Design and Analysis of Cryogenic Cooling System for Electric Propulsion System Using Liquid Hydrogen

: As the demand for eco-friendly energy increases, hydrogen energy and liquid hydrogen storage technologies are being developed as an alternative. Hydrogen has a lower liquefaction point and higher thermal conductivity than nitrogen or neon used in general cryogenic systems. Therefore, the application of hydrogen to cryogenic systems can increase efﬁciency and stability. This paper describes the design and analysis of a cryogenic cooling system for an electric propulsion system using liquid hydrogen as a refrigerant and energy source. The proposed aviation propulsion system (APS) consists of a hydrogen fuel cell, a battery, a power distribution system, and a motor. For a lab-scale 5 kW superconducting motor using a 2G high-temperature superconducting (HTS) wire, the HTS motor and cooling system were analyzed for electromagnetic and thermal characteristics using a ﬁnite element method-based analysis program. The liquid hydrogen-based cooling system consists of a pre-cooling system, a hydrogen liquefaction system, and an HTS coil cooling system. Based on the thermal load analysis results of the HTS coil, the target temperature for hydrogen gas pre-cooling, the number of buffer layers, and the cryo-cooler capacity were selected to minimize the thermal load of the hydrogen liquefaction system. As a result, the hydrogen was stably liqueﬁed, and the temperature of the HTS coil corresponding to the thermal load of the designed lab-scale HTS motor was maintained at 30 K.


Introduction
Currently, a large part of the world's energy demand depends on fossil fuels, and exhaust gases are causing many environmental problems.To solve this problem, regulations on gas emissions are being strengthened, and interest in new and renewable energy technologies for global energy replacement is growing.In response to this trend, hydrogen energy technology is being researched as a new energy.Producing electricity using hydrogen fuel cells is environmentally friendly because the by-product is pure water.In addition, hydrogen has a high specific energy density that can provide three times more energy per unit mass than fossil fuel-based systems.Hydrogen can be extracted from various substances in a variety of ways and used as a power source for power grid or power appliances [1][2][3][4][5].
In order to use hydrogen as an energy source, it should be easy to store and transport.In the past, there were economic problems for hydrogen generation, transportation, and storage, and there were also technical problems with ortho-para conversion of the hydrogen liquefaction process.Recently, with the development of various catalyst technologies, the ortho-para conversion has been overcome, and the performance of liquid hydrogen storage containers has been improved.Moreover, along with the improvement of the conversion Energies 2023, 16, 527 2 of 21 efficiency of fuel cells, the development of systems related to the use of hydrogen in various fields is being actively carried out [6][7][8][9][10][11][12].
In superconducting application, operating temperature is a main design condition that determines performance and reliability.In a typical indirect cooling system, using liquid hydrogen as the refrigerant can cool the helium gas to a lower temperature than when using liquid nitrogen or liquid neon, improving system performance and reliability.Although conceptual designs using liquid hydrogen for superconducting systems were being attempted, practical designs and methods were not sufficient [13][14][15][16][17][18][19].Additionally, there is no case of performance verification through manufacturing and experimentation of a cooling system using liquefied hydrogen.It is very difficult to maintain and utilize hydrogen in its liquid state.In order to confirm whether liquefied hydrogen can be used as a refrigerant, the following research was conducted.In order to design various nextgeneration propulsion systems using hydrogen energy in the future, it is essential to develop and verify basic design technology to utilize liquefied hydrogen.
This paper describes the design and analysis of a cryogenic cooling system for an electric propulsion system using liquid hydrogen as a refrigerant and energy source.The proposed aviation propulsion system (APS) consists of a hydrogen fuel cell, a power distribution system, a battery, and a motor.The fuel cell and battery capacity of the proposed APS were selected in consideration of aircraft operation characteristics [20].For the lab-scale 5 kW superconducting motor using 2G high-temperature superconducting (HTS) wire, the HTS motor and cooling system were analyzed for electromagnetic and thermal characteristics using a finite element method-based analysis program.The HTS motor was analyzed with magnetic field distribution, temperature distribution, and thermal load.To verify the performance of the cooling system, an HTS coil needed to be operated under the same heat-load conditions that the HTS motor was designed, manufactured, and tested with.The liquid hydrogen-based cooling system consists of a pre-cooling system, a hydrogen liquefaction system, and an HTS coil cooling system.To minimize the thermal load of the hydrogen liquefaction system, the target temperature for hydrogen gas precooling, the number of buffer layers, and the cryogenic cooler capacity were selected.In addition, the current leads of the HTS coil cooling system were constructed of HTS wire to minimize thermal loads.
As a result, the total heat load of the designed lab-scale 5 kW HTS motor was analyzed to be 55 W. The heat load of the hydrogen liquefaction system was 9.27 W, and the bottom temperature was maintained at a temperature sufficient for the hydrogen to be liquid.The heat load of the HTS coil cooling system was calculated to be 7.92 W, and it was realized through the operating current and heater to be the same as the heat load of the 5 kW HTS motor.The temperature of the HTS coil corresponding to the thermal load of the designed lab-scale HTS motor was maintained at 30 K by the helium gas passing through the liquid hydrogen-helium heat exchanger.These results can be utilized in the design of application systems that use hydrogen as both an energy source and refrigerant.

