KR20230077035A - Liquid hydrogen storage tank to which 3D printing-epoxy composite material is applied and its manufacturing method - Google Patents

Abstract

The present invention relates to a liquid hydrogen storage tank to which a 3D printing-epoxy composite material is applied and a method of manufacturing the same, and more specifically, to a liquid hydrogen storage tank to which a 3D printing-epoxy composite material is applied, which includes predetermined pressure resistance performance and insulation performance to be applied to a drone using liquid hydrogen as fuel, and a method of manufacturing the same. According to an embodiment of the present invention, there is provided a liquid hydrogen storage tank to which a 3D printing-epoxy composite material is applied, comprising: an internal tank (1) that is 3D-printed to withstand a predetermined pressure so as to store liquid hydrogen; a first epoxy adhesive having a predetermined viscosity and applied to the outside of the internal tank (1); an external tank (2) that is 3D-printed to be spaced by a predetermined distance from an outer circumferential surface of the internal tank (1); a second epoxy adhesive having a viscosity differing from that of the first epoxy adhesive and applied to the outside of the external tank (2); and a supply pipe (3) connected to one side or more of the internal tank (1), wherein predetermined vacuum is generated in a space between the internal tank (1) and the external tank (2).

Description

Liquid hydrogen storage tank to which 3D printing-epoxy composite material is applied and its manufacturing method}

The present invention relates to a liquid hydrogen storage tank to which a 3D printing-epoxy composite material is applied and a method for manufacturing the same, and more particularly, to a 3D printing device having predetermined pressure resistance and insulation performance so that it can be applied to drones using liquid hydrogen as fuel. It relates to a liquid hydrogen storage tank to which a printing-epoxy composite material is applied and a manufacturing method thereof.

One of the issues that has recently been an issue at CES (Consumer Electronics Show), the world's largest IT/home appliance international electronics expo in 2020, is Urban Air Mobility (UAM). UAM emerged in the aspect of solving the traffic and environmental problems of the city amidst the continuing population density in the city, and means the urban air transport ecosystem using the air.

Hydrogen is attracting attention as an energy source of such UAM, and accordingly, the need for hydrogen storage technology is increasing.

Most of the hydrogen storage tanks for drones currently commercialized and used are hydrogen storage tanks filled by compressing gaseous hydrogen at high pressure.

When gaseous hydrogen is liquefied at atmospheric pressure and -253°C, its volume can be reduced by about 1/780, so its energy density is about 780 times higher than that of gaseous hydrogen. In addition, the density of liquid hydrogen is 70.8 g per 1 L, which is about 1.75 times higher than that of gaseous hydrogen stored in a gaseous state of 39.6 g per 1 L at room temperature of 700 bar. Therefore, when liquid hydrogen is used as an energy source, it has advantages of high energy density and light weight.

In order to store hydrogen in a liquid state, a separate insulated storage container is required. Heat penetration into the storage tank is unavoidable due to the temperature difference between cryogenic liquid hydrogen and external room temperature, and liquid hydrogen continuously vaporizes through this heat penetration, thereby minimizing heat penetration from the outside and loss due to evaporation. A container with excellent insulation performance is required.

As a general insulation method of a cryogenic fluid storage tank, vacuum insulation that can minimize heat transfer by convection, multi-layer insulation (MLI) through layered insulation by coating aluminum on a thin film ), and a vapor-cooled shield that blocks radiant heat using cold gaseous hydrogen evaporated from liquid hydrogen.

If these various insulation methods are applied to maximize the insulation performance, the weight of the liquid hydrogen storage tank increases. E-mobility and aerospace vehicles such as drones and unmanned aerial vehicles have a decrease in specific thrust as the weight of the aircraft increases, so a lightweight storage tank design is required to improve specific thrust and increase payload.

Since the gaseous hydrogen storage tank filled with gaseous hydrogen at high pressure is charged at high pressure at room temperature, the change in material properties with respect to temperature can be relatively ignored. Therefore, rather than thermal design, the pressure design inside the vessel is focused.

On the other hand, in the case of a liquid hydrogen storage tank filled with cryogenic fluid, since the material properties change in a cryogenic environment, structural/thermal design considering the changes in material properties is essential.

Korean Patent Registration No. 10-2105883 proposes a technology for preventing backflow of liquid gas as a liquid hydrogen container for application to drones.

According to the prior art, such as the above-mentioned patent documents, constructing a storage container with excellent insulation performance can be seen as a widely known technology, but technology that can improve the specific thrust of a drone by securing sufficient insulation performance while reducing the weight of the storage container is very rare. .

The problem to be solved by the present invention is to reduce the weight by about 30% compared to the conventional aluminum container through a composite material in which an epoxy adhesive is applied to the 3D printing material in order to improve the limitations of the conventional liquid hydrogen storage container as described above. It is to provide a liquid hydrogen storage tank to which a 3D printing-epoxy composite material is applied and a manufacturing method thereof.

The present invention, in order to improve the limitations of the conventional liquid hydrogen storage container as described above, is 3D printed inner tank 1 to be able to withstand a predetermined pressure so that liquid hydrogen is stored; a first epoxy adhesive having a predetermined viscosity and applied to the outside of the inner tank 1; an outer tank (2) 3D printed to be spaced apart from the outer circumferential surface of the inner tank (1) by a predetermined distance; a second epoxy adhesive having a viscosity different from that of the first epoxy adhesive and applied to the outside of the external tank 2; And a supply pipe (3) connected to at least one side of the inner tank (1), wherein a predetermined vacuum is formed in the space between the inner tank (1) and the outer tank (2), 3D printing-epoxy Provides a liquid hydrogen storage tank applied with a composite material.

