Hence, the tank’s shape parameters have to be chosen in accordance with the mass of the LH2 required to be carried, its nominal temperature, the operating pressure of the vessel, and the insulation thickness ^[13][19]. At the time of its design, a trade-off has to be considered between the volume of the tank and the space available on the carrier.

Figure 2 sThows the tank volume (on the left vertical axis) and the stored hydrogen mass (on the right vertical axis) as functions of the tank radius; the lightlight-blue solid line refers to a spherocylinder with a length of just one meter, while the dark blue solid line represents a tank 20 m in length. The round-dotted lines indicate the performance of similar tanks with compressed gaseous hydrogen inside, at a pressure of 70 MPa. The increase in density with the transition from the gaseous to the liquid state is very clear here; a capsule tank 20 m in length can transport twice the mass of hydrogen if it is liquid instead of gaseous. Note that the dimensions shown here are of the interior vessel; nowadays’ cryogenic trailers may have these dimensions for the exterior tank, thus having much smaller interior capsules than the ones shown in this graph.

Understanding the differences in size between the interior vessel and the exterior tank is understanding insulation; the storage of hydrogen at such low temperatures requires special high-efficiency insulated tanks. This can be achieved by applying internal and/or external insulation. Internal insulation is always hard to implement since the materials are in direct contact with hydrogen in cryogenic temperatures; heat transfer phenomena cause the transition of hydrogen from a liquid state to a gas state at these contact boundaries, diffusing gaseous H2 into the walls of the tank. This then increases the thermal conductivity of the insulation material, thus damaging its efficacy ^[6][12]. Therefore, the system would have to be impermeable to CGH2 ^[12][18].

In the case of external insulation, the problem is not so much the direct contact with extremely low temperatures, but rather the contractions and expansions that the materials undergo, which come respectively from the charge and discharge of liquid hydrogen at these temperatures. Moreover, there is also an attachment issue for the support systems as well as possible mechanical damage from impact loads ^[12][18]. Nonetheless, these problems are more easily solved than those of internal insulation. Hence, the main challenge involving liquid hydrogen storage is the boil-off phenomenon; as previously explained, this occurs when heat is transferred to the tank’s interior, consequently warming the liquid hydrogen and effectively turning it into a gas (depending on several factors, up to 1 wt% per day due to heat in-leakages from the environment alone, as disclosed by Sherif et al. and confirmed by C. Yang and J. Ogden. ^[14][15][21,22]). Boil-off management is then an extremely relevant matter with regard to cryogenic hydrogen storage since it can lead to an unwanted pressure increase inside the rigid vessel; once the nominal maximum pressure is reached, the boil-off valve opens and ventilates part of the gas to the exterior. To reduce this effect—and thus reduce energy losses—several solutions have been adopted: Notardonato et al. ^[16][23] proposed an integration of cryo-coolers in the frame, while J. Wiley ^[17][24] recommended a storage combination with the chemical metal hydrides; Ho and Rahman favoured a cooling combination with liquid nitrogen (which is used to 'shield' the insulated tank, hence drastically reducing the heat transfer to the environment) and the installation of passive insulation ^[18][25]. This last solution can branch into several techniques, as pointed out by A. Züttel and J. Reijerkerk ^[19][20][26,27], with the use of vacuum, foam, and multi-layer insulation:

• Vacuum insulation. While a perfect vacuum may seem to be the best solution to eliminate the boil-off effect, it is also extremely difficult to achieve; as demonstrated by A. Colozza ^[21][28], peripheral equipment and venting devices are required to actively maintain the vacuum region. A vacuum chamber also raises issues about the thickness of the tank walls—they must be wide enough to withstand the buckling effect caused by external ambient pressure; however, as Millis et al. ^[22][29] reiterated, large walls in conjunction with additional stiffeners (required in-between the vacuum jacket shell) will inevitably increase the overall weight of the tank.
• Foam insulation. Despite not having insulation levels as good as those of vacuum chambers, foam insulation is a passive technique, easier and cheaper to implement. Usually, these foams are very light, have low density, and have low thermal conductivity; in his PhD thesis, I. Cumalioglu ^[23][30] studied how these are applied, sandwiched between two metal plaques, in order to improve structural stability and to better protect against external forces and impacts. Another great advantage of foam insulation over vacuum-jacketed insulation is its resistance to catastrophic failure; a pierced vacuum chamber would probably cause the overall collapse of the insulation layer, whereas foam insulation would not ^[19][26].
• Multi-layer insulation. This method can be seen more as a compliment to the previous two rather than as an alternative—although it is also possible to implement it as a stand-alone. Multi-layer insulation systems are commonly a set of reflective foil made of thermal radiation shields aligned perpendicularly to the direction of the heat flux; they are usually wrapped over the external layer of the tank in order to impede radiation heat transfer. These radiation shields mostly consist of alternate layers of thin aluminum foil and an insulating material such as fiberglass or polyester ^[3][8]. While additional layers may improve insulation from heat radiation, it also increases heat transfer by conduction; to cite Khandelwal and his co-authors ^[6][12], the optimal recommended number of layers is then between 60 and 100. Naturally, more layers will also add to the overall weight of the tank. Another drawback of multi-layer insulation sheets is their sensitivity to pressure gradients during manufacturing, as studied by Allideris and Janin ^[10][16]; this leads to the need for a very specialized type of production, which is more expensive. Further attention needs to be given to damage use accumulation resultant from external loads such as impacts during operation or unpredictable accidents, which can lead to fibre breaks, delamination, and matrix cracking ^[3][8].

