A wind turbine is a device that converts wind power into mechanical energy. This energy may in turn be used for pumping water, grinding grain or generating electricity. The first two types appeared in Iran and Afghanistan as early as in the 7th century, eventually making it to Europe in the middle ages. It was on these that the first wind generator was based when it was designed in 1887, though up until about 1980 it was only really used in remote locations or areas with scattered populations. It was not until Denmark - one of the countries previously reliant on windmills to a not insignificant degree - begun manufacturing modern turbines around 1980 that they came to be more widely used all over the world.
Present-day wind generators belong either to vertically or horizontally rotating turbines. The horizontally rotating type, largely based on old-fashioned windmills, is most commonly employed for electricity generation. They tend to have their main rotor shaft and electrical generator fitted at the top of a tower. They are often also fitted with a wind sensor and servo motor that serve to align the turbines with the wind. Normally the blades are placed upwind of the tower to avoid the turbulence behind it. Blades are thus made strong and stiff and sometimes even tilted forward so as to prevent them from being pushed into the tower by heavy winds. As an added measure against such mishaps the blades are placed a considerable distance away from the tower. Downwind version need neither of these, nor a mechanism for controlling their alignment, but as repeated turbulence exposure may lead to unnecessary wear and tear there are not many wind turbines of this type.
The large wind farm turbines of modern days are an improved version of the horizontally revolving turbines. These sit at tubular steel towers between 60 and 90 meters tall and sport blades between 20 and 40 meters long that can rotate between 10 and 22 times per minute, or at a speed of over 320 kilometres per hours. A gear function is generally used to control the pace of the rotation, whereas the motors pointing the turbines into the wind are computerized. Unlike their predecessors they feature brakes and other protection systems against heavy winds that serve to slow or even stop the rotation of the blades, thereby reducing the strain upon them.
Horizontally rotating wind turbines are usually positioned at places of high wind power density; that is to say the mean annual power available per square meter of its swept area in terms of wind velocity, air density and height of the turbine. They are rarely placed in densely populated areas, instead tending to be features of countryside scenery or, in the case of offshore turbines, the middle of the ocean.
Vertical axis turbines instead have a motor shaft arranged vertically, which allows the turbine to be effective regardless of wind direction. This is beneficial in places of highly variable winds, such as on high buildings in cities or larger towns. The low rotational speed and subsequently lesser efficiency is a drawback of these models, but on the flip side they can be placed nearer the ground or rooftop, making maintenance an easier task. That said they are far from as common as their taller cousins, which in all likelihood comes down wind power being significantly hampered in city environments.
Both types of wind turbines have increased in size over time because of the greater energy yield that such improvements have resulted in. Bigger yet are currently under development, promising greater energy production per unit in the foreseeable future.