*3.3. Restrictions on CO2 Emissions from LH2 Carriers*

The attained EEDI indicates the CO2 emissions per unit of deadweight divided by the ship speed, which is calculated using Equation (10) for each ship [36].

$$\text{EEDI}\_{\text{att}} = \frac{\text{P}\_{\text{ME}} \cdot \text{C}\_{\text{ME}} \cdot \text{SFC}\_{\text{ME}} + \text{P}\_{\text{AE}} \cdot \text{C}\_{\text{AE}} \cdot \text{SFC}\_{\text{AE}}}{\text{DWT} \cdot \text{V}\_{\text{ref}^\*}} \tag{10}$$

PME: Power of the main engine

PAE: Power of the auxiliary engine

CME: Conversion factor of the main engine between the fuel consumption and CO2 emissions

CAE: Conversion factor of the auxiliary engine between the fuel consumption and CO2 emissions

SFCME: Specific fuel consumption of the main engine

SFCAE: Specific fuel consumption of the auxiliary engine

DWT: Deadweight of the ship

Vref: Speed of the ship

A diesel electric propulsion obtained using LNG is assumed for LH2 carriers. Specific fuel consumption is assumed as 175 g/kWh. The conversion factor between the fuel consumption and CO2 emissions is 2.75 [37]. According to the Marine Environment Protection Committee (MEPC) issued by the IMO, the power of the main engine for diesel electric propulsion is calculated using Equation (11). The parameter η is taken as 91.3 %, which indicates the product of the electrical efficiencies of the generators, transformers, converters and motors. Considering ships whose rated output of the motor is larger than 10,000 kW, the power of the auxiliary engine is calculated using Equation (12) [36].

$$P\_{\rm ME} = 0.83 \times \frac{\rm MPP\_{\rm motor}}{\eta} \tag{11}$$

$$P\_{\rm AE} = 0.025 \times \text{MPP}\_{\rm motor} + 250 \text{ kW} \tag{12}$$

MPPmotor: Rated output of the motor

η: Product of the electrical efficiencies of the generator, transformer, converter, and motor

The PEMFC system uses the BOH to generate electricity, which is then utilized for propulsion in conjunction with the electricity from the main engine. Therefore, the required power of the main engine is calculated using Equation (13). In this study, the efficiency of the PEMFC system is assumed to be 42% compared with lower heating value of hydrogen. In the case of LNG carriers with a BOG re-liquefaction system, the power required for the BOG re-liquefaction is added to the auxiliary engine power, as shown in Equation (14).

$$\text{P}\_{\text{ME}} = 0.83 \times \frac{\text{MPP}\_{\text{motor}}}{\eta} - \text{P}\_{\text{PEMFC}} \tag{13}$$

$$P\_{\rm AE} = 0.025 \times \text{MPP}\_{\rm motor} + 250 \text{ kW} + P\_{\rm re-liq} \tag{14}$$

PPEMFC: Electricity generated from the PEMFC Pre−liq: Power required for re-liquefaction

The required EEDI indicates the criteria that the ship under EEDI regulations must satisfy. Equations (15)–(17) show the methodology for calculating the required EEDI [36].

$$\text{EEDI}\_{\text{ref}} = a \cdot b^{-c} \tag{15}$$

$$\text{EEDI}\_{\text{ref}} = (1 - \chi) \times \text{EEDI}\_{\text{ref}} \tag{16}$$

$$\text{EEDI}\_{\text{att}} \le \text{EEDI}\_{\text{med}} \tag{17}$$

EEDIref: Reference EEDI EEDIreq: Required EEDI

The parameters *a* and *c* in the required EEDI equation are determined based on the type of ships. The variable *b* is the deadweight of the ship. *X*, which is between 0 and 1, is a reduction factor that indicates the reinforcement of the regulations over time. The time factor (referred to as the 'phase') represents the reinforcement of the regulations over time, which is determined using the value of *X*. For example, phase 3 indicates the year after 2025 and the factor *X* in this time is 0.3.

Because the EEDI regulations for LH2 carriers have not yet been designated, various perspectives should be considered before determining the final designation. This study considers the following EEDI candidates:

