Analysis of design parameters of large-sized wind turbines by non-dimensional model

This paper presents a simplified non-dimensional model to estimate capacity factor for megawatt-sized VSVP (variable speed variable pitch) wind turbines. Instead of cut-in (Vin), rated (Vr) and cut-out (Vout) speeds of a wind turbine a non-dimensional speed parameter (x) defined as the ratio of the wind speed to the mean wind speed (Vm) is employed. Then the non-dimensional speed parameters become: xin (=Vin/Vm), xr (=Vr/Vm), and xout (=Vout/Vm). The wind speed distribution is characterized by Weibull shape parameter k. It is shown that the CF (capacity factor) depends only on k and Vr. In this way the model is independent of the mean wind speed of the site. However, these simplifications does introduce small error in the estimation of the capacity factor, but of little significance. The model shows three important points: 1) There is an optimum relation between xr and k represented by a six-degree polynomial indicating optimum value of the CF. This implies that the design parameters of a wind turbine should be selected on the basis of k; 2) The capacity factor increases with the decreasing value of the xr. This decrease in xr can either be achieved by increasing Vm or by decreasing Vr.