# A Comprehensive Literature Review on Hydrogen Tanks: Storage, Safety, and Structural Integrity
## 2 Hydrogen Tanks Classification  
### High-Pressure Hydrogen Tanks  
The initial technique employed for hydrogen storage was based on the compression of gases. Approximately 140 years ago, the initial hydrogen tanks were developed for military applications. The tanks were constructed entirely of metallic material, specifically steel, and could withstand an operating pressure of 120 bar. In the early sixties, the initial fiberglass composite systems were developed for military and aerospace applications, extending to the civilian market [36]. In approximately the nineties, the initial composite tanks for methane storage at 250 bar were introduced. Subsequently, ongoing research on material optimization and production processes has facilitated the expansion of composite tank technology to hydrogen storage, culminating in the advent of modern systems at 700 bar [23]. As illustrated in Figure 3 and detailed in Table 2, the current classification of high-pressure gas storage tanks is based on the materials utilized and the diverse construction solutions employed in engineering applications. These include the following five types [49; 50; 51]:  
* Type I: tanks entirely made of metallic material (steel or aluminum) [52; 51].
* Type II: Tanks with a main metallic structure, with added circumferential wraps of composite material. They are the evolution of the Type I tanks. The composite reinforcements provide greater strength, allowing a reduction in the thickness of metal walls and, consequently, weight [52; 51].
* Type III: Tanks with a predominantly composite structure covering the entire surface. They feature a thin internal metallic layer (steel or aluminum), which provides both structural support and gas impermeability [24; 38].
* Type IV: Tanks made of fully composite structure. They include a thin internal layer of polymer material, with the function of preventing gas leakage from the system [24; 25; 38; 53].
* Type V: Tanks entirely made of composite material without any internal coating, characterized by specific resin and fiber orientation choices. This is the most modern storage system, currently reserved for aerospace applications [36; 43; 50; 52].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Type**} & \multicolumn{3}{c}{**Materials**} & \multirow{2}{*}{**Max. Pressure (bar)**} & \multirow{2}{*}{**Applications**} & \multirow{2}{*}{**Structural (Load [51]**} \\ \cline{2-2} \cline{6-6}  & **Metal** & & & & & \\ \hline \multirow{2}{*}{**I**} & Steel/Al & \multirow{2}{*}{-} & \multirow{2}{*}{-} & Al: 175 & Submarine & Metal body \\  & & & & Steel: 200 & Applications [54] & withstands \\ \hline \multirow{4}{*}{**II**} & Steel/Al & \multirow{2}{*}{
\begin{tabular}{c} Filament \\ windings \\ \end{tabular} } & Al/Glass: & Stationary & Steel and \\  & liner & around the cylinder & - & Steel/Carbon & hydrogen & materials \\ \cline{1-1}  & & part & fiber: & (FCH) & share the \\ \cline{1-1}  & & part & 299 & technologies & load equally \\ \hline \end{tabular}
\end{table}
Table 2: Hydrogen tanks classification and main features [50; 51].  
Type I and Type II tanks feature a low cost. Type III and Type IV tanks, despite higher production costs, are currently adopted by commercial FCEVs due to their superior performance [24; 25; 38; 50]. Type V tanks, reserved for aerospace applications, offer remarkable performance in terms of weight and cost [50; 52; 55].