Torsten Berning

Three-dimensional computational analysis of transport phenomena in a PEM fuel cell

Abstract A comprehensive non-isothermal, three-dimensional computational model of a polymer electrolyte membrane (PEM) fuel cell has been developed. The model incorporates a complete cell with both the membrane-electrode-assembly (MEA) and the gas distribution flow channels. With the exception of phase change, the model accounts for all major transport phenomena. The model is implemented into a computational fluid dynamics code, and simulations are presented with an emphasis on the physical insight and fundamental understanding afforded by the detailed three-dimensional distributions of reactant concentrations, current densities, temperature and water fluxes. The results show that significant temperature gradients exist within the cell, with temperature differences of several degrees K within the MEA. The three-dimensional nature of the transport is particularly pronounced under the collector plates land area and has a major impact on the current distribution and predicted limiting current density.

A 3D, multiphase, multicomponent model of the cathode and anode of a PEM fuel cell

A computational fluid dynamics multiphase model of a proton-exchange membrane ~PEM! fuel cell is presented. The model accounts for three-dimensional transport processes including phase change and heat transfer, and includes the gas-diffusion layers ~GDL! and gas flow channels for both anode and cathode, as well as a cooling channel. Transport of liquid water inside the gas-diffusion layers is modeled using viscous forces and capillary pressure terms. The physics of phase change is accounted for by prescribing local evaporation as a function of the undersaturation and liquid water concentration. Simulations have been performed for fully humidified gases entering the cell. The results show that different competing mechanisms lead to phase change at both anode and cathode sides of the fuel cell. The predicted amount of liquid water depends strongly on the prescribed material properties, particularly the hydraulic permeability of the GDL. Analysis of the simulations at a current density of 1.2 A/cm 2 show that both condensation and evaporation take place within the cathode GDL, whereas condensation prevails throughout the anode, except near the inlet. The three-dimensional distribution of the reactants and products is evident, particularly under the land areas. For the conditions investigated in this paper, the liquid water saturation does not exceed 10% at either anode or cathode side, and increases nonlinearly with current density. The operation of proton-exchange membrane ~PEM! fuel cells depends not only on the effective distribution of air and hydrogen, but also on the maintenance of an adequate cell operating temperature and fully humidified conditions in the membrane. The fully humidified state of the membrane is crucial to ensuring good ionic conductivity and is achieved by judicious water management. Water content is determined by the balance between various water transport mechanisms and water production. The water transport mechanisms are electro-osmotic drag of water ~i.e., motion of water molecules attaching to protons migrating through the membrane from anode to cathode!; back diffusion from the cathode ~due to nonuniform concentration!; and diffusion and convection to/from the air and hydrogen gas streams. Water production depends on the electric current density and phase change. Without control, an imbalance between production and removal rates of water can occur. This can result in either dehydration of the membrane, or flooding of the electrodes, which are both detrimental to performance. A common water management technique relies on the humidification of the air and hydrogen gas streams. At higher current densities, the excess product water is removed by convection via the air stream, and the rate of removal is controlled by adjusting moisture content in concert with pressure drop and temperature in the flow channels. Thermal management is also required to remove the heat produced by the electrochemical reaction in order to prevent drying out of the membrane, which in turn can result not only in reduced performance but also in eventual rupture of the membrane. Thermal management, which is performed via forced convection cooling in larger stacks, is also essential for the control of the water evaporation or condensation rates. The operation of a fuel cell and the resulting water and heat distributions depend on numerous transport phenomena including charge-transport and multicomponent, multiphase flow, and heat transfer in porous media. The complexity and interaction of these processes and the difficulty in making detailed in situ measurements have prompted the development of a number of numerical models. The theoretical framework was laid out in early one-dimensional numerical models of the membrane-electrode. 1-3 A quasi-twodimensional model based on concentrated solution theory was also proposed by Newman and Fuller, 4 and a full two-dimensional model including flow channels but no electrodes was also presented by Nguyen and White. 5 This model was refined in a number of subsequent studies to account for the porous electrodes and interdigitated

Three-dimensional computational analysis of transport phenomena in a PEM fuel cell—a parametric study

This paper presents the results of a parametric study conducted with a previously described three-dimensional, non-isothermal model of a polymer electrolyte membrane (PEM) fuel cell. The effect of various operational parameters such as the temperature and pressure on the fuel cell performance was investigated in detail. It was found that in order to obtain physically realistic results experimental measurements of various modelling parameters were needed. The results show good qualitative agreement with experimental results published in the literature. In addition, geometrical and material parameters such as the gas diffusion electrode (GDE) thickness and porosity as well as the ratio between the channel width and the land area were investigated. The contact resistance inside the cell was found to play an important role for the evaluation of the impact of such parameters on the fuel cell performance. The results demonstrate the usefulness of this computational model as a design and optimization tool.

A Computational Analysis of Multiphase Flow Through PEMFC Cathode Porous Media Using the Multifluid Approach

A three-dimensional multiphase model that describes the liquid water flux through the porous media and into the gas flow channel of a proton exchange membrane fuel cell (PEMFC) cathode is presented. The model is based on the multifluid approach, thus solving one complete set of transport equations for each phase. It is used to investigate the effect of different material parameters on the predicted liquid water saturation in various layers, i.e., catalyst layer (CL), microporous layer (MPL), and gas diffusion layer (GDL). Each layer can be characterized by its porosity, permeability, and contact angle, while the Leverett function was used to describe the capillary pressure vs saturation. The irreducible saturation can be specified for each layer independently. An expression for the channel/GDL interface condition was developed, which accounts for the number of droplets per unit area at the GDL interface. Describing the different porous media by Leverett equations leads to a jump condition in saturation across each interface. In accordance with previous studies it is found that the MPL remains at a low saturation level, while the CL might become flooded depending on the fraction of hydrophilic pores. Under-the-land compression leads to an increased level of flooding compared to the channel section.

