What Is Wind?
Wind is simply air in
motion. It is caused by the uneven heating of the earth's surface by the sun.
Since the earth's surface is made up of land, desert, water, and forest areas,
the surface absorbs the sun's radiation differently.
During the day, air above
the land heats more quickly than air above water. The hot air over the land
expands and rises, and the heavier, cooler air over a body of water rushes in
to take its place, creating local winds. At night, the winds are reversed
because air cools more rapidly over land than over water.
Similarly, the large
atmospheric winds that circle the earth are created because land near the
equator is heated more by the sun than land near the North and South Poles.
Today people can use wind
energy to produce electricity. Wind is called a renewable energy source
because we will never run out of it.
History of Wind Machines
Throughout history people
have harnessed the wind. Over 5,000 years ago, the ancient Egyptians used wind
power to sail their ships on the Nile River. Later people built windmills to
grind their grain. The earliest known windmills were in Persia (the area now
occupied by Iran). The early windmills looked like large paddle wheels.
Centuries later, the
people in Holland improved the windmill. They gave it propeller-type blades and
made it so it could be turned to face the wind. Windmills helped Holland become
one of the world's most industrialized countries by the 17th century.
American colonists used
windmills to grind wheat and corn, to pump water, and to cut wood at sawmills.
In this century, people
used windmills to generate electricity in rural areas that did not have
electric service. When power lines began to transport electricity to rural
areas in the 1930s, the electric windmills were used less and less.
Then in the early 1970s,
oil shortages created an environment eager for alternative energy sources,
paving the way for the re-entry of the electric windmill on the American
Today's Wind Machine
Today's wind machine is
very different from yesterday's windmill. Along with the change in name have
come changes in the use and technology of the windmill.
machines were used primarily to convert the wind's kinetic energy into
mechanical power to grind grain or pump water, today's wind machines are used
primarily to generate electricity.
windmills, today's wind machines still use blades to collect the wind's kinetic
energy. Windmills work because they slow down the speed of the wind. The wind
flows over the airfoil shaped blades causing lift, like the effect on airplane
wings, causing them to turn. The blades are connected to a drive shaft that
turns an electric generator to produce electricity.
Modern wind machines are
still wrestling with the problem of what to do when the wind isn't blowing.
Large turbines are connected to the utility power network-some other type of
generator picks up the load when there is no wind. Small turbines are often
connected to diesel/electric generators or sometimes have a battery to store
the extra energy they collect when the wind is blowing hard.
Types of Wind
Two types of wind
machines are commonly used today:
horizontal, which has blades like airplane propellers; and
vertical, which looks like an egg-beater.
machines are used the most. They make up 95 percent of all wind machines. A
typical horizontal wind machine stands as tall as a 10-story building and has
two or three blades that span 60 feet across. The largest wind machines in the
world have blades longer than a football field! Wind machines stand tall and
wide to capture more wind.
machines make up just five percent of the wind machines used today. The typical
vertical wind machine stands 100 feet tall and 50 feet wide. The most popular
vertical wind machine today is the Darrieus wind turbine, named after its
inventor, J.G.S. Darrieus, a French engineer.
Which Type Is
Each wind machine has its
advantages and disadvantages. Horizontal-axis machines need a way to keep the
rotor facing the wind. This is done with a tail on small machines. On large
turbines, either the rotor is located down wind of the tower acting like a
weather vane, or drive motors are used. Vertical-axis machines accept wind from
Both types of rotors are
turned by air flowing over their wing shaped blades. Vertical axis
blades lose energy as they turn out of the wind, while horizontal-axis blades
work all the time. Also, at many sites, the wind increases as you go higher-above
the ground, giving an advantage to tall horizontal-axis turbines. The small
tower and ground mounted generators on vertical-axis turbines make them less
costly and easier to maintain.
Wind Power Plants
Wind power plants, or wind farms as they are
sometimes called, are clusters of wind machines used to produce electricity. A
wind farm usually has hundreds of wind machines in all shapes and sizes.
Unlike coal or nuclear
plants, most wind plants are not owned by public utility companies. Instead they
are owned and operated by business people who sell the electricity produced on
the wind farm to electric utilities. These private companies are known as
Independent Power Producers.
Operating a wind power
plant is not as simple as plunking down machines on a grassy field. Wind plant
owners must carefully plan where to locate their machines. They must consider
wind availability (how much the wind blows), local weather conditions, nearness
to electrical transmission lines, and local zoning codes.
Wind plants also need a
lot of land. One wind machine needs about two acres of land to call its own. A
wind power plant takes up hundreds of acres. On the plus side, farmers can grow
crops around the machines once they have been installed.
After a plant has been built,
there are still maintenance costs. In some states, maintenance costs are offset
by tax breaks given to power plants that use renewable energy sources. The
Public Utility Regulatory Policies Act, or PURPA, also requires utility
companies to purchase electricity from independent power producers at rates
that are fair and nondiscriminatory.
