Wind power describes the process by which wind is gused to generate electricity. Wind turbines convert the wind’s kinetic energy into mechanical power.
Wind is caused by the uneven heating of the atmosphere by the sun, variations in the earth’s surface, and rotation of the earth.
Mountains, bodies of water and vegetation influence wind flow patterns.
Wind turnbines convert the wind’s energy into electricity by rotating propeller-like blades around a rotor.
The rotor turns the drive shaft, which turns an electric generator.
Three key factors affect the amount of energy a turbine can harness from the wind: Wind speed, Air density, Swept area (the size of an area through which the rotor spins)
Equation for Wind Power
- Wind speed
The amount of energy in the wind varies with the cube of the wind speed, in other words, if the wind speed doubles, there is eight times more energy in the wind.
Small changes in wind speed have a large impact on the amount of power available in the wind.
Density of the air
The more dense the air, the more energy received by the turbine. Air density varies with elevation and temperature. Air is less dense at higher elevations than at sea level, and warm air is less dense than cold air. All else being equal, turbines will produce more power at lower elevations and in locations with cooler average temperatures.
Swept area of the turbine
The larger the swept area (the size of the area through which the rotor spins), the more power the turbine can capture from the wind.
Since the swept area A is proportional to the square of the rotor radius r, a small increase in blade length results in a larger increase in the power available to the turbine.
Wind turbines can be separated into two basic types determined by which way the turbine spins. Wind turbines that rotate around a horizontal axis are more common (like a wind mill),
while vertical axis wind turbines are less frequently used (Savonius and Darrieus are the most common in the group).
Horizontal axis wind turbines have the main rotor shaft and electrical generator at the top of a tower, and they must be pointed into the wind. Small turbines are pointed by a simple wind vane placed square with the rotor (blades), while large turbines generally use a wind sensor coupled with a servo motor to turn the turbine into the wind. Most large wind turbines have a gearbox, which turns the slow rotation of the rotor into a faster rotation that is more suitable to drive an electrical generator. In the last few years, direct drive generators have been adopted to simplify the nacelle systems, increase reliability, increase efficiency and avoid gearbox issues.
Since a tower produces turbulence behind it, the turbine is usually pointed upwind of the tower. Wind turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted up a small amount.