Exergetic analysis of low gram nano-refrigerant charge for transport application

Limited gram charge is beneficial in transport refrigeration by sustenance of flammability limits for safety. Herein, an experimental investigation of 0.11 vol. % of titanium IV oxide doped with 30 g each of R600a and LPG as R6 ooaAl2O3 and LPG Al2O3 nano-refrigerants were retrofitted into a 102g R134a vapour compression refrigerating system. At steady state, the performance of the system using unblended R134a, R600a, LPG refrigerants, R600a Al2O3 and LPG Al2O3 nano-refrigerants were examined as a function air evaporator temperature and compressor characteristics; discharge pressure, power consumption, compressor exergetic defect, coefficient of performance and exergetic efficiency. At 80g charge, R600a, LPG, R6 ooaAl2O3 and LPG Al2O3 had higher COP; Exergetic efficiency; but lower Total exergetic defect of 2.9, 11.3, 9.7 and 14.1%; 19.4, 22.1, 31.8 and 29.4%; and 34.1, 43.6, 56.3 and 58.6% than R134a. R134a has higher discharge pressure of 0.25, 0.12, 0.77 and 0.84 Psia/ 0.77, 0.12, 0.84 and 0.25 Psia; higher power consumption of 14.79, 12.23, 27.83 and 26.51 Watts; higher compressor exergetic defect of 0.03, 0.03, 0.06 and 0.04 Joule, lower coefficient of performance of 0.06, 0.23, 0.71 and 1.18; and lower exergetic efficiency of 0.012, 0.008, 0.056, 0.062 Joule than R600a, LPG, R6 ooaAl2O3 , LPG Al2O3 . motor compression work of unblended R134a refrigerant is higher than unblended R600a, unblended LPGAl 2 O 3 blended in R600a and LPG by 15.28%, 16.08%, 37.73% and 40.43. All the selected refrigerants met the chill compartment temperature, ISO 8187 compartment standard of -2 ≤ tcc min, max ≤ +3, while R6 ooaAl2O3 and LPG Al2O3 nano-refrigerants showed improved compressor characteristics.


Introduction
Transport refrigeration exists across refrigerated trucks and vans to refrigerated containers and reefers (refrigerated cargo ships for temperature-sensitive goods), comprising condenser, compressor, expansion valve. Optimal refrigeration cycle is achievable by minimization of irreversibility in system components by the use of exergy method. Exergy analysis gives information on how, where and how much system performance is degraded and the potential for improvement unlike the energy analysis. Several studies on the use of nanoparticles in conventional lubricant and refrigerant enhancements have been conducted, and some are reported. The concept of blending conventional fluids with nanoparticles has been reported to possess great benefits [1]. These includes enhanced solubility of the lubricant and the refrigerant, improvement in the heat transfer characteristics of the refrigerant/lubricant, and reduction in the frictional coefficient and wear rate of the nano-lubricant. Several researches have been conducted on the use of nanoparticle in to improve performance of refrigeration [2], and some of them includes Wang et al. [3] investigated the solubility of appended N-Al2O3 in a mineral lubricant using R134a in refrigerating system. They reported that the mineral oil appended with Al2O3 nanoparticle returned more lubricant oil back to the compressor indicating better solubility, and had similar performance compared to systems of polyol-ester lubricant and R134a. Examination of the viscosity effect of TiO2 and Al2O3 blended into capella mineral oil and ethylene glycol lubricant was examined by Ajuka et al., [4]. They noted that at 0.0025g/ml, both TiO2 and Al2O3 nanoparticle blends in capella mineral oil were less viscous by 9.7 and 2.98%, while the nanoparticle blends were less viscous by 14.1 and 7.1% than unblended ethylene glycol. Thermal conductivity measurement and modeling of carbon nanotube (CNT) R113 nano-refrigerants by Jiang et al. [4], reported an improved 82%, 104%, 43% and 50% on the measured thermal conductivities of four sets of 1.0 vol.% of the CNT-R113 nano-refrigerants. The boiling heat transfer characteristics experimental examination of Al2O3 nanoparticles dispersed in R22 refrigerant as reported by Wang et al. Wang et al. [6] showed a reduction in bubble size and its rapid move towards the heat transfer surface. They concluded that nanoparticles can enhance heat transfer characteristic of the refrigerants. The pool boiling heat transfer of the R11 refrigerant blended with Al2O3-nanoparticles at particle loading up to 0.01 g/L was investigated by Wu et al. [7]. They reported that the heat transfer enhancement was up to 20%. The migrated mass characteristics of nanoparticles in the pool boiling process of both nano-refrigerant and nano-refrigerant-oil mixture was examined by Ding et al. [8]. They reported that the nanoparticle migrated mass and nano-refrigerant migration ratios were larger than the nano-refrigerant-oil mixture. The dispersion behavior, thermal conductivity and flow boiling heat transfer of Al2O3-R134a nano-refrigerant in a domestic refrigerator was conducted by Bi and Shi [9]. They reported to have added nanoparticles to lubricant to form nano-lubricant, and then added the mixture in the compressor as lubricant. In another experimental investigation, Bi and Shi reported to have blended nanoparticles and refrigerants together simultaneously to have nano-refrigerant [10]. They reported that both concept of nanoparticle addition gave better performance in the refrigerator. An energy and exergy analysis of a refrigeration system with vapor injection using reciprocating piston compressor was carried out by Tang et al. [11]. They modified a reciprocating piston compressor by building a vapor injection device and reported that the intermediate injection system can minimize exergy loss in the system and expand the application range of reciprocating piston compressor. The study by Bi et al. [12] on the performance of Al2O3-R600a nano-refrigerant considered Al2O3-R600a nano-refrigerant in a household refrigerator with no system reproduction. They reported that the coefficient of performance was higher to the unblended R600a-based system, with 9.6% less energy utilized with 0.5 g/L Al2O3-R600a nano-refrigerant. They inferred that the consequences of the energy utilization and the freeze limit test demonstrate that Al2O3-R600a nano-refrigerants worked typically and securely in the fridge, subsequently, idealized that the utilization of Al2O3-R600a nano-refrigerant in residential refrigerator is practicable. Considering the various researches that has been mentioned, none to the best of the authors knowledge has considered the exergetic effect of limited refrigerant charge with emphasis on compressor parametric performance using Al2O3 nanoparticles.

