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Abstract: The cycle frosting and defrosting performance of two types of microchannel heat exchangers were investigated. All the processes were observed using a CCD camera to better understand the cycle frost mechanism. Ice blockage formed in the fin root gaps of the horizontal-tube sample because of water retention. Cycle operation increased the blockage severity until the fin space was completely blocked. The amount of water retained and its impact on frosting time, pressure drop, and capacity were investigated. With increasing water retention, frosting time decreased, air pressure-drop and capacity could not return to the initial value after each defrosting time. Approximately 800g of water was retained on the heat exchanger after four operating cycles, causing the ice blockage that shortened the effective operating time by 40min compared with that of the vertical-tube sample at the end of the test. At the beginning of the fifth frost cycle, air pressure-drop had reached thrice the initial pressure drop, even when no frost was on the surface. The capacity decreased by 27% compared with the initial value. However, the vertical-tube sample exhibited no obvious water retention on the surface; as such, pressure drop and capacity experienced a similar degradation process during each cycle. The distribution of ice crystals on the fin surface was also studied, and the frosting process was divided into three periods: initial, developing, and fully grown. With increasingly serious water retention, frost only formed at the fin front-end surface, and could only reach the initial period because the ice blockage rapidly increased the pressure drop, thereby causing the defrosting process. [Copyright &y& Elsevier]

The heat pump is increasingly popular for the electric vehicles (EVs), however, there are no appropriate refrigerants. To provide long-term and environmental-friendly refrigerants in the heat pump for EVs, a test bench that can investigate the performance of an R410A prototype was built. The drop-in tests of three refrigerants (M1, M2, M3) developed previously with ultra-low GWP were carried out. The cooling and heating cycle performance were measured for R410A and alternative refrigerants. Experimental results showed the optimal refrigerant charge for the alternatives was less by 36-44% than that of the R410A system. For M1 and M2, the heating capacities showed 85-99% of those of the R410A system, with COP being 94-112%; the cooling capacities showed 94-109% of those of the R410A system, with COP being 99-107%. M3 showed much better COP than that of R410A system. The pressure drops were lower than those of R410A up to 21.6%, 25% and 55.7% for M1, M2 and M3, respectively. Furthermore, the alternative refrigerants showed better temperature match with air in the evaporator and condenser, the irreversible loss in the heat exchangers was also analyzed. This research will contribute to providing appropriate refrigerant substitutes in heat pump for EVs. [ABSTRACT FROM AUTHOR]

• Robustness of two lubricants, PAG and POE, was compared in an EV heat pump. • The PAG shows poorer compressor lubrication robustness than the POE. • Insoluble salts appeared in the PAG-R134a mixture at temperature lower than −10 °C. • Insoluble salts generated from additives and moisture could block the oil flow. Lubricating oil is the key to electric compressor performance, reliability, and durability in electric vehicle (EV) heat pumps. Previous investigations have revealed the lubricant's impact on fluid properties, component and system performance in vehicle climate control systems, but there have been few studies on heat pump application from the "robustness" perspective. In this study, we experimentally evaluated the robustness of two commercially available lubricants, polyalkylene glycol (PAG) and polyol ester (POE), for an electric scroll compressor in an electric vehicle heat pump by lubricant-refrigerant miscibility test, heat pump bench test and retrieved oil composition analysis. We showed that even if the PAG-R134a mixture kept miscible at low temperature, some insoluble substances would appear in the suction side accumulator below −10 °C, resulting in insufficient oil-return, increased compressor discharge temperature and superheat, and thus poor compressor lubrication. The mass spectrum analysis showed that the insoluble substances were reaction products of moisture and additives in the PAG oil. However, the POE oil used in this study did not show any insoluble substance under the same test condition. By comparison, the POE oil showed better robustness than the PAG. The experimental results indicate that the chemical composition of the lubricant should be carefully considered to ensure the robustness for compressor lubrication in EV heat pumps. [ABSTRACT FROM AUTHOR]

• HFC-152a was tested in a SL system and compared with baseline in a DX system. • The SL system showed heating performance attenuation compared with the DX system. • A heat exchanger was added in the SL system to use waste heat in the passenger cabin. • The COP of the improved SL system achieved the comparable level to the DX system. In mobile air conditioning (MAC) systems, the secondary loop (SL) structure was usually adopted when using flammable refrigerants such as R1234yf, R290 and R152a. The introduction of intermediate heat exchangers in SL system inevitably increased the temperature difference between the condensation side and the evaporation side, thereby increased the compressor power consumption and caused the performance attenuation. To overcome the attenuation, this study designed a heat recovery device which collects the waste heat from the passenger compartment to increase the evaporation temperature and reduce the compressor power consumption. Validation experiments were completed in a secondary loop heat pump (SLHP) MAC system using R152a and was compared with a baseline direct expansion (DX) system. According to experimental results, under the -10 °C outdoor temperature, the SLHP system showed performance attenuation compared with the DX system. With the increase in indoor temperature, the attenuation gradually increased to 8.96% at the indoor temperature of 20 °C. In this case, a heat recovery design was used in the SLHP system, which increased the coefficient of performance (COP) by 9.29% compared with the original SLHP system and achieved similar performance to the DX system. [ABSTRACT FROM AUTHOR]