Design of the Prototype APS 2.1. Configuration of the APS
Figure 1 shows the proposed APS.The proposed APS consists of a converter, power distribution system, battery, hydrogen fuel cell, and HTS motor.Hydrogen was stored as a liquid to minimize the volume and weight of the fuel.LH 2 is three times the energy density of jet fuel.Liquid hydrogen has a boiling point of 20.28 K, which was high enough to drive HTS motors.The hydrogen gas that cools the HTS motor was transferred to the hydrogen fuel cell and converted into electrical energy.The by-product generated in this process is pure water, which is environmentally friendly.The power distribution system included a hydrogen fuel cell and a battery, which was an auxiliary energy source, and they were connected to supply stable power to the APS system.The HTS motor was operated and controlled through a DC-AC converter.
included a hydrogen fuel cell and a battery, which was an auxiliary energy source, and they were connected to supply stable power to the APS system.The HTS motor was operated and controlled through a DC-AC converter.Table 1 shows the peak power rate and energy consumption requirements for each operating mode of the aircraft [21].The aircraft has five operating modes: take-off, climbout, approach-landing, taxi in/out, and cruise operations.Each operating mode has different requirements for peak power rate and energy consumption.Aircraft require maximum power in take-off mode, and most of the energy is consumed in cruise mode.The proposed APS operates with two energy sources: a hydrogen fuel cell as the main power source and a battery as the auxiliary power source.The overall weight of the APS was minimized by selecting an appropriate capacity in consideration of the specific power ratio and transient response characteristics of the battery and hydrogen fuel cell output.The capacity of the proposed lab-scale APS is summarized in Table 2.The battery and hydrogen fuel cell capacity were selected in consideration of energy conversion efficiency.Figure 2 shows the configuration of a 5 kW HTS motor.The HTS motor consists of a rotor body, HTS field coil, cryogenic vessel, armature winding, stator teeth, and magnetic shield.The type of motor designed was a rotating field-type synchronous motor.The motor was cooled by the refrigerant passing through the inside of the rotor body, and the Table 1 shows the peak power rate and energy consumption requirements for each operating mode of the aircraft [21].The aircraft has five operating modes: take-off, climbout, approach-landing, taxi in/out, and cruise operations.Each operating mode has different requirements for peak power rate and energy consumption.Aircraft require maximum power in take-off mode, and most of the energy is consumed in cruise mode.The proposed APS operates with two energy sources: a hydrogen fuel cell as the main power source and a battery as the auxiliary power source.The overall weight of the APS was minimized by selecting an appropriate capacity in consideration of the specific power ratio and transient response characteristics of the battery and hydrogen fuel cell output.The capacity of the proposed lab-scale APS is summarized in Table 2.The battery and hydrogen fuel cell capacity were selected in consideration of energy conversion efficiency.Figure 2 shows the configuration of a 5 kW HTS motor.The HTS motor consists of a rotor body, HTS field coil, cryogenic vessel, armature winding, stator teeth, and magnetic shield.The type of motor designed was a rotating field-type synchronous motor.The motor was cooled by the refrigerant passing through the inside of the rotor body, and the HTS coil was cooled to the operating temperature by heat conduction.The shape of the HTS coil was a racetrack form that can secure a sufficient length of a straight line in order to maximize the output of a radial-type synchronous motor.In addition, each pole of the field coil was stacked with two HTS coils to increase the magnetic flux density.A cryostat for maintaining the cryogenic state of the rotor part and the vacuum state surrounds the rotor.The material of the stator teeth was GFRP, and a non-magnetic material was used to minimize the influence of harmonics.A magnetic shielding layer was placed on the outer layer of the motor to minimize leakage flux.The material properties of the HTS motor components are summarized in Table 3.For a practical HTS motor design to be applied in an electric propulsion system, an air-gap magnetic flux of 2 to 3 T should be applied.In addition, an iron core must be applied to the armature to reduce the size of the motor and increase the magnetic flux density.In addition, the large iron loss of the iron core must be included in the design process.However, since the main purpose of this paper is the applicability of the cooling system using liquid hydrogen, it is difficult to reach 2~3 T in the air-gap magnetic flux of the HTS 5 kW motor applied to the prototype APS, and it is designed as an air-core type to minimize iron loss.
HTS coil was cooled to the operating temperature by heat conduction.The shape of the HTS coil was a racetrack form that can secure a sufficient length of a straight line in order to maximize the output of a radial-type synchronous motor.In addition, each pole of the field coil was stacked with two HTS coils to increase the magnetic flux density.A cryostat for maintaining the cryogenic state of the rotor part and the vacuum state surrounds the rotor.The material of the stator teeth was GFRP, and a non-magnetic material was used to minimize the influence of harmonics.A magnetic shielding layer was placed on the outer layer of the motor to minimize leakage flux.The material properties of the HTS motor components are summarized in Table 3.For a practical HTS motor design to be applied in an electric propulsion system, an air-gap magnetic flux of 2 to 3 T should be applied.In addition, an iron core must be applied to the armature to reduce the size of the motor and increase the magnetic flux density.In addition, the large iron loss of the iron core must be included in the design process.However, since the main purpose of this paper is the applicability of the cooling system using liquid hydrogen, it is difficult to reach 2~3 T in the air-gap magnetic flux of the HTS 5 kW motor applied to the prototype APS, and it is designed as an air-core type to minimize iron loss.The detailed design specifications of the lab-scale 5 kW HTS motor are summarized in Table 4.When voltage and current of 380 V and 7.8 A were applied through an external power supply, the rotational speed and torque, which are mechanical outputs, were 400 rpm and 123 N•m, respectively, in steady state.The maximum bending diameter of the HTS field coil wound in the shape of a racetrack was 35 mm, the number of turns was 130, and the effective length corresponding to the straight section was 174 mm.The mechanical and electrical air gaps of the designed HTS motor were 15 mm and 65 mm, respectively.The operating current was designed considering the 70% margin of the critical current,  The detailed design specifications of the lab-scale 5 kW HTS motor are summarized in Table 4.When voltage and current of 380 V and 7.8 A were applied through an external power supply, the rotational speed and torque, which are mechanical outputs, were 400 rpm and 123 N•m, respectively, in steady state.The maximum bending diameter of the HTS field coil wound in the shape of a racetrack was 35 mm, the number of turns was 130, and the effective length corresponding to the straight section was 174 mm.The mechanical and electrical air gaps of the designed HTS motor were 15 mm and 65 mm, respectively.The operating current was designed considering the 70% margin of the critical current, and the operating temperature of the HTS motor was 30 K. Figure 3 shows the distribution of the magnetic field for a 5 kW HTS motor.The magnetic field of the air gap that affects the output of the HTS motor was 0.27 T, and the perpendicular magnetic field that has the greatest effect on the performance degradation of the HTS coil was 1.17 T. greatest effect on the performance degradation of the HTS coil was 1.17 T.