The Effect of Inhomogeneous Compression on Water Transport in the Cathode of a Proton Exchange Membrane Fuel Cell

A three-dimensional, multicomponent, two-fluid model developed in the commercial CFD package CFX 13 (ANSYS Inc.) is used to investigate the effect of porous media compression on water transport in a proton exchange membrane fuel cell (PEMFC). The PEMFC model only consist of the cathode channel, gas diffusion layer, microporous layer, and catalyst layer, excluding the membrane and anode. In the porous media liquid water transport is described by the capillary pressure gradient, momentum loss via the Darcy-Forchheimer equation, and mass transfer between phases by a nonequilibrium phase change model. Furthermore, the presence of irreducible liquid water is taken into account. In order to account for compression, porous media morphology variations are specified based on the gas diffusion layer (GDL) through-plane strain and intrusion which are stated as a function of compression. These morphology variations affect gas and liquid water transport, and hence liquid water distribution and the risk of blocking active sites. Hence, water transport is studied under GDL compression in order to investigate the qualitative effects. Two simulation cases are compared; one with and one without compression.

Multiphase Simulations and Design of Validation Experiments for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells directly convert into electricity the chemical energy of hydrogen and oxygen from air. The by-products are just water and waste heat. Depending on the operating conditions the water may be in the liquid or gas phase, and liquid water can hence plug the porous media in the fuel cell, and, more importantly, the flow channels and outlet ports of a single cell in a stack. These problems may be avoided if the fuel cell operates in a way that both the anode and cathode outlet stream are exactly fully humidified, i.e. the relative humidity is at 100 %. Such operation can conceivably be obtained by adjusting the operating conditions using dew point diagrams. In this paper numerical results will be presented of two different flow field arrangements, both using the interdigitated flow field. It will be shown that arranging the gas streams in a counter-flow, “x-flow” modus is the preferred option. Moreover, a detailed analysis of the preferred channel width and land area shows that the finest pitch is predicted to yield the highest membrane hydration levels and is thus preferred.Copyright

Parametric Study of Transport Phenomena in PEM Fuel Cells Using a 3D Computational Model

This paper presents the results of a parametric study conducted using a three-dimensional, non-isothermal model of a PEM fuel cell. The effect of various operational, geometric and property parameters was investigated in detail. The availability of reliable data for setting various modeling parameters was found to be critical to obtain physically realistic results. In addition to the effect of temperature and pressure, geometrical and material parameters such as the gas-diffusion thickness and porosity as well as the ratio between the channel width and the collector plate land area were investigated in detail, and it was found that the contact resistance plays an important role for the evaluation of the impact of such parameters on the fuel cell performance. The results demonstrate the usefulness of this computational model as a design and optimization tool.Copyright

Modelling multiphase flow inside the porous media of a polymer electrolyte membrane fuel cell

Transport processes inside polymer electrolyte membrane fuel cells (PEMFC’s) are highly complex and involve convective and diffusive multiphase, multispecies flow through porous media along with heat and mass transfer and electrochemical reactions in conjunction with water transport through an electrolyte membrane. We will present a computational model of a PEMFC with focus on capillary transport of water through the porous layers and phase change and discuss the impact of the liquid phase boundary condition between the porous gas diffusion layer and the flow channels, where water droplets can emerge and be entrained into the gas stream.

A Numerical Algorithm and a Graphical Method to Size a Heat Exchanger

This paper describes the development of a numerical algorithm and a graphical method that can be employed in order to determine the overall heat transfer coefficient inside heat exchangers. The method is based on an energy balance and utilizes the spreadsheet application software Microsoft Excel™. The application is demonstrated in an example for designing a single pass shell and tube heat exchanger that was developed in the Department of Materials Technology of the Norwegian University of Science and Technology (NTNU) where water vapor is superheated by a secondary oil cycle. This approach can be used to reduce the number of hardware iterations in heat exchanger design.Copyright

Computational Modelling and Simulation of Proton-Exchange Membrane Fuel Cells (Keynote)

Fuel cells (FC’s) are electrochemical devices that convert directly into electricity the chemical energy of reaction of a fuel (usually hydrogen) with an oxidant (usually oxygen from ambient air). The only by-products in a hydrogen fuel cell are heat and water, making this emerging technology the leading candidate for quiet, zero emission energy production. Several types of fuel cell are currently undergoing intense research and development for applications ranging from portable electronics and appliances to residential power generation and transportation. The focus of this lecture is Proton-Exchange Membrane Fuel Cells (PEMFC’s). An electrolyte consisting of a “solid” polymer membrane, low operating temperatures (typically below 90 °C) and a relatively simple design combine to make PEMFC’s particularly well suited to automotive and portable applications. The operation of a fuel cell relies on electrochemical reactions and an array of coupled transport phenomena, including multi-component gas flow, two phase-flow, heat and mass transfer, phase change and transport of charged species. The transport processes take place in variety of media, including porous gas diffusion electrodes and polymer membranes. The fuel cell environment makes it impossible to measure in-situ the quantities of interest to understand and quantify these phenomena, and computational modelling and simulations are therefore poised to play a central role in the development and optimization of fuel cell technology. We provide an overview of the role of various transport phenomena in fuel cell operation and some of the physical and computational modelling challenges they present. The processes will be illustrated through examples of multi-dimensional numerical simulations of Proton-Exchange Membrane Fuel Cells. We close with a perspective on some of the many remaining challenges and future development opportunities.Copyright