Performance Optimization for Vertical-Axis Wind Turbines with Blade-Element Method Models

A numerical optimization of the torque generated by a vertical-axis wind turbine was completed using a double-multiple-streamtube momentum model. Aerodynamic force functions were obtained from curve fitting to experimental airfoil data. An optimal pitch function was found such that the torque output of the turbine was maximized. The numerical optimization procedure computes an optimal blade pitch trajectory based on the aerodynamic characteristics of a particular airfoil. This procedure was generalized in order to gain insight into the fundamental behavior regarding the relationship between the airfoil characteristics and turbine performance. The effect of the airfoil polars on the optimized turbine power output was studied and characterized. It was found that turbine performance depended only upon the drag polar rather than the individual lift and drag curves. Therefore, the effect of manipulating the drag polar was characterized by the resultant optimal power coefficient and corresponding tip-speed ratio at which this optimum was obtained for a low solidity turbine.