Special Issue on “Liquid Hydrogen Production and Application”

1. Introduction

Globally, researchers are actively advancing the low-carbon transformation of energy systems. Due to its clean, carbon-free nature, abundant sources, and high energy density, hydrogen energy is widely regarded as a core medium for building sustainable energy systems for the future. However, the efficient management of hydrogen across variable scales and large ranges remain critical bottlenecks which are impeding its industrial development. In this context, liquid hydrogen (LH₂) has emerged as a promising solution by enabling high-density storage and transport. LH₂ has attracted increasing attention in the field of energy engineering, drawing researchers worldwide to continuously investigate various aspects of its related technologies.

This Special Issue, entitled “Liquid Hydrogen Production and Application”, brings together the latest research findings in this field. It systematically documents theoretical modeling, analytical results, and practical data related to various LH₂ technologies. The issue comprises six studies covering multiple research directions for LH₂: two-phase flow and heat transfer mechanisms [1], precooling of large-scale tanks [2], fast filling of vehicle vessels [3], gas–liquid behavior under microgravity [4], insulation and pressure control technologies [5], and leakage and protection strategies [6] for LH₂. All the included studies have undergone a rigorous peer-review process. Building on these contributions, the latest research progress regarding hydrogen liquefaction processes, key liquefaction equipment, and LH₂ applications in aerospace and civil transportation are reviewed and synthesized.

2. Liquid Hydrogen Production

The core of liquid hydrogen production lies in complex cryogenic liquefaction cycles. Common hydrogen liquefaction processes include the precooled Linde–Hampson cycle (PLH), the Claude cycle, and the reverse Brayton cycle. The operation of these hydrogen liquefaction processes relies on the coordinated function of multiple key components, such as compressors, expanders, heat exchangers, and ortho–para hydrogen conversion (OPHC) devices. Currently, LH₂ production faces dual challenges of high energy consumption and relatively low efficiency. This section provides an overview of recent research advances in hydrogen liquefaction technologies and key equipment.

2.1. Hydrogen Liquefaction Process

In recent years, research on hydrogen liquefaction technologies has been primarily directed toward enhancing energy efficiency, reducing specific energy consumption (SEC), and optimizing system integration. Investigations in this domain have been characterized by process innovation, deep integration of intelligent optimization methods, and parallel advancements in both large- and small-scale systems.

In terms of hydrogen liquefaction processes, the research focus has evolved from performance assessment of individual cycles to comparative analyses of multiple cycles and the development of innovative hybrid processes. Omori et al. [7] evaluated the influence of high-pressure conditions on energy efficiency and energy destruction distribution across equipment in conventional cycles such as the Claude, PLH, single mixed refrigerant, and dual mixed refrigerant cycles. Im et al. [8] proposed a novel large-scale liquefaction process utilizing captured carbon dioxide (CO₂) as a refrigerant, effectively integrating hydrogen liquefaction with carbon capture and utilization (CCU), and thereby demonstrating its synergistic emission reduction potential. Moreover, Kumar et al. [9] integrated thermodynamic simulation, multi-criteria decision making, and machine learning clustering to optimize mixed refrigerant composition during the precooling stage; they were thus able to significantly improve system energy efficiency and established a data-driven framework for process optimization.

In the context of large-scale hydrogen liquefaction, technological development has been oriented toward scaling up production capacity while continuously lowering SEC. In pursuit of these goals, Yang et al. [10] presented a systematic quantitative comparison and demonstrated that, for a 120 t/day liquefaction capacity, the Claude cycle exhibited marginally lower exergy destruction and SEC relative to the Brayton cycle. Liu et al. [11] developed a cascaded liquefaction process based on three mixed refrigerant cycles, and used a genetic algorithm to optimize the key parameters of the process; thereby, the authors achieved SEC values as low as 6.07 kWh/kg LH₂ and quantitatively showed that OPHC accounted for approximately 16.3% of the total energy demand.

Regarding small-scale hydrogen liquefaction technologies, Xie et al. [12] successfully designed and validated a micro-scale direct liquefaction device with a capacity of 0.5 L/h based on a two-stage G-M refrigerator; this proposed solution addressed critical engineering challenges related to thermal insulation, heat transfer, and pressure maintenance, thereby providing valuable operational insights for compact liquefaction systems. Similarly, Bi et al. [13] designed and experimentally tested an open small-scale liquefaction system employing stepwise cooling. The proposed system achieved gradual hydrogen cooling through two-stage precooling and three-stage heat transfer, offering key technical references for the design of small-scale liquefaction units.