In addition, a heating member attached to the bottom surface of the inner tank 1 to heat the inner tank 1 to adjust the amount of evaporation of liquid hydrogen; and a laminated insulating material provided to surround the outside of the inner tank 1 so that the heating member is accommodated therein.

And, the walls of the inner tank 1 and the outer tank 2 may be formed in a triangular infill structure in the range of 30% to 40%.

In addition, outputting the inner tank 1 with a 3D printer so that the outer wall is formed in a triangular infill structure in the range of 30% to 40%; removing a support formed inside the inner tank 1 through a through hole formed on one or more sides of the inner tank 1; inserting a pipe fixing part (4) and a supply pipe (3) into a through hole formed on at least one side of the inner tank (1); Applying a first epoxy adhesive having a predetermined viscosity to the outside of the inner tank (1) and to the pipe fixing part (4); drying the first epoxy adhesive; Attaching a heating member to the lower surface of the inner tank (1); stacking eight or more layers of laminated insulation to surround the outside of the inner tank (1); Printing the upper and lower parts of the tank with a 3D printer so that the outer wall is formed in a triangular infill structure ranging from 30% to 40%; inserting an inner tank (1) between the upper tank and the lower tank; forming an external tank (2) by applying a second epoxy adhesive having a different viscosity from that of the first epoxy adhesive to outer circumferential surfaces of the upper and lower portions of the tank and adhering the upper and lower portions of the tank; And drying the second epoxy adhesive; provides a method for manufacturing a liquid hydrogen storage tank to which a 3D printing-epoxy composite material is applied, including.

In addition, it provides a liquid hydrogen storage tank to which the 3D printing-epoxy composite material is applied, manufactured according to the method for manufacturing a liquid hydrogen storage tank to which the 3D printing-epoxy composite material is applied.

And, it provides a drone that uses the liquid hydrogen storage tank to which the 3D printing-epoxy composite material is applied as a fuel tank.

According to an embodiment of the present invention, a 3D printing-epoxy composite that has sufficient pressure resistance and insulation performance to store and transport liquid hydrogen in an outdoor space, and is lighter by more than 30% compared to a conventional aluminum storage container (about 1200g). It is possible to configure a liquid hydrogen storage tank to which the material is applied.

In addition, since the inner and outer tanks are printed with a 3D printer, the exterior can be freely modified and designed according to the structure/performance/characteristics of the drone.

In addition, by forming a vacuum in the space between the inner tank and the outer tank, it is possible to block convective heat intrusion and prevent surface icing from occurring.

1 is a schematic diagram of a cryogenic tensile tester capable of performing a tensile test in a cryogenic environment.
2 is a graph showing the results of tensile tests at room temperature/cryogenic temperatures for Onyx, Nylon, and Loctite EA 9394 Epoxy.
3 is a graph showing the modulus of elasticity at room temperature/cryogenic temperature of 3D printing materials and epoxy.
4 is a schematic diagram of a cryogenic heat shrinkage rate measurement test.
5 is a graph showing the results of measuring cryogenic heat shrinkage of Onyx, Epoxy, and AL 6061 materials.
6 is a shape of a specimen manufactured to proceed with a permeability test.
7 is a schematic diagram and experimental photos of the permeability test of the fabricated Onyx/epoxy specimen.
8 (a) and (b) are room temperature and cryogenic fluid permeability measurements at pressures from 1 Bar to 5 Bar of Onyx/Epoxy specimens, and FIGS. This is the result of the 9-hour leak test at room temperature / cryogenic temperature.
9 is a cross-sectional view of a liquid hydrogen storage tank to which a 3D printing-epoxy composite material is applied according to an embodiment of the present invention.
10 is a 3D printing-epoxy composite material according to an embodiment of the present invention is applied to the inner tank shape and material of the liquid hydrogen storage tank.
11 is a thermal stress analysis result of a solid fill and a triangular fill.
12 is a thermal stress distribution result when the liquid nitrogen level is filled to about 90%.
13 is a result showing the storage tank temperature distribution and maximum thermal stress values at liquid nitrogen levels of 10 mm and 90 mm.
14 is a photograph of measuring the weight before and after applying the epoxy adhesive to the inner tank.
15 is a photograph showing a state in which a temperature sensor, a heat wire (heating member), and a laminated insulation material are attached to the inner tank.
16 is a photograph of the weight measured by coupling the outer tank to the inner tank and applying an epoxy adhesive.
17 is a photograph showing an experimental apparatus for evaluating characteristics of a liquid hydrogen storage tank.
18 is a result of measuring the temperature of the upper and lower parts of the storage tank wall and the flow rate of gaseous nitrogen according to liquid nitrogen injection.

Hereinafter, various embodiments of this document will be described with reference to the accompanying drawings. However, this is not intended to limit the technology described in this document to specific embodiments, and should be understood to include various modifications, equivalents, and/or alternatives of the embodiments of this document. . In connection with the description of the drawings, like reference numerals may be used for like elements.

