Review of the Liquid Hydrogen Storage Tank and Insulation System for the High-Power Locomotive

Hydrogen has been attracting attention as a fuel in the transportation sector to achieve carbon neutrality. Hydrogen storage in liquid form is preferred in locomotives, ships, drones, and aircraft, because these require high power but have limited space. However, liquid hydrogen must be in a cryogenic state, wherein thermal insulation is a core problem. Inner materials, including glass bubbles, multi-layer insulation (MLI), high vacuum, and vapor-cooled shields, are used for thermal insulation. An analytic study is preferred and proceeds liquid hydrogen tanks due to safety regulations in each country. This study reviewed the relevant literature for thermodynamic modeling. The literature was divided into static, dynamic, and systematic studies. In summary, the authors summarized the following future research needs: The optimal design of the structure, including suspension, baffle, and insulation system, can be studied to minimize the boil-off gas (BOG). A dynamic study of the pressure, mass flow, and vaporizer can be completed. The change of the components arrangement from the conventional diesel–electric locomotive is necessary.


Introduction
Mobile vehicles require independent energy carriers, for which alternatives to fossil fuels, such as batteries and hydrogen, are currently under research and development, as shown in Table 1. Electric propulsion systems using motors have been gradually introduced into the large transportation sector, such as trains, ships, and aircraft. Therefore, the vehicles can be arranged according to the required power. Conventional internal combustion engines (ICEs) or fuel cells can be viewed as power generators.
Hydrogen can be stored in gaseous, liquid, or chemical forms [1]. Metal hydrate, which can store and take out hydrogen through a thermal reaction, has also been researched [2]. Liquid organic hydrogen carriers (LOHCs) and ammonia, which are chemical forms, have the advantages of safe use but are not suitable for vehicles, because they require largevolume equipment to input energy for conversion. Gaseous hydrogen storage provides a fast response, but the energy content per weight and volume remains low, even if the tank pressure is high (350-700 bar). The liquid hydrogen (LH 2 ) form has the highest energy density and can be easily converted to hydrogen gas through a vaporizer. Hence, LH 2 , rather than gaseous hydrogen, could be preferred as a fuel for vehicles requiring a highspeed rail or enormous power. The use of LH 2 saves space and reduces the locomotive's weight, allowing more cargo to be loaded.
The battery's energy density is 300-500 Wh/kg and 700-1000 Wh/L, and the energy density of LH 2 is 2000-2500 Wh/kg and 110-140 Wh/L. Therefore, for the same energy storage system (ESS), hydrogen is lighter in weight than the battery, and the battery is smaller in volume than hydrogen. Due to these characteristics, batteries are considered for low-powered vehicles with a relatively short mileage, whereas hydrogen is considered for high-powered vehicles with a long mileage. For example, hydrogen or battery trains can replace diesel trains in the train sector. Using catenary lines is energetic and economically efficient, but the electrification rate of the railway is not high worldwide. The electrification rate of Northeast Asia and Europe is over 60%, but the rest of the Earth, including the Americas, Africa, and Australia, is under 10%. As an alternative to the diesel trains operating on routes without catenary lines, battery trains can be used as short-distance trams in cities, and hydrogen trains can be used as multiple-unit or long-distance, high-powered locomotives.
The replacement of diesel trains can be helpful for international environmental regulations. In Korea, 265 diesel locomotives existed as of 2018. They emit air pollutants equivalent to 300 commercial diesel vehicles. By converting diesel locomotives to hydrogen locomotives, direct air pollutants (particulate matter (PM)) and indirect air pollutants (CO and NO x ) are expected to be reduced.
Hydrogen trains have been actively studied since the commercialization by Alstom in France in 2014, and their model 'Coradia iLint' was unveiled in 2016. Beyond the railcar and tram, hydrogen locomotive and shunter have been researched by CP in Canada, CRRC in China, CZ in the Czech Republic, PESA in Poland, Wabtec in the United States, and Korean Railroad Research Institute (KRRI), as shown in Table 2. Especially, KRRI is working on the world's first LH 2 locomotive. tank requires a high level of thermal insulation to avoid thermal loss. The significant temperature difference between LH 2 (as low as 20 K) and the surrounding atmosphere induces boil-off gas (BOG), which leads to high pressure and the loss of stored hydrogen. In addition to heat leakage, flashing, sloshing (especially after thermal stratification), and ortho-to-parahydrogen conversion can cause boil-off [3]. This study focuses on heat leakage in the static state.
