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Choose the most effective components for H2 fuel cell vehicles
Special Focus: Advances in H2 in technology
C. HAYES, Swagelok, Cleveland, Ohio
As part of the industry-wide drive to develop high-efficiency, low-emissions vehicles, manufacturers are investing significant revenue in H2 mobility and its related infrastructure. H2 fuel cell technology has proven particularly promising, particularly in regard to heavy-duty vehicle applications. These fuel cells operate by combining H2 and oxygen (O2) gas to form an electrical current. The current drives a motor, providing the necessary power and torque to move a vehicle while producing no CO2 emissions.
The quick evolution and evident potential of H2-based technologies have produced a rapidly growing market. Barriers to adoption still exist, but both government and industry leaders are working to eliminate these obstacles while putting research and development funds toward creating H2 transportation infrastructure. In Europe, for example, multiple countries have agreed to participate in the Joint Initiative for H2 Vehicles across Europe (JIVE), which is designed to bring H2-powered buses into the mainstream. The program is expected to put nearly 300 such buses on the streets of 22 European cities (FIG. 1) by the early 2020s. JIVE also aims to help manufacturers lower the cost of building H2-powered buses, making them more cost competitive with natural gas and diesel-powered vehicles. The key to reducing manufacturing costs is the use of more efficient and effective components. Choosing the right components is essential to reliable infrastructure and vehicle operation.
FIG. 1. Europe’s JIVE project aims to promote the manufacture and use of H2-powered buses
Material selection.
Though corrosion prevention is important wherever tube fittings are involved, H2 containment is uniquely challenging. H2 gas can diffuse into metal; this diffusion can change the metal’s mechanical properties, reducing its ductility, impact strength, fracture toughness and resistance to failure by fatigue (FIG. 2). This results in the phenomenon known as H2 embrittlement. Metal that has been compromised in this way is more likely to fail when subjected to significant static or cyclic tensile strength.
FIG. 2. This H2 embrittled diaphragma shows evidence of intergranular corrosion that could lead to its failure.
H2 embrittlement occurs when H2 atoms, which are smaller than those of any other element, lodge in the cubic lattice network of the metal. This weakens the metal’s molecular bonds and compromises its integrity. In certain applications, H2 embrittlement can cause lesser grades of stainless steel to behave like cast iron: extremely brittle and susceptible to cracking. To prevent weakening of tubing and other components used for H2 gas fueling systems, designers of H2 infrastructure must specify materials that resist the infusion of H2 atoms, such as austenitic stainless steels (e.g., 316/316L stainless steel and 6-moly) that contain between 10% and 30% nickel.
Performance under pressure.
Though leak prevention is important in all fluid systems, it is critical in the componentry of H2 vehicles and infrastructure. H2 storage occurs under high pressure—there is a direct correlation between H2 storage pressure and the expected range of the vehicle that H2 is intended to fuel. Short-range vehicle fleets often use H2 stored at 350 bar because they frequently return to a central location for refueling. By contrast, long-range fleets require the use of H2 stored at 700 bar because manufacturers are aiming for a range of ~400 mi (~644 km) without refueling.
It is crucial to work with manufacturers that have completed a product certification process to remain in compliance with local laws and regulations. In Europe, for example, standard fittings do not meet the stringent requirements for H2 fixtures set out in Regulation EC 79. The higher the pressure, the higher a component’s performance must be. Fittings and connections used in H2 fuel cell components must handle both the high pressures required for H2 storage and the continuous vibrations created by a moving vehicle. Some manufacturers have addressed this challenge by developing fittings with a twin ferrule design intended to provide leak-tight performance despite constant vibration and mitigate the danger of components separating under vibrational stress.
Avoiding leaks: Efficiency and safety.
Even the smallest H2 leak can cause significant issues for both vehicle function and safety. Twin ferrule compression fittings (FIG. 3) have the lowest risk of developing leaks when compared with either traditional cone and thread or O-ring face seal fittings—of the latter two, one is connected manually (meaning an increased chance of developing leaks), and the other requires additional specialized tools and multiple steps to install (meaning a longer installation time and more points at which an error could occur). To install a twin ferrule compression fitting, an assembler only needs to pre-swage a nut, place the ferrules onto the tubing, and tighten these components by turning. The action of the two ferrules securely grips the tube as the compression fittings are tightened; the back ferrule provides direct and axial support to the tube gripping function. This results in a more efficient installation and provides leak-tight performance, enhancing overall system safety and saving on time and labor.
FIG. 3. A twin ferrule tube fitting. The hardened back ferrule (light gray) provides direct and axial support.
Application-specific components.
Different designs throughout the H2 fueling ecosystem require different component configurations. The H2-powered municipal bus fleets referenced earlier in this article are an example—because the buses have H2 storage tanks on their roofs, they require a more flexible construction than typical long-haul vehicles. Reliable hoses that can handle 350 bar (5,000 psi) are therefore a better choice for bus applications than rigid steel tubing.
As with the choice of what steel formulation to use for tubes and fittings, not all hoses are created equal. For example, designers are advised not to choose polymer-core hoses with metal overbraiding since they will not stand up to the rigors of this application.
Ongoing training.
Since H2 fuel systems contain hazardous, high-pressure gas, training installers properly in fuel delivery system assembly is crucial. They must understand system inspection protocols, as well as proper tube fitting installation to prevent safety incidents. Well-conducted training should emphasize the following:
- Correct and efficient installation of fittings
- Tube selection
- Tube bending (particularly important, as the systems are manufactured to tight tolerances)
- Tightening fittings by hand
- Verifying the installation using a gap inspection gauge (FIG. 4).
It is recommended to take advantage of comprehensive tube fitting training courses provided by some manufacturers. Likewise, fuel delivery system inspectors should attend manufacturer-sponsored training on how to pressure test a system and how to identify common installation errors and faults. Inspectors should perform functional tests at varying pressures to check for leaks and ensure the system’s overall integrity.
Working with appropriate partners.
Utilizing suppliers with relevant expertise—in component and materials science, working with gaseous systems in general and working with H2 specifically—will allow manufacturers and designers to create leak-tight, easy-to-assemble H2 fuel cell systems capable of revolutionizing the transport industry, enabling the full potential of long-range H2 transportation to be reached.H2T
NOTES
a Inconel® X-750 diaphragm
CHUCK HAYES is Principal Engineer for Swagelok Company.
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