Abstract>This paper introduces an annular elliptic finned tube evaporative condenser. Through the performance test experiment system of the evaporative condenser, the change rules of the heat and mass transfer performance and operation characteristics of the evaporative condenser under different circulating water ratio, different regional environments and four different operating modes: "air cooling", "water cooling", "evaporative condensation" and "full working" are studied. The results show that the heat flux of "full working" mode is higher than that of "air cooling" and "evaporative condensation" modes under the same conditions, the "water cooling" mode has the highest heat flux because its heat transfer area is much smaller than that of other modes; under the condition of "full working" mode maintaining a certain amount of air volume and the total amount of circulating water, the heat flux increases first and then decreases with the increase of the circulating water ratio, the best flow ratio of spray water and cooling water is 2:5; The total heat flux of the "full working" mode in dry cold area, cold area and hot summer and warm winter area is 5879.64 W/m2, 3964.28 W/m2 and 2933.05 W/m2, so the heat transfer performance of the "full working" mode varies greatly in different regions, and the "full working" mode of the condenser has better heat transfer performance when the spray and cooling water temperature ranges from 13 to 18 ℃. Under the same conditions, the mass transfer coefficient of the water film in the "evaporative condensation" mode is higher than that in the "full working" mode, and both increase and then decrease with the increase of the spray density.
Abstract>The influences of expansion valve opening on the stability and system performances of R32 heat pump system under the variable heat source inlet temperature conditions and variable hot water inlet temperature conditions were studied based on the water source heat pump device. The results show that the heat pump system has different superheat oscillation intervals under different operating conditions, and the heat source inlet temperature has a significant effect on the initial oscillation superheat, while the hot water inlet temperature has no obvious effect on it; under the variable heat source inlet temperature conditions, heating capacities of the system show different changing trends with the change of superheat, while the COP show the same changing trends, and the maximum COP of the system are obtained in the superheat oscillation interval under three operating conditions; under the variable hot water inlet temperature conditions, the heating capacities and COP of the system show the same trends with the change of superheat, and both of them reach the maximum values in the superheat oscillation range; both the increase of heat source inlet temperature and the decrease of hot water inlet temperature can increase the maximum heating capacity and the maximum COP of the heat pump system.
Abstract>The problem of inconsistency between the measurement results of the cooling capacity of room air conditioners between different enthalpy difference chambers still plagues China"s air-conditioning industry, and it is a metrological problem that easily leads to conflicts in international trade. A standard cold source device that can calibrate the laboratory measurement results of enthalpy difference needs to be studied urgently. In this paper, a standard cold source device based on ice slurry refrigeration is proposed, including ice slurry preparation system and heat exchange system, which based on studying several mainstream cooling capacity measurement methods for room air conditioners and the research status of standard cold source at home and abroad, combined with the in-depth understanding of the physical process of ice slurry refrigeration. And carry out experimental research and refrigeration capacity uncertainty analysis to verify whether the standard cold source device has the ability to calibrate the enthalpy difference laboratory. The experimental results show that the relative uncertainty of the output refrigerating capacity of the standard cold source device can reach 1.2% under the working condition of m=70g/s, which is about 1/3 of the air enthalpy method. It can be used as a standard cold source to calibrate the refrigerating capacity measurement results of the enthalpy difference chamber, which is helpful to improve the international competitiveness of the refrigeration industry.
