Experimental study of a small wind turbine for low- and medium-wind regimes

The results of an experimental assessment of a small prototype battery charging wind turbine designed for low- and medium-wind regimes are presented. The turbine is based on a newly designed axial flow permanent magnet synchronous generator and a three-bladed rotor with variable twist and taper blades. Overspeed control is performed by a furling mechanism. The turbine has the unique feature of being capable of operating at either 12, 24 or 48 V system voltage, requiring no load control in any case. In the 48 V configuration, the system is capable of providing 2 kWh day(-1) for an average wind speed as low as 3.5 m s(-1) and an air density of 85% of the standard pressure and temperature value. The experimental assessment has been conducted under field conditions with the turbine mounted on a 20 in guy-wired tubular tower. The experimental power curves are shown to be in good agreement with a detailed aerodynamical and electromechanical model of the turbine for non-furling conditions and for wind speeds above the theoretical cut-in speed. In the case of the rapidly spinning load configurations, a finite power production at wind speeds below the theoretical cut-in speed can be observed, which can be explained in terms of inertia effects. During the measurement campaigns with high loads, we were able to observe bifurcations of the power curve, which can be explained in terms of instabilities arising in situations of transition from attached to separated flow. A full experimental C-p(lambda)-curve has been constructed by operating the turbine under different load conditions and the findings are in good agreement with a variable Reynolds-number blade-element momentum model. The three proposed system configurations have been found to operate with a high aerodynamic efficiency with typical values of the power coefficient in the 0.40-0.45 range. Copyright (C) 2009 John Wiley & Sons, Ltd.