As a clean energy source, LH₂ is poised to play a pivotal role in future energy supplies. Currently, international hydrogen liquefaction facilities suffer from high energy consumption, high liquefaction costs, and low exergy efficiencies. In contrast, the development of cryogenic hydrogen liquefaction technologies and equipment in China is still in its infancy, significantly lagging behind advanced global standards. Against this backdrop, this paper summarizes the recent research advancements in hydrogen liquefaction technology, encompassing both process design and practical facilities. It delves into the latest developments in steady-state process simulation and dynamic characteristic studies, evaluating performance metrics across various liquefaction processes. Additionally, this paper provides an overview of the technical features and equipment layouts of large-scale hydrogen liquefaction plants and small-scale laboratory setups. Finally, it consolidates the key development priorities and future directions for hydrogen liquefaction technology, aiming to provide valuable guidance for technological progress and accelerate the widespread adoption of hydrogen energy.

An electric vertical take-off and landing flying vehicle (eVTOL) is a potential technology for future urban air mobility. A major challenge for thermal management systems is the high cooling requirement and the variable application scenarios. To overcome this challenge, a multi-scene eVTOL-integrated thermal management system was developed. In this study, an eVTOL thermal management simulation platform based on Amesim simulation software was developed to investigate the effects of flight conditions on thermal management and range. The simulation results show that increasing the cruise altitude can reduce the thermal management energy consumption when the ground temperature is high. The maximum reduction of energy consumption for thermal management energy is 4 kW when the cruising temperature ranges from 10 ℃ to 26 ℃. When the hovering rescue duration is more than 150 s during the emergency rescue operation, the temperature difference inside the battery becomes too pronounced. A reduced payload improves the range, with the unloaded range being 1.33 times greater than the fully loaded range.

R1270 and R290 are alternative refrigerants with great potential. In this study, the influence of the refrigerant charge on the performance of the heat-pump water heater with R1270 and R290, the temperature distribution of the heat transfer fluids in the condenser, and the feasibility of replacing R22 with natural refrigerants were investigated. The results indicate that the refrigerant charge has a considerable influence on the cycle performance and the temperature distribution of the heat-transfer fluids in both systems. At the same optimal charge (0.90 kg), the R1270 and R290 systems achieved a maximum coefficient of performance (COP) of 4.443 and 4.317, respectively. At different refrigerant charges, two heat transfer pinch points and two maximum heat transfer temperature differences occurred in the condensers of both systems, and the locations of the first pinch point and the second maximum temperature difference point showed similar migrations with changes in refrigerant charge. Compared with the R22 system, both the R1270 and R290 systems, at optimal charge, achieved significantly better COP and discharge temperatures, exhibited relatively equivalent discharge pressures, and their heating capacities increased by 7.05% and decreased by 10.65%, respectively. Hence, R1270 can be preferred over R290 for replacing R22 in a heat-pump water heater.

This study investigates the in-orbit liquid hydrogen management capability of screen channel tanks in cryogenic propulsion systems by developing a three-dimensional multiphysics model that integrates filling ratios (5%-50%) and microgravity disturbances (10^-3 g). The competition mechanism between capillary and inertial forces, as well as the fluid retention stability during tank reorientation, was systematically analyzed. The key findings include the following. The fluid retention capability was attributed to the structural synergy between liquid collection channels and tank walls, ensuring continuous liquid coverage at the channel inlets under all operating conditions. At low filling ratios, surface tension dominated the phase distribution with a quasi-static interfacial evolution. Increased filling enhanced inertial forces, inducing phase oscillations via momentum transport. The directional sensitivity analysis revealed that bottom acceleration induced the largest centroid depression, top acceleration had a minimal impact on the relative centroid height, and lateral disturbances caused larger centroid oscillation amplitudes and higher frequencies than oblique lateral disturbances.

Heat pump technology is an energy-saving solution that could potentially combat global warming and reduce carbon emissions. Industrial heat pumps recover waste heat from the heating process to heat water or air, thereby reducing electricity consumption and carbon emissions. Industrial heat pumps are energy-saving, environmentally friendly, and provide stable heating. They have been widely used at all stages of production and life. This study analyzes the compressor types and characteristics of domestic and international high-temperature heat pumps (HTHP) that recover industrial waste heat and analyzes their application and technical status with a focus on twin-screw and centrifugal compressors. Twin-screw heat pump compressors should adopt an open structure when the evaporation temperature is high, and high-speed and oil-free design can be used when the condensation temperature is high. In addition, centrifugal heat pump compressors should prioritize highly efficient impellers, high-temperature-resistant motors, and oil-free lubricated bearings. Screw steam compressors need to solve the problems of rotor thermal deformation and shaft seal, and target for large temperature lifts.

