AMF #21 - Innovative Designs for Liquid Hydrogen (LH?) Containment Systems: A Technical Overview
Muammer Akturk
Advancing a Greener Future: Specializing in Alternative Fuels, Decarbonization, and Gas Carriers
1. INTRODUCTION
The storage of hydrogen in its liquid state necessitates the maintenance of an ultra-cryogenic environment at approximately -253°C, significantly below the boiling point of LNG. This extreme temperature requirement demands the implementation of cutting-edge insulation technologies, such as vacuum insulation and Multi-Layer Insulation (MLI), at their most advanced level. The utilization of conventional insulation materials would necessitate an excessively thick insulation layer, further exacerbating the challenges posed by hydrogen's inherently low volumetric energy density.
To address these unique challenges, cargo containment systems for hydrogen are typically designed with a dual-layer insulation structure: an inner and an outer insulation layer. The inner layer spans the temperature range from near-cryogenic (close to hydrogen's boiling point) to slightly above oxygen's boiling point, while the outer layer manages the thermal gradient from this point to ambient temperature. This ensures that the extreme cold of liquid hydrogen does not compromise the integrity of the ship’s structure or pose operational hazards.
Vacuum insulation is generally preferred for the inner insulation layer due to its superior thermal resistance, however its application in large-scale shipboard cargo containment systems presents significant structural challenges. As the size of a vacuum vessel increases, the demands on its structural integrity become increasingly severe. Consequently, there is a pressing need for alternative insulation systems for larger containment structures. Hybrid insulation techniques, such as aerogel composites combined with MLI and vacuum spaces, are being explored to enhance thermal efficiency while maintaining structural feasibility.
In response to these challenges, ongoing research and development efforts are focused on creating large-capacity liquefied hydrogen carriers with innovative cargo containment systems. These designs incorporate materials such as polyurethane foam for the inner insulation layers within dedicated inner insulation spaces. To prevent condensation on the inner shell, these spaces are typically filled with gas such as hydrogen gas, which minimizes convective heat transfer and maintains pressure stability.
The development of these novel containment systems necessitated the establishment of new safety standards. To this end, the International Maritime Organization (IMO) developed the Interim Recommendations, as outlined in resolution MSC.420(97). These recommendations were crafted to facilitate the creation of tripartite agreements for pilot ships, specifically designed for researching and demonstrating the safe long-distance overseas transport of bulk liquefied hydrogen. The Interim Recommendations initially specified requirements for cargo containment systems utilizing vacuum insulation. However, recognizing the interest for larger hydrogen storage capacities, several initiatives have been undertaken to expand these recommendations to encompass new types of cargo containment systems, ensuring that safety requirements keep pace with technological advancements in the field. Additionally, fatigue assessment models for materials exposed to ultra-cryogenic conditions are under development to enhance regulatory compliance and operational safety.
2. CONTAINMENT SYSTEM DESIGN AND CONSIDERATIONS FOR LIQUID HYDROGEN STORAGE
The design of liquid hydrogen (LH?) storage tanks for maritime applications presents a multifaceted engineering challenge. These systems must not only provide exceptional insulation performance but also demonstrate resilience against the dynamic loads imposed by vessel motions. As integral components of a ship's fuel system, these tanks must address several operational factors:
To effectively and safely utilize LH? for large scale marine applications, innovative designs focusing on enhanced thermal insulation performance are being explored. These advancements aim to minimize boil-off and maintain the cryogenic state of the hydrogen more efficiently. Additionally, the development of advanced composite materials for cryogenic storage, such as carbon fiber-reinforced polymer (CFRP) structures with integrated nanotechnology-enhanced insulation layers, is under investigation.
Concurrently, research is being conducted on the feasibility of incorporating newly developed stainless-steel alloys with superior yield strength and austenite stability for LH? containment. These materials show promise in significantly reducing the overall weight of fuel tanks, a critical factor in vessel design and performance optimization. Cryogenic testing and long-term hydrogen exposure evaluations are essential to qualify these materials for marine applications.
