High-Load Operating Characteristics of Planar Membrane Humidifiers of Polymer Electrolyte Membrane Fuel Cell Systems for Heavy-Duty Applications

<div class="section abstract"><div class="htmlview paragraph">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 achieved by the use of membrane humidifiers. These are passive components that transfer the product water of the cathode exhaust air to humidify the supply air. Due to the passive design, controllability is achieved via a bypass. It is possible to use map-based control strategies to avoid the use of humidity sensors. 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 gas, 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 of a humidifier. We used a perfluorosulfonic acid membrane without a gas diffusion layer. It was shown that condensed liquid fractions, even isolated droplets, at the cathode outlet significantly enhance the water transfer. The influence of water mass flow rate, pressure level, temperature, and gas flow rate on humidifier’s water transfer rate was investigated. Static and dynamic measurements were obtained, with the presence of droplets also leading to characteristic enhancements in mass transfer during dynamic operation. The analyzed data show that if liquid water is not taken into account: a) the risks of flooding, which lead to irreversible ageing processes and thus to permanent performance loss of the fuel cell are not identified and b) opportunities to improve the membrane humidifier in terms of design, operating strategies and model-based control strategies in heavy-duty applications remain unused.</div></div>