Effect of Flow Rate Control Modes on a Vanadium Redox Flow Battery Based on a Numerical Model

This paper studies the effect of flow rate control modes on VRB performance based on a validated numerical model. Four modes were put forward, i.e., constant flow rate, variable flow rate with equal anolyte and catholyte (Variable modes I and III) and variable flow rate with unequal anolyte and catholyte (Variable mode II). Under the optimal condition (80 mA/cm² current and 85% state of charge, SOC), the charging efficiency under the Variable mode II is 0.59% higher than that under the constant flow, while 0.45% and 0.85% higher than those under Variable modes I and III, respectively. The charging efficiency improvement could be largely ascribed to anolyte and catholyte flow adjustment independently, which balances the pump power needed and concentration overpotential loss, benefitting the reduction of total power loss. Moreover, when the charging current is 120 mA/cm², the maximum SOC of tank under Variable modes I, II, and III increase by 7.63%, 6.41%, and 13%, respectively, in comparison with that under the constant flow rate. The significant increase under Variable mode III is due to an introduction of new objective function (pseudo power loss function), which limits the concentration overpotential difference between anolyte and catholyte, extending time to reach cut-off voltage.