Investigation of operational characteristics of vanadium redox flow batteries
Date of Issue2017-04-12
School of Electrical and Electronic Engineering
Redox flow based batteries are becoming increasingly popular because of the numerous advantages they offer. These systems are modular, highly efficient, and the cost is relatively low, making it easy to scale up. The battery, however, is not commercially widespread as lithium-ion or lead-acid batteries. One of the major reasons for this is not having enough ageing studies for this comparatively recent technology and lack of battery specific unique optimization techniques. This thesis focuses on the two important issues in batteries, namely capacity decay and charging methodology. The former is essential in the longevity and latter in short term efficiency and the rate of ageing. These are some of the key aspects that are less researched, but important in making the commercial implementation of Vanadium Redox flow Battery (VRB) viable. The capacity loss due to vanadium ion diffusion was simulated for multiple cycles of operation and the effect that flow rate has, on concentration flux in the cell was investigated. The concentration difference between two half cells is one of the major reasons for species crossover. As a consequence of diffusion, the charge transfer current density in the electrode varies. The relationship between charge transfer current density and the concentration of one species of vanadium was analysed at different temperatures. Further, the parameter dependence was analysed during the charging process of VRB for a system-aware building, which has distributed sources of energy production and amenable to flexible load scheduling. Preliminary studies on multi-modal VRB charging characteristics was done using an equivalent circuit model. The normalized energy and time efficiencies were studied for their operation in buildings. The results obtained show that there is a trade-off between the energy capacity and the charging time, depending on the charging current and flow rate. The concentration change of different species of vanadium ion was modelled with temperature influence for each of these vanadium ion variants. The concentration change of vanadium electrolyte was simulated for different temperatures. The change in capacity of VRB was calculated for different temperature profiles and the total loss was partially contributed by intrinsic loss and partly due to diffusion. It was also observed that the difference in the loss at different temperatures was higher during the initial 50% of the total life cycle and stabilizes as it proceeds. At the end of 200 cycles, the capacity loss at 40C was higher (14.23%) than that of 30C (13.37%) and 20C (12.67%) of which, a major part was contributed due to diffusion in the first 100 cycles of operation. Further, an extended dynamic model of the vanadium ion transfer was developed, in which, the effect of temperature and transfer of bulk electrolyte across the membrane was included. The model was used to simulate capacity decay for a range of different ion exchange membranes that are being used in the VRB. The model was made more comprehensive by including the effect of bulk electrolyte transfer. A volume change of 19% was observed in each half-cell for Nafion 115 membrane based on the simulation parameters. The effect of this change in volume directly affects concentration, and the characteristics were analysed for each vanadium species as well as the overall concentration in the half-cells. Following the initial feasibility study on multi-modal charging, a more detailed analysis of charging methodology and a profile based charging optimization for VRB was proposed. Individual over-potentials and the losses due to the pump were separately modelled for accuracy. A fuzzy logic controller was used to utilize the normalized loss values and the state of health of VRB from earlier studies on ageing to provide an optimal value for current and flow rate for the selected profile. The optimization was done in real-time to include flexibility of control due to external factors like temperature, humidity, etc., which can be added as a parameter based on the operator requirement. Three modes were proposed in this study namely, autonomous, time bias and energy bias. These modes, however, are a non-exhaustive fragment of the many control modes that can be achieved.
DRNTU::Engineering::Electrical and electronic engineering::Electric power