Aerodynamics of Clustered Wind Lens Turbines

Wind lens turbines, developed by Kyushu University have increased performance due to the duct surrounding the rotor [1]. A Multi rotor system is a promising concept to upscale wind turbine systems beyond 20MW [2]. Results of experimental research on a seven turbine array have shown no loss in performance with closely spaced rotors [3]. In the presented research we analyze the feasibility of using wind lens turbines in multi rotor systems. In a wind tunnel experiment we placed three wind lens turbines in a triangle array normal to smooth flow. At various spacing maximum output power was measured and compared to the values of a stand-alone setup. Optimum tip speed ratio didn’t change when the turbines are set closely next to each other. In a side-by-side setup of two turbines it was found that the rotation direction of the rotors had no influence on the turbines performance. In the three turbines in triangle arrangement the performance of the turbines is influenced by the width of the gap between the brims. At the closest possible spacing (brims of neighboring turbines are in direct contact) the cumulative power of all three turbines decreases compared to the cumulative power of three stand-alone turbines. The brims of the wind lens turbines shed vortices that establish a low pressure region behind the turbine. When the brims are in direct contact vortex shedding is inhibited and hence power drops. When increasing the gap between the turbines the power output restores to the same value, in some cases the power even exceeds the value of the three stand-alone turbines. Further, it was observed that the individual power output doesn’t follow the trend of the cumulative power output. These phenomena can be explained with flow patterns observed in gap flow analysis of bluff bodies where biased wake flow changes drag coefficients of flat plates [5]. However, smoke wire technique used to visualize the flow field couldn’t give us any indication of these known patterns. Further research is necessary to fully understand which mechanisms in three dimensional gap flow cause the effect on the performance of the turbines.