Fig. 8b shows the heat duties of the various components for the 65kWe FCS. Compared to the results for the 120kWe FCS, the heat duties of the high-temperature radiator at 6.5% grade and at the top sustained speed are almost double even though the FCS is 50% smaller in rated power. At these driving conditions, the stack for the 65kWe FCS operates at close to the rated power and system pressure (2.5 atm), and the system efficiency is 50%; the stack for the 120kWe FCS,on the other hand, is at 50% of the rated power and 1.7 atm pressure, and the system efficiency is 55%. The combination of lower efficiency and higher pressure implies that the high-temperature coolant in the 65kWe FCS absorbs more waste heat and latent heat of condensation in the stack for the65kWe FCS (72 kW) than in the stack for the 120kWe FCS (55 kW). Also, heat duties of the air/fuel humidifiers/preheaters are lower in the 65kWe FCS than in the 120kWe FCS (12kW versus 23 kW). Thus, at these driving conditions, the heat duty of the high-temperature radiator for the 65kWe FCS is nearly twice that for the 120kWe FCS (72kW minus 12kW versus 55kW minus 23 kW).Fig. 8 indicates that the peak heat duties of the lowtemperature radiator (10 kW) and thecondenser (8 kW) in the 65kWe FCS are less than one-half of those in the 120kWe FCS.In fact, the heat duty of the condenser for the 65kWe FCS is zero at the top sustained speed (2.5 atm operating pressure) implying that the water added in the humidifiers condenses out in the stack itself. The peak heat duty of the condenser for the 65kWe FCS occurs at lower vehicle speed (70 mph) than for the 120kWe FCS (85 mph). In general, water management is easier in the 65kWe FCS than in the120kWe FCS.