Following the space race, MSFC researched long-term LH 2 storage in the 1990s, rather than temporary storage for a rocket launch, with the NASA Glenn Research Center (GRC) and Lockheed Martin Space Systems [4,5]. The multi-purpose hydrogen test bed (MHTB) included an 18.09 m 3 tank, insulated with a 3.53-cm-thick spray-on foam insulation (SOFI) and a 3.75-cm-thick multi-layer insulation (MLI) with 45 aluminum blankets and spacers, as shown in Figure 1. The MLI was simulated using layer-by-layer, Lockheed, and modified Lockheed models, and the differences in the methods are described in the following Section 2.3 [6]. A vacuum gap separated the outer and inner vessels, and the two vessels were fixed through the ground-hold or orbit-hold method. The results showed a heat leakage of 0.22 W/m 2 for the orbit-hold method and 305 K room temperature, which is 50% less than when no MLI is used. The ground-hold method showed a heat leakage of 63 W/m 2 , which is 238 times that of the orbit-hold method. This result shows that the heat conduction at the suspension between the two vessels, which is the structure that maintains the distance between the two vessels and fixes them, is more excellent than the heat leakage of the wall surrounded by insulation schemes. Therefore, this result showed the necessity to study the method of fixing two vessels with minimal structure to avoid heat conduction. Another example of long-term LH 2 storage other than NASA is the first LH 2 fueling station built at Munich Airport in Germany in 1999. However, the LH 2 tank was not directly used to fuel LH 2 , and a vaporizer was used to fuel the gas tank vehicles [7].
In addition to heat leakage, flashing, sloshing (especially after thermal stratificatio ortho-to-parahydrogen conversion can cause boil-off [3]. This study focuses on hea age in the static state. Following the space race, MSFC researched long-term LH2 storage in the 1990s than temporary storage for a rocket launch, with the NASA Glenn Research Center and Lockheed Martin Space Systems [4,5]. The multi-purpose hydrogen test bed (M included an 18.09 m 3 tank, insulated with a 3.53-cm-thick spray-on foam insulation and a 3.75-cm-thick multi-layer insulation (MLI) with 45 aluminum blankets and s as shown in Figure 1. The MLI was simulated using layer-by-layer, Lockheed, and fied Lockheed models, and the differences in the methods are described in the fol Section 2.3 [6]. A vacuum gap separated the outer and inner vessels, and the two were fixed through the ground-hold or orbit-hold method. The results showed a he age of 0.22 W/m 2 for the orbit-hold method and 305 K room temperature, which less than when no MLI is used. The ground-hold method showed a heat leakag W/m 2 , which is 238 times that of the orbit-hold method. This result shows that th conduction at the suspension between the two vessels, which is the structure that tains the distance between the two vessels and fixes them, is more excellent than th leakage of the wall surrounded by insulation schemes. Therefore, this result show necessity to study the method of fixing two vessels with minimal structure to avo conduction. Another example of long-term LH2 storage other than NASA is the fi fueling station built at Munich Airport in Germany in 1999. However, the LH2 tan not directly used to fuel LH2, and a vaporizer was used to fuel the gas tank vehicle Following the stationary LH2 storage, mobile LH2 vessels have been research 1999, BMW showed the first LH2 vehicle, 750hl, in Germany. In 2018, Hylium bu first mobile refueling truck in Korea. In 2021, Linde provided LH2 tanks to Hjelm Ferry in Norway. This study reviewed the literature necessary for the system modeling of LH2 s tanks for application in the transportation sector. First, static modeling studies, inc heat transfer coefficients and thermal conductivity, were reviewed. Second, dy modeling studies were reviewed, including phase changes and values with chang time. Third, systematic modeling studies for suitable sizing and arrangement of the system for a vehicle were reviewed. Lastly, the future research needs were summa Following the stationary LH 2 storage, mobile LH 2 vessels have been researched. In 1999, BMW showed the first LH 2 vehicle, 750hl, in Germany. In 2018, Hylium built the first mobile refueling truck in Korea. In 2021, Linde provided LH 2 tanks to Hjelmeland Ferry in Norway.
This study reviewed the literature necessary for the system modeling of LH 2 storage tanks for application in the transportation sector. First, static modeling studies, including heat transfer coefficients and thermal conductivity, were reviewed. Second, dynamic modeling studies were reviewed, including phase changes and values with changes over time. Third, systematic modeling studies for suitable sizing and arrangement of the power system for a vehicle were reviewed. Lastly, the future research needs were summarized.

