η second law = Second law efficiency

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1 EXERGY ANALYSIS OF VAPOUR COMPRESSION REFRIGERATION CYCLE BASED AUTOMOTIVE AIR CONDITIONER USING ECO FRIENDLY REFRIGERANTS Vijender Singh Saini 1, Balvinder 2, Pardeep Kumar 3 1 M.Tech Scholar, 2,3 Assistant Professor, Department of Mechanical Engineering 1,2 Om Institute of Technology & Management, Hisar, 3 R.N. College of Engg. & Tech, Panipat Abstract--This research work deals with the exergy analysis of a vapour compression refrigeration system with CFC12, HFC134a, HFO1234yf refrigerants. The various parameters computed are COP and exergetic efficiency in the system. Effects of condenser temperature, evaporator temperature and sub-cooling of condenser outlet, Evaporator temperature and superheating of evaporator out let and effectiveness of vapour liquid heat exchanger are also computed and discussed. In this study, it was found that HFO1234yf has the better performance in all respect. Keywords--COP, Exergetic efficiency, Ozone depletion potential, Global warming potential, Liquidvapour heat exchanger, Superheating, Subcooling. Nomenclature: EX w = Work done by or on the system I = Exergy at inlet o = Exergy at outlet S gen = Entropy generation W c = Work consumption I destroyed = Irreversibility T k = Temperature of heat source or sink m = mass flow rate major part of Heating ventilation and air conditioning (HVAC) syste m. Most Heating ventilation and air conditioning (HVAC) system based on Vapour Compression Refrigeration System. Figure 1 Automotive Air Conditioning System( η second law = Second law efficiency INTRODUCTION During the past few decades, Chlorofluorocarbons (CFCs) and hydro chlorofluorocarbon (HCFCs) have been used extensively in Automotive air Conditioners. The CFCs& HFCs have been related to depletion of the ozone layer and a lot of efforts Figure 2 Components of Automotive Air Conditioning System on International level are going on to reduce the ( production and utilization of these working substances. Chloroflorocarbons (CFCs) has been used in domestic Automotive air conditioning system consists therefrigerators and air conditioner over a long period of time. They are molecules composed of carbon, chlorine ISSN Page 99

2 and fluorine. It contributes to the destruction of the ozone layer. Hydrochloroflorocarbons (HFCs) are other refrigerants used in Vapour compression refrigeration system having Zero ozone depletion potential. (Agrawal and Matani, 2012) HFO-1234yf is a new environment friendly sustainable refrigerant for automobile air conditioner which has 99.7% better Global Warming Protocol than HFC134a. Table 1 showing the Global warming potential comparison between HFC134a and HFO1234yf (Dupont, 2015) Table 1: Global warming potential comparison between CFC12, HFC134a and HFO1234yf (Dupont, 2015) Global warming Refrigerant potential(gwp) Number CFC HFC-134a 1300 HFO-1234yf 4 Mobile air conditioning Directive Requirement 150 or less Table 2: Physical properties of Refrigerants for car air Chemical formula conditioner (Kri, 2015) Molecular mass Normal boiling point CFC12 CF 2 Cl HFC134a CF 3 CH 2 F HFO1234yf C 3 H 2 F T c P c Substances that Deplete the Ozone Layer must be banned. The production of such kind of substances must be phase out to protect the ozone layer. India signed to Montreal protocol on September 17, 1992 (Ozonecell, 2015). According to the Kyoto Protocol the industrialised countries must advance them to reduce emissions of greenhouse gases. India signed to the Kyoto Protocol on 26 August 2002(Envfor, 2015).It has been observed that most of the analysis of refrigeration cycle regarding performance analysis is done with energy approach e.g coefficient of performance. Energy approach are not able to make the real energetic losses is refrigeration cycle (Arora and Kaushik, 2008).Energy analysis is the most commonly used method in the analysis of thermodynamic systems. Energy analysis shows only with the conservation of energy. The purpose of this study is to investigate the theoretical performance analysis of CFC12, HFC134a and HFO1234yf based on COP and exergy terms. Exergy Analysis of Vapour compression refrigeration System: Exergy is a term related to quality which always gets lower and lower because of irreversibility in thermodynamic system. For steady state flow, the exergy balance can be given by ignoring kinetic and potential energy changes. The exergy balance equation is given below: EX w = 0/ k) Q k + i- ) o - T 0 S gen (Cengel, 2012) (1) Therefore, the refrigerants having high ODP and GWP are required to be replaced with eco friendly refrigerants to protect the environment from Ozone depletion & Global warming with refrigerants having low ODP and GWP. The hydro fluorocarbon (HFC) having zero ODP can be considered as alternatives (Ma ni and Selladurai, 2008). According to the Montreal Protocol the ISSN Page 100

