最新刊期
卷 43 ， 期 3 ， 2022
- 摘要：Radiative sky cooling cools an object on the Earth’s surface by emitting thermal radiation through the atmospheric window to the cold universe. As a passive technique to reduce cooling energy requirements without power input, radiative cooling provides a new avenue to address the challenges of the energy crisis and global warming. Over the past few decades, radiative cooling has been limited to nighttime applications. With recent technological advancements in nanophotonics and metamaterials, daytime radiative cooling has been demonstrated. In this review, the current research status of radiative sky cooling is discussed, including its fundamental principles, advanced materials, and structures. Furthermore, applications and prospects in radiative sky cooling are summarized and the challenges are analyzed to promote the development of radiative sky cooling. As the energy security and environmental issues escalate, exploring the application of radiative sky cooling in different scenarios, such as energy saving in buildings, mitigation of the heat island effect, and improving the efficiency of solar cell generation, will considerably contribute to energy saving and emission reduction.关键词：radiative sky cooling;spectral selectivity;atmospheric radiation;infrared radiation2478|1707|5发布时间：2024-07-18
- 摘要：Thermal management technology for electric vehicles guarantees safety and comfort of driving, and it is one of the key technologies in the development of electric vehicles. The research progress and development trends of thermal management technology were introduced considering the thermal management demand, development history, and key components. Trends and prospects of the future technological development of thermal management in electric vehicles are discussed in the work, including the perspective of eco-friendly refrigerants, intelligent control, and improvement of passenger cabin comfort. We hope this review serves as a comprehensive reference for the advances in this field.关键词：electric vehicle;thermal management;heat pump;system configuration;technical progress1505|1422|4发布时间：2024-07-18
- 摘要：With the continuous development of data centers, the energy consumption and heat flux of information technology (IT) equipment has gradually increased, and the energy consumption of supporting refrigeration equipment increases proportionally. Consequently, efficient and energy-saving cooling technologies have gradually attracted considerable attention. In this study, existing two-phase cooling techniques were summarized and classified, and the distributed two-phase cooling method was proposed. By contrast, the research progress of two-phase cooling technology was introduced in three aspects: the properties of working fluids for data center cooling, enhanced heat transfer methods, and thermodynamic evaluation of cooling systems. The research on the working fluids mainly focused on the liquid properties, and the gaseous properties were rarely reported. The heat transfer of boiling and condensation was mainly strengthened by the formation and separation of bubbles and droplets. Research on enhanced heat transfer methods for liquid cooling was insufficient. The two-phase cooling system had an excellent thermodynamic performance. However, the pressure change caused by the phase change is undesired.关键词：data center;two-phase cooling;boiling heat transfer;condensation heat transfer1204|1752|1发布时间：2024-07-18
- 摘要：A review of domestic and international research progress on phase change materials (PCM) for cold thermal energy storage in air-conditioners is presented in this paper. The performance requirements and basic classification of PCMs are briefly introduced. The shortcomings of existing PCMs for cold thermal energy storage in air-conditioners are summarized and recent approaches to optimize the properties of PCMs are discussed, especially to remedy the supercooling and phase separation of inorganic materials, as well as the poor thermal conductivity of organic materials. The advantages and disadvantages of inorganic and organic phase change materials in cold thermal energy storage were compared and recommendations for future research and development directions of PCMs for cold thermal energy storage in air-conditioners were proposed. This paper focuses on materials for room-temperature air-conditioning. It equally conducts an in-depth investigation and review of typical materials, which provides a basis for improving the properties of PCMs and promoting their practical applications.关键词：phase change materials;cold thermal energy storage;phase separation;thermal conductivity1209|2044|4发布时间：2024-07-18
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Review on Research Status and Prospects of Twisted Tape Inserts in Boiling Heat Transfer Enhancement
