Fluid dynamics of mixing in the tanks of small vanadium redox flow batteries: Insights from order-of-magnitude estimates and transient two-dimensional simulations

Abstract

This paper investigates the fluid dynamics of mixing in the tanks of small-scale vanadium redox flow batteries. These systems use two redox pairs dissolved in separate electrolytes to convert electrical energy into chemical energy, a process that can be reversed in an efficient way with little or negligible chemical losses. After flowing through the electrochemical cell, the electrolytes are stored in separate tanks, where they discharge as submerged jets with small temperature and composition changes compared to the electrolyte already present in the tanks. The subsequent mixing process is critical for battery performance, as imperfect mixing tends to reduce energy capacity and lead to asymmetric battery operation. The analysis uses order-of-magnitude estimates to determine the conditions under which the mixing process is dominated by either momentum or buoyancy. Transient two-dimensional simulations then serve to illustrate the different flow regimes that emerge under laminar flow conditions. The results indicate that, contrary to the common assumption, the electrolytes do not mix well in the tanks. For high-momentum, and, specially, positively buoyant jets, a significant fraction of the electrolyte remains unmixed and unreacted for long periods, thereby reducing the energy capacity. The results highlight the importance of having reliable electrolyte properties for the accuracy of the numerical simulations, since small density variations can significantly impact the long-term mixing of the electrolytes. Specifically, in momentum-dominated flows the cumulative impact of density changes promotes flow instabilities that significantly enhance mixing. These findings underscore the importance of considering density changes over time in future studies aiming to optimize tank design in redox flow batteries.