Mechanical vapor compression (MVC) systems are energy-saving technologies that recover and reuse low-temperature waste heat resources, achieving energy conservation and carbon reduction. As the core equipment in MVC systems, the compressor directly affects the overall performance of the system. This article primarily reviews the thermodynamic and structural performance of vapor compressors, proposes relevant enhancement suggestions and improvement ideas, and provides a reference and assistance for the subsequent optimization of vapor compressor performance.

Improving indoor air quality in homes requires fresh air; however, this is a strain on air conditioning systems. To address this issue, the use of energy-efficient fresh air units equipped with exhaust air heat recovery is recommended. These units include both passive and active types, with prominent examples being air-to-air enthalpy heat exchangers and heat pump units. Currently, the evaluation of the energy efficiency in fresh air units predominantly revolves around air-to-air enthalpy heat exchangers, rendering the commonly used heat exchange efficiency inapplicable to heat pump units. The concept of exhaust air heat recovery is perplexing and contradictory. Furthermore, the assessment of fresh air units primarily focuses on the units themselves, without considering their impact on air conditioning units and the overall system performance once combined. This study aims to establish a unified definition of exhaust air heat recovery for fresh air units, elucidating its intrinsic meaning. Additionally, it proposes a comprehensive energy efficiency evaluation method for a combined fresh air and air conditioning system. Through a case study of seven existing fresh air unit types, the necessity of exhaust air heat recovery is highlighted, and the energy efficiency levels of different unit types are compared.

The high redundancy of the measured data from heating, ventilation, and air conditioning (HVAC) systems significantly reduces the computational efficiency of model calibration. To address this challenge, a model calibration method based on mining feature operating conditions and a priori probability guidance was introduced in this study. Correlation analysis was conducted on the operational data for mining feature operating conditions. Feature variables related to HVAC system operation were selected, and a grid sampling technique based on these characteristic variables was employed to obtain representative operating conditions, enhancing the efficiency of the model calculations. Additionally, a prior probability model was established for the parameters to be calibrated during the model calibration process. A priori interval estimation was then performed, and the objective function was improved based on the prior probability to guide the model towards faster convergence. The proposed method was validated using a one-month operational dataset from a cooling plant in an industrial building located in Wuhan, China. The results indicated that the proposed method achieved significant improvements in performance metrics. Specifically, mean absolute percentage error (MAPE) and cross-validated root mean square error (CV-RMSE) were reduced by 16.0% and 12.0%, respectively, compared to the K-means clustering-based method, and by 20.9% and 15.2%, respectively, compared to the baseline data-based method. Furthermore, the normalized mean bias error (NMBE) was closer to zero, and the coefficient of determination (R²) increased by 4.7% and 8.5%, respectively, compared to the two aforementioned methods. Additionally, our method enhanced the computational efficiency by approximately 39.3%. This method provides technical guidance and data support for achieving an efficient and accurate modeling of HVAC systems.

In cryogenic hydrogen storage systems, an accurate calculation method for the heat release from different ortho-para hydrogen catalytic conversions is important to determine the load of hydrogen storage systems. The objective of this study is to present a precise calculation method for the conversion heat of ortho-para hydrogen. By establishing a conversion model, the methods of using a smoothing spline curve to fit the experimental data and energy balance calculations are used to derive the released heat in adiabatic conversion multistage converters and the released heat during continuous conversion. The heat release properties of different conversion methods are analyzed in this study. Notably, the heat release amount of continuous conversion is the smallest, that of isothermal conversion is the largest, and the conversion heat of adiabatic conversion is in between, which is related to the number of conversion stages. In addition, a specific method and procedure are programmed to solve the implicit differential formula for continuous conversion. Finally, the calculation results of the different methods are consistent with a maximum deviation of only 0.22%, indicating that the calculations in this study are valid and accurate.

In this study, an experimental platform for cryo-adsorption and hydrogen storage systems was constructed to investigate the cryo-adsorption hydrogen storage law in the system and explore the kinetic and thermodynamic properties of cryo-adsorption hydrogen storage and the hydrogen storage performance of the entire system. The experimental results demonstrated that the adsorbent material exhibited an excellent hydrogen storage capacity under liquid nitrogen temperature zone conditions, and the adsorbent material demonstrated a high hydrogen storage capacity reaching mass-weight ratio of 5.02%, equivalent to the total hydrogen storage density of 16.63 kg/m³ under a charging pressure of 5 MPa and final storage pressure of 3.04 MPa. Through experimental research, the key factors affecting the hydrogen storage performance were revealed. These factors include the microstructural properties of the adsorbent materials, thermodynamic effects during the adsorption process, and experimental operating conditions. This experimental basis provides a foundation for optimizing the performance of hydrogen storage systems.

Seasonal thermal energy storage (STES) can effectively mitigate the supply and demand imbalance of solar energy between winter and summer. Large-scale water pit thermal storage systems require efficient and accurate computational simulations to avoid investment waste. This study proposes a simplified numerical analysis method and establishes a cylindrical underground pit with a total volume of 11 304 m³ to describe the operation of a STES system. The model establishes a one-dimensional heat transfer model for the water body and a two-dimensional heat transfer model for the soil, separately solving for the water and the soil temperature field. The two models are connected through the temperature boundary at the pool wall to simulate the entire system. To comprehensively verify the accuracy of the numerical simulation model, validation was conducted under standby, charging, and discharging modes. The results indicate that the developed model has good accuracy and reliability. Under the standby mode, the temperature error of the five water layers in the sandbox test is less than 10%, with the highest accuracy in the middle and lower-middle water layers, with an average absolute error of 1.75% and 1.24%, respectively. Under the charging mode, the average relative error is 1.57%, and the average temperature error is 0.44 ℃. Under the discharging mode, the average relative error is 0.46%, and the average temperature error is 0.24 ℃.

