**6. Conclusions**

In this work, the authors have performed a comparison between the steam methane reforming and steam methanol reforming technologies combined with HT-PEMFC and carbon capture systems for hydrogen-fueled ship applications. To find the most suitable technologies, an energy/exergy analysis, along with a space and fuel cost investigation, have been conducted. All the simulations have been conducted at a fixed Wnet, electrical (475 kW).

It is shown that, at the base condition, the energy and exergy efficiencies of the methanol-based system are 7.99% and 1.89% higher than those of the methane-based system, respectively. The different efficiencies between systems mainly arises from the reforming temperature difference. For fuel and CO2 storage, the methanol-based system requires a space 1.1 times larger than that of the methane-based system for the total navigation time, although the methanol-based system has higher electrical efficiency. Accordingly, the methanol-based system has 2.2 times higher fuel cost than the methane-based system for 475 kW of net electricity generation during the total navigation time. In the parametric study,

both systems show a similar trend, in which with increasing reforming temperature and S/C ratio, the electrical, exergy, and cogeneration efficiencies gradually decreased.

The comparative analysis reveals that the methanol-based system has many technological advantages directly related to its low reforming temperature, which leads to better integration to the HT-PEMFC. However, the methane-based system showed economic advantages from the perspective of fuel cost and better availability in the maritime sector.

Furthermore, this work shows the feasibility of combining reforming, HT-PEMFC, CO2 capture, and liquefaction systems for both methanol and methane fuels in heat and power integration point of view. Excessive heat from HT-PEMFC and reformer in both methane-, methanol-based system are enough for CO2 capture unit, which require a large amount of heat to regenerate the amine solvent. In addition, for the methane based-system, cold energy of LNG, which should be vaporized, can be utilized for CO2 liquefaction, therefore power consumption for compressors can be reduced. Although separate ammonia refrigerant cycle is required for CO2 liquefaction for methanol based-system and power consumptions for compressors are slightly high, methanol based-system has still higher efficiency.

Several limitations were identified for consideration in the future study. In the present study, constant current density of 0.2 A cm−<sup>2</sup> was assumed and resulted in a little higher electrical efficiency. More simulations in several current density within the operating window of HT-PEMFC are required in the future study. In addition, future study should use output voltage with real reformate gas for the detailed assessment. Present study compared two systems in the process simulation level, however, future study should include sizing and on-board arrangement of systems for reforming, CO2 capture, and liquefaction systems since those systems may take large spaces and lead to different results. Furthermore, other fuels such as ethanol and liquefied petroleum gas (LPG) which is getting attention together with methanol and LNG in maritime industry should be assessed in the future study. Although the present study has some limitations, the concepts suggested in this study can give other perspectives on applying hydrogen fuel cell on board and can be a good reference for the further development of hydrogen fuel cell ship.

**Author Contributions:** Formal analysis, H.L.; Investigation, Y.N.; Methodology, I.J. and G.R.; Supervision, H.K.; Writing—original draft, H.L.; Writing—review and editing, H.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This material is based upon work supported by the Technology Innovation Program (20004659, the development of LNG Fuel Gas Supply System for Coastal Ships) which was funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea) and Core Technology Development and System Construction for LNG Bunkering program (20180048, Test evaluation for LNG bunkering equipment and development of test technology) which was funded by the Ministry of Oceans and Fisheries (Korea).

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