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• Boiling coefficient increased with nozzle inlet pressure and specific enthalpy. • Boiling coefficient ranged from 300 to 4000 in the investigated trans-critical region. • Modified HNB showed an accuracy of 3 % in predicting the nozzle flow rates. • Proposed method for determining saturation pressure was superior to constant value. A modified homogeneous non-equilibrium boiling approach (HNB) was proposed for high-fidelity modelling of the trans-critical flashing process in two-phase R744 ejector nozzles. In the modified approach, the method of determining the metastable liquid saturation pressure was improved by integrating the metastable liquid enthalpy. Experimental tests were conducted on two different nozzles to calibrate the lumped accommodation coefficient in the modified HNB. The calibration dataset consisted of 30 trans-critical test points, with the motive inlet pressure ranging from 7.5 MPa to 11.5 MPa and temperature varying from 21 °C to 45 °C. It was demonstrated that the boiling coefficient was strongly related to nozzle inlet pressure and specific enthalpy. The coefficient was therefore fitted as a function of the operating condition. The modified HNB with the fitted function was then validated against another set of test data composed of 29 test points. The results demonstrated that the relative errors of the nozzle mass-flow rates were within 3 %. Finally, the modified HNB with the improved metastable liquid saturation pressure was compared and discussed with the existing HNB approach in the literature. [ABSTRACT FROM AUTHOR]

• A simulation model of automobile CO 2 micro-channel gas cooler was developed and validated. • The inlet temperature of refrigerant has a great influence on heating performance of CO 2 gas cooler. • Unfavorable air temperature inhomogeneity appears on the airside outlet surface of gas cooler. • Expanding the depth of the gas cooler from 32 to 64 mm can improve the heating capacity by 29%. This study aims to investigate the supercritical heat transfer characteristics of compact micro-channel gas coolers applied in automobile CO 2 heat pump systems. A simulation model of automobile gas cooler was developed by using segment-by-segment method, and validated by experimental results. Using this model, under the typical heat pump operation conditions, the temperature distributions along the CO 2 refrigerant flow path were predicted, and the variation of heat transfer coefficient was analyzed. Furthermore, the impacts of operation conditions such as refrigerant inlet pressure and temperature, as well as configuration parameters such as depth and pass number on the heating performance were discussed. Results show that the established model has a good prediction accuracy for heating capacity of gas cooler. It is found that there is unfavorable air temperature inhomogeneity on the airside outlet surface of gas cooler. [ABSTRACT FROM AUTHOR]

Natural refrigerant CO 2 (R744) is one of next generation refrigerants in mobile air conditioner (MAC) application with superior environmental and heating performance. This paper investigated the charge characteristics and refrigerant migration behavior of a trans-critical CO 2 MAC. A novel visualized accumulator with five observation mirrors was adopted to set up this CO 2 MAC. A series of steady-state system experiments were carried out in a calorimeter facility to reveal the effects of refrigerant charge and EXV opening on system performance and accumulator behavior. In addition, the transient migration behavior of refrigerant during shutdown and start-up and the flow behavior of lubricant oil with 0.5 mm oil return hole were studied experimentally. It is found that a plateau region with optimal system performance occurred between 1100 g and 1600 g refrigerant charge. When the return hole becomes small, stratification of lubricant oil and liquid refrigerant was found in the accumulator. • The refrigerant migration behavior under different charge and EXV conditions was distinguished. • A plateau region between 1100 g and 1600 g charge with optimal system performance was determined. • The criteria for determining the starting and ending points of plateau region are proposed. • Unnecessary liquid refrigerant evaporation during start-up may bring rapid transient response. • Lubricant oil was stratified in the accumulator when the diameter of oil return hole was 0.5 mm. [ABSTRACT FROM AUTHOR]