• Evaluated mixtures with GWP less than 150 to substitute R410A. • Carried out an extensive screening with five components at most. • Found 34 mixtures that fulfill all the criteria. • 4 mixtures have almost the same vapor pressure to that of R410A. To provide alternative refrigerants for R410A in those applications where the GWP (global warming potential) limit is extremely restrictive in the medium and long term. This work investigated refrigerant blends in both air-conditioning and heat pump applications that can provide similar theoretical cycle performance to R410A. We carried out an extensive screening of blends with a maximum of five components among a list of 12 pure refrigerants comprising three hydrofluoroolefins, five hydrofluorocarbons, two hydrocarbons, CO 2 and R13I1. These mixtures were subjected to a multi-parameter optimization process consisting of GWP, flammability and thermodynamic models. The screening metrics included low GWP (≤150), mild flammability, small temperature glide (≤10 K), maximum COP (coefficient of performance) and similar volumetric capacity. In total, 34 mixtures were identified to fulfill all the criteria, all of them presented mild flammability and a GWP value below 150, in which 11 mixtures were less than 10. Most of the selected mixtures operated with temperature glide less than 5 K, the mixtures composed by R32/R1123/R13I1 offered an increment of - 4% to 17% for the volumetric capacity, but the COPs decreased slightly from 5–10%. Finally, we found 4 mixtures that had almost the same vapor pressure to that of R410A, they were R32/R1123/R161/R13I1 (20/40/10/30), R1123/R161/R13I1 (65/5/30), R1123/R152a/R13I1 (65/5/30) and R1123/R1234ze(E)/R13I1 (65/5/30). The screening results fulfill all the restrictions for the first time, though a further experimental test is needed to confirm the real applicability. [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]

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]

• 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]

• Air conditioning load reduction strategy in vehicles using recirculated air is provided. • Utilizing the recirculated air can extent the annual driving range by 11−30%. • Energy saving of heat pump heating in electric vehicle air conditioning is provided. The stock of electric vehicle is booming recently. However, the range of the electric vehicle is still a main barrier in its adoption. Air conditioning system is the second largest energy consumption device on vehicle, which will cause significant range reduction during operation. In this paper, the energy-saving effect of utilizing the recirculated air in air conditioning system is evaluated. The outside air ventilation rate based on CO 2 limits and windshield anti-fog requirements are calculated, and the energy-saving effect of this strategy in 30 cities across China is evaluated using an annual energy consumption model. It is found that utilizing the recirculated air can save 14–46% heating energy when using PTC heater, and 33–57% heating energy in heat pump AC system. Throughout a year, utilizing the recirculated air can extend the driving range by 11–30%. Image, graphical abstract [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 • 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]

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 • R427A, R161 and R290 were firstly used to substitute R22 in EVI ASHP for heating. • Performances of four refrigerants were measured in high pressure ratio condition. • The capacity, COP, discharge temperature, IPLV (h) and LCCP were compared. • R290 is the most synthetically beneficial alternative. Abstract To help reduce the warming impacts, this study evaluates the drop-in performances of R-427A, R-161 and R-290 in R-22 air source heat pump with EVI (Economized Vapor Injection). The results show that the IPLV(H) of R-161 and R-290 arises 5∼8% relative to R-22 with EVI on. A reversal occurs between R-290 and the baseline with EVI on, making the IPLV(H) of R-290 superior to that of R-22 by 3.6∼5.2% from worse performance with EVI off. At -20/41°C condition, the capacity and COP of R-290 increase by 38% and 19%, respectively. Considering COP and discharge temperatures, R-290 is better-performed to replace R-22 in high pressure ratio conditions. Based on LCCP (Life Cycle Climate Performance) model, 11% of the equivalent CO 2 emission can be reduced by using R-161 and R-290. However, the discharge temperature of R-161 is high in the high pressure ratio condition. R-290 is the most synthetically beneficial alternative. [ABSTRACT FROM AUTHOR]

Highlights • A propane heat pump system is proposed as a solution for electric vehicle. • The heating performance of the propane heat pump system was studied. • Different technical solutions have been compared. • Propane heat pump system will be the most competitive solution for electric vehicle. Abstract To solve the problem of driving mileage reduction for electric vehicles during winter resulting from high power consumption for heating and to deal with the global warming issues, propane heat pump is proposed as a novel solution in this paper for the first time. The heating performance characteristics of a mobile propane heat pump system for electric vehicles in cold climate under various operation conditions have been experimentally studied. Different technical solutions for heat pump are also compared in the research. According to the results, propane (R290) system is the strongest competitor at an outdoor ambient temperature above −10°C among all the strategies while CO 2 system may have small advantages when the ambient temperature is −20°C. However, due to CO 2 's poor cooling COP (Coefficient of performance) at high ambient temperature and its high operating pressure, propane heat pump system will be the most competitive solution for electric vehicles. [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]

Electronic expansion valve (EEV) takes advantages of its fast response, wide working range and accurate control and is been widely used in CO 2 heat pump and air-conditioning system while the CO 2 mass flow characteristics is the most crucial parameter for EEV. Different from other refrigerants, at the inlet of EEV, CO 2 maintains at supercritical state. In this study, the effects of inlet pressure, inlet temperature, outlet pressure, EEV flowing area and EEV geometries on CO 2 mass flowing through EEVs have been investigated under a wide operation range for three different EEVs. Meanwhile, a new empirical correlation has been developed to predict the mass flow rate of CO 2 flowing through EEVs based on the experimental data. The proposed dimensionless correlation shows a good agreement with the test data. Approximately 91.8% of the experimental data within a relative deviation of ±5% for all EEVs. The average deviation and standard deviation of the predictions are 0.36% and 1.93% respectively. [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]