# Hydrogen Materials and Technologies in the Aspect of Utilization in the Polish Energy Sector
## 4 Hydrogen Storage  
The storage technologies for hydrogen presented schematically in Figure 2 can be categorised into the following three main types:  
* Physical methods that involve storing compressed or liquefied molecular hydrogen;
* Methods on the basis of the adsorption processes of molecular hydrogen on materials having a well-developed surface using weak intermolecular van der Waals interactions;
* Chemisorption methods, which involve the chemical binding (absorption) of atomic hydrogen. Currently, hydrogen storage in the gaseous or liquid phase is used on a large scale [28; 29; 30]. Hydrogen storage tanks have appropriate process safeguards, including pressure relief devices (PRDs). Commercially available fuel cell electric vehicles use high-pressure tanks that enable the storage of hydrogen at pressures of 350 and 700 atm. Nonetheless, these types of tanks are not cost-effective for stationary applications to store large volumes of hydrogen, which are typically stored at a pressure no greater than 100 bar in aboveground tanks and 200 bar in storage tanks located underground [31; 32]. The investment costs are much higher in the case of terrestrial options and therefore the latter solution is preferred, especially in areas where salt caverns [33; 34], depleted gas/oil deposits, or aquifers can be utilised [35]. Assuming that the use of hydrogen will significantly grow in the future, there will be a need for appropriate gas transport. The most economical means to transport large quantities of hydrogen over great distances is through pipeline networks [36; 37; 38; 39]. There are not many technical obstacles to building hydrogen storage pipelines. On the other hand, hydrogen pipelines are more expensive to build than natural gas pipelines, mainly because of the properties of hydrogen. The tendency of hydrogen to penetrate the walls of metallic materials increases their brittleness and deteriorates their mechanical properties. The modification of a natural gas pipeline to transport clean hydrogen necessitates solving issues related to the embrittlement of some steels due to the action of hydrogen and problems with sealing fittings, flanges, and threads to prevent uncontrolled hydrogen escape [40]. Hydrogen fuel is converted into a liquid state so as to increase its storage density. The volume ratio of hydrogen in the liquid state (L\({}_{\rm H2}\)) to hydrogen in the gaseous state (G\({}_{\rm H2}\)) is 1:848. Heating L\({}_{\rm H2}\) to ambient temperature in closed spaces leads to the creation of very high pressures. For safety reasons, it is important to provide buffer space in cryogenic tanks to accommodate the expansion of the gas volume increase as the temperature rises. A lack of buffer space may lead to overpressure in the tank. To reduce the rate of evaporation of liquid hydrogen, the surface-to-volume ratio of the storage tanks is minimised by giving them a spherical shape and through insulation that minimises heat transfer through the walls of the tank. Storage tanks for liquid hydrogen  
Figure 2: Hydrogen storage methods. most often have double walls, with high vacuum between them, which reduces heat transfer through convection and conduction to a minimum [41]. Additional materials, such as particles of aluminium, silica or perlite, or alternating layers of aluminium foil and fibreglass, are put in the space between the tank walls to protect against heat transfer via radiation [42]. It worth noting that evaporation is less of a problem in situations where the facilities for hydrogen liquefaction and the storage of its product are located near each other [43] and the hydrogen is utilised in hydrogen fuel cells or gas turbines to generate electricity when necessary. Currently, the largest tanks for storing liquid hydrogen are operated by NASA. The amount of hydrogen stored in these tanks is 230-270 tons.