Improving efficiency and discharge power of acid-base flow battery via a bi-objective optimisation

The implementation of effective storage systems is essential for a deeper market penetration of intermittent renewable sources. One promising, environmentally friendly energy storage technology is the Acid-Base Flow Battery (AB-FB). In the charge phase it stores electricity in the form of pH and salinity gradients via Bipolar Membrane Electrodialysis, while in the discharge phase it applies the reverse process for the opposite conversion. Despite the clear benefits over other osmotic batteries, the potential of the AB-FB has been poorly explored. This study presents the first bi-objective optimisation of the AB-FB in terms of net round trip efficiency (RTEnet) and average net discharge power density per membrane area ((NPDd) over bar). A comprehensive mathematical model previously developed by our research team was used to predict the battery performance. The epsilon-constraint approach was used to build curves of Pareto optimal solutions under various scenarios by letting several operating and design parameters to vary. Using current commercial membranes, optimal solutions yielded an RTEnet ranging from 32 % to 64 %, while the corresponding (NPDd) over bar ranged from 19.5 W m(-2) to 4 W m(-2). These results highlight the great potential of the AB-FB, as well as the need of a proper design of experimental stacks. Simulating hypothetical membranes with improved, yet realistic characteristics shifted the range of RTEnet and (NPDd) over bar to 59.1-76.3 % and 23.2-4.4 W m(-2), respectively, showing that the technological advancement in membrane manufacturing is essential for the development of high-performance AB-FB systems. Although the AB-FB performs similarly to other batteries, it can be made of non-critical materials that are not subject to supply disruptions or economic dependency, making the AB-FB a sustainability-friendly option and a good candidate for the future energy storage systems scenario.