Design Considerations of Rotor Cone Angle for Downwind Wind Turbines

Due to their potential to use light, flexible blades, downwind turbines are well suited for offshore floating platforms, for which there is a need to substantially lower the cost of wind-generated electricity. However, downwind rotors must operate in the presence of the tower's wakes with which are associated strong changes in flow incidence, and thus high fatigue loads. In order to guide the development of design rules for multimegawatt downwind turbines, a comprehensive experimental study has been conducted to better understand the characteristics of the unsteady rotor torque on downwind turbines. High-frequency measurements of the unsteady rotor torque on a model turbine that can be configured with rotors of different cone angles and operated either downwind or upwind in well-controlled flow conditions are conducted. The measurements show that in the case of the downwind turbine, the blade's passage through the tower's wake accounts for 56% to 61% of the variance of the rotor torque; the proportion of this unsteadiness is independent of the cone angle. For nonoptimum tip speed ratios (TSRs), the increase in unsteadiness is consistently less for downwind configurations than for upwind configurations. For the 5 deg-cone downwind configuration, the increase in rotor torque unsteadiness is 13–18% of the increase observed for the 5-deg-cone upwind configuration for nonoptimum TSRs. Thus from a design perspective, downwind rotor configurations offer above or below rated wind speed, a smaller increase in unsteadiness of the rotor torque compared to upwind turbine configurations. These characteristics differ from upwind turbines, on which broadband vortex shedding from the blade is the primary source of the unsteadiness, which may be reduced by increasing the rotor-tower clearance. It is suggested that given the strong periodic character of the blade's passage through the tower's wake, the turbine control system may be designed to reduce fatigue loads and there is a broader design space on downwind turbines that can be exploited for peak load mitigation by moderately adjusting the blade's stiffness.