How do wind turbines work?

Aerodynamic operating principles of wind turbines

According to the basic aerodynamic operating principles of a horizontal-axis wind turbine, wind passes over both surfaces of the airfoil-shaped blade. It passes more rapidly over the longer (upper) side of the airfoil, creating a lower-pressure area above the airfoil. The pressure differential between top and bottom surfaces results in a force, called aerodynamic lift. In an aircraft wing, this force causes the airfoil to rise, lifting the aircraft off the ground.

Because the blades of a wind turbine are constrained to move in a plane with the hub as its center, the lift force causes rotation around the hub. In addition to lift force, a "drag" force, perpendicular to the lift force, impedes rotor rotation. A prime objective in wind turbine design is for the blade to have a relatively high lift-to-drag ratio. This ratio can be varied along the length of the blade to optimize the turbine's energy output at various wind speeds.

Basic principles of wind turbine power production

The output of a wind turbine varies with the wind's speed through the rotor. The "rated wind speed" is the wind speed at which the "rated power" is achieved and generally corresponds to the point at which the conversion efficiency is near its maximum. In many systems, the power output above the rated wind speed is mechanically or electrically maintained at a constant level, allowing more stable system control.

At lower wind speeds, the power output drops off sharply. This is explained by the Cubic Power Law, which states that power available in the wind increases eight times for every doubling of wind speed (and decreases eight times for every halving of wind speed).

Using the power curve, it is possible to determine roughly how much power will be produced at the average or mean wind speed prevalent at a site. In the example above, the turbine would produce about 20 percent of its rated power at an average wind speed of 15 miles per hour (or 20 kilowatts if the turbine was rated at 100 kilowatts). This is somewhat lower than most modern wind turbines.