A windmill is a machine which converts the energy of wind into rotational energy by means of vanes called sails or blades. Originally windmills were developed for milling grain for food production. In the course of history the windmill was adapted to many other industrial uses. An important application was to pump water. Windmills used for generating electricity are commonly known as wind turbines.

Windmills in antiquity

The windwheel of the Greek engineer Heron of Alexandria in the 1st century AD is the earliest known instance of using a wind-driven wheel to power a machine. Another early example of a wind-driven wheel was the prayer wheel, which was used in ancient Tibet and China since the 4th century. It has been claimed that the Babylonian emperor Hammurabi planned to use wind power for his ambitious irrigation project in the 17th century BC.

Horizontal windmills

The first practical windmills had sails that rotated in a horizontal plane, around a vertical axis. They were invented in eastern Persia (what is now Afghanistan), as recorded by the Persian geographer Estakhri in the 9th century. The authenticity of an earlier anecdote of a windmill involving the second caliph Umar (AD 634–644) is questioned on the grounds that it appears in a 10th-century document. Made of six to twelve sails covered in reed matting or cloth material, these windmills were used to grind grain or draw up water, and were quite different from the later European vertical windmills. Windmills were in widespread use across the Middle East and Central Asia, and later spread to China and India from there.

A similar type of horizontal windmill with rectangular blades, used for irrigation, can also be found in 13th-century China (during the Jurchen Jin Dynasty in the north), introduced by the travels of Yelü Chucai to Turkestan in 1219.

Horizontal windmills were built, in small numbers, in Europe during the eighteenth and nineteenth centuries,

Vertical windmills

There is an ongoing debate among historians on whether and how the windmill from the middle East influenced the development of the early European windmill. In northwestern Europe, the horizontal-axis or vertical windmill (so called due to the plane of the movement of its sails) is believed to date from the last quarter of the 12th century in the triangle of northern France, eastern England and Flanders. The earliest certain reference to a windmill in Europe (assumed to have been of the vertical type) dates from 1185, in Weedley, Yorkshire, although a number of earlier but less certainly dated twelfth century European sources referring to windmills have also been found. These earliest mills were used to grind cereals.

Post mill

The evidence at present is that the earliest type of European windmill was the post mill, so named because of the large upright post on which the mill's main structure (the "body" or "buck") is balanced. By mounting the body this way, the mill is able to rotate to face the wind direction; an essential requirement for windmills to operate economically in North-Western Europe, where wind directions are variable. The body contains all the milling machinery. The first post mills were of the sunken type where the post was buried in an earth mound to support it. Later a wooden support was developed called the trestle. This was often covered over or surrounded by a roundhouse to protect the trestle from the weather and to provide storage space. This type of windmill was the most common in Europe until the 19th century when more powerful tower and smock mills replaced them.

Hollow-post mill

In a hollow-post mill the post on which the body is mounted is hollowed out, to accommodate the drive shaft. In this way it is possible to drive machinery below or outside the body while still being able to rotate the body into the wind. Hollow-post mills driving scoop wheels were used in the Netherlands to drain wetlands from the 14th century onwards.

Tower mill

By the end of the thirteenth century the masonry tower mill, on which only the cap is rotated rather than the whole body of the mill, had been introduced. The spread of tower mills came with a growing economy that called for larger and more stable sources of power though they were more expensive to build. In contrast to the post mill, only the cap of the tower mill needs to be turned into the wind, so the main structure can be made much taller, allowing the sails to be made longer, which enables them to provide useful work even in low winds. The cap can be turned into the wind either by winches or gearing inside the cap or from a winch on the tail pole outside the mill. A method of keeping the cap and sails into the wind automatically is by using a fantail, a small windmill mounted at right angles to the sails, at the rear of the windmill. These are also fitted to tail poles of post mills and are common in Great Britain and English-speaking countries of the former British Empire, Denmark and Germany but rare in other places. Tower mills with a fixed cap are found around the Mediterranean Sea. They are built with the sails facing the prevailing wind direction.

Smock mill

The smock mill is a later development of the tower mill where the tower is replaced by a wooden framework, called the "smock." The smock is commonly of octagonal plan, though examples with more, or fewer, sides exist. The smock is thatched, boarded or covered by other materials like slate, sheet metal or tar paper. The lighter construction in comparison to tower mills make smock mills practical as drainage mills as these often had to be built in areas with unstable subsoil. Having originated as a drainage mill, smock mills are also used for a variety of purposes. When used in a built-up area it is often placed on a masonry base to raise it above the surrounding buildings.

