Assessment of Bach-type internal rotor on the performance of a hybrid wind turbine: effects of attachment angle, tip speed ratio, and free-wind speed

The application of the Darrieus rotors in regions with lower wind potentials is one of the challenging issues in the operation of this type of vertical axis wind turbine (VAWT). To overcome this problem, various combinations of Darrieus and Savonius rotors, called the hybrid rotor, are proposed, which is one of the attractive subjects among wind turbine researchers. Using the computational fluid dynamics technique (CFD), this study investigates a new combination between the Savonius and Darrieus rotors at which a two-bladed Bach-type rotor is attached inside a two-bladed Darrieus rotor made by NACA 0018. In this way, various attachment angles (alpha) between the internal and external rotors such as 0 degrees, 45 degrees, and 90 degrees are studied to find the optimal case. Computations are performed for various tip speed ratios (TSR) such as 1.5, 2.5, and 3.5, and free-wind speeds (U-infinity) of 5 and 10 m/s. The obtained results revealed that the application of a hybrid wind turbine is more beneficial under lower TSR and U-infinity values. Additionally, it is demonstrated that the case with alpha = 90 degrees is the optimal case in TSR = 1.5 and 2.5 providing a maximum 21.43% improvement in comparison to the Darrieus rotor for TSR = 1.5 and U-infinity = 5 m/s.