Iron sulfide is a promising candidate anode material for high performance Na-ion batteries because of its high theoretical capacity and low price. Unfortunately, it usually shows frustrating performance because of its inherent low conductivity and large volume expansion during charge–discharge processes. Here, we have synthesized uniform yolk–shell structured FeS@mesoporous carbon spheres (FeS@meso-C) by combining a facile one-pot coating strategy with a sulfuration method. The resultant yolk–shell structured FeS@meso-C with a diameter of ∼200 nm possess ∼120 nm single crystalline FeS yolks, ∼40 nm void space and uniform mesoporous carbon shells (∼15 nm thickness and ∼5 nm mesopores). In this unique structure, the void spaces provide enough room to buffer the FeS expansion, the mesopores in the carbon shells facilitate the transport of Na ions and the mesoporous carbon shells can improve the conductivity of the active composites. When used as the anode for Na-ion batteries, the yolk–shell structured FeS@meso-C show a high specific capacity of 596 mA h g−1 even after 100 cycles at 0.2 A g−1 and show an excellent rate capability (capabilities of 578, 438, 420, 358, 320, 256 and 214 mA h g−1 at 0.2, 0.5, 1, 2, 4, 8 and 10 A g−1) [ABSTRACT FROM AUTHOR]

Highlights • Performance of a novel CO 2 heat pump system with Series Gas Cooler (SGC) has been experimentally tested in both Air Conditioning (AC) and Heat Pump (HP) mode. • A numerical model of CO 2 SGC HP system has been established and validated with experiment results. • The impacts of ambient temperature, discharge pressure and degree of superheat (DSH) on the HP system has been studied using the numerical model. • A comparison between conventional CO 2 system and SGC HP system is carried out based on the model. Abstract The present study aims to investigate the performance a novel CO 2 heat pump system designed for electrical vehicle with Series Gas Cooler (SGC) configuration. A numerical model is developed, and experiments in both Air Conditioning (AC) and Heat Pump (HP) mode are performed to validate this model. Based on the validated model, the impacts of ambient temperature, discharge pressure and degree of superheat on the system are discussed. Additionally, a comparison between conventional CO 2 system and the novel SGC heat pump system is carried out using the developed model. The model can predict system capacity and COP with average errors of 5.1% and 6.0% in AC mode, and average errors of 7.2% and 5.8% are achieved in HP mode. The introduction of SGC into the heat pump system can bring an average increase by 33.7% for the system heating capacity, and an average increase by 35% is achieved for the system COP. [ABSTRACT FROM AUTHOR]

The electric vehicles (EVs) are drawing much attention worldwide due to low fuel consumption and local pollution. However, the range of EVs is still not parallel to that of internal combustion engine vehicles. Moreover, the EV's range suffers a significant decrease in cold and hot climate due to the climate control load of the air conditioning system. These two combined making the range extension strategies a hot spot in EV research. In this paper, the review on EV range extension strategies is provided. First, the climate control load reduction strategies are reviewed. The characters of heating load in cold climate and the reduction methods are introduced. Second, the review on heat pump heating is conducted based on the different structures of heat pump systems. The combination of load reduction and heat pump heating seems to be a promising way in solving the range reduction problems. [ABSTRACT FROM AUTHOR]

The main purpose of this work is to enhance the energy efficiency of CO 2 automobile air conditioning system. Theoretical analysis demonstrated that the mixture of CO 2 and propane can improve its performance, thus, experiments have been carried out to see effects of various CO 2 -propane mass fractions of 100/0, 90/10, 80/20, 70/30, 60/40, 50/50 on the system performance at different ambient temperatures and gas cooler frontal air velocities. Experimental results show similar trends with those from the theoretical results. It has been shown that under the same compressor speed, system COP reaches highest at 60% of CO 2 mass fraction, which is 29.4% higher than pure CO 2 system and even achieves equal level of the R134a system, the optimum pressure and discharge temperature are reduced up to a maximum of 40% and 47   °C during the research range. Furthermore, comparison was carried out under the same cooling capacity by adjusting compressor speed for different mass fraction of CO 2 , results demonstrate that the use of CO 2 -propane mixtures yields a maximum COP rise of 22%even when cooling capacity is kept constant. A new optimum high pressure control algorithm for the transcritical CO 2 -propane mixture cycle has been developed based on the experimental data within a deviation of 5%. [ABSTRACT FROM AUTHOR]