As was notably mentioned by Kamiya and his co-authors ^[24][31], while heat inputs may be minimized through the use of the above-mentioned insulation techniques, sloshing is not. Sloshing occurs mainly during the acceleration/deceleration periods of trailer transport, where heat may be generated by the inherent vibration of the tanks; this can be overcome with the introduction of slosh-baffles—a device that dampens the adverse effects of sloshing and also increases the natural frequency of the tank ^[14][21].

Having seen the various methods of preferred exterior insulation, what happens in the interior of such tanks remains to be analyzed. An important matter to consider here is the material compatibility between hydrogen and the substance with which the vessel is made; the inner part of these vessels is usually made of stretched stainless steel. However, this raises concerns about the possible effects of hydrogen embrittlement, where hydrogen atoms diffuse into the structure of the metal ^[25][32]; by accumulating at locations with higher cracking potential, severe changes to the metal’s mechanical properties may happen—mainly a decrease in ductility, toughness, and load-bearing capability. At the same time, it also increases hardness and thus the propensity for premature brittle fractures. Note that this effect can happen with hydrogen in both the gaseous and the liquid states, and that this issue can have even greater impact when considering multi-material assemblies (as is the case with both compressed and cryogenic vessels). Among the methods for minimizing hydrogen embrittlement, Li et al. ^[26][33] indicated the use of special inner-surface treatments involving coating and surface material modifications as one of the most conventional approaches.

Regarding safety issues, Petipas and Aceves ^[27][34] believed that any explosion scenario is very unlikely to happen, since hydrogen has a low adiabatic expansion energy at very low temperatures; however, potential leakages can effectively damage nearby valves and devices that have not been rated to withstand such low temperatures, thus causing further malfunctioning.

3. Cryo-Compressed Hydrogen

Cryo-compressed hydrogen (CcH2) storage was introduced to overcome the disadvantages of both traditional storage methods mentioned above by combining their main characteristics, as explained by El-Eskandarany and Aceves et al. ^[28][29][35,36]. This storage occurs at cryogenic temperatures (not as low as with LH2) on a pressurized vessel, although not as much as with CGH2; it can comprise cold compressed hydrogen or hydrogen in a two-phase region (saturated liquid and vapor) ^[30][37]. Figure 3 illustrates hydrogen density as function of temperature and pressure, displaying the different storage methods discussed so far.

Figure 3. Correlation of hydrogen density and temperature for different conditions.

Usually, liquid hydrogen stored at ambient temperature is pumped through vacuum insulated piping to reach pressures of up to 85 MPa and densities as high as 85 g/L; an evaporator stage then converts it into a super-cooled gas before it is stored in special, carbon fibre-wrapped, metal, cryo-compressed vessels (similar to those of Type III for CGH2) ^[31][38]. Moreno-Blanco and his co-authors ^[32][39] found that storage density can be higher this way since liquid hydrogen is slightly more compressible: at −252 °C, it is 87 g/L at 240 bar compared to 70 g/L at atmospheric pressure.

Cryo-compressed hydrogen storage presents some advantages when compared to traditional methods, such as an overall higher energy density, volumetric efficiency and gravimetric capacities ^[31][38], a reduced boil-off effect ^[33][40], and thus reduced in-vessel over-pressurization and longer thermal endurance ^[34][41], among others. Stetson et al. ^[35][42] also demonstrated how cryo-compressed storage tanks are one of the most versatile since they are designed to endure both very low temperatures and very high pressures. However, there are still some limitations preventing this technology from becoming commercially viable; cryo systems are generally very complex and hard to implement, requiring the permanent and careful management and monitoring of their thermal insulation levels ^[31][38] as they have considerable maintenance costs, high energy needs for operation, and a short no-loss unused period ^[36][43].

Recently, numerous advancements have been made with respect to the geometry and the materials of the tanks used in these systems, since their performance highly depends on these aspects; thus far, an effort to compress the whole system was made, resulting in the halving of the vessel’s coating thickness to 1.5 cm. Nevertheless, Sdanghi et al. ^[31][38] emphasized that particular research should be carried out concerning the use of alloys of lighter density as coating materials.

The Table 1 following summary table displays the main attributes of the three physical ways of storing hydrogen discussed.

Table 1. Major features of physical hydrogen storage technologies.

Storage Method ⟶	CGH2	LH2	CCH2
Operational Parameters
Service pressure	Very High	Ambient	Very High
Service temperature	Ambient	Very Low	Very Low
Hydrogen density	Moderate	High	Very High
System Characteristics
Compression process	Required	Not Required	Required
Liquefaction process	Not Required	Required	Not Required
Thermal insulation	Not Required	Required	Required
Tank cost	Moderate	High	Moderate
Technology Features
Boil-off effect	Moderate	Very High	High
Hydrogen permeation	High	Moderate	High
Cooling capacity	n.a.	Very High	Very High

CGH₂: Compressed Gaseous Hydrogen; LH₂: Liquid Hydrogen; CCH₂: Cryo-compressed Hydrogen.