Self-Starting Hybrid ‘H’ Type Wind Turbine

Wind energy is one of the fastest growing alternative energy resources being taped into worldwide. Vertical axis Darrieus ‘H’ type lift turbines have the potential to compete effectively with the more widely used conventional horizontal axis wind turbines. However, a disadvantage is when the rotor is stationary, no net rotational force arises, even at high wind speed. The rotor must already be spinning to generate torque and the design is normally not self-starting. In this study a hybrid Darrieus ‘H’ type lift turbine was designed to operate as a self-starting drag type turbine at low rotational speed. With increasing rotational velocity the centrifugal force converted the drag type blades to full symmetrical airfoil sections making the turbine lift type. Design calculations, construction and testing of a two-blade prototype with 43 cm long blades, 14 cm wide, which formed a symmetrical airfoil of NACA 0018 profile when closed. The diameter for the turbine was 1.448 m and tests were conducted at six pitch angles with respect to the closed blade section (0°, 15°, 22.5°, 30°, 37.5° and 45°). Test results showed that the wind turbine was self-starting and the shaft speed increased linearly with wind speed at all angles of attack for moderate wind speed up to 10 m/s. However, the rate of change in shaft speed with wind speed showed an increase from 0° angle of attack to a critical 37.5° angle of attack and then decreased at 45° angle of attack. This observation is consistent with airfoil theory that greater drag forces exist at smaller angles of attack. A similar trend was observed for shaft power and turbine efficiency with maximum values of 49.5 W and 24%, respectively, at a wind speed of 8.29 m/s for the 37.5° angle of attack. The tip speed ratio increased from a minimum of 0.83 at 0° to a maximum of 2.66 at 37.5° and then decreased at 45°. This indicated drag type operation (t.s.r. < 1) at small angle of attack and lift type operation (t.s.r. > 1) at higher angle of attack. This trend was consistent with theory as the drag force decreased the aerodynamic lift force increased with increasing attack angle, causing the blade speed to exceed that of the wind speed.