Parametric study and flow rate optimization of all-vanadium redox flow batteries

The parametric study for an all-vanadium redox flow battery system was examined to determine the optimal operating strategy. As dimensionless parameters, the stoichiometric number and state of charge were used to apply the strategy to all scales of the flow battery system. In this study, we developed a transient model for this system, which is supported by experimental data, to analyze effect of parameters on the ion concentration and determine its optimal operating conditions. First, the performance of the flow battery system was analyzed in steady-state conditions to examine the changes of the ion concentration depending on different flow rates, current densities, and sizes of active area. As flow rate increases, the energy efficiency slightly increases, because faster flow rates can deliver more vanadium ions from the reservoir. The energy efficiency decreases according to current density, because large current results in large amount of ohmic loss of membrane. When the size of active area increases, the energy efficiencies remain constant, however, the cycle time decreases. Next, the transient response for the system was analyzed by changing the stoichiometric number and current density during the charge and discharge processes. Variation of the system’s energy efficiency was studied with changes in the stoichiometric number and state of charge as the current density was varied from 20 to 100 mA/cm2. Increasing the flow rate at the beginning of the charge–discharge process is more efficient in the low current density region. At a current density of 100 mA/cm2, however, it is better to increase the flow rate after the state of charge reaches 50%. Lastly, an operating strategy is suggested that involves controlling the mass flow rate of the electrolyte during the charge–discharge process. The operating strategy is presented as an empirical equation defined by the stoichiometric number and state of charge. Notably, this equation can contribute to improving the performance of all scales of the flow battery system by simply changing the electrolyte flow rate at right time.