Key Technologies for Composite Hydrogen Storage Bottles

The composite material hydrogen storage cylinder includes an inner lining material, a transition layer, a fiber winding layer, an outer protective layer, and a buffer layer from inside to outside. The filling cycle of hydrogen storage cylinders may be relatively long, and hydrogen has strong permeability under high pressure. Therefore, the lining material of hydrogen storage tanks should have good barrier function to ensure that most of the gas can be stored in the container.
 

The fiber winding layer uses carbon fiber as the reinforcing material. The composite gas cylinder made of high-strength and high modulus carbon fiber material through winding molding technology not only has a reasonable structure and light weight, but also has good processability and designability, which has broad application space in the preparation of hydrogen storage gas cylinders. The key technologies involved in the preparation of high-pressure hydrogen storage cylinders are summarized as follows:
 

1. Inner liner design technology
In the traditional strength design of aluminum inner fully wrapped gas cylinders, the load-bearing capacity of the inner cylinder is generally not considered, and theoretically, the internal pressure of the gas cylinder is completely borne by the reinforcing fibers. But in fact, the inner liner of the gas cylinder is always under tensile stress under working pressure, which is the key factor restricting the fatigue life of the gas cylinder. Choosing the appropriate shape and size of the inner liner is of great significance in order to meet the requirements of lightweight and good fatigue resistance of hydrogen storage cylinders.
 

The innovative and efficient layering mode of the Toyota 2nd generation plastic liner configuration results in a reduction of approximately 40% in carbon fiber usage

2. Interface connection technology between carbon fiber and resin matrix
The interface between carbon fiber and resin matrix is the key factor affecting the performance of composite materials, and interface debonding is one of the main failure modes of composite materials. Due to the significantly higher tensile strength and modulus of carbon fiber compared to resin matrix, carbon fiber serves as the main load-bearing structural material of composite materials, and the interface effect is reflected in the effective transfer of external loads to the carbon fiber after being applied to the resin matrix. There are high requirements for composite material interface technology in high-pressure hydrogen storage tanks due to their compressive and explosion-proof properties.
 

3. Fiber winding forming technology
The carbon fiber winding process can be divided into wet winding and dry winding. Among them, wet winding is widely used due to its low cost and good processability. Wet winding equipment mainly includes fiber frame, tension control equipment, dipping groove, spinning nozzle, and rotating core mold structure. The internationally advanced six dimensional winding technology can effectively control the fiber direction, achieving a combination of circumferential winding, rotary winding, and planar winding. In actual production, a combination of rotary winding and circumferential winding is often used. Circular winding can eliminate the circumferential stress caused by internal pressure on the gas cylinder, while rotary winding can provide longitudinal stress and improve the overall performance of the gas cylinder.
 

The design of fiber winding layer needs to consider the anisotropy of fibers. According to its structural requirements, layer theory and mesh theory are usually used to calculate the stress distribution of container head, inner lining, and fiber winding layer, and then determine the tension selection and linear distribution in the winding process. By alternating between circumferential winding and rotary winding to achieve a multi-level structure, selecting appropriate fiber stacking area, longitudinal winding angle, and rotary winding line type, not only can the strength requirements be met, but also the head can be reasonably laid.
 

4. Tension control technology for fiber winding forming
Reasonable use of tension control system is required in the winding forming process to ensure that the designed line shape can be correctly laid and the fiber content can be controlled. By reasonably controlling the winding tension, the compactness of the product can be improved, thereby leveraging the high strength and high modulus characteristics of the fibers, enhancing the product's resistance to internal pressure, and improving its fatigue resistance.
 

When using high tension, the fiber content can be increased, but high tension can cause the outer fibers to compress the inner fibers, reducing the adhesive content and affecting performance; Choosing a lower tension can lead to a decrease in the density of the gas cylinder, as well as the formation of bubbles and defects. Choosing the appropriate tension is one of the key points of winding forming technology. During the winding process, it is also necessary to follow the principle of decreasing tension. As the number of winding layers increases, the tension should be continuously reduced to avoid excessive tension on the outer fibers, which will compress and twist the inner fibers, prevent internal tightness and external looseness, and ensure that each layer of fibers can be uniformly stressed.
 

5. Preparation technology of high-performance resin matrix with high toughness and fatigue resistance
The resin matrix of carbon fiber hydrogen storage cylinders not only needs to meet the mechanical strength and toughness requirements of the cylinders, but also needs a high-strength, fatigue resistant resin system to ensure the service life of the cylinders due to the easy fatigue damage of the matrix in long-term inflation and deflation environments. The resin matrix used in wet winding molding not only needs to meet the corresponding performance requirements, but also requires a lower initial viscosity at the working temperature and a longer applicable period at that temperature. Epoxy resin has excellent mechanical and heat resistance properties, simple and diverse curing processes, and great room for modification. It is widely available and reasonably priced, making it suitable for wet winding process systems. The research on epoxy resin in China is quite mature, and resin systems suitable for different fiber interfaces and meeting corresponding application conditions can be produced. The interface adhesion and stress transmission ability between the resin matrix and fibers can be determined through NOL ring testing.