This study investigated the heating performance characteristics of a CO 2 heat pump system for an electrical vehicle in a cold climate. Experimental tests evaluated the effects on system performance of outdoor temperature, outdoor air velocity, indoor temperature, indoor air flow rate, compressor speed, and EXV opening. The results of heating experiments when both the indoor and outdoor temperatures were −20 °C showed a coefficient of performance (COP) of 3.1 and a heating capacity of 3.6 kW. The COP was 1.7 when the outdoor, indoor air inlet, and outlet temperature were −20 °C, 20 °C, and 40 °C, respectively. Therefore, the heat pump using CO 2 refrigerant achieved good heating performance in a cold climate. Additionally, a new secondary loop heat pump was also compared with the conventional heat pump, and the test results show that use of the secondary loop heat pump reduced COP by 19%. [ABSTRACT FROM AUTHOR]

This study investigates four influence factors of flow distribution in multi-circuit evaporators with distributor, including distributor inlet tube length, inclination angle, uneven airflow, and feeder tube length. Experimental tests and numerical simulations were carried out to evaluate the effect of these factors on superheat uniformity and refrigerant distribution performance. Additionally, a feeder tube compensation method for optimizing refrigerant distribution is presented that involves pressure drop fine-tuning of the feeder tube in each circuit coupled with a main expansion device to obtain more uniform superheat performance at all circuit outlets. To adopt this method, a model was developed to predict the best feeder tube length for each circuit. Experimental results show that when using feeder tubes with predicted lengths in condition of uneven airflow, the superheat uniformity was improved significantly, and up to 35% evaporator cooling capacity could be enhanced when airflow non-uniformity coefficient ( F a i r ) was 0.6. [ABSTRACT FROM AUTHOR]

CO2 (GWP = 1) is considered as a promising natural alternative refrigerant to HFC-134a in mobile air conditioning (MAC) applications. The objective of this study is to investigate the cooling performance characteristics of a CO2 MAC system. A prototype CO2 MAC system, consisting of a CO2 electrical compressor, CO2 parallel flow microchannel heat exchangers, and an electrical expansion valve, was developed and tested. Factor analysis experiments were conducted to reveal the effect of outdoor temperature on the cooling performance of this CO2 MAC system. Compared with a conventional R134a MAC system, the prototype CO2 MAC system achieved comparable cooling capacity, but had COP reductions of 26% and 10% at 27 °C and 45 °C outdoor conditions, respectively. In addition, based on refrigerant properties, theoretical cycle analysis was done to reveal the impact of evaporator, gas cooler and compressor, on the system cooling performance. It is concluded that the increase of overall compressor efficiency or the decrease of gas cooler approaching temperature could greatly improve the COP of this CO2 MAC system. [ABSTRACT FROM AUTHOR]

Highlights • The heating energy consumption of a battery electric vehicle was evaluated under the new Worldwide Harmonized Light-duty Test Procedure driving cycle. • A heating load prediction model was established and validated with an error within 5%. • Heat pump Scenarios lead to 41% - 72% lower energy consumption than PTC. • Utilizing moderate recirculation air is a smart way to reduce the overall consumed energy. • Driving range extension is between 14.2% and 31% higher than PTC for different heat pumpscenarios studied. Abstract The aim of this work is to evaluate the cabin heating energy consumption and driving range for a battery electric vehicle. The vehicle was tested in climatic wind tunnel laboratory under Worldwide Harmonized Light-duty vehicle Test Cycle. A heating load model was established and used to predict the heating requirements of the vehicle in different climate conditions. Three different heat pump systems and five refrigerants are also investigated. Results show an error within 5% for the established heating load model. Compared with PTC heating, the heat pump systems lead to much lower energy consumption between 41% and 72% for different scenarios. Besides, 20% and 30% recirculation air can reduce the overall consumed energy by an average of 28.5% and 36.6%, respectively. Finally, the driving range of all the heat pump systems are much higher than that of PTC, which is between 14.2% and 31%. [ABSTRACT FROM AUTHOR]