Wall, Vacuum Gap, Suspension, and Sensors
Various tank shapes have been studied for ships carrying LNG, and the expression "1st/2nd barrier of the tank" is used instead of "inner/outer vessel" of the LH 2 tank. The tanks are independent or integrated with the ship, which GTT operates in France.
Most materials become brittle under cryogenic conditions, such as stainless steel, aluminum alloy, titanium alloy, and composite materials, whose behaviors have been studied. Grade 316L stainless steel, which has a low carbon content, is commonly used [8]. Stainless steel has a thermal conductivity of approximately 10 W/(m·K) at 20 K in contact with LH 2 and approximately 16 W/(m·K) in contact with the air at room temperature. The thermal resistance equation can represent the inner wall for conduction in a cylindrical form [9].
Other materials, such as copper, which has a poorer insulation effect than steel, can be considered for the inner vessel. The existing studies have only focused on the insulation effect. Instead, the total efficiency should be considered when designing an LH 2 tank for a vehicle. The loss rate through vaporization during storage is approximately 0.1-5% per day, but the loss rate during charging is approximately 20% per charge if the vessel is at room temperature. Moreover, the storage period for a vehicle can be as short as a few days. Therefore, an economic-energetic approach must be formulated to determine the optimal point. The case of charging the vessel containing LH 2 at cold temperatures instead of the empty vessel at room temperature can also be considered.
The air layer in the gap between the inner and outer vessel walls can be configured as the heat conduction of gases and heat radiation between solids. The outer vessel wall was cooled via air convection, and radiation to the surroundings was assumed to be in the same condition as the air. The emissivity of stainless steel is 0.6 (dimensionless), the convective heat transfer coefficient of the air is approximately 2 W/(m 2 ·K), and the Stefan-Boltzmann number is 5.667 × 10 −8 W/(m 2 ·K 4 ). The air and surrounding temperatures are generally assumed to be equal.
The suspension structure connects the inner and outer walls, and heat conduction appears. G10, G10-CR, and G11, which are fiberglass, can be used for suspension and supports for small-sized stationary tanks [10]. Their thermal conductivity is approximately 0.2~0.3 W/(m·K). However, 316L stainless steel can be used for a big size of tank for mobility, and it will enlarge the heat leakage.
Sensors also connect the inner and outer walls. Level, pressure, and temperature sensors are typical. Their outer material can be 316L stainless steel, but the wire material can be copper. For insulating the wire, the polyimide film can be used as the shell material, and its thermal conductivity is approximately 0.1~0.7 W/(m·K) by product and temperature.

Inner Materials
As the inner materials (IMs), porous foam, SOFI, fiber-reinforced plastic (FRP), aerogel, glass bubble, and hollow glass microsphere (HGM) have been used. Polyurethane foam has been used to insulate liquified natural gas (LNG), but a higher insulation performance is required for the insulation of LH 2 . A thickness of several meters is required to insulate LH 2 by using polyurethane foam.
Unlike the thermal conductivity of MLI, which changes significantly with the change in the degree of vacuum, the vacuum pressure had little effect on that of SOFI. Fesmire researched the effects of spray foam and rigid foam in non-vacuum conditions for the insulation of the rocket fuel tanks [11]. Johnson et al. tested the heat flux according to the vacuum pressure in MLI for the long-term storage of liquid methane on the lunar surface and suggested a correlation formula [12].
SOFI has a thermal conductivity of 10 −3 -10 −2 W/m·K and can be simplified to a hypothetical layer in which the thermal conductivity ranges from 0.005 to 0.04 W/(m·K). Tseng et al. measured the thermal conductivity of polyurethane foam and suggested the shape of the test chamber and the experimental method [13]. Mekonnen et al. described the process from fabrication to coating aerogels and showed the thermal conductivity range [14].
MLI shows a 35-50% better insulation performance than SOFI [15]. By contrast, HGM has a thermal conductivity of 10 −3 -10 −4 W/m·K, which is even better than MLI [16,17]. A group of microspheres is easily assumed to be the same size and uniform arrangement, as shown in Figure 2. However, the particle size is all different in an actual experiment, and the accumulation is random.
x FOR PEER REVIEW 5 of 14 et al. measured the thermal conductivity of polyurethane foam and suggested the shape of the test chamber and the experimental method [13]. Mekonnen et al. described the process from fabrication to coating aerogels and showed the thermal conductivity range [14]. MLI shows a 35-50% better insulation performance than SOFI [15]. By contrast, HGM has a thermal conductivity of 10 -3 -10 -4 W/m·K, which is even better than MLI [16,17]. A group of microspheres is easily assumed to be the same size and uniform arrangement, as shown in Figure 2. However, the particle size is all different in an actual experiment, and the accumulation is random.