2.2. Key Equipment for Hydrogen Liquefaction Process

For key equipment in hydrogen liquefaction processes, significant progress has been achieved in enhancing energy efficiency and advancing integrated design approaches. A notable development in this context is the integrated design of OPHC with high-efficiency heat exchangers. Lv et al. [14] developed a heat exchanger model in Aspen HYSYS that incorporates OPHC functionality, and designed a 5 t/day hydrogen liquefaction plant and derived essential process parameters using this model. In a further refinement, Tao et al. [15] performed multi-objective optimization of a catalyst-filled plate-fin heat exchanger using the response surface methodology, which led to significant improvements in both the heat transfer performance and conversion efficiency. Concurrently, research focus on OPHC has shifted from conventional isothermal and adiabatic methods toward more efficient continuous conversion techniques. Teng et al. [16] demonstrated through numerical simulations that the continuous conversion reduces the SEC to as low as 11.38 kWh/kg LH₂, which is 21.8% and 28.7% lower than the adiabatic and isothermal ones. Overall, these studies show a clear trend of evolving from the performance optimization of individual devices to system integration design and intelligent collaborative regulation.

3. Liquid Hydrogen Application

LH₂ is increasingly being utilized across multiple critical sectors, with notable advancements achieved in recent years, particularly within the aerospace and transportation fields. Meanwhile, efficient storage and safe control technologies are also key elements in promoting the application of LH₂.

3.1. LH₂ Applications in Aerospace

LH₂ is a critical propellant for space propulsion and plays an increasingly essential role in aerospace applications. To support large-scale, long-term missions, LH₂ management under microgravity conditions has become a key research priority in recent years. For orbital pressure control of LH₂ tanks, Zhou et al. [5] experimentally compared active and passive thermodynamic venting system (TVS) strategies and demonstrated that an active TVS can reduce the mass venting rate by 87.3% relative to direct venting (Contribution of this Special Issue). Zuo et al. [17] established a comprehensive computational fluid dynamics (CFD) model of the TVS, revealing that condensation of superheated vapor is the dominant mechanism driving pressure reduction. For orbital fluid management, Liang et al. [4] explored the feasibility of utilizing momentum from evaporated exhaust gas to achieve tank reorientation in space, thereby offering a novel approach to propellant attitude control and fuel conservation (Contribution of this Special Issue). Ma et al. [18] numerically investigated the LH₂ retention stability of a screen channel liquid acquisition device (SCLAD) and its outflow characteristics under acceleration disturbances. For orbital transfer and filling of LH₂, Chen et al. [2] developed a mathematical model of the precooling process for a 300 m³ LH₂ tank and recommended a stepped cooling strategy to reduce both the cooldown time and propellant consumption (Contribution of this Special Issue). Ma et al. [19] proposed a four-node model for no-vent filling of LH₂ tanks under microgravity conditions, to support the development of orbital refueling technology of cryogenic propellants.

3.2. LH₂ Applications in Transportation

Research on LH₂ technologies in the transportation sector has progressively expanded to cover multiple specific scenarios, such as hydrogen refueling stations, vehicles, and maritime shipping.

LH₂ refueling stations demonstrate great potential in terms of energy efficiency and scalability. Zhai et al. [20] introduced an innovative pump-thermal synergistic pressurization process, lowering the SEC to 0.55 kWh/kg LH₂ and effectively reducing both the initial investment and energy consumption use of the LH₂ refueling station. Furthermore, Yang et al. [21] addressed a research gap in LH₂ cascade refueling systems utilizing gas–liquid mixed precooling, provided a design framework for energy-efficient LH₂ refueling infrastructure by developing and optimizing a thermodynamic model for system configuration.

