Experimental characterisation and modeling of a vanadium redox flow battery / Mohd Rusllim Mohamed

Thesis (PhD in Electrical Engineering) -- Universiti Malaysia Pahang – 2013

Bibliography : p. 152-168

This thesis presents the summarization of work on experimental characterisation and modeling of a vanadium redox flow battery (V-RFB). This thesis presents background material and motivation factors of the studies; research goals, a review of previous work and discussion on related issues with respect to energy storage technologies, emphasising on V-RFB system. The aim of the study is to investigate the performance of V-RFB through experimental characterisation of V-RFB at different operating parameters and develop electrical circuit modeling of V-RFB. Preliminary experiment on 100 cm2 unit cell laboratory unit V-RFB has helped in familiarising with V-RFB setup and its design weaknesses and factors leading the cell into failure mode are highlighted. Based on observation on 100 cm2 unit cells, new design of 25 cm2 unit cell laboratory unit V-RFB has been proposed with an improvement of efficiency and reduction of contact resistance are observed. Theoretically studies by using Faraday’s law of electrolysis and Nernst equation are used to relate the equilibrium cell’s potential with the concentration changes in vanadium species, back-up with experimental data from a divided, open-circuit potentiometric cell approach. Two different approaches has been presented, with newly proposed approach of a divided, open-circuit potentiometric cell, via Hg/Hg2SO4 reference electrodes and graphite rod working electrodes present superiorities in estimating the state-of-charge (SOC) of V-RFB. System characterisation has been carried-out for the new 25 cm2 unit cell laboratory unit V-RFB under different of operating parameters such as current densities, temperatures, flow rates, concentrations and material properties. The cell exhibits highest energy efficiency at 82.1 %, operating at 308 K, 60 mA cm-2 current density and 3 cm3 s-1 volumetric flow rate for 250 cm3 (each reservoir) of 1.6 mol dm-3 V(III)/V(IV) in 4 mol dm-3 H2SO4. Formation charge of mixture of vanadium species into single electro-active species at positive and negative electrodes are highlighted. A method for estimating the V-RFB to complete its formation charge using electrochemical calculation of Faraday’s constant are also presented. New equivalent electric circuit model for V-RFB has been proposed which consists of an open-circuit cell potential in series of ohmic internal resistance and the parallel n-RC network. Extended Kalman filter is used for parameter identification of dynamic characterisation of V-RFB. Numerical simulations are compared to experimental data at different pulse voltages at few SOCs and experimental charge-discharge characterisation of V-RFB system, demonstrating good agreement.