John C. Fagley

Thermal Modeling of a PEM Fuel Cell

Mathematical modeling of fuel cells can take place at many different levels of detail, from simplified spreadsheet representations to detailed CFD (computational fluid dynamics) models. All of these levels are utilized within General Motors Corporation Fuel Cell Activities. This paper describes the development and application of a model used for analysis of the thermal aspects of a PEM fuel cell. The model domain is a single cell in a fuel cell stack, which is broken into between ten and two hundred control volumes. Each control volume includes eleven lumps; one each for anode, cathode and coolant streams, three for diffusion media/membrane electrode assembly (DM/MEA), four for the cathode portion of the bipolar plate, and four for the anode portion of the bipolar plate. The resulting simulation has the following features; (1) Unlike most CFD representations which typically contain hundreds of thousands or even millions of elements, the model described here does not solve the equations of motion to determine velocity profiles in the anode, cathode or coolant channels. Rather, the flow rates in the anode and cathode flow fields are specified by the user. Typically, uniform flow profiles are assumed, although maldistributed flows may be specified as well. (2) By using between ten and two hundred control volumes, the model can represent the spatial variations of RH (relative humidity) and temperature, (3) The relatively low computational overhead of the modeling approach described here (as compared to a more detailed CFD approach) facilitates dynamic simulation of the cell, i.e. transient thermal response of the system can be simulated, (4) Heat effects simulated include heat released by electrochemical reaction, convection from the fluid streams to the solid lumps (DM/MEA, cathode and anode plates), and conduction in the bipolar plates. This paper also describes some of the ways the model has been used to analyze thermal aspects of fuel cell operations; (1) Sensitivity of the temperature difference between the DM/MEA and coolant plate thermal conductivity and contact resistance, and (2) Impact of coolant flow field (cross-flow and co-flow) on cathode RH. Other potential applications of this type of model are also outlined, including modeling of the cell during transient operation, and start-up.Copyright

PEM Fuel Cell Research Direction for Automotive Application

In recent years, there has been an increasing amount of PEM (proton exchange membrane) fuel cell-related research conducted and subsequently published by universities and public institutions. While a good deal of this research has been useful for understanding the underlying fundamental aspects of fuel cell components and operation, much of it is not as useful for a group working on automotive applications as it could be. The reason for this is that in order to be put to practical use in an automotive application, the system being studied must meet certain constraints; satisfying targets for projected system costs, system efficiency, volumetric and gravimetric power densities (packaging), and operating conditions. For example, numerous recent publications show studies with PEM fuel cells designed and built such that limiting current density is achieved at 0.9 A/cm2 or lower, and voltages of 600 mV can only be achieved at current densities less than 0.6 A/cm2. This type of performance is sufficiently below what is required for commercial application, that any conclusions drawn from these works are difficult to extrapolate to a system of commercial automotive interest. The purpose of this article is to show, through use of engineering calculations and cost projections, what operating conditions and performance are required in a commercial automotive fuel cell application. In addition, best known (public domain) performance and corresponding conditions are given, along with Department of Energy Freedom Car targets, which can be used for state-of-the-art benchmarking. Also, reference is made to a university publication where performance (500 mV at 1.5 A/cm2) close to automotive application targets was achieved, and important aspects of their components and flow field geometry are highlighted. It is our hope that through this publication, further PEM fuel-cell related research can be directed toward the region of greatest interest for commercial, automotive application.© 2005 ASME