Modellierung und Simulation von Hochtemperatur-Polymerelektrolyt-Brennstoffzellen

Fuel cells are electrochemical energy converters that convert chemical energy of constantly fed reactants directly into electricity. The most commonly used fuel gas in this respect is hydrogen, which is either produced in pure form by electrolysis, for example, or as a hydrogen-rich gas mixture (reformate gas), produced by reforming diesel or kerosene e.g.. However, a disadvantage of reformate gas is that it contains additional carbon monoxide (CO), which leads to catalyst poisoning in the fuel cell. Since higher operating temperatures also lead to a higher CO tolerance, the use of high-temperature Polymer-Electrolyte-Fuel- Cells (HT-PEFCs) is particulary suitable for reformate operation. The aim of the presented work is the modeling and CFD-simulation of HT-PEFC stacks with the intention of gaining a better understanding of multi-physical processes in the stack operation as well as the optimization and analysis of existing stack designs. The geometric modeling used is based on the Porous Volume Model, which significantly reduces the required number of computing elements. Furthermore, the electrochemical models for hydrogen / air and reformate / air operation, which were taking the CO poisoning effects into account, are developed in this work and implemented in the software ANSYS / Fluent. The resulting simulations indicated the optimal flow configuration for the stack operation in terms of the homogeneous current density distribution, which has a positive effect on the stack aging. Thus, the current densities showed a strong homogeneity regarding the stack configuration anode / cathode in counter-flow and anode / cooling in co-flow. The influence of cooling strategies was examined for the stack performance in a similar way. In the following, the local temperature distribution as well as temperature peaks within the stack could be predicted and validated with experimental measurements. Further on, the model scalability and thus the general validity of the developed modeling approach have been demonstrated. Consequently, the applied modeling approach as well as the obtained conclusions can be used as high-quality support in the development of HT-PEFC-stacks, which are particularly intended for the power supply for auxiliary power units (APUs) in lorries, ships and aircrafts

Performance and Durability of HT-PEFCs with Customized Flow Field Plates

Geometries of flow fields have a strong impact on the gas distribution in fuel cells. In this work, two different flow field geometries are compared for high temperature polymer electrolyte fuel cells, a serpentine like flow field and a spiral one. Computational fluid dynamics analysis yielded a mixed flow distribution over the active area of the spiral flow field. This cell exhibited a significantly higher voltage than the cell with the serpentine flow field. Additionally, the degradation rate of the cell with the spiral flow field was five times lower after an operation of 1000 h. Moreover, electrochemical impedance spectroscopy and cyclic voltammetry presented lower resistances and 50% larger specific electrochemical surface area after long term tests for the cell with the spiral flow field compared to the one with the serpentine flow field.</jats:p