**8. Conclusions**

Until now, most of the marine fuel cell research works have been limited to concentrating on the low power capacity of PEMFCs for small vessels or MCFCs for medium and large-sized vessels as auxiliary power sources.

However, in this paper, a hybrid power system was constructed along with a battery and a generator system, in order to apply the MCFC as the main power source for medium and large-sized ships. Their performance in parallel operation was monitored and investigated based on the actual experiment with the test bed.

Experiment results revealed that the deviation levels of voltage and frequency were kept within the standard ranges across all operational cases. Therefore, system synchronization was proven stable. It also confirmed that power quality of the built-in hybrid power source the test bed was compliant with the rules of the Korean Register of Shipping.

Therefore, the novelty of this research can be placed on this contribution to the design and the application of the hybrid power system using MCFC for the propulsion power system of the middle and large-sized ships.

**Author Contributions:** Conceptualization, K.Y. and H.J.; Methodology, K.Y., H.J. and S.K.; Formal analysis, H.J. and S.K.; Resources, H.J. and S.K.; X.X.; Writing—original draft preparation, K.Y. and H.J.; Writing—review and editing, K.Y., H.J. and S.K.; Visualization, H.J. and S.K.; Supervision, K.Y.

**Funding:** This research received no external funding.

**Acknowledgments:** We thank our colleagues from Kido Park, who provided insight and expertise that greatly assisted the research, although they may not agree with all of the conclusions of this paper. The authors would also like to thank POSCO Engineering co. for approval to use 'DFC300' model.

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

#### **References**


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