Thermal Design and Analysis of the 5 kW HTS Motor
Figure 4 presents the configuration of the rotor.In order to minimize the cooling capacity and secure the economic feasibility and cooling margin of the cooling system, it is essential to minimize the thermal load on the main rotor components.The current lead was a path that supplied power for excitation of the HTS field coil.The current lead was connected to the room temperature part and the cryogenic part, and it accounts for the largest heat load among the rotor parts due to the generation of Joule heat by applying current.Therefore, the cross-sectional area and length must be appropriately selected to minimize the thermal load of the current lead.The optimal design for minimizing the thermal load of the current leads can be calculated by reference [22].The design specifications of the current leads are summarized in Table 5.The current lead consists of a copper

Thermal Design and Analysis of the 5 kW HTS Motor
Figure 4 presents the configuration of the rotor.In order to minimize the cooling capacity and secure the economic feasibility and cooling margin of the cooling system, it is essential to minimize the thermal load on the main rotor components.The current lead was a path that supplied power for excitation of the HTS field coil.The current lead was connected to the room temperature part and the cryogenic part, and it accounts for the largest heat load among the rotor parts due to the generation of Joule heat by applying current.Therefore, the cross-sectional area and length must be appropriately selected to minimize the thermal load of the current lead.The optimal design for minimizing the thermal load of the current leads can be calculated by reference [22].The design specifications of the current leads are summarized in Table 5.The current lead consists of a copper block connected with the HTS coil, a feed-through to reduce the temperature difference between room temperature and cryogenic temperature, and a brass current terminal.
Torque tubes must be designed for stability against mechanical torque and minimal thermal load.Torque tube design specifications considering the minimization of thermal load and mechanical torque stability can be derived by reference [23].Table 6 summarizes the specifications of the torque tube.The material of the torque tube is GFRP, which has strength in a cryogenic environment and low thermal conductivity that can minimize conduction of heat load.
Torque tubes must be designed for stability against mechanical torque and minimal thermal load.Torque tube design specifications considering the minimization of thermal load and mechanical torque stability can be derived by reference [23].Table 6 summarizes the specifications of the torque tube.The material of the torque tube is GFRP, which has strength in a cryogenic environment and low thermal conductivity that can minimize conduction of heat load.Thermal analysis of the rotor, torque tube, and current lead was performed using the FEM program.The upper part of the torque tube and current lead was considered to be at room temperature, and an operating current of 190 A was applied to the current lead.Figure 5 presents the thermal analysis results of the rotor part and current leads.The temperature of the current lead connected to the cryogenic part was 23 K, and the maximum temperature of the upper part of the HTS coil was 23 K.The heat load of the current lead, which accounted for the largest proportion of the heat load on the rotor part, was calculated to be 20 W, and the conduction and Joule heat load on the rotor part was calculated  Thermal analysis of the rotor, torque tube, and current lead was performed using the FEM program.The upper part of the torque tube and current lead was considered to be at room temperature, and an operating current of 190 A was applied to the current lead.Figure 5 presents the thermal analysis results of the rotor part and current leads.The temperature of the current lead connected to the cryogenic part was 23 K, and the maximum temperature of the upper part of the HTS coil was 23 K.The heat load of the current lead, which accounted for the largest proportion of the heat load on the rotor part, was calculated to be 20 W, and the conduction and Joule heat load on the rotor part was calculated to be 44.1 W. Considering that the rotor surface was covered with 20 turns of multi-layer insulation, the thermal load due to radiation was 11.3 W. As a result, the total heat load of the rotor part was calculated as 55.4 W. Table 7 shows the calculated heat loads for each rotor component.to be 44.1 W. Considering that the rotor surface was covered with 20 turns of multi-la insulation, the thermal load due to radiation was 11.3 W. As a result, the total heat load the rotor part was calculated as 55.4 W. Table 7 shows the calculated heat loads for ea rotor component.Figure 6 shows the configuration diagram for verifying the hydrogen cooling syst performance.To verify the performance of the hydrogen cooling system, an HTS coil id tical to the HTS motor thermal load was designed.The total heat load of the 5 kW H motor was found to be 55.4 W. The specifications of the HTS coil for the cooling test summarized in Table 8.The shape of the wound HTS coil is O-shaped, and the outer ameter and number of turns are 105 mm and 50 turns, respectively.Figure 7 shows fabrication of the HTS coil and the experiment in liquid nitrogen conditions.As a res as shown in Figure 8, the critical current and magnetic field predicted through FEM an ysis were consistent with the experimental measurement results.Figure 6 shows the configuration diagram for verifying the hydrogen cooling system performance.To verify the performance of the hydrogen cooling system, an HTS coil identical to the HTS motor thermal load was designed.The total heat load of the 5 kW HTS motor was found to be 55.4 W. The specifications of the HTS coil for the cooling test are summarized in Table 8.The shape of the wound HTS coil is O-shaped, and the outer diameter and number of turns are 105 mm and 50 turns, respectively.Figure 7 shows the fabrication of the HTS coil and the experiment in liquid nitrogen conditions.As a result, as shown in Figure 8, the critical current and magnetic field predicted through FEM analysis were consistent with the experimental measurement results.presents the configuration of the cooling system for the APS.The components of the cooling system for the APS were a pre-cooling system, a hydrogen cooling system, and an HTS coil cooling system.In order to minimize the overall heat load of the cooling system, hydrogen gas at room temperature was pre-cooled using liquid nitrogen and then injected into the hydrogen cooling system.Hydrogen was cooled to 21 K by the cryo-cooler of the hydrogen cooling system and accumulated in the inner tank.The liquid hydrogen-helium heat exchanger was immersed in liquid hydrogen in a hydrogen cooling system.The cooled helium was moved to the HTS coil to cool it to the target temperature.Two cryo-coolers were used in the hydrogen cooling system for hydrogen liquefaction, and the total cooling capacity of the cooling system was 120 W. The design process of the pre-cooling system, O-P converter, and tank inner wall to calculate the heat load of the hydrogen cooling system was shown in Figure 10.The results presented in Section 3 were derived by relying on the FEM thermal analysis program that can secure results through reflection of the physical properties of the material and setting of heat transfer conditions, because it was difficult and unreliable to derive results through heat transfer equation-based calculations considering the phase change of hydrogen, fluid properties, and complex ambient conditions.The FEM analysis program used for the design and thermal analysis of the cooling system is COMSOL.In the case of analysis involving fluid, turbulence analysis including the gravity of the fluid was performed.Each component boundary condition for analysis is presented in the figure and table in Section 3.