Multi-Layer Insulation
MLI and variable density MLI (VDMLI) were studied. MLI consists of a reflector with low thermal emissivity and a spacer with low thermal conductivity. Spacers prevent reflectors from direct contact and avoid heat conduction. Alumina-coated polyester sheets, alternating layers of aluminum foil and glass fiber, aluminum, silica, or perlite particles, were used for MLI [18]. Typically, Mylar or Kapton can be used as a reflector, and Dacron nets can be used as the spacer [19]. The external film can be coated by painting or material such as glass-reinforced cloth. An example of MLI for insulating spacecraft is the same in Figure 3 [20]. Previous studies have shown that the ratio of these heat flux components depends on the temperature distribution [21]. Thus, VD-MLI, which allows for the thickness of spacers to be modified, provided a better insulation performance than the basic MLI [22]. Many studies have suggested the optimization of the layer density and arrangement via thermodynamic modeling [23][24][25]. Optimization based on a combination of foam and MLI was also researched, and the combination of foam and MLI showed a 10-50% higher performance than MLI alone [26,27].
MLI can be simplified to a hypothetical layer in which the thermal conductivity varies from 0.00001 to 0.072 W/(m·K), owing to material differences and pressure variations [28]. MLI can be simulated by layer-by-layer, Lockheed, and modified Lockheed models.

Multi-Layer Insulation
MLI and variable density MLI (VDMLI) were studied. MLI consists of a reflector with low thermal emissivity and a spacer with low thermal conductivity. Spacers prevent reflectors from direct contact and avoid heat conduction. Alumina-coated polyester sheets, alternating layers of aluminum foil and glass fiber, aluminum, silica, or perlite particles, were used for MLI [18]. Typically, Mylar or Kapton can be used as a reflector, and Dacron nets can be used as the spacer [19]. The external film can be coated by painting or material such as glass-reinforced cloth. An example of MLI for insulating spacecraft is the same in Figure 3 [20]. et al. measured the thermal conductivity of polyurethane foam and suggested the shape of the test chamber and the experimental method [13]. Mekonnen et al. described the process from fabrication to coating aerogels and showed the thermal conductivity range [14] MLI shows a 35-50% better insulation performance than SOFI [15]. By contrast, HGM has a thermal conductivity of 10 -3 -10 -4 W/m·K, which is even better than MLI [16,17]. A group of microspheres is easily assumed to be the same size and uniform arrangement, as shown in Figure 2. However, the particle size is all different in an actual experiment, and the accumulation is random.

Multi-Layer Insulation
MLI and variable density MLI (VDMLI) were studied. MLI consists of a reflector with low thermal emissivity and a spacer with low thermal conductivity. Spacers prevent reflectors from direct contact and avoid heat conduction. Alumina-coated polyester sheets alternating layers of aluminum foil and glass fiber, aluminum, silica, or perlite particles were used for MLI [18]. Typically, Mylar or Kapton can be used as a reflector, and Dacron nets can be used as the spacer [19]. The external film can be coated by painting or material such as glass-reinforced cloth. An example of MLI for insulating spacecraft is the same in Figure 3 [20]. Previous studies have shown that the ratio of these heat flux components depends on the temperature distribution [21]. Thus, VD-MLI, which allows for the thickness of spacers to be modified, provided a better insulation performance than the basic MLI [22] Many studies have suggested the optimization of the layer density and arrangement via thermodynamic modeling [23][24][25]. Optimization based on a combination of foam and MLI was also researched, and the combination of foam and MLI showed a 10-50% higher performance than MLI alone [26,27].
MLI can be simplified to a hypothetical layer in which the thermal conductivity varies from 0.00001 to 0.072 W/(m·K), owing to material differences and pressure variations [28]. MLI can be simulated by layer-by-layer, Lockheed, and modified Lockheed models Previous studies have shown that the ratio of these heat flux components depends on the temperature distribution [21]. Thus, VD-MLI, which allows for the thickness of spacers to be modified, provided a better insulation performance than the basic MLI [22]. Many studies have suggested the optimization of the layer density and arrangement via thermodynamic modeling [23][24][25]. Optimization based on a combination of foam and MLI was also researched, and the combination of foam and MLI showed a 10-50% higher performance than MLI alone [26,27].
MLI can be simplified to a hypothetical layer in which the thermal conductivity varies from 0.00001 to 0.072 W/(m·K), owing to material differences and pressure variations [28]. MLI can be simulated by layer-by-layer, Lockheed, and modified Lockheed models. The Lockheed model is based on a semi-empirical formula and can quickly solve the overall heat flux, but the selection of empirical parameters depends on the conditions. By contrast, the layer-by-layer method is based on an iterative algorithm and can calculate the temperature of each layer in MLI. The layer-by-layer method divides the heat transfer into three forms: solid heat conduction, gaseous heat conduction, and radiation.

