Greene Tweed investigating green hydrogen seals technology for aviation industry

LANSDALE, Pa.—The aviation sector is increasingly focusing on the use of green hydrogen (H2) generated by using renewable energy sources, such as wind and solar, according to Greene Tweed.

Interest in the fuel, it said, is being driven by studies showing hydrogen can supply up to three times the energy than what jet fuel offers on a mass basis.

However, hydrogen brings environmental challenges to storing and transporting it, both on the ground and on the aircraft, the company noted in an online article.

Researchers and engineers are exploring advanced materials that can support new aircraft design and operations, airport infrastructure, and fuel supply chain.

Among the main challenges is permeation, particularly as hydrogen is an exceptionally light, low-density gas, which can penetrate any type of polymer materials and metals.

When combined with high pressure or pressure cycling applications, hydrogen permeation can generate rapid gas decompression (RGD) issues.

Depending on pressure and temperature levels, Greene Tweed said it recommends special O-rings made from FKM or metal spring energized (MSE) PTFE lip seals that offer "exceptional" resistance to hydrogen exposure.

Greene Tweed is currently evaluating new sealing solutions and thermal insulation materials for extreme temperatures found with liquid hydrogen.

A further issue is that the low lubricity of the hydrogen molecule can lead to excessive wear and friction in equipment, like valves and compressors.

To address such problems, Greene Tweed offers a PFA composite with unique dry-running properties and a special cross-linked PEEK polymer material.

Looking at weight, a key issue in aircraft fuel-consumption, Greene Tweed noted that energy density of hydrogen (by volume) is around three times lower than jet fuel. Reservoir sizes must, therefore, be dramatically increased to transport the same volume of energy with hydrogen as with jet fuel.

As a lightweighting solution for hydrogen aircraft, Greene Tweed pointed to its Xycomp thermoplastic composite, which is already extensively used in the aerospace industry as metal replacement.

Carbon/PEEK composites are permeable to hydrogen due to their molecular structure and relatively large space between the molecules and the graphite planes. So, 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, "use of our Xycomp solutions makes even more sense in the low-carbon emissions aircraft of the future," Greene Tweed stated.