Thermodynamic Analysis of Eco-Friendly Refrigerant Mixtures as an Alternative to HFC-134a in Household Refrigerator

Environmental pollution is aggravated by the excessive use of refrigerators and air conditioners worldwide, along with automobiles. The ozone layer is damaged by releasing refrigerants containing chlorine into the atmosphere. Due to this, dangerous ultraviolet radiations are coming to the surface of the earth. As a result, the earth’s surface temperature is increasing rapidly, leading to weather change. The effect of these greenhouse gases can be expressed in terms of GWP. In the last 30 years, CFCs and HFCs are widely used in refrigerators and air conditioners. However, ODP and GWP values are very high for these refrigerants, which cause environmental pollution. According to Montreal protocol, chlorofluorocarbons and HCFC are entirely prohibited in the air conditioning and refrigeration sector due to this higher ODP value. Therefore, in place of these refrigerants, HFC refrigerants are introduced, but the main problem with these refrigerants is that they have a higher GWP value. Therefore, these should be banned in the coming years based on the Kyoto Protocol. Therefore, R134a has to be phased out by 2021. In addition, most of the developing countries are drastically reducing their HFC production and consumption. Therefore, there is a greater demand for an adequate replacement for HFC-134a to adapt to existing and new systems.

Hoe et al. [1] experimented with R600a, which is a substitute to the R12 in a household refrigerator. They analyzed theoretically with the help of software REFPROP, and then performed a series of tests with this refrigerant substitute to R134a in a fridge. Jung et al. [2] conducted an experiment with a mixture of HC290 / HC600a (60:40 by mass) as a direct substitute for R12 in a refrigerator and concluded that COP and power efficiency improved by 2.5 and 3.8%. Fatouh and Kafafy [3] studied the performance of the household refrigerator that works with the refrigerant mixture (consist of HC290 / HC600 / HC600a in the ratio 60:20:20 by mass) a substitute to HFC-134a. It has been reported that the power consumption of compressor operating with an LPG blend was 5.1% lower than HFC-134a with 7.5% higher COP. Garland and Hadfield [4] studied the environmental impact of the R600a natural refrigerant installed in the hermetic compressor of the household refrigerator. The results showed that the R600a is superior to the R134a, with the compressor having its 15-year cycle.

Dalkilic and Wongwises [5] conducted a theoretical analysis on the refrigerator using various alternative refrigerants and refrigerant mixtures as an alternative to R12 and R22. They concluded that HFC and HC refrigerants could be used as alternatives to the above refrigerants from that theoretical analysis. Naushad et al. [6] had conducted an energy and exergy analysis of R1234yf, R1234ze (E) and R134a in a domestic refrigeration system. Finally, they concluded that HFO-1234yf could be used as a good substitute for HFC-134a at a higher value of the evaporator temperature, and R1234ze (E) can be used as a suitable replacement after specific modification. Rastietal [7] conducted an experiment on R600aand R436a consisting of 46% and 50% of isobutane and propane) as an alternative to R134a in a household refrigerator. The results concluded that the compressor energy consumption and volumetric cooling capacity was reduced by about 15% and 8%, respectively. Joybari et al. [8] carried out the exergy analysis to find the optimal load of HC-600a as a substitute for HFC-134a, the optimum load required for HC-600a was 0.050kgand 65% lower than HFC-134a. Bilen et al. [9] investigated theoretical analysis of the automobile air conditioning system using R152a, R22 and R12 to find out possible alternatives to R134a. From the results, they concluded that the performance does not change significantly by using R152a as compared with R134a. Bolaji et al. [10] made a performance comparison of low GWP refrigerants like R152a and HC600 theoretically, an alternative to HFC134a in a refrigerator. By observing these results, they concluded that R152a shows a higher volumetric cooling capacity (VCC) and co-efficient performance compared to HFC-134a. The average COPs achieved for HC-600a and HC-152a were 6% lower and 12.9% higher than HFC-134a. They concluded that HFC-152a shows the best results as compared with R134a. Morsi [11] performed a theoretical analysis of a VCR system using pure natural refrigerants to substitute HFC-134a. Results revealed that LPG gives a lower COP, and Isobutene gives a higher COP than HFC-134a by 11% and 5%, respectively.

Meng et al. [12] have done thermodynamic investigation for HFO-1234ze (E), R152a and HFO-1234ze (E)/R152a blends as a direct substitute to HFC-134a in a refrigerator system without making any modifications to the system. Sanchez et al. [13] led an experiment with low GWP refrigerants like HFO-1234yf, 1234ze (E), R290, R152a and HC-600a in the refrigeration system and experimental results were compared with HFC-134a. From that experiment, they concluded that HFO-1234yf and R152a have a perfect substitute for HFC-134a. Makhnatch et al. [14] examined the performance ofR450Awhich is a mixture of R134a/R1234ze (E) (42:58% by mass) as a substitute to HFC-134a in household refrigerators. It has been revealed that the Refrigerating effect and COP of the refrigerant mixture were approximately 10% and 3% lower than HFC-134a. At the same time, the outlet temperature of the compressor is more inferior to HFC-134a. Hasheer and Srinivas [15] conducted a theoretical investigation on low GWP refrigerants as a direct substitute to R134a in a domestic refrigerator. They concluded that R1234yf could be used as a natural substitute to R134a. Mohammad Hasheer Sk et al. [16] performed a thermodynamic analysis of low gwp refrigerant mixtures as alternative to R134a in refrigerator. From that they concluded that R290/600(60/40), ARM42, ARM42a, R440A, and R430A have better COP execution and volumetric cooling limit than R134a, which makes it the best substitute to R134a shows a favorable conditions.

