Numerical and experimental investigation of Darrieus vertical axis wind turbines to enhance self-starting at low wind speeds

Wind energy, being renewable, cost-effective, and environmentally friendly, has attracted global attention. However, due to suboptimal performance and limited research, vertical axis wind turbines (VAWTs) lag behind horizontal axis wind turbines (HAWTs) in commercial applications, particularly for large-scale installations. This study aims to improve the self-starting capability of the Darrieus VAWT. While some parameters, such as the number of blades (N) and solidity (sigma), have been studied extensively, the airfoil shape has not received as much attention. This study compares the performance of National Advisory Committee for Aeronautics (NACA) airfoils and Selig airfoils at a Reynolds number (Re) of 40,673. The investigation revealed that the NACA0015 airfoil exhibited the highest peak power coefficient (Cp). Further analysis utilizing an advanced double multiple stream tube (DMST) code in MATLAB increased the peak Cp by adjusting the thickness-to-camber ratio (t/c) of the NACA0015 airfoil, resulting in a 12.50 % increase in the maximum achievable Cp at a Re of 40,673. This study compared four modes of VAWT operation, utilizing the NACA0015 airfoil and a modified NACA0015 airfoil for both straight-bladed and embossed-bladed VAWTs. The results showed that the modified NACA0015 airfoil for embossed-bladed VAWTs exhibited the best self-starting capability and rotation at wind velocities of 1 to 9 m s-1. Additionally, the self-starting force required by embossed-bladed VAWTs was lower than that needed by straightbladed VAWTs due to the ability of the embossed material to enhance airflow attachment to the VAWT and suppress turbulence.