Optimizing Small Wind Turbine Performance in Battery Charging Applications

Many small wind turbine generators (10 kW or less) consist of a variable speed rotor driving a permanent magnet synchronous generator (alternator). One application of such wind turbines is battery charging, in which the generator is connected through a rectifier to a battery bank. The wind turbine electrical interface is essentially the same whether the turbine is part of a remote power supplyfor telecommunications, a stand-alone residential power system, or a hybrid village power system, in short, any system in which the wind generator output is rectified and fed into a DC bus. Field experience with such applications has shown that both the peak power output and the total energy capture of the wind turbine often fall short of expectations based on rotor size and generator rating.Other authors have discussed the performance limitations of permanent magnet wind turbine generators (WTG) in battery charging applications, which are caused by the poor match, over most of the operating wind speed range, of the rotor, generator, and load characteristics [German 1984, Lawrance et al. 1994, Seale 1983]. In this; paper, we present a simple analytical model of the typical windgenerator battery charging system that allows one to calculate actual power curves if the generator and rotor properties are known. The model clearly illustrates how the load characteristics affect the generator output. In the second part of this paper, we present four approaches to maximizing energy; capture from wind turbines in battery charging applications. The first of these is to determinethe optimal battery bank voltage for a given WTG. The second consists of adding capacitors in series with the generator. The third approach is to place an optimizing DC/DC voltage converter; between the rectifier and the battery bank. The fourth is a combination of the series capacitors and the optimizing voltage controller. We also discuss both the limitations and the potential performance gainassociated with each of the four configurations.