In this document, expressions such as "has," "may have," "includes," or "may include" indicate the existence of a corresponding feature (eg, numerical value, function, operation, or component such as a part). , which does not preclude the existence of additional features.

In this document, expressions such as “A or B,” “at least one of A and/and B,” or “one or more of A or/and B” may include all possible combinations of the items listed together. . For example, “A or B,” “at least one of A and B,” or “at least one of A or B” (1) includes at least one A, (2) includes at least one B, Or (3) may refer to all cases including at least one A and at least one B.

Expressions such as "first," "second," "first," or "second," as used in this document may modify various elements, regardless of order and/or importance, and refer to one element as It is used only to distinguish it from other components and does not limit the corresponding components. For example, a first user device and a second user device may represent different user devices regardless of order or importance. For example, without departing from the scope of rights described in this document, a first element may be called a second element, and similarly, the second element may also be renamed to the first element.

A component (e.g., a first component) is "(operatively or communicatively) coupled with/to" or "connected" to another component (e.g., a second component). When referred to as "connected to", it should be understood that the certain component may be directly connected to the other component or connected through another component (eg, a third component).

Terms used in this document are only used to describe a specific embodiment, and may not be intended to limit the scope of other embodiments. Singular expressions may include plural expressions unless the context clearly dictates otherwise. Terms used herein, including technical or scientific terms, may have the same meaning as commonly understood by a person of ordinary skill in the technical field described in this document. Among the terms used in this document, terms defined in a general dictionary may be interpreted as having the same or similar meaning as the meaning in the context of the related art, and unless explicitly defined in this document, an ideal or excessively formal meaning. not be interpreted as In some cases, even terms defined in this document cannot be interpreted to exclude the embodiments of this document.

Of course, various modifications can be made by those skilled in the art without departing from the gist of the present invention claimed in the claims of the present invention, and these modifications are the technical spirit of the present invention or It should not be understood individually from the perspective.

In the embodiment of the present invention, 3D printing technology of FDM (Fused Deposition Modeling) method was used, and Markforged's Onyx filament was used as a printing material (filament).

The filament may be a thermoplastic resin in which 80% of PA6-based tough nylon and 20% of chopped carbon are mixed. Since these materials contain about 20% of carbon, they are 1.4 times stronger than general ABS materials, as well as strong against heat and excellent in chemical resistance. Therefore, it is suitable for use as a liquid hydrogen storage tank due to low heat shrinkage in a cryogenic environment.

In the embodiment of the present invention, EA 9394 Aero, an epoxy for bonding composite materials from Loctite, was used as the first epoxy adhesive applied to the inner tank. This epoxy is an aerospace structural adhesive and is widely used as a structural adhesive for composite materials due to its excellent adhesion, mechanical properties and chemical resistance. In addition, unlike other epoxy resin products, adhesive strength is relatively excellent, and heat shrinkage is similar to Onyx material (3D printing material), so thermal stress caused by shrinkage is less generated in a cryogenic environment. Therefore, it is suitable for use in adhesion and leakage prevention of internal storage containers to be exposed to cryogenic environments.

In the embodiment of the present invention, EA 9396 Aero, an epoxy for bonding composite materials from Loctite, was used as the second epoxy adhesive applied to the external tank. EA 9394 Aero and EA 9396 Aero have similar mechanical properties, but EA 9396 has a relatively lower viscosity than EA 9394. Since the outer tank is not a cryogenic environment like the inner tank, and the pressure is not high inside (the space between the outer tank and the inner tank), it is only necessary to secure airtightness and create a vacuum environment. Therefore, epoxy with the lowest viscosity and fewer pores is suitable, and EA 9396 is used as the second epoxy adhesive for the external tank because it has a very low viscosity and can be degassed to remove bubbles existing between the epoxy.

1. Measurement of physical properties of liquid hydrogen storage tank

① Tensile test

A tensile test was performed at a temperature of -196 °C using liquid nitrogen.

As a tensile tester, QRUTS-S100, a universal material tester from Curo, was used.

Since the tensile tester can only perform a tensile test at room temperature, a water tank was installed inside to secure a space for injecting liquid nitrogen so that a tensile test can be performed inside the liquid nitrogen.

1 is a schematic diagram of a cryogenic tensile tester capable of performing a tensile test in a cryogenic environment.

The upper and lower jigs and specimens are placed in a liquid nitrogen bath, and temperature sensors are located in the center of the upper and lower jigs and the specimen. The temperature sensor used an E-type Thermo couple.

After filling the inside of the water tank with liquid nitrogen, wait until the upper and lower jigs and the specimen converge to the liquid nitrogen temperature to achieve thermal equilibrium. After that, a tensile test is performed, the force generated during tension is measured through the load cell located at the top, and the displacement is measured through the crosshead.

The standard of the specimen to be used for the test was the ASTM D 638 standard, and it was printed with a 3D printer.

When liquid nitrogen is injected, the temperature decreases from the temperature sensor of the lower jig over time and gradually converges to 77 K to the upper temperature sensor. After all three temperature sensors converged, a tensile test was performed, and a load was applied at a rate of 2 mm/min.

For the force-displacement curve obtained from the tensile test, the strain can be calculated as follows.

