Challenges include permeation, as hydrogen can penetrate any type of polymer material and metals
Lansdale, Pennsylvania – The aviation industry is increasingly focusing on the use of green hydrogen (H2) generated by the use of renewable energy sources, such as wind and solar, according to Greene Tweed.
Interest in the fuel, he said, is driven by studies showing that hydrogen can deliver up to three times more energy than jet fuel offers by mass.
However, hydrogen presents environmental challenges for its storage and transportation, both on the ground and in aircraft, the company noted in an online post.
Research and engineers are exploring advanced materials that can support the design and operation of new aircraft, airport infrastructure and the fuel supply chain.
One of the main challenges is permeation, especially since hydrogen is an exceptionally light and low density gas, which can penetrate all types of polymeric materials and metals.
When combined with high pressure or pressure cycling applications, hydrogen permeation can cause rapid gas decompression (RGD) problems.
Depending on pressure and temperature levels, Greene, Tweed recommends special O-rings made from either FKM or Metal Spring (MSE) PTFE lip seals which provide “exceptional” resistance to exposure to hydrogen.
Since permeation can also occur at low temperatures, the company suggests that FKM O-rings or MSE lip seals might offer the best solution.
Meanwhile, Greene Tweed is currently evaluating new sealing solutions and thermal insulation materials for the temperature extremes encountered with liquid hydrogen.
Another problem is that the low lubricity of the hydrogen molecule can lead to excessive wear and friction in equipment, such as valves and compressors.
To solve these problems, Greene Tweed offers a PFA composite with unique dry-running properties and a special cross-linked PEEK polymer material.
Regarding weight, a key issue in aircraft fuel consumption, Greene Tweed notes that hydrogen’s energy density (by volume) is about three times lower than that of jet fuel. The size of the tanks must therefore be considerably increased to transport the same volume of energy with hydrogen as with jet fuel.
As a lightweight solution for hydrogen aircraft, Greene Tweed mentions its Xycomp thermoplastic composite, which is already widely used in the aerospace industry as a replacement for metal.
Carbon/PEEK composites are permeable to hydrogen due to their molecular structure and the relatively large space between the molecules and the graphite planes. Thus, they cannot accumulate hydrogen as can be seen with metals, according to the company.
Considering that hydrogen embrittlement is therefore not possible with a polymer composite, “using our Xycomp solutions makes even more sense in the low-carbon aircraft of the future,” said Greene Tweed.