Experimental evaluation of a vapour compression refrigeration system using a nano-oil mixture of TiO2, Al2O3, and SiO2

Nanolubricants were used to improve the performance of refrigeration systems in a household refrigerator. In this study, a pure lubricant is used as the base compressor lubricant, and nanoparticles of TiO2, Al2O3, and SiO2 at 0.4g/L concentrations are used as nanolubricants in 50g isobutane (R600a) refrigerants. Power consumption was investigated alongside the COP. TiO2 nanolubricant has the highest COP of 2.8 and the lowest power consumption of 11.3% when compared to based lubricant. However, in terms of power consumption, SiO2 and Al2O3-nanolubricant outperformed the based lubricant by 4.5% and 7.7%, respectively. This suggests that a nanolubricant could be used in a refrigeration system to replace a based lubricant with R600a refrigerant.


Introduction
Energy demands are usually associated with environmental concerns which have recently become a major issue worldwide due to the increased use of fossil fuels.To create systems that are energy efficient and to protect the environment, numerous researchers have studied improvements in a variety of technologies in refrigeration [1]- [3].A new strategy to minimize the energy demand and environmental challenges in refrigeration without compromising its predominance is needed.The performance of refrigerators can be improved by adding nanoparticles to the lubricant and refrigerant oil [4]- [7].Nanorefrigerant and nanolubricant are the names given to these specialised nanofluids.Nanorefrigerants have drawn a lot of attention from heat transfer researchers as a result of their potential in a variety of industries, including air conditioning, heat pumps, and refrigeration.To reduce energy consumption and enhance heat transfer in a reirrigation system lubricity and wear resistance are two tribological properties that nanolubricants must be considered.Nanolubricants only lubricate the compressor while nanorefrigerants serve as heat absorbers in a refrigeration system, functioning as separate systems [8]- [10].The use of nanoparticles as colloidal material is one of the techniques to increase the thermal transport in heat transfer [11].The main obstacles to the development of efficient and sustainable systems in the field of heat transfer are low thermal conductivity and low heat transport capacities.Hence, improving the thermophysical and heat transport properties of the fluids from an energy-saving standpoint is critical.Apart from the shape and size performance of nanoparticles in heat transfer system depends on the nanoparticles type [4], [12] Furthermore, to address environmental issues in refrigeration systems.The choice of working fluid should also be considered.A new class of refrigerants known as hydrocarbons (HCs) exhibits exceptional qualities such as being environmentally friendly, having no ozone depletion potential (ODP) and having a global warming potential (GWP) of 3, which is lower than HFCs refrigerants [13], [14].By using nanolubricant and nanorefrigerants refrigeration systems energy savings can be significantly increased.
Properties like thermal conductivity and pool boiling can be significantly enhanced by adding nanoparticles to the refrigerant-lubricant mixture.There are two ways to introduce nanoparticles into refrigeration systems: directly into the refrigerant or by dispersing them in compressor lubricant which transports them inside closed systems as a mixture of refrigerant, oil, and nanoparticles.With this mechanical and thermodynamic performance of refrigeration systems can be significantly improved by nanoparticles.M.Anish et al. [15] investigated R22 refrigerators using a combination of Al2O3 and CuO nanoparticles.0.05 CuO/Al2O3 nanolubricants were used to achieve an improved COP.Additionally, it increases refrigeration capacity and reduces the power consumed by the compressor.When using a nanolubricant containing CNT nanoparticles, Madyira et al. [16] evaluated R600a's performance in the refrigerator.To replace the R134a refrigeration system, two nanolubricants of 0.4 and 0.6 g/L were used.With nanolubricants, the highest COP recorded is 2.9 and the compressor was 63.9 W of power compared to 90 W of R134a.Additionally, in a detailed investigation, they tested graphene/nanolubricants in an R600a refrigeration system [7].They used three different mass fractions of refrigerants with concentrations of 0.2, 0.4, and 0.6 g/L.The graphene/nanolubricants enhanced cooling capacity to 2054 W with a highest COP of 3.2 and lowest power consumption of 65.3W.Wang et al. [17] tested the performance of fullerene (70) and NiFe2O4 nanolubricants in a VCRS and compared them to the standard lubricant.The system's performance coefficient increased by 23 % as a result of the study.R600a is one of the utilized hydrocarbon refrigerants.Therefore, this research focuses on using TiO2, Al2O3 and SiO2 nanolubricants as a replacement for the based lubricant in home refrigerators using R600a.R600a is less expensive and easily accessible in most countries, and it has zero ODP and a very low GWP.R600a is a better option for this experiment considering R600a is compatible with mineral oil [13], [18].

