Aerodynamics: turning wind into mechanical motion

The aim of a wind turbine is to transform the kinetic energy in the wind into electrical power. The kinetic energy per time (power) that passes an area, A, perpendicular to the wind velocity and the available aerodynamic efficiency is thus naturally defined as the fraction of the actual produced power to the available power in the so-called power coefficient. To remove kinetic energy from the wind, it is necessary to design a rotor that produces an upstream force, denoted by the thrust T, which reduces the wind speed behind the rotor, as sketched in Figure 1.1. The wind speed through the rotor is gradually reduced from far upstream, Vo, to, u, at the rotor plane and finally to u 1 in the wake. Due to conservation of mass, the streamlines that divide the airflow going through the rotor from the one passing are expanding as shown in Figure 1.1. In order to transform the extracted power into useful work and not simply dissipate it into internal heat, a mechanical torque should also be produced by the rotor as input to the shaft of an electrical generator. Figure 1.2 shows how the aerodynamic loads produce a normal load on the blades to reduce the wind speed and at the same time a tangential load to drive a generator.