Performance Studies on Low GWP Refrigerants as Eco-friendly Alternatives for R134a in Household Refrigerator

Abstract


Introduction
The climate change is a significant environmental issue and it increases the global temperature.In December 2015, the Paris Climate Conference set out an international action plan to bring down global warming below 2°C.Therefore, the main environmental targets are to minimize carbon emissions and mitigate the impact of climate change (Guo et al., 2018).The household refrigerator is one of the most important emitter of greenhouse gases because they use high GWP refrigerants as working fluids (Aprea et al., 2017).The greenhouse gases emits by the refrigeration system in two ways such as leakage of high GWP refrigerants are done by directly during the maintenance and charging process and indirectly through the generation of energy used to supply the system (Tassou et al. 2010).Moreover, the direct release of refrigerants is calculated to be responsible for 2% of the overall emissions of the equivalent CO2, while indirect releases represent about ten times the total direct releases, the total contribution to global warming by the refrigeration sector is approximately 20% (Mota-Babiloni et al., 2020).
Due to its excellent thermophysical properties, R134a is the most commonly used refrigerant in household refrigerators but its high GWP will have to be phased out earlier.The European Union recently announced that alternative refrigerants would have a GWP of less than 150 in order to reduce greenhouse gas emissions (Saji et al., 2019).In order to meet global environmental targets, conventional refrigerants must be replaced by eco-friendly and energy-efficient refrigerants.The experimental tests carried out on household refrigerator to replace R134a with R1234yf, the refrigeration capacity improved slightly, making R134a an appropriate substitute for household refrigerator (Aprea et al., 2016).The R1234yf, R1234ze (E), and R600a are low-GWP refrigerants that can be used to replace R134a.Among the refrigerants, R1234yf may be an alternate solution for R134a due to similar efficiency and the same mass flow rate in household refrigerators (Righetti et al., 2015).The experimental investigation carried out on R1234yf and R1234ze in an R134a system with various evaporation and condensation temperatures.It indicates that R1234ze has a lower average refrigeration capacity and COP than R134a by around 30% and between 2% and 8%, respectively.Similarly, R1234yf has a less cooling capacity about of 9% and between 5% and 30% less COP than R134a (Navarro et al., 2013).Furthermore, the flammability of these refrigerants is a big concern.Owing to its flammability, one of Europe's major car manufacturers declined to use R1234yf.
There are various applications such as household refrigerator, beverage coolers and mobile air conditioners, an azeotropic mixture R1234yf /R134a is offered to replace R134a.Adding 10-11% of R134a to R1234yf, the mixture is non-flammable and it's COP, discharge temperature and refrigeration capacity are like that of R134a (Yohan et al., 2013).The HFO/HFC mixtures are suitable alternatives of HFC refrigerants and it has low GWP (GWP<150).It has been observed that adding 10% R134a to R1234yf will make the refrigerant mixture to be non-flammable and it is suitable alternative for R134a due its energy efficiency (Aprea et al., 2016).
The HFO/HFC mixtures have improved performance, and their expected COP and exergy efficiency are 4% to 8.3% and 5.1% to 10.5% respectively higher than the HFO (Saji et al., 2019).
Therefore, the energy analysis on a household refrigerator operating with low GWP refrigerant mixtures as environmental friendly alternatives for R134a has been studied using mathematical simulation.The household refrigeration system has been modeled and the performance has been studied to find the best composition of the mixture to operate the system.This simulation was carried out using MATLab software, and the REFPROP database was used to obtain thermophysical properties of the refrigerants.Due to the environmentally friendly properties and flammability aspects, R1234ze/R134a (90/10) could be a good substitute for R134a in the refrigerator to satisfy the Montreal and Kyoto Protocol expectations.

Refrigerant Selection
One of the essential methods of identifying the suitable refrigerant is the selection of refrigerants based on its thermo-physical properties.For operating temperatures -30 o C to 50 o C, the properties of refrigerants obtained from REFPROP 9.0 have been plotted.The vapor pressures of HFO refrigerant mixtures are lower than that of R134a as shown in Fig. 1.The vapor pressure of R1234ze/R134a is 21.5% lower than that of R134a among the different mixtures.This can cause a reduced energy consumption.Fig. 2. shows that the latent heat of R1234ze/R134a/R744 and R1234ze/R134a are 6.4% and 6.8% lower than R134a respectively.In the case of R1234ze/R32/R152a mixture, the latent heat is 1.3% greater than the R134a.The thermo-physical properties used in this analysis are shown in Table 1.

