Determination of the Maximum Aerodynamic Efficiency of Wind Turbine Rotors with Winglets

The present work contains theoretical considerations and computational results on the nature of using winglets on wind turbines. The theoretical results presented show that the power augmentation obtainable with winglets is due to a reduction of tip-effects, and is not, as believed up to now, caused by the downwind vorticity shift due to downwind winglets. The numerical work includes optimization of the power coefficient for a given tip speed ratio and geometry of the span using a newly developed free wake lifting line code, which takes into account also viscous effects and self induced forces. Validation of the new code with CFD results for a rotor without winglets showed very good agreement. Results from the new code with winglets indicate that downwind winglets are superior to upwind ones with respect to optimization of Cp, and that the increase in power production is less than what may be obtained by a simple extension of the wing in the radial direction. The computations also show that shorter downwind winglets (>2%) come close to the increase in Cp obtained by a radial extension of the wing. Lastly, the results from the code are used to design a rotor with a 2% downwind winglet, which is computed using the Navier-Stokes solver EllipSys3D. These computations show that further work is needed to validate the FWLL code for cases where the rotor is equipped with winglets.