Development and validation of a real time flow control integrated MPPT charger for solar PV applications of vanadium redox flow battery

In this paper a real time flow control integrated solar PV Maximum Power Point Tracking (MPPT) charge controller for Vanadium Redox Flow battery (VRFB) is developed and its performance is demonstrated under practical dynamic insolation profiles. Unlike the conventional Lead acid and Li-ion batteries the major challenge in designing the solar MPPT charger for VRFB lies in simultaneous control of charging current and corresponding flow rate for maintaining the maximum overall system efficiency of the VRFB besides achieving the maximum charging efficiency. In this work, the usual Perturb & Observe (P&O) algorithm for designing MPPT charger for conventional batteries has been significantly modified in case of VRFB charge controller by including the real time control of flow rate along with the charging current. The proposed charging algorithm has been validated by a practical 1 kW 6 h VRFB system operation. The modified MPPT based three stage constant current constant voltage (CC-CV) charging topology is found to be the most efficient for VRFB charging from solar PV. Solar MPPT charging of VRFB with a constant flow rate leads to premature thermal shut down of the charge controller resulting incomplete charging of VRFB which is prevented by the use of dynamic flow control integrated solar PV MPPT charging. Two practical case studies of sunny weather and cloudy weather conditions have been adopted for validation of the integrated MPPT charge controller. The proposed VRFB charging system is a generalised one and thus can be very useful for large scale VRFB application in solar PV power systems.