Statement of problem
Refrigeration of vehicles in motion needs to consider possibilities of cutting down on refrigerant charge percentage (limited charges) especially for flammable refrigerants like hydrocarbons, while hoping to achieve expected refrigerating rational (exergetic) efficiency, low volume hydrocarbon (HC) charge far less than 0.1kgm -3 of HC lower flammability limit (LFL) erases the potential of flammability particularly when there are no leaks, according to ASHRAE Standard 34 and ISO 817. There are no reports from literature establishing the limits within which a low charged compressor gives unique acceptable performance before standard refrigerant recharge.

Aim and Objectives
The aim of this paper is to investigate the chill compartment temperature (tcc) performance of selected low volumetric charge of Alumina hydrocarbon and compressor associated exergy performance of nano-refrigerant against -2≤ tcc ≥+3 ISO standard as can be applied in transport refrigeration. The specific objectives are to utilize R600a, LPG, R6ooaAl2O3, LPGAl2O3 and R134a refrigerant to evaluate the  Power consumption  Discharge pressures  COP, Exergetic defects and Efficiencies Of the vapour compression refrigeration system.

Material and methods
A typical domestic refrigerator running on capella oil was used in this study.

Rig set-up and Experimentation
In this study, a 48 Litre refrigerator test rig with a single door, evaporator cabinet, compressor, condenser, expansion valve and a capillary tube at no load condition using R134a was retrofitted with 30 g mass charges of LPG and R600a refrigerants in Al2O3 nano-lubricant. Using the two-step method, the 0.11vol.% nano-lubricant (mixture of nanoparticle and capella oil) was thoroughly homogenized with an ultrasonic vibrator (Branson M2800H) and the mixture was observed for 48 hours to examine coagulation or sedimentation before experimentation. As shown in the Figure 1, the temperatures and pressures at state points 1-4 as T1, T2, T3, T4 and Tair were measured according to ISO 8187 chill compartment temperature standard of -2 ≤ tcc min, max ≤ +3 using type K thermocouples as well as the suction and discharge pressures P1 and P2 using pressure gauges and recorded. The test was carried out using 30 g charge for each refrigerant: unblended R134a, R600a, LPG (50/50 Propane-Butane), R6ooaAl2O3, LPGAl2O3. The system compressor was flushed with unblended capella oil after each test to avoid leftover nanoparticle interaction while sigma B-42: ¼ HP Vacuum pump was utilised in the extraction of pent refrigerant after each trial. The schematic and pictorial diagrams of the system and materials are given in Figures 1 and 2, the specifications of the test unit, measurement ranges and the uncertainty of the measuring instrument are shown in the Table 1

Experimental Analysis
The thermodynamic properties of the selected refrigerants which were recorded were determined using NIST REFProp 9.1 according to Lemmon et al. [13]. The results were then used to determine the intended performance characteristics which equations are given below: Volume fraction of nanoparticle in nano-lubricant is given by Фn; Where mn, ρn, ρo are the nanoparticle mass, density and oil density.

