Optimal Flow Factor Determination in Vanadium Redox Flow Battery Control

The optimization of vanadium redox flow batteries (VRFBs) is closely related to the flow rate control: a proper regulation of the electrolyte flow rate reduces losses and prolongs battery lifetime. To this end, a flow factor control strategy in VRFBs was proposed in the literature provided with numerical/experimental validations. Yet, a theoretical justification of this approach was lacking. The respective control law is a generalization of Faraday's law of electrolysis since it employs a special scaling parameter referred to as the flow factor. In this paper, we show that this coefficient is directly related to the conversion rate of electrolyte in the cell. Furthermore, we pose an optimal control problem with maximization of total battery power integrated over time to determine an optimal flow factor. To this end, we use a simple stochastic policy gradient algorithm. The case studies illustrate the application of the computed optimal controller under various load currents and demonstrate that there is no single flow factor for all modes, the optimal performance of the battery can only be guaranteed by different values of this parameter. As a result, the proposed control strategy can be used for advanced control and monitoring tools for industrial VRFB systems.