On the modelling of an Acid/Base Flow Battery: An innovative electrical energy storage device based on pH and salinity gradients

Abstract

Electrical energy storage can enhance the efficiency in the use of fluctuating renewable sources, e.g. solar and wind energy. The Acid/Base Flow Battery is an innovative and sustainable process to store electrical energy in the form of pH and salinity gradients via electrodialytic reversible techniques. Two electromembrane processes are involved: Bipolar Membrane Electrodialysis during the charge phase and its opposite, Bipolar Membrane Reverse Electrodialysis, during the discharge phase. For the first time, the present work aims at predicting the performance of this energy storage device via the development of a dynamic mathematical model based on a multi-scale approach with distributed parameters. Four models, each one at a different scale, are fully integrated in a comprehensive process simulator. The model was preliminary validated by a comparison with experimental data and a good agreement was found. A sensitivity analysis was performed to identify the most detrimental phenomena. Results indicate a loss of 25–35% of Round Trip Efficiency caused by parasitic currents in the manifolds. Therefore, they may represent the main limit to the present technology performance in scaled-up stacks converting more power. Suitable geometries and operating conditions should be adopted to tackle this issue (e.g. isolated blocks), thus enhancing the battery Round Trip Efficiency.