High-load operating characteristics of planar membrane humidifiers of polymer electrolyte membrane fuel cell systems for heavy-duty applications

To gain high efficiencies and long lifetimes, polymer electrolyte membrane fuel cell systems require precise control of the relative humidity of the cathode supply air. This is usually accomplished with membrane humidifiers. These are passive components that use the humidity of the cathode exhaust air to humidify the supply air. Due to the passive design, controllability is achieved via a bypass. To avoid the use of humidity sensors, map-based control strategies are possible. Such map-based control requires deep insights into the humidifier behavior in all possible thermodynamic operating states, including various water loads. This paper focuses on typical operating conditions of heavy-duty application at high load, specifically on the occurrence of liquid water in the cathode exhaust stream, which has not been sufficiently investigated in the literature yet. In order to simulate these conditions, we built a test rig with an optically accessible single-channel set-up. To measure the humidifier membrane influence, we used a pure perfluorosulfonic acid membrane without the influence of a gas diffusion layer. It was shown that condensed liquid fractions, even isolated droplets, at the cathode outlet significantly improve the water transfer. Geometry and alignment show a very strong influence on these operating points. The influence of mass flow, pressure level and temperature were investigated. We obtained both static and dynamic measurements, whereas the occurrence of droplets also led to characteristic improvements in mass transfer during dynamic operation. Data and analysis serve for the improvement of the membrane humidifier regarding design, operating strategies and model-based control in heavy-duty applications.