**6. Conclusions**

The optimum component size for an FCREx depends on the cost of hydrogen and the powertrain components, the length of the ownership period, and VMT. Optimum component sizing for an FCHEV is less sensitive to these factors. The optimum design for an FCREx relies primarily on onboard hydrogen storage for energy. The proposed sizing process finds economically optimum design solutions for fuel cell vehicles while ensuring that there is no tradeoff in performance. For class 4 delivery trucks, optimized FCRExs and FCHEVs have comparable component sizes and RCO estimates. The FCREx ownership costs is slightly less than that of FCHEV by 1.3%. For a class 8 linehaul truck, the RCO of an FCHEV is 7.5% lower than that of an FCREx design. The energy cost may be equal to the initial cost. Therefore, a higher hydrogen cost could affect the solution. In this study, we do not consider the cost associated with infrastructure or downtime for charging. They need to be factored in later when comparing the RCOs of different powertrains. We expect that the proposed sizing process will use representative real-world cycles for sizing and cost estimates and for linking fuel cell cost to the power and operating conditions of load levels, duration, and so on.

**Author Contributions:** Conceptualization, R.V.; methodology, R.V. and N.K.; software, K.S.; validation, R.V., N.K. and K.S.; formal analysis, R.V.; investigation, K.S.; resources, N.K.; data curation, K.S.; writing—original draft preparation, K.S.; writing—review and editing, K.S., N.K, and R.V.; visualization, K.S.; supervision, R.V.; project administration, A.R.

**Acknowledgments:** The authors would like to acknowledge the financial support of Jason Marcinkoski (Fuel Cell Technologies Office, U.S. Department of Energy) to conduct this work. The submitted manuscript has been created by the UChicago Argonne, LLC, Operator of Argonne National Laboratory (Argonne). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

**Conflicts of Interest:** The authors declare no conflict of interest.