Methods
The test rig employed for this experiment was a domestic refrigerator.The system was evacuated and flushed before use.A litre of the based lubricant was first introduced into the compressor from the suction side.After this 30g of R600a was charged into the compressor from the suction side of the refrigerator compressor.The inlet and outlet temperature and pressure of the components of the system were captured at 30-minute intervals until the steady state temperature was achieved in the system as shown in Figure 1.This procedure was also repeated for 50g and 70g of R600a in the system.From the experiment, it was observed that R600a with 50 g charge has the lowest evaporator temperature and pull-down time in the refrigerator system.Therefore, it was safe to use 50 g of R600a as the experimental baseline.In the second step of the experiment.The 50g of R600a was further investigated in the nanolubricant consisting of TiO2, Al2O3, and SiO2 nanolubricants separately.The nanolubricant preparation procedure is presented in the flow chart in Figure 3. 0.4g of each nanoparticle sample was suspended in a separate 1 litre (1 L) of compressor lubricant.Firstly, the nanoparticles were mixed with the lubricant and then homogenised with an ultrasonicator.After this, it was agitated together with a magnetic stirrer to ensure the nanoparticles were uniformly mixed and well blended with the lubricant.

Figure 3. Nanolubricant preparation flow chart
The refrigerator was evacuated and then flushed.The 0.4 g/L of Al2O3 nanolubricant was introduced into the refrigerator from the suction side of the compressor.The 50 g mass charge of R600a was also introduced into the compressor from the suction line.The test was therefore carried out on the system considering the pressure and temperature at the inlet and outlet of the system as shown in Figure 2 above until the steady state is achieved in the system.The system parameters such as temperature, pressure and power consumption were recorded.The same procedure was repeated for SiO2 and Al2O3 nanolubricant in the system using 50 g of R600a in the based lubricant as the baseline of the experimental test.The pressure and temperature were used as parameters to determine the enthalpy at various inlet and outlet of the system's component which were used to calculate the system's cooling capacity.Also, the system's COP was calculated using the cooling capacity to power consumption ratio (W/W).Equation 1 was used to calculate cooling capacity.
Equation 2 is used to express the system's compressor power consumption.
Equation 3 is used to calculate the COP.