Abstract>To solve the problem of poor thermal conductivity and low thermal storage efficiency of paraffin wax, of carbon-containing materials with high thermal conductivity are used to improve the thermal storage performance of paraffin wax. Three carbon nanotube powders of different diameters, 8-12 nm, 10-20 nm and 20-40 nm, were added into paraffin wax with a mass fraction of 5 wt% for evaluating the influence of carbon nanotube diameter on the thermal storage properties and the anisotropic thermal conductivity of carbon nanotube/paraffin wax composite phase change material(CPCM). The experimental results showed that the specific heat capacity and latent heat value of CPCM decreased as the diameter of carbon nanotubes decreased, and the phase change temperature decreased by about 1°C, the latent heat value decreased by 0.5%-3.8%, and the specific heat capacity decreased by 16.2-16.5% compared with those of pure paraffin wax. The improved thermal conductivity and thermal diffusivity shortened the paraffin wax melting time, and the melting time of 8-12 nm composites saved by 18.2%-46.7%. The temperature uniformity in the vertical direction was also effectively improved. In the external measurement points, the temperature fluctuation of 8-12 nm composites was 47.7% compared with pure paraffin, while in the internal measurement points, the temperature fluctuation of 10-20 nm was 32.9% of pure paraffin. The smaller diameter carbon nanotubes have a significant improvement on the thermal storage properties of CPCM.
Abstract>The vapor injection heat pump (VIHP) technology has become one of the research hotspots in the field of energy and environment because of its high efficiency, energy saving and environmental protection. In this paper, inter-stage configuration, control strategy and performance optimization are discussed. The research status and progress of VIASHP system in existing public literature are summarized, and the development trend of the vapor injection heat pump (VIHP) system is predicted. Firstly, two kinds of inter-stage configurations of vapor injection heat pump system, flash tank vapor injection system and subcooler vapor injection system, are discussed, and the effect of different inter-stage configurations on the heating performance is illustrated. Secondly, the optimal control strategies of intermediate pressure under different conditions are reviewed and the improvement measures of defrosting methods in existing studies are discussed. Finally, the improvement effects of alternative refrigerants, optimization of compressor injection structure and inter-stage configuration on system performance are discussed. Matching and optimizing the refrigerant flow distribution is an effective way to improve the performance of the system. As a necessary way to improve the heating performance, injection structures, alternative refrigerants and the optimization control of intermediate pressure will also promote the further development of the vapor injection heat pump (VIHP) technology.
Abstract>Energy saving and renewable energy utilization in buildings is one of the keys to achieve the carbon neutrality in China. Radiative sky cooling, as a passive cooling technology that requires no external energy input nor refrigerant, has significant impacts in increasing energy efficiency and reducing carbon emissions in buildings. To this end, this paper explores the prospective carbon emission reduction if passive radiative cooling technology is applied to existing buildings in China, especially in the operational phase. The current total building area and the stock area by building type in different provinces are obtained from the statistical yearbook. Nine types of buildings were modeled according to energy efficiency standards, and typical meteorological year weather data were selected in each province. With all those inputs, the carbon emissions during operational phase of each building type in each province were simulated using the building energy consumption software EnergyPlus, as well as the amount and rate of the carbon emission reduction after the large-scale application of radiative cooling. The spatial distribution of the carbon reduction rate in China was analyzed. The total annual carbon reduction was calculated to be 230 million tCO2, accounting for 10.90% of the building operation carbon emissions, and 2.39% of the total energy carbon emissions, which can be a great contributor to achieving “carbon peaking” and “carbon neutrality” goals in China.
Abstract>o improve the performance of the CO2 system in the application of building space heating and cooling throughout the year, a transcritical CO2 system combined with ejector and dedicated mechanical subcooling (EJ-DMS) is proposed. The thermodynamic model of the system is developed. Then, the discharge pressure and subcooling degree are optimized by taking the coefficient of performance (COP) as the objective function, where the genetic algorithm is utilized. Taking 5 typical cities as application scenarios, the energy consumption and annual performance factor (COPann) of the system are further analyzed. The results show that compared with the conventional mechanical subcooling system and the conventional ejector system, the COP of EJ-DMS is enhanced by 10.90%, 5.58% in the heating mode and 8.99%, 18.12% in the cooling mode, and the COPann is improved by 7.95% and 5.98%, respectively. Compared with the conventional ejector system, the discharge pressure of the EJ-DMS is reduced by 0.47 MPa and 0.77 MPa in the heating and cooling modes, respectively. In addition, the COPann improvement ratio of the EJ-DMS system is the highest when operating in Guangzhou and Harbin, indicating that the EJ-DMS system is more suitable for the regions with relatively high or low ambient temperature, such as hot summer and warm winter, and severe cold regions. This study can provide a theoretical reference for the development and optimization of combined cooling and heating CO2 system.
