Braided thermoplastic composite H2 tanks with co-consolidated molded boss areas to fit EV battery space

The use of hydrogen (H₂) to decarbonize aviation is moving forward. CW has reported on ZeroAvia’s (Kemble, U.K. and Hollister, Calif., U.S.) lead, receiving investment and certification collaboration from Airbus, additional orders pushing its secured total past 2,000 and $4.2 million in funding by the FAA to further develop and validate its 2-5 megawatt electric powertrains for 10-20 seat aircraft by 2025 and 40-80 seat aircraft by 2027. Meanwhile, development of composite tanks for compressed gas (CGH₂) and liquid H₂ (LH₂) storage tanks also continues, including projects such as:

• COCOLIH2T aiming for two thermoplastic composite (TPC) demonstrators and TRL 4 by 2025;
• Lockheed Martin and Omni-Tanker’s collaboration on Type 4 and 5 tanks;
• OVERLEAF for a TPC LH₂ tank with 40% storage efficiency, 60% less weight and 25% boost in storage capacity;
• Airbus’ new ZEROe development center (ZEDC) in Stade, Germany, to develop CFRP cryogenic LH₂ tanks;
• LeiWaCo (2022-2025) for economic series production of TPC LH₂ tanks;
• THOR, completed in 2022, for industrialized production of Type 4.5 TPC tanks;
• Efforts at the National Composites Centre in the U.K. and the Netherlands LH₂ composite tank consortium.

Another key project is BRYSON (BauRaumeffiziente Hydrogen Speicher Optimierter Nutzbarkeit or “space-efficient hydrogen storage with optimized usability”) funded by the German Federal Ministry of Economics and Energy. (03ETB019D). It ran from 2020-2023 and was completed by a German consortium comprising:

• BMW AG (Munich)
• The Institute of Lightweight Engineering and Polymer Technology (TUD-ILK) at TUD Dresden University of Technology (Dresden)
• ILK spin-off, engineering and development firm Leichtbau-Zentrum Sachsen (LZS) GmbH (Dresden)
• ILK spin-off, thermoplastic composites fabricator herone GmbH (Dresden)
• Composites distributor WELA Handelsgesellschaft mbH (Geesthacht)
• Department of Mechanical, Automotive and Aeronautical Engineering, Munich University of Applied Sciences (Munich)

The project’s work has been described in multiple papers, including (linked references at bottom): “Hydrogen permeability of thermoplastic composites and liner systems for future mobility applications” (April 2023),¹ “Thermoplastic multi-cell pressure vessels for hydrogen storage - design, manufacturing and testing” (June 2022)² and “New design approach for multi-cell pressure vessels …” presented at SAMPE Europe 2023.³

The goal of the project was to develop new Type 4 H₂ storage tanks that could fit into the same space as a battery for electric vehicles (EV). Within the project two concepts were pursued — the Munich University of Applied Sciences developed a tensile strut-reinforced conformable tank while the partners from Dresden (LZS, herone and ILK) developed a multicell storage approach. “The idea was to use multiple small pressure vessels,” explains Jan Condé-Wolter, researcher and project manager at ILK. “Per Barlow’s formula, pressure vessel wall thickness depends linearly on the diameter. Thus, if we look at just the tube section — neglecting boss area effects — we see that if we store the same amount of H₂ in a lot of small pipes that are thinner, in theory, they’re going to weigh the same as a larger, thicker walled tank which can store the same volume. But the smaller tanks can offer the possibility to use available installation space much more efficiently.”

Automated TPC tube production

Unfortunately, the higher volumetric efficiency of such an approach is accompanied by a significantly larger manufacturing effort to produce the high number of individual pressure vessels required. To minimize the weight and cost of using many smaller tanks, BRYSON explored a design that could exploit an automated production process for tubular TPC structures already developed at the ILK and commercialized by its spin-off company, herone GmbH. This technology comprises automated processing of carbon fiber-reinforced TPC tapes in a braiding process and consolidation in an internal bladder-assisted molding process. Herone has already demonstrated aircraft struts and a line segment for transporting H₂ — all made from 100% TPC. This is possible because the TPC tubes and profiles can then be injection overmolded and formed to integrate TPC load transfer elements such as the threaded boss outserts (see “Injection-forming for high-performance, unitized thermoplastic structures”).

