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.

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.

The drying process of a heat pump clothes dryer (HPCD) has complex characteristics such as strong coupling (the thermal cycle of the refrigerant side is coupled with the drying cycle of the air side), time variation (the system operating parameters change with drying time), and integration (limited space integrating evaporator, condenser, fan, compressor, etc.), and such complexity makes theoretical analysis of the drying performance of HPCD difficult. Based on certain simplifications, this study analyzes the effects of different compressor capacities and fan airflows on the moisture extraction rate per unit time (MER) and moisture extraction rate per unit energy consumption (SMER) of the HPCD. Under the same airflow, the SMER increases first and then decreases with the condenser discharge air temperature and evaporator discharge air temperature, when the discharge air temperature of the condenser is between 20 ℃ to 80 ℃ and the discharge air temperature of the evaporator is between 10 ℃ to 50 ℃. For the HPCD analyzed in this paper, when drying a half load (5 kg) of clothes, theoretical calculations identified an optimal condenser discharge air temperature of 53 ℃ and an evaporator discharge air temperature of 27 ℃ that maximized the SMER. The optimal temperatures are related to the COP of the heat pump system and the mass and heat transfer capacity of air with clothes. Under the same evaporator discharge and condenser discharge temperatures, within the airflow range of 0.02~0.08 kg/s, the SMER first increases and then decreases with the airflow. There is an optimal working airflow of 0.047 kg/s that maximizes the SMER, which is related to the drum power and the airflow resistance characteristic of the clothes dryer. According to methods for measuring the performance of tumble dryers for household use, testing verified that the theoretical analysis results were consistent with experimental tests. This research method and its conclusions provide theoretical guidance for the design and optimization of HPCD.

Helium throttling refrigeration technology is a key cooling method used in liquid helium temperature zones in space. Research on the rapid cooling of chillers coupled with large heat capacity loads is important for the efficient operation of large heat capacity loads. To clarify the cooling characteristics of helium throttling chillers under different rapid cooling schemes, cooling experiments with no additional measures scheme, room-temperature valve bypass scheme, and thermal switch scheme were conducted based on GM pre-cooled helium Joule-Thomson chillers under different heat capacity loads. The experimental results show that the load-free pull-down times of the three schemes were 39.8 h, 20.5 h, and 19 h, respectively. Based on thermodynamics and heat transfer theories, the changes in the radiation, convection, heat conduction, and throttling source terms during no-load cooling were quantitatively analyzed, and the reasons for the difference in cooling time of helium throttling chillers under different schemes were explained. With a simulated load of 0.136 kg of copper, schemes of the room-temperature valve bypass and hot switch were adopted, and the corresponding cooling times were 25.5 h and 20 h, respectively. The experimental results show that the cooling effects of the thermal switch and room-temperature valve bypass scheme are essentially the same for the cooling of a small heat capacity load. Therefore, thermal switch cooling has significant advantages for large-heat-capacity load cooling.

This study investigates the magnetocaloric properties and refrigeration performance of batch-prepared (La, Ce)(Fe, Mn, Si)₁₃H_y alloys. After heat treatment and hydrogenation, the Curie temperatures of M1, M2, and M3 were 292.9 K, 287.8 K, and 283.9 K, respectively, decreasing with higher Mn content. Arrott plots indicated an itinerant-electron metamagnetic transition. M2 exhibited the highest isothermal magnetic entropy change of 12.0 J/(kg·K) under a 2 T magnetic field, with a full width at half maximum of 11 K. Relative cooling capacities (RCP) were 110.2 J/kg, 132.0 J/kg, and 110.0 J/kg for M1, M2, and M3, respectively. Adiabatic temperature changes measured under a 1.5 T magnetic field were 3.48 K, 3.14 K, and 2.96 K for M1, M2, and M3, respectively. A maximum refrigeration temperature span of 16.9 K was achieved by cascading the alloys at an ambient temperature of 295 K.

