Energy and Exergy Analysis of Eco Friendly Refrigerants based Domestic Refrigerator System AbstractIn this paper energy and exergy performance of different refrigerants like CFC12, HFC134a, HFC143a, HFC152a, HC290, HC600a and HO1270 used in vapour compression refrigeration system have been compared. Vapour compression refrigeration systems exergetic modelling has been presented. The effects of evaporator and condenser temperature on the coefficient of performance, exergy flow destruction, and entropy generation in the major components of refrigerator has been studied and presented. It has been found from the study, refrigerant HC600a is the best based on second law analysis.Keywords Exergetic efficiency; Heat exchanger effectiveness; Subcooling; Superheating; Ozone depletion Potential; Global warming potential. NomenclatureEXw=Work done by or on the system second law=Second law efficiencyi=Exergy at inlet m=mass flow rateo=Exergy at outlet I= IrreversibilitySgen=Entropy generation = Rate of heat transfer in condenserWc=Rate of energy = rate of heat transfer in evaporatorT0=Ambient temperature

Tk=Temperature of heat source or sink IntroductionThe domestic refrigerator is an appliance that is found in almost all households for storing food, vegetables, beverages and much more. Domestic refrigerators were among the first appliances to be targeted for energy efficiency improvements, as it was the largest energy user at homes. As against the general perception that global warming impact is caused due to refrigerant leakage, it is interesting to note that 80% of it happens actually because of energy consumption. Overall the refrigeration accounts for about 15% of the total electricity consumed in the world, which in turn is mostly produced by burning of fossil fuels (coal, oil and gas). Fossil fuels gives rise to strongly elevated greenhouse gas emission as well as environmental damage, both of which have extreme impacts on the respective ecosystems. Unlike fossil fuels, renewable energies are conventional energy resources, which are inexhaustible so we can switch to renewable energy resources. In the present scenario commercial enterprises, residential estates and private households all over the world are increasingly in need of reliable, affordable, and environment friendly energy to fulfill their demand for electricity, heat, cooling and mobility. Renewable energies can make a considerable contribution to fulfill this demand, both in private households as well as in commercial enterprises. To ascertain how much energy is required by the system and how efficiently it is utilizing that energy, it is necessary to perform exact analysis. And exergy analysis is the means to perform exact analysis. Exergy analysis seems to be an effectual method using the conservation of mass and conservation of energy principle in proximity with second law of thermodynamics for the design and analysis of energy systems. It is an acceptable technique for promoting the goal of more efficient energy resource use and ascertaining the locations, types and true magnitudes of waste and losses. It is an efficient technique divulging whether or not and by how much it is feasible to design more efficient energy systems by reducing the inefficiencies of the system. The damage caused by CFCs was discovered bySherry Rowland andMario Molinawho, after hearing a lecture on the subject of Lovelock's work in 1974 (Ozonecell, 2013). (Sentence incomplete)To prevent environment from impact of these refrigerants many treaties came into action. First one was Montreal Protocol according to which production of such kind of substances that deplete the ozone layer must be phased out. Then Kyoto Protocol was negotiated in December 1997, at the city Kyoto, Japan and came into force on February 16th, 2005. Its objective was to reduce greenhouse gases emissions. The properties of the refrigerants which are taken into consideration are given in table 1.Table 1 Refrigerants properties (Ozonecell, 2013)RefrigerantsOzone depletion potential(ODP)Global warming potential(GWP)

HFC134a01300

HFC143a04470

HFC152a0120

HC290020

HC600a020

HO1270020

CFC12110900

It has been observed that energy analysis is the most common analysis of refrigeration system. This approach fails to make out the real energetic losses in refrigeration cycle (Arora and Kaushik, 2008).Exergy analysis provide the improved and deeper insight view into the process (Saidur, 2007).Chen & Prasad (1999) did the simulation of a vapour-compression refrigeration cycle and compared the performance of HFC134a and CFC12. Sencan et al. (2005) has performed computer based energy and exergy analysis and observed the effect of subcooling and superheating on the vapour compression refrigeration cycle containing R134a, R407C and R410A. Mani & Selladurai (2008) compared the performance of R290/R600a refrigerant mixture as drop-in replacement of CFC12 and HFC134a in vapour compression refrigeration cycle. Arora & Kaushik (2008) presents a detailed exergy analysis of an actual vapour compression refrigeration (VCR) cycle and concluded that R507A is a better substitute to R502 than R404A. Mohanraj et al. (2008) discussed the possibility of using R152a and hydrocarbon refrigerants (such as R290, R1270, R600a, and R600) as alternative of R134a in domestic refrigerator. Padilla et al. (2010) calculated the exergy and compared the performance of domestic refrigerator based on R12 with the retrofitted zeotropic mixture of R413A. The purpose of this study is to find out the best refrigerant on the basis of four parameters i.e. GWP, ODP, first law analysis and second law analysis. The thermophysical properties of selected refrigerants are given in table 2. Table 2 Physical properties of Refrigerants

