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%.

Superhydrophobic surfaces, a new type of green material, exhibit promising application prospects in the field of anti-/de-icing. In this paper, the kinetic behavior of impinging droplets on surfaces with different temperatures (-25-16 ℃), different inclination angles (0°-60°), and different wettability (hydrophilic and superhydrophobic surfaces) is investigated through experimental comparisons. The variations of the droplet morphology, spreading factor, spreading time, and contact time are analyzed. The results show that the impinging droplets exhibit different kinetic behaviors after spreading due to the different inclination angles and wettability. The maximum spreading factor and spreading time on hydrophilic surfaces increase with the inclination angle. The variation of the spreading time on superhydrophobic surfaces follows the same trend as that on hydrophilic surfaces, while the maximum spreading factor decreases with an increase in the inclination angle, especially at T_s-25 ℃; Compared to hydrophilic surfaces, the impinging droplets have shorter spreading times on superhydrophobic surfaces, which can reach about 10 times at T_s=-25 ℃. Increasing the wall inclination angle breaks the symmetric bounce of the droplets on the horizontal superhydrophobic surface, thereby shortening the contact time of the droplets. This suggests that increasing the inclination angle can effectively inhibit the freezing of water droplets.

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.

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.

Large cold storage systems play a significant role in economic development with substantial energy consumption and environmental impacts. To promote the green, low-carbon, and efficient development of cold storage, this study mainly focuses on refrigerant substitution, refrigeration system optimization, and the application of transcritical CO₂ systems with ejectors in large cold storage systems. The performance and energy consumption characteristics of different refrigeration systems were compared through a comprehensive annual hourly energy consumption analysis based on the cold storage demands at different temperatures and under various climatic conditions. The results show that the COPs of a transcritical CO₂ system integrated with specifically optimized ejectors are higher than that of the R507A system in all four cities for low-temperature (-32 ℃), medium-temperature (-8 ℃), and high-temperature (0 ℃) cold storages. However, it exhibited performance advantages over the R717 system only in cold climate zones, with the highest system COPs of 2.45, 4.86, and 5.98 for low, medium, and high-temperature cold storages, respectively. Considering the system′s annual energy consumption, the application of CO₂ transcritical systems in low, medium, and high-temperature cold storages in Beijing achieved energy savings of 7.9%, 10.1%, and 10.5%, respectively, compared to the R507A system. The energy savings of the R717 system were slightly higher than that of the CO₂ system in low-temperature cold storage, but the CO₂ system had more obvious advantages in medium- and high-temperature cold storage. The energy consumption of the CO₂ transcritical system also varied across climate zones. In the cold climate zone, the energy savings reached 9.3%, outperforming the R717 system, while in the hot climate zone, its energy savings dropped to 2.8%, slightly lower than that of R717. With the appropriate selection of temperature range and climate zone, the overall operational efficiency and energy-saving performance of the transcritical CO₂ system can surpass those of the R717 system. This study conducted a comprehensive analysis of the operational performance and energy consumption distribution characteristics of the CO₂ system and highlighted its applicability in different scenarios, providing important references for promoting and applying the system, which is crucial for achieving dual carbon goals.

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.

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.

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.

The application of latent thermal energy storage with heat pumps has been extensively studied in recent years. The combination of phase change heat storage and a heat pump can improve the performance of the heat pump and the utilization of renewable energy; however, further cost reduction and efficiency increase are required. Therefore, this study reviews the progress of heat pumps coupled with solid-liquid phase change materials and summarizes the applicable conditions and characterization methods for phase change materials applied to heat pumps. The optimization approaches for the performance of the heat pump system are summarized, including the selection and improvement of phase change materials, the optimal setting of the heat exchanger, and the dynamic optimization control strategy of the system. The outstanding performance of heat pumps with cascade heat storage in improving the supply-side comfort and utilization rate of renewable energy indicates the broad prospect of cascade heat storage being applied to heat pump energy storage systems. Herein, mixed, non-eutectic phase change materials are proposed as alternative materials for cascade heat storage. Notably, summarizing and developing new methods for adjusting the thermophysical properties of phase change materials for energy storage is necessary for adapting the selection and improvement of phase change materials to the optimization of the thermodynamic cycle of cascade heat storage devices and further improving the heating decarbonization ability of latent heat storage heat pumps.

The air-source heat pump capillary radiant floor heating system directly heats the floor using a refrigerant as the heat transfer fluid, which employs a simple system and promotes good heat transfer. This study developed an experimental device for capillary floor radiant heating with an air-source heat pump, and a heating experiment with parallel capillary floor radiant terminal was conducted at different outdoor ambient temperatures. The experimental results demonstrate that a longer time is required for the temperature of the capillary floor radiation terminal to reach steady as the outdoor temperature decreases. When the outdoor temperature was -5 ℃, the required time was 120 min. The temperature difference between the discharge inlet end and the condenser outlet end on the same capillary wall was large. The temperature difference on the capillary wall reached 6.40 ℃, while that on the surface of the 20 mm-thick cement floor reached 4.20 ℃. Conversely, the vertical temperature difference from the capillary wall to the cement floor surface was small, not exceeding 0.40 ℃, and the vertical heat transfer effect of the floor was good. The temperature difference at the same position of different capillaries was within 0.80 ℃, and the temperature uniformity was good. When the outdoor temperature was -5 ℃, the heating coefficient of performance of the unit reached 4.61 with good heating performance.