*5.1. Overall Composition of Power Generation System of the Hybrid Power System for Ships*

The power generation system can be divided according to the power use purpose, as shown in Figure 6: main power, emergency power, auxiliary power, and alternative maritime power (AMP). The fuel cell, generator, and battery supply power to the main and auxiliary power sources. While AMP is generally supplied from onshore sources through cold ironing, in the hybrid power system, depending on the anchoring period, fuel cells with low greenhouse gas emissions can supply power on board.

**Figure 6.** Power generation system of the hybrid power system for a ship.

### *5.2. Classification of Fuel Cell Components*

A molten-carbonate fuel cell (MCFC) generally consists of a regulator, desulfurizer, humidifier, pre-converter, super heater, recycle blower, fresh air blower, inline heater, and catalytic oxidizer [41]. However, MCFCs for ships are comprised of the following components as shown in the block diagram of Figure 7: an air supply system, fuel supply system, water process system, pre-reformer system, fuel cell stack, fresh water system, auxiliary boiler and steam system, and cargo handling system [42].

**Figure 7.** Fuel cell system of the hybrid power system for a ship.

Figure 8 is the configuration of the fuel cell for ships. The MCFC fuel supply system for ships must be connected to the pre-reformer system in the liquified natural gas (LNG) fuel supply chain, the fuel supply system of LNG propulsion ships was selected.

The electrolyte of the molten carbonate fuel cell (MCFC) is alkali metal carbonate, which is a mixture of lithium and potassium or lithium and sodium carbonate contained in a ceramic matrix of LiAlO2. In general, it operates at a high temperature of 600–700 ◦C and carbonate ions (CO3 <sup>2</sup>−) act as a charge carrier. Figure 9 and Equations (9)–(11) show a schematic diagram and chemical reactions occurring in MCFC [41].

$$\text{Total Reaction}: \text{ H}\_2 + \frac{1}{2}\text{O}\_2 + \text{CO}\_2 \rightarrow \text{H}\_2\text{O} + \text{CO}\_2.\tag{9}$$

Anode Reaction : H2 + CO2<sup>−</sup> <sup>3</sup> → CO2 + H2O + 2e−. (10)

$$\text{Cathode Reaction}: \frac{1}{2}\text{o}\_2 + \text{CO}\_2 + 2\text{e}^- \rightarrow \text{CO}\_3^{2-}.\tag{11}$$

**Figure 8.** Composition of the fuel cell system for a ship.

**Figure 9.** The schematic diagram and chemical reactions for the molten-carbonate fuel cell (MCFC) using hydrogen fuel.

MCFC needs to be supplied carbon dioxide together with oxygen to the cathode. The supplied carbon dioxide is converted into carbonate ions and becomes a means of moving ions between the cathode and the anode. The transferred carbonate ions are converted back to carbon dioxide by reaction with hydrogen at the anode side, and water and electricity are generated together as a result. In MCFC, not only hydrogen but also carbon monoxide can be used as fuel. Figure 10 schematic diagram and chemical reactions for MCFC using carbon monoxide fuel.

**Figure 10.** The schematic diagram and chemical reactions for the MCFC using carbon monoxide fuel.

In case of using carbon monoxide as fuel, the chemical reaction of the cathode is the same as that of using hydrogen as fuel. Oxygen and carbon dioxide supplied to the cathode react with each other to be converted to carbonated ions, which are transferred to the anode through the electrolyte. The transferred carbonate reacts with carbon monoxide supplied to the anode side and is converted back to carbon dioxide.