摘要：The demand for compact and miniature heat exchangers in refrigeration equipment has led to the development of new heat transfer enhancement technologies. Twisted tape inserts are cheap and easy to manufacture, and can be classified as a passive heat transfer enhancement technology. They have great application potential in the evaporator of refrigeration systems. Inserting twisted tapes into a tube with two-phase flow boiling can increase the surface coefficient of heat transfer and pressure drop in the tube. The analysis in this study revealed that changes in mass flow and vapor quality positively correlated with the the surface coefficient of heat transfer and pressure drop, while changes in pipe diameter, twist ratio, and saturation temperature negatively correlated with the surface coefficient of heat transfer and pressure drop. The complexity of the boiling heat transfer process, different evaluation criteria, and limited experimental conditions are the main reasons why the best conditions for twisted tape insertions are inconsistent as explained by different scholars. In addition, this study also summarized the correlations for the surface coefficient of heat transfer and pressure drop of boiling heat transfer in tubes with twisted tape inserts as explained by different researchers. Finally, we believe that the best operating conditions of the twisted tape inserts need to be further clarified, and the twisted tape insert needs to be studied in real evaporators or inverter compressors.关键词：twisted tape inserts;boiling heat transfer;heat transfer enhancement;pressure drop1076|1418|0发布时间：2024-07-18 - 摘要：R1233zd(E) has attracted extensive attention owing to its good environmental performance and cycle characteristics. To solve the matching problem between R1233zd(E) and lubricating oils, the miscibility and solubility properties of R1233zd(E) using common lubricating oils were investigated in this study. The critical miscibility temperatures of R1233zd(E) using mineral oils, such as SUNISO 3GS, polyol ester (POE) RL32H, RL68H, and polyvinyl ether (PVE) FVC68D were measured in the temperature range from -40 ℃ to 40 ℃. The results showed that R1233zd(E) was miscible with mineral oil under standard conditions, although flocs and stratifications appeared at -20 ℃. The miscibility of R1233zd(E) with POE and PVE oil was good, and the critical miscibility temperature was lower than -40 ℃. Considering that R1233zd(E) is usually used under high-temperature conditions, it is recommended to choose POE68 oil, which has a higher flash point and viscosity grade. Based on the isochoric saturation method, the solubility of R1233zd(E) in POE68 oil was tested between 40 ℃ and 120 ℃. The solubility increased with increasing pressure and decreased with increasing temperature.关键词：refrigerant;lubricating oil;miscibility;solubility1177|3038|1发布时间：2024-07-18
- 摘要：Microchannel technology is one of the most promising technologies for chip cooling. In this study, the flow and heat transfer characteristics in manifold microchannels with different porosities (e=0.2-0.8, e-rise, and e-drop) of porous fins and solid fins were compared and analyzed using numerical simulation. The results showed that porous fins considerably reduced the flow resistance in the manifold microchannel by more than 20%. However, the pressure drop reduction effect decreased at larger inlet velocity. In the studied porosity variation range, the porous fins with e=0.4 and e=0.6 exhibited a better comprehensive performance. Based on the evaluation of the maximum chip temperature, e-rise had a lower thermal resistance and higher pressure drop than e=0.4. As the inlet velocity increased, the thermal resistance advantage of the porous fins with e-rise was strengthened, and its pressure drop gradually reduced. Compared to the e=0.4 case, there was an 8% reduction in thermal resistance in the e-rise case, at the cost of only a 1.4% increase in pressure drop.关键词：electronics cooling;manifold microchannel;porous medium;porosity;flow and heat transfer1314|1192|4发布时间：2024-07-18
- 摘要：A performance test system for an evaporative condenser was developed in this study. The effects of different parameters on the heat transfer performance of an annular elliptic finned tube evaporative condenser were studied using a control variable method, including the face velocity, spray density, wet-bulb temperature, circulating water temperature, and flow rate of the cooling water. The experimental results showed that the optimal face velocity and spray density of the condenser was 3.1 m/s and 0.0056 kg/(m/s), respectively. The air pressure drop increased rapidly with an increase in the face velocity. With an increase in the wet-bulb temperature, the heat flux of the external heat transfer (that is, external heat flux) decreased by 67.5%, while that of the internal heat transfer (that is, internal heat flux) increased by 47.5%. The performance of the condenser was good because the internal heat-transfer process was enhanced. With an increase in the circulating water temperature, the internal heat flux of the condenser decreased by 64.6%, while the heat-transfer performance of the condenser decreased sharply. With an increase in the cooling water flow rate, the internal heat flux of the condenser increased greatly by 2.92 times, while the total heat flux increased by 21.1%. Consequently, the heat-transfer performance of the condenser was enhanced considerably. The condenser exhibited excellent heat transfer performance at low wet-bulb temperatures, low circulating water temperatures, and large cooling water flow rates.关键词：evaporative condenser;annular elliptic tube;finned tube heat exchanger;heat transfer performance1145|1059|1发布时间：2024-07-18
