TiO2, CNTs, and SiO2 nanolubricant: An approach to improving the efficiency of a domestic refrigerator

To enhance a home refrigerator’s refrigeration system’s performance, three types of nanoparticles (TiO2, CNTs, and SiO2) were used to produce nanolubricants. The experiments were conducted with different charges of 50 and 60 g for R600a refrigerants, which a digital charging scale was used to measure. The inlet and exit temperature of the system’s components were measured using K-type thermocouples. The system was outfitted with two pressure gauges for measuring the pressures at suction and discharge. The obtained results show the system’s COP increases with nanolubricant-enhanced refrigerants, with the power of the based lubricant, TiO2, CNTs, and SiO2 nanolubricant being 0.0821, 0.0612, 0.0673, and 0.0702 kW, respectively. Therefore, nano-enhanced refrigerant can successfully replace R600a in the based lubricant in domestic refrigerators.


Introduction
Among household appliances, the air conditioning and refrigeration systems use the most energy.These typically result in increased fuel consumption, and and energy costs.The most urgent issue facing the world today is global warming which is caused by fossil fuel and greenhouse gases.This greenhouse effect is also caused by refrigeration and air conditioning systems [1], [2].A vapour compression refrigeration system uses the vapour compression refrigeration cycle.To keep a refrigerator cool, the refrigeration system consumes a lot of energy.Nanoparticles as additives in compressor's lubricants can improve the compressor's dependability and performance, helping refrigerator systems use less energy [3].Refrigerant and compressor lubricant make up the refrigeration system's working fluid.Fuel consumption can be significantly reduced by using nanolubricant in place of based or pure lubricant [4].To achieve the necessary cooling, energy consumption and emissions can be lowered by increasing efficiency of refrigeration system [5]- [8].To mitigate the impact on the environment and lessen reliance on global fuel consumption, the efficiency of the vapour compression refrigeration system (VCRS) needs to be enhanced [9], [10].Even with its high energy usage and adverse impact on the environment, VCRS is still an important home appliance [11].By adding dispersed nanoparticles to the refrigeration system, also known as nanolubricant and nanorefrigerant [12], [13], the compressor in the VCRS can operate more efficiently.Frequent use of nanolubricant and nanorefrigerant in VCRS has demonstrated good potential for enhancing the performance of the mechanical and thermodynamic system.In VCRS, nanoparticles are dispersed using two different techniques.Nanoparticle dispersion to refrigerant is the first step in the nanorefrigerant technique.Dispersing nanoparticles in compressor lubricant is the second technique for using nanolubricant.Nanolubricant has several advantages.Because of the distinct properties nanoparticles possess such as large suface area, thermal conductivity, tribology, their application in heat transfer systems is capable of improving their performance.Also, nanoparticle application in the lubricant tends to reduce friction and wear.Hence, its application in refrigeration systems is currently under study by numerous researchers.Considering various works on nanolubricant in VCRS Redhwan et al. [14] [7] conducted an experiment to test the Al2O3-PAG nanolubricant.According to their research, the best Al2O3-PAG nanolubricant had a 0.01% Vol and provided the highest performance with an average of 28%, energy reduction Al2O3-PAG nanolubricant demonstrated a 31% COP increase.Zhang et al. conducted a test of TiO2 nanoparticles with lubricant in a VCRS.POE lubricant and R134a refrigerant were used.With TiO2 nanoparticles the refrigerator consumed 26.1% less energy compared to the based lubricant.Similarly, Subramani and Prakash [26] achieved a 25% decrease in power consumption with Al2O3-MO nanolubricants in VCRS.Al2O3 and CuO nanoparticles were combined in an investigation of R22 refrigerators conducted by Anish et al. [15].The COP increased with 0.05 CuO/ Al2O3 nanolubricants were employed.They also observed a decrease in compressor power consumption and increased refrigeration capacity.In a refrigerator that was charged with R134a, Mahbubul et al. [16] used Al2O3 nanoparticles.The refrigerant's thermal and physical properties improved and the COP rose to 15% with 5% Al2O3 nanolubricant.Sharif et al. [17] studied thermophysical properties of SiO2 and Al2O3 and nanoparticle mixture to produce the Al2O3-SiO2/PAG hybrid nanolubricant, the Al2O3 and SiO2 were mixed in lubricant.At 0.1 % volume concentration the hybrid nanolubricants were reported to gain thermal conductivity increase.Nabil et al. [18] observed the use of TiO2 nanoparticles in the heat transfer system with ethylene glycol water between 30 and 80 °C.The Heat transfer increased by 15%, and the nusselt number increased by 28.95%.Based on previous studies, different researchers have reported the effect of nanoparticles in heat transfer systems.Different from previous studies this study compares the performance of TIO2, carbon nanotubes (CNTs) and SiO2 in refrigeration systems.In addition to the safe use of refrigerant in refrigeration systems a natural refrigerant was selected for this study.R600a is a hydrocarbon (HC) refrigerant in this category.As such, this study emphasises using TiO2, CNTs, and SiO2 nanoparticles dispersed in a pure-based lubricant as a substitution for the pure lubricant which is the based lubricant in refrigerator systems.R600a is less expensive and easier to obtain.Also, HC-R600a possess zero ODP and a very low GWP and has good compatibility with the based lubricant in the refrigerator system [1], [19].

