How windmills have evolved By John Traveler

Very long ago, ancient humans discovered they could harness wind and water to do work for them and lessen their toil. “The earliest-known references to windmills are to a Persian millwright in ad 644 and to windmills in Seistan, Persia, in ad 915. “ (Encyclopedia Britannica ) Since then, windmills have been used around the world, most notably to drive gristmills, hence the name “wind-mill.”

The water wheel long preceded the windmill dating well back into the first dynastic period of Egypt. In fact, the waterwheel may well have been the impetus for the windmill, perhaps some waterwheel being observed set in motion by a strong wind. The Sumerians are attributed as the first culture to use wind power to propel boats, but never took the idea much further. Some sources promote the idea the windmill was invented by the Chinese, but the evidence to support this notion is elusive. Nevertheless, by the end of the first millennium of the common era (CE), windmills in one form or another were in operation pretty much around the world.

The Dutch, of course, made windmills famous, from the fourteenth century onward. Early windmills in Holland and Europe were generally of the post type, requiring the whole building to be turned to keep the sails of the windmill facing the wind. In the later tower type, the cap or head turned independent of the tower beneath it; a significant advantage over turning the whole building.

Later in the 18th century, Edmund Lee, an English engineer, invented the fantail (like the vertical stabilizer on the tail of an aircraft) which automatically kept the windmills sails turned into the wind. Both post and tower type windmills were typically outfitted with four sails. In addition to gristmills to turn grain into flower, the Dutch put windmills to work as pumps to keep a rising sea level from flooding their fields with saltwater. Over the next few centuries, the waterwheel would become a much more reliable source of mechanical energy and would replace windmills to run factories up until the introduction of steam power, and the incipience of the industrial revolution.

In the 19th century, as the American frontier pushed westward, a new kind of windmill called a cambered plate bladed rotor windpump became popular, and they are still used today where electricity is not available to pump water from wells. They are most often found on farms and ranches.

The rotor can be 6 feet in diameter or larger, and usually have 10  to 20 pitched blades which catch the wind energy. The rotor shaft is mounted on a crank shaft which thrusts a long rod connected to a positive displacement pump up and down. Each time the rod goes down, water flows up through one way valves to successively higher stages in the pump and is finally expelled at the top.

In 1888 a new type of windmill came on the scene in America, and its proper nomenclature is wind-turbine. Wind turbines are used specifically to turn and alternator or generator to produce electricity. Unfortunately, hydropower and steam turbines were much more reliable than the wind and wind turbines never became too popular.

That is, until the 1980’s when the need for more environmentally friendly methods of producing electricity were needed. Today wind turbines cane be found in growing numbers around the world, anywhere the wind blows regularly and with sufficient velocity to generate electrical power.

Today’s wind turbines are a far cry from any of their windmill predecessors, their sleek designs and 21st century technology tweaked to extract every bit of power the wind can muster, and convert it into megawatts of electrical energy. Summing it all up, windmills have been around for a long time, and wind turbines probably will too.

How is electricity generated from wind?

Humans have been harnessing the power of the wind for at least 4,000 years, the earliest application being the sail boat. Later on, “windmills” were used directly to provide mechanical power to run pumps, grist mills and other machinery. It was not until 1888 that Charles F. Brush built the first wind turbine employing wind power to generate electricity. Almost another hundred years elapsed before the first wind turbine farm was erected at Altimont Pass in California’s San Joaquin valley and wind power became a viable source for commercial electricity generation.

Windmills and wind turbines are essentially the same thing, the former used to produce direct mechanical energy and the latter electrical energy. The principal of operation is really quite simple. The wind passes by the turbine blades, set at an angle (pitch), causing greater air pressure on the side pitched into the wind than on the back side. This air pressure pushes the blade towards the low pressure side. A vertical stabilizer, similar to that on the tail of an airplane, keeps the wind turbine facing into the wind, and the velocity at which the turbine spins is directly proportional to the wind speed. The larger the circumference of the turbine the more energy that can be captured from the wind.

Wind power, converted into mechanical energy imparted to the turbine, is transferred to a shaft at the center or hub of the turbine. Since the circumference of the turbine is greater than the circumference of the shaft, the torque in the shaft is substantially increased while the relative speed at the outside circumference of the shaft is proportionally decreased. This increase in torque is important because the torque is needed to overcome the electromagnetic resistance of the generator or alternator.

Generators and alternators are fundamentally the same device. They both have a center shaft upon which is wound coils of wire known as an armature. Around the armature and separated from it, are two, or in some cases more, coils of wire called field coils. When an electric current passes through the field coils a magnetic field develops. As the armature rotates, driven by the turbine shaft, its wire coils pass through these magnetic fields and a secondary current is induced into the armature windings. In an alternator, the current is drained off directly through devices called slip rings. Alternators produce alternating current (A/C). In a generator, a segmented mechanical switch called a commutator converts A/C into Direct Current (D/C) which is drained off through brushes.

Wind causes the turbine to spin, converting wind power into mechanical energy which is then delivered via the shaft to a generator or alternator coupled to it. Smaller wind turbines usually use a generator, employing an electronic inverter to convert D/C into A/C. Of course, there is some power loss involved, but for a small single home system it’s not a big deal. In commercial applications, alternators are used for a number of reasons, the main one being that the electricity must be in A/C form to be transmitted over existing transmission lines (the power grid). Alternating current for commercial use in the United States and Canada must be held to very tight tolerance maintaining a frequency of 60 hertz, or in Europe and other parts of the world 50 hertz. Maintaining this tolerance requires a more complex design for commercial wind turbines.

Essentially, the frequency of the A/C electricity produced is directly controlled by how fast the alternator spins. Since the coupling between alternator and turbine is mechanical, the alternator rotational velocity is directly proportional to how fast the turbine is spinning, and the turbines velocity, as reported earlier, a direct attribute of wind velocity. Since wind speed can not be controlled, some other method of adjusting the turbines rotational velocity is required. This is accomplished by varying the pitch of the turbine blades to offset fluctuations in air pressure, thus governing the turbines rotational velocity quite effectively and maintaining a constant frequency at the alternator output. As wind speed increases, blade pitch is decreased, and as wind speed decreases, blade pitch is increased. Further speed buffering is provided when larger diameter turbines are used, because like a flywheel, the turbine builds up a significant amount of centrifugal force that resists changes in rotational velocity.

Today, wind farms consisting of numerous wind turbines are popping up all over the country side and around the world, wherever the wind blows regularly and with enough force to make electrical power generation from wind power viable.

By John Traveler,