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Windmill, machine
that converts wind into useful energy. This energy
is derived from the force of wind acting on
oblique blades or sails that radiate from a shaft.
The turning shaft may be connected to machinery
used to perform such work as milling grain,
pumping water, or generating electricity. When the
shaft is connected to a load, such as a pump, the
device is typically called a windmill. When it is
used to generate electricity, it is known as a
wind turbine generator.
Wind-driven mills are of
ancient origin. Simple windmills may have been
used in Persia (now Iran) as early as the 7th
century AD. They were used for
irrigation and milling grain. The wheel bearing
the wind sails of the earliest windmills was
horizontal and supported by a vertical shaft.
These machines were relatively inefficient.
Nevertheless, this type of windmill spread to
China and throughout the Middle
East.
The earliest European
windmills appeared in France and England in the
12th century and quickly spread throughout Europe.
These early wood structures, called post mills,
were rotated by hand around a central post to
bring the sails into the
wind.
The tower mill was developed
in France during the 14th century. It consisted of
a stone tower topped by a rotatable wooden cap
that supported the windshaft and the upper portion
of the mill gearing.
Early windmills all share
certain features. A horizontal shaft protrudes
from the cap, or upper portion of the mill
building. Four to eight wind sails, each about 3
to 9 m (about 10 to 30 ft) in length, radiate from
the shaft. The wood frames of the sails are either
covered with canvas or fitted with wood shutters.
The power of the turning shaft is transmitted
through a system of gears and shafts down to the
mill machinery at the base of the
building.
Besides milling grain and
irrigating farmland, windmills developed from the
15th century to the 19th century were adapted to a
variety of tasks, including pumping seawater from
land below sea level, sawing wood, making paper,
pressing oil from seeds, and grinding many
different materials. By the 19th century the Dutch
had built about 9000
windmills.
Of the major
improvements on the windmill, the most important
was the fantail, a mechanism invented in 1745 that
automatically rotates the sails into the wind. In
1772 the spring sail was developed. This type of
sail consists of wood shutters, the openings of
which can be controlled either manually or
automatically to maintain a constant sail speed in
winds of varying speeds. Other improvements
include air brakes to stop the sails from rotating
and the use of propellerlike airfoils in place of
sails, which increases the usefulness of mills in
light winds.
Water-pumping windmills were
widely employed during the settlement of the
western United States. The use of wind turbines
for generating electricity was pioneered in
Denmark late in the 1890s. Small wind turbine
generators supplied electricity to many rural
communities in the United States until the 1930s,
when power lines were extended across the nation.
Large wind turbines were also built during this
time. The largest was the Smith-Putnam generator,
installed in 1941 at Grandpa's Knob, near Rutland,
Vermont.
Modern wind turbines are
propelled by one of two effects: drag, by which
wind pushes the blades; and lift, by which the
blades are moved in the same way an airplane's
wing rises on an air current. Turbines operated by
lift turn more rapidly and are inherently more
efficient. Wind turbines can be classified as
horizontal-axis machines, with their main shafts
parallel to the ground, or vertical-axis machines,
with shafts perpendicular to the ground.
Horizontal-axis turbines used to generate
electricity have one to three blades; those used
for pumping may have many more. The most common
vertical-axis machines, named after their
designers, are the Savonius, used primarily for
pumping, and the Darrieus, a higher-speed machine
resembling an eggbeater.
The water pumper is a
high-torque, low-speed windmill common in rural
areas of the United States.
Continue
article...
Water pumpers are
used mainly to draw water from underground. These
machines use a rotor, usually from 2 to 5 m (from
6 to 16 ft) in diameter, with a number of oblique
blades radiating from a horizontal shaft. The
rotor is mounted on a tower high enough to catch
wind. A large, rudderlike vane directs the wheel
into the wind. The wheel turns gears that operate
a piston pump. When wind velocities become
excessive, safety devices automatically turn the
rotor out of the wind to prevent damage to the
mechanism.
Wind turbine generators
consist of a variety of components. The rotor
converts the power of the wind to the rotating
power of the shaft; a gearbox increases speed; and
a generator converts the shaft power into
electrical power (see Electric
Motors and Generators). In some
horizontal-axis machines, the pitch of the blades
can be adjusted to regulate the speed during
normal operation and also to shut down the machine
when wind speeds are excessive. Others use stall,
an aerodynamic phenomenon that naturally limits
the power at high wind speeds. Usually, modern
machines start operating when wind speeds reach
about 19 km/h (about 12 mph), achieve their rated
power at about 40 to 48 km/h (about 25 to 30 mph),
and shut down in wind speeds of about 100 km/h
(about 60 mph).
The best sites for turbine
generators have annual average wind speeds of at
least 21 km/h (13 mph). Scientists have estimated
that as much as 10 percent of the world's
electricity could be provided by wind generators
by the middle of the 21st century. See
also Electric
Power Systems.
The most successful wind
turbine generators for large-scale power
generation have been of medium size (from 50 to
100 ft in diameter, with power ratings of 100 to
400 kw). These are sometimes installed in groups
or arrays, known as wind farms. The world's
largest wind farms are in California, where wind
turbines can generate power up to about 1120 MW. A
typical nuclear plant has a rating of about 1100
MW. The cost to produce wind power in such
applications is competitive with many other forms
of power generation. Denmark now obtains more than
2 percent of all its electricity from wind
turbines. Wind turbines are also being used to
increase the power supply to communities on
islands or in other remote locations. Wind energy,
which contributes very little pollution and few
greenhouse gases to the environment, is a valuable
alternative to nonrenewable fuel, such as
oil.
Contributed By:
James F.
Manwell
Director of Renewable Energy Resources
Laboratory, Department of Mechanical Engineering,
University of Massachusetts.
See
an outline for this article.
Further
Reading
HOW TO CITE THIS
ARTICLE
"Windmill," Microsoft® Encarta®
Online Encyclopedia 2001
http://encarta.msn.com
© 1997-2000 Microsoft Corporation. All Rights
Reserved.
© 1993-2001 Microsoft Corporation. All
Rights Reserved.
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