Modelling and simulation of all-vanadium redox flow batteries

Abstract

Properties and applications of all-vanadium redox flow batteries are discussed and a two-dimensional model is developed. The model, which is based on a comprehensive description of mass, charge, energy and momentum transport and conservation, is combined with a global kinetic model for reactions involving vanadium species. Gas evolving reactions are then incorporated into the modelling frame work. Bubble formation as a result of evolution at the negative/positive electrode is included in the model, taking into account the attendant reduction in the liquid volume and the transfer of momentum between the gas and liquid phases, using a modified multiphase-mixture approach. Comparisons to simulations with negligible gas evolution demonstrate the effect of gas evolution on the efficiency of the battery. The effects of reactant concentration, flow rate, applied current density and gas bubble diameter on gas evolution are investigated. Significant variations in the gas volume fraction and the bubble velocity are predicted, depending on the operating conditions. The construction of a cell and charge/discharge experiments are described. Numerical simulations are compared to experimental data for different vanadium concentrations and mean linear electrolyte flow rates, demonstrating good agreement. Numerical simulations demonstrate the effect of changes in the operating temperature on performance of the all- vanadium redox flow battery and the extent of oxygen evolution. It is shown that variations in the electrolyte flow rate and the magnitude of the applied current substantially alter the charge/discharge characteristics, the temperature rise and the distribution of temperature. The effects of heat losses on the charge/discharge behaviour and temperature distribution are investigated. Conditions for localised heating and membrane degradation are discussed

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