Wind Resources and Energy Production
Where is the best place
to build a wind plant? There are many good sites for wind plants in the United
States including California, Alaska, Hawaii, the Great Plains, and mountainous
regions. Scientists say there is enough wind in 37 states to produce
electricity from the wind. Generally, an average wind speed of 14 mph is needed
to convert wind energy into electricity economically. The average wind speed in
the United States is 10 mph.
Scientists use an
instrument called an anemometer to measure how fast the wind is blowing.
An anemometer looks like a modern-style weather vane. It has three spokes with
cups that spin on a revolving wheel when the wind blows. It is hooked up to a
meter that tells the wind speed. (By the way, a weather vane tells you the
direction of the wind, not the speed.)
As a rule, wind speed
increases with altitude and over open areas with no wind breaks. Good sites for
wind plants are the tops of smooth, rounded hills, open plains or shorelines,
and mountain gaps that produce wind funneling. The three biggest wind plants in
California are located at mountain gaps.
Wind speed varies
throughout the country. It also varies from season to season. In Tehachapi,
California, the wind blows more from April through October than it does in the
winter. This is because of the extreme heating of the Mojave desert during the
summer months. The hot desert air rises, and the cooler, denser air from the
Pacific Ocean rushes through the Tehachapi mountain pass to take its place. In
a state like Montana, on the other hand, the wind blows more during the winter.
By happy coincidence,
these seasonal variations perfectly match the electricity demands of the
regions. In California, people use more electricity during the summer when air
conditioners are used for cooling. Conversely, more people use electricity in
Montana during the winter heating months.
How much energy can we
can get from the wind? We will use two terms to describe wind energy
production: efficiency and capacity factor.
Efficiency refers to how
much useful energy (electricity, for example) we can get from an energy source.
A 100 percent energy efficient machine would change all the energy put into the
machine into useful energy. It would not waste any energy. (You should know
there is no such thing as a 100 percent energy efficient machine. Some energy
is always "lost" or wasted when one form of energy is converted to
another. The "lost" energy is usually in the form of heat.)
How efficient are wind
machines? Wind machines are just as efficient as coal plants. Wind plants
convert 30 percent of the wind's kinetic energy into electricity. A coal-fired power
plant converts about 30-35 percent of the heat energy in coal into electricity.
It is the capacity factor
of wind plants that puts them a step behind other power plants. Capacity factor
refers to the capability of a plant to produce energy. A plant with a 100
percent capacity rating would run all day, every day at full power. There would
be no down time for repairs or refueling, an impossible dream for any plant.
Wind plants have about a
25 percent capacity rating because wind machines only run when the wind is
blowing around nine mph or more. In comparison, coal plants typically have a 75
percent capacity rating since they can run day or night, during any season of
One wind machine can
produce 275-500 thousand kilowatt-hours (kWh) of electricity a year. That is
enough electricity for about 50 homes per year.
In this country, wind
machines produce about three billion kWh of energy a year. Wind energy provides
0. 12% of the nation's electricity, a very small amount. Still, that is enough
electricity to serve more than 300,000 households, as many as in a city the
size of San Francisco or Washington, D.C.
California produces more
electricity from the wind than any other state. It produces 98 percent of the
electricity generated from the wind in the United States. Some 16,000 wind
machines produce more than one percent of California's electricity. (This is
about half as much electricity as is produced by one nuclear power plant.) In
the next 15 years, wind machines could produce five percent of California's
Why is California
outproducing every other state? More than any other reason, wind energy has
taken off in this state because of California's progressive state policies that
support renewable energy sources. Other states have just as good wind resources
The United States is the
world's leading wind energy producer. The U.S. produces about half of
the world's wind power. Other countries that have invested heavily in wind
power research are Denmark, Japan, Germany, Sweden, The Netherlands, United
Kingdom, and Italy.
What does the future look
like for wind energy? Using a best-case scenario, the American Wind Energy
Association (AWEA) estimates wind energy could produce more than 10 percent of
the nation's electricity within the next 30 years.
So, wind energy may be an
important alternative energy source in the future, but it will not be the sole
answer to our energy problems. We will still need other energy sources to meet
our growing demand for electricity.
Wind Energy Economics and the Environment
On the economic front,
there is a lot of good news for wind energy. First, a wind plant is far less
expensive to construct than a conventional energy plant. Wind plants can simply
add wind machines as electricity demand increases.
Second, the cost of
producing electricity from the wind has dropped dramatically in the last two
decades. Electricity generated by the wind cost 30 cents per kWh in 1975, but
now costs less than five cents per kWh. In comparison, new coal plants produce
electricity at four cents per kWh.
Wind and the
In the 1970s and 1980s,
oil shocks and shortages pushed the development of alternative energy sources.
In the 1990s, the push may come from something else, a renewed concern for the
Wind energy is clean.
Wind machines produce no air or water pollution because no fuel is burned to
The only environmental
drawbacks to wind energy may be a wind plant's effect on bird populations and
its visual impact on the surrounding landscape. To some, the glistening blades
of wind machines are an eyesore; to others, they're a beautiful alternative to