Performance prediction and enhancement of Darrieus wind turbine

The straight-bladed Darrieus wind turbine is a highly promising vertical axis wind turbine (VAWT). By further improving its efficiency, it is expected to receive a wide adaptation in small to medium size applications, especially, due to its simple design. Since this turbine is a lift-based wind turbine, its performance highly depends on the occurrence of the dynamic stall and the high lift forces associated with it. In this paper, the “moving surface” method is applied to the turbine blades to delay the occurrence of the dynamic stall by controlling the boundary layer separation from the turbine blade surface. The method aims to inject external momentum into the region of separation by moving the blade surface – or wall – to prevent, or at least, minimize the boundary layer separation, and consequently, the dynamic stall. Initially, the study utilizes a robust method to evaluate the performance of the typical turbine, by making use of the available static data of the NACA 0021 airfoil used for the turbine blades. Numerical simulations are then carried out using Ansys-Fluent software to solve for the Reynolds-averaged Navier–Stokes (RANS) equations using the Transition-SST turbulence model, which proved to be highly accurate upon validating the results. Several Numerical simulations are carried out to study the effect of moving 80% of the upper surface of the blade one time, and the lower surface the other time. The results show that moving the lower surface is very effective in delaying the stall and enhancing the overall turbine efficiency. Meanwhile, moving the upper surface has no significant effect on the turbine performance. The remaining of the study focused on moving the lower surface of the turbine blade at various speeds, setting the value of the ratio U mv / ω R to 0.5, 1.0 and 1.5. The results show that the efficiency of the turbine, represented by the overall average value of the power coefficient ( C p ), increases almost linearly as the velocity of the moving surface increases, when the power required by the moving surface is not considered. At a ratio of U mv / ω R = 0 . 61 , the efficiency increases by 59%. • A robust method was suggested and used to predict the turbine performance at low tip speed ratios. • The boundary layer separation was reduced using a moving surface method. • The dynamic stall was delayed and the turbine efficiency was improved.