Numerical Investigation on the Effect of the Azimuthal Deviation on Performance of Equal Speed Co-Rotating Double Rotor Small-Scale Horizontal-Axis Wind Turbine

Abstract

A double-rotor setup is a promising approach to increase wind turbine power extraction. One common double-rotor configuration is the co-rotating-equal-speed arrangement. The performance of this setup is affected by the azimuthal deviation between the two rotors, which remains constant during rotation. To identify the impact of azimuthal deviation, a numerical investigation was conducted using 10 m/s input wind speed and 0.9 m turbine diameter. The separation distance between the two rotors varied for two values of 0.14 and 0.25 rotor-diameter. The power coefficient of both rotors and the overall turbine were analyzed at different azimuthal deviations using Reynolds-averaged Navier–Stokes k-ω SST equations. The azimuthal deviation was positive when the front rotor preceded the rear and negative when the rear preceded the front. At 0.14 rotor-diameter separation, positive deviation increased the front rotor power coefficient but decreased the rear’s, while negative deviation had the opposite effect on both rotors. The maximum changes in the power coefficient of the front and rear rotors at tip speed ratio of 5 were ΔC_P=0.058 and ΔC_P=0.066, respectively. However, the net harvesting power by the double-rotor wind turbine exhibited slight marginal change of ΔC_P=0.008 at a tip speed ratio of 5. In contrast, at the greater separation, the power coefficients of both rotors and the overall turbine showed slight change with the variation of the azimuthal deviation with a marginal change of ΔC_P=0.012 at a tip speed ratio of 5. Moreover, the highest increase in the power coefficient was 15% compared to single rotor.

Keywords

• wind turbine
• small horizontal axis wind turbine
• double rotor
• tip speed ratio
• power coefficient