Experimental Performance Evaluation of Small-Scale Vertical-Axis Sail-Type Wind Turbines

Conventional vertical-axis wind turbines (VAWTs) rely on rigid blades that are heavy and require expensive materials and manufacturing processes to withstand aerodynamic loads. This study explores flexible sail-type blades as a low-cost alternative capable of achieving comparable performance. Small-scale sail-type VAWTs employing blades made out of fabric similar to maritime sails were designed, fabricated, and tested in a wind tunnel. One edge of each sail was hemmed to a vertical rod, while the opposite edge was secured at its end to top and bottom end disks. The turbine radius and height were fixed at 19 cm and 21 cm, respectively. Experiments were conducted in a wind tunnel and measurements included wind speed, turbine rotational speed, and the generator current and voltage. A total of 36 configurations were evaluated by varying four design parameters: number of sails (2, 3, and 6), chord length (10 cm and 15 cm), pitch angle (0°, 15°, 30°, and 45°), and blade shape (rectangular or trapezoidal). Performance was assessed using power output (P), power coefficient (Cp), and tip-speed ratio (TSR). Experimental results were validated by comparison with test data for a conventional rigid-bladed VAWT. The best-performing configuration was a three-sail turbine with rectangular blades, 15 cm chord length, and 0° pitch angle, producing 547 mW at 177 RPM, corresponding to a Cp of 0.053 at a TSR of 0.59. Numerical predictions using state-of-the-art codes were used to further assess sail-type VAWT performance. Under the test conditions considered, the sail-type VAWT not only showed better performance but also comparable self-starting capability compared to a rigid-bladed VAWT. However, due to the maximum wind speed limitation of the test facility, the complete Cp-TSR characteristic curve could not be determined. Moreover, preliminary estimates indicate a potential 24% reduction in total turbine capital cost and an overall 30% reduction in turbine mass of sail-type turbines over conventional HAWTs, demonstrating that flexible sail blades are a promising low-cost option that needs further investigation to realize their full potential.