Abstract>The natural evaporation of microdroplets was presented as a new way to add cryoprotectants. The effects of initial volume and concentration on cell permeation characteristics and damages during cryoprotectant loading by natural evaporation of microdroplets were investigated systematically using image processing techniques, combined with theoretical analysis of cell permeation and damage models, and compared to traditional cryoprotectant loading methods (one-step loading and two-step loading).The results showed that: (1) The faster the evaporation rate, the easier it was for the cryoprotectant to achieve the required concentration, which might effectively cut the loading time (4907s for 1μL against 5μL); (2) As the initial concentration of the protectant decreased, the loading time increased gradually (2460s and 1346s for 0.5M and 2.5M, respectively), but the cell volume change caused by it extremes was significantly reduced (14% and 43% of the initial volume for 0.5M and 2.5M, respectively), as was the cumulative toxic damage (15.19 and31.53 for 0.5M and 2.5M, respectively); (3) When compared to the usual loading approach, microdroplet loading by natural evaporation considerably enhanced cell survival and proliferation (P<0.05). Cell survival was 96.73%±0.54%, a 16% increase over one-step loading, and cell wall adherence was considerably better at 48h than with two-step loading. This innovative technique substantially reduced cell damage while simplifying the cryoprotectant loading process.
Abstract>The energy and environmental problems require the water chiller to operate with energy conservation and environmental pro-tection. Efficient compressor and refrigerant replacement become the key. The use of centrifugal refrigeration compressor with gas bearings and environmental friendly refrigerant is an important direction, in which motor cooling is the guarantee of performance and reliability. The mathematical model of centrifugal refrigeration compressor with gas bearings is established, and the influence of different refrigerants (R134a, R1234yf, R1234ze (E)) on the motor cooling process and refrigeration system is analyzed. The results show that the internal temperature of the motor is the highest when R1234ze (E) is used, and the maximum temperature of the permanent magnet of the motor when R1234ze (E) is 60 ℃ - 90 ℃ higher than that when R134a and R1234yf are used; The average winding temperature of the motor decreases with the increase of the cooling inlet temperature; When R134a and R1234yf are used, the maximum temperature of the permanent magnet decreases with the increase of the cooling inlet temperature. When R1234ze (E) is used, the maximum temperature of the permanent magnet increases first and then decreases with the increase of the cooling inlet temperature. When the cooling inlet temperature is about 25 ℃, the maximum temperature is reached. The outlet dryness of the motor cooling circuit decreases by about 3% - 5% when the cooling inlet temperature increases by 4.5 ℃. Compared with the traditional system, the system with motor cooling branch controlled the motor temperature within the safe operation range, and COP decreased by 1.23% - 1.82%, 1.23% - 1.65% and 1.14% - 1.17% respectively.?
Abstract>At present, the precise linear cooling of microfluidic is still a challenge in the development and application of microfluidics technology in medicine and life sciences. In order to realize linear cooling of the target area of microfluidic chip, a thermoelectric refrigeration-based microfluidic chip temperature control system was designed, and linear coupling between current, cooling capacity and microfluidic temperature was achieved through numerical simulation and experimental tests. The results show that when the thermoelectric cooler is driven by conventional constant current, the linearity of the cooling process of the microfluidic sample cell is only 0.598. The use of single-function unsteady current can significantly improve the linearity of microfluidic cooling, but there are still cold surpluses and gaps, which cannot meet the accuracy requirements of linear cooling. Finally, an unsteady transition current mechanism to realize linear cooling of thermoelectric cooled microfluidic is obtained by coupling a variety of current waveforms. Ana, linear cooling of microfluidic between 26°C ~ -19°C and 24°C/min ~ 42°C/min is realized, and the linearity is above 0.9981. Meanwhile, the linear cooling mechanism of thermoelectric cooled microfluidic is successfully verified by experimental tests.