- 摘要：Aiming at extensive applications and frosting problems of wavy finned-tube heat exchangers and based on the phase change driving force to analyze its frosting mechanism, the frosting characateristics on wavy aluminum surfaces with different cold surface temperatures (-5-15 ℃), air temperatures (11-17 ℃), and air velocities (1.5-2.5 m/s) were experimentally analyzed in this study. The micromorphology of frost growth on wavy aluminum surfaces was observed. In addition, the environmental effects on frost formation were analyzed using the Taguchi experimental method. The results showed that a change in experimental conditions affected the frosting characteristics on wavy aluminum surfaces. Using a wavy plate with a wavy angle of 11.3° as an example, when the frosting time was 60 min, compared to a cold surface temperature of﹣5 ℃, the frosting weight increased by 12.20% and 31.28%, and the frost layer thickness increased by 19.95% and 47.24%, when the cold surface temperature was﹣10 ℃ and﹣15 ℃, respectively. Based on the Taguchi experimental method, when compared to the air temperature and velocity, the cold surface temperature and air relative humidity greatly influence the frosting characteristics on wavy surfaces. These analyses that were based on the Taguchi experimental method revealed that the contribution rates of cold surface temperature and air relative humidity to the frosting weight of wavy surfaces were 37.3% and 31.8%, respectively, and the contribution rates to the frost layer thickness were 61.1% and 22.6%, respectively. Compared to the air temperature and velocity, the cold surface temperature and air relative humidity had a greater impact on the frosting characteristics on wavy surfaces. It is worth noting that the contribution rate of air velocity to the frost weight was 22.6%, while the contribution rate of frost layer thickness was only 4.2%, revealing that the air velocity played a considerable role in the densification of the frost layer.关键词：frost;phase change driving force;wavy surface;micromorphology;Taguchi method1299|959|0发布时间：2024-07-18
- 摘要：For the spiral tube gas cooler in a CO2 heat pump system, a simulation model was established in MATLAB. The effects of the inlet water temperature, CO2 pressure, and mass flow rate of the gas cooler heat transfer, entransy dissipation, exergy loss, exergy efficiency, and outlet water temperature were studied using a single factor analysis method. Compared to the experimental data, the overall error of the model was within ±10% when the inlet temperature was 24.5-35.0 ℃, the CO2 pressure was 8.4-10.7 MPa, and CO2 mass flow rate was 0.0326-0.0476 kg/s. Compared to the inlet water temperature and CO2 mass flow rate, CO2 pressure had considerable effects on the performance of the gas cooler, thereby causing the optimal pressure. At inlet water temperature of 20 ℃ and inlet refrigerant temperature of 90 ℃, the exergy efficiency of the gas cooler reached the peak when CO2 pressure was 10 MPa, and the heat transfer rate of the gas cooler was the highest when CO2 pressure was 11 MPa. When the inlet temperature was lower than 20 ℃ and the CO2 pressure was 10.5 MPa, the outlet water temperature was the highest.关键词：CO2 heat pump;gas cooler;heat transfer rate;entransy dissipation;exergy1182|1134|0发布时间：2024-07-18
- 摘要：In this study, an experimental bench was built to conduct experimental tests on the heat transfer performance of fillers in evaporative cooling air-conditioners. The effects of the filler material, air flow rate, and pressure drop on the heat and mass transfer performance of the fillers were investigated. The experimental results showed that the direct evaporative cooling efficiency of plant fiber packing, foam ceramic packing, PVC packing, and metal packing under optimal working conditions were 82.93%, 71.29%, 54.85%, and 55.34%, respectively. The pressure drop of the plant fiber packing was the highest, and that of the metal packing was the lowest. The most suitable air flow rate does not necessarily correspond to the highest heat transfer efficiency of the packing, since the energy consumption of the fan, due to the pressure drop in the packing, requires further consideration. According to projections, an air-conditioning system with direct evaporative cooling will be used in Xi'an City’s communication room for 4,126 h or 47.10% when plant fiber packing is used, 3,115 h or 35.56% when foam ceramic packing is used, and 2,582 h or 29.47% when PVC and metal packing are used.关键词：heat and mass transfer;direct evaporative cooling;fillers;pressure drop;applicability1052|1121|1发布时间：2024-07-18
- 摘要：In this study, the harmonic method was introduced to solve the inner wall temperature problems of an envelope structure with heat storage characteristics in a large-space building with stratified air conditioning. The transient experimental results were used to calculate the heat storage of the indoor air. Combined with the diffusion characteristics of the indoor moisture, the transient full-heat six-zone Block-Gebhart (B-G) model was established under summer operating conditions. The results showed that the average absolute errors were between 0.44-1.12 ℃ and 0.63-1.96 ℃, in terms of the differences between the predicted and experimental values of the air and wall temperatures in the four regions above ground level, respectively, while the indoor air enthalpy was 1.85-1.96 kJ/kg. However, the errors in air and wall temperature, as well as the air enthalpy in the fifth and sixth zones close to the roof were higher and the predictions lower than the experimental results. The results showed that it was feasible to use the harmonic method and experimental results to analyze the transient solution of the thermal environment in large-space buildings.关键词：large space building;summer thermal environment;unsteady state;total heat B-G model;stratified air-conditioning1022|949|0发布时间：2024-07-18
