A model predicting transient responses of proton exchange membrane fuel cells

There has been a recent interest in modelling the transient behaviour of proton exchange membrane (PEM) fuel cells. In the past, there have been several electrochemical models which predicted the steady-state behaviour of fuel cells by estimating the equilibrium cell voltage for a particular set of operating conditions. These operating conditions included reactant gas concentrations and pressures, and operating current. Steady-state behaviour is very common and in some cases is considered as the normal operating standard. Unsteady-state behaviour, however, is becoming more of an issue, especially for the transportation applications of fuel cells where the operating conditions will normally change with time. For example, system start-up, system shut-down, and large changes in the power level may be accompanied by changes in the stack temperature, as well as changes in the reactant gas concentrations at the electrode surface. Therefore, both mass and heat transfer transient features must be incorporated into an electrochemical model to form an overall model predicting transient responses by the stack. A thermal model for a Ballard Mark V 35-cell 5 kW PEM fuel cell stack has been developed by performing mass and energy balances on the stack. The thermal characterization of the stack included determining the changes in the sensible heat of the anode, cathode, and water circulation streams, the theoretical energy release due to the reaction, the electrical energy produced by the fuel cell, and the heat loss from the surface of the stack. This thermal model was coupled to a previously-developed electrochemical model linking the power produced by the stack and the stack temperature to the amount and method of heat removal from the stack. This electrochemical model calculates the power output of a PEM fuel cells stack through the prediction of the cell voltage as a complex function of operating current, stack temperature, hydrogen and oxygen gas flowrates and partial pressures. Initially, a steady-state overall dynamic model (electrochemical model coupled with the thermal model) was developed. This was then transformed into a transient model which predicts fuel cell performance in terms of cell voltage output and heat losses as a function of time due to various changes imposed on the system.