OPTIMAL CHARGING AND CONVERTER DESIGN FOR VANADIUM REDOX FLOW BATTERIES

The battery energy storage system (BESS) has become an indispensable part of the current electricity network due to the vast integration of renewable energy sources (RESs). The vanadium redox flow battery (VRB) is considered the most suitable for grid-tied BESSs due to its long cycle-life, deep discharging capability and decoupling capability of rated power and energy capacity. The charging power from RESs is time-varying, which makes the real-time determination of the optimal charging current challenging, considering the extraction of the maximum amount of power. To address these challenges, an advanced charging control scheme for the VRB storage system is proposed in this thesis. The proposed approach determines the appropriate charging current and the optimal electrolyte flow rate based on the available time-varying input power. Moreover, the charging current is bounded by the limiting current, which prevents the gassing side reactions and protects the VRB from overcharging. The charging/discharging currents fluctuations might be harmful to conventional non-flow (e.g. lead-acid and lithium-ion) batteries. Redox flow batteries, on the other hand, are not subject to slow solid-state reaction kinetics or to the formation of any insulating films on the electrode surface. Therefore, high-frequency ripples may not significantly impact the battery, and some results in existing literature support the claim. However, a complete study on the effects of current ripples on the VRB cell is still missing. This thesis reports results from simulation and experimental studies conducted using a laboratory-scale single vanadium redox flow cell subjected to different ripple currents during charging and discharging. The results confirmed that the ripple currents does not significantly impact the VRB cell. The control techniques of the interfacing converters for VRB systems need to control the average charging/discharging current and the ripple magnitude of this current simultaneously. This introduces significant complexities in the control strategies. A capacitor-less bidirectional synchronous dc-dc converter with a modified control scheme was proposed in this thesis to overcome these limitations. The efficacy of the proposed converter was verified through the experimentations, which confirmed that the proposed converter control the average charging/discharging current and its ripple magnitudes are bounded within the preset limit