Innovative design method and experimental investigation of a small-scale and very low-tip speed ratio wind turbine

Small horizontal axis wind turbines operating at low wind speeds face the issue of low performance compared to large wind turbines. A high amount of torque is required to start producing power at low wind speed to overtake friction of mechanical parts. A low design tip-speed ratio (λ) is suitable for low power applications. The relevance of the classical blade-element/ momentum theory, traditionnaly used for the design of large wind turbines operating at high tip-speed ratio, is controversial at low tip-speed ratio. This paper presents a new design methodology for a 300 mm horizontal axis wind turbine operating at very low tip-speed ratio. Chord and blade angle distributions were computed by applying the Euler's turbomachinery theorem. The new wind turbine has multiple fan-type blades and high solidity. The rotor was tested in wind tunnel and compared to a conventionnal 3-bladed horizontal axis wind turbine designed according to the classical blade-element/ momentum theory. It was found that the new wind turbine achieved a maximum power coefficient of 0.31 for λ = 1. The conventionnal wind turbine achieved similar performance but at higher tip-speed ratio λ = 3. At low tip-speed ratio, the torque coefficient (C τ) is higher for the new wind turbine than for the conventional one and decreases linearly with the tip-speed ratio. The high magnitude of torque at low tip-speed ratio allows it to have lower instantaneous cut-in wind speed (2.4m.s −1) than the conventionnal wind turbine (7.9m.s −1). The analysis of the wake by stereoscopic particle image velocimetry shows that the new wind turbine induced a highly stable and rotating wake that could drive a second contra-rotating rotor. The magnitude of the axial and tangential velocities in the near wake shows a good correlation with the design requirements. Wake expansion and decceleration of the fluid are less significant with the new wind turbine than the conventionnal one.