Desing of the the Cooling System
3.1.1.Configuration of the Cooling System for the APS Figure 9 presents the configuration of the cooling system for the APS.Th nents of the cooling system for the APS were a pre-cooling system, a hydroge system, and an HTS coil cooling system.In order to minimize the overall heat l cooling system, hydrogen gas at room temperature was pre-cooled using liquid and then injected into the hydrogen cooling system.Hydrogen was cooled to 2 cryo-cooler of the hydrogen cooling system and accumulated in the inner tank.T hydrogen-helium heat exchanger was immersed in liquid hydrogen in a hydro ing system.The cooled helium was moved to the HTS coil to cool it to the targe ature.Two cryo-coolers were used in the hydrogen cooling system for hydrogen tion, and the total cooling capacity of the cooling system was 120 W. The desig of the pre-cooling system, O-P converter, and tank inner wall to calculate the he the hydrogen cooling system was shown in Figure 10.The results presented in were derived by relying on the FEM thermal analysis program that can secu through reflection of the physical properties of the material and setting of hea conditions, because it was difficult and unreliable to derive results through hea equation-based calculations considering the phase change of hydrogen, fluid p and complex ambient conditions.The FEM analysis program used for the design mal analysis of the cooling system is COMSOL.In the case of analysis involv turbulence analysis including the gravity of the fluid was performed.Each co boundary condition for analysis is presented in the figure and table in Section 3

Design of the Pre-cooling System
The copper pipe was directly cooled by immersion in a pre-cooling system filled with liquid nitrogen at a temperature of 77 K.The inside of the copper pipe was half filled with FeO, a catalyst for ortho-para conversion of hydrogen.Hydrogen gas at room temperature passed through the inside of the copper pipe and was cooled.Otho-para conversion of hydrogen proceeded at a rate according to the temperature of hydrogen.The configuration of the pre-cooling system is shown in Figure 11, and the analysis conditions and specifications are summarized in Table 9.The target outlet temperature of the pre-cooling system was less than 80 K, and it was confirmed to be 78.6K in FEM analysis as shown in Figure 12.

Design of the Pre-Cooling System
The copper pipe was directly cooled by immersion in a pre-cooling system filled with liquid nitrogen at a temperature of 77 K.The inside of the copper pipe was half filled with FeO, a catalyst for ortho-para conversion of hydrogen.Hydrogen gas at room temperature passed through the inside of the copper pipe and was cooled.Otho-para conversion of hydrogen proceeded at a rate according to the temperature of hydrogen.The configuration of the pre-cooling system is shown in Figure 11, and the analysis conditions and specifications are summarized in Table 9.The target outlet temperature of the precooling system was less than 80 K, and it was confirmed to be 78.6K in FEM analysis as shown in Figure 12.

Design of the Pre-cooling System
The copper pipe was directly cooled by immersion in a pre-cooling system filled with liquid nitrogen at a temperature of 77 K.The inside of the copper pipe was half filled with FeO, a catalyst for ortho-para conversion of hydrogen.Hydrogen gas at room temperature passed through the inside of the copper pipe and was cooled.Otho-para conversion of hydrogen proceeded at a rate according to the temperature of hydrogen.The configuration of the pre-cooling system is shown in Figure 11, and the analysis conditions and specifications are summarized in Table 9.The target outlet temperature of the pre-cooling system was less than 80 K, and it was confirmed to be 78.6K in FEM analysis as shown in Figure 12.

Design of the Hydrogen Cooling System
The hydrogen cooling system consists of a liquid nitrogen shield layer, a buffer layer, a cooling pipe, and an O-P converter.There is a liquid nitrogen shielding layer on the outer wall to minimize radiant heat in the hydrogen cooling system, and there are several buffer layers to minimize convective heat loss in the inner tank.The inside of the O-P converter was filled with FeO, a catalyst for ortho-para conversion of hydrogen.The precooled hydrogen gas was cooled inside a heat pipe connected to a cryo-cooler and was liquefied through an O-P converter and accumulated in an inner tank.The configuration of the hydrogen cooling system is shown in Figure 13.
In order to analyze the heat load and temperature distribution of the hydrogen cooling system, the temperature of the inner wall, the outlet temperature, and the pressure and flow rate of hydrogen liquefied through the O-P converter were required.The temperature distribution of the inner wall was determined by the flange at room temperature and the liquid nitrogen shield layer.The temperature distribution of the inner tank wall is shown in Figure 14.As a result, the temperature of the inner tank wall converged to 86 K after 150 mm from the top flange.

