# Analysis of Hybrid Ship Machinery System with Proton Exchange Membrane Fuel Cells and Battery Pack
## 5 Results and Discussion  
In Figure 8, the SoC, total resistance of vessel, and PEMFC power are shown for six voyages, where all components within the system respond to each other with the hysteresis mechanism implemented for the PEMFC stack, but for this configuration, it was not activated. This is better displayed in Figure 9, which has same parameters as in Figure 8 but is given for only one voyage, where it is seen that total resistance follows the pattern defined on Table 1, with additional time between recharging of the battery and repeating the voyage. It can be seen that the SoC and PEMFC power tracked the ship resistance, as the PEMFC used the battery system for compensation while getting to the work load, and when the load reached the required 600 kW, battery system started to discharge; after the first part of trip (Port of Split to Port of Resnik), the PEMFC continued to work on maximum load until the battery system reached the required SoC, as shown in Figure 5. To show the hysteresis mechanism there was a simulation of one cycle of the battery pack of 100 battery packages connected in parallel with 100 batteries connected in series (total 10,000 batteries) with a total power of 296 kWh; results are shown in Figure 10, with the hysteresis mechanism activated after the 89 th minute and kept until the end of the route. As a consequence. the total distance passed changed from 13.8 nm to 11.6 nm.  
Figure 7: Velocity and required propulsion power for duration of one voyage.  
The results in Figures 8 and 9 show that the system with the PEMFC stack of 300 kWe and battery pack with 424 kWh with an initial SoC of 87% at the start of the voyage can replace the previous configuration of two diesel engines with an output power of 300 kW each and meet all required demands for this route. With the required velocity and duration of voyage shown in Figure 8, the SoC increased to 88.14%, which ensures that route is repeatable, as the final SoC is higher than the initial one. Total consumption for one voyage was 47.45 kg, and for six voyages with the PEMFC as only source of power, it was 284.7 kg of hydrogen in comparison to diesel engines, which require 254 kg for one voyage and 1524 kg of diesel for six voyages. Fuel consumption of the hybrid system can be reduced on the last voyage by 14.6 kg, as it is end of the day, so the battery pack can be recharged with an outer electric energy supply. For 284.7 kg of hydrogen, the storage capacity required is 7200 L under a pressure of 700 bar. All greenhouse gases, including CO\({}_{2}\), are removed in comparison to the 4886 kg of CO\({}_{2}\) generated from diesel engines, which was calculated from the IMO Resolution MEPC.308(73) [28].  
Figure 8: Comparison of power, SoC, and total ship resistance for six voyages.  
Figure 9: Comparison of power, SoC, and total ship resistance for one voyage.  
If the time that the ship is moored in the harbor increases, the battery pack capacity could be reduced, and approximately 2 more minutes in full load increases the total consumption of energy up to 3.5% with the increase of distance traveled from 13 nm to 13.8 nm per voyage. In conclusion, this means that the realistic power consumption is smaller than in the numerical model. As the system is 100% electric and compatible with a conventional electric energy grid, it is possible to implement the system with photovoltaics and plug-in charge of batteries, which will consequently significantly reduce the required battery pack power, or it is possible to implement another PEMFC stack as an auxiliary or upscaled power system to meet electric energy requirements.