Here, L is the gauge length, in the case of ΔL, the length increased compared to the initial length, and ε is the strain. In addition, in the case of stress, it can be calculated as follows for the cross-sectional area of the specimen in the tensile direction of 19 mm 2 .

Then, Young's modulus can be calculated by the relationship between stress, Strian, and Young's modulus in Equation 3.

2 is a graph showing the results of tensile tests at room temperature/cryogenic temperatures for Onyx, Nylon, and Loctite EA 9394 Epoxy.

In all three materials, the strain decreased in the cryogenic environment compared to room temperature, which can be seen that the material became brittle as the temperature decreased. In addition, the strength increased in all materials, and the strength increase of Onxy was slightly greater than that of EA 9394 Epoxy and Nylon.

3 is a graph showing the modulus of elasticity at room temperature/cryogenic temperature of 3D printing materials and epoxy. As the temperature decreased, the modulus of elasticity increased in all materials.

Overall, it was confirmed that the tensile strength of the 3D printing material and the epoxy increased by about 1.2 to 3 times compared to room temperature in a cryogenic environment, and the modulus of elasticity increased by about 3 to 9 times. Accordingly, among Onyx and Nylon, Onyx, which has 30 to 40% higher tensile strength and modulus of elasticity in a cryogenic environment, was selected as the material for the cryogenic fluid storage tank.

② Measurement of heat shrinkage rate

In the case of Onyx and Epoxy, there is no data related to thermal shrinkage in a cryogenic environment, so measurement is required.

The variables that need to be measured to determine the heat shrinkage rate are the displacement and temperature of the material. Displacement was measured using a strain gauge, and a strain gauge from Kyowa, which can be used in a cryogenic environment, was used.

The temperature of the specimen was measured through a measuring instrument using an E-type Thermo couple temperature sensor.

In the case of the measurement specimen, the Onyx material was printed with a 3D printer, and the Epoxy specimen was manufactured by putting it in a specimen mold.

In general, materials expand as temperature increases and contract as temperature decreases. The amount of shrinkage of a material as the temperature decreases can be expressed as:

Here, L is the initial length of the strain gauge, ΔL is the reduced length compared to the initial length, and ε is the strain.

4 is a schematic diagram of a cryogenic heat shrinkage rate measurement test. In order to measure the degree of shrinkage of a material as the temperature decreases, it is necessary to establish a cryogenic environment for the material. In addition, the heat shrinkage rate should be measured at a sufficient time interval to obtain an accurate value. Therefore, a method of simulating a cryogenic environment was adopted by spraying liquid nitrogen little by little into the sealed container.

Two liquid nitrogen spray nozzles and a circulator are located at the top of the container, and liquid nitrogen is sprayed little by little through the spray nozzles. The injected liquid nitrogen is phase-changed into gaseous nitrogen through the circulator, lowering the internal temperature of the container. At the same time, the temperature and displacement of the slowly cooled specimen are simultaneously measured in real time. The test is terminated when the temperature of the specimen converges to near 77 K, which is the temperature of liquid nitrogen.

5 is a graph showing the results of measuring cryogenic heat shrinkage of Onyx, Epoxy, and AL 6061 materials.

For reliability of thermal contraction rate, the thermal shrinkage rate of AL 6061 metal, whose thermal shrinkage rate is known at cryogenic temperatures, was measured and compared to verify reliability. In both materials, the shrinking displacement gradually increased as the temperature decreased, and when the temperature dropped to around 77 K, Onyx showed a shrinkage of about 0.4% compared to the initial length, and in the case of Epoxy, the shrinkage rate was about 0.8% compared to the initial length. has been measured

③ Fluid permeability measurement

One of the essential considerations for using liquid hydrogen in a cryogenic environment is the fluid permeability of the material.

Since hydrogen is an explosive gas, there is a risk of explosion if the gas leaks out of the tank. Therefore, it is necessary to check the fluid permeability of the material used in the room temperature and cryogenic environment.

이하로 기준을 설정하였다. 이 값은 약 10시간 동안 누설되었을 때, 가로 x 세로 x 높이 3.6 cm의 부피에 해당하는 양이다. 또한 내부압력에 대한 누설률을 확인하기 위해 헬륨 압력은 1 bar ~ 5 bar로 설정하였다.Since there is no exact standard for the standard of fluid permeability, referring to the standard applied to the particle accelerator and KSTAR,

The criteria were set as follows. This value corresponds to a volume of 3.6 cm in width x length x height when leaked for about 10 hours. In addition, to check the leak rate for the internal pressure, the helium pressure was set to 1 bar to 5 bar.

Figure 6 and Table 1 are the shape and material of the specimen manufactured to proceed with the permeability test.

In order to match the conditions of the actual composite tank, specimens were prepared by applying EA 9394 Aero Epoxy to Onyx. Epoxy was applied to the inner side after producing a round disc-shaped Onyx specimen with a 3D printer.

Parameter Value Printed filament thickness [mm] 0.1 Epoxy application thickness [mm] 1.25 Assemble part thickness [mm] 4 Measuring plane printing thickness [mm] 1.5 Measuring plane total thickness [mm] 2.75 Measuring plane cross sectional area [mm^2] 2,551

7 is a schematic diagram and experimental photos of the permeability test of the fabricated Onyx/epoxy specimen.

Helium was used as a fluid used for permeability testing. Helium is an atom with a smaller particle size next to the hydrogen atom, and its permeability is easy to determine.

