Experimental and Simulative Investigations of the PEM Fuel Cell System and the Thermal Management of the Overall Vehicle

Hydrogen-powered Fuel Cell Electric Vehicles (FCEV) are considered a highly promising option for heavy-duty applications due to their short refueling times and long ranges. Within these vehicles, the integration of an efficient and intelligent thermal management system is of crucial importance to optimize efficiency and lifetime at both component and vehicle level. In addition, there is optimization potential to reduce system complexity and packaging volume by simplification of the Balance of Plant (BoP) topology, for example by eliminating the external reactant humidifier. In this work, on the one hand, a co-simulation for the holistic analysis of FCEV thermal management at the system level is developed. On the other hand, experimental investigations on the cathode exhaust gas condition of a fuel cell system without external reactant humidification were carried out and are described in this work. The findings related to stack thermal management are intended to be integrated into the full vehicle co-simulation, thus combining simulation and testing.
Generally, Proton Exchange Membrane (PEM) fuel cells for electrochemical energy conversion represent a key component for sustainable mobility, only emitting heat, liquid water and oxygen-reduced air mixed with water vapor and small quantities of hydrogen. The in-depth investigation of the exhaust gas composition regarding water and oxygen content as well as mass flow is highly significant not only for general fuel cell research but also for the operation of the stack thermal management, in particular for systems without external humidification. This is due to the fact, that stack and cathode air cooling influence the water vapor absorption capability of the cathode gas. The fuel cell thermal management can therefore be used to control the formation of liquid water inside the stack and plays a major role in preventing membrane drying out.
Direct sensor based measurement of the cathode exhaust gas mass flow and relative humidity is difficult due to the high humidity environment and the potential formation of water droplets. For this reason, a mathematical thermodynamic model was developed and used to calculate the exhaust gas mass flow and relative humidity. This model was also validated based on the gas species balancing method between the cathode inlet and exhaust and using data measured on the fuel cell system test bed. The determined correlations between the cathode exhaust gas relative humidity and the fuel cell current density serve as an indicator for the saturation state of the cathode exhaust gas and will be integrated into the full vehicle co-simulation to ensure intelligent operation of the stack thermal management system throughout the whole fuel cell operating range.