R1270 and R290 are alternative refrigerants with great potential. In this study, the influence of the refrigerant charge on the performance of the heat-pump water heater with R1270 and R290, the temperature distribution of the heat transfer fluids in the condenser, and the feasibility of replacing R22 with natural refrigerants were investigated. The results indicate that the refrigerant charge has a considerable influence on the cycle performance and the temperature distribution of the heat-transfer fluids in both systems. At the same optimal charge (0.90 kg), the R1270 and R290 systems achieved a maximum coefficient of performance (COP) of 4.443 and 4.317, respectively. At different refrigerant charges, two heat transfer pinch points and two maximum heat transfer temperature differences occurred in the condensers of both systems, and the locations of the first pinch point and the second maximum temperature difference point showed similar migrations with changes in refrigerant charge. Compared with the R22 system, both the R1270 and R290 systems, at optimal charge, achieved significantly better COP and discharge temperatures, exhibited relatively equivalent discharge pressures, and their heating capacities increased by 7.05% and decreased by 10.65%, respectively. Hence, R1270 can be preferred over R290 for replacing R22 in a heat-pump water heater.

With the development of computer technology and the application of artificial intelligence, electronic chips are becoming increasingly miniaturized and integrated, leading to a rapid increase in their volumetric heating power, thus affecting their normal operation. To address this problem, a heat sink with an array of finned porous microjets was designed, and HFE-7100, which has good thermal stability and electrical insulation, was selected as the cooling medium. Through a combination of numerical simulations and experimental research, the influence of factors such as the longitudinal aspect ratio of the slotted fins, inlet subcooling, inlet volumetric flow rate, and jet Reynolds number on the heat transfer process of microjet boiling was investigated. The results showed that the optimized structure with an aspect ratio of 0.5 met the requirements of chip cooling and had a better cooling effect. In the single-phase convection heat transfer stage, under the same working condition, the inlet subcooling degree had little effect on heat transfer, and increasing the volume flow rate or jet Reynolds number could strengthen the convection heat transfer, and the maximum heat transfer coefficient could reach 15 724.40 W/(m²·K). However, in the jet boiling stage, the heat flux corresponding to the onset of nucleate boiling (ONB), and it decreased with a decrease in the inlet subcooling degree. Increasing the inlet volume flow rate or jet Reynolds number inhibited the occurrence of boiling, thus weakening the heat transfer. However, compared with the single-phase convective heat transfer stage, the heat transfer coefficient increased by 20.6%.

Frosting is one main adverse factor hindering the application of finned tube evaporators in refrigeration systems. To accurately predict the growth characteristics of the frost layer, its growth behavior on the surface of a three-dimensional finned tube under forced convection conditions was numerically simulated based on the coupled VOF (volume of fluid) multiphase flow and phase change mass transfer rate model method. The maximum difference between the simulated frost layer thickness and the experimental results was within 15%. The estimated value of the frost layer density was within a confidence interval of up to 90%; this is in good agreement. By building a visualization experimental platform for frosting on the surface of the finned tube of the evaporator, the influences of ambient temperature, relative humidity, and frontal wind speed on frost growth characteristics under single-factor changes were analyzed. The results showed that the thickness of the frost layer gradually decreased from front to back in the direction of the airflow. The lower the air temperature, the greater the wind speed, the higher the relative humidity, the larger the thickness of the frost layer, the more frost, and the larger the thermal resistance of the frost layer. The thickness of the frost layer reached its highest value of 2.344 mm at 85% relative humidity. According to the Morris sensitivity analysis, the relative humidity had the greatest influence on the frost layer thermal resistance, and the sensitivity coefficient reached 2.41. The correlation of the frost layer thermal resistance with different environmental parameters was obtained using the least squares regression method.

The combination of a super-long gravity heat pipe and a heat pump system for harvesting deep geothermal heat has the advantages of low cost, high efficiency, and no groundwater contamination. Direct heat exchange between the evaporator of the heat pump system and the condenser of the gravity heat pipe can simplify the heat exchange process and improve the heating efficiency of the system. Therefore, a U-shaped evaporator-condenser was developed, and its heat transfer performance was studied by building an experimental platform combining a heat pump and a heat pipe. Notably, the heat transfer coefficient of the U-shaped evaporator-condenser reached 2 037.92 W/(m²·℃) when the working fluid on the heat pump side passed through the tube. Based on the homogeneous flow model, a one-dimensional steady-state evaporator-condenser heat transfer model was established by integrating the mass, energy, and momentum conservation equations with empirical formulas for condensation outside the tube and boiling heat transfer inside the tube. Using Python, the simulation results were compared with experimental data. Notably, the average deviation of the heat transfer in the evaporator-condenser was 18.91%, confirming the accuracy of the model and providing a theoretical calculation method for designing an efficient evaporator-condenser.