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Abstract

Fuel cell systems are electrochemical energy converters which can be applied for both, stationary as well as mobile power supply. The complexity of these systems is due to the fact that they consist of three-dimensional stacks of membrane-electrode-assemblies and that they interact with a number of peripheric components such as reforming reactors, gas purification units, electric motors etc. Moreover, fuel cells can exhibit a strongly nonlinear behaviour because the electrochemical reactions not only depend on the reactant concentration and the temperature, but also on the electrostatic overpotentials along the electrodes. For the design and analysis of fuel cell systems, there is an increasing need for adequate modelling and simulation tools. In the present contribution, a modular modelling strategy is proposed which is based on the network theory for chemical processes. According to this network theory, a fuel cell process is decomposed into elementary units on several hierarchical levels (process unit level, phase level, storage level, molecular level). After decomposition, model formulation starts on the molecular level which accounts for e.g. electrochemical reaction mechanisms at the catalyst particles. From these mechanistic considerations electrochemical source terms are derived and combined with the diffusive and convective transport phenomena taking place on the phase level. These phases are aggregated to a single fuel cell unit consisting of fluid compartments, electrode backings, catalyst layers, and the membrane electrolyte. Finally, the single cells are aggregated to fuel cell stacks. For the described modular modeling procedure, the Process Modeling Tool ProMoT is used. The generated sets of equations form DAE-systems which are solved efficiently with numerical solvers avaliable within the simulation environment DIVA. The implemented models can be used to analyse the steady-state and the dynamic process behaviour. This modelling procedure is exemplified for proton exchange membrane fuel cells (PEMFC) operated with hydrogen and air.