CFD performance enhancement of a low cut-in speed current Vertical Tidal Turbine through the nested hybridization of Savonius and Darrieus

•OpenFoam code validation against the experimental investigations on Savonius and Darrieus.•Performance assessment of Savonius, Darrieus and the nested hybrid turbines.•Proposal of new designs of a low cut-in speed hybrid turbine.•The hybrid asynchronous coupling with an identical rotational direction is stable on low TSR range. This paper aims to enhance through two-dimensional numerical CFD simulations the hydrodynamic performance of a low cut-in speed current Vertical Axis Tidal Turbine (VATT) by means of the nested hybridization of Savonius and Darrieus. In fact, the hybrid turbine offers several coupling scenarios. In this work, the nested coupling has been used for Darrieus and Savonius to design the hybrid turbine model. Moreover, the effect of the rotational velocities of Savonius and Darrieus turbines, weather synchronized/desynchronized, identical/inverse rotational direction of the assessed rotors has been investigated. OpenFoam code was used to evaluate the hydrodynamic performance of the nested hybrid turbines, and to investigate their torque and power coefficients. This code is based on an Unsteady Reynolds Averaged Navier Stocks (URANS) approach coupled with k-ω Shear Stress Transport (SST) turbulence model. URANS models were validated against the literature results by performing 2D simulations on Savonius and Darrieus wind turbines. Therefore, the turbine’s behavior was studied and the overall performance efficiency of the nested hybrid design was enhanced over a defined Tip Speed Ratio (TSR) range. The main founding of this study was that the hybrid asynchronous coupling of Savonius and Darrieus (Identical rotational direction for the two rotors) is providing the effective performances compared to the other hybrid studied configurations.