Effects of the Airfoil Section, Chord and Twist Angle Distributions on the Starting Torque of Small Horizontal Axis Wind Turbines

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Abstract
Small wind turbines usually suffer from poor efficincy, low power, and lack of public incentives. This study is focused on investigating the effects of the geometry of the airfoil sections and blades on the starting torque and minimum wind speed for energy generation. The blade element momentum theory is used to develop a numerical code where the airfoil S832 is used as a reference for comparison and validation. The investigated parameters include three airfoil sections, Joukowski J9.513, Gottingen GO447, and S832, linear and elliptic chord distributions, linear twist angle distribution, and multiple airfoil sections along the blade. The results show that large local solidity ratio at the intermediate region of elliptic chord distribution produces significant reduction in the local generated torque of about 5–21% and that the linear chord distribution along the blade length increases the torque by about 27–77% and thus permits lower starting wind speeds. For rotors with high solidity ratio as in the case of elliptic chord distribution, the distribution of twist angle for constant angle of attack reduces the generated torque by about 13–19%. The J9.513 airfoil-based rotor shows 20–35% more start torque than the S832 and GO447 airfoils-based rotors. The linear chord distribution shows better results for all the three airfoils-based rotors. The linear twist angle distribution increases significantly the start torque of the rotors with the proposed airfoils sections. The three airfoils S832, GO447, and J9.513 with linear twist angle distribution are viable options for small wind turbines. The J9.513 with linear chord and linear twist angle distribution shows the lowest wind speed for electricity generation. The use of multiple airfoils on the blade length shows marginal improvement of the starting torque.

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