Fig. 1 shows the schematic of the pressurized direct hydrogen fuel cell systems analyzed in this study. At the rated power point, the PEFC stack operates at 2.5 atm and 80 ◦C to yield an overall system efficiency of 50% (based on lower heating value of hydrogen).Compressed hydrogen and air are humidified to 90% relative humidity at the stack temperature using processwater and heat from the stack coolant. The system pressure is lower than 2.5 atm at part load and is determined by the operating map of the compressor–expander module.