Experiment
For this experiment, a refrigerator with HC-R600a serves as the working fluid , the based lubricant serves as the pure lubricant.The system was entirely evacuated employing a vacuum flusher as shown in Figure 1.On a digital scale, the 0.4 g TiO2, CNTs, and SiO2 -nanoparticles were weighed and dispersed into a litre of mineral-based lubricant oil (pure lubricant) and each of the three nanoparticle samples was used to prepare 0.4g/L of TiO2, CNTs, and SiO2 nanolubricant concentration.To produce the nanolubricant, the ultrasonic oscillators and a magnetic stirrer were made used to agitate and stir TiO2, CNTs, and SiO2 -nanolubricant samples until they were well blended using a two-step method.The temperature of the system's various components was measured at the intake and outlet using four types of thermocouples.In order to determine the discharge and suction pressures of the compressor, pressure gauges were attached to the discharge and suction ports.The flow chart for preparing the nanolubricant and the experimental test rig setup is displayed in Figures 2 and 3.The baseline test for the HC-R600a with the pure lubricant was carried out by introducing the pure lubricant and 50g of HC-R600a into the system from the compressor's suction side.The continuous test was carried out at 30-minute intervals for 5 hrs and repeated 5 times.The temperature and pressure were observed at the various inlet and outlet of the system until the steady state was attained in the evaporator as shown in Figure 3.The wattmeter was used to capture power consumption of the compressor.After this, the system was evacuated and flushed and repeated for 0.4g/L of TiO2, CNTs, and SiO2 nanolubricant concentration.The results obtained were compared with the based l which serves as the pure lubricant.The refrigerant's entropies and were computed with pressure and temperature data output.These values were used to calculate power consumption, cooling capacity, and coefficient of performance.Performance assessment of experimental result was calculated using the subsequent equations, 1-3.
Equation 1 was used to calculate cooling capacity.
Equation 3 is used to calculate the COP.

