Two-Phase Pressure Drop Characteristics During Low Temperature Transients in PEMFCs

Abstract

Proton Exchange Membrane fuel cells are being considered as the powertrain of choice for automotive applications. Automotive fuel cells experience transients during start-up, shut-down and changing load conditions, which constitute a significant part of the drive cycle. Transient behavior of PEMFCs can be classified into three categories: electrochemical, thermal and two-phase flow. Two-phase transients require a longer time to return to steady state than the electrochemical transient (which typically requires less than 1 second). Experiments have shown two-phase transients to be more prominent at the lower temperatures due to the increased presence of liquid water. Overshoot / undershoot behavior of current and voltage has been observed during investigations of electrochemical transients. This study investigates similar overshoot / undershoot behavior in the two-phase pressure drop in the reactant channels. An increase in the current drawn from the PEMFC is accompanied by larger air flow rates and greater water generation. An in situ setup is utilized to measure the pressure drop in the reactant channels across the length of the channel, when the electrical load drawn from the PEMFC is changed. This pressure drop measurement along the length of the reactant channels is used to characterize the overshoot / undershoot behavior. A parametric study is conducted to identify the factors which influence the overshoot / undershoot in two-phase flow pressure drop. The transient behavior is explored at the temperatures of 40, 60 and 80°C. Transient behavior is more pronounced at the lower temperature. Five different ramp rates have been used to show that faster ramp rates results in larger overshoot. The effect of magnitude of current change is investigated using four levels of load change. It was observed that increased magnitude of change results in increased overshoot behavior. However, no direct relationship has been observed between the magnitude of overshoot and the time required to return to steady state.