Air masses that move are called wind. It is created by radiation from the sun onto the earth. Incident irradiation causes the air to heat up, where air with higher temperature has a lower density. The result is the creation of high and low pressure areas.

Windrad
© Petra Bork / Pixelio

Wind power

The compensation of the pressure drop between high and low pressure area sets air into motion. Wind is therefore a pressure compensation mechanism. Moving air holds kinetic energy, which humankind already harvests since thousands of years (e.g. windmills or sailing boats). The utilization of wind for electricity generation started in the 19th century.

Operating principle

A body situated in a streaming fluid (e.g. air or water) experiences two forces:

  • Resistance force and
  • buoyant force

The magnitude of those forces depends on the density of the fluid, its velocity, the surface of the body acting onto the fluid and a coefficient. The coefficient denotes how strong the forces act on the body. The resistance force “pushes” the body into the direction of the streaming fluid.

The buoyant force (or for wind the lift force) however acts perpendicular to the direction of the fluid. It is the same force that allows planes to fly. When utilizing wind power, two different designs, each of the using one of the two different forces, are distinguished:

Drag turbines and lift turbines

Lift turbines have an advantage compared to drag turbines, as their rotational speed can be higher than the velocity of the streaming fluid. The inflow velocity or velocity of approach is decisive for the rotational speed. The velocity of approach results from the velocity of the wind and the rotational speed.

The force upon a drag turbine acts into the same direction from which the wind approaches. The velocity of approach is therefore the difference of the wind velocity and the velocity of the drag turbine’s moving parts. The maximum efficiency of a drag turbine is roughly 19 percent.

For a lift turbine the force acts perpendicular to the flow, and both velocities are added (vector addition). As the rotational speed is higher, the efficiency is higher as well. That is why the lift turbine has established itself for commercial electricity generation.