Accelerated Testing of the End-plate Assembly of a Redox Flow Battery

Abstract: As the world transitions to intermittent renewable energy sources like solar and wind, the need for long-duration energy storage technologies is becoming more and more prominent. In this regard, flow batteries are seen as a promising solution, owing to their inherent advantages like decoupling of power and energy, extremely high cycle life and negligible self-discharge. However, there are multiple engineering challenges to overcome before the widespread application of flow batteries.   This study, carried out at a leading manufacturer of vanadium-based flow batteries, VoltStorage GmbH, addresses one of those challenges related to the hydraulic sealing of the endplate assembly of the battery. The endplate assembly is prone to losing its structural integrity over the continuous operation, thus failing to achieve its intended purpose of hydraulic sealing. Additionally, it is susceptible to enhanced contact resistance during operation, thus harming the battery performance. Therefore, the primary objective of this study was to develop a modular test rig that could evaluate the endplate assembly's performance in an accelerated manner but without using electrolytes to eliminate the complications of dealing with the sulfuric acid solution (i.e. electrolyte). So, air was chosen as the working fluid to offer clean and highly repeatable testing.   The study began with a literature review of the flow batteries. It was found that the literature concerning the engineering aspects of a flow battery was limited. Therefore, it was followed by an in-depth analysis of the stack design of VoltStorage and the engineering challenges linked to the endplate assembly. Importantly, the root cause of the problem of hydraulic sealing was identified, which was the pressure cycling of the monopole. After that, the test rig was designed and developed based on the understanding of the engineering challenge and to realize the objective of a modular design. The design modularity was desirable to test multiple assemblies simultaneously without increasing the floor footprint. Three parameters were chosen to characterize the assembly: monopole deflection, internal resistance and air leak rate. Due to the system's complexity, experiments to monitor these parameters were divided into two phases, i.e., rig qualification and full-scale testing. The first phase aimed to characterize their baseline behaviour and evaluate the rig's robustness; the next phase aimed at monitoring their behaviour evolution with continuous operation.   The monopole deflection measurements during the first phase indicated a maximum deflection of 0.3 mm. The air-electrolyte equivalence was also established by making the deflection behaviour similar during air and electrolyte operation. Much higher pressure had to be applied with air (~1.6 bar gauge) than water (~0.8 bar gauge) to achieve this equivalence. Moreover, the internal resistance and air leak rate measurements conducted during the first phase provided baseline values (6.341 ± 0.731 mΩ and 1.241 ± 0.091 Pa∙l/s, respectively, with a 95% confidence level) against which any change during continuous operation could be differentiated. However, the full-scale testing could not be performed due to the global supply chain disruptions and the limited time frame of the project. Nevertheless, a vital objective of the design, to modularize the rig so that it could be scaled up quickly and test multiple assemblies simultaneously to facilitate the rapid prototyping of different designs, was realized in the project.   Flow batteries are a promising technology for long-duration energy storage, although there are some challenges to overcome. In addition, to be defined as a truly sustainable solution, the problems linked with vanadium mining and the high capital costs of the system have to be eliminated. With the rapidly expanding development and deployment of these systems, it is expected that they will be an essential part of our future grids.   To conclude, in this project, a testing system was developed which could perform a dry mechanical and electrical integrity check of the endplate assembly of a flow battery in an accelerated manner. The system could prove to be vital in enhancing the reliability of stack-based systems and hence foster their widespread applicability. The future work that can benefit this system is assembling the set of 5 short stacks and performing a continuous operation to monitor the behaviour evolution of the stacks. This step would help assess the testing system's shortcomings and subsequently make the required modifications.