Shunt Current Analysis of Vanadium Redox Flow Battery System with Multi-Stack Connections

Abstract

Vanadium redox flow battery (VRFB) systems with multiple stacks due to long lifetime, low self-discharge, and flexible design, are commonly used in large-scale electrical energy storage applications In a VRFB system, pumps deliver positive and negative electrolytes to each stack through a piping system including channels and manifolds. However, 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, which 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 the 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 serial, parallel, and mixed connections.Then, the Coulomb efficiencies (CEs) were investigated with operating currents at 36 and 54A. The results show that shunt currents are generally more significant at the center cell of a stack than at 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 SOC. In addition, the CEs of the system are higher at 54A compared to those at 36A due to smaller shunt current losses regardless of the stack configurations. Furthermore, regardless of the operating current levels, the multi-stack system connected by parallel loads achieves the highest CEs compared to the series and mixed connected systems due to the absence of shunt current losses in the piping system. Figure 1