Redox flow batteries are a promising technology for large-scale energy storage. 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 because of the slight changes in temperature and state of charge (SoC). Three different flow regimes may be found in the system, each dominated by one effect or the other, or by both when they are comparable. All these phenomena are governed by the current intensity applied (I), the volumetric flow (q) and the properties of the vanadium electrolyte.

In particular, it is seen that an accurate description of the effect of the temperature and state of charge on the density and specific heat of the electrolytes is essential to compute the buoyancy induced flow and heat transfer taking place in the tanks. A parametric sweep is presented to show the impact of these dependences on the mixing process, which calls for the need of accurate experimental measurements of the variation of the physical properties of the electrolytes with their temperature and state of charge.

Our numerical simulations show that the electrolytes are never perfectly mixed, and that the homogeneity of the species concentration is strongly dependent on the operation conditions and on the tank design, with details of the inlet and outlet ducts being particularly relevant. Results on the charging and discharging operations, heating process and the rising of the state of charge in the cell are presented and discussed.

Acknowledgments

This work has been partially funded by the Agencia Estatal de Investigación (PID2019-106740RB-I00 and RTC-2017-5955-3/AEI/10.13039/501100011033), and by Grant IND2019/AMB-17273 of the Comunidad de Madrid.