Discussion
The pull-down time is depicted in Figures 4 and 5.The test started by measuring the evaporator temperature and pull-down time of pure R600a in 20 increments of 30, 50, and 70 g.50 g charge of R600a has the fastest pull-down time and the lowest evaporator temperature, which was -5 o C, compared to 30 g and 70 g charges, which had 1 and -3 o C, respectively.50 g of R600a was tested as the optimal mass charge of R600a in the system.Furthermore, to test different TiO2, Al2O3, and SiO2 nanolubricants.50 g of pure R600a served as the baseline in this experiment.In this study, the based lubricant is also referred to as the pure lubricant which is the baseline lubricant for this study.
Figure 4 shows the impact of the R600a mass charge on the system's pull-down time and evaporator temperature.Figure 5 shows the impact of the nanolubricant on the system's pull-down time.
In Figure 6, the system's power consumption is shown.In pure TiO2, Al2O3 and SiO2-nano lubricant, R600a consumed 82.1, 72.9, 78.4, and 75.8W of power respectively.TiO2, Al2O3 and SiO2-nano lubricant use respective amounts of power that are 11.3%, 4.5%, and 7.7% less than R600a in based lubricant.The decrease in compressor system friction force may be responsible for the decrease in power consumption [17], [20].In addition, a decrease in the evaporator chamber brought on by the nanoparticles' effect may also be a contributing factor in the power consumption decrease.The outcome is consistent with research by Vipin et al., who tested R134a/Al2O3/nano oil against R134a in a VCRS [21].The system's capacity for cooling is shown in Figure 7.In terms of pure lubricant, TiO2, Al2O3 and SiO2nano lubricant, the R600a has cooling capacities of 172.4,203.1, 186.2, and 207.7 W, respectively TiO2, Al2O3 and SiO2-nanolubricant concentrations have higher cooling capacities than R600a in based lubricant with higher cooling capacity of 17.8%, 8.5 %, and 20.5 4%.This is brought on by the system's increased thermal conductivity, which enhances heat transport [6], [18] This is consistent with the findings of Kumar et al. [22], who discovered that using ZnO nanoparticles with LPG increased cooling capacity by increasing thermal conductivity.It similarly fits with the outcomes of Khairat et al. [23], who used CuAlO2 nanofluid to improve the turbulent flow's thermal conductivity inside a hollow pipe.The discrepancy in system COP for different nanoparticles, as well as associated power usage and cooling capacity, are shown in Figure 7.The system's power consumption decreases as the evaporator temperature decreases, increasing the COP as a result.Additionally, as shown in Figures 6 and 7, an increase in cooling capacity also contributes to an increase in COP.
Figure 8. Impact of nanolubricant on the system's COP Figure 9. Impact of nanolubricant on the system's R600a discharge pressure As can be seen in Figure 8, the recorded COP of the R600a/nano-oil mixture was higher than the COP noticed with pure R600a.At evaporator temperatures of -10, -7, and -11 °C, the COP of the system obtained with R600a/nano-oil combination was 2.8, 2.4, and 2.7, respectively.Whereas the COP recorded without nanoparticles was 2.1.The TiO2 nanolubricant had the highest COP of the R600a/nano-oil combination in the system.Although, SiO2 has the highest cooling capacity its power consumption was higher than TiO2 in the refrigerator which made it a bit lower in COP than TiO2-nanolubricant.Figure 8 TMREES shows that during the test, the pressure discharge with nanolubricant decreased as well, suggesting a long lifespan for the refrigerator system's compressor [13].

Conclusion
In a home refrigerator system, the performance of R600a, an eco-friendly refrigerant made up of TiO2, Al2O3, and SiO2-nanolubricants, was tested and compared to pure R600a.The use of nanolubricants improves the performance of vapour compression refrigeration systems.The R600a system's COP was increased by using TiO2 nanolubricant up to 2.8.The compressor power has a minimum output of 72.9 W, which was 11.3% less than the baseline (pure) lubricant used in the study.TiO2, Al2O3, and SiO2nanolubricants reduced the system's evaporator temperature and pull-down time when compared to the R600a in the base lubricant, with SiO2 concentration producing the lowest evaporator temperature.The performance of R600a with Al2O3, and SiO2 nano-mixture outperforms R600a in based lubricant.While R600a has the best performance in the system, Al2O3 and SiO2 nanolubricants are also potential substitutes for R600a-base lubricant in the system.

Figure 1 .
Figure 1.The system charging operation

Figure 4
Figure 4 depicts the R600a refrigerant's pull-down time.TiO2 has the lower temperature in the evaporator, at -10 o C after 210 minutes.At 210, 240, and 240 minutes of pull-down time, the based (pure) lubricant, Al2O3, and SiO2 nanolubricants, respectively, reached evaporator temperatures of -5, -7, and -11°C.This is supported by the findings of Murshed et al.[19] and Madyira et al. [16] found that adding Al2O3 and CNTs-based lubricant enhanced heat transfer rate and reduced pull downtime.

Figure 6 .Figure 7 .
Figure 6.Impact of the nanolubricant on the system's power consumption