Windmill
Wind-energy conversion system operated by wind acting on oblique blades or sails radiating 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 coupled 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 vertical-axis machines existed in Persia perhaps as early as the 7th century
AD, where they were used for irrigation and milling grain. The wheel bearing the wind sails of the earliest machines was horizontal and supported by a vertical shaft. These machines were relatively inefficient because wind energy was transferred only during one-half of one sail's rotation. Nevertheless, this type of windmill spread to China and throughout the domain of Islam.
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 manually, in their entirety, around a central post to bring the sails into the wind.
The tower mill, developed in 14th-century France, comprises stationary machinery inside a masonry tower topped by a rotatable woodsided cap bearing the windshaft and the upper portion of the mill gearing. The so-called smock mill is generally an octagonal tower mill, built of wood, resting on a masonry base.
These 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 exterior end of 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 were adapted to a variety of tasks from the 15th to the 19th century, including pumping seawater as part of land reclamation projects in the Low Countries, sawing wood, making paper, pressing oil from seeds, and grinding many different materials. By the 19th century the Dutch had built about 9000 windmills, many of which remain in use today.
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 in the settlement of the western U.S. 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 U.S. 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, Vt.
In the operation of modern wind turbines, the force of the wind is exerted by drag (pushing) or lift (as on an airplane wing). Those 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 considerably 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.
High-torque, low-speed machines, such as the common water pumper that can be seen in rural areas of the U.S., use a rotor, usually from 2 to 5 m (about 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 permit unobstructed action by the wind. A large, rudderlike vane directs the wheel into the wind; when wind velocities become excessive, safety devices automatically turn the rotor out of the wind to prevent damage to the mechanism. The coupling of the multibladed rotor to a piston pump provides relatively low overall efficiency. Nonetheless, because of its ruggedness and simplicity it is still used in many rural areas.
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 . 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 ambient wind speeds reach about 19 km/hr (about 12 mph), achieve their rated power at about 40 to 48 km/hr (about 25 to 30 mph), and shut down in wind speeds of about 100 km/hr (about 60 mph).
The power available in the wind varies as the cube of the wind speed--that is, as the wind speed multiplied by itself twice. A wind turbine can theoretically extract 59.3 percent of this power. More typically, overall efficiencies range from 30 to 45 percent.
The most attractive sites have annual average wind speeds of at least 21 km/hr (13 mph). It is estimated that 10 percent of the world's electricity could be provided by wind generators by the middle of the next 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 arrays, known as wind farms. The world's largest wind farms are in California, where total rated wind turbine power exceeds 1400 MW. A typical nuclear plant has a rating of 1000 MW. The cost to produce wind power in such applications is competitive with many forms of power generation. Denmark now obtains over 2 percent of all its electricity from wind turbines. Wind turbines are also being used to augment the power supply to communities on islands or in other remote locations.
Wind energy, which contributes very little pollution or toxic by-products and few greenhouse gases to the environment, is a significant alternative to nonrenewable fuels. J.F.M.