Pressure-alternating gas impedance: A new pathway for estimating recirculation flow rate of ejectors to avoid fuel cell degradation

Abstract

An ejector-based gas recirculation system (EBGRS) has emerged as a mainstream configuration for hydrogen subsystems of proton exchange membrane (PEM) fuel cells. However, this configuration poses challenges in regulating anode water and hydrogen concentration owing to the passive characteristics of the ejector. To avoid anode flooding and hydrogen starvation and improve the lifetime of PEM fuel cells, precise closed-loop control of anode purge is essential. Such control relies on the feedback of the ejected recirculation flow rate (ERFR). However, existing methods to measure or estimate ERFR, including sensor measurement, model-based estimation, and pressure-drop-based estimation, do not meet the needs of vehicle applications. To address this bottleneck, this study proposes a new pathway for estimating ERFR, which is named the gas impedance method. Two gas impedance variables are defined and a theoretical model of EBGRS is proposed. By systematically analyzing the modeling methods, the lumped parameter method is adopted to model the recirculation pipeline. The flow rate–pressure characteristics of the ejector are locally linearized to couple with the recirculation pipeline model. The analytical expressions of the two gas impedance variables are derived from the model. Moreover, the analytical model is validated via numerical simulations and experiments. Validation results demonstrate that the analytical model effectively describes the frequency response of gas impedance and its correlation with ERFR for various operating conditions and frequencies, paving the path for estimating ERFR accurately using gas impedance. Future research will focus on improving the accuracy of the analytical model and fully exploiting the potential of gas impedance in estimating ERFR.