Sails

Common sails consist of a lattice framework on which a sailcloth is spread. The miller can adjust the amount of cloth spread according to the amount of wind available and power needed. In medieval mills the sailcloth was wound in and out of a ladder type arrangement of sails. Post-medieval mill sails had a lattice framework over which the sailcloth was spread, while in colder climates the cloth was replaced by wooden slats, which were easier to handle in freezing conditions. The jib sail is commonly found in Mediterranean countries, and consists of a simple triangle of cloth wound round a spar. In all cases the mill needs to be stopped to adjust the sails. Inventions in Great Britain in the late 18th and 19th century led to sails that automatically adjust to the wind speed without the need for the miller to intervene, culminating in Patent sails invented by William Cubitt in 1813. In these sails the cloth is replaced by a mechanism of connected shutters. In France, Berton invented a system consisting of longitudinal wooden slats connected by a mechanism that lets the miller open them while the mill is turning. In the 20th century increased knowledge of aerodynamics from the development of the airplane led to further improvements in efficiency by German engineer Bilau and several Dutch millwrights. The majority of windmills have four sails. Multi-sailed mills, with five, six or eight sails, were built in Great Britain (especially in and around the counties of Lincolnshire and Yorkshire), Germany and less commonly elsewhere. Earlier multi-sailed mills are found in Spain, Portugal, Greece, parts of Romania, Bulgaria and Russia A mill with an even number of sails has the advantage of being able to run with a damaged sail and the one opposite removed without resulting in an unbalanced mill.

Machinery

Gears inside a windmill convey power from the rotary motion of the sails to a mechanical device. The sails are carried on the horizontal windshaft. Windshafts can be wholly made of wood, or wood with a cast iron poll end (where the sails are mounted) or entirely of cast iron. The brake wheel is fitted onto the windshaft between the front and rear bearing. It has the brake around the outside of the rim and teeth in the side of the rim which drive the horizontal gearwheel called wallower on the top end of the vertical upright shaft. In grist mills the great spur wheel, lower down the upright shaft, drives one or more stone nuts on the shafts driving each millstone. Post mills sometimes have a head and/or tail wheel driving the stone nuts directly, instead of the spur gear arrangement. Additional gear wheels drive a sack hoist or other machinery. The machinery differs if the windmill is used for other applications than milling grain. A drainage mill uses another set of gear wheels on the bottom end of the upright shaft to drive a scoop wheel or Archimedes' screw. Sawmills use a crankshaft with to provide a reciprocating motion to the saws. Windmills have been used to power many other industrial processes, including papermills, threshing mills, and for example to process oil seeds, wool, paints and stone products

Spread and decline

The total number of wind powered mills in Europe is estimated to have been around 200,000 at its peak, compared to some 500,000 waterwheels. With the coming of the industrial revolution, the importance of wind (and water) as primary industrial energy source declined and was eventually replaced by steam (in steam mills) and internal combustion engines, although windmills continued to be built in large numbers until late in the 19th Century. More recently windmills have been preserved for their historic value, in some cases as static exhibits when the antique machinery is too fragile to put in motion, and in other cases as fully working mills. There are around 50 working mills in operation in Britain as of 2009.

Of the 10,000 windmills in use in the Netherlands around 1850, about 1000 are still standing. Most of these are being run by volunteers though there are some grist mills still operating commercially. Many of the drainage mills have been appointed as backup to the modern pumping stations. The Zaan district has been said to have been the first industrialized region of the world with around 600 operating wind powered industries by the end of the 18th century. Economic fluctuations and the industrial revolution had a much greater impact on these industries than on grain and drainage mills so only very few are left.

Construction of mills spread to the Cape Colony in the 17th century. The early tower-mills did not survive the gales of the Cape Peninsula, so that in 1717 the Heeren XVII sent carpenters, masons and materials to construct a durable mill. The mill was completed in 1718 and became known as the ''Oude Molen'' and was located between Pinelands Station and the Black River. Long since demolished, its name lives on as that of a Technical school in Pinelands. By 1863 Cape Town could boast eleven mills stretching from Paarden Eiland to Mowbray.