Phase Change of Liquid Hydrogen
The heat transfer coefficient (HTC), critical heat flux (CHF), and thermal conductivity of the hydrogen have been studied. Kumar et al. solved a homogeneous two-phase model and showed that the effect of evaporation on stratification increases as the aspect ratio, which means the ratio of width and height of the tanks, increases by Ansys Fluent [29]. Horie et al. experimentally summarized the film boiling heat transfer coefficient of LH 2 , according to the pressure and the subcooling temperature [30]. Baldwin Figure 4 [35]. In Figure 4, unlike other studies that modeled only the insulation system, the liquid and gaseous hydrogen were assumed to be a single node with mass, temperature, pressure, and volume. The thermal resistance diagram, including hydrogen, can be expanded to dynamic modeling with venting, filling, and vaporizing, with level and pressure changes in the tank. On the other hand, the study of high-pressure vessels with phase changes is similar to the steam generation heat pump [36]. In the industrial sector, high-temperature and high-pressure steam are used, and heat pumps with higher energy efficiency than boilers are introduced. In a flash tank, water and steam coexist, and water is added as much as steam is used. The Lockheed model is based on a semi-empirical formula and can quickly solve the overall heat flux, but the selection of empirical parameters depends on the conditions. By contrast, the layer-by-layer method is based on an iterative algorithm and can calculate the temperature of each layer in MLI. The layer-by-layer method divides the heat transfer into three forms: solid heat conduction, gaseous heat conduction, and radiation.

Phase Change of Liquid Hydrogen
The heat transfer coefficient (HTC), critical heat flux (CHF), and thermal conductivity of the hydrogen have been studied. Kumar et al. solved a homogeneous two-phase model and showed that the effect of evaporation on stratification increases as the aspect ratio, which means the ratio of width and height of the tanks, increases by Ansys Fluent [29].  Figure 4, unlike other studies that modeled only the insulation system, the liquid and gaseous hydrogen were assumed to be a single node with mass, temperature, pressure, and volume. The thermal resistance diagram, including hydrogen, can be expanded to dynamic modeling with venting, filling, and vaporizing, with level and pressure changes in the tank. On the other hand, the study of high-pressure vessels with phase changes is similar to the steam generation heat pump [36]. In the industrial sector, hightemperature and high-pressure steam are used, and heat pumps with higher energy efficiency than boilers are introduced. In a flash tank, water and steam coexist, and water is added as much as steam is used.

Changes due to Phase Change of the Hydrogen
One of the characteristics of LH2 is that its sensible heat is more significant than its vaporization heat until it reaches a gaseous state at room temperature. The ratio of the two increases with the increase in pressure, and thus, the effect of the tank pressure was studied. The ratio is 7.82 at 100 kPa and 23.70 at 1.2 MPa [37]. The enthalpy changes by 3860 kJ/kg while vaporizing from -253 to 20 °C, as shown in Table 3 [38]. The heat of the

Changes due to Phase Change of the Hydrogen
One of the characteristics of LH 2 is that its sensible heat is more significant than its vaporization heat until it reaches a gaseous state at room temperature. The ratio of the two increases with the increase in pressure, and thus, the effect of the tank pressure was studied. The ratio is 7.82 at 100 kPa and 23.70 at 1.2 MPa [37]. The enthalpy changes by 3860 kJ/kg while vaporizing from -253 to 20 • C, as shown in Table 3 [38]. The heat of the phase change and sensible heat of hydrogen can be obtained using REFPROP (version 10, National Institute of Standards and Technology). A commercial electrolyzer requires 45-50 kWh of electricity to produce 1 kg of hydrogen, higher than the higher heating value (HHV) of hydrogen (39.4 kWh/kg), and emits 5-10 kWh/kg of heat. A commercial fuel cell generates 15-20 kWh/kg of electricity from 1 kg of hydrogen, which is lower than the hydrogen's lower heating value (LHV) (33.4 kWh/kg) and emits 13-18 kWh/kg of heat. The vaporization heat of LH 2 is insufficient to cool the fuel cell, but it can help insulate the LH 2 tank.
Additionally, the expansion ratio of the vaporization is approximately 848, which means that 100 kPa of LH 2 can be vaporized to 172 MPa of vapor hydrogen. The typical fuel cells for vehicles use 300 kPa-1.6 MPa of gaseous hydrogen. Therefore, appropriate pressure control is required from the vessel to the vaporizer and the fuel cell inlet.