Data presentation
The result of coefficient of performance (COP), exergetic efficiency (Ex. Eff.) and total exergetic defect (T.E.D) using 80 g charge of LPG, LPGAl2O3, R600a, R600Al2O3 and R134a are given in the Table 2 below. It can be inferred that R134a had the least COP and Ex. Eff. But highest T.E.D. value. Total exergetic defect is the sum total of exergetic defects in the compressor, condenser, expansion device and the evaporator. The effect of titania nanoparticle is also seen.

Discussion of findings
The result of the experiment investigating the compressor associated exergy performance using unblended LPG, R600a and blend of Al2O3 on each (R6ooaAl2O3 and LPGAl2O3) are discussed below;

Discharge Pressure
Compressor discharge pressure is a function of the performance of the refrigerating system's mean temperature difference. In the discharge pressure variation graphical illustration shown in Figure 3, the discharge pressure of unblended R134a (baseline) refrigerant is 3.0%, 3.0%, 6.01% and 4.2% higher than that of unblended R600a, unblended LPG, R6ooaAl2O3, LPGAl2O3. Since the size of the discharge line is unchanged for all the refrigerants, it can be concluded that at low volume of refrigerants, there was lower percentage variation in the baseline discharge pressure and the selected refrigerants.

Compressor Work
The compressor work for the refrigerants as a function of evaporator air temperature is given in Figure 3. The compressor work of unblended R134a is higher than unblended R600a, unblended LPG, R6ooaAl2O3, LPGAl2O3 by 18.3%, 15.6%, 41.9% and 39.7%, respectively. There is a slight increase in compressor work with increase in Evaporator air temperature.

Coefficient of Performance (COP)
Coefficient of Performance of R134a refrigerant was lower than unblended R600a, unblended LPG, Al2O3 blended in R600a and LPG by 6.01%, 12.04%, 48.02% and 50.5% as shown in the Figure 4 with varied evaporator air temperature. The entire selected refrigerant had similar trend as the figure shows that COP increased with increase in evaporator temperature.

Compressor Exergetic Defect (C.E.D)
The Figure 5 shows that the Compressor exergetic defect increases as evaporator temperature increases. Unblended R134a showed the highest compressor defect and the least exergetic defects were LPG and R600a blendsed with Alumina. This may be due to the transport properties nanoparticle.

Exergetic Efficiency (Ex. Eff.)
Figure 5 also shows the exergetic efficiency of unblended R134a and hydrocarbons were the least and had close value, whereas the effect of alumina nanoparticle was observed in the LPG and R600a blends which had the highest exergetic efficiencies. Figure 6 below is a representation of the defect of compression work in the compressor (Exc, motor) for the refrigerants. It can be inferred that the highest work of compression is done in the system when unblended R134a refrigerant is used and the least work was done with LPG (Al2O3) refrigerant. The work of motor compression of unblended R134a refrigerant is higher than unblended R600a, unblended LPG, Al2O3 blended in R600a and LPG by 15.28%, 16.08%, 37.73% and 40.43%, respectively. Work of compression decreased with increase in motor efficiency and it can be concluded that the nano-refrigerants reduced the compression work since the unblended refrigerants had higher associated exergy defect due to work of compression. This may be due to the unique transport properties of the Al2O3 nanoparticle.

Conclusion
Technically, transport refrigeration involves some motion of refrigerant in the compressor during navigation. This puts such vehicles at higher risk when flammable refrigerants are used. Limited refrigerant charges, energy savings and better cooling is essential without affecting the environment. Exergetic investigation of the impact of low volume (30 g) refrigerant charge using capella oil was carried out and the following were deduced:  The 30 g refrigerant charges worked normally and efficiently in the refrigerating system with all the refrigerants reaching the -2 ≤ tcc min, max ≤ +3 ISO standard, with higher COP record of 6.1% and 23.3% using unblended R600a and LPG, and 48.02% and 50.5% using R6ooaAl2O3 and LPGAl2O3 when compared to the R134a refrigerant.  There was energy savings within the range 27.8% and 26.5 using R6ooaAl2O3 and LPGAl2O3 nano-refrigerants.  1The nano-refrigerants operated at lower discharge pressures compared to unblended R134a which is a benefit for the compressor life.
The COP, exergetic efficiencies and energy savings attributed to the nano-refrigerants may be due to better heat transport of particle/ particle surface area, lesser compressor work from the alumina nanoparticle.

Contributions to knowledge
In light of the study, the following recommendations are suggested:  Alumina hydrocarbon nano-refrigerants meets ISO standard, even, at lower charge rate.  Energy savings of alumina nano-refrigerant is not limited to only high charge rates.  Service life of nano-refrigerant charged compressor should be higher due to lesser discharge pressure.  R134a low gram charge is not recommended for use in VCRS (Incorrect refrigerant), otherwise;  Utilizing lower gram charges of R134a: Installation of compound Cut-out to limit the upper and lower pressure limits.