A review of existing literature shows that much research has been done to find suitable alternative refrigerants from different groups individually. But the combination of two or more refrigerants from HC, HFC and HFO groups have not tried so far by the previous researchers. Also, the performance evaluation with LSHX was not done extensively. So the present work mainly focuses on filling that research gap. So the investigation was carried out with the refrigerants AC5, R440A and R430A. They are the combination of two or more refrigerants from the above-said groups. So the performance of a domestic refrigerator involving LSHX was evaluated with these three refrigerants, and the best alternative refrigerant to replace R134a have been identified and suggested.

The thermodynamic Analysis of AC5, R440A and R430Aas a direct substitute to HFC-134a in a refrigeration system by varying the working conditions, i.e. when changing the temperature of the evaporator from -200℃ to 100℃ at different condenser temperatures. The complete analysis has been carried out by using an internal heat exchanger.

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Figure 1. Refrigerator with LSHX

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Figure 2. Pressure-enthalpy diagram of a refrigerator with LSHX

Data (from the literature review) used for analysis are given below. The results are plotted as shown in Figures 3 to 8.

1. Condensing temperatures: 40℃ and 50℃ 

2. Evaporating temperatures: -20℃ to 10℃

3. Loss of Pressure in the evaporator: 0.03 MPa

4. Loss of Pressure in the condenser: 0.02 MPa

5. Isentropic efficiency of a compressor: 0.70

6. Volumetric efficiency: 0.75

7. Compressor had a swept volume: 8.16cm³/rev

8. Compressor Speed: 30rev/sec

9. Effectiveness of the heat exchanger: 0.6.

The components of a domestic refrigerator with LSHX in the position shown in Figure 1. The pressure-enthalpy diagram with the heat exchanger is shown in Figure 2. At the entry to the compressor, the refrigerant is in superheated condition, and pressure losses are considered. At the same time, it passes through the evaporator and condenser and is also represented in Figure 2. REFPROP 9.1 software is used to calculate the properties at each state, which is very accurate software for calculating properties.

Pressure ratio, volumetric cooling capacity, COP, outlet compressor temperature, Refrigeration effect and compressor power consumption is the main parameters to accept a direct substitute to a domestic refrigerator.

3.1 Performance parameters

The pressure ratio can be expressed as

$r_{p}=P_{\text {cond }} / P_{\text {evap_act }}$               (1)

Compressor power consumption can be calculated from

$\dot{\mathrm{W}}_{\text {comp }} \quad \dot{\mathrm{m}}_{\mathrm{r}}\left(\mathrm{h}_{2}-\mathrm{h}_{1}\right) \mathrm{kW}$               (2)

Here $h_{1} \& h_{2}$ are the enthalpies of the refrigerant at entry and exit of the compressor. The relation between these two can be obtained by defining isentropic efficiency $\left(\eta_{\text {isen }}\right)$ of the compressor as follows

Where $h_{2}=h_{1}+\left(h_{2 s}-h_{1}\right) / \eta_{\text {isen }}$                    (3)

The refrigeration effect of a refrigerator can be calculated by

Refrigeration effect, $Q_{r}=\left(h_{1}-h_{5}\right) \quad \mathrm{kJ} / \mathrm{kg}$          (4)

Here $h_{5}$ is the enthalpy of the refrigerant at the entry to the evaporator.

The cooling capacity of a refrigerator can be calculated by

Cooling Capacity, $\dot{Q}_{c}=\dot{m}_{r} Q_{r}=\dot{m}_{r}\left(h_{1}-\right.\left.h_{5}\right) \mathrm{kW}$             (5)

The Coefficient of Performance (COP) of the refrigerator is given by

COP $=\frac{\text { Cooling capacity }}{\text { Compressor power consumption}}\quad=\frac{\dot{m}_{r}\left(h_{1}-h_{5}\right)}{\dot{m}_{r}\left(h_{2}-h_{1}\right)}=\frac{\left(h_{1}-h_{5}\right)}{\left(h_{2}-h_{1}\right)}$               (6)

The Volumetric Cooling Capacity (VCC) is given by 

$Q_{\text {vol }}=\left(h_{1}-h_{5}\right) \times \eta_{\text {vol }} / v_{1} \mathrm{~kJ} /$             (7)

Here $\eta_{\text {vol }}$ is the volumetric efficiency of the compressor and $v_{1}$ is the specific volume of the refrigerant at the compressor inlet.

The mass flow rate of the refrigerant ( $\dot{m}_{r}$ ) is given by

$\dot{m}_{r}=V_{\mathrm{c}} \times \rho_{1} \times R P M \times \eta_{\text {vol }} / 60 \mathrm{~kg} / \mathrm{s}$                (8)

The AC5 refrigerant shows a much lower volumetric cooling capacity compared to HFC-134a of approximately 9.1%. For a direct replacement, the value must be between -8% and 8%. Therefore, it is not suitable for direct use as a substitute for HFC-134a in a household refrigerator. R440A and R430A had given good results in VCC, power consumption of a compressor, COP and pressure ratio. But R440A shows a high compressor output temperature that affects the properties of lubricating oil for refrigerator compressor. When comparing all results with R134a, R430A can be used as a direct substitute for HFC-134a in the household refrigerator without changing the refrigerator. Therefore, it is concluded that R430A can be used as an alternative to HFC-134a in a household refrigerator. At the same time, when comparing the results with the literature (without LSHX) there is an improvement in the performance of a household refrigerator.