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Pressure vessels are a significant market for composites, led by Type 4 pressure vessels made by filament winding carbon fiber-reinforced polymer (CFRP) over a plastic liner. Hexagon Composites (Ålesund, Norway) has a long history in this market, first producing cylinders for storing compressed natural gas (CNG) in 1992. Over the next 20-plus years, it became a leader in the market, and by 2019 had organized its CNG and renewable natural gas (RNG) pressure vessels and systems within its Hexagon Agility business and established Hexagon Purus (Oslo, Norway) to deliver zero-emission solutions, including battery systems for electric vehicles and Type 4 pressure vessels for storing hydrogen (H₂) in mobility and infrastructure applications.

CW has written extensively about Hexagon Purus. In 2024, it achieved revenue of 1.9 billion NOK, a 42% increase over 2023, and completed its global capacity build-out, including facilities in Westminster, Maryland, and Shijiazhuang, China. This tour is of the company’s state-of-the art production in Kassel, Germany, with capacity for 40,000 cylinders/year.

Cylinder products for mobility

Kassel produces cylinders for bus, truck and rail systems that store compressed H₂ gas at a range of pressures, including 300, 500, 700 and 950 bar. These cylinders can measure up to 760 millimeters in diameter and 4,000 millimeters long, weighing up to 450 kilograms. The exact number of cylinders produced depends on the mix of tank sizes and pressures ordered by various customers. Higher pressure cylinders have a thicker wall and thus require longer cycle times.

Cylinders for trucks are typically 350 or 700 bar. The latter store a maximum of 18.4 kilograms of H₂ for a total of 74 kilograms in a four-tank system mounted behind the cab. Tanks can also be mounted on the side rails. Customers include Ford, while pressure vessels for Toyota are produced in the U.S. facility in Westminster, Maryland.

For buses, cylinders are 350 bar storing 10 kilograms of H₂/cylinder and 50 kilograms/system. Customers include Solaris, Caetano Bus, Nesobus, Arthurbus and Karsan in Europe. Gillig and New Flyer are supplied from the U.S. “We are developing the next generation of cylinders per the new R134 regulation,” says Heiko Chudzick, responsible for operations at the facility. “This is for transit buses and we have developed a completely new system with new cylinders and less steel in the frame to minimize weight and increase possible payload, which also improves cost. We also have better packaging and maintenance access.”

“For rail, we started with leading players like Stadler and Alstom,” he notes. Tanks here are 350 bar and store up to 8.4 kilograms of H₂/tank and several hundreds of kilograms/system. These are designed for a 30-year service life. Rail customers include Alstom for its Coradia iLint trains, Stadler and Linsinger for railway maintenance trains. There is also interest in locomotives, which will likely use H₂ internal combustion engines (ICE) instead of fuel cell-based electric power systems.

“We are seeing growth in H₂ mobility, first in transit buses and heavy-duty equipment, where batteries struggle to provide enough energy density, and then longer term in rail and maritime applications,” says Chudzick. “Real use and growth of H₂ in passenger cars won’t start until after 2030, with increased H₂ production and refueling infrastructure.

H₂ infrastructure and strategy

From its inception, Hexagon Purus has focused on the H₂ infrastructure market, which involves truck transport of H₂ from production facilities to end users or H₂ refueling stations (HRS) as well as stationary storage at HRS and mobile refueling modules. Distribution and refueling modules comprise multiple cylinders loaded into a custom build ISO container or similar module. These cylinders can vary in size by customer and end use but are produced in Kassel on the same production line as those for mobility applications.

Chudzick describes four sets of players in this market:

• H₂ producers who also consume it internally (such as Shell, Equinor, BP, Sabic, BASF, Yara, Fertiberia)
• Merchant H₂ producers (such as Air Products, Linde, Air Liquide)
• Emerging producers that are one-stop shops for decarbonization (such as Plug, Everfuel, Lhyfe, GP Joule, Norwegian Hydrogen)
• Industrial processes (such as steel manufacturing) as well as distributed power.

“All of these need distribution and some part of that will be using trucks,” he notes. “Right now, there are not many pipelines, and though there are legacy steel tube trailers, they are heavy, with limited payload due to over-the-road weight limits. Also, once the tubes corrode, they cannot be returned to service.” In contrast, Type 4 cylinders can be tested and recertified for extended service. “We are seeing more and more older Type 4 tanks being recertified for another 10 years of use,” says Chudzick. “The distribution market is continuing to expand as demand grows for using H₂ to reduce emissions for industrial facilities and power generation.”

Location, pilot line/testing

Why Kassel? “We have a legacy plant here, now part of Hexagon Agility,” says Chudzick. “So, we had a local workforce with filament winding experience and other specific skills that are hard to find elsewhere. Kassel is also well situated logistically in the center of Germany and Europe, and this site offers room for expansion.”

Production in Kassel operates in synergy with the company’s systems facility in Weeze, Germany. “We send composite cylinders to Weeze,” explains Chudzick, “where they are assembled into complete H₂ fuel systems enclosed in steel frames and equipped with pipework, valves and fuel management controls. These integrated systems are then tested and delivered to customers.”

