Abstract
Butterfly wind turbine (BWT) is a vertical axis wind turbine (VAWT) of a new concept with multiple looped blades making a double-blade structure. A micro BWT model (diameter: D=0.4m, height: H=0.3m) was manufactured by stereolithography to carry out the wind tunnel experiments. Good self-starting characteristics and poor maximum power coefficients of the model were demonstrated by the experiments. The performance prediction based on the Blade Element Momentum (BEM) theory using Quadruple-Multiple Streamtube (QMS) model showed large difference from the experimental results. To investigate the effects of blade section, two-dimensional CFD analyses were carried out for two double-blade rotors of a symmetry or cambered airfoil and were compared with the BEM-QMS predictions for the 2D rotor of the same size. Although the 2D-CFD analysis could not make clear that the BEM simulation with aerodynamic data of a symmetrical blade in a parallel flow would correspond to the performance of a rotor with cambered blades in curvilinear flow, it showed that the cambered blades will generate more torque than the symmetrical blades at least as for the outer rotor.
