Compressed hydrogen. The compression of hydrogen into tanks provides the benefits of quick charge and discharge times. However, the volumetric density of hydrogen is much lower than that of other energy sources, for example four times lower than natural gas. Therefore, hydrogen needs to be compressed to very high pressures (700-800 bar or higher) to achieve reasonable volumetric density. Thick walled steel tanks with steel or aluminum (Type-3) or polymer linings (Type-4) therefore become necessary. The tanks are accompanied by inherent safety risks including explosion and fire, and troubled by the permeability of hydrogen and embrittlement of the tank walls. A recent study of 350-bar and 700-bar polymer-lined H2 storage tanks has shown that the carbon fiber-reinforced composite needed to provide the structural strength for these fuel tanks. For 700-bar H2 storage tanks, the high-density polyethylene (HDPE) liner fully wrapped with carbon fiber composites is required. Liquified hydrogen. Liquefaction is an option if an application requires hydrogen volume to be reduced further than compression can achieve. MHI Group and the space industry have used liquefied hydrogen to fuel rockets for many years, however liquid hydrogen storage is energy intensive, technically complex, and very costly. Hydrogen must be cooled to -253°C (near to absolute zero) and stored in insulated tanks to maintain this extremely low temperature. To minimize the losses due to vaporization, transfer of liquid hydrogen should be performed in a vacuum-insulated system. In addition, this transfer should be conducted in a closed system with a proper safety relief device to avoid a flammable atmosphere or explosive mixture of air and liquid hydrogen. Cryo-Compressed hydrogen. The two techniques -compression and liquefaction -can also be combined.