Thermal modeling and temperature control of a PEM fuel cell system for forklift applications

Temperature changes in PEM fuel cell stacks are considerably higher during load variations and have a negative impact as they generate thermal stresses and stack degradation. Cell hydration is also of vital importance in fuel cells and it is strongly dependent on operating temperature. A combination of high temperature and reduced humidity increases the degradation rate. Stack thermal management and control are, thus, crucial issues in PEM fuel cell systems especially in automotive applications such as forklifts. In this paper we present a control-oriented dynamic model of a liquid-cooled PEM fuel cell system for studying temperature variations over fast load changes. A temperature dependent cell polarization and hydration model integrated with the compressor, humidifier and cooling system are simulated in dynamic condition. A feedback PID control was implemented for stack cooling. The stack energy balance was reduced to a first order differential equation using a lumped approach. The first-order Linear Time-Invariant system was used to obtain the transfer function which was determined based on experimental data at different stack loads. The developed model approach can assist designers in choosing the required coolant mass flow rate and radiator size to minimize the stack temperature gradients.