CRYOGENIC LIQUID HYDROGEN STORAGE

Liquefying hydrogen increases its storage density; hence it is possible to store more energy than when stored as a gas. LH₂ (liquid H₂) is stored at the temperature of -253°C, and the main concern is the energy-intensive process of liquefying using 30% of energy sourced by burning liquid hydrogen. Another problem represents a 3% volume loss of stored hydrogen daily. Therefore, it is crucial to develop new methods of liquefying to decrease energy demands and increase the efficacy of the process.

 

 

The energy demands mainly lye in ortho-form (o-H₂) to para-form (p-H₂) conversion. Ortho-form is a hydrogen molecule where the atoms have spin symmetry and para-form molecules atoms have spin asymmetry. The p-H₂ is more stable in lower temperatures and has a lower enthalpy. Due to this characteristic, heat is released during the conversion increasing the energy demand of the process. The other factor is the purity of the gas; all gases except helium need to be removed, especially CO₂, CO, CH₄ and O₂. Oxygen concentration above 1 mg per kg can cause explosion.

 

LH₂ is stored in multilayer containers with superior insulation properties equipped with overpressure protection. The overpressure protection releases the evaporated LH₂ to avoid pressure build-up in the tank. Usual daily losses of LH₂ are 3% of the volume per day. Sometimes, the evaporated LH₂ is captured and stored in additional pressure cylinders.[1]

 

Comparison of conventional methods

The following table shows the comparison of weight and volume characteristics for a full fuel tank of a C-segment car with a 500 km range representing the equivalent of 6 kg H₂ to 45 l of petrol.

 

Container type	Container weight [kg]	Container volume [l]
Fuel tank	55	45
Cryogenic tank	100	180
Steel tank 350 bar	360	290
Composite tank 350 bar	120	290
Composite tank 450 bar	130	230
Composite tank 700 bar	140	200

 

A hydrogen-powered car with 500 km range would be equipped with almost four times larger and 2-3 times heavier fuel tank compared to a petrol-powered car (in case of steel tank up to 7 times heavier than fuel tank).