Numerical investigation of the effects of increased and mixed chord lengths for a 4-bladed darrieus vertical axis wind turbine

Increasing amounts of study and research on vertical axis wind turbines (VAWTs) have shown that they are a competitive option in wind energy power generation. However, the VAWT’s primary drawback is low power efficiencies. Although there are several studies on the effects of solidity on Darrieus VAWT performances, few focus on the effect of the aerofoil chord length. Hence, in the present study, 2D numerical simulations are performed to explore the effects of different aerofoil chord lengths on the performance of a Darrieus VAWT. The simulation was first validated with the experimental data from the literature. The studied turbine is a 4-bladed VAWT fitted with NACA0021 blades with an original chord length, c of 85.8 mm and another with an increased chord length of 1.2 c (102.96 mm). Additionally, a modified rotor geometry with mixed chord lengths of c and 1.2 c to improve turbine performance is proposed and investigated. The coefficients of power (C P) and torque (C T) for tip speed ratios (TSRs) between 1.4 and 3.3 for each of the turbines are evaluated and comparatively analysed. All the data was obtained using the computational fluid dynamics (CFD) software ANSYS Fluent in conjunction with the shear stress transport (SST) k−ω turbulence model. The findings show that the turbine with 1.2 c chord length and hence larger solidity outperforms those with smaller chord lengths at low TSRs. However, their performances decrease significantly at TSRs above 2.5, resulting in up to 86.1% lower C P values. The mixed chord lengths case was successful at achieving significantly higher C P values at both TSR ranges with only a decrease of 3.03% in maximum C P at its optimum TSR.