(226b) Design and Optimization of a Vanadium Redox Flow Battery for Load-Following Applications

Sai Pushpitha Vudata, Debangsu Bhattacharyya, Richard Turton

Department of Chemical and Biomedical Engineering, West Virginia University, Morgantown, WV 26506, USA

The vanadium redox flow battery (VRFB) is a rechargeable flow battery that is one of the most promising large-scale energy storage systems. These batteries have the potential for storing very large quantity of energy that is readily dispatchable making them suitable for grid energy storage. Compared with conventional rechargeable batteries, the VRFB battery stores energy in two distinct electrolyte reservoirs, thus readily increasing the battery capacity by simply adding more storage volume. VRFB can be left completely discharged for long periods of time with no undesired degradation. Since vanadium can exist in solution in four different oxidation states, it makes the life of electrolyte practically unlimited by eliminating cross-contamination. The power and energy ratings of VRFB are independent of each other and each may be optimized separately for a specific application. However, the self-discharge reactions along with the undesired side reactions can significantly reduce the capacity of VRFBs. For maximizing the utilization and overall energy efficiency of VRFBs under load-following applications, their design and operation need to be optimized with due consideration of the undesired efficiency loss mechanisms.

In this work, a detailed dynamic model of the VRFB is developed. Self-discharge reactions, caused by the diffusion, convection, and migration of the vanadium ions from one half-cell to the other, lead to an imbalance between the state-of-charge of the two half-cell electrolytes and consequently cause a capacity drop. As a result of evolution of hydrogen or air oxidation of V²⁺, the side reactions also affect the capacity of the VRFB. Likewise, inadequate flow rate reduces the cycling time, thus effecting the capacity. To overcome the effect of side reactions, Tang (2011) suggested maintaining the operating range of the cell between 10% and 90% state-of-charge (SOC). However, even when the overall SOC is constrained in this desired region, there can be considerable local inhomogeneity causing capacity loss. To date no membrane or treatment process has been found to completely eliminate the capacity drop. A dynamic model-based approach is proposed in this work that can estimate the capacity profile as a function of time and location by considering the ionic flux of all vanadium species in both half cells due to vanadium crossover and self-discharge reactions. The model is used first for design and later for optimal operation in the face of varying load-following scenarios. Since the convective loss mainly arises from viscosity differences between the positive and negative electrolytes, the model is also used to predict and mitigate this loss by manipulating the flow through the two half-cells. A multi-objective optimization problem is solved to maximize the utilization and energy efficiency of the VRFB under load-following operation.

Reference

• Tang, Ao, Bao, Jie, and Maria Skyllas-Kazacos, “Dynamic modelling of the effects of ion diffusion and side reactions on the capacity loss for vanadium redox flow battery”, Journal of Power Sources 196 (2011) 10737– 10747.