Self-starting of a small urban Darrieus rotor

Abstract

Like many vertical-axis wind turbines (vawts) of the Darrieus type, Turby fails to achieve a fully passive start-up. Experiments indicate that there is a band of negative torque—often referred to as dead band in literature—somewhere in the tip speed range 0.5 B l B 1.6. The reason behind this is that, unfortunately, the power coefficient obtained at the rated wind speed is all but constant. The combination of low wind speeds and the small scale of the device leads to very low Reynolds numbers; well in the order of < 105. The lift production and stall behavior of the airfoils aggravates in these conditions, with low starting torque as a result. The design case revolved around Turby – a small 1.6 kW, 2.20 m diameter vawt with a projected cut-in wind speed of 3 m/s. The purpose of this master thesis is to investigate the cause of the poor start-up performance and to find possible solutions for the problem. This eventually led to a design proposal.