Displacer-type pulse-tube cryocoolers use a displacer for phase adjustment and recovery of acoustic power, leading to high refrigeration efficiency. Despite their potential, there is limited research on highly efficient displacer-type pulse-tube cryocoolers with large cooling capacities near -100 ℃. This paper presents the design of a 100-watt displacer-type pulse-tube cryocooler for low-temperature freezers and evaluates its performance, focusing on displacement motion and compressor efficiency. A validated numerical model was employed to analyze the coupling between the pulse tube and compressor and the internal phase relationship of the cold finger. Results show that under operating conditions of a 3.0 MPa charging pressure, 64.9 Hz frequency, 20 ℃ cooling water temperature, and 500 W input power, the cryocooler achieved a cooling capacity of 160.3 W at -100 ℃ with a relative Carnot efficiency of 22.2%. The displacer led the compressor piston by 59°, the internal phase distribution of the pulse tube was optimal, and the compressor exhibited a high efficiency of 78%. This cryocooler is the most efficient displacer-type pulse-tube cryocooler in its temperature range.

Transcritical CO₂ heat pump air-conditioning systems have gained prominence in new energy vehicle thermal management due to their energy-saving and environmentally friendly characteristics. However, the relatively low coefficient of performance (COP) in cooling mode remains a significant obstacle to developing transcritical CO₂ heat pump air conditioning systems. To enhance system performance, five technical approaches are proposed: internal heat exchangers (IHX), expanders, vortex tubes, ejectors, and combined multiple evaporation steps with vapor injection. The performances of these methods were evaluated through one-dimensional theoretical calculations under vehicle operating conditions. Results indicate that optimizing discharge pressure is critical for all methods, with varying degrees of COP improvement. Expanders provide the most comprehensive benefits, ejectors perform well under specific design conditions, IHX shows notable enhancements in cooling mode, and vortex tubes and combined multiple evaporation steps with vapor injection exhibit broad adaptability across working conditions. These findings offer valuable insights for practical engineering applications and support the adoption of transcritical CO₂ heat pump systems in new energy vehicles.

Battery thermal management systems are crucial components of pure electric vehicles. The promising application of liquid immersion technology in electronic equipment has also garnered increasing attention for its potential in battery thermal management. Power battery immersion liquid-cooling technology involves directly immersing the battery in dielectric liquid to dissipate heat through convection or phase-change heat transfer. This study analyzes the impact of temperature on battery performance and compares the advantages and limitations of different thermal management systems. The importance of immersion-based battery thermal management is emphasized. Key technical challenges and recent research advancements are reviewed in detail, including coolant selection, module design, and considerations for battery life and safety. Finally, commercially developed immersion cooling products for demonstration and exploration are introduced.

The rapid development of microelectronic devices has driven a trend toward miniaturized and lightweight electronic devices with high heat flux. Porous structures are increasingly used in heat dissipation due to their ability to expand the heat transfer area, enhance nucleation sites for boiling, and regulate surface wettability, significantly improving boiling heat transfer. Microchannel heat dissipation technology based on porous structures has emerged as an effective and promising method to enhance heat sink performance. Recent advancements highlight three common configurations: porous structures on microchannel surfaces, porous materials within microchannels, and porous microchannel skeletons. These structures encompass coatings, microcavities, metal foams, porous fins, and ribs. This article reviews progress in microchannel heat dissipation using porous structures, evaluates the benefits and drawbacks of these configurations, addresses challenges such as balancing heat transfer and pressure drop, and proposes optimization strategies to overcome these issues.

In northern China, public buildings commonly use direct-expansion air-conditioning systems for cooling and district heating networks for heating, necessitating separate terminals for each function. This study proposes a multi-connected air conditioning system capable of utilizing refrigerants and water for direct heating with hot water from district heating systems. The system integrates three-fluid heat exchangers within indoor units, enabling seamless switching between direct-expansion air-conditioning and district heating systems. Using an office building in Beijing as a case study, the system was evaluated under summer cooling design conditions, and its heating performance during winter and transitional seasons was analyzed. Results reveal that during winter, the system requires user-side water temperatures below 54 ℃, while in transitional seasons, the direct-expansion mode delivers unit capacities exceeding peak heating demands with energy efficiency surpassing 3.5 over 49% of operating hours. This system simplifies existing configurations by providing a single terminal for year-round heating and cooling, enhancing efficiency and thermal comfort.

