Hydrogen Storage Vessels of Type 4 and Type 5

Abstract

This chapter explores the optimization of type 4 pressure vessels used for hydrogen storage, focusing on carbon fiber-reinforced composites produced through filament winding. Many studies delve into the intricacies of the winding process to enhance the structural integrity of the vessels. Progressive failure analysis is employed to identify potential weak points and failure modes, guiding the development of optimal designs for improved safety and performance. Additionally, the chapter highlights the importance of considering recycling strategies in the design phase to address environmental concerns associated with composite materials. The findings contribute to advancing sustainable practices in the production and life cycle management of hydrogen pressure vessels.

1. Introduction

Composite materials including high-performance fibers (glass, carbon, aramide, or organic) have emerged as alternative materials. Those composites with a polymer matrix, and fibers such as carbon fiber are viable candidates known as composite fiber-reinforced polymers (CFRP). They are being used in making hydrogen storage vessels due to their lightweight, high-strength, and corrosion-resistant properties.

In this chapter, we will expose the main storage vessel’s features, which are the key stones of a high-performance hydrogen storage vessel. We will build this chapter similarly to the industrial process in which a type 4–5 storage vessel is built. In the first section, we will expose why a storage vessel made of composite is predominant. Then, in the second section, we will explain the choice of epoxy resin as a major material for designing the tank. The third section will deal with the process of filament winding. Here, the process of continuous fiber reinforcement will be exposed. The winding filament methodology will be highlighted. The choice of carbon fiber for winding over all other types of fibers will be justified with regard to the mechanical performance, among many other factors. The last section will treat the unavoidable question of recycling. The end-of-life of each component of the tank will be reviewed.

2. Why the composite storage vessel: a game changer

The composite storage vessels (CSV) have gained importance since the moment that an important part of energy transition will be built around hydrogen storage [1]. In fact, it is highly appealing for autonomy in the car industry to store a large amount of gas by just applying high compression. The most attractive feature of hydrogen tanks is their lightweight.

2.1 Description and orders of magnitudes

The CSVs of type (4) see 1 are refer to references [1, 2] used to store gaseous hydrogen under pressure of 700 bar. Physical consideration concerning the Thermos-chemical properties like phase diagram and energy densities can be found in this paper [3]. Those CSVs are made of the following components:

1. Cylindrical composite shell: tasked with facilitating the mechanical structuring of the reservoir, the external wall consists of a shell measuring several centimeters in thickness, composed of a carbon fiber-based composite material (see Figure 1) and a polymer resin.

2. Liner: inner wall, with a thickness of a few millimeters, serves the purpose of containing hydrogen at 700 bar and 23 K and providing sealing; it bears multiple tens of thousands of refills without developing cracks. Thermoplastic liners are commonly employed to ensure gas tightness and chemical compatibility. Commonly utilized thermoplastic liners include polyethylene (PE), polypropylene (PP), high-density polyethylene (HDPE), and polytetrafluoroethylene (PTFE).

3. Boss: At each end of the liner are two metal fittings called boss, which serve to connect the tank to the fuel cell and filling system. Functioning to provide mechanical structuring of the tank.

4. Carbon filament: this part is the most crucial because it will decide for the future behavior of the entire CSV [4] (see next paragraph §5). The carbon fiber takes the form of thin filaments with diameters ranging from 5 to 10 μm made of over 90% carbon. It is derived through the carbonization process of fibers from a polymer known as a precursor, with polyacrylonitrile (PAN) being the most commonly utilized as a precursor (see next Figure 2).

Figure 1.

(a) Perspective representation. (b) Vertical section representation of a type IV tank composed of metallic bases in gray to connect the tank to the fuel cell and filling system, in yellow a polymer material liner for hydrogen sealing, and in black of a composite shell with carbon fiber for mechanical structuring (adapted from Ref. [2]).

Figure 2.

Fabrication steps of carbon fibers from polyacrylonitrile, oxidation, and cross-linking of the PAN fibers, pyrolysis occurring at the higher temperatures under inert atmosphere or vacuum conditions (adapted from Ref. [5]).