Vapor-Cooled Shield or Vent Line
A vapor-cooled shield (VCS) is a vent pipe made from the same material as the inner vessel and located between the inner wall and the end of MLI. For production convenience, the VCS can be placed just outside the inner wall by winding around it. The VCS can be simplified to a single layer and considered as intercepting the heat penetration at the middle.
The VCS, which involves recovering the sensible heat of the self-evaporated gaseous hydrogen owing to heat penetration, is a technology at the boundary between passive insulation and active insulation. The insulation effect of the VCS with LH 2 , nitrogen, oxygen, and methane was studied using a model validated against the existing experimental data [39]. Subsequently, the optimal location of the VCS within the foam and MLI and its thermal behavior was studied [37,40,41]. In addition, another material, liquid nitrogen, which has a higher boiling point than LH 2 , was studied for use as an insulation shield around the LH 2 tank [37].
Based on the energy conservation, the heat transfer in the VCS is the same as the difference in heat entering into and exiting out of the VCS, as illustrated in Figure 5. The heat penetration into the inner tank (Q in ) is assumed to be the latent heat of the vaporization of hydrogen. In contrast, the heat transfer through the VCS (Q vcs ) is assumed to be sensible heat caused by the difference between the saturated vapor temperature and the outside temperature.  A commercial electrolyzer requires 45-50 kWh of electricity to produce 1 kg of hydrogen, higher than the higher heating value (HHV) of hydrogen (39.4 kWh/kg), and emits 5-10 kWh/kg of heat. A commercial fuel cell generates 15-20 kWh/kg of electricity from 1 kg of hydrogen, which is lower than the hydrogen's lower heating value (LHV) (33.4 kWh/kg) and emits 13-18 kWh/kg of heat. The vaporization heat of LH2 is insufficient to cool the fuel cell, but it can help insulate the LH2 tank.
Additionally, the expansion ratio of the vaporization is approximately 848, which means that 100 kPa of LH2 can be vaporized to 172 MPa of vapor hydrogen. The typical fuel cells for vehicles use 300 kPa-1.6 MPa of gaseous hydrogen. Therefore, appropriate pressure control is required from the vessel to the vaporizer and the fuel cell inlet.

Vapor-Cooled Shield or Vent Line
A vapor-cooled shield (VCS) is a vent pipe made from the same material as the inner vessel and located between the inner wall and the end of MLI. For production convenience, the VCS can be placed just outside the inner wall by winding around it. The VCS can be simplified to a single layer and considered as intercepting the heat penetration at the middle.
The VCS, which involves recovering the sensible heat of the self-evaporated gaseous hydrogen owing to heat penetration, is a technology at the boundary between passive insulation and active insulation. The insulation effect of the VCS with LH2, nitrogen, oxygen, and methane was studied using a model validated against the existing experimental data [39]. Subsequently, the optimal location of the VCS within the foam and MLI and its thermal behavior was studied [37,40,41]. In addition, another material, liquid nitrogen, which has a higher boiling point than LH2, was studied for use as an insulation shield around the LH2 tank [37].
Based on the energy conservation, the heat transfer in the VCS is the same as the difference in heat entering into and exiting out of the VCS, as illustrated in Figure 5. The heat penetration into the inner tank (Qin) is assumed to be the latent heat of the vaporization of hydrogen. In contrast, the heat transfer through the VCS (Qvcs) is assumed to be sensible heat caused by the difference between the saturated vapor temperature and the outside temperature.

Boil-Off Ratio
The Boil-off Ratio (BOR) means the ratio of BOG generated in a day from the size of the entire tank. The analysis conditions for the calculation of the BOR are shown in Figure 6. For example, Jeon et al. predicted the BOR of the stationary tanks by a multiphase thermal simulation [42]. A C-type liquid nitrogen tank and membrane LNG tank were analyzed by the computational fluid dynamics (CFD) software STAR-CCM+, and the results showed that heat conduction through suspension is significant. The Boil-off Ratio (BOR) means the ratio of BOG generated in a day from the size of the entire tank. The analysis conditions for the calculation of the BOR are shown in Figure 6. For example, Jeon et al. predicted the BOR of the stationary tanks by a multiphase thermal simulation [42]. A C-type liquid nitrogen tank and membrane LNG tank were analyzed by the computational fluid dynamics (CFD) software STAR-CCM+, and the results showed that heat conduction through suspension is significant. However, the mobile vessels for transport show a higher BOR due to sloshing by vibration compared to the stationary vessels. Liu et al. did a series of studies on numerical models of dynamic meshes in sloshing conditions [43][44][45]. The authors used the software ICEM surf with the volume of fluid (VOF) method and applied the horizontal sinusoidal vibration to the LH2 tank. Slosh baffles in the tank decreased the sloshing, and BOR increased the stratification thickness. The authors additionally studied the force, momentum, pressure, and amplitude of the vibration in sloshing. Wei et al. analyzed the effect of horizontal sinusoidal sloshing on vertical tanks using the software Fluent 17.0 (ANSYS Inc., Pittsburgh, PA, USA) [46]. The authors showed that the pressure-time curve increases if the vibration condition exceeds the critical amplitude and frequency.
Ghafri et al. summarized LH2 boil-off modeling and implemented it in the software package BoilFAST [47]. The software is validated by multiple sets of industrially relevant data, including NASA, and provides static and dynamic modeling. Besides, the pathway's heat penetration, including pipes and cryopump, was studied. Petitpas et al. showed that the losses along the pathway of LH2 could vary by up to 15% and calculated an economicenergetic analysis [48].

