A windmill is a structure or machine that converts wind into usable energy through the rotation of a wheel made up of adjustable blades. Traditionally, the energy generated by a windmill has been used to grind grain into flour. Windmills are designed by skilled craftsmen and can be constructed on site using hand tools. Windmills developed steadily over the centuries and achieved their most prominence in Europe during the eighteenth century. They were largely replaced as a power generating structure when steam power was harnessed during the nineteenth century. Today, windmill technology is experiencing a renaissance and the wind turbine promises to be an important alternative to fossil fuels in the future.
Man has used wind to power machines for centuries. The earliest use was most likely as a power source for sail boats, propelling them across the water. The exact date that people constructed windmills specifically for doing work is unknown, but the first recorded windmill design originated in Persia around A.D. 500-900. This machine was originally used for pumping water then it was adapted for grinding grain. It had vertical sails made from bundles of lightweight wood attached to a vertical shaft by horizontal struts. The design, known as the panemone, is one of the least efficient windmill structures invented. It should be noted that windmills may have been used in China over 2,000 years ago making it the actual birthplace for vertical-axis windmills. However, the earliest recorded use found by archeologists in China is A.D. 1219.
The concept of the windmill spread to Europe after the Crusades. The earliest European designs, documented in A.D. 1270, had horizontal axes instead of vertical ones. The reason for this discrepancy is unknown, but it is likely a result of two factors. First, the European windmills may have been patterned after water wheels that had a horizontal axis. The water wheel had been known in Europe for long before this. Second, the horizontal axis design was more efficient and worked better. In general, these mills had four blades mounted on a central post. They had a cog and ring gear that translated the horizontal motion of the central shaft into vertical motion for the grindstone or wheel which would then be used for pumping water or grinding grain.
The European millwrights improved windmill technology immensely over the centuries. Most of the innovation came from the Dutch and the English. One of the most important improvements was the introduction of the tower mill. This design allowed for the mill's blades to be moved into the wind as required and the main body to be permanently fixed in place. The Dutch created multi-story towers where mill operators could work and also live. The English introduced a number of automatic controls that made windmills more efficient.
During the pre-industrial world, windmills were the electric motors of Europe. In addition to water pumping and grain grinding, they were used for powering saw mills and processing spices, dyes, and tobacco. However, the development of steam power during the nineteenth century, and the uncertain nature of windmill power resulted in a steady decline of the use of large windmill structures. Today, only a small fraction of the windmills that used to power the world are still standing.
Even as larger windmills were abandoned, smaller fan-type windmills were thriving. These windmills were used primarily for pumping water on farms. In America, these designs were perfected during the nineteenth century. The Halladay windmill was introduced in 1854 followed by the Aermotor and Dempster designs. The later two designs are still in use today. In fact, between 1850 and 1970 in the United States over six million were constructed.
There are two classes of windmill, horizontal axis and vertical axis. The vertical axis design was popular during the early development of the windmill. However, its inefficiency of operation led to the development of the numerous horizontal axis designs.
Of the horizontal axes versions, there are a variety of these including the post mill, smock mill, tower mill, and the fan mill. The earliest design is the post mill. It is named for the large, upright post to which the body of the mill is balanced. This design gives flexibility to the mill operator because the windmill can be turned to catch the most wind depending on the direction it is blowing. To keep the post stable a support structure is built around it. Typically, this structure is elevated off the ground with brick or stone to prevent rotting.
The post mill has four blades mounted on a central post. The horizontal shaft of the blades is connected to a large break wheel. The break wheel interacts with a gear system, called the wallower, which rotates a central, vertical shaft. This motion can then be used to power water pumping or grain grinding activities.
The smock mill is similar to the post mill but has included some significant improvements. The name is derived from the fact that the body looks vaguely like a dress or smock as they were called. One advantage is the fact that only the top of the mill is moveable. This allows the main body structure to be more permanent while the rest could be adjusted to collect wind no matter what direction it is blowing. Since it does not move, the main body can be made larger and taller. This means that more equipment can be housed in the mill, and that taller sails can be used to collect even more wind. Most smock mills are eight sided although this can vary from six to 12.
Tower mills are further improvements on smock mills. They have a rotating cap and permanent body, but this body is made of brick or stone. This fact makes it possible for the towers to be rounded. A round structure allows for even larger and taller towers. Additionally, brick and stone make the tower windmills the most weather resistant design.
While the previous windmill designs are for larger structures that could service entire towns, the fan-type windmill is made specifically for individuals. It is much smaller and used primarily for pumping water. It consists of a fixed tower (mast), a wheel and tail assembly (fan), a head assembly, and a pump. The masts can be 10-15 ft (3-15 m) high. The number of blades can range from four to 20 and have a diameter between 6 and 16 ft (1.8-4.9 m).
Windmills can be made with a variety of materials. Post mills are made almost entirely of wood. A lightweight wood, like balsa wood, is used for the fan blades and a stronger, heavier wood is used for the rest of the structure. The wood is coated with paint or a resin to protect it from the outside environment. The smock and tower mills, built by the Dutch and British prior to the twentieth century, use many of the same materials used for the construction of houses including wood, bricks and stones.
The main body of the fan-type mills is made with galvanized steel. This process of treating steel makes it weather resistant and strong. The blades of the fan are made with a lightweight, galvanized steel or aluminum. The pump is made of bronze and brass that inhibits freezing. Leather or synthetic polymers are used for washers and o-rings.
Windmills are always erected on site using pre-made parts. The following description relates to the fan-type windmill. The basic
Making the tower parts
- 1 The tower parts are made from galvanized steel. This process begins with a roll of coiled sheet metal. The coils are put on a de-spooling device and fed to the production line. They are run under a straightener to remove any kinks or twists. The pieces are cut to the appropriate size and shape. In some cases, pieces may be put on a machine that rolls them and welds the seam. The ends are passed under a crimping machine and the pieces are moved to the finishing station.
- 2 At the finishing station, holes are drilled in the metal parts at specific places as required by the windmill design. The parts may also be painted or coated before being arranged in the final windmill kit.
Making the gearbox
- 3 The gearbox is an intricate assembly made up of various gears, axles, rotors, and wheels. The parts are die cast and assembled by hand. The are placed in an weather resistant housing that is designed to accommodate the gearbox parts and the attached wheel and tail assembly.
Making the fan
- 4 The fan is made up of a metal rim with slightly curved blades attached. The rim is produced on a machine that rolls steel strips into circular hoops. A hole is drilled in both ends, and they are connected with a small clamp and screw after the fan blades are attached. A center axle is then connected to the rim and attached with small steel spokes. A typical design will have five pairs of spokes attached a evenly spaced intervals along the rim.
5 The fan blades and tail are cut from pieces of sheet metal. The blades
are then run through a machine that gives them a slight curve. They are
attached to the metal rim with small bolts and metal clamps. They are
attached in such a way that they can be raised or lowered depending on
the wind conditions.
Preparing the site
- 6 Finding and preparing the construction site is a crucial step in creating a functional windmill. First, an area with a prevailing wind of at least 15 mph (24 km/hr) is needed. Then the area needs to be cleared of trees and other structures that may block wind. In some cases, a dirt mound or concrete base is erected to raise the windmill off the surface to catch more wind.
- 7 The parts of the main body are connected first. They are bolted together on the ground and then raised up vertically. The outer poles are joined with the connecting rods. Clamps are bolted at each joint for stability. After the tower is raised it is loosely bolted to the solid base. Next stay wires are strung from the frame down to the ground and attached to tensioners and ground anchors. When the structure is level, the bolts are tightened and the structure integrity is tested. In some cases a ladder is built into the frame design to allow access to the fan on top which makes cleaning an maintenance easier.
- 8 The fan wheel, gearbox, and main shaft are next attached. The gearbox is first clamped and bolted to the top of the tower. The main shaft is then inserted into the bottom of the gearbox. Next, the fan and its attached axle are connected to the gearbox. Finally, the tail section is attached to the gearbox. The pump is then hooked up to the main shaft and the windmill is operational.
Various tests may be done to ensure that each part of the windmill meets the specifications laid out in the design phase. The most basic of these are simple visual inspections. These will catch most of the obvious production flaws. Since windmills are erected by hand, the quality of each part goes through an additional visual inspection. The quality of workmanship that goes into construction of the windmill will be primarily responsible for the quality of the finished product. To ensure that it remains efficient during operation, regular maintenance checks are necessary.
Windmills have changed little over the last hundred years. In fact, one basic design conceived in the 1870s is still sold today. The major improvements have come in the types of materials used in construction. This trend will likely continue in future windmill products. However, the future of harnessing wind power is not in traditional windmills at all. The United States government has spent millions of dollars researching and developing wind turbines for electricity generation. In California, numerous wind farms are already in operation. Various other states and cities have plans for creating similar wind farms. In the future, wind power promises to be an environmentally friendly substitute for fossil fuels.
Where to Learn More
Baker, T. Lindsay. North American Windmill Manufacturers' Trade Literature. University of Oklahoma Press, 1998.
Clegg, Alan John. Windmills. Horseshoe Publications, 1995.
Hills, Richard L. Power from Wind: A History of Windmill Technology. Cambridge University Press, 1994.
Hooker, Jeremy. In Praise Of Windmills. Circle Press Pubns, 1990.
Watts, Martin. Water and Wind Power. Shire Publications, 2000.