Typically, for a hydrogen tank containing 62 L and 2.5 kg of hydrogen, the outer diameter measures approximately 40 cm, with a length of 90 cm; carbon fiber constitutes 60% of the total mass of the tank. Its total mass is approximately 55 kg, of which 33 kg are carbon fibers. One kilogram of hydrogen provides a range of 100 kilometers; therefore, each vehicle requires at least 5 kilograms of hydrogen to achieve a range of at least 500 kilometers. Knowing that at 700 bar and 15°C, a volume of 25 liters is required to store 1 kg of gaseous hydrogen, an internal storage volume of 125 L is needed. In the industry of automobile, it will be implemented in the form of two CSVs.

Table 1 summarizes the main features of hydrogen CSTv of type IV.

CSVs	Pressure (bar)	Volume (L)	Weight (kg)	Volume density(MJ/L)	Cost ($/kg)
Type IV	700	62	55	6	600
Type V	700	62	50	8	800

Table 1.

Projected performance and cost of CSVs of both type IV and type V.

2.2 Physical justification: permeability

The physical basement of choosing composite is permeability effect. Two mechanisms contribute to gas leakage in composite materials: diffusion and microcracking, according to Humpenëoder et al. [6].

2.2.1 Diffusion

This effect is permanently present as long as both fiber and material have a certain rate of porosity. Many studies have been conducted to shed light on it. In the early 2000, experimental works [7] on polymer diffusion were done on three semicrystalline polymers: polyethylene (PE), polyamide 11 (PA11), and poly(vinylidene fluoride) (PVF2). They were studied in the presence of helium (He), argon (Ar), nitrogen (N2), methane (CH4), and carbon dioxide (CO2) for temperatures ranging from 40° to 80°C in the case of PE, and from 70° to 130°C for both other. The applied pressures were, in the majority of tests, 10 MPa for He, Ar, N₂, which is far from the 70 MPa standard pressure of the present hydrogen CSVs. In the last years, many interesting experimental works have been oriented toward realistic condition of pressure [8, 9]. It has been found by Fujiwara et al. that the polymers used as liners (LDPE, LLDPE, HDPE, and MDPE) suffered fracture during the decompression process after hydrogen exposure was found. The permeability coefficient decreased with the decrease of diffusion coefficient under higher pressure condition. The second main result pointed out was that the shrinkage in free volume caused by hydrostatic effects of the applied hydrogen gas pressure decreases diffusion coefficient, resulting in the decrease of permeability coefficient with the pressure rise. In the following Figure 3 is summed-up the main experimental result obtained by Fujiwara et al. [8].

Figure 3.

(a) Matrix representation of n of the transmitted light image of six different polymers. Evaluation of destruction revealed when exposed to 10–90 MPa hydrogen for 24 h at 30°C. (b) Hydrogen gas permeation characteristics Relationship with specific volume in atmospheric pressure environment and in high-pressure environment (adapted from Ref. [8]).

They are bringing proofs that at steady-state high-pressure hydrogen gas permeation test (HPHP) under 90 MPa, the polypropylene (PP) suffers more destruction at higher exposure pressures, and the destruction in materials with smaller crystallinity was more severe. Another main fact is that hydrogen gas permeability increases with hydrogen pressure, but the increase ratio slows down with increasing pressure. Besides, the compressive effect of free volume after the application of hydrogen lowered the gas diffusion, and the permeation coefficient was also reduced in high-pressure environments.

In the last years, a noticeable study run by Conde-Wolter and co-workers [9] was achieved concerning permeability. Figure 4 schematizes the experimental set-up that was used to measure permeability.

Figure 4.

(a) Material composition – thermoplastic liner films only necessary for type C specimens. (b) Manufacturing process. (c) Sample dimensions and different type of samples – type A – unreinforced samples of matrix polymer; type B – CFRTP sample with sealing, type C – CFRTP sample like type B with applied multilayer liner (adapted from Ref. [9]).

High-pressure hydrogen permeation tests were carried out on various thermoplastic matrix materials and on continuous fiber-reinforced thermoplastic composites (PA6, PA12, PA410, PPA, and PPS). Thin layers of liner made of ethylene vinyl alcohol (EVOH) thermoplastic films were added to PA6 composites. They studied how they affected the permeation rate. They also conducted pressure tests and examined micrographs to check for any manufacturing issues, pore size, and other defects like microcracks. Their experiments found that these thermoplastic composites can achieve low permeation rates as long as the microstructure remains intact.

The numerical approach is also very promising. According to this article [10], it is possible to determine effective diffusion coefficients numerically for CSVs. In a framework of inhomogeneous composite laminate, three different homogenization methods were used. In order to assess the effective permeation coefficients for hydrogen permeation through composite laminates, the Wiener bounds, the Hashin-Shtrikman bounds, and a numerical finite element calculation of a representative volume element (RVE) were compared. It was possible to estimate the influence of the crack volume on the effective permeability. The results show that microcracking in a composite pressure vessel significantly affects the leak tightness, even if the crack volume is very small. The pressure vessel should therefore be free of matrix cracks if a liner-less design is aimed at. The crack volume fraction and the assumed permeability coefficient of the crack have no significant influence on the real permeability coefficient as long as they are within a plausible range. The main influencing factors are the fiber volume fraction and the crack configuration, respectively (Figure 5).

Figure 5.

(a) Influence of the crack configuration on the effective permeability. (b) Influence of the crack configuration on the effective permeability. Approximation of the numerical calculation is done using the geometric mean. In gray the bounds by use of parallel series and the numerically calculated result of the RVE in (a) (adapted from Ref. [10]).

2.2.2 Microcracking

Figure 6 represents the enhancing effect of microcracking on hydrogen particle diffusion.

Figure 6.

Schematizing of the leakage mechanism due to intersecting matrix microcracks (adapted from Ref. [11]).

Basically, matrix microcracking occurs when mechanical and thermal stresses are applied to the composite laminate. These cracks form within each layer and run parallel to the fibers. Over time, these cracks can create pathways through the vessel’s walls, allowing gas to escape. This is the initial sign of failure in a composite tank. While these cracks may not immediately cause the tank to fail completely, they weaken the material and can eventually lead to catastrophic failure. Therefore, it is crucial for designers to comprehend how these cracks form and their impact on the tank’s performance.

This phenomenon has been found to result in significantly higher leakage compared to diffusion [12, 13]. Various factors, such as temperature, pressure, matrix properties, fiber distribution, fiber type, and ply stacking sequence, affect permeability. However, material-based properties only cause a minor change in permeability. Understanding permeation in polymers at high pressures, like 700 bar, remains limited. Testing at such pressures is challenging due to equipment limitations, and most measurements are conducted at lower pressures. Fujiwara et al. [8] addressed this by creating a 900 bar permeability cell, but it focused on polymers, not composites. Additionally, typical permeation tests are performed with the sample unloaded, which is not representative of the actual operating conditions of a pressurized vessel.

3. Matrix materials: epoxy resin

The majority of existing hydrogen pressure vessel (HPV) designs are manufactured using a thermoset matrix. Those classes of material are easier and more reliable to produce [14]. Due to their lightweight characteristics, resistance to corrosion, and gas permeability, resin epoxy is considered as the market standard thermoset. In this article [15], the authors focused on the specific application of epoxy resin in cryocompressed hydrogen storage vessels. The incorporation of polyethylene glycol-modified epoxy resin is highlighted for its role in enhancing the properties of the composite layer. This modification showcases the adaptability of epoxy resin to address the unique challenges associated with cryogenic conditions, underscoring its importance in ensuring the integrity and durability of hydrogen storage vessels. The study demonstrates how tailored modifications to epoxy resin contribute to the improvement of crucial properties, such as thermal stability and mechanical strength.

4. Carbon fiber synthesis

Carbon fiber is a material composed of thin filaments with diameters ranging from 5 to 10 μm, consisting primarily of carbon (over 90%). Carbon fiber is produced through the carbonization of precursor fibers, typically polyacrylonitrile (PAN), which accounts for over 90% of global production. In the next Figure 2 a PAN is synthesized via radical polymerization of acrylonitrile and then spun through a spinneret with thousands to hundreds of thousands of holes. The resulting fibers undergo a series of thermal treatments to convert them into carbon fibers. Oxidation heating renders the fibers infusible, followed by an initial carbonization treatment under an inert atmosphere to obtain carbon fibers. For higher modulus fibers, a third thermal treatment called graphitization is performed at temperatures around 2000–2500 °C. Additionally, a thin polymer layer, known as sizing, is applied to the fiber surface to protect it during storage and transportation and to enhance the interface with the matrix during composite material fabrication.

Carbon fibers possess superior mechanical properties, including tensile strength and Young’s modulus, compared to most materials, making them highly desirable for various applications. They are primarily used to manufacture composites with high mechanical properties by integrating them into a polymer matrix to bind and maintain the fibers. The quality and properties of these composites depend not only on the fiber and matrix quality and nature but also on the architecture used. Architecture refers to controlling the fiber orientation within the material to achieve desired mechanical properties in multiple directions. Since fibers act as one-dimensional elements, they only contribute their properties along their length, resulting in anisotropy in the composite. To obtain composites with desirable mechanical properties in multiple directions, it is essential to control fiber orientation within the material. This architecture is typically achieved through braiding, weaving, or winding techniques.

One crucial factor in fiber selection is the size effect, as investigated by Hwang et al. [16]. They found that as the component size increases, there is a reduction in the strength of the fiber, affecting vessel performance. They also developed the ring burst test, which accurately predicts failure strain results similar to full-scale vessels. Cohen et al. [17] demonstrated the impact of fiber volume fraction (FVF) on vessel strength, noting that increasing FVF in hoop plies improves ultimate strain-to-failure and failure pressure. According to this extensive review [18], there is a classification of the different mechanical properties of carbon fibers based on their chemical synthesis methods. Table 2 summarizes the most important carbon fiber used for hydrogen storage vessels.

Carbon fiber	Tensile strength(MPa)	Young’s modulus(GPa)	Elongation at break(%)	Ref
T700	4900	230	1.8	[19]
M40	3650	230	1.4	[20]
IM7	4500	275	1.5	[21]
T800	4600	240	1.6	[22]
T800S	4950	230	2.0	[23]

Table 2.

Mechanical properties of carbon fibers used in high-pressure storage vessels.

5. Filament winding for carbon fiber

This aspect of industrial processing was a major ax of development during the last decades [24]. It involves wrapping around a composite cylinder (named mandrel) in addition to the two hemispherical domes at each side (see Figure 7). This process, known since the 1940s, can be described in three major steps

1. Filament winding holds the fiber under tension during placement. It will only wrap around convex surfaces.

2. The fiber band is kept continuous throughout the winding process, which causes excess thickness build-up around the polar boss.

3. Placed under tension, the fibers self-align to geodesic paths (see next section). To achieve non-geodesic paths, friction is required to prevent fibers from slipping [24].

Figure 7.

Illustration of step of automated deposition of one ply of filament.

This continuous process of depositing tapes of carbon fibers will end up formatting multiple ply. Each of those stratified ply is different from the previous one by an angle value that will make a sequence.

Figure 8 illustrates a sequence of ply that make up the external reinforcement of CSV. Many approaches [11, 26] were utilized to investigate the effects of winding angle on filament-wound pressure vessels.

Figure 8.

The stacking sequences for 55° winding angle (adapted from Ref. [25]).

5.1 Winding patterns: laminated ply

A geometric approach for filament winding pattern generation on surface of revolution has been developed [27]. When winding fibers are applied on a cylindrical surface, there are three common types of paths: geodesic, non-geodesic, and semi-geodesic.

• Geodesic: These paths are the shortest routes between two points on the surface and are stable, requiring no external force to stay in place.

• Non-geodesic: Paths offer more design flexibility but are unstable and need frictional forces to prevent slipping. The choice between them depends on the shape of the CSV.

• Semi-geodesic: The way of winding involves a slight deviation from the geodesic path, which depends on the necessary friction to maintain the fiber in its intended position. This technique, also known as stable non-geodesic winding, offers flexibility in optimizing fiber paths.

Geodesic winding is preferred for most designs. Geodesic winding has zero lateral force on the fiber, while non-geodesic winding has some lateral force. Semi-geodesic winding is a stable variation of non-geodesic winding, offering flexibility while still maintaining stability.

Wet and dry winding are two methods used for winding fibers. In wet winding, fibers are soaked in resin and wrapped around a rotating mandrel, while in dry or prepreg winding, pre-impregnated fiber tows are used. Wet winding is commonly preferred, especially for making filament-wound composite cylinders, due to its advantages such as lower material costs, shorter winding periods, and easily modifiable resin formulations to meet specific requirements. Recent enhancements in 700 bar type 4 containers have shown promising results, including increased cycling resistance, burst pressure, hydrogen tightness, and improved storage capabilities (Figure 9) [28].

Figure 9.

(a) Winding pattern architecture on composite tubes. (b) Laminate structure – pattern 2/1 with 18 bands (adapted from Ref. [24]).

5.2 Mozaic pattern

The last decades have seen a more sophisticated process of deposition of the filament settled. Thanks to robotizing, filament winding combines accurate fiber lay-up with a high degree of automation. When filament-wound parts are manufactured, obtaining a pattern geometry is unavoidable [27]. The cyclic positioning of the fiber band on the rotating mandrel creates the so-called mosaic pattern. An integer number defines this pattern, indicating how many diamonds are on the circumference of the part. The diamond regions can then be divided into different areas for analysis. First, two triangular laminate areas with laminate sequences −α / +α and +α / −α can be observed 9. A concentration of interweaving can be seen in this area. It is then concluded that the pattern number directly influences the number of interweaving and undulation areas [29]. A schematic presentation is provided to better understand the creation of interlaces. Finally, the winding pattern architecture on composite tubes is presented. Literature [29, 30, 31] suggests that filament winding parameters have been studied in various configurations. The mechanical response of the composite of a ring shape was evaluated to assess the effects of winding angle, diameter-to-thickness ratio, and stacking sequence. A novel hoop ring test facility was used to find fiber and burst properties [12]. Another study investigated impact damage development in composite pipes stacked at different angles. It was found [32] that internal pressure was effective in damaged pipes, and pressure increase reduced impact damage. Additionally, the influence of winding angle on fatigue damage development was studied in glass-fiber-reinforced polymer (GFRP) pipes. The most common failure mode was delamination with small off-axis cracks and fiber/matrix debonding [31]. In this research [32], a genetic algorithm was used to find the best stacking sequence for internally pressurized filament-wound tubes. A novel damage model was developed to predict the response of filament-wound pipes under radial compression and external pressure [33].

An ongoing researcher is still interested in the impact of mosaic patterns on progressive failure. Studies examined pressure tests on glass/epoxy pipe specimens with a ± 55° and ± 75° winding angle [34, 35]. It was concluded that higher pattern numbers, indicating higher degrees of interweaving, influence damage growth. However, this only impacts closed-ended internal pressure loading and weeping tests, with no significant variations found in tensile or pure internal pressure tests. Subsequent significant work was done on pattern issues. Experiments were accompanied by numerical evaluations [36, 37]. Finally [38], an attempt was made to incorporate the mosaic pattern into the cylindrical part and dome area of the numerical investigation.

5.3 Fiber under tension

The preload from pre-tensioning is crucial for enhancing pressure capacity and reducing tank weight and volume [39]. Maintaining this preload at a specific level is important due to varying friction forces between fibers and the mandrel. Higher fiber tension improves rigidity and resilience, while lower tension allows for greater flexibility. Filament-wound composite pressure vessels are designed to stack fibers with high tension for high-performance applications, typically reaching 60–70% of the fiber’s ultimate strength. However, excessive fiber tension can lead to significant elongations of the composite and severe resin matrix splitting between fibers. Resin crazing, occurring between 10% and 40% of final fiber strength, is critical to composite stress, yet it remains lower than operating stress in high-performance CSV. Experimental studies [40] have shown that tubular part strength depends on fiber stress levels, with higher winding tension providing better resistance against failure under fiber-dominated loading conditions. Conversely, reduced fiber tension delays failure under matrix-dominated loading. Additionally, the filament winding process is highly customizable and ideal for automation, particularly in controlling fiber tension and stress.