- 摘要：Behavior modes, such as self-regulating water supply temperature and switching valves in heating circuits, have significant effects on the energy-saving and comfort of household air-source heat pump floor heating systems. In this study, using an actual residence as an example, the heating characteristics of a household air-source heat pump floor heating system under different water supply temperatures and different numbers of heating circuits were experimentally studied. The indoor air temperature, ground temperature, and system power consumption were measured and recorded. When the average outdoor temperature and the maximum water supply temperature of the unit was -2.3 ℃ and 55 ℃, respectively, the ground temperature was very high and the local dissatisfaction rate of the floor surface temperature was more than 20%. Compared to the water supply temperature of 30 ℃, the power consumption increased by 191% and the temperature difference between the living room and the study area increased by more than 2.5 ℃. When the average outdoor temperature was -1.7 ℃, after closing the corresponding floor heating loop valves in the second bedroom and the guest bedroom, the room temperature of the master bedroom, living room, and the study decreased by 1.4 ℃, 2.9 ℃, and 0.5 ℃, respectively. Also, the power consumption of the system was reduced by 18.7%.关键词：air-source heat pump;radiant floor heating;behavior mode;water supply temperature;energy consumption972|943|0发布时间：2024-07-18
- 摘要：A curved baffle-type inlet structure was designed for a water storage tank to improve the performance of an air-source heat pump water heater system. A heat pump water heater test bench was set up to compare the extraction efficiency and COP of the curved baffle inlet structure to the direct-entrance and side-entrance inlet structures. The experimental results showed that the thermal stratification of water along the height direction was effectively released by the curved baffle-type inlet structure. Compared to the direct-entrance inlet structure water heater, the extraction efficiency increased from 52.87% to 96.60% with a curved baffle-type inlet heat pump water heater when the inlet flow rate ranged from 5 L/min to 10 L/min, and the COP improved from 7.14% to 7.40%. Contrary to the side-entrance inlet structure water heater, the extraction efficiency increased from 38.37% to 76.99% with a curved baffle-type inlet heat pump water heater when the inlet flow rate ranged from 5 to 10 L/min, and the COP improved from 3.22% to 5.53%. The performance improvement increased as the inlet flow rate increased. The curved baffle-type inlet structure of the heat pump water heater simultaneously increased the extraction efficiency and the system’s COP.关键词：inlet structure;curved baffle;extraction efficiency;COP;double high efficiency1107|1044|0发布时间：2024-07-18
- 摘要：A solar-assisted ground-source heat pump system is an excellent way of supplying clean energy in extremely cold regions. In this system, the solar energy collection and storage system has different connection and operation modes from the ground-source heat pump system. The running time of the solar system directly affects the thermal recovery characteristics of the soil. In this study, a solar-assisted ground-source heat pump system with annual heat storage was established based on the TRNSYS platform. A simulation calculation method for a soil heat accumulator with double independently buried pipes was proposed. The simulation results were compared to the field test data. Based on the actual project of a public building in Dalian, the operating parameters, which greatly influence the total operating energy consumption and the soil temperature change rate of the system, were obtained using an orthogonal experimental design and TRNSYS simulation. Their influence laws were subsequently analyzed. The results showed that the total operating energy consumption of the system positively correlated with the water supply temperature of the heat pump in winter, load-side water flow rate, and soil-side water flow rate. It negatively correlated with the water supply temperature of the heat pump in summer. When the cumulative heating and cooling capacity ratio was 1.31, the total operating energy consumption of the system negatively correlated with the start-up temperature of the heat storage. When the ratio was 2.32, it positively correlated with the start-up temperature of heat storage and when the ratio was 1.77, the start-up temperature of heat storage was 35 °C, and the energy consumption of the system was the lowest. The change rate in the soil temperature is negatively correlated with the start-up temperature of heat storage. The operating time and parameters of the solar system were adjusted according to the heating and cooling ratios of the system.关键词：solar energy;ground-source heat pump;TRNSYS;model establishment;influence factors988|1061|1发布时间：2024-07-18
