Transient Modeling of a Vanadium Redox Flow Battery and Real-Time Monitoring of Its Capacity and State of Charge

The vanadium redox flow battery (VRFB) is a rechargeable flow battery that is one of the most promising large-scale energy storage systems making it suitable for grid-level energy storage. However, the self-discharge reactions along with the undesired side reactions and water transfer through the membrane cause imbalance in the electrolyte and state of charge, which can significantly reduce the capacity of these batteries. A first-principles, 2D electrochemical dynamic model of all-VRFB is developed. The electrochemical model includes self-discharge reactions caused by diffusion, convection, and migration of the vanadium ions from one half-cell to the other. In addition, side reactions leading to evolution of hydrogen and oxygen are modeled. Water transport through the membrane is modeled by considering transport of water along with vanadium ions due to electro-osmotic drag and diffusion between two electrolyte half-cells. The model is validated with the experimental data. A filtering approach is developed for co-estimation of state of charge and capacity. Using a state-space model identified using the data generated by the pseudo-random binary sequence in the current and electrolyte flowrates, the filtering approach is found to yield satisfactory estimation of the state of charge and capacity by using voltage as the only measured output variable.