Highlights • CO 2 /R41 blends in mobile air conditioning and heat pump system are conducted. • 14.5% and 25.7% efficiency improvement in heating and cooling mode are obtained. • System pressure, gas cooler outlet temperature and pressure drop are reduced. • Flammability of CO 2 /R41 blends is studied to certificate its safety application. • An optimum pressure correlation for trans-critical CO 2 /R41 cycle is developed. Abstract This study primarily aims to evaluate the cooling and heating characteristics and provide a performance comparison of CO 2 , R41, and several CO 2 /R41 blends for applications in automobile air-conditioning and heat pump systems. Experiments were conducted to reveal the effects of the refrigerant charge, compositions, ambient temperatures, and compressor speeds among the trans-critical and subcritical cycles. The performance merits of the environmental friendliness, energy efficiency, and safety of using CO 2 /R41 blends are emphasized. The results demonstrate that, at an optimum charge amount, the coefficient of performance of pure CO 2 in the heating and cooling modes can be improved up to a maximum of 14.5% and 25.7%, respectively, by increasing the R41 mass fraction. The heating and cooling capacities were both decreased with an increasing R41 mass fraction, as the system mass flow rates decreased owing to the reduction in suction density. The overall system operation pressure, gas cooler outlet temperature, and high- and low-side pressure drops in both the heating and cooling modes were reduced significantly when the mass fraction of R41 increased from 0% to 100%. Furthermore, the effect of the CO 2 mass fraction on the flammability of CO 2 /R41 blends was studied to discuss the advantages of its safety application. Finally, based on the experimental data, a correlation of the optimum high-side pressure for the trans-critical CO 2 /R41 cycle was developed, showing a deviation of ±5% from the data. [ABSTRACT FROM AUTHOR]

Highlights • A new modified autocascade heat pump system for electric vehicle heating in cold climate is proposed. • Both transcritical and subcritical cycle characteristics using CO 2 -propane are theoretically evaluated. • Results show the new system has much better performance than conventional cycle. • The feasibility of the ACHP system is verified and further optimization locations in the ACHP system is pointed out with exergy analysis. Abstract Heat pump and its efficiency are of great significance to the energy saving of electric vehicle in cold climate. This paper proposes a modified autocascade heat pump (ACHP) system using natural CO 2 -propane mixture. The main purpose is to enhance the heating performance compared with that of conventional heat pump. Using a thermodynamic model of the proposed ACHP cycle, the transcritical and subcritical cycle performance under various conditions are carried out from the perspective of first and second law of thermodynamics, and the transcritical operating characteristics of ACHP system is introduced for the first time. Additionally, an exergy performance comparison has been carried out. Results indicate that the ACHP system can produce more heating capacity, lower compression pressure ratio, lower heat rejection temperature for low ambient temperature conditions and it consumes less compressor work for all the temperature range researched. For transcritical ACHP cycle, the best COP is 2.7 obtained when CO 2 mass fraction is 0.8 at −20 °C, which is 12.3% higher than that of CO 2 single stage heat pump. For subcritical ACHP cycle, the best COP is 3.97 while heating capacity is much lower than that of transcritical cycle in the same condition. Finally, the exergy analysis confirms the feasibility of the ACHP system and points out exactly the further optimization locations in the ACHP system. [ABSTRACT FROM AUTHOR]

