**6. Conclusions**

This study analyzed the fuel consumption and CO2 emission reduction rates when a fuel-cell-based hybrid power source instead of a conventional commercial diesel power source was used in ships. The results showed that under the rated output on a test bed with a load bank of 180 kW, the conventional commercial diesel generator consumed fuel at 43.5 kgoe/h and emitted CO2 at 148.5 kg/h, whereas the fuel-cell-based hybrid power source consumed fuel at 35.6 kgoe/h and emitted CO2 at 57.7 kg/h, as given in Table 11. The hybrid power source reduced fuel consumption by 18% and CO2 emissions by 61% at part load in the port period. These results indicate that it is possible to reduce CO2 emissions by up to 61% if a hybrid power source of the same capacity is used to power a ship.

In this study, the actual electric load analysis values of the 5500 TEU Reefer Container, 13 k TEU Container, 40 k Bulk Carrier, 130 k DWT LNG Carrier, and 300 k DWT Crude Oil Tanker were scaled down according to the operation mode, and the control logic and systems of the test bed developed in this study were operated normally according to the respective load scenarios. The experimental results of applying the developed five load scenarios to the test bed are shown in Table 15.

Because the output characteristics and control time of the diesel generator, according to the power source of the hybrid system, were reduced, according to the load variation pattern of the ship and the ship's type, the CO2 emissions of the hybrid system, as compared with the case of the diesel generator, alone operated for each load scenario with an average of 70%~74% less.

In order to apply the hybrid system to ships, it is possible to maximize the CO2 emission reduction effect by setting the capacity of the fuel cell + battery to be able to take charge of the base load of the ship through analysis of the base load of each ship type.

**Author Contributions:** Conceptualization, G.R., H.K. and K.Y.; Methodology, G.R., K.Y. and H.J.; Formal analysis, G.R. and H.J.; Supervision, H.K. and K.Y., Visualization, H.J; Writing-original draft preparation, G.R. and K.Y.; Writing-review and editing, K.Y.

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

**Acknowledgments:** We thank our colleagues from Jong Su Kim who provided insight and expertise that greatly assisted the research, although they may not agree with all of the conclusions of this paper.

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

#### **Nomenclature**