Design of the Hydrogen Cooling System
The hydrogen cooling system consists of a liquid nitrogen shield layer, a buffer layer, a cooling pipe, and an O-P converter.There is a liquid nitrogen shielding layer on the outer wall to minimize radiant heat in the hydrogen cooling system, and there are several buffer layers to minimize convective heat loss in the inner tank.The inside of the O-P converter was filled with FeO, a catalyst for ortho-para conversion of hydrogen.The precooled hydrogen gas was cooled inside a heat pipe connected to a cryo-cooler and was liquefied through an O-P converter and accumulated in an inner tank.The configuration of the hydrogen cooling system is shown in Figure 13.
In order to analyze the heat load and temperature distribution of the hydrogen cooling system, the temperature of the inner wall, the outlet temperature, and the pressure and flow rate of hydrogen liquefied through the O-P converter were required.The temperature distribution of the inner wall was determined by the flange at room temperature and the liquid nitrogen shield layer.The temperature distribution of the inner tank wall is shown in Figure 14.As a result, the temperature of the inner tank wall converged to 86 K after 150 mm from the top flange.

Design of the Hydrogen Cooling System
The hydrogen cooling system consists of a liquid nitrogen shield layer, a buffer layer, a cooling pipe, and an O-P converter.There is a liquid nitrogen shielding layer on the outer wall to minimize radiant heat in the hydrogen cooling system, and there are several buffer layers to minimize convective heat loss in the inner tank.The inside of the O-P converter was filled with FeO, a catalyst for ortho-para conversion of hydrogen.The precooled hydrogen gas was cooled inside a heat pipe connected to a cryo-cooler and was liquefied through an O-P converter and accumulated in an inner tank.The configuration of the hydrogen cooling system is shown in Figure 13.
In order to analyze the heat load and temperature distribution of the hydrogen cooling system, the temperature of the inner wall, the outlet temperature, and the pressure and flow rate of hydrogen liquefied through the O-P converter were required.The temperature distribution of the inner wall was determined by the flange at room temperature and the liquid nitrogen shield layer.The temperature distribution of the inner tank wall is shown in Figure 14.As a result, the temperature of the inner tank wall converged to 86 K after 150 mm from the top flange.The contact area of the O-P converter was determined by the inlet temperature, pressure, and flow rate of hydrogen gas.The shape of the O-P converter was designed as a fin type for maximum heat transfer in minimum volume.The dimensions and analysis conditions of the O-P converter are summarized in Table 10, and the temperature distribution is shown in Figure 15.?The outlet temperature of the O-P converter was 21.3 K.The contact area of the O-P converter was determined by the inlet temperature, pressure, and flow rate of hydrogen gas.The shape of the O-P converter was designed as a fin type for maximum heat transfer in minimum volume.The dimensions and analysis conditions of the O-P converter are summarized in Table 10, and the temperature distribution is shown in Figure 15.The outlet temperature of the O-P converter was 21.3 K.The contact area of the O-P converter was determined by the inlet temperature sure, and flow rate of hydrogen gas.The shape of the O-P converter was designed type for maximum heat transfer in minimum volume.The dimensions and analys ditions of the O-P converter are summarized in Table 10, and the temperature distri is shown in Figure 15.?The outlet temperature of the O-P converter was 21.3 K.The dimensions and thermal analysis results of the hydrogen cooling system are summarized in Tables 11 and 12.The total heat load of the hydrogen cooling system was calculated to be 9.27 W. Figure 16 shows the thermal analysis results of the hydrogen cooling system.As a result of re-cooling the evaporated hydrogen gas by the heat pipe, it was confirmed that liquid hydrogen was maintaining a liquid state.The dimensions and thermal analysis results of the hydrogen cooling system are summarized in Tables 11 and 12.The total heat load of the hydrogen cooling system was calculated to be 9.27 W. Figure 16 shows the thermal analysis results of the hydrogen cooling system.As a result of re-cooling the evaporated hydrogen gas by the heat pipe, it was confirmed that liquid hydrogen was maintaining a liquid state.

Design of the LH2-He Heat Exchanger
The copper pipe was directly cooled by immersion in a LH2-He heat exchanger filled with liquid hydrogen at a temperature of 21 K. Helium gas at room temperature passed through the inside of the copper pipe and was cooled.The shape of the LH2-He heat exchanger is shown in Figure 17.Dimensions and analysis conditions are summarized in Tables 13 and 14.The results of thermal analysis of the pre-cooling system are shown in Figure 18.It was confirmed that the temperature at the outlet of the pre-cooling system was 27 K.The copper pipe was directly cooled by immersion in a LH 2 -He heat exchanger filled with liquid hydrogen at a temperature of 21 K. Helium gas at room temperature passed through the inside of the copper pipe and was cooled.The shape of the LH 2 -He heat exchanger is shown in Figure 17.Dimensions and analysis conditions are summarized in Tables 13 and 14.The results of thermal analysis of the pre-cooling system are shown in Figure 18.It was confirmed that the temperature at the outlet of the pre-cooling system was 27 K.Table 13.Specifications of the LH2-He heat exchanger.Table 13.Specifications of the LH2-He heat exchanger.

Design of the HTS Coil Cooling System
Figure 19 shows the configuration of the cooling system of the HTS coil.The structure for the HTS coil cooling system includes a current lead, a support, and a copper plate.The material of the support was G10 with low thermal conductivity and high strength to minimize the conduction heat load and maintain the stability of the HTS coil.The copper plate was the heat transfer path between the HTS coil and the heat exchanger.In order to increase the cooling efficiency, OFHC with a high heat transfer rate was used for the copper plate.The thermal load was minimized by using an HTS wire as the current lead.Dimensions and analysis conditions are summarized in Tables 15 and 16.