3. NOVEL DESIGN SYSTEMS FOR LH? STORAGE
Several innovative designs have been developed to enhance the efficiency and safety of LH? storage systems for maritime applications, such as:
3.1. Optimized Support Structure
The primary heat ingress in LH? tanks is considered to occur at the interconnection points between inner and outer shells. To mitigate this, a novel support structure has been proposed using topology optimization. This method is intended to maximize structural stiffness while minimizing material usage, thereby reducing thermal conduction pathways. The design is based on G10 composite material, known for its low thermal conductivity, in conjunction with stainless steel structures. Advanced computational fluid dynamics (CFD) simulations have been examined to optimize heat flux distribution and mechanical stress points in support structures.
3.2. Specialized Baffle Design
Unlike stationary LH? storage, shipboard tanks must contend with dynamic conditions such as varying fill levels and sloshing-induced forces. To address these challenges, a specialized baffle has been considered to account for LH?'s unique properties. This baffle design is intended to minimize liquid movement across various fuel levels and ship motions, thereby reducing vaporization due to sloshing. Computational modeling and physical testing in cryogenic environments are regarded as essential for evaluating baffle efficiency.
3.3. Vapor Cooled Shield (VCS)
Given the extended operational periods of ships compared to other modes of transport, continuous hydrogen supply to the propulsion system is crucial. The VCS system is intended to address the challenge of warming vaporized hydrogen (initially at -253°C) to the ambient temperatures required by propulsion systems like PEMFCs. The VCS, installed within the vacuum insulation layer, is designed to utilize high thermal conductivity copper to transfer penetrating heat to the cold vaporized hydrogen as it flows to the propulsion system. A spiral-shaped vent pipe design is considered to increase contact area, enhancing performance. Performance evaluations using LH? have been conducted to assess the VCS's effectiveness, with discharged hydrogen temperatures reaching approximately -160°C. This system not only aims to improve insulation performance but also seeks to reduce the size and capacity requirements of vaporizers.
3.4. Advanced Material Selection
STS316LH, a novel stainless-steel alloy, has been tested for LH? storage applications. This material exhibits approximately 1.8 times higher yield strength than conventional STS316L, allowing for significant weight reduction in fuel tanks. Moreover, STS316LH demonstrates enhanced austenite stability compared to STS316L, effectively mitigating concerns related to hydrogen embrittlement in cryogenic environments. Additionally, research into alternative alloys, such as aluminum-lithium composites, is ongoing to further reduce tank weight while maintaining durability under cryogenic conditions.
4. MEMBRANE CARGO CONTAINMENT SYSTEMS FOR LIQUEFIED HYDROGEN
Research and development on membrane-type cargo containment systems for LH? is currently being pursued by several organizations. These systems are considered a promising solution for large-scale LH? carriers, drawing parallels with their successful application in the LNG industry. However, the implementation of membrane systems for LH? containment necessitates the establishment and adherence to appropriate safety requirements.
A major shipbuilding company has made significant progress in this field for its membrane-type LH? containment system with plans for a pilot ship project utilizing this containment system within the next few years. Advanced numerical simulations and cryogenic impact resistance tests are being conducted to evaluate membrane containment performance
A key distinguishing feature of LH? membrane containment systems, compared to their LNG counterparts, is the operation of the inter-barrier space in a vacuum environment. The IGC Code stipulates that primary and secondary insulation spaces of membrane cargo containment systems should be inerted with a suitable dry inert gas. However, for LH? containment systems, such inerting is not practically feasible for the inter-barrier space (primary insulation space) due to the ultra-low temperature of the cargo.
Consequently, maintaining the area under vacuum instead of using dry inert gas has been proposed. This operational method is not currently addressed in either the existing IGC Code or the Interim recommendations for the carriage of liquefied hydrogen in bulk (resolution MSC.420(97)) for membrane-type cargo containment systems.
The membrane cargo containment system for LH?, as illustrated in Figure 2, comprises thin liquid and gastight layers (primary and secondary barriers) supported through insulation by the adjacent hull structure. The primary and secondary insulation spaces are predominantly filled with insulation materials. The primary insulation space is maintained under vacuum during system operation, while dry inert gas is circulated throughout the secondary insulation space.
