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

Abstract

SUMMARYThe 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 À1for an average wind speed as low as 3.5 m s À1 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 m 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 (l)-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.

Design, application, and power performance analyses of a micro wind turbine

“…They reached the conclusion that SWTs with blade diameter of over 3 m have higher AEPs. Elizondo et al [24] executed power performance analyses of their own designed MWT giving power of 1.4 kW at 10.5 m/s wind speed by using truck and the field tests.…”

“…In this case, the power coefficient of the MWT reaches the maximum value. The TSR is defined by [6,24,30]:…”

“…The power obtained from a MWT at a certain wind speed is called "nominal power" and the wind speed is called the "nominal wind speed". Ac Power performance analyses of manufactured MWTs can be carried out by truck [24], field [25], and tunnel [26] tests. Though the truck and the tunnel tests can be performed in a short time, the field test takes 1-2 years.…”

Small Wind Turbine Technology

“…Most small wind turbines are variable-frequency devices, allowing for an optimal operation at all wind speeds below the threshold for the onset of the overspeed and power control mechanism. While in the case of battery-charging applications the use of a passive rectifier together with the selection of an appropriate voltage level may be sufficient to maintain the operating point close to the system optimum, especially when the aerodynamic efficiency curve (section 6) is broad (Elizondo et al, 2009, some commercial systems rely on the use of an active load control in order to maintain the system at the optimal operating point for each wind speed (Martínez et al, 2006). In the case of a coupling to the electric grid, a full wild AC/fixed frequency AC conversion is generally feasible through the use of a back-to-back AC/DC/AC converter, as opposed to large wind turbines where a direct full conversion is still rather the exception than the rule and most commercial large wind turbines rely on doubly fed induction generators (DFIG) where only a fraction of the total power is passed through a converter.…”

“…3, where the experimental results of the aerodynamic power coefficient C p vs. the tip speed ratio (TSR) λ of a turbine rated at 1.4 kW (swept diameter 3m), obtained from a field characterization, have been plotted together with the predictions of a mathematical model of the turbine. The experimental data was obtained by operating the turbine under different controlled load conditions, including direct connection to a battery bank with a voltage of 48V, 24V, or 12V; to provide higher load conditions, the 12V battery bank was shunted with a resistance whose value was varied from 2.1Ω to 1.1Ω (Elizondo et al, 2009). It can be seen that for low values of the tip speed ratio all power coefficient values fall onto a universal curve, while for higher TSR values a greater spread between the recorded values exsist, as predicted by the mathematical model based on a combination of a Blade Element Momentum (BEM) and an electromechanical model of the generator/rectifier.…”