Design of the HTS Coil Cooling System
Figure 19 shows the configuration of the cooling system of the HTS coil.The structure for the HTS coil cooling system includes a current lead, a support, and a copper plate.The material of the support was G10 with low thermal conductivity and high strength to minimize the conduction heat load and maintain the stability of the HTS coil.The copper plate was the heat transfer path between the HTS coil and the heat exchanger.In order to increase the cooling efficiency, OFHC with a high heat transfer rate was used for the copper plate.The thermal load was minimized by using an HTS wire as the current lead.Dimensions and analysis conditions are summarized in Tables 15 and 16.   20 and 21 show the thermal analysis results of the current lead and HTS coil.A current of 150 A was flowing through the current lead, and the temperatures at the top and bottom of the current lead were 78.2 K and 30.9 K, respectively, which was within the allowable temperature range for the HTS current lead to operate.Table 17 shows the heat load of the HTS coil cooling system.The heat load of the designed current lead was 7.92 W. The heat load was minimized through the application of HTS wire in the straight part.The total heat load of the HTS coil cooling system was calculated to be 9.26 W.  Figures 20 and 21 show the thermal analysis results of the current lead and HTS coil.A current of 150 A was flowing through the current lead, and the temperatures at the top and bottom of the current lead were 78.2 K and 30.9 K, respectively, which was within the allowable temperature range for the HTS current lead to operate.Table 17 shows the heat load of the HTS coil cooling system.The heat load of the designed current lead was 7.92 W. The heat load was minimized through the application of HTS wire in the straight part.The total heat load of the HTS coil cooling system was calculated to be 9.26 W.

Hydrogen Liquefaction Test
Figure 22 shows the position of the temperature, pressure, and level sensors in the hydrogen liquefaction system.As a result of the hydrogen liquefaction system cooling test, hydrogen gas was successfully liquefied at a temperature of 21 K and accumulated in the inner tank.Figures 23 and 24 show the inner tank temperature and pressure of the hydrogen liquefaction system.It was confirmed that the inner tank pressure decreased after 300 min due to hydrogen liquefaction.When hydrogen gas was vaporized in the hydrogen cooling system, it was cooled again by a heat pipe to maintain a liquefied state.
The liquefaction amount of hydrogen was calculated using the inner tank volume and the measured level sensor data.The information on the inner tank volume and liquefaction amount of the hydrogen is shown in Figure 25.As a result of cooling for 900 min, a total of 49 L of liquid hydrogen was produced.

Hydrogen Liquefaction Test
Figure 22 shows the position of the temperature, pressure, and level sensors in the hydrogen liquefaction system.As a result of the hydrogen liquefaction system cooling test, hydrogen gas was successfully liquefied at a temperature of 21 K and accumulated in the inner tank.Figures 23 and 24 show the inner tank temperature and pressure of the hydrogen liquefaction system.It was confirmed that the inner tank pressure decreased after 300 min due to hydrogen liquefaction.When hydrogen gas was vaporized in the hydrogen cooling system, it was cooled again by a heat pipe to maintain a liquefied state.
The liquefaction amount of hydrogen was calculated using the inner tank volume and the measured level sensor data.The information on the inner tank volume and liquefaction amount of the hydrogen is shown in Figure 25.As a result of cooling for 900 min, a total of 49 L of liquid hydrogen was produced.Figure 22 shows the position of the temperature, pressure, and level sensors in the hydrogen liquefaction system.As a result of the hydrogen liquefaction system cooling test, hydrogen gas was successfully liquefied at a temperature of 21 K and accumulated in the inner tank.Figures 23 and 24 show the inner tank temperature and pressure of the hydrogen liquefaction system.It was confirmed that the inner tank pressure decreased after 300 min due to hydrogen liquefaction.When hydrogen gas was vaporized in the hydrogen cooling system, it was cooled again by a heat pipe to maintain a liquefied state.The liquefaction amount of hydrogen was calculated using the inner tank volume and the measured level sensor data.The information on the inner tank volume and liquefaction amount of the hydrogen is shown in Figure 25.As a result of cooling for 900 min, a total of 49 L of liquid hydrogen was produced.

Cooling Test of the Prototype HTS Coil
Figure 26 shows the temperature sensor position in the HTS coil cooling system.The HTS coil was in contact with the copper plate.The copper tube that cooled helium passes was embedded inside the copper plate.As a result, the HTS coil was cooled by conduction heat transfer of the cooled copper tube, and the target temperature was 30 K. Figure 27 and Table 18 show the cooling test results of the HTS coil cooling system.The temperature of the HTS coil cooling system converges at 300 min from the start of cooling by helium.As a result, the temperature of the HTS coil was 29.9 K, which was below the target temperature.

Cooling Test of the Prototype HTS Coil
Figure 26 shows the temperature sensor position in the HTS coil cooling system.The HTS coil was in contact with the copper plate.The copper tube that cooled helium passes was embedded inside the copper plate.As a result, the HTS coil was cooled by conduction heat transfer of the cooled copper tube, and the target temperature was 30 K. Figure 27 and Table 18 show the cooling test results of the HTS coil cooling system.The temperature of the HTS coil cooling system converges at 300 min from the start of cooling by helium.As a result, the temperature of the HTS coil was 29.9 K, which was below the target temperature.

Cooling Test of the Prototype HTS Coil
Figure 26 shows the temperature sensor position in the HTS coil cooling system.The HTS coil was in contact with the copper plate.The copper tube that cooled helium passes was embedded inside the copper plate.As a result, the HTS coil was cooled by conduction heat transfer of the cooled copper tube, and the target temperature was 30 K. Figure 27 and Table 18 show the cooling test results of the HTS coil cooling system.The temperature of the HTS coil cooling system converges at 300 min from the start of cooling by helium.As a result, the temperature of the HTS coil was 29.9 K, which was below the target temperature.

