Shunt current analysis of vanadium redox flow battery system with multi-stack connections

In vanadium redox flow batteries (VRFBs), the electrolyte flowing between cells through channels and manifolds and the electrolyte flowing between stacks through pipes are electrically conductive. Shunt currents are generated due to the voltage difference between the cells and between the stacks, and reduce the energy efficiency of the battery system. In particular, under different load connections, the shunt currents of a multi-stack VRFB system have different distributions and cause different impairments to the system efficiency. Therefore, it is important to predict the shunt currents of a multi-stack system under different load connections before the actual construction of the system. In this paper, a multi-stack VRFB system with 120 cells was explored by circuit-based modeling to evaluate the shunt currents according to the stack configuration such as having serial, parallel, and mixed connections. Thereafter, the Coulomb efficiencies (CEs), voltage efficiencies (VEs), and energy efficiencies (EEs) were evaluated at the operating current of 36 A and 54 A. The results show that shunt currents are generally more significant at the center cell of a stack than at the other cells under these stack configurations and exhibit a positive correlation with the battery state of charge (SOC), i.e., larger shunt currents increase with the SOC. In addition, the CEs of the system are higher at 54 A compared with those at 36 A due to smaller shunt current losses regardless of the stack configurations while the VEs and EEs show the opposite trend to the CEs. Furthermore, regardless of the operating current levels, the multi-stack system connected by parallel loads achieves the highest CEs and EEs compared with the series and mixed connected systems due to the absence of shunt current losses in the piping system.