Windpumps

Windpumps are used extensively on farms and ranches in the central plains and South West of the United States and in Southern Africa and Australia. These mills feature a large number of blades so that they turn slowly with considerable torque in low winds and be self regulating in high winds. A tower-top gearbox and crankshaft convert the rotary motion into reciprocating strokes carried downward through a rod to the pump cylinder below. The farm wind pump was invented by Daniel Halladay in 1854. Eventually steel blades and steel towers replaced wooden construction, and at their peak in 1930, an estimated 600,000 units were in use. The multi-bladed wind turbine atop a lattice tower made of wood or steel hence became, for many years, a fixture of the landscape throughout rural America. Firms such as Star, Eclipse, Fairbanks-Morse and Aermotor became famed suppliers in North and South America.

Wind turbine

A windmill used to generate electricity is commonly called a wind turbine. The first windmills for electricity production were built by the end of the 19th century by Prof James Blyth in Scotland (1887), Charles F. Brush in Cleveland, Ohio (1887–1888) and Poul la Cour in Denmark (1890s). La Cour's mill from 1896 later became the local powerplant of the village Askov. By 1908 there were 72 wind-driven electric generators in Denmark from 5 kW to 25 kW. By the 1930s windmills were widely used to generate electricity on farms in the United States where distribution systems had not yet been installed, built by companies like Jacobs Wind, Wincharger, Miller Airlite, Universal Aeroelectric, Paris-Dunn, Airline and Winpower and by the Dunlite Corporation for similar locations in Australia.

Forerunners of modern horizontal-axis utility-scale wind generators were the WIME-3D in service in Balaklava USSR from 1931 until 1942, a 100 kW generator on a 30 m (100 ft) tower, the Smith-Putnam wind turbine built in 1941 on the mountain known as Grandpa's Knob in Castleton, Vermont, USA of 1.25 MW and the NASA wind turbines developed from 1974 through the mid 1980's. The development of these 13 experimental wind turbines pioneered many of the wind turbine design technologies in use today, including: steel tube towers, variable-speed generators, composite blade materials, partial-span pitch control, as well as aerodynamic, structural, and acoustic engineering design capabilities. The modern wind power industry began in 1979 with the serial production of wind turbines by Danish manufacturers Kuriant, Vestas, Nordtank, and Bonus. These early turbines were small by today's standards, with capacities of 20–30 kW each. Since then, they have increased greatly in size, with the Enercon E-126 capable of delivering up to 7 MW, while wind turbine production has expanded to many countries.

As the 21st century began, rising concerns over energy security, global warming, and eventual fossil fuel depletion led to an expansion of interest in all available forms of renewable energy. Worldwide there are now many thousands of wind turbines operating, with a total nameplate capacity of 194,400 MW. Europe accounted for 48% of the total in 2009.

A wind turbine looking like a windmill is De Nolet in Rotterdam.

See also

List of windmills

Mill machinery

Millstone

Molinology

Éolienne Bollée

Renewable energy

Watermill

References

Further reading

Ahmad Y Hassan, Donald Routledge Hill (1986). ''Islamic Technology: An illustrated history''. Cambridge University Press. ISBN 0-521-42239-6.

Chartrand, ''French Fortresses in North America 1535–1763'': Quebec, Montreal, Louisbourg and New Orleans.

Dietrich Lohrmann, "Von der östlichen zur westlichen Windmühle", Archiv für Kulturgeschichte, Vol. 77, Issue 1 (1995)

A.G. Drachmann, "Heron's Windmill", Centaurus, 7 (1961).

Needham, Joseph (1986). ''Science and Civilization in China: Volume 4, Physics and Physical Technology, Part 2, Mechanical Engineering''. Taipei: Caves Books Ltd.

Hugh Pembroke Vowles: "An Enquiry into Origins of the Windmill", ''Journal of the Newcomen Society'', Vol. 11 (1930–31)

Roy Gregory and Laurence Turner (2009) ''Windmills of Yorkshire'' ISBN 978-1-84033-475-3

Edwin Tunis (1999), ''Colonial living'', The Johns Hopkins University Press ", ISBN 0-8018-6227-2, pp. 72 and 73

External links

Earth Science Australia, Wind Power and Windmills

The International Molinological Society

Windmills at Windmill World

Mill Database, Belgium and Netherlands

A Geograph article and photo-record of Windmills in Great Britian

Category:Sustainable technologies Category:Agricultural buildings Category:Wind power Category:Industrial buildings

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