Increasing the operational capability of a horizontal axis wind turbine by its integration with a vertical axis wind turbine

Abstract

A major difficulty encountered by a horizontal axis wind turbine is the limit of aerodynamic torque that it can withstand at high wind speeds. A novel strategy is proposed to improve the operational capability of a prototype scale system by increasing its rated wind speed for power generation. This is achieved by integrating its drivetrain with that of a vertical axis wind turbine supported on a common tower. Excess torque is transferred from the horizontal axis rotor to the vertical axis rotor’s drivetrain by coupling them using a continuously variable transmission. In this article, firstly, the concepts of motion transfer that facilitate this combined operation are discussed. A combination of a 12-kW horizontal axis rotor and a 10-kW vertical axis wind turbine is studied to estimate the increased benefit of increments in rated wind speed. Performance of this hybrid system is predicted at potential wind sites and is shown to exceed the standalone mechanical power output of both subsystems under different wind regimes. The critical criterion of the system’s aerodynamic feasibility is then investigated. Turbulence modelling is performed for a configuration which involves a combination of the NREL Phase VI rotor and a NACA 0021 profiled vertical axis H-rotor. A 3-D simulation, using a validated k-ω (Shear Stress Transport) computational fluid dynamics model helps confirm the ability of both turbines to operate aerodynamically independent of each other. Further, by this methodology, a safe clearance between the two rotors is pre-determined. Analysis of turbulent flow scenarios reveals the characteristic effects of aerodynamic torque ripple experienced by the vertical axis wind turbine and its impact on combined power output. Parameters outlined in this article will be of assistance in the practical implementation of the integrated axes wind turbine.