Predictable tidal current resources make marine current turbine (MCT) generation system highly attractive as an electricity supply source for coastal areas and remote islands. However, the tidal speed varies greatly due to the flood and ebb tides during one day period. This results large mismatch between MCT produced power and grid-side (or load-side) demanded power. This paper focuses on a grid-connected MCT system and proposes using vanadium redox flow battery (VRB) energy storage system to manage the combined output power and to follow grid-side demand on a daily basis. The VRB model and parameter calculation process are detailed in this paper. The diesel generator (DG) system is considered as a backup power supply source in case of low battery state of charge (SoC) caused by losses during long-time battery operation. Simulations are carried-out on a grid-connected MCT system with VRB ESS to follow a given power demand profile during one day period. The results valid the proposed VRB sizing and control strategy. The DG system is demonstrated as a feasible solution to avoid VRB reaching its low SoC limitation and to guarantee the expected power injection to the local grid.