The primary purpose of maintaining a vacuum in the primary insulation space is to prevent condensation and/or solidification of gas components due to the ultra-low temperature of LH?. Only hydrogen or helium gas can be used in this space, as gases with higher boiling points are unsuitable. However, hydrogen's flammability and helium's limited commercial availability pose challenges as well.
The secondary insulation space, experiencing relatively lower heat conduction from the hydrogen cargo, is typically filled with circulating dry inert gas during normal operation. However, in the event of significant hydrogen leakage (exceeding vacuum pump capacity) detected in the primary insulation space, the secondary space should be switched to vacuum mode. Alternatively, the space may be operated in vacuum mode from the outset.
Given the complexity of these novel designs, comprehensive evaluations considering multiple factors are essential for their successful implementation and safe operation.
5. SAFETY ASSESSMENT
Membrane containment systems employ a vacuum environment, necessitating additional safety measures compared to any traditional design, such as:
6. STRUCTURAL INTEGRITY ASSESSMENT
A comprehensive strength assessment is must for all components of the cargo containment system, including:
This assessment must account for:
Advanced FEA and cryogenic material testing should be employed to ensure the system's resilience under all operating conditions. Large-scale validation tests simulating real shipboard conditions may also be required to be demonstrated.
7. VACUUM CONTROL SAFETY MEASURES
7.1 Vacuum Degree Specification: The required vacuum level must be precisely determined and approved by the relevant authorities, such as Administration and Class Societies. Factors influencing this specification may include:
7.2 Pipe Routing Through Insulation Spaces: Some recommendations have been made on this issue, proposing that liquid and gas hydrogen pipes may pass through insulation spaces, provided that:
7.3 Vacuum Monitoring and Maintenance: A sophisticated monitoring system should be implemented to:
7.4 Atmosphere Management Protocol: To mitigate the risk of flammable mixture formation:
8. EMERGENCY CONTROL PROTOCOLS
8.1. Primary Barrier Leakage Management
Detection Methods:
Response Protocol:
8.2. Vacuum Loss Due to External Leakage
8.3. Secondary Insulation Space Vacuum Loss
9. Final Thoughts
The advancement of LH? containment systems marks a significant step toward sustainable maritime solutions. As the industry explores hydrogen as a viable fuel, maintaining ultra-cryogenic conditions remains a critical challenge, necessitating continuous improvements in insulation efficiency, material durability, and structural integrity. Hybrid insulation techniques and optimized structural designs are being examined to enhance performance while minimizing risks such as sloshing and heat ingress.
Safety remains at the core of LH? adoption, requiring refinements to regulatory frameworks and the integration of advanced monitoring systems. The shift toward membrane-type containment systems introduces new operational complexities, particularly in vacuum management and leakage detection, underscoring the need for further research and real-world trials. As industry stakeholders work to align technological advancements with evolving regulations, collaboration between shipbuilders, classification societies, and hydrogen technology developers will be key to ensuring reliability and scalability.
Looking ahead, the maritime sector must focus on bridging the gap between technological advancements and regulatory alignment. Collaborative efforts between stakeholders will be essential in refining best practices for LH? containment and transportation.
While substantial progress has been made, the full-scale commercialization of large capacity LH? storage in shipping requires continued investment in research and development. Efforts to improve cost efficiency, scalability, and regulatory adaptation will determine the pace at which large capacity containment systems become a mainstream reality.
Disclaimer: This article reflects the author's personal views and does not represent ABS in any way. It is not official communication from ABS, and the information here should not be taken as professional or legal advice.
Director, Head of Global Gas Solutions at American Bureau of Shipping
2 周Quite insightful review of LH2 containment systems and their challenges. Let’s not forget that the biggest challenge for LH2 transportation by sea is mainly commercial. Basically, the percentage of energy developed to produce hydrogen, then liquefying it, then maintaining it in liquid condition contained ie adressing the fugitive leakages almost equals the energy contained in the LH2 as cargo making it very expensive compared with other forms of energy. The best yield, so far, in the hydrogen value chain, is to use it next to its production sites for local communities.
Senior Manager Operation
2 周Insightful. Thank you.