Capacity of the Power Required and the Fuel Tank
The capacity of the fuel cell and fuel tank are determined according to the purpose of the hydrogen vehicle. The typical capacity ranges of the hydrogen vehicle are listed in Table 4. The Hyundai group in Korea modularized its fuel cells to 100 kW and produced more than 20,000 fuel cells per year. Buses, trucks, and trams require two to four fuel cell modules, whereas locomotives require one to two modules for each of the six wheels. With the increase in the weight, including cargo and passengers, the fuel efficiency decreases, and the size of the fuel tank is determined to meet the required distance according to the purpose of the vehicle. However, the mobile vessels for transport show a higher BOR due to sloshing by vibration compared to the stationary vessels. Liu et al. did a series of studies on numerical models of dynamic meshes in sloshing conditions [43][44][45]. The authors used the software ICEM surf with the volume of fluid (VOF) method and applied the horizontal sinusoidal vibration to the LH 2 tank. Slosh baffles in the tank decreased the sloshing, and BOR increased the stratification thickness. The authors additionally studied the force, momentum, pressure, and amplitude of the vibration in sloshing. Wei et al. analyzed the effect of horizontal sinusoidal sloshing on vertical tanks using the software Fluent 17.0 (ANSYS Inc., Pittsburgh, PA, USA) [46]. The authors showed that the pressure-time curve increases if the vibration condition exceeds the critical amplitude and frequency.
Ghafri et al. summarized LH 2 boil-off modeling and implemented it in the software package BoilFAST [47]. The software is validated by multiple sets of industrially relevant data, including NASA, and provides static and dynamic modeling. Besides, the pathway's heat penetration, including pipes and cryopump, was studied. Petitpas et al. showed that the losses along the pathway of LH 2 could vary by up to 15% and calculated an economic-energetic analysis [48].

Capacity of the Power Required and the Fuel Tank
The capacity of the fuel cell and fuel tank are determined according to the purpose of the hydrogen vehicle. The typical capacity ranges of the hydrogen vehicle are listed in Table 4. The Hyundai group in Korea modularized its fuel cells to 100 kW and produced more than 20,000 fuel cells per year. Buses, trucks, and trams require two to four fuel cell modules, whereas locomotives require one to two modules for each of the six wheels. With the increase in the weight, including cargo and passengers, the fuel efficiency decreases, and the size of the fuel tank is determined to meet the required distance according to the purpose of the vehicle. Apart from the vehicles, the LH 2 tank for drones weighs 0.3-1 kg, the ground tank for refueling stations is 20-450 kg, and the trailer tank for transport weighs 1000-3000 kg. The Saturn V rocket used in the space race carried 69 metric tons of LH 2 , but this is not relevant to this study, because its tank was meant for immediate launch and not for insulated storage.

Arrangement of the Fuel Tank
Smith et al. designed a conceptual LH 2 carrier ship from a conventional LNG carrier ship [49]. Ship sizing, power consumption, including electrified propulsion system and reliquifying the hydrogen system, and sloshing are reviewed. The authors predicted that only 40.2% of energy could be loaded at the same volume, and the fuel loss rate could be reduced by 38.7% through reliquefication.
Similarly, KRRI is considering the arrangement of the LH 2 locomotive from a conventional diesel locomotive. GE PowerHaul is a major diesel-electric locomotive with 2.8 MW power since 2007, and it is called Class 70 in the UK, DE 36000 in Turkey, and Class 7600 in Korea. Its standard subclasses include PH37ACi in mainland Europe and PH37ACmi in the UK. A sample of its configuration is shown in Figure 7. The components related to the engine and mass flow will be changed to the fuel cell and the hydrogen supply system. The heating, ventilation, and air-conditioning (HVAC) system will remain. However, its sizing and arrangement will be changed, because the amount of heat generation for power generation (engine to fuel cell) is different. New technologies, including liquid-or phase-change cooling, can be applied to decrease the volume of the HVAC system. A conventional diesel fuel tank is located under the diesel engine at the bottom of the vehicle. The side view of the tank is trapezoidal, and its capacity is 8600 L of diesel, as shown in Figure 8. Firstly, the location of the fuel tank can be considered. Hydrogen leaks upwards in the case of fire, so it may be good to put the tank on top of the vehicle. A conventional diesel fuel tank is located under the diesel engine at the bottom of the vehicle. The side view of the tank is trapezoidal, and its capacity is 8600 L of diesel, as shown in Figure 8. Firstly, the location of the fuel tank can be considered. Hydrogen leaks upwards in the case of fire, so it may be good to put the tank on top of the vehicle. that will be changed.
A conventional diesel fuel tank is located under the diesel engine at the bottom of vehicle. The side view of the tank is trapezoidal, and its capacity is 8600 L of diesel shown in Figure 8. Firstly, the location of the fuel tank can be considered. Hydrogen le upwards in the case of fire, so it may be good to put the tank on top of the vehicle. Secondly, the design of the LH2 tank can be considered. A vessel with a low surfa to-volume ratio shows a good insulation effect. However, the cylinder type is preferred the sphere type because of the efficiency in space utilization. The cylindrical tank wit diameter of 0.72 m and a length of 2.7 m has a capacity of 500 L or 35 kg. If the tank sp Secondly, the design of the LH 2 tank can be considered. A vessel with a low surfaceto-volume ratio shows a good insulation effect. However, the cylinder type is preferred to the sphere type because of the efficiency in space utilization. The cylindrical tank with a diameter of 0.72 m and a length of 2.7 m has a capacity of 500 L or 35 kg. If the tank space is a rectangular parallelepiped, 14 tanks (490 kg) can be stored. To enlarge the volumetric and energetic efficiency, the diameter of the cylinder tank can be 1 m or more. A larger tank can also offer better insulation (for example, 0.1 and 100 m 3 tanks show boil-off ratios of 2 and 0.06%, respectively [50]). The tank can be made in a spherical shape and placed inside the vehicle to maximize the thermal insulation effect. David et al. briefly reviewed the surface-volume ratio, boil-off ratio, and underlying assumptions of the LH 2 tank [51].

