Electrolyte mixing in vanadium flow battery tanks: Effects on capacity utilization

This work investigates the fluid dynamics of electrolyte mixing within the tanks of vanadium flow batteries. Custom axisymmetric tanks are used to study the different flow regimes that emerge during battery cycling at various flow rates. A dual online diagnostic tool based on UV-vis spectroscopy enables precise tracking of the state of charge (SoC) of both electrolytes. The experimental campaign is supported by a mathematical model that implements accurate empirical fits for the density and viscosity obtained in previous work, which qualitatively reproduces the observed behavior. The results show that at high flow rates, momentum-dominated flows induce strong mixing in both tanks, yielding homogeneously mixed electrolytes. As the flow rate decreases, buoyancy effects start to play a role, and each electrolyte exhibits different flow regimes as a result of different SoC-dependent density changes. During charge, the positive tank exhibits a submerged jet that crosses the tank directly to the outlet, causing a decrease in capacity. In contrast, the negative tank shows a stratified piston flow that becomes more prevalent as the jet momentum decreases. During discharge, the negolyte flows directly to the outlet, causing a marked capacity drop, while the positive tank maintains relatively well-mixed conditions. These findings highlight the critical impact of tank-level mixing on battery performance and indicate the need for further investigations carried out under operational conditions that better represent industrial settings, with an emphasis on large-scale flow battery systems.