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The use of vapor compression system-based air-conditioning units is predominant in conventional fossil-fueled and electric vehicles. Mobile air-conditioning (MAC) units play a significant role in direct and indirect CO2 emissions from vehicles. Refrigerant and component efficiency are the major factors that determine CO2 emissions from air-conditioning units. Therefore, it is essential to enhance the performance of MAC systems using environmentally friendly refrigerants that satisfy environmental requirements. In this study, we demonstrated the measures taken to minimize CO2 emissions by replacing high global warming potential HFC-134a, charge optimization methods, and performance enhancement strategies. HFO-1234yf could be better alternative for retrofitting HFC-134a, although it exhibited a slightly poor performance with mid-flammability. Mixtures of hydrocarbon-based refrigerants are suitable for secondary loop applications, and R-744 is the best option in cold climates. During charge optimization, the subcooling and superheating plateau methods were mostly adopted. To enhance system performance, most researchers have used variable displacement compressors, efficient microchannel evaporators, integrated received drier condensers, and suction line heat exchangers. Recently, Nano-seeded lubricants have been used in some MAC applications; however, further research is required on the condensation and evaporation aspects. The present review could be very helpful for researchers and manufacturers in developing MAC systems with lower CO2 emissions. [ABSTRACT FROM AUTHOR]

In this work, the performance enhancement of a HFO-1234yf mobile air conditioning (MAC) system with a suction/liquid line heat exchanger (SLHX) was carried out experimentally by tuning the thermostatic (constant superheat) expansion valve (TXV) and its impact on the environment was also evaluated. The optimum charge of HFO-1234yf and HFC-134a systems was found to be 670 g and 740 g, respectively. The results showed that the HFO-1234yf system with SLHX had better coefficient of performance (COP) and exergy efficiency when compared to HFC-134a system with SLHX at idling condition, whereas it had reduced performance at other speed conditions. The tuning of the TXV in the HFO-1234yf system had a positive influence on the COP, cooling capacity, and exergy efficiency and those were higher than that of existing HFC-134a system by 4.3–8.6%, 6.5–10.1%, and 3.7–5.1%, respectively, at idling and city speed conditions, whereas those were slightly lower at high-speed conditions. The total CO2 equivalent emission of tuned and un-tuned HFO-1234yf system was 27.98% and 24.64% lower than that of the existing HFC-134a system. The outcome of this study indicated that the SLHX implementation in the HFO-1234yf MAC system with tuned TXV could be a possible option to replace HFC-134a. [ABSTRACT FROM AUTHOR]

The flow boiling heat transfer coefficients of R-134a/R-290/R-600a (91%:4.068%:4.932% by mass) refrigerant mixture are experimentally arrived in two tubes of diameter 9.52 and 12.7 mm. The tests are conducted to target the varied heat flux condition and stratified flow pattern found in evaporators of refrigerators and deep freezers. The varied heat flux condition is imposed on the refrigerant using a coaxial counter-current heat exchanger test section. The experiments are performed for mass flow rates of the refrigerant mixture between 3 and 5 g s-1 and entry temperature between -8.59 and 5.33°C which are bubble temperatures corresponding to a pressure of 3.2 and 5 bar. The influences of heat flux, mass flow rate, pressure, flow pattern, tube diameter on the heat transfer coefficient are discussed. The profound effects of nucleate boiling prevailing even at higher vapor qualities in evaporators are highlighted. The heat transfer coefficient of the refrigerant mixture is also compared with that of R-134a. [ABSTRACT FROM AUTHOR]

In this study, an attempt was made to extend the comfort of a passenger car cabin during the compressor off cycle using thermal energy storage (TES) in an HFO-1234yf mobile air conditioning (MAC) unit for idle stop/start vehicles. Fatty acid (OM08), as a phase change material (PCM), with 0.1–0.5 vol% of graphene nanoplatelets (GnPs) was used in this study. It was found that the inclusion of GnPs increases the thermal conductivity and dynamic viscosity of the liquid PCM nanocomposites by ~ 46% and ~ 53%, respectively, with 0.5 vol% of GnPs. During the pull-down cycle, the enhanced thermal conductivity outweighs the increased dynamic viscosity, resulting in a quicker decrease in PCM temperature. The test results revealed that the cabin temperature increases through the addition of TES, with a marginal decrease in the coefficient of performance. The addition of TES with the use of pure PCM increases the compressor power consumption of the MAC system by less than 1%. However, with the inclusion of graphene the power consumption increases with respect to the volume fraction. Without TES, the cabin comfort is extended by 78 s, 60 s, and 43 s for heating loads of 500, 1000, and 1500 W, respectively, and with the inclusion of TES, using pure PCM, the cabin comfort increased by up to 106 s, 87 s, and 63 s, respectively. The inclusion of 0.5 vol% GnPs extends the cabin comfort further by up to 189 s, 147 s, and 105 s for heating loads of 500, 1000, and 1500 W, respectively. Further, the CO2 equivalent emissions of the MAC system with TES using a pure PCM and a PCM nanocomposite are 10.54% and 5.64% lower than that of the system without TES, respectively. [ABSTRACT FROM AUTHOR]