Numerical simulation of the electrolyte flow in the tanks of vanadium redox flow batteries

Redox flow batteries are a promising technology for large-scale energy storage. An energy-conversion cell stack converts electrical energy into chemical energy of two redox couples that are stored in independent tanks, reversing the process when power is needed. The flow of the electrolyte in the tanks is a relevant factor for battery optimization that has been largely overlooked to date, with departures from perfect mixing associated with an effective capacity loss of the system. The flow in the tanks is driven by the competing effects of inertia and buoyancy, the former associated with the momentum flux of the discharging jet and the latter with the density changes suffered by the electrolyte as it passes through the cell. Three different flow regimes can be found in the system, each dominated by one effect or by both when they are comparable. Our numerical simulations show that the electrolytes are never perfectly mixed, and that the homogeneity of the concentration field is strongly dependent on the tank design, with details of the inlet and outlet ducts being particularly relevant.