrons away. These excess electrons are pushed into the metal wire back to
the bottom P-layer, which has lost some of its electrons.
This electrical current
will continue flowing as long as radiant energy in the form of light strikes
the cell and the pathway, or circuit, remains closed.
Current PV cell
technology is not very efficient. Today's PV cells convert only about 10 to 14
percent of the radiant energy into electrical energy. Fossil fuel plants, on
the other hand, convert from 30-40 percent of their fuel's chemical energy into
electrical energy. The cost per kilowatt-hour to produce electricity from PV
cells is presently three to four times as expensive as from conventional
sources. However, PV cells make sense for many uses today, such as providing
power in remote areas or other areas where electricity is difficult to provide.
Scientists are researching ways to improve PV cell technology to make it more
competitive with conventional sources.
Like solar cells, solar
thermal systems use solar energy to make electricity. But as the name
suggests, solar thermal systems use the sun's heat to do it.
Most solar thermal
systems use solar collectors with mirrored surfaces to concentrate sunlight
onto a receiver that heats a liquid. The super-heated liquid is used to make
steam that drives a turbine to produce electricity in the same way that coal,
oil, or nuclear power plants do.
Solar thermal systems may
be one of three types: central receiver, dish, or trough. A central receiver
system uses large mirrors on top of a high tower to reflect sunlight onto a
receiver. This system has been dubbed a "solar power tower." Another
system uses a dish-shaped solar collector to collect sunlight. This system
resembles a television satellite dish. A third system uses mirrored troughs to
collect sunlight. Until recently, trough systems seemed the most promising.
The world's first solar
electric plant used mirrored troughs. LUZ, as the plant was called, was
perfectly situated in the sunny Mojave desert of California. LUZ was the only
solar plant to generate electricity economically. Dollar for dollar, it had
always been cheaper to use conventional sources of energy (coal, oil, nuclear)
to generate electricity. But the LUZ solar plant turned that around, producing
electricity as cheaply as many new coal plants, and with no hidden pollution
costs. The future looked bright for this pioneering solar plant and then the
dream cracked. LUZ closed its doors at the end of 1992 because of a drop in oil
prices and an over-budget construction project at LUZ's home-base.
LUZ may be gone, but most
solar energy engineers believe solar power towers will be ready to take the
place of trough systems very soon.
Solar Energy and the Environment
In the 1970s, the push
for renewable energy sources was driven by oil shortages and price increases.
Today, the push for renewable energy sources is driven by a renewed concern for
Solar energy is the
prototype of an environmentally friendly energy source. It consumes none of our
precious energy resources, makes no contribution to air, water, or noise
pollution, does not pose a health hazard, and contributes no harmful waste
products to the environment.
There are other
advantages too. Solar energy cannot be embargoed or controlled by any one
nation. And it will not run out until the sun goes out.