Thermal Modeling of a PEM Fuel Cell

Abstract

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.