Analytical study
Figure 3 shows a schematic diagram of the refrigeration system used throughout this analysis.It consists of a reciprocating compressor, air-cooled condenser, capillary tube, and evaporator.The simulation of the above components was performed using MATLAB software.To obtain the thermo-physical properties of refrigerants, REFPROP is directly interlinked with MATLAB.A 190 l household refrigerator has been considered in this modeling.The property variations are one dimensional, steady state condition, constant mass flow rate, polytropic process in compression, efficiency of electrical motor is 85% and pressure drop in the heat exchanger is negligible.
The above assumptions are taken into account in this analysis to minimize the complexities of the simulation.

Compressor model
A 100W reciprocating type compressor has been taken for the study and it is divided in to three control volumes such as compressor shell, swept volume and discharge tube (William and Doyle, 1988).The following equation is used to determine the swept volume: The displacement volume is calculated at each time step using the following equation: The following equation is used to calculated compressor work:

Condenser model
The condenser is divided into de-superheated region, two-phase region, and sub-cooled region.The following correlations are used to measure the refrigerant side heat transfer coefficient in the single-phase region (Minor et al., 2010): In laminar region ( < 2100) In turbulent region ( > 10,000) The air-side heat transfer coefficient of the condenser is calculated using the correlations below (Marques et al. 2014):

Capillary tube model
The flow of a capillary tube can be separated into single-phase and two-phase.Pressure drops in the capillary tube causes due to friction, momentum loss, and gravitational effect.Sudden contraction happened by the pressure drop, is calculated by following equation (Perry and Chilton, 1984).

Exergy Analysis
The aim of the exergy analysis is to assess the irreversibility of each component of a household refrigerator.
Fig. 4 depicts the T-S diagram and the exergy balance equations are taken from the previous studies (Bayrakci HC and Ozgur, 2009) and shown in Table 2.The total exergy destruction of the system is determined by:   =   +   +   +   (13)  The figure indicates that the simulation COP deviates from the experimental COP by 9% to 12% for various ambient conditions at the evaporator temperature of -12°C.Since this difference is nominal, the validity of the mathematical approach has been verified.

Results and Discussion
The energy analysis on a household refrigerator through environmentally friendly refrigerant mixtures was theoretically studied for different ambient temperatures at an evaporator temperature of -18 o C. The findings would examine the output parameters including the compressor work, COP, total exergy destruction, exergy efficiency and TEWI.
As seen in Fig. 7, the function of the compressor work increases as the atmospheric temperature increases.This is because the input pressure of compressor and mass flow rate increases.The R1234ze/R134a mixture has been observed to reduce compressor work from 4.3% to 8.5 % than other two mixtures.Because of low evaporator pressure and high molecular weight of the mixture.The difference of COP with ambient temperature was calculated and plotted using with HFO mixtures in Fig. 8.It has been noted that the COP of R1234ze/R134a mixture is higher than the other two mixtures by about of 16.4%.This may be because, the refrigerant mixture has a high latent heat.et al., 2012).The overall irreversibility of the R1234ze/R134a mixture is decreases from 5.2% to 12.4% than the other two mixtures that can be attributed to the high compression ratio.Since the exergy efficiency is used to determine the quality of energy used by a system, it has been calculated for HFO refrigerant mixtures and shown in Fig. 10.The exergy efficiency of the R1234ze/R134a mixture improves from 4% to 16% as compared to the other two mixtures, and it also has a lower exergy efficiency at high ambient temperatures.Among all the HFO mixtures, R1234ze/R134a mixture shows better performance and it is very similar to the R134a.
Table 4. shows that the R1234ze/R134a has a lower TEWI than R134a by about of 5.9%.This is because of less energy efficiency, but direct emissions are significantly lower than R134a.

Conclusion
The low-GWP refrigerants such as R1234ze/R134a, R1234ze/R32/R152a, and R1234ze/R134a/R744 have been studied theoretically in household refrigerator.Among the mixtures, R1234ze/R134a (90/10) performs best, with estimated COP and exergetic efficiency 3.7% to 16.4% and 4% to 16% higher than the other mixtures.The R1234ze/R134a has a lower TEWI than R134a by about of 5.9%.This is because of less energy efficiency, but direct emissions are significantly lower than R134a.Even though the performance of the mixture R1234ze/R134a is slightly lower than R134a, but it is superior to other HFO mixtures.
It may also be an appropriate alternative for R134a in the household refrigerator to address environmental problems according to the Kyoto Protocol.

Fig. 1
Fig. 1 Variation of vapour pressure as a function of temperature

Fig. 5
Fig. 5 Flow chart for simulation model

Fig. 6
Fig. 6 Variation of theoretical COP with ambient temperature

Fig. 7
Fig. 7 Variation of compressor work with ambient temperature

Fig. 9
Fig. 9 Variation of total exergy destruction with ambient temperature Figures

Table 1 .
Thermophysical properties of the refrigerants

Table 3 .
Operating conditions

Table 4 .
TEWI for the both refrigerants