A Blade Element Estimation of the Cut-in Wind Speed of a Small Turbine

Abstract

As the wind speed increases from zero, small turbines will cut-in only when the aerodynamic torque acting on the blades exceeds the resistive torque of the generator and drive system. This Note describes the use of standard blade element theory to calculate the cut-in wind speed, at which rotation commences, using the apparent near-universality of aerofoil lift and drag at high incidence. The starting torque is shown to depend on the “chord-pitch” integral that, in turn, is a function only of the blade geometry. This integral is evaluated for two small blades designed by the author for which the cut-in wind speed has been determined. In both cases, the calculations considerably over-estimate the actual cut-in speed and some possible reasons for this are advanced.