ABSTRACT
It is possible to exceed the Betz limit by placing the wind turbine in a diffuser. If the cross-section of the diffuser is shaped like an aerofoil, a lift force will be generated by the flow through the diffuser as seen in Figure 5.1.
As shown in de Vries (1979), the effect of this lift is to create a ring vortex, which by the Biot-Savart law will induce a velocity to increase the mass flow through the rotor. The axial velocity in the rotor plane is denoted by V2, and ε is the augmentation defined as the ratio between V2 and the wind speed Vo, i.e. ε = V2 /Vo. A 1-D analysis of a rotor in a diffuser gives the following expression for the power coefficient:
