Type IV Hydrogen Storage Cylinder

Analysis: Large-medium hollow plastic inner liner molding process

Blow molding process

Defects:

1. Poor dimensional accuracy; 2. Unstable wall thickness; 3. Small thickness range

Rotomolding process

Defects:

1. Poor dimensional accuracy; 2. Long molding cycle; 3. High labor intensity; 4. High energy consumption

Because the performance requirements of Type IV hydrogen storage cylinders are very high, it is not ideal to use these two processes for one-time molding production.

Analysis: Breakthrough - Combined plastic inner liner molding process

Analysis: Combined plastic inner liner molding process

Extrusion of the inner liner body + injection molding of the inner liner cap + hot melt welding

Plastic extrusion and injection molding processes, the product has high dimensional accuracy, stable wall thickness, large size range, and can achieve efficient continuous production, successfully solving the manufacturing problem of Type IV hydrogen storage cylinder inner liners. It can also achieve the selection and production of different diameters and lengths, breaking through the limitations of traditional manufacturing processes on the volume and size of Type IV hydrogen storage cylinder inner liners.

Contribution: Successfully developed extrusion equipment for the Type IV hydrogen storage cylinder inner liner body

Typ	Output (kg/h)	Material	Diameter range (mm)	Extrusion speed (m)	Min. wall thickness (mm)
HSC250	130-160	PA	110-250	0.8	3
HSC450	180-210	PA	200-450	0.7	3.5
HSC630	230-280	PA	315-630	0.6	4

Successful case 1: XXX Hydrogen Energy Storage Tank Company, Qingdao, Shandong Province

Product model: HSC630

Order date: June 2024

Production line acceptance status:

1. Acceptance date: October 10, 2024

2. Trial production materials: Imported plastic particles provided by the customer

3. Trial production results: After xx hours of debugging, the dimensional accuracy and color of the extruded products meet customer requirements

Production line delivery status:

1. Delivery date: December 2024

2. Installation and debugging status

Customer usage status:

1. Customer production start date: Debugging and trial production were carried out from February 29 to February 5, 2025, and the results met expectations.

2. Production line usage status

a. Production speed or capacity status

b. The production line is simple and convenient to operate, stable in operation, and the product quality is stable. It is currently in normal operation.

c. Energy consumption, etc.

Successful case 2: XXX Storage Tank Company, Ningxia

Product model: HSC630

Order date: October 2024

Production line acceptance status:

1. Acceptance date: February 22, 2025

2. Trial production materials: Imported plastic particles provided by the customer

3. Trial production results: After xx hours of debugging, the dimensional accuracy and color of the extruded products meet customer requirements

Production line delivery status:

Delivery date: Due to the relocation of the customer's original factory, the equipment is expected to be shipped at the end of April.

Currently, Zhejiang Laneng under Shanghai Shenneng, Suzhou Zhongcai, and Germany Voith are negotiating orders.

Hydrogen energy, as a zero-carbon energy source, possesses a series of advantages such as abundant sources, cleanliness and environmental friendliness, high calorific value, no pollution, and storability/transportability, earning it the reputation as the most promising secondary energy source of the 21st century. The forms of hydrogen energy utilization are extensive, with typical hydrogen energy products like hydrogen fuel cell vehicles, fuel cell forklifts, fuel cell power stations, and communication base station emergency backup power supplies gradually being promoted, which is of great significance for solving the world's energy and environmental problems. The complete chain of hydrogen energy utilization includes production, storage, transportation, and application. The key to whether hydrogen energy will be widely used is safe and reliable hydrogen storage technology. Onboard hydrogen storage technologies mainly include high-pressure gaseous hydrogen storage, cryogenic liquid hydrogen storage, solid-state hydrogen storage, and organic liquid hydrogen storage. Among them, high-pressure hydrogen storage is highly valued due to its simple equipment structure, low energy consumption for compressed hydrogen preparation, and fast filling and discharge speeds, making it currently the absolutely dominant method for hydrogen energy storage and transportation. Considering factors such as compression energy consumption, driving range, infrastructure construction, and safety, the nominal working pressure of high-pressure hydrogen storage cylinders is generally 35-70MPa. High-pressure hydrogen storage cylinders are mainly divided into four types: All-metal cylinders (Type I), metal liner hoop-wrapped fiber cylinders (Type II), metal liner fully wrapped fiber cylinders (Type III), non-metal liner fully wrapped fiber cylinders (Type IV). Among them, Type I and Type II have a large weight-to-volume ratio, making it difficult to meet the hydrogen storage density requirements of hydrogen fuel cell vehicles. Type III and Type IV cylinders, due to their full fiber-wrapped structure, have a small weight-to-volume ratio, and single-

unit mass high hydrogen storage density, and other advantages, and are currently widely used in hydrogen fuel cell vehicles.

I. Comparison of Various Types of Hydrogen Storage Cylinders

With the rapid development and industrialization of hydrogen fuel cells and electric vehicles, the challenge of hydrogen storage and transportation is becoming a global research hotspot. Hydrogen storage cylinders are a very important storage and transportation medium, and the table below lists the performance comparisons of different hydrogen storage cylinders.

Type I~IV Hydrogen Storage Cylinders

Type	Type I	Type II	Type III	Type IV	Type V
Material	Pure Steel Metal Cylinder	Metal Liner (Steel ) Fiber Hoop Wrapped	Metal Liner (Steel /Aluminum) Fiber Fully Wrapped	Plastic Liner Fiber Fully Wrapped	No Liner Fiber Fully Wrapped
Working Pressure (MPa)	17.5-20	26-30	30-70	30-70	Country Internal External Research Development In progress
Medium Compatibility	Hydrogen embrittlement, Corrosive	Hydrogen embrittlement, Corrosive	Hydrogen embrittlement, Corrosive	Hydrogen embrittlement, Corrosive
Weight Volume (kg/L)	0.9-1.3	0.6-1.0	0.35-1.0	0.3-0.8
Service Life (Years)	15	15	20	20
Cost	Low	Medium	Highest	High
Onboard Capability	No	No	Yes	Yes
Market Application	Fixed hydrogen storage such as hydrogen refueling stations		Fuel Cell Vehicles

 

With the rapid development and industrialization of hydrogen fuel cells and electric vehicles, Type IV hydrogen storage cylinders are becoming a global research hotspot due to their lightweight and fatigue resistance. Type IV hydrogen storage cylinders from Japan, South Korea, the United States, and Norway have already entered mass production, and other countries also have relevant plans to increase research efforts on Type IV cylinders.

The manufacturing cost of Type IV hydrogen storage cylinders ranges from US$3000 to US$3500, mainly including: composite materials, valves, regulators, assembly inspection, hydrogen, etc. Among these, the cost of composite materials accounts for over 75% of the total cost, while the cost of hydrogen itself only accounts for about 0.5%. The development trend of hydrogen storage cylinder technology is lightweight, high pressure, high hydrogen storage density, and long service life. Compared with traditional metal materials, polymer composite materials can reduce the tank wall thickness while maintaining the same pressure rating, thereby increasing capacity and hydrogen storage efficiency, and reducing energy consumption costs during long-distance transportation. Therefore, the performance and cost of composite materials are key to the preparation of Type IV hydrogen storage cylinders.

II. Structure and Materials of Type IV Hydrogen Storage Cylinders

Composite hydrogen storage cylinders consist of, from inside to outside, a liner material, an intermediate layer, a fiber winding layer, an outer protective layer, and a buffer layer. The charging cycle for hydrogen storage cylinders can be long, and hydrogen under high pressure has strong permeability, so the liner material of hydrogen storage tanks must have good barrier properties to ensure that most of the gas can be stored in the container.

The structure of Type IV hydrogen storage cylinders mainly includes the following parts:

(1)Inner liner

The total thickness of the bottle wall is approximately 20~30mm. The innermost layer in direct contact with hydrogen is the gas barrier layer, with a thickness of approximately 2~3mm. The material is PA6, PA612, PA11, HDPE, etc., which acts as a barrier to hydrogen.

(2)Middle layer

A relatively thick pressure-resistant layer. The material is CFRP (carbon fiber reinforced polymer), composed of carbon fiber and epoxy resin. While ensuring the pressure resistance level, the thickness of this layer is minimized to improve hydrogen storage efficiency.

(3)Surface layer

The outermost layer is a surface protection layer, with a thickness of approximately 2~3mm. The material is GFRP (glass fiber reinforced polymer), composed of glass fiber and epoxy resin.

Raw materials and forming process of the inner liner:

For all-composite type IV hydrogen cylinders with plastic liners, high-polymer materials are used for the inner liner, and carbon fiber composite materials are wound as the load-bearing layer. The hydrogen storage mass ratio can reach more than 6%, up to 7%, thus further reducing the cost.

(1)Hydrogen permeability resistance and heat resistance

Hydrogen molecules easily permeate the shell material of the plastic inner liner. When selecting materials, the hydrogen barrier performance of the raw materials must be considered. In addition, after the throttling effect of the valve, the gas temperature will increase, and then the gas is compressed to the working pressure of the cylinder, and the temperature also increases. The inner liner raw materials need to have suitable hydrogen permeability and heat resistance.

PA6 resin has excellent performance in preventing hydrogen permeation and has excellent mechanical properties, including durability against sudden temperature changes in the tank during hydrogen filling and discharge, and impact resistance at low temperatures. The permeability of PA6 material can be modified at the raw material level, and the softening temperature of the material can be increased to around 180℃ to meet the requirements.

(2)Good low-temperature mechanical properties

In order to avoid damage to the inner liner raw materials due to excessively high filling temperatures, the gas source is usually cooled, generally to -40 ℃. When low-temperature hydrogen is filled into the cylinder, the inner liner will become hard and brittle at low temperatures, easily cracking. Therefore, the low-temperature mechanical properties of the inner liner raw materials are particularly important.

(3)Good processability Plastic inner liner forming technologies include injection molding, rotational molding, and blow molding. Currently, the inner liner forming process for type IV hydrogen storage cylinders used in fuel cell vehicles by Toyota and Hyundai is injection molding. Injection molding is a relatively low-cost and widely used inner liner forming method, and it must be combined with subsequent welding procedures to form the inner liner.

2.2 Type IV Hydrogen Cylinder Inner Liner Forming Process

In the traditional strength design of aluminum inner liner all-wound cylinders, the inner liner load is generally not considered. Theoretically, the internal pressure of the cylinder is completely borne by the reinforcing fibers. However, in fact, the cylinder inner liner is always under tensile stress under working pressure, which is the key factor restricting the fatigue life of the cylinder. In order to meet the requirements of light weight and good fatigue resistance of hydrogen storage cylinders, selecting the appropriate shape and size of the inner liner is of great significance.

Type IV cylinder liners mostly use PA6, high-density polyethylene (HDPE), and PET polyester plastics, and the corresponding forming processes are mainly injection molding, blow molding, and rotational molding. Toyota and Hyundai's mass-produced Type IV cylinders are injection molding + welding processes. This forming method has low cost and is widely used, but the yield rate is also low, and it must be combined with subsequent welding procedures.

Hydrogen Storage Cylinder Inner Liner Forming Process

Project	Injection Molding	Rotational Molding	Blow Molding
Process Introduction	Melt the plastic material and then inject it into the mold cavity. Once the molten plastic enters the mold and cools, it will form into a certain shape according to the mold cavity.	Put the powdered plastic into the mold, then rotate and heat it at the same time. The powder in the mold gradually melts and adheres to the mold cavity, and is cooled and shaped to obtain a plastic product.	A processing method in which a preform of semi-molten plastic obtained by extrusion or injection molding is inflated with compressed air in a closed mold and then cooled to obtain a hollow workpiece.
Advantages	Stable product dimensions, low cost; free design of sealing structure, high impact toughness and environmental stress cracking resistance.	Uniform product wall thickness, simple forming process, low process cost, can form large parts.	High production efficiency, high impact toughness and environmental stress cracking resistance, low cost.
Disadvantages	Requires welding process support, low yield rate, difficult to form large sizes.	Poor dimensional stability, low density, easy to produce defects, high requirements for the melt flow rate of the material, and low production efficiency.	Poor wall thickness uniformity, difficult insert molding, requirements for melt flow index

III. Type IV Hydrogen Cylinder Resin Matrix

The resin matrix of carbon fiber hydrogen storage cylinders not only needs to meet the requirements of the cylinder for mechanical strength and toughness, but also because the matrix is prone to fatigue damage in the long-term inflation and deflation environment, a high-strength, tough, and fatigue-resistant resin system is required to ensure the service life of the cylinder. The resin matrix used in wet winding molding, in addition to meeting the corresponding performance, also requires it to have a low initial viscosity at the operating temperature and a long service life at this temperature. Epoxy resin has the advantages of high bonding strength, low curing shrinkage, no small molecule volatiles, good processability, good heat resistance, chemical stability, and low cost. It also has great modification space, and its sources are wide and the price is reasonable, suitable for wet winding process systems.

The resin primarily used in the composite layer of Type IV high-pressure hydrogen storage cylinders is epoxy resin. Epoxy resin is one of the commonly used thermosetting resin matrices in resin-based composite materials. It is widely used in fiber winding processes due to its advantages such as high bonding strength, low curing shrinkage, no small molecule volatiles, good processability, good heat resistance, good chemical stability, and low cost.

(1) Good mechanical properties

The role of resin in composite materials is to fix the fibers and transfer the load through the interface between the resin and the fibers, maximizing the fiber strength. The resin needs to have high toughness and strength, but these two are contradictory, and the balance between them is a key technical difficulty in resin modification.

(2) Good thermal stability

For Type IV hydrogen storage cylinders, the curing temperature needs to be lower than the softening temperature of the plastic inner liner to protect the inner liner structure. To ensure that the cylinder is completely safe during actual use, the glass transition temperature of the resin needs to be greater than 105℃. Generally, the lower the curing temperature, the lower the glass transition temperature after curing, which contradicts the stability of the plastic inner liner structure, requiring corresponding modification of the resin.

(3) Good processability

Appropriate resin pot life and moderate viscosity are important aspects of resin processability. The composite layer thickness of vehicle-mounted hydrogen storage cylinders is generally between 20 and 30 mm, with a long winding time. A short resin pot life will lead to poor resin wettability, affecting the composite material performance. The curing furnace is heated by air convection, and heat radiation heats the cylinder to cure it. If the viscosity is not appropriate, it will make it difficult to remove air bubbles from the resin, and the heat transfer from the surface to the inside will create a temperature gradient between the inside and outside, resulting in defects such as bubbles on the surface and pores inside after curing, which may seriously affect product performance.

IV. Type IV Hydrogen Cylinder Fiber Winding Process

Carbon fiber winding processes can be divided into wet winding and dry winding. Wet winding is more widely used due to its lower cost and better processability. Wet winding equipment mainly includes a fiber frame, tension control equipment, a resin impregnation tank, a fiber delivery nozzle, and a rotating mandrel structure. Internationally advanced six-axis winding technology can effectively control the fiber direction and combine hoop winding, helical winding, and planar winding. In actual production, helical winding and hoop winding are often combined. Hoop winding can eliminate hoop stress caused by internal pressure in the cylinder, and helical winding can provide longitudinal stress, improving the overall performance of the cylinder.

The design of the fiber winding layer needs to consider the anisotropy of the fiber. According to its structural requirements, the layer theory and grid theory are usually used to calculate the stress distribution of the container head, liner, and fiber winding layer, and then determine the tension selection and line type distribution in the winding process. By alternately performing hoop winding and helical winding to achieve a multi-layer structure, selecting the appropriate fiber stacking area and longitudinal winding angle and helical winding line type, not only meets the strength requirements, but also allows for reasonable coverage at the head.

4.1 Dry Winding Process

The dry winding process uses prepreg tape that has undergone pre-impregnation treatment as raw material. It is wound onto the mandrel after being heated and softened to a viscous state on the winding machine. Its main advantages are:

(1) Professionally produced prepreg yarns/tapes can ensure strict control of the fiber and resin content ratio (accurate to within 2%), resulting in high and stable product quality;

(2) High production efficiency, winding speed can reach 100-200 m/min;

(3) The winding equipment and production environment are clean and easy to clean, and the service life of the winding machine is also longer.

 

4.2 Wet Winding Process

The wet winding process is a molding method in which carbon fiber tows are impregnated in a specific impregnation device, then wound onto a mandrel under tension control, and finally cured. Its main advantages are as follows:

(1) Lower production cost, about 40% lower than dry winding. The process equipment involved is relatively simple, with small equipment investment and relatively low requirements for raw materials.

(2) Good product airtightness. During the winding process, excess resin can be squeezed out and gaps filled by tension control.

(3) The resin impregnated on the surface of the carbon fiber can effectively reduce fiber wear.

(4) Good fiber parallel arrangement.

 

4.3 Main Winding Methods

(1) Hoop Winding

Hoop winding is winding along the circumference of the container. During winding, the mandrel rotates at a uniform speed around its own axis, and the guide head moves in the cylindrical section parallel to the mandrel axis. For each rotation of the mandrel, the guide head moves a distance equal to the width of a yarn. This cycle continues until the yarn is evenly distributed on the surface of the mandrel cylinder section.

The characteristic of hoop winding is that winding can only be performed on the cylindrical section and not on the head. Adjacent yarns are joined without overlapping, and the fiber winding angle is usually between 85° and 90°.

(2) Helical Winding

Helical winding is also called geodesic winding. During winding, the mandrel rotates at a constant speed around its own axis, and the wire head reciprocates along the axis of the mandrel at a specific speed. This achieves helical winding on the mandrel body and cap, with a winding angle of approximately 12°~70°. In helical winding, fiber winding is performed not only on the cylindrical section but also on the cap. The winding process is as follows: the fiber starts from a certain point on the circumference of the pole hole at one end of the container, winds around the cap along a curve tangent to the pole hole circle on the cap surface, winds around the cylindrical section in a helical trajectory, enters the cap at the other end, then returns to the cylindrical section, and finally winds back to the cap where it started, repeating this cycle until the mandrel surface is uniformly covered with fiber. In this way, when the fiber is uniformly wound around the mandrel surface, a double-layer fiber layer is formed. To ensure that the pressure vessel after winding meets the pressure requirements for use, the winding method generally combines hoop winding and helical winding.

 

In summary, the production process of Type IV hydrogen storage cylinders is shown in the figure below. The production process of high-pressure hydrogen cylinders mainly includes:

(1) Liner processing (making the inner liner from thermoplastic olefin polymer)

(2) Fiber winding molding

(3) Inspection and testing

Type IV Hydrogen Storage Cylinder Production Process

V. New Challenges for Type V Hydrogen Storage Cylinders

The development of pressure vessels and tanks used for storing gas under high pressure has gone through four distinct stages. The fifth stage of pressure vessels—the all-composite linerless tank (Type V)—refers to a pressure vessel that does not contain any liner and is entirely made of composite materials. For a long time, Type V pressure vessels have been considered the pinnacle of products and technology in the pressure vessel industry.

Compared to the three-layer structure of Type IV cylinders, which consists of a resin liner, a carbon fiber reinforced resin layer, and a glass fiber reinforced resin layer, Type V cylinders have a two-layer structure without a liner: a carbon fiber composite shell and a dome protective layer. Compared to Type IV cylinders, Type V cylinders have advantages such as working pressures up to 70-100 MPa, no hydrogen embrittlement, no corrosion, a service life of over 30 years, and lower costs, and can also be used in aerospace and automotive applications. The technology for Type V cylinders is still in its infancy, and various industries are closely watching the development and opportunities of this technology.

The mandrel is made of water-soluble core material cast and bonded in two parts, with a wall thickness of 30 mm. It has annular stiffeners inside to help withstand the torsional loads generated during the automatic fiber laying process and the pressure generated during fiber curing.

The prepreg is precisely cut into 6.35 mm wide narrow strips, with a total winding length of 22,000 meters. The winding process uses specialized software to control helical and hoop winding. 24 layers are wound, reaching a thickness of 5.5 mm.

 

Conclusion

The R&D of Type IV hydrogen storage cylinders not only needs to be linked with composite materials but also needs to be closely linked with plastic processing and manufacturing processes and plastic sealing structures. For China, the relevant technologies for Type IV cylinders are still in a stage of continuous development and progress, and continuous efforts are needed to improve the relevant theoretical technologies to lay a solid foundation for the future development of Type V cylinders.