5.4 Fiber pre-impregnation

In recent years, the use of pre-impregnated fibers (thermoset or thermoplastic) has been introduced to enhance quality control and increase adhesion between laminated ply. Before wrapping around the mandrel, the fibers are soaked in resin (see Illustration 10) and then solidify together with the fiber. Once the fiber wrapping process is complete, the entire assembly, consisting of the mandrel and the layers of composite overwrap, is placed in an oven and heated to the necessary temperatures for curing (Figure 10).

Figure 10.

Schematic diagram of filament winding technique (adapted from Ref. [41]).

Filament-wound composite cylinders exhibit notable advantages with wet winding compared to dry winding. These advantages include reduced material costs, shortened winding duration, and a resin formulation adaptable to specific requirements [42]. Additionally, wet winding offers superior fiber volume control.

6. End-of-life: recycling

The recycling of composite hydrogen storage vessels is crucial because they could offer a more eco-friendly option compared to traditional fuel storage methods. Yet, recycling these vessels can be tricky due to their complex composition and the presence of hydrogen. Recent research [43] has explored various recycling techniques, for instance [44, 45], a mechanical method like grinding and shredding to break down the composite material into smaller pieces, which can then be processed to recover the fibers and resin. Other studies have focused on thermal treatments, such as pyrolysis, to break down the composite material into its basic components. Besides physical recycling methods, there have been investigations into reusing composite hydrogen storage vessels by repurposing them for different applications or refurbishing and refilling them with hydrogen. However, ensuring the integrity and safety of these vessels remains a key consideration for these methods.

In addition to traditional fibers, sustainable and natural fibers have been explored for composite pressure vessels. Bouvier et al. [46] investigated alternative fiber choices for type IV CSVs, considering factors such as mechanical performance, cost, and recyclability. They found that hybrid combinations like E-glass/T700S carbon offer cost advantages, while basalt/recycled T700S and flax/recycled T700S hybrids show potential for reducing greenhouse gas emissions in different pressure vessel applications. However, vessels entirely made of T700S carbon fiber remain the preferred option for maximum mechanical performance.

7. Conclusion

In conclusion, the development and implementation of composite materials, particularly composite fiber-reinforced polymers (CFRP), have revolutionized the field of hydrogen storage vessel manufacturing. These materials, often incorporating high-performance fibers such as carbon fiber, offer a lightweight, high-strength, and corrosion-resistant alternative to conventional storage methods. This review has outlined the critical features of high-performance hydrogen storage vessels, with a focus on the type 4–5 vessels commonly used in various applications.

The primary advantages of composite storage vessels, including their lightweight nature and high strength-to-weight ratio, have been elucidated. These vessels, constructed with a composite shell, inner liner, boss fittings, and carbon filaments, exhibit superior performance in terms of both mechanical strength and gas containment capabilities. Understanding the physical properties and performance metrics of these vessels, such as pressure and volume specifications, is crucial for their successful implementation in hydrogen storage applications.

Furthermore, the choice of epoxy resin as a matrix material for these vessels has been discussed, emphasizing its importance in ensuring the structural integrity and durability of the composite structure. The synthesis and application of carbon fibers in filament winding processes have also been explored, highlighting their role in enhancing mechanical properties and structural performance.

Lastly, the review has addressed the critical issue of end-of-life management for composite storage vessels. Recycling techniques, including mechanical grinding, shredding, and thermal treatments, have been investigated to recover valuable materials such as fibers and resin. Additionally, alternative fiber choices, including sustainable and natural fibers, have been explored for their potential environmental benefits.

Composite hydrogen storage vessels represent a promising solution for the storage and transportation of hydrogen fuel, offering significant advantages in terms of performance, durability, and environmental impact. Continued research and development efforts in this field are essential to further optimize the design, manufacturing, and recycling processes of these vessels, ensuring their continued contribution to the advancement of hydrogen energy technologies.