Trust is Good, Control is Better: A Review on Monitoring and Characterization Techniques for Flow Battery Electrolytes

Flow Batteries (FBs) currently are one of the most promising large-scale energy storage technologies for energy grids with a large share of renewable electricity generation. Among the main technological challenges for the economic operation of a large-scale battery technology is its calendar lifetime, which has to cover a few decades without significant loss of performance. This requirement can only be met when key parameters representing the performance losses of the system are continuously monitored and optimized during the operation. In contrast to conventional battery technologies, nearly all the performance parameters of the FB are related to the two electrolytes as the electrochemical storage media. In this review, we therefore first survey the literature on the available characterization methods for the key FB electrolyte parameters. Based on these, we comprehensively review the available approaches for assessing the most important battery state variables: The State-of-Charge (SOC) and the State-of-Health (SOH). We furthermore discuss how monitoring and operation strategies are commonly implemented as online tools to optimze the battery performance and to recover lost battery capacity as well as how their automation is realized via Battery Management Systems (BMSs). Our key findings on the current state of this research field are finally highlighted and the potential for further progress is identified.