Structure Design and Analysis of a Novel Forward-folding Rotor used in a Downwind Horizontal-axis Turbine

Abstract

To alleviate the stiffness constraints of the conventional blade and thus reduce the rotor mass, a novel forward-folding rotor (Downwind Forward-folding Rotor, DFFR) used in a downwind horizontal-axis turbine is presented. This novel rotor is designed to align the combination of gravitational, centrifugal, and thrust forces along the blade path, resulting in primarily tensile loads instead of cantilever loads on the blades. The DFFR blades fold forward at a power-limited condition, which induces the change of the blade pitch angle and cone angle and thus maintains a constant power output. To quantify the mass savings, a 5-MW DFFR was designed based on the NREL 5-MW reference rotor. According to the results calculated by an improved BEM method, the rotor power and torque of the 5-MW DFFR have a slight increase compared with those of the NREL-5MW reference rotor, while the rotor thrust of the 5-MW DFFR is smaller than that of the NREL-5MW reference rotor. Furthermore, based on the finite element analysis, the blade of the 5-MW DFFR had an 18.9% mass saving and an 8.4% peak stress reduction compared with the blade of the NREL-5MW reference rotor, over a range of operating wind speeds and azimuthal angles.