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• 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]

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]

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]

• 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]