But to use the same manufacturing technology for pressure vessel, some adjustments were necessary. “Even if we reduce the diameter of our vessel cells to 50 or 100 millimeters,” explains Condé-Wolter, “for 700-bar pressure vessels we end up with quite thick structures compared to most struts and driveshafts. To ensure a good consolidation and fiber orientation, we needed to reduce fiber movement during the consolidation process.

This was achieved through an in-line debulking process developed at the ILK, which reduces the thickness of the braided preform. This helps to constrain fiber movement during consolidation and improves laminate quality. “Before this, our 100-millimeter-diameter vessel demonstrator had wall thickness up to 7 millimeters in some locations,” notes Condé-Wolter. “Without an in-line debulk process, the preform would be too thick to achieve good consolidation quality.”

Using TPC materials also enables integrating additional functions or parts directly into the structures. “Essentially, our process is based on co-consolidation,” he explains. “We can add inserts, outserts or even liners to our preform and they will be co-consolidated into the final structure.” For example, an extruded short fiber-reinforced TPC tube was added to the outside of the braided tube preform. The preform assembly is then inserted into the cavity of the matched toolset and heated to the TPC melt temperature with a bladder installed on the inside.

In the subsequent bladder-assisted molding process, the insert or outsert are fusion bonded with the braided continuous fiber-reinforced structures — resulting in material interdiffusion between the parts, which creates an integrated structure. Although this does not require the same matrix in the parts, it does require matrix compatibility. The short-fiber reinforced areas can then be threaded, enabling screwable boss concepts to close the individual storage cells.

Two demonstrator concepts

To compensate for the increased production costs, the BRYSON project had to purse a continuous production process. That meant the traditional strongly necked design of most filament-wound vessels would be difficult to manufacture. “We would have to braid onto a necked mandrel and then find a way to remove this mandrel from the necked preform,” explains Condé-Wolter. “It was much easier to keep everything as straight as possible.” The two final demonstrator concepts were a straight tube with a 50-millimeter diameter and threaded insert and a slightly necked tube with a 100-millimeter diameter and threaded outsert.

While the straight tube was fairly simple to manufacture, the slightly necked tube required some development. It used a multilayered preform with a constant diameter sized to achieve the final necked diameter. This preform also featured a specifically designed, smaller fiber angle than the target fiber angle of 54.7°. “In the consolidation process, the bladder expands the vessel in the tube section to the desired outer diameter of 100 millimeters,” explains Condé-Wolter, “with the braiding angle reaching the desired 54.7° to withstand the inner pressure. This design and process allows us to use continuous braiding and still achieve the desired and complex geometry, wall thickness and fiber angle through the bladder-assisted molding process.”

He notes it’s complex because everything is changing at the same time. “Our preform is getting shorter and wider and the braiding angle changes — but for certain diameters it fits our machines and process quite well. And we can even add short fiber-reinforced threads on the outside, enabling the use of simple screwed endcaps for our pressure vessels.”

Materials and demonstrators

BRYSON did not consider PAEK and similar polymers for the TPC tape matrix due to their high cost, but instead looked at polyamide (PA) and polyphthalamide (PPA). Both have good mechanical properties, with melting temperatures of 200-300°C, and were shown to have good permeability resistance, which is important for H₂ storage. 

“We did a lot of material testing and identified a PPA material with a T_g [glass transition temperature] of around 130°C, which had a good balance in terms of cost and performance,” says Condé-Wolter, “but during the project that material was still at a prototype stage.” He notes that PPA materials are being studied intensively for a large variety of applications because they are priced similarly to PA but the higher T_g eliminates issues with creep, which extends the in-service temperature range. However, the development of the manufacturing process was largely carried out with a PA6 tape that TUD-ILK had completely characterized. “So, its behavior was well known,” he adds, “which made it ideal for process development.”

