Effect of the number of stages on the performance of savonius vertical axis wind turbines: Part I- using straight semicircular blades

The Savonius vertical axis wind turbine offers several benefits as simple construction, low manufacturing, installation and maintenance costs, low operating wind speed, and independence of wind direction. However, the conventional single-stage Savonius rotors have negative torque values at a certain range of rotor angles and large torque variation over the complete cycle which led to reduction of the power coefficient and the self-starting abilities. To overcome these drawbacks, a multi-stage Savonius wind rotor is proposed to obtain the optimum number of stages that enhances both the power coefficient and the self-starting capabilities. In the current study, one-, two-, three-, and four- stage Savonius wind rotors with straight semicircular blades are investigated. In two stages rotor, one single-stage rotor is mounted over another single stage with a phase shift of 90°. In the three stages rotor, the three singlestage rotors are mounted one above the other with a phase shift of 60° relative to one another while with a phase shift of 45° for the four stages. All rotors have overall rotor diameter (D) of 200 mm, a thickness of 2 mm, a stage aspect ratio of 1.0, and an overlap ratio (d) of 0.0. The diameter of the circular end plates (Do) attached to the rotor tips is 1.1 of the rotor diameter (D). All studies are performed at a wind speed (V) of 6 m/s. The variations of torque and power coefficients are estimated for all the studied multi-stages rotors. Therefore, 3D incompressible unsteady Reynolds-Averaged Navier-Stokes equations along with the k-w shear-stress transport turbulence model is developed and numerically simulated using ANSYS Fluent. The numerical model is validated using the available measurements and numerical results. The predicted flow field characteristics such as streamlines and pressure fields around the studied rotors with various numbers of stages are presented and analyzed. Accordingly, results indicated that with the increase in the number of stages, a significant enhancement of the torque and the power coefficients is attained. In addition, the rate of percentage gain in the torque and the power coefficients is higher for two stages rotor than that of three and four stages rotors. The maximum torque and power coefficients for the two stages rotor are 0.336 and 0.194, respectively. The power coefficient gain obtained by using the two stages Savonius rotor is 17.5 % compared to the conventional single stage which has a power coefficient of 0.165 at a wind velocity (V) of 6 m/s. Furthermore, using a multi-stage rotor significantly smooths the variations in the torque coefficient and produces positive static torque values at all rotational angles resulted in enhancing the self-starting capabilities of the Savonius rotor.