# A Comprehensive Literature Review on Hydrogen Tanks: Storage, Safety, and Structural Integrity
## 5 Fundamental Issues
### Design Technology of Tanks
#### 5.2.1 Pressure of Tanks  
Tanks designed for high pressure can hold compressed hydrogen gas at up to 700 bar pressure. Furthermore, hydrogen can be cryogenically cooled in insulated tanks to about 20 K at pressures between 6 and 350 bar [158]. In effect, 700 bar and 350 bar hydrogen storage tanks differ primarily in the pressure at which hydrogen is stored, which has significant implications for vehicle design. A 700 bar tank stores hydrogen at a higher pressure, allowing for greater energy density, which translates into longer driving ranges without increasing the tank's size [36; 43; 98]. This makes 700 bar tanks ideal for passenger vehicles where space is limited. However, the higher pressure requires more robust materials and advanced manufacturing techniques, which can increase the cost and complexity of the tank. On the other hand, 350 bar tanks, commonly used in larger vehicles like buses or trucks, are less expensive to manufacture and maintain, but offer a shorter driving range due to the lower energy density. The choice between 700 bar and 350 bar systems affects vehicle design in terms of weight, cost, and available space for storage, with 700 bar tanks typically favored for light-duty vehicles and 350 bar for heavy-duty applications [159]. Table 9 shows the main different pressure levels properties. Storing hydrogen at very high pressures (up to 700 bar for automotive applications) requires tanks to be designed with structures that can withstand these pressures without compromising safety. Additionally, ensuring that the tanks are lightweight and robust enough to prevent leaks or ruptures is a significant engineering challenge. The complexity of designing tanks that can maintain integrity under extreme conditions, including rapid temperature and mechanical stress changes, further complicates this issue. Another problem is the need for tanks to have a shape and size that can be easily integrated into vehicles or other applications without occupying too much space or adding excessive weight. The work of [PERSON] [160] shows an innovative way for automotive applications (designed explicitly for FCEVs) to create a hydrogen tank that respects these conditions. It is a portable hydrogen tank leveraging material-based storage. A three-layer insulation system effectively maintains cryogenic temperatures for over 12 days without external cooling. Operating at pressures below 100 bars and a temperature of 77 K, this tank presents a safer alternative to conventional Type IV tanks operating at 700 bars. By exploiting the \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Property**} & **700 bar** & **350 bar** \\  & **Hydrogen Storage Tank** & **Hydrogen Storage Tank** \\ \hline
**Pressure** & 700 bar & 350 bar \\ \hline \multirow{2}{*}{**Energy Density (by volume)**} & Higher (more hydrogen stored per unit volume) & Lower (less hydrogen stored per unit volume) \\ \hline \multirow{2}{*}{**Storage Temperature**} & Ambient (\(-40\) °C to 85 °C, depending on conditions) & Ambient (\(-40\) °C to 85 °C, depending on conditions) \\ \hline \multirow{2}{*}{**Gravimetric Efficiency**} & Lower (due to stronger, heavier materials needed for high pressure) & Higher (due to lower pressure requirements and lighter materials) \\ \hline \multirow{2}{*}{**Property**} & **700 bar** & **350 bar** \\  & **Hydrogen Storage Tank** & **Hydrogen Storage Tank** \\ \hline \multirow{2}{*}{**Vehicle Range**} & Longer (more hydrogen stored, better for long-distance travel) & Shorter (less hydrogen stored) \\ \hline \multirow{2}{*}{**Tank Weight**} & Heavier (due to stronger tank materials) & Lighter (due to lower pressure requirements) \\ \hline \multirow{2}{*}{**Applications**} & Passenger vehicles, small fleets (where space and range are critical) & Buses, trucks, heavy-duty vehicles (where cost is a bigger factor) \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Comparison of 350 bar and 700 bar hydrogen storage tanks [36; 43; 98; 159].