Conclusions
This paper describes the design and analysis of a cryogenic cooling system for electric propulsion system using liquid hydrogen as the refrigerant and energy sou The proposed APS consists of the power distribution system, the battery, the hydro fuel cell, and the HTS motor, and the capacity was selected in consideration of the airc operating characteristics.The lab-scale 5 kW HTS motor and cooling system were a lyzed for electromagnetic and thermal characteristics using a finite element method-ba analysis program.To verify the performance of the cooling system, the HTS coil nee to be operated under the same conditions used to test, design, and manufacture the h load of the lab-scale 5 kW HTS motor.The liquid hydrogen-based cooling system cons of the hydrogen liquefaction system, the hydrogen gas pre-cooling system, and the cr stat surrounding the 2G HTS coil.To achieve the hydrogen gas pre-cooling target tem ature, the number of buffer layers and the cryogenic cooler capacity were selected.In dition, in order to achieve the target temperature of the HTS coil, a design to minim thermal load was carried out by using G10 material and applying superconducting rent leads.As a result, the heat loads of the hydrogen liquefaction system and the H coil cooling system were minimized to 9.27 W and 7.92 W, respectively.The bottom face of the hydrogen liquefaction system was cooled to a temperature sufficient to k the hydrogen in a liquid state.Hydrogen gas was cooled stably, and 49 L of liquid hyd gen was successfully generated, and the HTS coil was cooled and maintained at the ta temperature by means of liquid hydrogen and helium gas.These results can be used designing a storage system that liquefies gaseous hydrogen or an application system

Conclusions
This paper describes the design and analysis of a cryogenic cooling system for an electric propulsion system using liquid hydrogen as the refrigerant and energy source.The proposed APS consists of the power distribution system, the battery, the hydrogen fuel cell, and the HTS motor, and the capacity was selected in consideration of the aircraft operating characteristics.The lab-scale 5 kW HTS motor and cooling system were analyzed for electromagnetic and thermal characteristics using a finite element method-based analysis program.To verify the performance of the cooling system, the HTS coil needed to be operated under the same conditions used to test, design, and manufacture the heat load of the lab-scale 5 kW HTS motor.The liquid hydrogen-based cooling system consists of the hydrogen liquefaction system, the hydrogen gas pre-cooling system, and the cryostat surrounding the 2G HTS coil.To achieve the hydrogen gas pre-cooling target temperature, the number of buffer layers and the cryogenic cooler capacity were selected.In addition, in order to achieve the target temperature of the HTS coil, a design to minimize thermal load was carried out by using G10 material and applying superconducting current leads.As a result, the heat loads of the hydrogen liquefaction system and the HTS coil cooling system were minimized to 9.27 W and 7.92 W, respectively.The bottom surface of the hydrogen liquefaction system was cooled to a temperature sufficient to keep the hydrogen in a liquid state.Hydrogen gas was cooled stably, and 49 L of liquid hydrogen was successfully generated, and the HTS coil was cooled and maintained at the target temperature by means of liquid hydrogen and helium gas.These results can be used for designing a storage system that liquefies gaseous hydrogen or an application system that uses liquefied hydrogen as a refrigerant.

Figure 1 .
Figure 1.Simplified block diagram of the proposed APS.

Figure 1 .
Figure 1.Simplified block diagram of the proposed APS.

Figure 2 .
Figure 2. Simplified block diagram of the proposed APS.

Figure 2 .
Figure 2. Simplified block diagram of the proposed APS.

Figure 3 .
Figure 3. (a) Magnetic field distribution and (b) perpendicular magnetic field of the 5 kW HTS motor.

Figure 3 .
Figure 3. (a) Magnetic field distribution and (b) perpendicular magnetic field of the 5 kW HTS motor.

Figure 4 .
Figure 4. Configuration of the rotor part.Table 5. Specifications of the current lead for the 5 kW HTS motor.Items Value Room temperature (TH) 300 K Operating temperature (TL) 30 K Operating current (I) 190 A Cross section area (A) 75 mm 2 Length of the current lead (L) 300 mm

Figure 4 .
Figure 4. Configuration of the rotor part.

Figure 5 .
Figure 5. Temperature distribution of the (a) rotor and (b) current leads.

Figure 5 .
Figure 5. Temperature distribution of the (a) rotor and (b) current leads.

Figure 6 .
Figure 6.Configuration diagram for verification of liquid hydrogen cooling system performance.

Figure 7 .
Figure 7. Fabrication and performance test of the HTS coil for cooling test.

Figure 8 .
Figure 8. Critical current measurement result of the HTS coil.

Figure 6 . 21 Figure 6 .
Figure 6.Configuration diagram for verification of liquid hydrogen cooling system performance.

Figure 7 .
Figure 7. Fabrication and performance test of the HTS coil for cooling test.

Figure 8 .
Figure 8. Critical current measurement result of the HTS coil.

Figure 7 . 21 Figure 6 .
Figure 7. Fabrication and performance test of the HTS coil for cooling test.

Figure 7 .
Figure 7. Fabrication and performance test of the HTS coil for cooling test.

Figure 8 .
Figure 8. Critical current measurement result of the HTS coil.Figure 8. Critical current measurement result of the HTS coil.

Figure 8 . 3 .
Figure 8. Critical current measurement result of the HTS coil.Figure 8. Critical current measurement result of the HTS coil.

Figure 9 .
Figure 9. Configuration of the cooling system for the APS.

Figure 9 .
Figure 9. Configuration of the cooling system for the APS.

Figure 10 .
Figure 10.Design process of the hydrogen cooling system.

Figure 11 .
Figure 11.Configuration of the pre-cooling system.

Figure 10 .
Figure 10.Design process of the hydrogen cooling system.

Energies 2023 , 21 Figure 10 .
Figure 10.Design process of the hydrogen cooling system.

Figure 11 .Table 9 .
Figure 11.Configuration of the pre-cooling system.Table 9. Specifications and analysis conditions of the pre-cooling system.Items Value Diameter of pipe 12.7 mm Thickness of pipe 1.24 mm Diameter of pre-cooling tank 175 mm Length of pre-cooling tank 345 mm Inlet temperature (H2) 300 Inlet pressure (H2) 3 bar Inlet volume flow (H2) 45 L/min

Figure 11 .
Figure 11.Configuration of the pre-cooling system.

