Post mill: Oldland Mill, West Sussex. Wikimedia Commons

Wind power is as old as the oldest sail boats–thousands of years old.

About a thousand years ago, the Persians put half a dozen or so sails together radiating from a common vertical axis.

When the wind blew, the entire contraption rotated and provided power for pumping water or grinding grain.

The earliest European windmills (from ca. 1200) appear to have been the more familiar horizontal-axis type.

They more nearly resembled water wheels than sails, but performed basically the same functions as the Persian model. The horizontal windmill is a much more efficient technology than vertical ones, but the earliest ones had two problems to overcome: stability and pointing the blades into the wind.

Post mills

A simple post mill comprises a wooden rotor mounted on a wooden post. A mill large enough to accomplish any real work is top-heavy and can easily blow over in a strong wind. Soon enough the posts were stabilized by trestles of assorted designs, which eventually rested on raised brick piers to keep the foundation from rotting. It was a fairly uncomplicated evolution from a simple trestle to a mill house useful for storage and protecting the machinery from the weather.

A horizontal windmill must face the wind in order to operate, and wind can come from any direction. Therefore, it requires some kind of beam attached to the post in order to turn it. As mill houses replaced the simpler trestle, they required considerable muscle (often supplied by animals) or some sort of other machinery to rotate the entire body.

Tower mills

Tower mill: Molen De Noord in Schiedam. Wikimedia Commons

Why turn the whole structure if you can detach part of the roof and simply rotate that? Although the post mill remained the most common kind in Europe until the 19th century, tower mills, where only the cap need be turned, started to appear as early as 1295.

Freeing the post from the supporting structure enabled tower mills to become larger and more powerful than ever before. Not only that, the mill house no longer needed to be wooden.

Brick or stone mill houses could better withstand the weather and, no less important, allow more artistic creativity in windmill design.

With the invention of the windmill fantail in England in 1745, it was no longer necessary to turn the blades manually. The fantail, a structure mounted behind the blades at a right angle, used the power of the wind itself to turn the cap automatically. Although the tower mill represented a tremendous technological advance over the post mill, it was much more expensive to build. No one would ever erect a tower mill where a post mill provided adequate power.

At the same time the outer structure of windmills developed from the simple post to the tower, the blades themselves underwent their own evolution. By 1800, they were designed to generate aerodynamic lift, which enabled them to turn the rotor faster. Although still mostly wooden, they developed important features critical to the working of modern wind turbines:

• The width of the blade is turned so that one side, the leading edge, faces the wind the the other side trails one quarter the width of the blade behind the leading edge.
• The center of gravity is located at the same “quarter chord” position.
• The leading edge has a slightly arched shape called a camber.
• The entire blade has a slight and non-linear twist along its length.

Throughout their entire evolution up until the 19th century, European windmills had four blades. The classic windmill of the American planes had many more than that. Modern wind turbines have three. Over the past thirty years, they have grown from 25′ blades to 125′ blades. By 2015, blades will be mass produced so large that a large passenger plane could easily fit between them. Increasing the size of windmill blades has reached a point of diminishing returns. Engineers have begun to experiment with entirely new shapes. It could be that a concept documented as early as the 13th century is finally coming to the end of its development.

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