Roger Rivera
October 28, 2002
Wind Power
In the society we live in today, many people have the misconception that there is a
never-ending supply of energy available for our wasteful use. However, people must
realize that the fossil fuels that we have come to depend on for our energy are quickly
being depleted. There are several renewable energy resources, most of which come from
the sun, that are available for our use. One of these abundant energy resources is the
wind. By taking advantage of the wind, and harnessing its power to supply useful
energy, people can ensure that they will have energy for as long as the sun continues to
heat the earth.
The wind is a direct product of the sun. The earth receives 1.74 x 10^17 Watts of
power (per hour) from the sun (Tour 1). The sun causes differential heating of earth’s
surface and atmosphere, inducing vertical and horizontal air currents that are affected by
the rotation of the earth and the contours of the land (Ristinen 133). As the land is
heated, the warm air near the surface rises into the cooler atmosphere, causing a pressure
gradient between the surface and the upper atmosphere. The lower pressure near the
surface causes an inward current of air (wind) from the higher-pressure surroundings. A
great example of this is the Land Sea Breeze Cycle, which we feel when we step on the
beach (Tour 1). As the warmer air rises into the atmosphere, the cooler air over the ocean
rushes onto the shore, and this is the refreshing sea breeze we have come to expect
whenever we step on the beach.
About one to two percent of the energy that the earth receives from the sun is
converted to wind energy (Tour 1). The amount of energy that the wind transfers to the
rotors of a wind turbine depends on the density of the air, the rotor area, and the wind
speed (Tour 7). The power of wind (kW/ m^2) is equal to 6.1 x 10^-4 V^3 (Ristinen
134). Notice that the power of wind is proportional to the cubic velocity. This is true
because the kinetic energy (0.5 x M x V^2) of a body in motion is proportional to V^2,
and since the flow rate (F= A x V ) of a fluid is proportional to the velocity, multiplying
these two factors causes the power of wind to be proportional to the cubic velocity.
Therefore, the greater the wind speed, the greater the power that can be captured from the
wind, depending on the efficiency of the wind turbine.
The process of harnessing the power of the wind is not extremely difficult. What
is difficult is increasing the efficiency of this process. When dealing with wind energy,
people are concerned with the surface winds, which are those that flow between the
ground and about 100 meters in altitude (Wind 1). In general, the process of converting
wind energy into electrical energy requires the use of a wind turbine and a generator.
Therefore, the more efficient the designs of the wind turbines and the generators are, the
more efficient the process of converting wind energy into electrical energy will be.
The design of the wind turbine is probably the most essential part of converting
wind energy to electrical energy. When dealing with wind turbines, one must take into
account how high it is, how big the rotors are, and exactly how much overall area the
rotors will take up. The rotor area determines how much energy a wind turbine is able to
harvest from the wind (Tour 7). Since the rotor area increases with the square of the rotor
diameter, a turbine that is twice as large will receive twice as much energy. However,
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one must be careful not to build a turbine too large, for it can disrupt the airflow through
its cross section, thus reducing the effective wind velocity at its location and diverting
much of the airflow around itself, minimizing the power extracted from the wind
(Ristinen 135). At the other extreme, one must be careful not to build the turbine too
small, reducing the wind velocity by only a small amount, thus extracting only a small
amount of the power from the wind (Ristinen 135). Therefore, one must build a mid-size
rotor, which will not disrupt the airflow, but will be able to efficiently extract a large
amount of the wind’s power.
When planning on the size of the turbine, one can choose between a large or small
wind turbine. Large turbines are well suited for offshore wind plants, where the ocean
currents and differential heating of the ocean can produce gusting winds that can produce
a great amount of electricity. Large turbines are also suited for areas where it is difficult
to find sites because one large turbine on a tall tower can use the wind extremely
efficiently. In addition, large turbines can deliver electricity at a lower cost than small
turbines because foundation costs and planning costs for a wind tower are independent of
turbine size, and since a large turbine can produce more electricity than a small turbine,
the electricity produced will cost less (Tour 7). On the other hand, there are times when
small turbines are preferred. Small turbines might be preferred in areas where the local
electrical grids may not be able to handle the large electrical output from a large turbine.
Also, due to the high costs needed to make the towers taller and stronger in order to hold
larger turbines, it might be more economical in some areas to use smaller turbines.
Moreover, when building a windmill one must consider the landscape of the surrounding
area, and many people argue that small turbines do not disrupt the landscape as much as
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large turbines (Tour 7). As one can see, the size of the turbine is an essential part of its
design, but it is not the only factor that must be taken into account.
Moreover, another factor that must be taken into account when designing a
turbine is the number of blades (rotors). Wind turbines have been built with one, two, or
three rotors. The three-blade design is the most popular because it is the most stable
when it comes to the lift caused by the wind (Tour 68). As the top blade gets pushed
back due to force of the wind, the other two blades are lifted and caused to turn.
Repeating this process every time a new blade is in top position causes the fast rotation of
the wind turbine. On the other hand, a two-blade design is less stable because as the top
blade is pushed back, the other blade is lifted, but it must travel a further distance before
it is in the top position. Also, although it will cost less to build two blades instead of
three, the money saved will have to be invested into placing better shock absorbers for
the blades due to the increased instability of the design. Moreover, some people have
even used a single-blade design for the wind turbine. However, this design is extremely
unstable due to the same lift principles explained previously. In addition, although one
can save money by building one blade instead of three or two, this money must go into
building a safe and stable weight on the opposite end of the blade in order to balance the
weight of the turbine. Therefore, as one can see, the reason that the three-blade design is
the most popular is because it is the most stable design when taking into account the lift
force of the wind.
