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 is 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 J9.513with 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.
Small wind turbines are mainly used in rural areas, farms and isolated communities. These machines are supposed to be able to work within a wide range of wind speeds and usually are not equipped with pitching control mechanisms. The main objective of this study is to investigate Joukowski airfoil as an alternative for use in small power windmills (compared with the Solar Energy Research Institute, SERI airfoil as a reference) and investigate the effects on the aerodynamic performance caused by varying the tip-speed ratio and the number of blades as well as the use of blades composed of multiple airfoils. The SERI airfoils were treated using Xfoil software while the Joukowski airfoils were calculated by analytical transformation and then treated by Xfoil to estimate the aerodynamic characteristics for the different operating conditions. The data from the selected airfoils were used in a home-built numerical code based on the blade element momentum (BEM) theory. Predictions from this numerical code were compared with available experimental and numerical results to validate the numerical code. The validated code was then used to investigate parametrically two rotors of 10 kW each and verify the effects of varying the blade geometry and the tip-speed ratio on the aerodynamic loads. The Joukowski blades show higher values of torque and power at low speeds and the short chord makes these blades light and easy to startup when compared with the SERI airfoil.
The high cost and poor performance of small wind turbines make them not widely used. In an attempt to meliorate this situation the authors propose to investigate alternative airfoils with different chord and pitch angle distributions that permit low manufacturing, installation and maintenance costs as well as high efficiency. To achieve these goals two airfoil sections, Gottingen and Joukowski, together with different chord and pitch angle distributions were simulated by using a validated numerical code based on the BEM method. The chord geometry includes constant, linear and elliptic distributions while the twist angle includes constant and linear distributions. The results reveal that the linear pitch distribution reduces the thrust in the intermediate region of the blade and the bending moment at the root and reduces the power coefficient for both rotors. Rotors with elliptic chord distribution show increased forces in the intermediate region. Joukowski based blades with elliptic chord distribution show lower thrust compared to those with linear chord distribution. The linear chord distribution increases the thrust in the intermediate region and reduces it at the tip and root regions. Blades with multiple airfoils show marginal improvement. The Gottingen and Joukowski based rotors have similar annual energy production. The Joukowski based rotor with linear pitch and linear chord distribution shows better performance at low velocities and easy to manufacture which make it a good candidate for small power wind turbines.
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