Understanding the Effects of Ripple Currents on Vanadium Redox Flow Batteries

Battery energy storage systems are increasingly deployed to support modern power grids by providing (grid) services such as backup power, peak shaving, frequency regulation and load balancing. However, when flow batteries are coupled with power converters, the power converters inevitably introduce current and voltage ripples into the battery system. Ripple current refers to the periodic alternating current (AC) component on a direct current (DC) signal, commonly resulting from the rectification of AC power or the operation of power electronic circuits. In conventional batteries, such as lead-acid and lithium-ion batteries, ripple currents can accelerate degradation and require external counter measures such as filters to mitigate ripple currents. This added filtering increases the size, cost and can reduce system reliability. Compared to Li-ion and lead-acid batteries, Vanadium redox flow batteries (VRFBs) offer a promising alternative for grid-scale applications owing to their long cycle life, deep-discharge capability and independent scaling of power and energy. Studies on the effects of ripple currents on the electrodes of VRFBs have been conducted. However, the overall influence of converter-induced ripple currents on VRFB performance, especially with focus on vanadium electrolytes are largely unexplored. This thesis examines how an AC-signal pulse and an electrical signal consisting of a DC-bias and an AC-wave affect a full-cell VRFB under regulated state-of-charge and flow conditions. Performance indicators such as discharge capacity, power, coulombic, voltaic and energy efficiency were compared to a no-ripple baseline. The results will provide insights into VRFB cell behaviour when exposed to ripple currents and an AC signal pulse. The framework provides guidance on converter design and control strategies for VRFB-based storage systems.