New solutions for hydrogen pressure vessel manufacturing
Momentum for hydrogen is rapidly developing with many potential applications in the industrial, automotive, aerospace, rail and maritime sectors. By all accounts, investment in hydrogen fuel cells and related technologies will grow significantly as the clean energy transition accelerates.
Composite pressure vessels are already playing a key role in hydrogen storage, both in vehicles and for transport and distribution, and the potential for future growth in this product segment is enormous. However, performance and safety standards in key markets such as aviation and heavy-duty transport are stringent, and manufacturers need to be supported by resin suppliers to choose the optimal chemistry and production process. The development of pressure vessels becomes easier and faster when supported by extensive testing and process simulation, as shown in Figure 1.
Pressure vessels come in five different design types, three of which use filament-wound composites: Type 3 has a composite overwrap with a metal liner; Type 4 uses all-composite construction with a polymer liner; and Type 5 features all-composite construction with no liner.
A tough regulatory landscape
The emergence of hydrogen storage has led to technical challenges, especially considering increased pressure requirements, which can range from 350 to 700 bar. Today, the Global Technical Regulation on Hydrogen and Fuel Cell Vehicles (GTR No. 13) demands not only pressure testing with high safety margins, but also impact resistance, chemical exposure, and numerous temperature/pressure-cycling tests, as shown in Figure 2.
Meeting stringent requirements
The pressure resistance of hydrogen vessels is mainly determined by the performance of the reinforcement fibers, but the resin matrix plays a key role in providing environmental exposure protection (thermal, chemical, impact) as well as fatigue/pressure-cycling resistance to withstand the constant filling and emptying cycles.
The severe pressure cycling test at 85° C required by the GTR 13 standard means that, in practice, resin systems must have a glass transition temperature (Tg) of at least 115-120° C to avoid premature failure, even in hot/wet conditions.
Studies have shown that resin systems featuring high mechanical strength and high elongation at break can better support the dimensional changes (strain) induced by pressure cycling, preventing crack initiation within the laminate even at maximum rated pressure.
Furthermore, a high fracture toughness can enhance fatigue performance and impact resistance by reducing damage initiation and crack propagation. Standard epoxy systems have fracture toughness (KIc) values around 0.7 to 0.9 MPa.m1/2, whereas Huntsman’s toughened epoxy systems can offer values as high as 1.7 MPa.m1/2, significantly improving pressure cycling and impact performance.
Static, dynamic, and hot/wet performance of both the pure resin and composite parts can be fully characterized in Huntsman’s ISO 17025-accredited laboratories. The key properties of several of our toughened ARALDITE® systems for pressure vessel manufacture are shown in Table 1.
A range of manufacturing processes
Wet filament winding is a well-established manufacturing method, but manufacturers are increasingly considering resin transfer molding (RTM) and towpreg winding in order to meet the need for increased productivity and greater part consistency.
The RTM process consists of forming a braided fiber preform over an inliner, followed by fast injection of a fast-cure resin system within a closed mold. Equipment investment and process complexity may be high, but there are potential advantages for the mass production of smaller pressure vessels.
Towpreg winding, which uses a fiber tow that is pre-impregnated with resin, is gaining popularity for mass production of hydrogen tanks, mainly due to clean processing, improved part quality, fast winding speeds, and short cure cycles. Towpregs can be manufactured on site, and available equipment supports speeds up to 100 meters per minute, which is both cost-effective and reduces the need for cold transportation and storage. Towpreg resin systems must offer low initial viscosity for fast impregnation, high storage stability at 23° C, and the correct level of tack to enable bobbin unwinding and subsequent mandrel winding up to 5 meters per second without slippage. Curing can be via the classical oven process or by infrared exposure.
Table 2 compares the advantages of the most popular processes for composite pressure vessel manufacturing.
Faster, more robust processing
Achieving short cycle times while properly curing laminates up to 50 mm thick can be a major challenge. The exothermic curing reaction can lead to temperature overshoots, damaging thermoplastic liners and creating high internal stresses. Establishing optimum cure cycles often requires a large number of experimental trials.
Computer simulation capabilities provide a valuable tool to quickly assess the effects of changes to cure cycles, reducing the number of costly experimental trials. Providing that a detailed material data model is available, accurate predictions of exotherm temperature, degree of cure and Tg can be made.
Figures 3 and 4 shows how simulation makes it possible to visualize the temperature evolution at any point during curing of the pressure vessel. Tg build-up and degree of cure can be visualized in a similar manner, as shown in Figure 5.
The future of hydrogen
As the need for clean energy technologies grows, hydrogen will increasingly be adopted for applications in which it offers advantages over batteries and other energy storage options—aerospace, rail, maritime sectors, heavy-duty commercial vehicles, and off-grid energy storage (for example, electric vehicle charging stations in remote areas).
All of these applications will require pressure vessels, and composites will be the materials of choice. Selecting the right resin systems and the most appropriate, cutting-edge processes will be an ongoing process, and manufacturers need to work in close partnership with resin suppliers.
Building on strong experience in natural gas pressure vessel technology, Huntsman Advanced Materials offers a comprehensive range of epoxy resin systems that address the emerging challenges and manufacturing requirements of hydrogen storage.
In addition, Huntsman’s proven expertise in material characterization and process simulation offers a powerful set of tools to accelerate product development and optimize manufacturing, leading to increased part quality and minimum production cycle times. Huntsman has developed in-house expertise not only in the complex material characterization needed to generate an accurate data model, but also in cure simulation of real customer parts, providing tailored guidance for process optimization. Huntsman’s ISO-certified lab testing ensures compliance with the most stringent regulations, speeds up the approval process and minimizes time to market.