Referring to FIG. 7, the test specimen is placed on a path composed of an inlet and an outlet and is fastened through the CF Flange so that gas does not leak out of the specimen. Helium is pressurized at the inlet and helium is detected at the outlet side through a leak detector.

The pressure of the internal pipe and the temperature of the specimen were measured through a temperature sensor and a pressure sensor, and a vacuum pump was used to maintain a high degree of vacuum on the exit path for precise measurement of the leak detector.

의 Leak rate를 유지하였다.8 (a) and (b) are room temperature and cryogenic fluid permeability measurement results at pressures ranging from 1 Bar to 5 Bar of Onyx/Epoxy specimens. The helium pressure was increased from 1 Bar to 5 Bar at intervals of about 1000 seconds, and up to 5 Bar in both room temperature and cryogenic temperatures.

The leak rate was maintained.

의 Leak rate를 유지하였다.8 (c) and (d) show the results of a 9-hour leak test at room temperature/cryogenic temperature at 5 bar of Onyx/epoxy specimens. Due to the nature of liquid hydrogen storage containers for drones that require long-term operation, it is necessary to check the leak rate for a sufficiently long time. Therefore, the leak rate was measured at room temperature and cryogenic temperature at the maximum set pressure of 5 Bar. Measurement results for 9 hours at both room temperature and cryogenic temperature

The leak rate was maintained.

The room temperature and cryogenic fluid permeability measurement results are summarized in Table 2.

Parameter Room Cryogenic 1-5 Bar Max.
Leak rate (mbar L/s) One 1.9 X 10^-10 1.8 X 10^-10 2 1.9 X 10^-10 1.8 X 10^-10 3 2.0 X 10^-10 2.4 X 10^-10 4 2.1 X 10^-10 2.0 X 10^-10 5 2.3 X 10^-10 2.0 X 10^-10 1-5 Bar Max. Vacuum rate (torr) 2.3 X 10^-6 2.3 X 10^-6 5 Bar, 9 hr Max. Leak rate (mbar L/s) 3.4 X 10^-10 2.4 X 10^-10 5 Bar, 9 hr Vacuum rate (torr) 5.9 X 10^-6 2.8 X 10^-6

2. Design specifications of liquid hydrogen storage tank

① Target specification design

9 is a cross-sectional view of a liquid hydrogen storage tank to which a 3D printing-epoxy composite material is applied according to an embodiment of the present invention.

10 is a 3D printing-epoxy composite material according to an embodiment of the present invention is applied to the inner tank shape and material of the liquid hydrogen storage tank.

Since there is a limit to the size that can be printed through a 3D printer, the goal was set to be applicable to small applications. In the case of wall thickness, it was set to 4.8 mm in consideration of manufacturability, and the capacity of the liquid hydrogen storage tank was calculated to be about 1.1 L.

The weight was minimized by setting the fill level of the inner filament of the wall thickness to Triangular fill (37%). The total weight of the liquid hydrogen storage tank to which the 3D printing-epoxy composite material was applied according to an embodiment of the present invention was measured to be about 870 g.

The rated power of fuel cell batteries commonly used from small drones to large unmanned aerial vehicles ranges from 300 W to 1000 W. Based on the above specifications, the flow rate of gaseous hydrogen required by the fuel cell and the corresponding maintenance time can be calculated. Table 3 is the specification of the fuel cell battery applicable from 300 W to 1000 W when the water level is charged to 90% for the 1.1 L liquid hydrogen storage tank.

Fuel cell power (W) Required H 2 (g/s) Operating time (hours) 300 0.0043 10 400 0.0057 7.6 500 0.0071 6 600 0.0085 5 700 0.01 4.4 800 0.011 3.8 900 0.013 3.4 1000 0.014 3

Liquid hydrogen in the filled liquid hydrogen storage tank continuously evaporates, and the amount of evaporation is proportional to the amount of heat intrusion from the outside.

In order to maximize the liquid hydrogen retention time in the storage tank and minimize the amount of gaseous hydrogen that is wasted, heat intrusion from the outside must be minimized. Basically, the conduction heat intrusion through the inlet/outlet piping and the radiant heat intrusion to the outer container area become the base heat load.

First, conduction heat penetration is heat penetration from room temperature to the inlet/outlet pipe, and can be calculated by the following Fourier's heat conduction law.

Here, for the sus 316L material, the conduction heat penetration of each inlet/outlet pipe of 1/4 inch size was calculated to be about 0.5 W.

Since radiant heat penetration accounts for a relatively large proportion compared to conduction or convection, it was minimized through multi-layer insulation (MLI).

Radiant heat transfer occurring between the two temperatures from the room temperature of the external vacuum container to the cryogenic temperature of the internal storage container is expressed by Equation 6.

Here, σ is the Stefan-Boltzmann constant, ε is the emissivity, T1, T1 are the temperature of the heat transfer target, and A is the heat transfer area. Typically, T2 will be the ambient temperature.

Without the MLI, the radiant heat penetration cannot be calculated because the emissivity of Onyx, the internal storage material, is not known. However, when the MLI is constructed, the radiant heat intrusion can be calculated because the amount of radiant heat intrusion according to the number of MLI is known. The amount of radiant heat penetration according to the number of MLI is shown in Equation 7

Referring to Equation 7, for the radiant heat penetration of 12 W/m^2 when about 10 MLIs are constructed, and the radiant heat penetration corresponding to the total area of 0.067 m^2 of the liquid hydrogen storage container, the amount is about 0.8 W was calculated as

In the case of convective heat penetration, it was assumed that there was no convective heat penetration by maintaining the inside of the container in a high vacuum state.

