Studies on dynamic responses and impedance of the vanadium redox flow battery

This paper studies the feasibility of using the vanadium redox flow battery (VRB) for power quality control applications. This work investigates the dynamic voltage and current responses of the VRB to load changes over a range of frequencies (up to 5 kHz), through experimental studies on a laboratory scale testing system. Experiments were carried out under different operating conditions to examine the effects of system SOC, discharging current and temperature. The analysis shows that the magnitude of battery impedance is higher at low frequencies but lower at high frequencies. These results suggest that the VRB has the ability to handle charging-discharging power fluctuations in a frequency range up to a kHz level. By using the concept of fractional order systems, the transient behaviour of the VRB cell was modelled as an equivalent circuit that utilises a constant phase element to represent the electrochemical double layer and a Warburg element to describe the effect of concentration polarisation. This equivalent circuit model is useful for electrical interface design and power flow control applications.