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

TL;DR

This study combines order-of-magnitude estimates and transient 2D simulations to analyze mixing in small vanadium redox flow battery tanks, revealing that poor mixing can limit energy capacity and is influenced by flow regimes and electrolyte properties.

Contribution

It provides new insights into the flow regimes and mixing efficiency in battery tanks, emphasizing the importance of electrolyte properties and buoyancy effects for battery performance.

Findings

Electrolytes do not mix well in tanks, especially with high-momentum and buoyant jets.

Unmixed electrolyte can persist for long periods, reducing energy capacity.

Flow instabilities from density variations can enhance mixing over time.

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 to restore the initial electrical energy with 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 the energy capacity and may lead to asymmetric battery operation. The analysis starts using order-of-magnitude estimates to determine the conditions under which the mixing process is dominated by momentum or buoyancy. Transient two-dimensional simulations illustrate the different flow regimes that emerge in the tanks under laminar flow conditions. The results show that, contrary to the common assumption, the electrolytes do not mix well in the tanks. In the presence of high-momentum -- and, specially, positively buoyant -- jets, a significant fraction of the electrolyte remains unmixed and unreacted for long periods, thus reducing the energy capacity. The results also show that the availability of reliable electrolyte properties is crucial for the accuracy of the numerical simulations, as, under the mixed convection conditions that typically prevail in vanadium redox flow batteries, small density variations can significantly impact the long-term mixing of the electrolytes. In particular, in momentum-dominated flows the cumulative effect of density changes over time eventually leads to flow instabilities that significantly promote mixing; therefore, they should be taken into account in future studies.