Achieving low-carbon combined cooling and heating supply in distributed areas away from centralized cooling and heating networks is highly significant in the context of carbon neutrality. This study proposes a combined cooling and heating system based on an absorption heat pump, which uses a variety of clean and renewable energies, such as solar heat, geothermal, waste heat, biomass, and air-source energy, to achieve the combined cooling and heating in a wide temperature range from -20 ℃ to 90 ℃. Such systems are suitable for distributed areas, such as villages, cities, and industrial parks. The system model was constructed based on Aspen, and a prototype was developed. The prototype uses a vacuum tube collector to capture solar thermal energy and introduces natural gas as a supplementary heat source to balance fluctuations of solar energy. Multiple sets of indoor heating and cooling terminals can be driven through medium circulation and valve switching using a single set of absorption heat pumps and outdoor units. The environmental test of the prototype was performed in Jinan, and the solar thermal ratio reached 35% during the testing period. An all-weather stable energy supply was achieved by proportional control of natural gas. Moreover, a wide range of concentration adjustments was achieved by controlling the liquid level in the solution tank, enabling efficient system operation in a wider temperature range. The coefficient of performance (COP) of cooling reached 0.30-0.43 at -20 ℃ and 0.70-0.78 at 7 ℃, with cooling water temperatures varying from 30 ℃ to 20 ℃; the COP of heating reached 1.40-1.90 at 45 ℃ and 1.35-1.56 at 80 ℃, with evaporation temperature varying from -15 ℃ to 20 ℃. The study results demonstrated that introducing solar thermal energy and ambient energy recovery increased the fraction of renewable energy in the system to over 50%. Compared with the traditional method of gas furnace plus air conditioning, the annual operating cost and carbon emissions of the proposed system were reduced by over 54.3% and 44%, respectively, which has significant application potential.

HP-1 is an eco-friendly hydrofluoroolefin (HFO) refrigerant with favorable thermodynamic and environmental properties. The critical parameters and saturated vapor pressure of HP-1 are similar to those of R245fa, with HP-1 serving as a potential replacement for R245fa, which has a high global warming potential (GWP) in high-temperature heat pumps. The flammability, solubility, and material compatibility of HP-1 were mainly determined experimentally, with the results demonstrating that HP-1 has a flammability range of 9.75%-16.1%, exhibiting excellent solubility with MK220 lubricating oil at elevated temperatures, and good compatibility with materials used in high-temperature heat pump systems. When HP-1 is applied to high-temperature heat pump units, the condensing temperature of the unit can reach up to 125 ℃ when the evaporating temperature ranges between 50 ℃ and 70 ℃, with a heating capacity of 99.27-153.14 kW and a coefficient of performance (COP) of 2.25-4.85.

The two-phase flow pattern of hydrocarbon working fluids on the shell side of a helically baffled heat exchanger for liquefied natural gas determines its heat transfer performance. This study tested the two-phase flow patterns of propane and ethane/propane mixtures on the shell side of a helically baffled heat exchanger using a visualization experimental method. The test results demonstrated that with the increase in vapor quality, the experimental observations sequentially included stratified flow, stratified-spray flow, and spray flow; as the mass flux of propane increased from 20 kg/(m²·s) to 40 kg/(m²·s), the transition vapor quality from stratified flow to stratified-spray flow decreased from 0.7 to 0.3, while the transition vapor quality from stratified-spray flow to spray flow decreased from approximately 1 to 0.7; when the proportion of ethane increased from 0 to 50%, the transition vapor quality from stratified flow to stratified-spray flow increased from 0.30-0.45 to 0.43-0.55, while the transition vapor quality from stratified-spray flow to spray flow increased from 0.69-0.85 to 0.83-close to 1. The existing flow pattern map for water-air mixtures was inadequate for predicting the flow patterns of hydrocarbon working fluids. A new set of flow pattern transition criteria was established with prediction deviations of approximately 6.5%, 5.5%, and 4.2% for the experimental stratified flow, stratified-spray flow, and spray flow, respectively.

The refrigeration systems in high- and low-temperature test chambers face challenges of high energy consumption and low efficiency. This study developed an enhanced vapor injection system in a test chamber and conducted experiments using R448A and R404A refrigerants to improve the system efficiency and ensure its alignment with low-carbon environmental goals. The impact of refrigerant charge amounts and compressor frequencies on system performance was analyzed. The results demonstrated that the cooling capacity and coefficient of performance (COP) of the R404A and R448A systems initially increased and then decreased with increasing refrigerant charge amounts. The R448A system demonstrated an 11.3% higher maximum cooling capacity and a 10.4% higher COP than the R404A system. In addition, the compressor power consumption of the R448A system was lower than that of the R404A system. At a refrigerant charge amount of 2.0 kg, the R448A system consumed 7.5% less power than the R404A system. The refrigeration capacity of the R448A system exhibited a 7.7% higher increase compared with that of the R404A system, whereas the compressor power consumption increase was 1.9% lower than that of the R404A system.

The low-carbon transformation of data centers is highly significant for achieving carbon peaking and carbon neutrality. This study compared and analyzed the overall situation of data centers in China. Three variables—energy efficiency improvement rate, proportion of non-fossil energy—and negative emission technology intensity were introduced based on the CO₂ emission and intensity targets of China in key years, and the total CO₂ emissions of the data centers were projected via scenario analysis. The results demonstrated that the power consumption of the data centers increased gradually; the carbon emissions first increased and then decreased, and the power usage effectiveness (PUE) of the data centers decreased gradually. The carbon peak time of the three scenarios is 2030, and the expected times to achieve carbon neutrality are 2059, 2057, and 2055 in the three scenarios. In light of the goal to achieve carbon neutrality by 2060, the data center industry should further improve the energy efficiency utilization rate, increase the proportion of non-fossil energy, strengthen the technological innovation of carbon capture and storage, and enhance the level of carbon sinks.