A Fluid Dynamic Analysis of Mixing Phenomena inside Electrolyte Tanks to Enhance Performance of Industrial-Scale Vanadium Redox Flow Batteries

Abstract

The control of electrolyte concentration supplied to the stack of a redox flow battery is crucial to determine voltage output and so power performance of Redox Flow Batteries (RFBs). In common lab-scale RFBs systems, in particular vanadium ones, perfect mixing hypothesis is usually assumed regarding mixing in tanks, due to small volumes involved. The State of Charge (SOC) of large-scale vanadium redox flow batteries is usually measured by means of Open Circuit Voltage (OCV) cell. The Industrial Scale Vanadium Redox Flow Battery (IS-VRFB) which is currently in operation at the Electrochemical Energy Storage and Conversion Laboratory (EESCoLab) of the University of Padua (Padova, Italy) with a rated power/energy of 9 kW/27 kWh is equipped with two 550 L electrolyte tanks. This experimental facility is suitable for the evaluation of differences between the measured SOC and the real one in the tanks. In fact, galvanostatic constant flow factor charge and discharge operations show unpredicted behaviour in the SOC curves measured through the OCV cell upstream the stack. In particular, a step size variation results from low current charge operations after a long quasi-flat plateau whose duration is correlated to current and flow rate values considered. Due to the lack of similar studies in scientific literature, a computational fluid dynamic analysis has been performed by using commercial software. Common adopted transient models where flow-field is considered time-independent resulted in numerical plateau which is orders of magnitude smaller than the experimentally measured. Therefore, it is reasonable to hypothesize that other time-dependant physical phenomena are involved in mixing and should be taken into account. Thermal effects due to electrochemical reactions were considered negligeable with respect to buoyancy phenomenon that is triggered from electrolyte density variation in charging-discharging operations. Considering high estimated Richardson number for tank, natural convection inside stored electrolyte volume is modelled through Boussinesq hypothesis, identifying a fictitious temperature gradient between bulk and inflow electrolyte. Taking advantage from buoyancy driven segregation phenomena can lead to control concentration distributions in the electrolyte volume and therefore manage power performance of the tank in charge and discharge. This study intends to provide skills for the RFB designer in order to proper size tanks according to their needs.