RefrigerantChemical formulaMolecular mass(g/mol)Normal boiling point(C)Tc(C)Pc(bar)

CFC12CF2CL2120.9-29.811241.15

HFC134aCF3CH2F102.03-26.1101.140.64

HFC143aCH3CF384.04-47.272.937.8

HFC152aCH3CHF266.05-24.0113.345.2

HC290C3H844.1-42.196.842.5

HC600a(CH3)3 CH58.12-11.7313536.6

HO1270C3H6

42.09-47.791.946.1

Modelling of Vapour compression refrigeration systemDomestic refrigerator is based on vapour compression refrigeration cycle. The schematic diagram of vapour compression cycle is shown in fig.1. The T-s chart of vapour compression cycle is shown in fig.2. For steady state flow, the exergy balance can be given by ignoring kinetic and potential energy changes

EXw=Qk+-T0Sgen (Cengel, 2012) (1) Exergy balance equation is employed for various components of VCR system.

Exergy balance for compressorT0Sgen= m1 (2-3) + Wc (2)

I destroyed= T0Sgen= m1T0 (S3-S2) (3)

second law =1- (I destroyed/ Wc) (4)

Figure 1: Vapour Compression Refrigeration cycle

Figure 2: Temperature Entropy (T-s) diagram of Vapour Compression CycleExergy balance for condenserT0Sgen=m3 ( 3- 4) - 0/k) Qk (5)

I destroyed= T0Sgen= (m1 (h3-h4)-T0 (m3 (S3-S4)) - 0/k) Qk (6) Exergy balance for heat exchangerT0Sgen=m4 ( 4- 5)-m1 ( 2- 1) (7)

I destroyed= T0Sgen= (m4 (h4-h5)-m1 (h2-h1))-T0 (m4 (S4-S5)-m1 (S2-S1)) (8)

second law =1- (I destroyed/ m4 ( 4- 5) (9)

Exergy balance for Throttle valveT0Sgen= m5( 5- 6) (10)

I destroyed= T0Sgen= m5 ((h5-h6)-T0 (S5-S6) (11)

Exergy balance for evaporatorI destroyed= T0Sgen= (m6 (h1-h6)-T0 (m6 (S1-S6)) - 0/k) Qk (12)

Energy analysis (Ballaney, 2012) COP = (h1-h6) *F*iso*g / (h3-h2) (13)

Exergy analysis (Cengel, 2012) second law=COP/(T1/(T4-T1)) (14)Energy analysisThe objective of first law analysis is to predict the variation of COP with evaporator temperature, condenser temperature, subcooling temperature, superheating temperature and effectiveness of liquid suction heat exchanger. The operating temperature range for domestic refrigerator is taken from 248K to 328K (Arora, 2000). Assumptions observed during this work are summarized in table 3.Table 3 Assumptions for domestic refrigeratorm(kg/s)1(Constant)

T0(K)278K

Te(K)216K to 263K

Tc(K)318K to 338K

TSub(K)5K

TSuper(K)5K

Effectiveness0.8

F0.8

c0.85

motor0.9

The COP variation is shown in figures 9, 10, 11, 12 and 13. COP decreases with rise in condensing temperature as work done by compressor increases, HO1270 shows the maximum drop in COP and CFC12 shows the minimum.

Figure 9: Variation of COP with condenser temperature

The evaporator temperature is varied from 216 K to 263 K. COP increases with rise in evaporator temperature as pressure ratio decreases which cause reduction in compressor work. HC600a shows the highest COP among the refrigerants, closely followed by HFC134a.

Figure 10: Variation of COP with evaporator temperature

The subcooling temperature is varied from 256.2 K to 260.2 K. COP is showing decreasing trend with subcooling temperature. Maximum drop in COP is seen in HO1270.

Figure 11: Variation of COP with sub cooling temperature

The superheating temperature is varied from 248.2 K to 253.2 K. COP increases with superheating as it increases the refrigeration effect. Maximum COP is observed in HC600a.

Figure 12: Variation of COP with superheating temperature

The effectiveness of heat exchanger is varied from 0.6 to 1. COP increases with effectiveness, maximum increase is shown by HO1270.

Figure 13: Variation of COP with liquid suction heat exchanger effectiveness

We have also studied the exergy destruction in each component with selected refrigerants. The irreversibility variations of compressor, condenser, heat exchanger, expansion valve and evaporator with selected refrigerants are presented in fig.4, 5, 6, 7 and 8 respectively. From the graphs we can observe that CFC12 is having the lowest exergy destruction as compared to other refrigerants and HO1270 is highly irreversible in all components except heat exchanger.

