A design study of a 1 MW stall regulated rotor

The main objective of the present work has been to design a 1 MW stall regulated rotor and investigate the potential improvements by using special tailored airfoils. The target rotor should have an improved cost performance compared to existing rotors. Cost performance is the annual production of energy seen relative to the material consumption.

A newly developed numerical optimization tool and an aeroelastic code have been used in the study. Design parameters have been the blade chord, twist, tip pitch angle, angular velocity and airfoil characteristics. The objective function for the optimization has been the annual production of energy in the Danish roughness class 1.

Constraints have been put on mean and extreme blade root flapwise moments, rated power, tip speed and blade geometry. By performing aeroelastic calculations on the optimized designs with different levels of constraints on the mean blade root flapwise moments, an almost linear correlation between the mean blade root flapwise moment and the equivalent fatigue loads appears.

The optimum ratio of rated power to swept area appears to be around 400 W/m2 having the mean flapwise blade root moment constrained to 80%. The maximum annual production of energy has been obtained for the airfoil section maximum lift coefficient, CLmax, being high over the entire blade independent on constrained loads.

The direct improvement from the use of special tailored airfoils has been found to be around 4% on the annual energy production and 1.5% on the material consumption. When the entire rotor geometry is included as optimization design variables, the choice of CLmax becomes less important since the design space is flat in the neighbourhood of the optimum.

Therefore other qualities like roughness insensitivity can be given more attention in the design process at the expense of a specific CLmax. The results indicate a potential improvement of the cost performance of about 11% of which 5.5% can be achieved with the use of traditional airfoils. Whereas the proposed methodology have been very beneficial to constrain the fatigue loads, the extreme loads have not yet been entirely included.