POWER OF SOLAR ENERGY
Learning about renewable energy
What is renewable energy?
Solar water heating
Solar thermal electric power
Energy from trash
Renewable energy in your future
Energy Efficiency and Renewable Energy Network)
Can you imagine life without television, cars, or computers?
What if you had to cook your dinner over a fire or fetch
water from a river? It might be fun for a camping trip, but
you probably would not want to do it every day. But that's
how life was before scientists and inventors discovered ways
to use energy to make our lives easier.
of the energy we use comes from fossil fuels. Coal, oil, and
natural gas are all fossil fuels. Over millions of years,
the decay of plants, dinosaurs, and other animals was formed
into fossil fuels. These fuels lie buried between layers of
earth and rock. The only way to get them out is to drill or
mine for them. While fossil fuels are still being created
today by underground heat and pressure, they are being
consumed more rapidly than they are created. For that
reason, fossil fuels are considered nonrenewable; that is,
they are not replaced as soon as we use them. So, we could
run out of them sometime in the future. Or, we might someday
use so much fossil fuel that we won't be able to drill or
mine fast enough to keep up with the demand.
world depends so much on energy, we need to find sources of
energy that will last a long time. What if there was a type
of energy that never ran out? There is. It is called
because there are so many people on the earth using fossil
fuels, we create a lot of pollution. So, we should also use
energy sources that produce as little pollution as possible.
While all energy sources cause some pollution in their
creation or their consumption, renewable energy systems
generally are less polluting than fossil fuel systems.
energy systems use resources that are constantly replaced
and are usually less polluting. Examples of renewable energy
systems include solar, wind, and geothermal energy (getting
energy from the heat in the earth). We also get renewable
energy from trees and plants, rivers, and even garbage.
We can use
the energy in sunshine to warm and light our homes, heat our
water, and provide electricity to power our lights, stoves,
refrigerators, and other appliances. This energy comes from
processes called solar heating, solar water heating,
photovoltaic energy (converting sunlight directly into
electricity), and solar thermal electric power (when the
sun's energy is concentrated to heat water and produce
steam, which is used to produce electricity).
Have you ever
sat in a car that was closed up on a sunny day? Did you
notice how hot it was in the car? This warmth is just one
example of solar heating. We can use the sun to heat other
things, including our homes. Today, more than 200,000 houses
in the United States have been designed to use features that
take advantage of the sun's energy. These homes often use
passive solar designs, which do not normally require pumps,
fans, or other mechanical equipment to store and distribute
the sun's energy. In contrast, active solar designs need
additional mechanical components.
solar home or building naturally collects the sun's heat
through large, south-facing windows, which are just one
aspect of passive design. Once the heat is inside, we need
to capture and absorb it. Think about a sunny spot on the
floor of your house on a cold day. That "sun spot"
is nice and warm, right? It is warm because it holds the
sun's heat, and we call such things absorbers.
buildings, sunspaces are built onto the south side of the
structure and act as large absorbers. The floors of
sunspaces are usually made of tiles or bricks that absorb
heat throughout the day, then release heat. When the air is
colder than the floor, the tiles or bricks release the heat
to the air.
with solar heating is keeping the heat inside the house. One
way to do this is to use special windows that reflect the
heat back into the house. Another aspect of solar heating is
that the house absorbs heat even during hot weather, when
the last thing you need is more heat! So, passive solar
homes need to be designed to let the heat in during cold
months and block the sun in the hot months. How can you do
You can use
deciduous trees or bushes in front of the south-facing
windows. These plants lose their leaves in the winter and
allow most of the sun in, while in the summer, the leaves
will block out a lot of the sunshine and heat. Or, you can
design your house to have overhangs above the south-facing
windows. This will block out the summer sunshine when the
sun is high in the sky but let it in when the sun is lower
in the winter.
The sun also
can heat water for bathing and laundry. Most solar
water-heating systems have two main parts: the solar
collector and the storage tank. The collector heats the
water, which then flows to the storage tank. The storage
tank can be just a modified water heater, but ideally it
should be a larger, well-insulated tank. The water stays in
the storage tank until it is needed for something, say a
shower or to run the dishwasher.
collector is called a flat-plate collector, and is usually
mounted on the roof. This collector is a rectangular box
with a transparent cover that faces the sun. Small tubes run
through the box, carrying the water or other fluid such as
antifreeze to be heated. The tubes are mounted on a metal
absorber plate, which is painted black to absorb the sun's
heat. The back and sides of the box are insulated to hold in
the heat. Heat builds up in the collector, and as the fluid
passes through the tubes, it heats up.
solar-designed buildings, solar water-heating systems can be
either active or passive. The most common systems are
active, which means they use pumps to move the heated fluid
from the collector and into the storage tank.
While a solar
water-heating system can work well, it can't heat water when
the sun isn't shining—and we all know it can be cloudy for
days at a time! For that reason, homes also have a
conventional backup system that uses fossil fuels.
energy can also be made directly into electricity using
photovoltaic (PV) cells, sometimes called solar cells. PV
cells make electricity without moving, making noise, or
polluting. They are used in calculators and watches. They
also provide power to satellites, electric lights, and small
electrical appliances such as radios. PV cells are even
being used to provide electricity for homes, villages, and
businesses. Some electric utility companies are building PV
systems into their power supply networks.
PV cells used in calculators and watches are tiny—less
than a half inch (1.2 centimeters) in diameter—PV cells
for larger power systems are about 4 inches (10 centimeters)
in diameter. When more power is needed, PV cells can be
wired together to form a module. A module of about 40 cells
is often enough to power a small light bulb. For more power,
PV modules are wired together into an array. PV arrays can
produce enough power to meet the electrical needs of your
house—or for even larger uses.
systems are mostly used for water pumping, highway lighting,
weather stations, and other electrical systems located away
from power lines. For example, if you had a cabin on a
mountain top, a PV system would allow you to read some of
your favorite books before you went to sleep!
systems can be expensive, they are not used in areas that
have electricity nearby. But if someone needs electricity in
a remote place, PV can be quite economical. Another aspect
of PV power is "intermittency," which means that
if the sun isn't shining, the system can't make electricity.
Because PV systems only produce electricity when the sun is
shining, these remote systems need batteries to store the
systems can also change sunlight into electricity, but not
in the same way as PV cells. In most cases, solar thermal
systems concentrate (focus) sunlight to produce heat. This
heat boils water to make steam. The steam rotates a turbine,
which is made of several rows of blades mounted on a large
shaft. The steam's pressure flows through the turbine,
pushes against the blades, and causes the shaft to turn,
much like you can make a pinwheel spin by blowing on it. The
turbine is attached to a generator that makes electricity.
electricity from PV systems, solar thermal power can be
intermittent. To avoid this problem, many systems use a
backup system that relies on natural gas to heat the water.
Because solar thermal systems concentrate the sun's energy,
they need to be located in areas of the world that receive a
lot of intense sunshine.
Did you know
that wind is considered an indirect form of solar energy?
This is because the wind is driven mainly by temperature
differences on the surface of the earth that are caused by
centuries, the wind has been used to sail ships, grind
grain, and pump water. Now, people use the wind to generate
electricity. The windmills built long ago had many blades,
but today's wind turbines usually have just two or three
blades that turn when the wind blows. But the blades on wind
turbines are much longer than those you might see on a
windmill. In fact, wind turbine blades can be up to 82 feet
(25 meters) long!
drive a generator that produces electricity, much like steam
turbines. The longer the blades and the faster the wind
speed, the more electricity the turbine generates. Wind
turbines are placed on towers because the wind blows harder
and more steadily above the ground.
the most electricity, wind turbines need to be located in
areas where the wind blows at a constant speed, which it
does not do in all parts of the world. Wind speed is
described by seven "classes." For example, Class 7
winds are extremely strong, while Class 2 winds are mild
breezes. Generally, Class 4 winds and above are considered
adequate for a wind turbine to produce electricity.
of wind turbines, called wind farms or wind plants, are
connected to electric utility power lines and provide
electricity to many people. New turbine designs now take
advantage of less windy areas by using better blades, more
electronic controls, and other improvements. Some new
turbines can also operate efficiently over a wide range of
of wind turbines over some forms of renewable energy is that
they can produce electricity whenever the wind blows (at
night and also during the day). In theory, wind systems can
produce electricity 24 hours every day, unlike PV systems
that can't make power at night. However, even in the
windiest places, the wind does not blow all the time. So,
while wind farms don't need batteries for backup storage of
electricity, small wind systems do need backup batteries.
And, we're still learning about local wind patterns and how
they affect wind turbines and blades.
We can also
get energy directly from the heat in the earth. This is
known as geothermal energy, from "geo" for earth
and "thermal" for heat. Geothermal energy starts
with hot, molten rock (called magma) miles below the earth's
surface that heats a section of the earth's crust. The heat
rising from the magma warms underground pools of water known
as geothermal reservoirs. Sometimes the water can even boil
to produce steam. If there is an opening through the rock to
the surface, the hot underground water may seep out to form
hot springs, or it may boil to form
such geyser that you may have seen is Old Faithful in Yellowstone National Park.
of years, people have been using hot springs for bathing and
for cooking food. With today's technology, we do not have to
wait for the hot water to come to the earth's surface.
