Dynamic model for estimation of hydrogen flow rate in hydrogen recirculation system for the PEM fuel cell stack

Hydrogen utilization rate is critical for hydrogen-electricity conversion efficiency of the PEM fuel cell system. Significant part of hydrogen is inevitably wasted as a result of essential periodical vent of accumulated gaseous and liquid impurities, which degrades hydrogen flow rate and hydrogen concentration, in hydrogen recirculation system (HRS) for anode reaction chamber of the stack. Estimation of hydrogen flow rate is the key to improving hydrogen utilization rate, because impurities can be vented only when actual flow rate is lower than acceptable range. This paper investigates dynamic model of the HRS to construct connection between hydrogen flow rate and obtainable parameters. Firstly, lumped-parameter dynamic model is established for the recirculation pumpdriven and ejector-driven HRS. Coupling mechanism between hydrogen flow rate and pressure of each recirculation apparatus is introduced to dynamic model, and then transfer function between pressure at inlet and outlet of anode chamber is derived for estimation of hydrogen flow rate in comparison. According to Nyquist plot, the recirculation pump-driven HRS behaves as a common first-order or second-order inertial system while the ejector-driven HRS behaves as a novel shifted first-order system. Secondly, effect of purge valve action on flow rate of the ejector-driven HRS is analyzed in analogical way based on transition between operating points on ejector characteristic curve. It shows that opening purge valve contributes to larger flow rate, even if pressure at backflow inlet is decreased. Thirdly, experiment on plant of an ejector-driven HRS shows that Nyquist plot of transfer function in complex coordinate is a circle with origin included and it's in consistent with that by dynamic model. Besides, relation is found between circle radius and flow rate at ejector outlet. Finally, experiment result on a 120 kW fuel cell system validates explanation to effect of opening purge valve on hydrogen flow rate of the ejector-driven HRS.