Torque maximization of vertical axis wind turbines with variable pitch law

In the last years, an increasing interest in Vertical Axis Wind Turbines has been experienced because of their intrinsic advantages with respect to the horizontal-axis layout, such as the capability to work with omni- directional asymptotic wind with no yaw mechanism. On the other hand, a Vertical Axis Wind Turbine is generally less efficient than a Horizontal Axis Wind Turbine, due to the presence of complex unsteady aerodynamic effects, such as dynamic stall. These phenomena occur when a turbine blade operates at high incidence with steep variations of the angle of attack, increasing the noise and reducing the fatigue life of the turbine components. A valid strategy to delay the dynamic stall, increasing the power, is a dynamic variation of the blade pitch angle. In this paper, an optimization procedure for achieving the optimal variable pitch angle law of a H-Darrieus wind turbine, able to maximize the torque, is presented. Aerodynamic performance was evaluated using an in-house modified version of the open source code QBlade, which implements the Nonlinear Lifting Line Free Vortex Wake method. The optimization is carried out by using a genetic algorithm. The variable pitch law is generated by combining properly up to six harmonics whose fixed frequencies are multiple of the fundamental one, while amplitudes and phases represent the design variables in the optimization process. The results show a considerable increase in the torque by implementing the optimized pitch law compared to a conventional design with no pitch law.