Effect of the number of stages on the performance of savonius vertical axis wind turbines: Part II- using twisted blades

Wind energy comprises one of several renewable resources of energy engineered to contain the global energy crisis. Although horizontal axis wind turbines (HAWTs) have proven to be effective in low turbulence and steady wind conditions, vertical axis wind turbines (VAWTs) potentially have the advantage in highly variable and turbulent regions. The Savonius vertical axis wind turbine has several advantages such as simple design, low manufacturing costs, low operating wind speed, low noise, and Omni-directional capability. However, the Savonius rotor requires further design optimization to improve its aerodynamic performance before becoming competitive with other turbine designs. Thus, the main objective of the current study is to numerically investigate the aerodynamic performance of a multistage Savonius rotor to enhance the power coefficient and the ability of self-starting. In the current study, one-, two-, three-, and four- stage Savonius rotors with twisted blades are investigated. In a two-stage rotor, one single-stage rotor is mounted over another single- stage with a phase angle of 90°. In a three-stage rotor, the three single-stage rotors are mounted one above the other with a phase angle of 60° relative to one another while with a phase angle of 45° for the four stage-rotor. The blades of the studied Savonius rotor are twisted with a twist angle (f) of 45o. This is the first contribution to understand how multi-stages influence the aerodynamic performance of the twisted-bladed Savonius rotor. Moreover, variations of torque and power coefficients are computed for all the studied rotors with various numbers of stages. The developed numerical model is simulated using ANSYS Fluent and validated using the available experimental and numerical results. Results showed that the coefficients of torque (CT) and power (CP) increase with rising the number of stages. Increasing the number of stages from 1 to 2 significantly increases the CT and CP of the rotor. However, with a further increase in the number of stages to 3 and 4 stages, both the CT and CP remains almost the same as the rotor with 2 stages. The maximum coefficient of torque (CT, max) and power (CP, max) for a two-stage rotor are 0.42 and 0.253, respectively. The gain in the coefficient of power obtained by using the twostage Savonius rotor with twisted blades is 53.5% compared to the conventional single-stage which has a coefficient of power 0.165 at a wind velocity of 6 m/s. Moreover, using multi-stages and twisted blades significantly smooth the variations in the generated torque and produce positive values at all rotor angles resulted in improving the self-starting ability of the Savonius rotor.