On the Effect of Complex Permeability and Thermal Material Properties for 3D-CFD Simulation of PEM Fuel Cells

Abstract

Fuel cells are considered a key technology to decarbonize the power generation sector, thanks to the absence of pollutants emissions related to the direct chemical-electric energy conversion, their high global efficiency, and the possibility for on-board electricity production, overcoming the storage limits of batteries. An example of the renewed interest towards fuel cells is the research in Proton Exchange Membrane Fuel Cell (PEMFC) in the automotive sector, as a candidate alternative to fossil fuels-fed internal combustion engines (ICEs). The complex interplay of electrochemical and physical phenomena concurring in PEMFC makes their understanding and optimization a challenging task. This is a field of active research thanks to the development of advanced CAE tools, e.g., 3D-CFD simulations of non-isothermal reactive flows, in which all the relevant physics is numerically solved, allowing to identify governing mechanisms as well as system bottlenecks. Among the multiple complex aspects, the material property characterization of PEMFC components is one of the major modelling challenges for modern CAE tools. This is usually provided as a set of boundary conditions for the numerical model, having a large impact on the simulated results which is often motivated by an oversimplification of materials characteristics. Examples of commonly overlooked aspects are direction-independent thermal/flow properties for fibrous materials, the neglection on the deformed (compressed) status, and the simplified contact approach. All of these might alter the key parameters (e.g., water management) and mislead designers’ conclusions on PEMFC optimization. In this paper three-dimensional CFD simulations are used to weight the impact of orthotropic diffusion layer properties on both flow distribution and heat transfer. In the first part, a simplified test case from literature is created and used to investigate the flow convection/diffusion balance in the gas diffusion layer considering the orthotropic permeability typical of pressed fibrous layers. Differences with respect to the still widely used isotropic permeability will be assessed, and implications on channel bypass and mass transport to the catalyst layer will be provided. In the second part, the analysis moves to the use of orthotropic thermal conductivity for the fibrous gas diffusion layers, which is another commonly discarded aspect despite being well documented in literature. A critical analysis of heat transfer routes between parts of different heat capacity (membrane, diffusion layers, solid plates) and thermal field for all the components will be assessed. Finally, thermal contact resistance between adjacent pressed materials will be applied. The altered thermal pathways for heat removal will be critically analyzed, as well as the differences in temperature distribution and their implication on electricity production and water management. This hierarchical flow/thermal analysis will provide guidelines for more accurate 3D-CFD models for a deeper understanding of flow and heat dynamics in PEMFC.