Compound Fuel Cell Thermodynamic Model for System Simulation

Abstract

A simulation model of a fuel cell system is created using a discretized image of the fuel cell membrane and the liquid cooled bipolar plate. Thus it is possible to link processes at cell level (overvoltage, diffusion processes in the membrane) with processes at cell stack level (coolant mass flow, exhaust gas recirculation). The simulation model achieves short computing times and thus enables sensitivity analyses to be performed efficiently for different operating parameters. Coupled heat and mass transfer processes take place in a liquid cooled low-temperature cell stack, while an electrical DC power is produced by the stack. The time-dependent equation systems of the mass and power balances for the fuel cell system are derived and discretized to determine the exact distribution of the waste heat in the outflowing media. As diffusion of water through the membrane is taken into account, the system behaviour of a low-temperature PEM cell stack is finely resolved that the effects of different system configurations and different operating strategies (gas temperature control, gas humidification, exhaust gas recirculation, variable environmental conditions, cold start) can be investigated with sufficient accuracy in a dynamic simulation. As the equation system of the mass balance includes the water diffusion term, the value of the membrane humidity is explicitly calculated as a function of time and coordinate. The modelling approaches are validated with the help of laboratory tests for a fuel cell cold start. A 4 kW fuel cell system with hydrogen and air or with hydrogen and oxygen will be used for validation. Simulation results for different cold start temperatures are presented and compared with the measured values (figure 1).