Numerical examination of the performance of a vanadium redox flow battery under variable operating strategies

A two-dimensional multi-physical model was applied for analyzing the performance of a vanadium redox flow battery under variable operating strategies. The study indicated that for constant flow rate and current, it is inconsistent to give consideration to both the capacity and efficiency of the battery. In contrast, variable flow rate strategy can enhance the overall battery performance. Considering the complicity in controlling the pump, the step-up flow rate strategy is suggested. When the flow rate is stepped up from 0.5 to 3.5 ml s⁻¹ at state of charge (soc) over 0.85 for charge process or soc below 0.15 for discharge process, the system efficiency can reach 82% while maintaining both the capacity and net discharge work at high levels. Besides, it was indicated that the overall battery performance can be further improved with simultaneous step-up flow rate and step-down current strategy. For all cases studied, the simultaneous step-up flow rate from 0.5 to 3.5 ml s⁻¹ and step-down current density from 80 to 60 mA cm⁻² is preferred, which further increases the capacity and net discharge work by over 6.5% while maintaining high efficiency as compared with the strategy of flow rate step-up only.