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Windmill , 11k Wind Turbines , 14k
Article Outline

Introduction, The First Windmills, Uses and Improvements, Modern Wind Turbines

I. Introduction Print section

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.

II. The First Windmills Print section

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.

III. Uses and Improvements Print section

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.

IV. Modern Wind Turbines Print section

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.

A. Water Pumper Print section

The water pumper is a high-torque, low-speed windmill common in rural areas of the United States.
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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.

B. Electricity Generator Print section

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.

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Further Reading

"Windmill," Microsoft® Encarta® Online Encyclopedia 2001 © 1997-2000 Microsoft Corporation. All Rights Reserved.

© 1993-2001 Microsoft Corporation. All Rights Reserved.

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