Environmental Engineering Reference
In-Depth Information
(35,000 new jobs), which supported a 60 percent surge in new wind
capacity installation over 2007. 41 The increase was 8,358 megawatts,
5,000 new turbines, which accounted for 40 percent of all new electricity
capacity. Wind capacity in the United States now totals over 25,000
megawatts.
The rapid increase in wind power installations has affected the eco-
nomics of wind power. Between 1992 and 2007, the costs of wind-
generated electricity dropped by 50 percent as effi ciency, reliability, and
power rating experienced signifi cant improvements. But costs have
increased in recent years due to a shortage of turbines and parts, rising
costs of materials, and increasing manufacturing profi tability. Despite
the higher costs, wind power remains competitive with new natural gas
plants, and it will become increasingly competitive with coal if Congress
puts a price on carbon emissions, which seems likely.
Four main variables determine how much electricity a turbine can
produce: wind speed, blade radius, tower height, and air density. For a
turbine to function, wind speed must be at least 8 mph, and energy can
be produced economically at 15 mph; maximum electricity is generated
at velocities between 25 and 55 mph. Most turbines are programmed to
shut down at about 55 mph because they can be damaged by stronger
winds.
The energy carried by the wind increases as the cube of the wind
speed, so a wind speed of 25 feet per second yields 15,625 (25 3 ) watts
(15.6 megawatts) for every square foot exposed to the wind, and at 50
feet per second, energy production is 125,000 (50 3 ) watts (125 mega-
watts) per square foot. This relationship puts a premium on sites having
the strongest winds and explains why the wind industry strives to
increase the scale of the equipment it installs. This relationship also
explains why the area swept by the turbine blades is so important. The
most effi cient turbines have increased in size over the years, with each
of the three large blades longer than a football fi eld. In 1980 turbine
blades were about 50 feet in diameter; today they can exceed 400 feet.
Electrical output has increased correspondingly, from 50 kilowatts to a
possible 10 megawatts, 200 times as much. The average generating
capacity of existing wind turbines in 2008 was 1.67 megawatts. The
blades spin a shaft that connects to a generator that produces the elec-
tricity. Wind velocity is lowest at ground level because of friction
between the moving air and the ground, and the velocity increases with
height. Hence, turbines are placed high above the ground. The effect of
friction is zero above about 300 feet.
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