2.3.2. Thermal Management System

The thermal management system (TMS) consists of two different types of cooling loops. One loop uses the liquid hydrogen to absorb the heat from components, while the other uses a conventional cooling cycle [47]. Since the difference between the temperature of the components and the surrounding air is small compared to other appliances, the airflow in the cooling inlet has to be higher to achieve sufficient cooling. To estimate the amount of thermal energy that must be removed from the system, the energy distribution between power used at the propellers and generated heat was taken from Hartmann et al. [47] for the cruise flight segment.

The TMS load is defined as the ratio between heat energy displaced by the TMS and the electric energy provided by the fuel cell. To calculate the TMS load for the aircraft, first, the difference in fuel cell efficiency compared to the reference in Hartmann et al. [47] was added/subtracted to the reference TMS load. These values are listed in Table 5. The second step is multiplying the resulting number with the required propulsive power to result in the heat power distributed to the TMS for the aircraft. Conventional cooling systems consist of an air intake, a radiator and a nozzle to expel the air efficiently. Most applications have an additional fan fitted to keep the performance of the radiator high even if the airspeed is low, for example during the taxi phase.

The additional drag from the radiator can be reduced with the Meredith effect [48]. Kellermann et al. [49] concluded in their studies that the drag increased by 0.7% due to the radiator. Since the study was conducted for a 30% hybridized 180 seat aircraft, the drag can be scaled with the reference heat load and a reference MTOM. To calculate the additional drag for the aircraft, an increase in heat load and a decrease in MTOM result in a higher drag penalty. The reference values from Kellermann et al. [49] are listed in Table 6.

**Table 5.** Selection basis for thermal management system load [47].


**Table 6.** Reference values for the thermal management system drag by [49].


Chapman et al. [50] conducted simulations on the transient behavior of fluid-based TMS. Their findings concluded that designing the TMS for the maximum system power as a steady state can result in oversized cooling systems. Their simple model was tested for a much smaller load and fluid mass compared to this aircraft; however, the results showed that maximum power for less than 2 min leads to a much smaller TMS size. Since the takeoff phase takes less than a minute for regional aircraft, the cruise power was used to size the TMS. The TMS fluid capacity should be sized to allow this simplification in follow up projects.

#### 2.3.3. Fuel Tanks

Liquid hydrogen is stored at temperatures below 20 K for multiple hours within the aircraft's tanks, resulting in the need for an insulated fuel tank. Compared to gaseous hydrogen, which is used in the automotive industry, the tank pressure is low, thereby, resulting in lower strength requirements. The wall thickness and resulting empty tank mass were derived from Silberhorn et al. [51] for their tank concept in the rear of the aircraft and are listed in Table 7. Their fuel tank mass was scaled cubic with the block energy. This was chosen because the tank volume, which is directly proportional to the stored energy, scales with the power of 3 with the radius. The resulting mass from the cubic scaling was further reduced since the mission range in the TLAR is only 14% of the aircraft used by Silberhorn et al. [51], resulting in an estimated reduction of wall thickness of 60%.

**Table 7.** Reference values for liquid hydrogen fuel tanks by [51].

