Hydrogen storage systems

Hydrogen is stored in two ways: metal hydrides and compressed gas.

Metal hydrides

Hydrogen gas can be stored at low pressure through a chemical reaction with an alloy that absorbs hydrogen and which forms a solid metal hydride: various metal alloys (mainly based on magnesium) and intermetallic compounds react with hydrogen to form metal hydrides. In other words, metals can be charged and discharged with hydrogen. These systems take up a very small amount of space and the pressures reached in them vary between 1 and 25 bar; on the down side, they are extremely heavy and need a special cooling system since heat is exchanged when charging and discharging.

Compressed gas

Storing hydrogen as compressed gas is done using pressurised, high-strength steel containers. Composite cylinders, lighter than metal ones, are used and operate at pressures between 350 bar and 700 bar (increasing the pressure allows more hydrogen to be stored).

There are four types of container suitable for hydrogen: Type I, Type II, Type III and Type IV, which vary in terms of the constituent material and the gas pressures reached inside them.

A type I tank is more advantageous in economic terms but is extremely heavy. A type IV storage tank can reach 700 bar with a lower weight but costs twice as much as a type I. A key parameter that characterises the storage system are the kilogrammes of H₂ stored at the pressure at which you decide to work.