A windmill is a machine or structure which converts wind energy into rotational energy with the help of blades known as sails. From over the period of time, the energy which is produced by windmill is used to grind grain into flour. Windmills also provide energy to windpumps, hammermills, saw mills and paper mills for obtaining fresh water from underground.
Windmills are made by skillful people known as craftsmen and can be constructed on site by using hand tools. During the period of eighteenth century, windmills developed very steadily and made their presence in Europe. In current scenario windmill technology is at its best and the wind turbine promises to be a vital alternative to fossil fuels in future.
Before the twelfth century, windmills were not known in Europe, but afterward they became famous landmarks in England, France, Germany and Holland. In Holland, a windmill is also called as a tower type which is a huge tower made up of stone, brick or wood; on the other hand German mills are called as post mills which revolve around a single central post and at the top it has rotating device to which 4-6 arms are attached. The arms are usually six-twelve meters long which are constructed by light wood or a canvas attached to frame and a small fan serves as a rudder to keep wheel facing to the wind.
Windmill involves various parameters like-
- Location- Windmill should be constructed near seacoast, offshore islands or coasts of very large lakes as these are sources of wind energy.
- Wind characteristics- While constructing a windmill one should have to consider mean wind speed and its seasonal variations.
- The probability distribution of wind speed and extreme winds- The mean wind speed should be high and the distribution must be so that all the data points are similar.
- The height difference of wind speed and wind way- Wind cannot be too high or too low in relation to the ground.
- The gustiness of the wind field in both speed and direction- Gusty winds are harmful as they affect power output of windmills.
- The wind way, sharing and probability of unexpected large shifts in direction- The wind should blow in a same general direction.
- The seasonal density of the air and variation of density of the air with height- If density of the air is high then it will affect windmill.
- Hazardous conditions such as sandstorms, humidity, and salt-spray are very bad to windmills.
- Channeled wind through mountain and trade winds in subtropics are beneficial to windmill.
Efficiency of windmill depends on number of blades because more blades capture more energy but large windmills must use few blades due to stress on the metal. When diameter of rotation is 300 ft, two blade systems must be used and when diameter of rotation is 150 ft; three blade systems must be used. Over sea coast blades must be large and few in number but in smaller places it should be small and more in number.
Overall efficiency means the amount of electricity that can be generated over time on a cost basis. Two factors are very important in determining overall efficiency of a windmill one is its ability to use low velocity wind and second is conversion ability. The minimum wind speed required by windmill is more than 15 mph for production of electricity. Conversion efficiency requires more than two blades which allows lower rpm and results in efficient conversion.