• An automotive CO 2 air conditioning system with evaporative gas cooler was tested. • COP was improved by 13.1-52.9% with evaporative gas cooler. • Evaporative gas cooler was more efficient at lower air velocities and higher ambient temperatures. • Spraying in the area where wall temperature is low delivered a better COP enhancement. A well-known challenge in the application of carbon dioxide (CO 2) refrigerant for vehicles is the great energy efficiency attenuation in hot climates. In attempting to solve this problem, a novel design integrating evaporative cooling with automotive CO 2 air conditioning system, where water was sprayed to the gas cooler, was introduced and experimentally investigated in present work. Results illustrate that with this design the CO 2 temperature leaving gas cooler could be reduced to a lower level or even below the outdoor air temperature, thus inducing a significant performance improvement. A coefficient of performance (COP) enhancement percentage of 42.4% was achieved at 35°C outdoor condition. This percentage was increased up to 52.9% when outdoor air temperature increased to 45°C. With the decreased outdoor air velocity and the decreased outdoor air humidity, the merit of this design was more significant. These indicate that the evaporative gas cooler is a promising technology to alleviate the problem of performance deterioration in hot and dry weather conditions. A reduced spray water temperature could not cause a significant change in the COP enhancement. This may be attributed to that the sensible heat during droplet heat-up period was negligible compared to the latent heat of droplet evaporation. When the surface position of gas cooler chosen to be sprayed was moved from CO 2 inlet to CO 2 outlet, the COP enhancement increased, which suggests that water should be sprayed to the area where wall temperature is low. [ABSTRACT FROM AUTHOR]

• The front anti-collision beam in a car leads to air maldistribution. • A six-path condenser is an optimization of the traditional four-path condenser. • The new condenser shows better capacity with the anti-collision beam. • The cooling performance of automobile air conditioning system is improved. To improve the efficiency of automobile air conditioning and reduce the impact of non-uniform air velocity distributions caused by the front anti-collision beam on automobiles, we designed a new six-process condenser and tested it under different working conditions. A baffle was positioned to simulate the anti-collision beam during the experiment. We analyzed the condenser by itself and the system level performance of this newly designed condenser under three working conditions compared to a traditional four-process condenser. Condenser heat exchange capacity, condenser inlet pressure, system refrigerant flow rate, refrigerant flow resistance, thermal imaging of condenser outlet air, overall system cooling capacity, and coefficient of performance were compared. The coefficient of performance of the system with this new condenser improved by 8.5%, 7.8%, and 10.3% for the three tested working conditions. [ABSTRACT FROM AUTHOR]

Vapor injection technology has been applied to improve the performance of heat pump systems at low ambient temperatures. However, little attention has been paid to the application of heat pump systems combined with the economized vapor injection for electric vehicles. In this study, the performances of two electric vehicle system configurations were compared, including the upstream injection and the downstream injection. The effects of the injection pressure, the compressor speed, the condenser and outdoor inlet temperatures on the overall system performance and specific operating parameters were measured and analyzed in detail. The results indicated that the downstream injection exhibited better system performance. At the compressor speed of 6500 rpm, the maximum heating capacity of the downstream injection was 18.6% and 8.9% higher than that of the upstream injection at the condenser inlet and outdoor temperatures of −2 °C and −12 °C, −7 °C and −18 °C, respectively. At the compressor speed of 4500 rpm, the COP of the downstream injection was 12.9% higher at the condenser inlet and outdoor temperatures of −7 °C and −18 °C. [ABSTRACT FROM AUTHOR]

With carbon emission levels on the rise, rapid and far-reaching action is needed to counteract global warming. Among many available strategies, CO 2 is nowadays more and more often proposed as a solution for heating, cooling and refrigeration purposes since the extremely low global warming potential and being natural. In order to overcome the inherently low efficiency in high-temperature conditions and high operating pressure especially in transcritical cycles, an updated review of the advances on modified technologies to solve the drawbacks of CO 2 refrigeration is provided and recent progress on the energy efficiency improvement is summarized. First, the basic principles of the CO 2 refrigeration cycle and important performance characteristics are discussed. Then, a detailed discussion on different modified technologies as well as their operating fundamental, technical features and performance are provided, followed by a summary of previous studies. At the end of this review, conclusion and perspectives on the future development of this field are presented. • The principles of the CO2 refrigeration cycle and performance are discussed. • A detailed discussion on different modified technologies is provided. • Recent progress on the energy efficiency improvement is summarized. • Perspectives on the future development, research opportunities are presented. [ABSTRACT FROM AUTHOR]