The amount of evaporation of liquid hydrogen according to the heat load can be calculated according to Equation 8

은 외부 열부하이며,

의 경우 액체 수소의 증발잠열이다.here,

is the external heat load,

is the latent heat of vaporization of liquid hydrogen.

The latent heat of vaporization of liquid hydrogen is 443 kJ/kg, and when the liquid hydrogen evaporation rate is calculated for the basic heat load of 1.8 W of the liquid hydrogen storage tank calculated above, it is about 0.0041 g/s. This means that about 0.0041 g of gaseous hydrogen is evaporated per second by the latent heat of vaporization of liquid hydrogen, and is a range applicable to fuel cell batteries with a rated output of 300 W or more according to the fuel cell requirements in Table 3. For comparison with the actual experiment, when calculating the amount of evaporation for the latent heat of vaporization of liquid nitrogen (199 kJ/kg), it is calculated as about 0.009 g/s.

② Thermal stress analysis

When liquid hydrogen is filled inside the liquid hydrogen storage tank, evaporation of liquid hydrogen occurs due to heat transfer between the tank wall at the initial room temperature and the cryogenic liquid hydrogen, and the water level rises by the difference between the charged amount and the evaporated amount.

In addition, as the water level rises, a temperature gradient is formed on the tank wall, and thermal stress is generated according to the degree of contraction according to the temperature of each material. Since the shrinkage rates are different, the displacements at each location are all different, which can cause excessive thermal stress.

When fine cracks or destruction occur due to excessive thermal stress, internal hydrogen leakage occurs, and the tank cannot function as a storage tank. Therefore, thermal stress analysis is needed to evaluate the thermal integrity of the liquid hydrogen storage tank in a cryogenic environment. The analysis program used Comsol Multiphysics, which is suitable for simulation by coupling structural analysis and thermal analysis.

Thermal stress analysis of a cryogenic fluid storage tank can be largely divided into two types (assuming liquid nitrogen is filled).

The first is to identify the part where the maximum thermal stress occurs inside the storage tank. When the inside of the storage tank is filled with 90% or more of liquid nitrogen, the temperature of most of the inner walls of the storage tank is formed near 77 K, which is the temperature of liquid nitrogen. At this time, the point where the maximum thermal stress occurs due to heat transfer between the outer wall and the inlet/outlet hydrogen charging port is checked.

The second is to compare the thermal stress change according to the liquid nitrogen level during liquid nitrogen filling. As the liquid nitrogen is filled, the water level gradually increases, and when the water level is filled more than 90%, the temperature of the inner wall is formed around 77 K, but when the water level is about 10% formed, the temperature difference between the upper part and the lower part of the storage tank occurs. This creates a temperature gradient on the inner wall. If the change in shrinkage rate due to temperature change is large, a larger thermal stress may occur than when the water level is filled to 90% because the shrinkage displacement of the lower part of the storage tank is large.

The liquid hydrogen storage tank thermal stress analysis model was modeled the same as the actual size, and because it has symmetry around the axis, the analysis was performed as a 1/4 model for the entire container size.

As a constraint condition, a symmetric condition was added with the axis as the boundary, and a condition to fix the displacement in the z-axis was added to the end of the sus. As the temperature condition, a 293 K condition was added to the end of the sus tube, which is an inlet/outlet port, and a 77 K condition was added to the inner container wall. Since the actual experiment is conducted with liquid nitrogen, the temperature of liquid nitrogen was applied for comparison.

As materials applied, sus 316L for the inlet/outlet port, Onyx for the liquid hydrogen storage tank body, and Loctite EA 9394 Epoxy to be applied to the storage tank were applied. In the case of SUS 316L, the physical properties according to the temperature inside Comsol Multiphysics were applied, and in the case of Onyx and Epoxy, the measured thermal strain and Young's modulus experimental values were used, and the data provided by the manufacturer were used for the rest of the physical properties. The physical properties applied to the analysis for Onyx and EA 9394 Aero Epoxy are shown in Table 4 below.

Parameter Onyx EA 9394 Aero Epoxy Density [kg/m^3] 1200 1400 Thermal conductivity [W/m-K] 0.9 0.331 Young's modulus [GPa] 2.4 4.24 Thermal strain Experiment Experiment poisson's ratio 0.35 0.35

In order to minimize the weight of the liquid hydrogen storage container, the internal filling level was determined as triangular infill (37%) for the thickness of the liquid hydrogen storage tank. The triangular infill ratio can be adjusted within an appropriate range by comparing lightness and pressure resistance. In order to reduce the weight of the liquid hydrogen storage tank, it is advantageous to reduce the degree of internal filling, but when shrinking in a cryogenic environment, resistance to deformation is reduced and cracks may occur. Therefore, it is necessary to compare the thermal stress of solid fill (100%) and triangular infill (37%) as the degree of internal filling.

As shown in FIG. 11, the thermal stress analysis results of the solid fill and the triangular fill showed generally similar stress distributions.

