Hydrogen storage can basically take place in a gaseous or liquid state, but also in chemically bound form, in metal hydrides, or via adsorption. Hydrogen is gaseous under ambient conditions and is the lightest element in the periodic table with a very low volumetric energy density. These properties make transport and storage a challenge.
Hydrogen has the highest storage density to the pure storage volume if it is liquefied before storage. Hydrogen becomes liquid at -253 degrees Celsius. Cryotanks, are tanks for liquid, cryogenic gases, that have very high insulation properties, where the losses due to heating which leads to evaporation losses are kept low. The storage of hydrogen in liquid form is ideal for vehicles because the energy content of liquid hydrogen, based on weight, is the greatest possible. That is why liquid hydrogen is used as rocket fuel in space travel. In practice, however, liquid hydrogen also has disadvantages compared to the pressure variant. Evaporation losses cannot be completely avoided when vehicles are parked for a long time and the amount of energy required to provide liquid hydrogen is higher than that of pressurized hydrogen.
Gaseous hydrogen can be stored in a tank after compression at high pressure. In cars, for example, a pressure level of around 700 bar has become established. The high-pressure accumulator is cheaper than other types of storage for small amounts of storage and is therefore mainly used in mobile applications such as in cars and commercial vehicles.
Metal hydrides absorb gaseous hydrogen. You can think of it like a sponge soaking up water. When the hydrogen gas comes into contact with the solid surface of the storage material, the hydrogen molecules disintegrate into atomic hydrogen and penetrate the material. The loading and unloading of the metal hydride storage take place at a pressure level of approximately 30-60 bar. The main disadvantage is that these storage systems are relatively heavy in relation to the absorbed H2 content. Besides, these memories are comparatively expensive due to the high material costs. Metal hydride storage systems have clear advantages in terms of handling and safety. They work almost at normal pressure and show no evaporation losses. The hydrogen is released through heat, so the hydrogen remains bound if the storage tank is damaged. They are used for special applications such as small storage facilities and submarines.
Chemically bound hydrogen
In chemical hydrogen storage, excess energy is used to produce hydrogen electrolytically. This is bound to a low-energy carrier medium through a chemical reaction. Due to the enrichment with hydrogen, the medium stores its chemical energy. This hydrogen-enriched medium can be stored over long periods without loss, transported, and distributed with a high energy density. At the place where the energy is required, the hydrogen is separated from the carrier medium and can be used for various purposes. The carrier medium can now be charged again with hydrogen and circulated. This allows power fluctuations to be absorbed when generating energy. In this way, the energy originally available as electrical power can be stored, transported, and converted back. If it is suitable, the waste heat from the process can also be used.
The volume-related storage density is higher than that of pressurized hydrogen at the same temperature and pressure. Porous carbon and zeolites are examples of materials with suitable properties. Since only very little hydrogen is adsorbed at room temperature, it is necessary to operate adsorption storage at lower temperatures due to thermodynamic reasons. The cooling however requires a lot of energy.
With the increasing importance of hydrogen as an energy source, storage and transport options are constantly being developed and are becoming increasingly important.