Finished in 2023, the site in Kassel features two buildings: the left has offices and a test lab while the right has the main production line and room for expansion. We exit the office area and enter an open production hall. To our left is a pilot production line used for R&D, testing new products and new process improvements. There is also an area for performing load cycling and burst tests. “Having this dedicated area for R&D allows us to focus on production in the other side of the plant,” says Chudzick. “Here, we have a one-spindle and a three-spindle winding machine and three small ovens. We can do small-series production with a clear focus and most effectively run new trials.”

4.0 production line

The Kassel production line is based on decades of Hexagon’s experience in composite cylinders and Chudzick’s background in industrializing production for top tier manufacturers. “We spent a long time thinking about the design of this line,” he notes, “and had an amazing, multidisciplinary team that worked together well. The result is very different from how Hexagon has done cylinder production in the past. It is optimized for space and material flows with robotically controlled handover points and an IT system that meshes with our logistics.”

Production includes: (1) Preparing the plastic liner, (2) composite winding, (3) oven cure, (4) hydrostatic burst test and drying, (5) valve installation and leak test and (6) finishing (coating, bonding shoulder pads, marking) and shipping. “The line has a circular flow because it minimizes the distance between operations,” notes Chudzick. “For such a high output, it has an extremely small footprint. And that’s what you want for sustainability and efficiency. It’s also fully automated with no operators inside the main cell, designed with redundancy should we have any issues in the automation but also with a high level of safety. This includes reduced touch points for operators so that they’re not in the path of moving equipment or handling heavy tanks.” All movements of cylinders — which can be up to 4 meters long and weigh up to 450 kilograms — are done by robots or automated cranes.

The machines in the production line are also all connected and controlled so it can run cylinders for bus, truck, rail or distribution/refueling modules one after the other. “The line reads the QR code on the liner, sees what length, weight and type of cylinder it is and then sets how that cylinder will be filament wound and moved through subsequent steps per its specifications,” says Chudzick. “The whole layout was based on the takt times we measured for each operation per each tank type, including its length, diameter and pressure. These parameters determine how much fiber is wound and how long the cure and test cycles take.

“We also have online monitoring and can see if parameters are drifting,” he continues. “We have upper and lower limits built into the control system but the operators watch on top of that. Human experience is a very good input, and we don’t want to get rid of that. The operators have to approve and release each tank.

“We continue to add monitoring systems and capability, including for environmental data,” says Chudzick. “Any of this data can be pulled up on our company smartphones and also feeds the data exchange with our ERP and SCADA systems. So, we are continuously improving how we use the data, including our production planning and control, and our future vision to report the environmental footprint of our cylinders.” This is detailed further below.

“Our goal is to have a digital twin of the production. We’re in the process of completing this now,” adds Chudzick. “We then can take that to our pilot line and test new improvements, but it also allows us to look at how to develop new cylinder products for our customers.”

Liner bay, adaptors, loading

We exit the office/pilot line building and walk across to the main production  building. As we enter the open production hall, we walk first to the liner bay where thermoplastic polymer liners are prepared. To the left is an incoming liner station where all liners are pre-inspected before being sent to the liner bay. Polymer liners are first dried and then annealed in two large vertical stations. Liners can come with the metal boss molded in at the end of the cylinder for filling and refilling or the boss can be welded in during production.

Although this bay produces all the liners for this production line without a high degree of automated handling, notes Chudzick, a new liner welding station located in the heart of the liner bay is highly automated. “It can weld a liner in one shot and provides much more data collection and process control. All the data is shown in a digital display that also includes camera footage of each welding operation.”

Next, we walk to a station where QR codes are applied to each of the winder adaptors, used on each end of a liner to install it into the spindles of the filament winding machine. “We then check for pressure,” says Chudzick. “We maintain pressure inside the liner to prevent it from collapsing or changing shape during filament winding.”

At the next station, a large orange frame lays flat, supported a few feet above  the ground. Here, five liners with adaptors are loaded into the frame. Once fully loaded, the station lifts the frame into a vertical position and it is then moved by the production line’s cranes into the central production area, where it will be loaded into the filament winding station.

Composite winding, cure

Most composite cylinders produced in Kassel are filament wound using only carbon fiber. “We have fewer and fewer customers who want a glass fiber outer layer,” explains Chudzick. “Instead, it is more common for customers to specify an outer coating that meets their requirements.”

The filament winding station supplied by Roth Composite Machinery (Steffenberg, Germany) uses five spindles. The tows are fed through a tensioning station for efficient fiber feed and control and then a resin bath. Carts with spare parts sit just outside the winding station, part of the design to minimize downtime. “In the legacy facility, we had three winding machines,” notes Chudzick, “while here we have only one, yet it produces the same capacity.”