Regarding road vehicles and maritime transport, Yuan et al. [3] numerically simulated the fast-charging process of a high-pressure gaseous hydrogen vessel for fuel cell vehicles, and provided key technical support for the safety of GH₂ charging (Contribution of this Special Issue). Shen et al. [22] numerically investigated the thermodynamic behavior of vehicle-mounted LH₂ bottles under different motion states, thus providing critical insights for the dynamic safety design of vehicular hydrogen storage systems. Wang et al. [23] developed a thermodynamic model to evaluate an LH₂ carrier equipped with four 40,000 m³ spherical tanks. Their study indicated that, for relatively short journeys, the minimal pressurization of large-scale double-wall tanks is sufficient to achieve very low boil-off losses, confirming the techno-economic feasibility of maritime LH₂ transport under specific conditions.

3.3. Efficient Storage and Safety Control in LH₂ Applications

LH₂, characterized by its extremely low boiling point, high leak potential, rapid diffusivity, flammability, and explosion hazards, imposes stringent requirements on efficient storage and safety control across all application fields.

Research on LH₂ efficient storage has been increasingly focused on minimizing evaporation losses and extending storage duration. Wu et al. [24] developed a reduced-order model combining proper orthogonal decomposition and a neural network to rapidly optimize variable-density multi-layer insulation (VD-MLI) under complex constraints, achieving a lighter and higher-performance insulation solution for LH₂ tanks. Yu et al. [25] evaluated seven insulation schemes for a 4000 m³ spherical LH₂ tank using a non-constant heat flux method and found that a multi-layer configuration combining a vapor-cooled shield (VCS) with a liquid-nitrogen-cooled shield could reduce heat leakage to below 10 W. Li et al. [26] pioneered the synergistic integration of insulation systems with active TVS. Through transient modeling, they investigated the effects of various integration configurations and activation strategies of TVS and VCS on the tank’s “dormancy period”, marking the evolution of LH₂ storage technology from passive thermal insulation toward comprehensive thermal management and utilization.

The safety control of LH₂ mainly involves the prevention and risk control of such as leakage, electrostatic charging, and fire. Rong et al. [6] simulated LH₂ leakage at a refueling station and proposed an active protection strategy using an air curtain, providing a directly applicable technical measure to suppress hydrogen dispersion (Contribution of this Special Issue). Xie et al. [1] investigated the fundamental thermo-physics of the LH₂ boiling vaporization process on solid ground, revealing the non-uniform boiling behavior of LH₂ on concrete surfaces and providing essential experimental data for leakage risk assessment (Contribution of this Special Issue). Liu et al. [27] developed a theoretical model coupling charge conservation and Navier–Stokes equations, systematically revealing the electrostatic saturation behavior of long-distance LH₂ transfer. Nubli et al. [28] developed and validated a simulation model capable of accurately predicting pressure buildup in LH₂ tanks under fire exposure, delivering a critical scientific basis for designing fire and explosion prevention measures and emergency response protocols.

4. Conclusions

The production and applications of hydrogen have received increasing attention in recent years and many valuable research advancements have been achieved. This Special Issue effectively supplements existing knowledge in key areas such as the heat exchange mechanisms, aerospace and transportation applications, and safety control of liquid hydrogen, while also providing guidance for future research directions.

Regarding LH₂ production, studies have focused on the process optimization of hydrogen liquefaction, the system design for both large-scale and small-scale facilities, and OPHC devices. It is suggested that future research could focus on the following areas: enhancing the in-depth application of machine learning in the optimization and control of hydrogen liquefaction systems; constructing green, integrated hydrogen liquefaction systems that synergize with renewable energy and CCU; advancing core equipment technologies such as high-efficiency heat exchangers and cryogenic expanders; and deepening techno-economic analyses for both large-scale hydrogen liquefaction plants and small-scale liquefaction units.

With respect to LH₂ applications, microgravity propellant management for aerospace systems, LH₂ refueling stations, and LH₂-powered vehicles and ships have attracted sustained research interest. Studies on passive, active, and composite insulation schemes of LH₂, as well as its leakage behavior and electrostatic safety, have been conducted. Future efforts could focus on the following areas: theoretical and experimental studies under realistic, complex operational conditions, such as microgravity, ship motion, vehicle vibration, and extreme environments for different application scenarios; synergistic optimization of energy efficiency and economics across the entire LH₂ chain from production to application; innovation in LH₂ storage technologies towards achieving zero-boil-off storage; and the establishment of a comprehensive LH₂ safety technology framework, supported by large-scale testing and predictive modeling.