E.C.A.S

Numerical study on dynamic responses of a polymer electrolyte fuel cell vehicle was performed to understand the effect of operating parameters on heat and mass transfer. We developed a system-level model to analyze dynamic behavior of fuel cell car applied with a variable pressure system consisting blower and back pressure control valve at cathode. Increased hydrogen mass flow rate increased both the anode pressure and relative humidity; when the stoichiometric number increased from 1.5 to 2.5, the pressure increased from 1.6 bar to 3.4 bar and the relative humidity increased from 32% to 62%, respectively. Although pressure increased with air mass flow rate from the blower, relative humidity decreased because blower outlet temperature increased; the relative humidity is 90% at 1.9 bar, but 30% at 1.4 bar. For back pressure control valve, pressure changed without altering mass flow rate or temperature. Therefore, relative humidity is proportional to pressure; the relative humidity is 70% at 1.6 bar, but 98% at 1.9 bar. The operating temperature was changed by controlling the fan speed. Relative humidity was inversely proportional while vehicle performance was proportional to temperature; the relative humidity is 95% at 55 °C, but 58% at 70 °C. Under drastic load variations, changes in pressure by blower and pressure regulator affected dynamic behavior differently; changing the pressure by controlling blower resulted in over/under-shoot, but that by controlling pressure regulator resulted in smooth variation. This analysis can help to understand the dynamic behavior of fuel cell vehicles at the system level and provide the proper direction for the operating strategy of fuel cell vehicles.