To improve the efficiency of mobile air conditioning and to meet the need of protecting the environment, performance characteristics of the R1234yf heat pump were tested under various conditions in a cold climate. R1234yf is considered a promising replacement for R134a because of its low global warming potential (GWP). Many studies have examined the cooling performance of R1234yf, but little attention has been focused on its heating performance in cold climate. In this work, the system performance of R1234yf heat pump was analyzed and compared with that of R134a under different influencing factors such as charge, outdoor temperature, indoor temperature, outdoor wind speed, indoor air mass flow rate, compressor speed, inner condenser width, and economized vapor injection (EVI). The results obtained are presented in detail from the view of experimental analysis and comparison. [ABSTRACT FROM AUTHOR]

The environmental-friendly heat pump with low global warming potential (GWP), such as R1234yf, CO 2 , R290, and several potentially promising refrigerant mixtures, is increasingly essential for the electric vehicle (EV) to save energy consumption and extend the driving range. It is beneficial to achieve carbon neutrality by reducing both direct and indirect carbon emissions. Previously, the life cycle climate performance (LCCP) was a widely accepted metric to evaluate the carbon footprint of mobile air conditioning systems "from cradle to grave" for the internal combustion engine vehicle (ICEV), however, such LCCP analyses about EV heat pumps can hardly be found. To facilitate the EV industry and policymakers' better understanding of the environmental impacts of those low-GWP refrigerants, this study provided a comprehensive LCCP analysis for the EV heat pumps based on the system bench test results, local climates, local power supply characteristics, real-world driving patterns, vehicle cabin thermal sensation, and climate control load. Five low-GWP refrigerants, i.e., R1234yf, CO 2 , R290, binary blends of CO 2 and R41 (with GWP values of 49), M2(R410A substitute with GWP values of 137), were compared against R134a and R410A in 31 provinces of mainland China. We also considered the impacts of electric vehicle adoption rates and the carbon intensity of electricity from 2020 to 2060. Results show that the total life-cycle emissions per vehicle are highly related to climate conditions in different provinces, while R290 shows the lowest emission on the national average. The national mean life-cycle emissions per vehicle are projected to decrease 14% by 2030 and 45.4% by 2060 under the current technological trajectory of electricity generation. By using low-GWP heat pump systems in EVs, the cumulative emissions counting all vehicles can be saved up to 1450 Mt. CO 2 e from 2020 to 2060. • Developed a comprehensive LCCP model for electric vehicle heat pumps. • Figured out current and future benefits of low-GWP refrigerants in EV heat pump. • 31 provinces of mainland China were evaluated. • R290 showed lowest life-cycle emissions on the national average. • Heat pump can save 1450 Mt. CO2e cumulative emissions from 2020 to 2060. [ABSTRACT FROM AUTHOR]

• Performance of a two circuits CO 2 train AC system has been experimentally tested under different operating conditions. • An introduction of a numerical model of CO 2 train AC system has been presented. • Effects of operating conditions on the train AC system have been discussed in detail. • An error analysis between the test and the numerical has been presented. • A related equation between the gas cooler outlet temperature and the optimum high pressure is presented. This paper aims to investigate the effects of different operating parameters on the performance of a transcritical CO 2 air conditioning system for trains. Similar designs of automotive CO 2 air conditioning system are used in the train system such as internal heat exchanger (IHX) to improve the system efficiency. Experimental results are presented for the performance characteristics of the CO 2 train air conditioning system with two independent circuits under different test conditions such as different refrigerant charge amount, compressor working frequency, indoor air volume flow rate and outdoor chamber temperature. Two different operating modes of the two circuits are also included in this research. For mode I while only one circuit is working, the test results show that the maximum cooling capacity and coefficient of performance (COP) is 24.3 kW and 2.7, during 55 Hz and 40 Hz compressor working frequency conditions respectively. Also, the cooling capacity and COP reach the minimum, 16.2 kW and 1.5, when the outdoor chamber temperature is 45°C. For mode II while both two circuits are working, the results show that the cooling capacity and COP are 42.0 kW and 2.4, respectively. The system performance error between the test and numerical modeling which directs the design of the system is within ±5%. Furthermore, an optimum high pressure control algorithm for the transcritical CO 2 cycle of the train AC system is also presented. [ABSTRACT FROM AUTHOR]