Structure and Efficiency of the Fuel Tank
The LH 2 tank design for aircraft and heavy-duty trucks (HDV) has been studied. Gomez et al. suggest structural sizing and stress analysis for EASA CS-25 aircraft [52]. Abu Kasim et al. suggest a performance and failure analysis for Cessna 208 aircraft [53]. Choi et al. reviewed the ISO 13985 tank for HDV [54]. The authors suggested a design procedure involving material selection and a structural and thermal analysis.
Thermodynamic analysis and CFD modeling of the LH 2 tank venting under microgravity have been studied. Shuang et al. suggested pressure control by using the microgravity of the tank, which showed that it can enhance the efficiency [55]. Jiang et al. did a CFD simulation of the tank's heat transfer and phase change under microgravity conditions. [56] Zheng et al. also did a CFD simulation of the tank under microgravity, but the authors added the dynamic results of the venting system [57]. Zuo et al. suggested a thermodynamic model of the venting process of an on-orbit structure liquid hydrogen tank [58].

Future Research Needs
In future studies, the approaches, as follows, are predicted to be available: (1) The optimal suspension design between vessels and baffles in the inner vessel can be studied to minimize the BOG. Various CFD can be studied depending on the structure and vibration direction. (2) The optimal location and method of filling and drain can be studied. Depending on the circumstances, the BOG in the charging process can be larger than the BOG in storage.
(3) Materials and arrangement of the vessel wall and insulation system can be studied. Former studies only focused on the insulation effect, but the optimal arrangement can exist depending on the operation, including filling and draining. A 4E analysis (Energy, Economic, Enthalpy, and Entropy) can be used. (4) Dynamic control of the pressure, mass flow, and vaporizer (heat exchanger) system before the fuel cell can be studied. (5) Utilizing the heat of the fuel cell or battery for heating the hydrogen in the vaporizer can also be studied. (6) The arrangement of new components can be researched. For example, fuel cells and battery modules require a large cooling capacity. Liquid-or phase-change cooling can be applied. (7) Various LH 2 vessel shapes, locations, and arrangements can be studied. The charger design is needed to research because of the large loss and BOG generation in the filling process.

Conclusions
Heavy-duty mobilities, including trains, are expected to change their fuel from fossil fuel to hydrogen, renewable energy for carbon-neutralization and solving global warming. The use of liquid hydrogen is one way to solve the low energy density of gaseous hydrogen. However, liquid hydrogen storage requires a high level of thermal insulation to minimize the boil-off gas.
This paper shows the recent hydrogen train research trend and reviews the thermodynamic modeling of the hydrogen tanks by dividing them into static, dynamic, and systematic levels. In static studies, insulation schemes, including IMs, MLIs, vessels, and vacuums, were researched. Thermal resistance diagrams involving hydrogen can be directly applied to dynamic studies. Regarding dynamic studies, the phase change of hydrogen was calculated. The shape, structure, and direction of the vibration of the tank were studied to check the boil-off ratio. Finally, systematic studies reviewed the surface-volume ratio and arrangement. The change of gravity and venting according to the arrangement also improved the system efficiency.