An alternator always needs a prime mover to rotate it. In all kind of power plants where alternator is used to generate electricity, there is a requirement of rotating the prime mover of alternator by any means. In thermal power plant the prime mover is rotated by steam produced in boiler. In hydral power plant the prime mover is rotated by electric current of water flow. Similarly in wind power plant Wind turbines use kinetic energy, presents in wind to rotate prime mover of alternator (generator) to make electricity. When a sufficient wind hits the blades of these turbines, they rotate and shaft of the turbine rotor is so coupled with alternator by means of gear it also make rotating the alternator's rotor. Hence the alternator associated with the turbine generates electricity. In this way the kinetic energy present in the wind is first converted into mechanical energy then ultimately in electrical energy via alternator. In other words, Wind turbine is coupled with generator and next the energy travels along the shaft and drives generator where the electricity is produced. In daily life we see that in many place if we switch on the fan, it gives air that means electrical energy is converted to the mechanical energy and then air is produced by means of rotating blades of fan. Wind turbine also works on the same principal but in reverse order that means first the wind hits on blade of turbine then turbine produces mechanical energy after that this mechanical energy is convert into electrical energy through generator.
Working of Wind Turbine
As we know that wind does not present everywhere at the same speed that's why we set up a wind farm where, there speed of wind is sufficient to move the blade of turbine. Blades rotate when a wind strikes over them and blades are coupled with a rotor. So when blades move, rotor is also moved. In a wind turbine pitch system is present and it controls the speed of rotor. Rotor is connected to low speed shaft which rotates at the speed of 30 to 60 rpm (revolution per minute). This low speed shaft is connected to gear and it boosts up or raises the rotational speed of generator shaft from 30 to 60 rpm to about 1000 to 1800 rpm. This is the speed of common generators. This high speed generator produces electricity.
Apart from this wind turbine also consists controller which determines when to start or stop the machine means we determine some range of wind within that range, turbine is operated otherwise not. Generally wind speed of about 8 to 16 miles per hour for starting the machine and the machine is automatically stopped at about 55 miles per hour.
Anemometer which determines the wind speed and sends regularly information to controller whether the wind speed is high or not. Brake works in emergencies to stop the rotor mechanically, electrically, or hydraulically. Wind turbine also contains wind vane, yaw drive and yaw motor. Their functions are to measure the wind direction, and to adjust up wind turbines to stay in front of wind when the direction of wind changes.
Types of Wind Turbine
There are generally two kinds of wind turbines. Horizontal axis and vertical axis. Horizontal axis is divided as upwind and downwind whereas vertical axis is divided as a drag based and lift based as shown in below.
• Horizontal Axis Wind Turbine or HAWT – Up wind
• Horizontal Axis Wind Turbine or HAWT – Down wind
• Vertical Axis Wind Turbine or VAWT – Drag based
• Vertical Axis Wind Turbine or VAWT – Lift based
In Horizontal Axis Up Wind turbine, the shaft of turbine and alternator both are aligned horizontally and the turbine blades are placed at the front of the turbine that means air strikes the turbine blades before the tower. In the case of Vertical Axis Down Wind turbine the shafts of the rotor and generator are also placed horizontally but turbine blades are placed after the turbine that means the wind strikes the tower before the blades.
If we observe VAWT drag based turbine, the generator shaft is located vertically with the blades positioning up and the turbines are normally mounted on the ground or on a tiny tower. This type is also called the Savonius turbine, after its inventor, S.I. Savonius. In the case of VAWT lift based turbine, the generator shaft is placed vertically with the blade's position is up.
Now days Horizontal axis wind turbines are most popular because of high efficiency. Since the blades always move perpendicularly to the wind, and receive power through the whole rotation.
History of Wind Turbine
Using wind energy is not a new concept. It was being using from long past but for different purposes other than producing electricity. It was long before invention of electricity, the Chinese and Persian people used windmill for pumping water, breaking up grain and sawing lumber etc. It was long before invention of electricity.
There are mainly two types of wind turbine, namely vertical axis and horizontal axis turbine. The first wind turbine was designed as vertical axis where a number of sails attached around the vertical axes produce ration of rotor along the vertical axis of the system. The figure below shows a very old design of vertical axis wind turbine.
After that horizontal axis wind mill was designed in the British Isles, Northern Europe. Horizontal axis wind mills were most popularly utilized in Holland in 14th century. These windmills carried out lots of tasks for example timber milling, pumping water for farming etc. Netherlands is another European country which utilized windmill popularly at that time.
In the late 19th century in the American mid-west farmers came to put your faith in a leaner design characteristic a trestle tower topped by wooden or steel paddle-type blades. Between 1850 and 1970, more than six million mostly small means 1 horsepower or less mechanical output wind turbines were installed in the U.S. only and the most important use was water-pumping and the major purpose were store water for home water needs. In 1891 Danish meteorologist, Poul La Cour designed an electrical output wind turbine replicating the aerodynamic design principles that were used in European tower mills. In Denmark, 1900 the biggest machines were on 24 meter (79 ft) tower with four-bladed 23 meter (75 ft) diameter rotors and generating 30 MW. In the 1920s, wind generated electrical systems began to follow the design of airplane propellers and monoplane wings.
In 1950’s the world’s first alternating electric current wind turbines comes in the picture and credit goes to Johannes Juul, he is the best student of Paul La Cour (great scientist and known as his work on wind power). After that John Brown & Company in 1951, developed a first convenience grid-connected wind turbine which operated in the UK (United Kingdom).
An over view about wind turbines. The figure given below contains some components of wind turbines.