Figure 12 .
Figure 12.Temperature distribution of the pre-cooling system.

Figure 13 .
Figure 13.Cross-section view of the hydrogen cooling system.

Figure 12 .
Figure 12.Temperature distribution of the pre-cooling system.

Figure 12 .
Figure 12.Temperature distribution of the pre-cooling system.

Figure 13 .
Figure 13.Cross-section view of the hydrogen cooling system.Figure 13.Cross-section view of the hydrogen cooling system.

Figure 13 .
Figure 13.Cross-section view of the hydrogen cooling system.Figure 13.Cross-section view of the hydrogen cooling system.

Figure 14 .
Figure 14.Temperature distribution of the inner wall.

Figure 15 .
Figure 15.Temperature distribution of the O-P converter.

Figure 14 .
Figure 14.Temperature distribution of the inner wall.

Figure 14 .
Figure 14.Temperature distribution of the inner wall.

Figure 15 .
Figure 15.Temperature distribution of the O-P converter.

Figure 15 .
Figure 15.Temperature distribution of the O-P converter.

Figure 16 .
Figure 16.Temperature distribution of the hydrogen cooling system.

Figure 16 .
Figure 16.Temperature distribution of the hydrogen cooling system.

Figure 17 .
Figure 17.Temperature distribution of the O-P converter.

Figure 17 .
Figure 17.Temperature distribution of the O-P converter.

Figure 17 .
Figure 17.Temperature distribution of the O-P converter.

Figure 18 .
Figure 18.Temperature distribution of the LH 2 -He heat exchanger.

Figure 19 .
Figure 19.Temperature distribution of the LH2-He heat exchanger.

Figure 19 .
Figure 19.Temperature distribution of the LH 2 -He heat exchanger.

Figure 20 .
Figure 20.Temperature distribution of current leads.

Figure 21 .
Figure 21.Temperature distribution of the HTS coil.

Figure 21 .
Figure 21.Temperature distribution of the HTS coil.

Figure 21 .
Figure 21.Temperature distribution of the HTS coil.

Figure 22 .
Figure 22.Sensor position of the hydrogen cooling system.

Figure 23 .
Figure 23.Temperature of the hydrogen cooling system.

Figure 24 .
Figure24.Pressure of the hydrogen cooling system.

Figure 22 . 21 Figure 22 .
Figure 22.Sensor position of the hydrogen cooling system.

Figure 23 .
Figure 23.Temperature of the hydrogen cooling system.

Figure 24 .
Figure 24.Pressure of the hydrogen cooling system.

Figure 23 .
Figure 23.Temperature of the hydrogen cooling system.

Figure 24 .
Figure 24.Pressure of the hydrogen cooling system.

Figure 24 .
Figure 24.Pressure of the hydrogen cooling system.

Figure 26 .
Figure 26.Temperature sensor position of the HTS coil cooling system.

Figure 25 .
Figure 25.Liquid hydrogen production of the cooling system.

Figure 26 .
Figure 26.Temperature sensor position of the HTS coil cooling system.Figure 26.Temperature sensor position of the HTS coil cooling system.

Figure 26 .
Figure 26.Temperature sensor position of the HTS coil cooling system.Figure 26.Temperature sensor position of the HTS coil cooling system.

Figure 27 .
Figure 27.Temperature measurement result of cooling system of HTS coil.

Figure 27 .
Figure 27.Temperature measurement result of cooling system of HTS coil.

Table 1 .
Energy and power requirements for each operating mode of the aircraft.

Table 2 .
Capacity and efficiency of the proposed lab-scale APS.

Table 1 .
Energy and power requirements for each operating mode of the aircraft.

Table 2 .
Capacity and efficiency of the proposed lab-scale APS.

Table 3 .
The material properties of the 5 kW HTS motor.

Table 3 .
The material properties of the 5 kW HTS motor.

Table 4 .
Detail design specifications of the lab-scale 5 kW HTS motor.

Table 4 .
Detail design specifications of the lab-scale 5 kW HTS motor.

Table 6 .
Specifications of torque tube for the 5 kW HTS motor.

Table 5 .
Specifications of the current lead for the 5 kW HTS motor.

Table 6 .
Specifications of torque tube for the 5 kW HTS motor.

Table 7 .
Thermal analysis results of the 5 kW HTS motor.

Table 8 .
Specifications of the HTS coil for the cooling test.

Table 7 .
Thermal analysis results of the 5 kW HTS motor.

Table 8 .
Specifications of the HTS coil for the cooling test.

Table 9 .
Specifications and analysis conditions of the pre-cooling system.

Table 9 .
Specifications and analysis conditions of the pre-cooling system.

Table 10 .
Specifications and analysis conditions of the O-P converter.

Table 10 .
Specifications and analysis conditions of the O-P converter.

Table 10 .
Specifications and analysis conditions of the O-P converter.

Table 11 .
Dimensions of the hydrogen cooling system.

Table 12 .
Heat load of the hydrogen cooling system.

Table 11 .
Dimensions of the hydrogen cooling system.

Table 12 .
Heat load of the hydrogen cooling system.

Table 13 .
Specifications of the LH 2 -He heat exchanger.

Table 14 .
Analysis conditions and results of the LH 2 -He heat exchanger.

Table 15 .
Specifications of the exchanger for the HTS coil.

Table 16 .
Analysis conditions and results of the exchanger for the HTS coil.

Table 15 .
Specifications of the exchanger for the HTS coil.

Table 16 .
Analysis conditions and results of the exchanger for the HTS coil.

Table 17 .
Heat loads of the HTS coil cooling system.

Table 17 .
Heat loads of the HTS coil cooling system.

Table 17 .
Heat loads of the HTS coil cooling system.

Table 18 .
Cooling test results of the HTS coil.

Table 18 .
Cooling test results of the HTS coil.