(33f) Three Dimensional Modelling of PEM Fuel Cells: Heterogeneity Analysis of Pore Structure on Fuel Cell Hydrodynamics

In this work a three-dimensional mathematical model is presented for Polymer Electrolyte Membrane Fuel Cells (PEMFCs) that can be used for analysing the effects of pore structure heterogeneities on fuel cell hydrodynamics and in turn on fuel cell performance. Water management and flooding are critical issues in the design of (PEMFCs) which can greatly depend on the heterogeneous cathode and anode pore structures. Current three-dimensional PEMFC models either do not consider these effects or tend to over simplify the problem by making the questionable isotropic assumption regarding permeability structure. To overcome these deficiencies, an extended network permeability model has been developed in which a network of pores and capillaries is representative of the random porous media structure within the fuel cell. The network model has been developed in MATLAB and is capable of simulating heterogeneous pore distributions within the PEMFCs. To model the fuel cell flow dynamics the free flows in the gas channels have been modeled by the Navier-Stokes equations for laminar regimes whereas the flows in very low permeability porous cathode/anode structures are represented by the conventional Darcy equation. A computational sub-model describing the incorporation of electrochemical reaction source terms in the momentum balance equations for free and porous flow regimes within the fuel cell has been devised. The developed network permeability model and model have been incorporated in a general purpose single phase flow module to simulate the water flooding levels and dynamic flow field variables whilst the fuel cell is in operation. The network model is capable of simulating varying permeability in the porous regimes based on water saturation fluctuations. At increasing saturation, capillaries are blocked off progressively and the updated flow field is solved. Pore size distribution is a critical design parameter and henceforth a parametric study has been carried out to determine the effect of different pore-size distributions on the flow-field and cell efficiency in order to determine the optimum pore size structure. The simulated results for water saturation, dynamic flow field variables and fuel cell performance have been compared against the experimental data available in literature for purposes of model validation. The developed computer model offers a robust, cost-effective and user friendly design and analysis tool for fuel cell manufacturers, which can be easily used by engineers in industry.