Performance of Fuel Cell Systems for Heavy Duty Vehicles

Abstract

Polymer electrolyte membrane fuel cell (PEMFC) systems for heavy-duty vehicles (HDV) are attracting attention for several reasons including tighter emission regulations and, compared with light duty vehicles (LDVs), possibility of a gradual build-up of hydrogen infrastructure during the market introduction phase, and higher allowable cost (60 $/kWe vs. 30 $/kWe) and Pt loadings.1 However, competing with the incumbent diesel technology requires longer stack lifetimes than for LDVs (30,000 h vs. 8,000 h) and higher efficiencies to compensate for the likely price differential between diesel and hydrogen.2 Heat rejection in fuel cells for HDVs can also be an issue. We conducted simulations to compare radiator heat loads in two propulsion systems for a Class-8 HDV: a 450-hp turbocharged diesel engine, and a hybrid 275-kWe FCS with a 35-kWh battery. The simulations showed that the radiator heat load at rated power is higher for the hybrid propulsion system even though it is 20% more efficient than the diesel engine. Higher heat load combined with lower operating temperature makes heat rejection in an HDV radiator more challenging for the hybrid powertrain. We conducted system analysis to determine the stack operating conditions (cell voltage and temperature) and the radiator fan power required to reject the waste heat in the HDV radiator at rated power. Figure 1 summarizes the important results of the analysis of an FCS with a compressor-expander module (CEM) for air management, 2.5-atm stack inlet pressure, no cathode humidifier, de-alloyed PtCo cathode catalyst supported on a high surface-area carbon, and 0.25 mg/cm2 total Pt loading.3, 4 Figures 1a and 1b show that the heat load and radiator fan power are strong functions of the cell voltage and stack coolant temperature. These indicate a heat transfer limit that sets the minimum coolant temperature which only depends on the cell voltage. For example, at 0.7 V cell voltage, the minimum coolant temperature is 88oC corresponding to 265 kW radiator heat load and 30 kWe fan power. The cell voltage needs to be higher than 0.75 V for stacks that operate below 80oC. Figures 1c and 1d present the net stack power (stack power minus the parasitics including radiator fan power) density and system efficiency (net power produced divided by the lower heating value of H2 consumed) as functions of cell voltage and coolant temperature. Increasing the cell voltage at fixed coolant temperature improves the system efficiency at the expense of lower power density. For a fixed cell voltage, there is an optimum temperature at which the net power density is highest. The net power density is smaller below the optimum temperature because of higher fan power, and above it because of the adverse effects of low relative humidity on Ohmic losses and ORR kinetics. As a reference point, the stack can reach 1200 mW/cm2 gross and 1015 mW/cm2 net stack power density for 0.7 V cell voltage at rated power and 88oC optimum coolant temperature. The projected power density and operating conditions are based on a model formulated using the experimental data for d-PtCo/C cathode catalysts with 0.05, 0.1 and 0.15 mg/cm2 Pt loading. Further work is required to confirm these projections by building and optimizing electrodes with 0.2 mg/cm2 Pt loading. References Marcinkoski, R. Vijayagopal, J Adams, B. James, J. Kopsz, and R. Ahluwalia, DOE Advance Truck Technologies https://www.hydrogen.energy.gov/pdfs/19006_hydrogen_class8_long_haul_truck_targets.pdf L. Borup, A. Kusoglu, K. C. Neyerlin, R. Mukundan1, R. K. Ahluwalia, D. A. Cullen, K. L. More, A. Z. Weber, and D. J. Myers, “Recent Developments in Catalyst-related PEM Fuel Cell Durability,” Current Opinion in Electrochemistry, 21, 192–200 (2020) Ahluwalia, X. Wang and J-K Peng. "Fuel Cell System Modeling and Analysis," 2019 DOE Hydrogen and Fuel Cells Program Annual Merit Review and Peer Evaluation Meeting, Arlington, VA, April 29 - May 1, 2019. K. Ahluwalia, X. Wang, and A.J. Steinbach, “Performance of Advanced Automotive Fuel Cell Systems with Heat Rejection Constraint,” Journal of Power Sources, 309 (2016), 178-191. Figure 1