Blade height impact on self-starting torque for Darrieus vertical axis wind turbines

Self-starting torque (TSelf-starting) presents a significant challenge for Darrieus vertical axis wind turbines (DVAWTs), often necessitating external assistance to initiate rotation. This study addresses the issue by optimizing airfoil design, employing embossed blades (EBs), and adjusting blade height (H) to reduce TSelf-starting. From an analysis of 43 rotors at a chord-based Reynolds number (Rec) of 45,192, national advisory committee for aeronautics (NACA) 0015, NACA4412, and NACA4415 rotors were selected for their superior power coefficients (Cp). These rotors were optimized using double-multiple streamtube theory (DMST) and particle swarm optimization (PSO), focusing on the thickness-to-chord ratio (TCR). Among them, the NACA0015-Opt rotor achieved the highest Cp, demonstrating its effectiveness in enhancing DVAWT efficiency. This study also investigates the effect of H on the performance of EBs, comparing H of 35 cm and 75 cm. Experimental findings reveal that combining airfoil optimization with EBs, along with an increased H, leads to a substantial decrease in TSelf-starting. Specifically, higher H enhance the aerodynamic performance of EBs by improving airflow over the blade surface, further reducing drag and contributing to a significant reduction in TSelf-starting. At a H of 75 cm, the embossed blade Darrieus vertical axis wind turbine (EB-DVAWT) equipped with the optimized NACA0015-Opt rotor required 15.92 %, 17.04 %, 18.12 %, 21.23 %, 52.06 %, 49.23 %, 51.25 %, 35.20 %, 14.12 %, and 9.09 % less TSelf-starting at wind velocities (U∞) of 1, 2, 3, 4, 5, 6, 7, 8, 9, and 9.5 m/s, respectively, compared to the baseline smooth blade Darrieus vertical axis wind turbine (SB-DVAWT) with the original NACA0015 rotor.