Results and discussion
The experiment's pull-down time is depicted in Figures 4 and 5. Firstly, the based lubricant with 25 g, 50 g, and 75 g were subjected to test.The evaporator temperature of -5 °C is the lowest with a 50 g charge, while the evaporator temperatures for 25 and 75 g charge are 2 and -1 °C.After that 50 g of R600a was selected for further testing in various nanolubricants (TiO2, CNTs, and SiO2).The refrigerant's pull-down time of the pure 50g, TiO2, CNTs, and SiO2 is shown in Figure 5.At 210, 210, 210 and 240 minutes, pure, TiO2, CNTs, and SiO2 nanolubricant achieved evaporator temperatures of -5, -10, -8, and -11 respectively.Using nanolubricants, the drop in evaporator temperature was brought on by the critical heat flux and boiling heat transfer coefficient increase.The boiling surface's wettability and capillarity were enhanced by the thin layer of porous nanoparticles that were deposited on the evaporator.This is corroborated by Murshedet al. [20] who found that incorporating Al2O3 nanoparticles into the refrigeration system's base lubricant increased heat transfer rate and shortened pull-down times.Figure 6 shows the power consumption when using pure lubricant, TiO2, CNTs, and SiO2 nanolubricant, the system uses 0.082 kW, 0.073 kW, 0.061 kW and 0.076 kW of power, respectively.The TiO2, CNTs, and SiO2 nanolubricant use 11.0 %, 25.6%, and 7.3% less power than pure lubricant, respectively.SiO2 nanolubricant uses more power than CNTs and TiO2 nanolubricant, but less power than based lubricant, even though the system's evaporator temperature is lower (−11).Tribological improvements such as lubricity and low power needed for the compressor's compressor also contributed to the observed decrease in power consumption in TiO2, CNTs, and SiO2 nanolubricant.Furthermore, the lubrication of the compressor piston was more effective than the base lubricant due to its superior tribology properties.
The refrigeration system's energy-saving capacity is significantly increased [21], [22].The results align with the research conducted by Vipin Nair et al. [23] in 2020, wherein they tested R134a/Al2O3/nano oil-mixture in a VCRS against R134a.In Figure .6 reduction in the compressor's discharge pressure was experienced while using the nano oil mixture.The results of Babarinde et al. and Su et al, [24], [25] found that incorporating nanoparticles in vegetable oil as lubricating oil leads to a reduction in wear and friction The cooling capacity is shown in Figure 7.The cooling capacities of based lubricants, TiO2, CNTs, and SiO2 are 0.1724 kW, 0.185 kW, 0.203 kW, and 0.208 kW, respectively.Figure 7 shows adding TiO2, CNTs, and SiO2 enhanced the R600a's cooling capacity with an increase of 9.7%, 0.25%, and 12.5%, respectively.The system's thermal conductivity was enhanced by the nanoparticles dispersed in the lubricant.More heat removal from the system was attained through enhanced heat transfer convection in the system's condenser.This supports the research by Kumar et al., [26] who found that using ZnO/LPG enhanced cooling capacity.Moreover, it also aligns with research conducted by Khairat Dawood et al., [27] who enhanced turbulent flow thermal conductivity within a hollow pipe by using a CuAlO2 nanofluid.
The refrigerator system COP is expressed as ratio of the evaporator's cooling capacity to the power consumed by the compressor.A system with a higher cooling capacity with a lower power input is considered to have a good performance.Hence, the more energy that can be saved the more efficiently it operates.Figure 8 shows how the system COP varies for different nanoparticles.Compared to a conventional lubricant, the system's recorded COP with nanolubricants was higher.The based lubricant, as seen in Figure 8, has a COP of 1.9, while the TiO2, CNTs, and SiO2 nanolubricant.At temperatures of -10, -8, and 11 °C in the evaporator, COPs of 2.8, 2.5, and 2.7 were obtained respectively.

Conclusion
In a domestic vapour compression refrigerator system (VCRS), the efficiency of HC-R600a with TiO2, carbon nanotubes (CNTS), and SiO2 nano mixtures were evaluated.The nano mixture's efficiencies were evaluated using indicators like power consumption, cooling capacity, COP, and evaporator temperature.Based on the results and observations, TiO2, CNTs, and SiO2 -nanolubricant resulted in lower pull down time, evaporator temperature, when compared to pure lubricant.CNTs produced the lowest energy saving with an acceptable evaporator temperature of -7 while TiO2 delivered the highest COP.Generally, TiO2, CNTs, and SiO2 -nanolubricant possessed lower power consumption compared to HC-R600a in pure lubricant.However, because of the lower power consumption and lowest evaporator temperature in the system, R600a in the based lubricant can also be replaced with SiO2.Thus, adding nanolubricant to VCRS may increase the energy efficiency of a refrigerator system.

Figure 1 .
Figure 1.Charging operation of the system.