Design and Experimentation of Darrieus Vertical Axis Wind Turbines
A comparative analysis has been performed for vertical-axis wind turbines, including the straight, troposkien, and helical-bladed Darrieus configurations, to assess their aerodynamic efficiency and real-power performance. The experimentation process included numerical modeling, CAD design, 3D printing-fabrication, and wind tunnel testing of lab-scale prototypes with a maximum power point tracking (MPPT) control scheme under different wind velocities. Implementing a double multiple streamtube (DMST) model, aided in the delimitation of non-dimensional parameters, where the local Reynolds numbers are between Re_b = 32,000 and 190,000, finding the ideal solidity value to be σ < 1.7 for low tip-speed ratio conditions, λ < 2.5. The optimum rotor swept areas are S = 0.048 m2 and 0.093 m2 with a maximum rotational speed around ω ≅ 1100 RPMs. At the designed conditions, the best wind tunnel results are obtained from the troposkien configuration (T-v1), with a Cp_opt = 0.218 at λ_opt = 2.25, followed by the straight-bladed (SB-v2) with a Cp_opt = 0.118 at λ_opt = 1.31 and helical-bladed (H45-v3) with Cp_opt = 0.082 at λ_opt = 0.99. The implementation of a free-vortex wake (LLFVW) method demonstrated the artificial increases in Cp (13-17%) and TSR (4-6%) due to wind tunnel blockage ratios between BR = 18% and 26% with turbine curvature ratios c/R > 0.5. Nonetheless, the power predictions for the vortex model are not consistent with real experimental data varying around |∆Cp| ≥ 30%, while the DMST deviates on average by |∆Cp| ≥ 25%. As such, the best strategy for small-scale wind turbine experimentation resides on wind tunnel tests, whereas basic aerodynamic models are mainly taken as tools for parameter selection and wake-flow visualization in the downstream region.