Figure 4: Variation of Irreversibility in compressor with selected refrigerants

Figure 5: Variation of irreversibility in condenser with selected refrigerants

Figure 6: Variation of irreversibility in heat exchanger with selected refrigerants

Figure 7: Variation of irreversibility in throttle valve with selected refrigerants

Figure 8: Variation of Irreversibility in evaporator with selected refrigerantsExergy AnalysisThe second law analysis is based on an abstract idea of exergy. It can be defined as quantification of work potential or quality of different forms of energy relative to environmental state. When exergy analysis is applied to a system it describes all loses in the system components and in the whole system. The exergy method is a corelatively new evaluation technique in which the basis of assessment of thermodynamic loses follows from the second law rather than the first law of thermodynamics. Our aim of exergy analysis is to specify the variation of exergetic efficiency with evaporator temperature, condenser temperature, subcooling and superheating temperature and effectiveness of liquid suction heat exchanger.The figures 14, 15, 16, 17 and 18 shows the variation of exergetic efficiency. Exergetic efficiency decreases with increase in evaporator temperature. As evaporator temperature increases, COP increase causing minimum exergy intake to perform the given work. HC600a is showing highest exergetic efficiency.

Figure 14: Variation of Exergetic efficiency with evaporator temperature

The condenser temperature is varied from 318 K to 338 K. Exergetic efficiency increases with increase in condenser temperature. HC600a is showing maximum exergetic efficiency.

Figure 15: Variation of Exergetic efficiency with condenser temperatureThe subcooling temperature is varied from 256.2 K to 260.2 K. Exergetic efficiency is decreasing with subcooling and highest is given by HC600a.

Figure 16: Variation of Exergetic efficiency with sub cooling temperatureThere is slight decrease in exergetic efficiency with superheating. The temperature is varied from 248.2 K to 253.2 K. With increase of degree of superheating the total exergy loss increases due to which second law efficiency decreases.

Figure 17: Variation of Exergetic efficiency with superheating temperaturesThe effectiveness of liquid suction heat exchanger is varied from 0.6 to 1. Exergetic efficiency increases with the effectiveness of heat exchanger. HO1270 is showing maximum exergetic efficiency.

Figure 18: Variation of Exergetic efficiency with heat exchanger effectiveness

ConclusionThe performance of theoretical Vapour compression refrigeration cycle was assessed by both energetic and exergetic COPs, with the latter providing good guidance for system improvement. It is concluded that HC600a can be good drop-in replacement of HFC134a, as HC600a is having low value of global warming potential (GWP) as compared to HFC134a. For HC600a the coefficient of performance and exergetic efficiency is found to be maximum among the selected refrigerants. From the irreversibility viewpoint, worst component is condenser and worst refrigerant is HC1270. References1. Ahamed J.U, Saidur R, Masjuki H.H, A review on exergy analysis of vapor compression refrigeration system Renewable and Sustainable Energy Reviews 15 (2011) 15931600

2. Arora A, Kaushik SC (2008) Theoretical analysis of a vapour compression refrigeration system with R502,R404A,R507A. International Journal of Refrigeration 31:998-10053. Arora CP, Refrigeration and air conditioning; Tata McGraw-Hill; 2007.4. Ballaney PL, Refrigeration and Air conditioning; khanna publishers;20125. Cengel Yonus, Boles Michael, thermodynamics an engineering approach 7th edition Tata McGraw-Hill, 2012.6. Chen Qiyu, Prasad R.C, Simulation of a vapour-compression refrigeration cycles using HFC134a and CFC12 Int. Comm. Heat and Mass Transfer, Vol. 26, No. 4, pp. 513-521, 19997. Kreith Frank Ed ,Mechanical Engineering Handbook Boca Raton: CRC Press LLC, 19998. Mani K, Selladurai V, Experimental analysis of a new refrigerant mixture as a drop in replacement for CFC 12 and HFC 134a, International Journal of Thermal Science 47 (2008) 1490-1495.9. Mohanraj M, Jayaraj S, Muraleedharan C ,Comparative assessment of environment-friendly alternatives to R134a in domestic refrigerators Energy Efficiency (2008) 1:189198

10. Padilla M, Revellin R, Bonjour J. Exergy analysis of R413A as replacement of R12 in a Domestic Refrigeration system. Energy Conversion Management. 2010; 51: 2195201.11. Reddy Siva, Panwar N.L, Kaushik S. C, Exergetic analysis of a vapour compression refrigeration system with R134a, R143a, R152a, R404A, R407C, R410A, R502 and R507A Clean Techn Environ Policy (2012) 14:475312. www.envfor.nic.in/cc/india_unfccc.html (Accessed on 2013)13. www.ozonecell.com (Accessed on 2013)14. http://www.kri.cn(Accessed on 2013)

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