We walk past the creels to an area where feed lines from resin totes run up to a mezzanine over the creels which houses the system that meters and mixes the epoxy resin and dispenses that into the resin baths. The automated system requires a single operator and provides continuous control of flow, viscosity and temperature.

At the operator’s station, parameters are displayed for each spindle and resin bath. “We capture 3.5 gigabytes of data for every cylinder including process parameters,” says Chudzick. “We then calculate and track impregnation.” There are also cameras monitoring the process and two additional displays — one for the system handling the five-cylinder racks and another for the curing ovens.

Next, there is a handling portal where the rack from the winding station is moved by crane into one of four ovens that cure the composite cylinders at up to 150°C. “For efficiency, we filament wind five tank cylinders at a time per type,” explains Chudzick. “These will then move into the cure oven together. We have eliminated the waiting time for an oven because this automated line has optimized the operations per each tank type’s cycle time.”

Tank tests, finishing

After cure, a centralized robot on a rail completes all tank handling through the next steps. The winding adaptors are removed and replaced with proof test adaptors. These enable hydrostatic proof testing at pressures up to 1,050 bar. “Even though we test every tank, there is only one station to do this testing,” notes Chudzick. “However, there is a buffer station, so that while one tank is tested, another is loaded and prepared. When the first tank completes testing, the robot removes it and places it into one of multiple drying stations. While this is happening the buffer tank is flipped up and over into the test station. So, there is very little wait time and no need for a second test station.”

After the cylinders complete drying, the robot moves them to a station where the proof test adaptors are removed and the cylinders are inspected for cleanliness. Liners that have valves welded into them move directly to leak testing after drying. For cylinders with liners that don’t have valves pre-welded in, they are next moved into the valve installation station. This is a semi-automated operation that occurs inside a type of enclosed clean room where the boss valve is integrated with the cylinder. Movable arms are used to install the valves with location and torque recorded to ensure precise and repeatable installation per specification.

After valve installation, each tank is placed in a test chamber and undergoes a leak check using test gas pressurized up to 875 bar. Which cylinders are tested first is decided by the production line depending on the ship date for each customer order. Multiple chambers are used, says Chudzick, because it takes a long time to get the gas out of the tanks after the pressurized test. As that gas is vented, 85-90% of it is collected and reused — a small amount escapes during connection and disconnection with the pressurization equipment.

At this point, the tanks leave the production line and any coatings are added followed by adhesive bonding of shoulder pads as specified by the customer. Here, rows of tanks sit while moving through various finishing steps.

There is a lot of room to expand at this end of the building. “We’ve already thought all of this out and know how we would extend the robotic system and add modular units, depending on what type of cylinder production expansion is needed,” says Chudzick. “We’re trying to think 10 years ahead.”

Data-enabled, decarbonized future

This forward-thinking strategy is also seen in Kassel’s use of the data collected — for example emissions, energy use, waste — in efforts to improve the sustainability of its operations and publish that in its annual ESG report. “We issued the first ESG filing for this facility in 2024,” says Chudzick, “and have developed Scope 1 and 2 data, but are just now trying to expand our Scope 3 reporting with our supply chain. From 2025 onward, we will include data from suppliers in our online collection and are in the process of automating that within our ERP system. We are allocating this online data to each machine, each batch of cylinders and even each spindle within the batch.”

This data is then used for the facility’s own decarbonization strategy development. “We will set these goals in in the near future,” he explains, “because it’s part of our business model as a company trying to help decarbonize the planet.” For example, one issue is the use of carbon fiber because its manufacture uses a high amount of energy and thus produces significant CO₂.

“But having the data is key and necessary to develop an intelligent strategy,” adds Chudzick. “We started this process in 2021 with Hexagon Composites Kassel and then began developing our life cycle analysis [LCA] for cylinders being produced in Kassel. In the near future, we will publish our LCA internally and then publish it externally. In 2027, we plan to publish our Environmental Product Declaration, which certifies our LCA and we will then update that annually starting in 2028.”

The H₂ market is extremely dynamic and challenging, but Hexagon Purus has spent a decade validating its various segments, establishing a strong global customer base and then building production capacity focused on efficiency, flexibility and, of course, sustainability. This approach is exemplified in Kassel and part of what enables the Hexagon Purus overall to be profitable at a commercial scale via diversified products before any single segment reaches mass adoption.

“We are a decarbonization company and so we look at the overall H₂ economy and how fossil fuel is being replaced over a broad range of applications and sectors,” says Chudzick. He notes Kassel’s production approach has also been used in the company’s facility in China, adapted for the local variation in skilled labor and capable of scaling to 100,000 cylinders/year. “This approach allows us to expand in a modular way, meaning we can integrate improved equipment or adapt as needed, depending on how the market evolves.”

Decarbonization is vital to mitigate climate change and save our planet, thus the Kassel facility’s ability to help achieve this is of huge importance. But another key contribution is the ability of this well-considered, compact production line to take such a step forward in Composites 4.0 automated manufacturing. It provides a lesson to other CFRP parts producers that sustainable, smart processes are possible and necessary for our industry’s future.