In cryogenic hydrogen storage systems, an accurate calculation method for the heat release from different ortho-para hydrogen catalytic conversions is important to determine the load of hydrogen storage systems. The objective of this study is to present a precise calculation method for the conversion heat of ortho-para hydrogen. By establishing a conversion model, the methods of using a smoothing spline curve to fit the experimental data and energy balance calculations are used to derive the released heat in adiabatic conversion multistage converters and the released heat during continuous conversion. The heat release properties of different conversion methods are analyzed in this study. Notably, the heat release amount of continuous conversion is the smallest, that of isothermal conversion is the largest, and the conversion heat of adiabatic conversion is in between, which is related to the number of conversion stages. In addition, a specific method and procedure are programmed to solve the implicit differential formula for continuous conversion. Finally, the calculation results of the different methods are consistent with a maximum deviation of only 0.22%, indicating that the calculations in this study are valid and accurate.

Improving indoor air quality in homes requires fresh air; however, this is a strain on air conditioning systems. To address this issue, the use of energy-efficient fresh air units equipped with exhaust air heat recovery is recommended. These units include both passive and active types, with prominent examples being air-to-air enthalpy heat exchangers and heat pump units. Currently, the evaluation of the energy efficiency in fresh air units predominantly revolves around air-to-air enthalpy heat exchangers, rendering the commonly used heat exchange efficiency inapplicable to heat pump units. The concept of exhaust air heat recovery is perplexing and contradictory. Furthermore, the assessment of fresh air units primarily focuses on the units themselves, without considering their impact on air conditioning units and the overall system performance once combined. This study aims to establish a unified definition of exhaust air heat recovery for fresh air units, elucidating its intrinsic meaning. Additionally, it proposes a comprehensive energy efficiency evaluation method for a combined fresh air and air conditioning system. Through a case study of seven existing fresh air unit types, the necessity of exhaust air heat recovery is highlighted, and the energy efficiency levels of different unit types are compared.

Mechanical vapor compression (MVC) systems are energy-saving technologies that recover and reuse low-temperature waste heat resources, achieving energy conservation and carbon reduction. As the core equipment in MVC systems, the compressor directly affects the overall performance of the system. This article primarily reviews the thermodynamic and structural performance of vapor compressors, proposes relevant enhancement suggestions and improvement ideas, and provides a reference and assistance for the subsequent optimization of vapor compressor performance.

An energy-saving optimization operation strategy based on maintaining the high-energy-efficiency operation of chillers is proposed to address the prevalent issue of increased energy consumption in the application of cold-storage technology for economic optimization in the current air-conditioning industry. This strategy involves the storage and release of cooling using small cold-storage tanks to actively control the load ratio of the water chiller, thus ensuring that the unit operates efficiently for an extended period to achieve energy savings. A physical model of the air-conditioning system is established and simulated using operational data from the central air-conditioning system of a hospital. Performance curves of the water chiller under different environmental conditions are obtained to accurately depict the high-efficiency operational states of the unit at each moment. On a typical day with a peak cooling load of 9 979 kW, using an active chilled-water storage system, as compared with using the conventional chiller operation strategy without active storage, can reduce 2 777 kW·h of daily electricity consumption, which constitutes 6.0% of the daily electricity usage of the central air-conditioning system. Over the entire cooling season, this approach can save 2.35% of the total electricity consumption of the central air-conditioning system and 4.45% of the electricity consumption of the chiller.

The combination of a super-long gravity heat pipe and a heat pump system for harvesting deep geothermal heat has the advantages of low cost, high efficiency, and no groundwater contamination. Direct heat exchange between the evaporator of the heat pump system and the condenser of the gravity heat pipe can simplify the heat exchange process and improve the heating efficiency of the system. Therefore, a U-shaped evaporator-condenser was developed, and its heat transfer performance was studied by building an experimental platform combining a heat pump and a heat pipe. Notably, the heat transfer coefficient of the U-shaped evaporator-condenser reached 2 037.92 W/(m²·℃) when the working fluid on the heat pump side passed through the tube. Based on the homogeneous flow model, a one-dimensional steady-state evaporator-condenser heat transfer model was established by integrating the mass, energy, and momentum conservation equations with empirical formulas for condensation outside the tube and boiling heat transfer inside the tube. Using Python, the simulation results were compared with experimental data. Notably, the average deviation of the heat transfer in the evaporator-condenser was 18.91%, confirming the accuracy of the model and providing a theoretical calculation method for designing an efficient evaporator-condenser.