*2.3. Powertrain*

New propulsion technologies require new calculation approaches. By scaling the confirmed values of previous work, the component architecture can be calculated as laid out in the following sections.

#### 2.3.1. Fuel-Cell System

The sizing of the fuel-cell system was conducted based on the evaluation of the feasibility of dual use of liquid hydrogen in regional aircraft [47]. The focus of the research of Hartmann et al. [47] is an ATR 72, and thus the results of their baseline scenario are scaled down using the fuel-cell polarization curve for the baseline case in [47]. The ATR 72 is the larger version of the reference aircraft; however, since the powertrain is only influenced by the required power for the aircraft and not other characteristics, e.g., the wingspan and fuselage length, it is viable to scale the powertrain from the ATR 72 to a smaller aircraft.

The linearized section is assumed to be applicable from a current density of 0.2 up to 1.4 A/cm². Three different cases were investigated, and the resulting sizing of the fuel cell had to work for all of them. The first scenario is the maximum power during take-off. For this case, the highest current density is expected. The upper limit of 1.4 A/cm² results in the smallest size of the fuel cell, while it is desirable to achieve a lower current density since that correlates linearly with higher efficiency. The second case is cruise flight. The segment is the longest of the whole mission; therefore, a high efficiency in this segment results in a high efficiency for the total flight. The third case is the descent phase.

Since the powertrain components utilize superconducting technologies, it is necessary to provide a minimum flow of hydrogen to keep the components within their operating window. This hydrogen will not be vented but instead used to power the aircraft during descent, with the excess power being used to charge the battery. In this case, the current density should not be lower than 0.2 A/cm². The minimum flow is calculated in three steps:


Another limiting boundary condition for the fuel cell sizing is the maximum de-charge rate of the battery during take-off. For durability reasons, this is set as 2C. The mass of the fuel cell is calculated using the stack mass density given for the baseline scenario in [47] using the calculated fuel cell area for the aircraft. The mass of the compressor and the humidifier are linearly scaled from the results for the ATR 72 using the fuel cell power. To account for structures, etc., we introduced a Balance of Plant (BoP) factor. This was set as 20% by Hartmann et al. [47] and was used for the aircraft to add to the total mass. The reference values for the calculations are listed in Table 4.


