what is a windmill

What is a Windmill?

A windmill is a structure used to harness the power of the wind for purposes like grinding grain, pumping water, and generating electricity.

The power of the wind was first harnessed by sailors, who were able to understand lift and harness the winds power through sails. This knowledge led to the development of the first vertical axis sail-type windmill used by the ancient Persians and Chinese for grinding grain and pumping water.

They consisted of vanes called sails or blades that when prompted to turn by the wind, converted the wind’s energy into rotational energy that could be utilized.

Early European windmills with horizontal axis systems were the foundation for current wind turbine technology used for energy production.

This article is going to discuss the history of windmills and how they advanced to modern designs, as well as how they work.

Early History of Windmills

There is no concrete evidence on who exactly was the first to invent the windmill, whether it was the Chinese or the Persians is up for debate. Regardless, both cultures began using this technology around the same time for the same purposes.

Some believe that the windmill technology was brought to Northern Europe as a result of the crusades, however their horizontal axis design rather than the vertical axis system, makes it just as likely that the Europeans discovered their windmill independently. The first existing illustrations from 1270 A.D. show blue-prints of the post mill type of windmill.

The post mill was composed of a four bladed mill mounted onto a central post, which used wooden cog-and-ring gears to covert the motion of the horizontal shaft to vertical motion that turned a grind stone. According to the Illustrated History of Wind Development, the wooden cog-and-ring gear was used by Vitruvius, an engineer of the Augustan Age, to develop the first horizontal axis water wheel.

The tower mill design is believed to have come into existence around the late 1300s, with the earliest known illustration being of a Normandy Mill between 1430 and 1440. The tower mill was made with sloping walls, a cap that could be rotated, a horizontal wind shaft, and vertical sails.

The smock mill, developed by the Dutch in 1526 based on the tower mill, is a vertical tapered tower with four to six sides topped with a cap that rotates to bring the sails into the wind. The smock mill was named from its resemblance to the smocks worn by farmers at the time.

Over the next 500 years windmills gained many diverse applications beyond just water-pumping  and grinding grain including irrigation, drainage pumping, saw milling of timber, and processing tobacco, spices, cocoa, paints, and dyes.

Modern Advances

Mechanical water pumping mills were first developed in the United States in 1854. These mills originally consisted of four wooden blades, with steel blades coming into existence in 1870.

More than six million mechanical wind mills were installed in the United States between 1850 and 1970. Their main applications were for stock watering and water for farm houses. Very large windmills were used to pump water for steam trains.

The first windmill to generate electricity was invented in 1888 by an Ohio-based engineer named Charles Brush. The windmill could generate up to 12 kilowatts of electricity and was a sixty foot post mill with a 56 foot rotor.

In today’s society windmills used to generate electricity are referred to as wind turbines.

The wind turbine market evolved from the 1 to 25 kilowatt range for domestic and agricultural practices, to intermediate-scale machines in the 50 to 600 kilowatt range for interconnected utility wind farm applications between the years of 1973 and 1986.

Over this time period, more than 20 different designs where engineered and tested, most proving to be unfeasible and inefficient.

Wind turbines evolved from the four bladed systems first created in the United States, to two bladed systems, and finally to the three bladed systems commonly seen in wind farms today.

How Windmills Work

The earliest documented windmills were vertical axis systems, with vertical sails made from reeds or wood that was attached by horizontal struts to a vertical shaft with a stone for grinding purposes. The first windmill to appear in Europe was the post mill with a horizontal axis system, with a four-bladed mill attached to a central post.

Mostert’s Mill describes the post mill as a building balanced and pivoted on a vertical central post kept upright by crosstrees and angled quarter bars, whose ends carried four upright stone or brick pillars.

The mill consisted of a wallower, a large brake wheel located on the same shaft as the sails, which transferred power to a smaller gear. The wallower shared the vertical shaft with the great spur wheel, from which a stone nut drove the grinding stone.

The tower mill was an advancement of the post mill having multiple floors for storing grain, removing chaff, grinding grain, and living quarters for the windsmith and his family.

The most important feature of the tower mill was a cap (roof) that could pivot in response to changing wind patterns. Unlike the post mill where you had to move the entire structure to orient the mill, only the cap had to be moved in the tower mill.

Both the tower and post mills were originally designed to be manually oriented into the wind by pushing a lever located on the back of the mill. In 1745, Edmund Lee invented the windmill fan tail, which was mounted on the rear of the mill at right angles to the sails automatically turning the cap to bring the sails into the wind.

Smock mills are similar to tower mills differing mainly in their appearance, in which smock mills were octagonal or hexagonal with their six to eight sides, rather than circular. Smock mills have the same rotating cap design as the tower mills, but were typically much larger.

The first mechanical mills that appeared in the United States had four wooden blades that resembled paddles, and most were equipped with “tails” that oriented them into the wind. However, some of these mechanical mills were designed to operate downwind, called weather-vaning mills.

Some mechanical mills sported speed control provided by hinged blades that folded back in high winds reducing the thrust, by reducing the rotor capture area. When steel blades where introduced they required a reduction gear to compensate for their high speed, so that the standard reciprocal pumps were at the right speed for the mill to function properly.

The three-bladed turbine systems commonly seen today, function in much the same way as the original post mills. Wind turns two or three blades around a rotor that is connected to a main shaft that spins a generator to create electricity.

The rotor, which is the blades and hub combined, are mounted 100 feet or more above the ground to take advantage of faster, less turbulent wind. The rotor is attached to a pitch system that turns the blades out of the wind to control the rotor speed.

When winds are too high or too low to produce electricity the pitch system prevents the rotor from turning.

The pitch system is connected to a low speed shaft that turns at about 30-60 rotations per minute. The low speed shaft is connected to the gear box which turns the low rotational speeds into the high rotational speeds of 100-1800 rpms required by the generator, which is attached to the gearbox, to produce electricity.

Today’s turbines also consist of brake system that can mechanically, electrically, or hydraulically stop the rotor in emergencies, and a controller that starts the machine up at wind speeds that are between 6 to 16 mph and shuts the machine off when wind speed are above 55 mph to prevent damage. An anemometer measures the wind speed and transmits the data to the controller.

The electricity is then transferred down the tower and converted to the same voltage as the local energy grid.

Conclusion

Windmill’s have come a long way since there conception, and modern advances in turbine technology are proving wind power to be an environmentally friendly substitute for fossil fuels.

In the 21st century we are starting to see many advances in the wind power industry including bladeless technologies and generators that do not require a gear box. But they are still in their infancies.

Wind power is one of the best solutions to the world’s increasing energy needs, because it is clean and does not produce toxic emissions to generate the electricity, unlike fossil fuels.

If you have experience with windmills and/or modern wind turbines, please share it in the comments.

Resources

Featured Image Credit: Alter Wolf @ Flickr

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