Search

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]

HC-290 is a promising alternative for HCFC-22, which is an ozone depleting substance under Montreal Protocol. However, HC-290 charge is to be reduced because of its flammability. The objective of this study was to reduce the charge quantity of HC-290 in a split air conditioner, which was originally designed for HCFC-22. The effect of charge reduction on the performance of the HC-290 split air-conditioning system was studied experimentally by varying condenser tube diameter, length of capillary tube, and condenser air face velocity. Initially, simulation study was conducted using ART simulation software. In accordance with the simulation results, a 5.25 kW split air conditioner was modified and experimentally tested with HC-290 in a psychrometric test facility under standard test conditions as per IS 1391 part 2. The condenser tube diameter was reduced from 12.7 mm to 6.35 mm, while the length of capillary tube was increased from 650 mm to 820 mm with enhanced condenser air face velocities. It was found that with 300 g of HC-290 charge, the performance parameters such as cooling capacity, power consumption, and coefficient of performance were better than the baseline test with HCFC-22 in all conditions tested. [ABSTRACT FROM AUTHOR]

The performance comparison of two automotive air conditioning systems was experimentally studied under two typical cabin conditions viz; 27°C dry bulb temperature / 40% relative humidity and 40°C dry bulb temperature /40% relative humidity in a bench test rig for different blower speeds of the evaporator. Two mobile air conditioning systems viz; enhanced system having minichannel evaporator with integrated receiver-dryer condenser, and baseline system having conventional serpentine evaporator with parallel flow condenser were considered for the study. The compressor was the same for both the systems. The charge quantity, compressor speed, condenser air flow rate were suitably modified to maintain similar suction / discharge pressures, suction super heat and sub-cooling at all test conditions. The percentage of condensate retention during the dehumidification for both the evaporators were also compared. The comparison indicates that the average coefficient of performance of the enhanced system is higher in both low and high cabin temperature condition by about 15% and 8% respectively. The percentage of condensate retention in the evaporator of the enhanced system is lesser in the range of about 17–31% as compared to the base line system evaporator. It is also expected that a specific design of compressor for the enhanced system can yield better performance at all conditions. [ABSTRACT FROM AUTHOR]

Polyalkaline glycol is used as the lubricant with R134a in automobile air conditioning systems and it is well known that polyalkaline glycol oil is highly hygroscopic in nature. Presence of moisture in the system may deteriorate the system and its performance. Hence polialkaline glycol oil is replaced with conventional mineral oil. In the present study, a new refrigerant blend R134a/R290/R600a with mineral oil as lubricant has been tested in an experimental test rig with R12 as base line and compared with the published results. Pull down test from an average cabin temperature of 50 °C is performed while the compressor speed is varied successively from 1500-2250-3000-800 rpm with a constant heat load of 2500 W, and held at that speed for duration of 30 minutes. Both R12 and the HFC-HC blend is tested in an experimental test rig and it has been found that this new mixture is almost in par with R12, However, a slight loss in system performance is encountered. But, considering the service issues associated with R134a and polyalkaline glycol oil, this new mixture can have better performance than R134a and polyalkaline glycol combination. [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]

Abstract In this article, detailed work on the heat transfer and fluid flow behavior of carbon-based hybrid nanofluid (multi-walled carbon nanotubes + graphene nanoplatelets/H2O) was studied numerically. The carbon-based hybrid nanofluids have been prepared experimentally and their measured properties were used for this study. The numerical study was carried out by passing the nanofluids under laminar flow through the tube side of the microchannel heat sink (MCHS). A constant heat flux on the heat sink base was applied and a single-phase model approach was used for analyzing the heat transfer. The nanofluid volume concentration varied from 0.01% to 0.2%. The improvement in the heat transfer coefficient is observed to be a maximum of 73%, with a maximum of 18% pressure drop variation across the MCHS for 0.2 vol%. The pressure drop variation across the MCHS is minimal for all the considered volume fractions of the nanomaterials. The results confirmed that the hybrid nanofluid possesses a remarkable heat transfer characteristic for transporting heat effectively. [ABSTRACT FROM AUTHOR]

In this work, we experimentally investigated the solidification behavior of functionalized graphene-based phase change nanocomposites inside a sphere. The influence of graphene nanoplatelets on thermal transport and rheological characteristics of the such nanocomposites were also discussed. We adopted the covalent functionalization method to prepare highly stable phase change nanocomposites using commercially available phase change material (PCM) OM08 as the host matrix and graphene nanoplatelets (GnPs) with 0.1, 0.3, and 0.5 volume percentage as the nano inclusions. We report a maximum thermal conductivity enhancement of ~102 and ~46% with 0.5 vol% in the solid and liquid states, respectively. Rheological measurements show that the pure PCM shows Newtonian behavior, whereas the inclusion of GnPs leads to the transition to non-Newtonian behavior, especially at lower shear rates. Viscosity of the nanocomposite increases with an increase in the volume fraction of GnP. For 0.5 vol% of GnPs, maximum increase in viscosity was found to be ~37% at a shear rate of 1000 s−1. Time required for complete solidification decreases with the loading of GnPs. Maximum reduction in solidification time with 0.5 vol% of GnPs was ~40% for bath temperature of −10°C. [ABSTRACT FROM AUTHOR]