Analysis of Flow Field Scale-up, Design Tradeoffs, and Shunt Current in Redox Flow Battery Stacks

Abstract

Recently it has been demonstrated that operating conditions and applications have a significant impact on the effectiveness of different flow field designs commonly used in redox flow batteries (RFBs)1. Depending on the power and energy storage requirements, as well as the system operational capabilities, the optimal flow field design may vary between designs2. These results have only been presented concerning small laboratory scale designs, however. Due to the different transport mechanisms utilized by these various flow field designs, the mass transport performance and efficiency of each design will scale differently when the active area of the cell is increased to a size more representative of a commercial system. Additionally, the most common configuration of these cells in a commercial system is in an RFB stack where several cells are connected in series electronically and in parallel hydraulically. This further complicates the design tradeoffs seen in the smaller single cell design, while also introducing new design concerns in the form of shunt current, a self-discharge phenomenon only present when multiple cells are connected ionically through the liquid electrolyte. Utilizing the energy and efficiency-based analysis previously published by our lab, we have conducted a thorough study of the differences between four flow fields when increasing the active area of the cell. CFD simulations using COMSOL Multiphysics provide quantitative data for electrolyte velocity and pressure drop, while experimental data provides depth of discharge and efficiency results. This analysis has also been extended to a small RFB stack to examine how these design tradeoffs are impacted by placing multiple cells electronically in series and sharing an electrolyte manifold. To quantify the shunt current in this stack, a novel in-situ shunt measurement technique was developed, allowing for real-time measurement of the shunt current present during all phases of stack operation. The results of this study provide insights and guidelines for the future development of commercial RFB systems, specifically ways in which the fluid pathway can be optimized to improve system efficiency.