A validated multi-scale model of a novel electrodialytic acid-base flow battery

Abstract

Electrical energy storage is crucial for a deeper penetration of intermittent renewable energies, e.g. solar and wind. The Acid/Base Flow Battery (AB-FB) is a novel, sustainable, environmental-friendly storage technology with high energy density1. The process is based on reversible electrodialytic techniques that convert the electrical energy in the chemical energy associated to pH gradients and vice versa. The bipolar membrane electrodialysis process operates in the charge phase, while the bipolar membrane reverse electrodialysis in the discharge phase. The stack consists of repetitive units, called triplets, made up of an anion-exchange membrane, a bipolar membrane, and a cation-exchange membrane, separated by spacers forming the channels where the acid, base and salt solutions flow. This work presents for the first time an experimentally validated AB-FB process model along with a sensitivity analysis. The model is based on a multi-scale simulation strategy, where four different dimensional scales are integrated within a comprehensive simulation tool with distributed parameters. The lowest hierarchical level concerns the channels. It includes CFD simulations for the estimation of polarization phenomena and pressure losses, and the correlations for the physical properties of the solutions. The middle-low hierarchical level simulates the triplets, by computing mass balances, membrane fluxes, electrical resistance and electromotive force. The middle-high scale simulates the stack by an electrical submodel intended to compute the shunt currents, and by a hydraulic sub-model to calculate pressure losses. Finally, the highest hierarchical level simulates the external hydraulic circuit including dynamic mass balances in the tanks. The model was validated against an original experimental campaign, showing a good agreement. A broad sensitivity analysis was performed in order to explore the behavior of the battery under several scenarios. The model outcome illustrates how stack geometry, operating parameters and battery layouts (e.g. open-loop vs closed-loop operations) can affect the process performance. By adopting some measures to tackle the shunt currents and taking thermodynamic advantages from open-loop operations, the round trip efficiency reached values up to 70%. This original model will orient the identification of optimized and competitive AB-FB systems.