CHARACTERIZATION OF A PEM FUEL CELL STACK UNDER TRANSIENT CONDITIONS AND ITS USE IN SIMULATING A FC POWERTRAIN

Vehicles powered by fuel cells have received great interests in recent years because of their potential to reduce dependence on fossil fuels, reduce greenhouse gases and have some flexibility in power train configuration. Modeling of vehicle powertrains, using a combination of components and energy storage media, are widely used to predict vehicle performance under different duty cycles. A fuel cell vehicle has to meet the varying load demands for acceleration, deceleration and accessory loads. This paper deals with simulations of a fuel cell powered hybrid vehicle. The simulations were carried out using modification in the Powertrain Simulation Analysis Toolkit (PSAT) components. A small laboratory stack was used to characterize transient performance of a low temperature PEM fuel cell. The stack, with eight fuel cells, had a rating of about 1.0 kW. The transient characteristics of the PEM stack were obtained by loading and unloading the stack at different current densities, stack temperatures and reactant humidity. The information was used in the simulation to predict performance of a medium-sized crossover SUV when operated in different test cycles. Transient loads on the fuel cell produced undershoot in cell voltage when the loading exceeded 0.15 A/cm(2)-s. This impacts efficiency of the fuel cell. The results show that an appropriate size of fuel cell and battery combination produced best fuel economy. The optimum rating of the fuel cell varied somewhat depending on the drive cycle. The highway cycle required a larger fuel cell while a smaller fuel cell provided better fuel economy in the UDDS cycle. Incorporating transient characteristics of the fuel cell reduced fuel economy by about 3-4% over the steady state characteristics.