Analysis and design of vanadium redox flow battery

Abstract

In this study, the dynamic model of a vanadium redox flow battery (VRFB) was developed to analyze the battery performance and capacity degradation caused by an electrolyte imbalance from hydrogen and oxygen evolution and self-discharge side reactions. The model-based analysis of the VRFB performance revealed that the rate of battery capacity loss resulting from the electrolyte imbalance considerably depended on electrode and membrane material as well as operating conditions. Self-discharge reactions were controlled by the operational time of the battery. In addition, the rate of capacity degradation increased with an increase in the total vanadium concentration and operating temperature, affecting the increased rates of the gassing and self-discharge side reactions. It was also found that operating the VRFB with variable flow rate did not improve the battery capacity and efficiency during long-term operation due to the electrolyte imbalance. To solve this problem, the dynamic optimization was performed to determine an optimal electrolyte flow rate. The obtained optimal flow rate profile can maximize the system efficiency, regarding the variation in an open circuit voltage and concentration overpotentials, and the electrolyte imbalance level. To further improve the performance of the VRFB, an on-line dynamic optimization was proposed for updating the optimal flow rate when the battery is operated under the intermittent current density. The extended Kalman filter was integrated into the proposed on-line optimization to estimate the current state of the vanadium concentration in the VRFB from the measurement of modified open circuit voltage. The results showed that the on-line optimization approach can increase the VRFB system efficiency and prevent the battery voltage from reaching to the limited voltage before the battery achieve the desired state of charge.