Furthermore, the size of the generator is another integral part of the efficiency of a
wind turbine (Tour 69). Wind power generators convert wind energy (mechanical
energy) to electrical energy. The generator is attached at one end to the wind turbine,
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which provides the mechanical energy. At the other end, the generator is connected to
the electrical grid. An important part of the design of a generator is placing an effective
cooling system near the generator to prevent overheating. When building a turbine one
can choose between a large or small generator. Smaller generators require less energy to
turn than larger ones, but give much less power output. Therefore, one must not combine
a large turbine with a small generator because it will capture only a small fraction of the
energy content of the wind at high speeds, thus making it inefficient (Tour 69). On the
other hand, a large generator is extremely efficient at high wind speeds, but it is unable to
turn at low wind speeds (Tour 69). This is because a generator with larger coils and/or a
larger and more potent internal magnet requires more force to start in motion. Therefore,
one can clearly see that when designing a windmill, the size of the turbine, the size and
number of rotors, and the size of the generator must all be taken into account in order to
maximize its efficiency, as well as lower its costs and increase the profits.
One may wonder, what does an average wind turbine cost to develop? The
Danish Wind Industry Association considers an average turbine to be a 600 kW turbine
developed in an area with average annual wind speeds of at least 12 mph (Tour 102). An
average 600 kW turbine costs about $450,000 to build. In addition, the installation costs
are typically $125,000. When these two figures are added, the total costs ends up being
$575,000. As a general rule, one can calculate a wind turbine’s cost to be $1,000 per
kilowatt electrical power installed (Tour 102). Moreover, an average wind turbine’s
annual maintenance costs are about 1.5- 2.0 percent of the original cost (Tour 102).
Considering the need for the use of renewable energy resources, why is it that the
United States does not use wind power as a major source of energy? The United States
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currently has more than 1,600 MW of installed wind power capacity and produces about
3 billion kWh of electricity each year (Wind 1). This is barely enough electricity to meet
the annual residential needs of one million people. Even more interesting is the fact that
over ninety percent of this power is produced by three wind farms in California:
Altamont Pass, Tehachapi, and Palm Springs (Ristinen 139). So, is it that the United
States has no areas capable of providing more wind power? On the contrary, North
Dakota alone has enough suitable wind resource to supply thirty six percent of the
electricity consumed in the United States, and overall, the Great Plains contain a great
amount of wind resource (Wind 1). However, the United States government is yet to
begin building wind farms in this section of the country, as if there was no need of
alternate sources of energy. Increasing the use of wind power as a renewable source of
energy in the United States would decrease the dependency of the country on fossil fuels,
and create an estimated 440 to 460 jobs per each billion kWh of annual wind energy
generation, according to a recent study (Wind 1).
Therefore, why is the United States government holding back? Are there any
drawbacks, or any environmental risks due to wind power use? On the contrary, wind
power is a domestic renewable source of energy that generates no pollution and has little
environmental impact. Up to ninety-five percent of land used for wind farms can also be
used for other profitable activities including ranching, farming, and forestry (Wind 1).
Although wind power has many advantages, some people argue that wind power is not a
continuous source of energy due to the variability of the wind. They are correct in saying
that the wind does vary, but they do not realize that most rapid variations will to some
extent be compensated for by the inertia of the wind turbine rotor (Tour 7). In addition,
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unlike other renewable energy resources, such as solar power, the wind blows day and
night, which allows windmills to produce electricity throughout the day (Ristinen 133).
Worldwide, people also worry about wind turbines and the landscape, sound from wind
turbines, and the death of birds due to wind turbines. However, these are minor details
that can be taken care of. For example, if people worry that numerous wind turbines will
ruin a scenic landscape, a large turbine can be put in to replace the numerous small
turbines, and since a large turbine turns at a lower rate and speed than small turbines it
will attract less attention (Tour 88). Moreover, by just increasing the tip speed of the
wind rotors, the wind turbine sound can be greatly reduced. Many studies also reveal that
at a distance of 300m (100 ft) a neighbor will experience only about 35 decibels of sound
due to a wind turbine (Tour 89). This is about the same as the amount of background
sound that one experiences when he/she is in a quiet home (Urone 410). Finally, many
environmentalists around the world believe that windmills cause the death of numerous
birds annually (NWTC 1). However, the Danish Ministry of the environment performed
a study in 1998 that revealed that birds are more likely to die from flying into electric
power lines, or flying into the electric power line poles, than by flying into wind turbines
(Tour 94). Therefore, wind turbines do not place birds at a greater risk than they already
are due to overall human interaction.
To conclude, people must realize that fossil fuels will not last forever, maybe not
even another hundred years. Therefore, we must begin to make use of the alternate
energy sources that available for our use, one of these being wind power. Wind power is
a renewable energy source that can be used for as long as the sun continues to heat the
earth. It causes no pollution and has little effect on the environment. Therefore, due to
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the availability and advantages of wind power, the decreasing cost of wind power, and
the growing interest in renewable energy resources, one can be assured that wind power
will soon become a feasible energy source in the United States, as well as throughout the
world.
Works Cited
Ristinen, Robert A., Jack J. Kraushaar.. Energy and the Environment. New York: John
Wiley and Sons, Inc., 1999.
Urone, Paul P.. College Physics: Second Edition. California: Brooks/Cole, 2001.
Guided Tour on Wind Energy. 16 Oct. 2002 <www.windpower.org/tour/index.htm>.
The National Wind Technology Center (NWTC). 16 Oct. 2002 <www.nrel.gov/wind/>.
Wind Energy. 16 Oct. 2002 <www.nrel.gov/lab/pao/wind_energy.html>.
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Most of the information and data in this paper was taken from
www.windpower.org.
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