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Material behaviour at hydrogen conditions

Material behaviour at hydrogen conditions

Transporting or storing large amounts of hydrogen requires its liquefaction or storage at high pressures. Storage pressure and temperature play a crucial role in the amount of stored hydrogen. For example, in 1m3 volume, while in gas form only 0.08kg of hydrogen can be stored at 1 bar, it can be increased to 42kg if pressure is increased to 700 bar or to 73kg if temperature is reduced to -255 deg C.

These are very energy intensive processes, and therefore companies are working hard on improving efficiencies of these process steps. Storing hydrogen at high pressure or at cryogenic temperature requires dedicated storage tanks: pressure vessels or cryogenic insulated tanks. And this brings perhaps the most challenging technological issue and a research domain on its own, performance of materials at extreme conditions.

At high pressures, stainless steel or composites are typically used as tank material. Metal tanks are referred to as Type I tanks and full composite tanks with polymer liners are referred to as Type IV tanks. Type V tanks are linerless tanks, which is the holy grail for the composite hydrogen tank producers.

The purpose of the liner is to act as a gas barrier. However, liners are also the cause of various structural integrity issues for example in the form of buckling (seperation of the liner from the reinforcing composite layers) during depressurisation of the tank, or loss of elasticity when exposed to temperature and pressure cycles. These issues led the researchers to investigate linerless tank concepts, in which a tank wall has to act as a gas barrier as well as performing its original task as reinforcing wall. The challenge is the permeation of hydrogen molecules through the tank wall. Potential voids in the resin, debonding between the fiber and the resin or delamination between the plies might eventually form a network that will increase the permeation of hydrogen molecules that have a diameter of 120 pm (1 pm = 1×10^-12 m).

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