Computational fluid dynamics modelling of PEM fuel cells to investigate transport limitations.
Islam, Sheikh Zahidul
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Modern technological advancements in our lifestyle have caused a significant increase in the consumption of energy. With this growing demand, people are more concerned about the rational use of existing limited energy and searching for alternative forms of environmentally friendly energy sources to reduce polluting emissions. Proton Exchange Membrane (PEM) fuel cell has shown and demonstrated that potential to be a suitable alternative power source because of its simplicity of design, load following capabilities, efficiency, feasibility and quick start-up. Although having these splendid advantages, cost and durability of PEM fuel cells are one of the major challenges that needed to be overcome. Three-dimensional single-phase and multi-phase isothermal PEM fuel cell models have been developed to investigate the transport limitations of fresh reactants and its effect on cell performance. The governing equations (continuity, momentum and species transport) with appropriate source terms were solved using computational fluid dynamics (CFD) technique. A user defined function (UDF) code was developed considering source terms for porous zones, effective diffusivity models for species transport inside cells and electrochemical reactions at catalyst layers to predict cell voltage at an average current density. The average current density and net water transfer coefficient, used to calculate the source terms, were calculated using auxiliary equations and linked to the solver through UDFs. Parametric studies were performed to determine the optimal operating conditions and geometrical design of PEM fuel cell. The simulation results show that gas diffusion layer permeability has no effect on cell performance for a value lower than 10-11 m2. GDL porosity is one of the major design parameters which have significant influence on limiting current density, hence on cell performance. Land area width of PEM fuel cell shows influence on cell performance. Low membrane thickness provides higher cell performance and approximately 50% reduction in membrane thickness results approximately 100% improvement in cell performance at high current density of 1.0 Acm-2. Bruggeman correlation was used in most of previous modelling work for explaining the diffusion of species though porous GDL and CL, but this thesis considered other types of effective diffusion models and investigated the effect of diffusion models on cell performance at high current densities. Tomadakis and Sotirchos (1993) anisotropic model produces cell voltage much closer to the experimental values. Therefore, anisotropic diffusion model should be utilized in PEM fuel cell modelling to minimize modelling uncertainties. A two-phase flow, steady-state, three-dimensional PEM fuel cell model considering the phase change effect of water has been developed in the final phase of the thesis. Flooding inside the cell was captured at high current density using the model for a condensation value of 10.0 s-1. Finally, parametric studies were performed based on isotropic and anisotropic GDL permeability cases. Modelling results suggest that isotropic permeability cases have strong influence on cell performance compared to anisotropic cases at high current density.