Predicting Double-Blade Vertical Axis Wind Turbine Performance by a Quadruple-Multiple Streamtube Model

Abstract

Abstract Double-blade vertical axis wind turbines (DB-VAWTs) can improve the self-starting performance of lift-driven VAWTs. We here propose the quadruple-multiple streamtube model (QMS), based on the blade element momentum (BEM) theory, for simulating DB-VAWT performance. Model validity is investigated by comparison to computational fluid dynamics (CFD) prediction for two kinds of two-dimensional DB-VAWT rotors for two rotor scales with three inner-outer radius ratios: 0.25, 0.5, and 0.75. The BEM-QMS model does not consider the effects of an inner rotor on the flow speed in the upwind half of the rotor, so we introduce a correction factor for this flow speed. The maximum power coefficient predicted by the modified BEM-QMS model for a DB-VAWT is thus closer to the CFD prediction. Keywords : Wind turbine, Double-blade rotor, VAWT, BEM, CFD, QMS. 1. Introduction Vertical axis wind turbines (VAWTs) offer lower costs due to the simplified mechanics, because the wind direction is unimportant to performance. However, small-scale lift-driven VAWTs typically have poor self-starting performance [1]. Therefore, a variety of steps have been tried to improve self-starting performance: increasing blade chord length, adding more blades, using drag-driven wind turbines as a starter, and designing blade airfoils for high aerodynamic performance (for example, [2]). Double-blade vertical axis wind turbines (DB-VAWTs), such as the one shown in Fig. 1 (a), are an example for improving self-starting performance. DB-VAWTs can produce larger starting torque than conventional single-rotor VAWTs with the same blade chord length. Figure 1(b) shows a new type of VAWT with multiple looped blades forming a double-blade rotor structure, as in a DB-VAWT. This VAWT was named the butterfly wind turbine (BWT) [3] because as it rotates around the vertical axis it has the shape of a butterfly. The BWT may offer reduced costs, good self-starting performance, high energy efficiency, and good vibration handling because of the direct installation of the armless looped blades on the central axis. Although propeller-type horizontal axis wind turbines (HAWTs) are the primary turbines for large-scale wind power generation, large-scale VAWTs have been recently proposed and developed for offshore wind power because of their low operational and maintenance costs [4]–[10]. A large-scale VAWT requires struts or arms that connect the blades and central rotating axis, and the cross-section of the struts should also be airfoils to reduce drag. These airfoil struts produce lift force if they incline against the horizontal plane, and so the structure can be regarded as a partial double-blade rotor. Since the flow field of a double-blade rotor is very complex, it is difficult to predict performance by using conventional blade