- 摘要：A solar air dual-source heat collecting evaporator (condenser) using a micro heat pipe array and a dual-source direct expansion heat pump system was designed. This system effectively reduces the temperature of the photovoltaic module to improve its power conversion efficiency and simultaneously achieves heating and refrigeration functions. The multiple energy complements each other in such an energy system for building applications. An experimental platform for a solar-air energy direct expansion heat pump system was established and the performance of the power generation, cooling supply, and domestic hot water supply was tested and analyzed. When the heating system in the dual-source direct expansion heat pump system operated, the PV/T assembly had a significant cooling effect on the photovoltaic module. Compared to the rest conditions, the photoelectric power and photoelectric efficiency were improved by 56.43% and 53.15%, respectively. During the heating process, the dual-source direct expansion heat pump heated 180 L water from 20 ℃ to 50 ℃. The average system heating COP, heating power, total power generation, and total power consumption were 4.59, 3.20 kW, 1.43 kW·h, and 1.36 kW·h, respectively. The average refrigeration EER was 2.29 when cooling from 12 ℃ to 7 ℃.关键词：direct-expansion heat pump;heat collecting evaporator(condenser);micro heat pipe array;photovoltaic-thermal1092|1124|2发布时间：2024-07-18
- 摘要：This paper proposes a new solar sewage dual-source heat pump heating system and explores its application in winter heating in nearly zero-energy buildings in Changchun, Urumqi, Xining, Beijing, and Shanghai. This operation affects the economy of producing domestic hot water throughout the year. The average heating temperature of the system in the five cities is above 40 ℃ and the average temperature of domestic hot water is above 33 ℃, excluding electric heating. The operation performance reveals that the system is good, but electric heating is still needed to ensure the reliability of the operation. In winter, the energy efficiency of the system in the five cities is more than 3.5, and the energy-saving effect is remarkable. In addition to Urumqi (owing to the low gas price compared to other cities), this system is economically advantageous compared to the traditional electric hot water and natural gas hot water systems.关键词：solar energy;sewage-source heat pump;heating;domestic hot water;economical efficiency1150|1008|0发布时间：2024-07-18
- 摘要：Photovoltaic thermal heat pumps are a comprehensive utilization technology that combines photovoltaic-driven heat pumps and solar thermal collectors, which have the characteristics of a tri-generation (heating, cooling, and electricity) to meet a building’s diversified and distributed energy demand. In this study, the refrigeration performance of a PVT heat pump system in summer was tested, with an emphasis on the long-wave radiative cooling and convective cooling between the PVT unit and the ambient under the refrigeration mode. The system composition, working principle, and performance evaluation method were introduced and analyzed in detail. The refrigeration performance was tested under ambient conditions. The experimental results showed that the PVT heat pump system met the building’s cooling demand in summer, and it demonstrated the benefits of considerable refrigeration performance, simplicity, and long-term stability. Under clear weather conditions at night in summer, the average COPc of the system was 2.9 (producing chilled water) and 2.3 (producing ice). In overcast conditions in summer daytime, the average COPc of the system was 2.37 (producing chilled water) and the refrigeration performance of the system at night was approximately 20% higher than that in the daytime.关键词：PVT heat pump;roll-bond PVT unit;long-wave radiative cooling;system model1114|1243|0发布时间：2024-07-18
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Simulation and Annual Performance Analysis on a Direct-expansion Solar-assisted PVT Heat Pump System
摘要：Mathematical modeling and simulation of a solar-assisted heat pump system based on a direct-expansion photovoltaic/thermal (PVT) module were developed to analyze its annual performance in Shanghai. The direct-expansion collector/evaporator, which couples the heat pump system with the PV module, absorbed the waste heat of the PV module and increased its electricity generation efficiency by reducing its working temperature. By contrast, such coupling enhanced the evaporation temperature and thus the performance of the heat pump. The simulation model, which was developed based on the energy balance and heat transfer feature of the collector/evaporator, was compared to the experimental results and it presented a maximum error of 6.2%. A simulation was performed under the ambient temperature and irradiation conditions of Shanghai taking one day as a time step. The results showed that the COP of the system varied between 2.67 and 9.06, with an annual average value of 5.47. The annually-averaged evaporation temperature was 18.34 ℃, with an annually-averaged thermal efficiency of 100.06%. Additionally, PVT module presented a 20.21 ℃ drop in annually-averaged working temperature compared to the PV module, which benefits the electricity generation by 11.80%.关键词：direct-expansion;PVT;solar energy;heat pump;performance simulation1347|3087|0发布时间：2024-07-18
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