The stress of the epoxy part in the solid fill was higher than that of the epoxy part in the triangular fill. However, the overall stress in the onyx part was found to be slightly higher for the triangular fill.

12 is a thermal stress distribution result when the liquid nitrogen level is filled to about 90%.

The maximum thermal stress generation point occurred in the sus tube and was about 175 MPa. As Onyx and Epoxy surrounding the sus tube contract in a complex way, thermal stress occurs in the sus tube. The overall thermal stress of the storage tank except for the SUS tube was confirmed to be about 0 to 33 MPa.

As the fluid level in the liquid hydrogen storage tank gradually increases, the magnitude of the maximum thermal stress generated also changes. In order to confirm the change in maximum thermal stress according to the increase in water level, thermal stress analysis was performed according to the fluid level in the storage tank. The fluid level was set in increments of 10 mm from 10 mm to 90 mm.

13 is a result showing the storage tank temperature distribution and maximum thermal stress values at liquid nitrogen levels of 10 mm and 90 mm.

Table 5 shows the maximum thermal stress values at each water level and the maximum thermal stress values excluding SUS tubes.

Height (mm) Maximum thermal stress in tank (MPa) Maximum thermal stress in tank except for the sus tube (MPa) 10 111.5 32.3 20 124.6 33.4 30 130.3 33.2 40 135.4 32.2 50 140.4 32.8 60 145.6 32.5 70 151.7 33.1 80 158.8 32.8 90 170.8 33.0

It can be seen that the temperature distribution on the inner wall of the storage tank changes as the water level increases, and the tank wall is cooled from the bottom of the tank to the top by conduction heat transfer.

As the water level increased, the maximum thermal stress of the sus tube increased. As the water level increases, the shrinkage rate of Epoxy and Onyx surrounding the sus tube inside the storage tank increases, increasing the maximum thermal stress of the SUS tube. When the water level was 90%, a maximum of 170.8 MPa occurred, which is the thermal stress generated in the SUS tube.

Conversely, in the change of maximum thermal stress except for the SUS tube, as the water level increased, the thermal stress of the overall storage tank showed a similar tendency. This means that even if the water level increases, the overall maximum thermal stress of the tank remains below about 33 MPa.

3. Liquid hydrogen storage tank manufacturing method using 3D printing-epoxy composite material

① Manufacture of inner tank

Based on the previously designed liquid hydrogen storage tank model, the inner tank can be manufactured through a 3D printer.

Triangular infill (37%) was applied and printed for the wall thickness.

In order to prevent cracks and leakage of the inner tank as much as possible in a cryogenic environment, the inner tank was printed as a single body, and since the inner tank is cylindrical, a support is created inside the inner tank due to the characteristics of the 3D printer.

Therefore, when modeling the inner tank, a hole must be made in one side of the inner tank so that the support can be removed after printing, and all supports must be removed through the hole after printing is finished.

As shown in FIG. 14 (a), the Onyx weight of the inner tank after printing is about 210 g.

To prevent hydrogen leakage, Loctite's EA 9394 epoxy was applied to the Onyx side of the fabricated inner tank. In addition, hydrogen inlet/outlet pipes made of SUS 316L were constructed, and the final weight of the manufactured liquid hydrogen storage tank was confirmed to be 329 g as shown in FIG. 14 (b).

In addition, as shown in FIG. 15, temperature sensors are attached to the upper and lower sides of the inner tank to predict the temperature distribution and the inner water level of the inner tank. The temperature sensor used an E-type Thermo couple. In addition, a Nichrome wire heater was installed on the bottom of the storage tank to control the flow rate of evaporated gaseous hydrogen, and the resistance was 33 Ω for a total length of 1 m. 15 (c) is a state in which 10 layers of laminated insulation (MLI) are laminated on the inner tank.

② External tank production / combination

In order to improve the insulation performance of the liquid hydrogen storage tank, the external tank of the storage tank was produced with a 3D printer.

In order to minimize the amount of heat intrusion into the liquid hydrogen storage tank, it is important to minimize conduction, convection, and radiative heat intrusion.

Among them, in order to reduce the amount of heat intrusion by convective heat transfer, the external environment of the inner tank was configured as a vacuum to minimize the amount of heat intrusion by convection.

In order to secure the MLI construction space, each dimension was made about 10 mm larger than the inner tank (formed so that the inner circumference of the outer tank is separated from the outer circumference of the inner tank by a predetermined distance), and the outer tank also had a triangular fill (37%) on the wall thickness. was applied to minimize the weight.

The outer tank is made of two parts (tank upper part and tank lower part) to be combined with the inner tank. As shown in Figure 16 (a), the inner tank is placed under the manufactured tank, the upper part of the tank is covered, and epoxy adhesive is applied. was joined using

Loctite's EA 9396 epoxy was used for the external tank, and since this epoxy has low viscosity and almost no leakage, it is not only advantageous in forming a vacuum environment for a vacuum container, but also has sufficient bonding power for adhesion. In addition, to remove air bubbles inside the epoxy, epoxy was applied after degassing.

After combining the inner tank and the outer tank, the weight of the final liquid hydrogen storage tank was measured to be about 873 g, including the SUS 316L inlet/outlet pipe (supply pipe).

4. Evaluation of characteristics of liquid hydrogen storage tank applied with 3D printing-epoxy composite material

17 is a photograph showing an experimental apparatus for evaluating characteristics of a liquid hydrogen storage tank.