“We manufactured both concepts with our PA6 material,” says Condé-Wolter, “and because the straight tube was less complex and risky, we also made that with the desired PPA material.” He notes that burst tests will be performed on this PPA straight tube in the future but also that the concept with the 100-millimeter diameter is better suited to the needs of future applications. “This is because the number of 50-millimeter-diameter straight tube storage cells per vehicle would be too large.

”For now, we showed that the manufacturing process works,” he says, adding that herone’s process line enables large-scale manufacturing and short cycle times. “For the BRYSON project, most of the preliminary tests were done in our labs at ILK, but then we switched over to herone’s process line and showed that this approach works beyond the lab scale.” In addition to developing the new design and manufacturing process for the TPC pressure vessels, the team also investigated the permeability of composites and liner materials and set up a test rig for burst testing the small PA tube demonstrators.

Permeability studies

This is an issue for all H₂ pressure vessels, but is exacerbated in longer, narrow-diameter tanks because the surface area — where the gas can permeate through — is larger. “For a storage concept with a larger surface area-to-volume ratio, you need a thicker liner or a material with better barrier properties” says Condé-Wolter. In general, H₂ storage vessels should lose less than 46 milliliters of H₂ per hour, per liter water volume. He notes the liner thickness required to meet this can use up a lot of a vessel’s storage diameter, especially for the thinner vessels in multicell storage systems.

The BRYSON consortium decided that a standard PA6 liner would not work. “For our 50-millimeter-diameter concept, a PA6 liner would need a thickness of several millimeters,” says Condé-Wolter. “Our braided composite structure had a wall thickness up to 3 millimeters. With a liner  thickness in the same order of magnitude as this composite wall, we would lose too much storage volume and our concept would be too inefficient. So, we performed high-pressure permeability tests on many different polymers to identify better-suited materials. We tested them externally with H₂ and on our in-house developed permeability test rig with helium.”

The best performance was achieved with EVOH, a copolymer of polyethylene (PE) and polyvinyl alcohol (PVA). Condé-Wolter explains that EVOH has a polar molecular structure, which means its solubility rate for H₂ gas is very low. “So, you might get problems with moisture, but it’s an incredibly good barrier for all polar gases. I think it could be possible to develop multilayer composite structures where an EVOH layer can provide a permeability barrier and a PPA or other material could protect it from external moisture. As EVOH has 25 times better barrier properties than PA6, the necessary liner thickness would significantly drop — from millimeters to microns. Thus, the effect of volume loss and mass increase due to the liner is strongly reduced and almost negligible. And in the small set of adhesion tests we performed, it bonded well with our PA6 material.”

Condé-Wolter also noted that in comparisons of PPA with and without carbon fiber reinforcement, the latter shows 2.5 to 3 times better barrier properties. “By adding carbon fiber, which are close to impermeable, you create a torturous diffusion path for the H₂ which reduces permeability.”

Burst tests and other developments

While other partners within BRYSON developed flame-resistant coatings and burn tests, the team at ILK, LZS and herone worked to develop a test fixture and method to perform burst testing up to 1,700 bar on the tube-like specimens. Four specimens were tested with a maximum pressure of 1,200 bar achieved versus the design pressure of 1,400 bar. And though the desired burst was not accomplished, the team identified the main issue, which was damage to the liner during test clamping. Follow-on improvements and further testing are planned.

Other applications

The new Type 4 pressure vessel concept demonstrated in the BRYSON project is not limited to automotive applications, says Professor Gude, institute director of the ILK: “We have taken an essential step towards sustainable mobility concepts of the future. For example, in the projects Saxonhy and SWAT, we are already developing similar concepts with our partner APUS [Strausberg, Germany] for aviation applications.”

APUS is developing a family of zero-emission H₂-powered aircraft, which currently include the i-2 and i-5. Not needing LH₂ for their utilization scenarios, these aircraft will house CGH₂ tanks in the wings. The company reports this patented structurally integrated H₂ storage system increases specific energy density by 25% versus standard H₂ tanks and energy density by a factor 10 versus battery-electric aircraft. APUS has also participated in the SaxonHY project, announced in July 2022, with a consortium that includes TUD-ILK. The goal is to investigate Type 5 tanks without a liner to increase gravimetric storage density.