Instead, we can drill wells deep below the surface of the
earth to tap into geothermal reservoirs. This is called
direct use of geothermal energy, and it provides a steady
stream of hot water that is pumped to the earth's surface so
its heat can be used.
energy also is used to produce electricity. Similar to solar
thermal electricity, steam—either pulled directly from the
geothermal reservoir or from water heated to make steam—is
piped to the power plant. There, it rotates a turbine that
One source of
geothermal power is The Geysers geothermal field located in
northern California. This power plant is the largest source
of geothermal energy in the world and produces as much power
as two large coal or nuclear power plants.
geothermal energy is a good source of power, we could run
out of it by drawing so much energy out of the reservoir
that it is not able to replenish itself at the rate we're
using it. In addition, water from geothermal reservoirs
often contains minerals that are corrosive and polluting.
When you burn
a log in your fireplace or in a campfire, you are using
biomass energy. Because plants and trees depend on sunlight
to grow, biomass energy is a form of stored solar energy.
Although wood is the largest source of biomass energy, we
also use corn, sugarcane wastes, and other farming
three ways to use biomass. It can be burned to produce heat
and electricity, changed to a gas-like fuel such as methane,
or changed to a liquid fuel. Liquid fuels, also called
biofuels, include two forms of alcohol: ethanol and
methanol. Because biomass can be changed directly into a
liquid fuel, it could someday supply much of our
transportation fuel needs for cars, trucks, buses,
airplanes, and trains. This is very important because nearly
one-third of our nation's energy is now used for
can also be replaced by biodiesel made from vegetable oils!
In the United States, this fuel is now being produced from
soybean oil. However, any vegetable oil—corn, cottonseed,
peanut, sunflower, or canola—could be used to produce
biodiesel. Researchers are also developing algae that
produce oils, which can be converted to biodiesel.
commonly used biofuel in the United States is ethanol, which
is produced from corn and other grains. A blend of gasoline
and ethanol is already used in cities with high air
pollution. However, ethanol made from corn is currently more
expensive than gasoline on a gallon-for-gallon basis. And
even if we took all the corn that could possibly be grown in
the United States and used it to produce ethanol, it would
not make enough ethanol to power all our cars. So, it is
very important for scientists to find less expensive ways to
produce ethanol from other biomass crops.
have found new ways to produce ethanol from grasses, trees,
bark, sawdust, paper, and farming wastes. These processes
could greatly increase the use of biomass energy in the
United States. Imagine a new type of farm where energy
crops, such as fast-growing trees or grasses, might be grown
and harvested for their energy content!
like many resources, we need to manage our use of biomass or
we might consume it faster than we produce it. Also, like
any fuel, biomass creates some pollutants when it is burned
or converted into energy.
The water in
rivers and streams can be captured and turned into
hydropower, also called hydroelectric power. The most common
form of hydropower uses dams on rivers to create large
reservoirs of water. Water released from the reservoirs
flows through turbines, causing them to spin. The turbines
are connected to generators that produce electricity.
power plants in the United States generate enough
electricity to power whole towns, cities, and even entire
regions of the country. Hydropower currently is one of the
largest sources of renewable power, generating about 10
percent of the United States' electricity.
also inexpensive, and like many other renewable energy
sources, it does not produce air pollution. However, the
drawback to hydropower is that damming rivers can change the
ecology of the region. For example,
below the dam is often colder than what would normally flow
down the river, so fish sometimes die. The water level of
the river below the dam can be higher or lower than its
natural state, which affects the plants that grow along the
What you may
throw out in your garbage today just might become fuel for
someone else. That's right, whether you call it trash or
garbage, this municipal solid waste has the potential to be
a large energy source.
In 1993, the
Environmental Protection Agency estimated that the United
States generated 207 million tons (188 million metric tons)
of trash. Out of all that trash, however, only 32 million
tons (29 million metric tons) were converted to energy.
also an inexpensive energy resource. Unlike most other
energy resources, someone will collect garbage, deliver it
to the power plant, and pay to get rid of it. This helps
cover the cost of turning the garbage into energy. Garbage
is also a unique resource because we all contribute to it.
solid waste can be burned in large power plants to generate
electric power. Municipal waste-to-energy plants currently
generate about 2500 megawatts of electricity—the
equivalent of several large coal plants.
There is also
a way to use the energy trapped in landfill garbage. When
food scraps and other wastes decay, a gas called methane is
produced. Methane is the main ingredient in natural gas. We
can drill wells into landfills to release this gas. Pipes
from each well carry the methane gas to a central point
where it is cleaned. The gas can then be burned to produce
steam in a boiler, or it can be used to power generators to
with burning any type of fuel, municipal solid wastes can
produce air pollution when they are burned and turned into
energy in your future
One day, all
your home's energy may come from the sun or the wind. You
may not think twice about filling your car's gas tank with
biofuel. And your garbage might contribute to your city's
energy supply. As scientists push the limits of renewable
energy technologies and improve the efficiencies and costs
of today's systems, we will soon be to the point when we may
no longer rely mostly on fossil fuel energy.
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Section: More information about PV