The experiment was conducted using liquid nitrogen instead of liquid hydrogen, and the experimental equipment consists of liquid nitrogen, vacuum pump, mass flow meter, heat exchanger, vacuum gauge, vacuum monitor, temperature sensor instrument, and power supply.

First, the inside of the vacuum container of the liquid hydrogen storage tank is made into a vacuum environment through a vacuum pump, and then cold gaseous nitrogen is first injected into the storage tank to pre-cool the inside of the storage tank. After pre-cooling, liquid nitrogen is gradually injected to fill the storage tank with liquid nitrogen.

After charging is completed, close the Vent valve discharged to the atmosphere and measure the flow rate of gaseous nitrogen evaporated. The evaporated gaseous nitrogen transfers heat to the outside air through the heat exchanger and then is discharged through the flow meter.

18 is a result of measuring the temperature of the upper and lower parts of the storage tank wall and the flow rate of gaseous nitrogen according to liquid nitrogen injection.

In the beginning, it can be seen that the temperature decreases rapidly as gaseous nitrogen and liquid nitrogen are injected.

The temperature of the lower part of the storage tank converged around 85 K, which is higher than the liquid nitrogen temperature of 77 K. can be judged to have converged on

Gaseous nitrogen evaporated by external heat penetration passed through the heat exchanger and then discharged through the flowmeter, and the evaporation rate was maintained at about 0.005 g/s (0.25 LPM). Based on this, the liquid nitrogen injection amount was calculated by integrating the flow rate data measured for the total experimental time. The injected amount of liquid nitrogen was about 390 g, and about 42% of the total capacity was filled.

Based on the actually measured flow rate, inverse calculation was performed to calculate the external heat intrusion.

When calculating the external heat penetration of the liquid hydrogen storage tank through Equation 8, since the latent heat of vaporization of liquid nitrogen is 199 kJ / kg, an external heat penetration of about 1 W is calculated for an evaporation of 0.005 g / s.

This is about 45% of the 1.8 W, which is the sum of conduction and radiation heat penetration, which was previously expected. This can be judged to be an error in the expected conduction heat invasion and radiative heat invasion.

Conductive heat penetration from 300 K to 77 K was expected, but in the actual experiment, the temperature of the SUS tube would be lower due to liquid nitrogen, so it is expected that a slight difference occurred in the conduction heat penetration.

The experiment was conducted for about 20 hours, and a flow rate of about 0.005 g/s was continuously maintained. If the maximum operation time is calculated for 90% of the water level of the storage tank, it is expected that it can be maintained for about 42 hours.

1: Inner tank
2: External tank
3: supply piping
4: pipe fixing part

Claims (6)

An inner tank that is 3D printed to withstand a predetermined pressure to store liquid hydrogen;
a first epoxy adhesive having a predetermined viscosity and applied to the outside of the inner tank;
an outer tank 3D printed to be spaced apart from the outer circumferential surface of the inner tank by a predetermined distance;
a second epoxy adhesive having a viscosity different from that of the first epoxy adhesive and applied to the outside of the external tank; and
Including; supply pipe connected to at least one side of the inner tank,
A liquid hydrogen storage tank with a 3D printing-epoxy composite material in which a predetermined vacuum is formed in the space between the inner tank and the outer tank The method of claim 1,
a heating member attached to the lower surface of the inner tank to heat the inner tank to adjust the amount of evaporation of liquid hydrogen; and
A liquid hydrogen storage tank to which a 3D printing-epoxy composite material is applied, including a laminated insulation material provided to surround the outside of the inner tank so that the heating member is accommodated inside. The method of claim 1,
The walls of the inner tank and the outer tank are formed with a triangular infill structure in the range of 30% to 40%, a liquid hydrogen storage tank to which a 3D printing-epoxy composite material is applied Outputting the inner tank with a 3D printer so that the outer wall is formed with a triangular infill structure ranging from 30% to 40%;
removing a support formed inside the inner tank through a through hole formed on at least one side of the inner tank;
inserting a pipe fixing part and a supply pipe into a through hole formed on at least one side of the inner tank;
applying a first epoxy adhesive having a predetermined viscosity to the outside of the inner tank and to the pipe fixing part;
drying the first epoxy adhesive;
attaching a heating member to the lower surface of the inner tank;
stacking eight or more layers of laminated insulation to surround the outside of the inner tank;
Printing the upper and lower parts of the tank with a 3D printer so that the outer wall is formed in a triangular infill structure ranging from 30% to 40%;
inserting an inner tank between the upper tank and the lower tank;
forming an external tank by applying a second epoxy adhesive having a different viscosity from that of the first epoxy adhesive to outer circumferential surfaces of the upper and lower portions of the tank and adhering the upper and lower portions of the tank; and
Method for manufacturing a liquid hydrogen storage tank to which a 3D printing-epoxy composite material is applied, including drying the second epoxy adhesive A liquid hydrogen storage tank to which a 3D printing-epoxy composite material is applied, manufactured according to the manufacturing method of a liquid hydrogen storage tank to which a 3D printing-epoxy composite material is applied according to claim 4 A drone using a liquid hydrogen storage tank to which the 3D printing-epoxy composite material of any one of claims 1 to 3 and claim 5 is applied as a fuel tank

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