To reduce energy consumption in temperature- and humidity-independent air-conditioning systems and enhance solar energy utilization, this study developed a solar-assisted desiccant wheel and adsorption cooling system (SDCS-A) using TRNSYS 18. System performance under Guangzhou's climatic conditions was analyzed by varying collector areas and tank volumes together with evaluating metrics such as system coefficient of performance (COP_sys), solar fraction (F_s), and primary energy consumption (E_p). These results were compared with those of a solar-assisted desiccant wheel and vapor compression cooling system (SDCS-C). Findings indicate that changes in collector area significantly influence F_s and E_p, with F_s increasing by an average of 12.18%, while variations in tank volume predominantly affect COP_sys, with a maximum difference of 0.1. Compared to SDCS-C, SDCS-A achieved 6.51% higher monthly average COP_sys, a 21.05% increase in F_s, and a 21.45% reduction in E_p during the cooling season. Furthermore, the system's performance across different climates was evaluated, demonstrating that Guangzhou offers more stable and higher monthly COP_sys values than Beijing, Shanghai, and Lhasa.

Capillary mat heat exchangers are increasingly used in transportation energy tunnels owing to their large heat-transfer area and uniform temperature. Thermal energy tunnels, a new type of energy tunnel, differ from transportation energy tunnels because of the heat source inside such tunnels. To examine the feasibility of applying capillary mat heat exchangers in thermal energy tunnels under endothermic conditions, heat transfer performance was experimentally investigated using a 1∶1 intermittent operating mode. The results showed that the higher the initial air temperature (T₀) in the tunnel, the greater the heat flux. With the inlet temperature of circulating water (t_in) fixed at 5 ℃, as the temperature difference between T₀ and t_in increases by 10 ℃, the heat flux increases by 45.9%. The heat flux also increases with the increase of circulating water velocity (u); whereas u increases up to 0.1 m/s, the heat transfer rate saturates and approaches 187.22 W/m². The lower the t_in is, the greater the heat flux. When the T₀ is 50 ℃ and the u is 0.075 m/s, for every 1 ℃ increase in the t_in, the heat transfer rate decreases by 2.04%.

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.

Refrigerant leakage is a frequent and costly fault that deteriorates the normal operation of a chiller; however, it is difficult to measure directly. This study proposes a data mining- and key-feature-based approach for the soft measurement of refrigerant leakage. Random forest importance ranking and distance correlation coefficients were used to select the characteristic features, and a support vector regression (SVR) soft measurement model was established to measure leakage quantitatively. The proposed model was validated through a leakage experiment conducted on a screw chiller with a rated cooling capacity of 1 440 kW and a refrigerant charge of 330 kg. The results showed that the SVR soft measurement model established on the three selected key features achieved significantly improved performance. The model had a root mean square error (RMSE) of 0.844 kg and a mean absolute error (MAE) of 0.734 kg, outperforming the other three feature subsets.

To address the issue of unreasonable buried pipe length design in ground-source heat pump projects, a three-dimensional dynamic simulation platform was developed. The reasonableness of the buried pipe length was evaluated by comparing the simulated outlet temperature with the designed outlet temperature based on specifications. Using a building in Beijing as a case study, the effects of soil thermal properties and borehole-related parameters on the design error in buried pipe length were analyzed. A sensitivity analysis further examined the impact of these factors. Results indicate that the relative error in buried pipe design length increases with rising soil thermal conductivity, soil volumetric heat capacity, borehole depth, and borehole spacing. Relative error ranges were 10.7%-27.3%, 8.0%-23.8%, 7.3%-12.5%, and 12.5%-17.4% for the respective factors. Sensitivity analysis revealed soil thermal conductivity as the most significant factor influencing pipe length, with a quantitative index of 0.909. Other influential factors, in descending order, were soil volumetric heat capacity, borehole spacing, number of borehole columns, and borehole depth.