Windmill

I

Introduction

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

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.

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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

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

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.

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