3 Figure 3: Vapour Compression Refrigeration System Figure 4: Temperature Entropy (T-s) diagram of Vapour Compression Cycle MATHEMATICAL MODELING The exergy balance equation (1) is employed for various components of Vapour compression refrigeration system. Exergy balance for compressor: T 0 S gen = m 1 ( 2-3)+ W c (2) I destroyed = T 0 S gen =m 1 T 0 (S 3 -S 2 )(3) From here we can find out Entropy generation (S gen ) and Irreversibility (I destroyed ) occurring within the Compressor The second law =1- (I destroyed / W c )(4) From this equation we can find out the Second law efficiency of the compressor. Exergy balance for condenser: T 0 S gen =m 3 ( 3-4)- 0/ k) Q k (5) I destroyed = T 0 S gen =(m 1 (h 3 -h 4 )-T 0 (m 3 (S 3 -S 4 ))- 0/ k) Q k (6) From here we can find out Entropy generation (S gen ) and Irreversibility (I destroyed ) occurring within the condenser. Exergy balance for Heat exchanger: T 0 S gen =m 4 ( 4-5)-m 1 ( 2-1) (7) I destroyed = T 0 S gen =(m 4 (h 4 -h 5 )-m 1 (h 2 -h 1 ))-T 0 (m 4 (S 4 - S 5 )-m 1 (S 2 -S 1 )) (8) From here we can find out Entropy generation (S gen ) and Irreversibility (I destroyed ) occurring within the Heat exchanger. second law =1- (I destroyed /m 4 ( 4-5) (9) From this equation we can find out the Second law efficiency of the heat exchanger. Exergy balance for Throttle valve: T 0 S gen =m 5 ( 5-6) (10) I destroyed = T 0 S gen =m 5 ((h 5 -h 6 )-T 0 (S 5 -S 6 ) (11) From here we can find out Entropy generation (S gen ) and Irreversibility (I destroyed ) occurring within the throttle valve. Figure 5: Pressure Enthalpy diagram (P-h) of Vapour Compression Cycle Exergy balance for Evaporator: I destroyed = T 0 S gen =(m 6 (h 1 -h 6 )-T 0 (m 6 (S 1 -S 6 ))- 0/ k) Q k (12) ISSN Page 101

4 Energy analysis: This is done in the form of evaluating COP. COP =(h 1 -h 6 )*F* iso * g / (h 3 -h 2 ) (Ballaney,2012) (13) Exergy analysis: This is done in the form of evaluating Second law efficiency. Flow Chart:A program has been prepared in Engineering Equations solver version 9.904D and algorithm in terms of flowchart as been given in figure 6: second law= COP/(T 1 /(T 4 -T 1 )) (Cengel,2012) (14) Vapour compression refrigeration Energy analysis is done by calculating the Coefficient of Performance(COP) and Exergy analysis is done by calculating the value of second law (Second law efficiency. RESULTS & DISCUSSION A computational model was developed for carrying out the energy and exergy analysis of the system using Engineering Equation Solver software ( S.A. Klein, 2015). To access the performance of selected refrigerants in vapour compression refrigeration cycle, following assumptions were made: Table 3: Assumptions for Automotive air conditioner m(kg/s) T 0 (K) T e (K) T c (K) T Sub (K) T Super (K) 1(Constant) Ambient temperature 233K to 293K 308K to 328K 5K(286.2K to 291.2K) 5K (288.2K to 293.2K) Effectiveness 0.8 motor 0.9 Isentropic efficiency of compressor 0.85 Fig 6: Flowchart for energy and exergy analysis Effect of condensing temperature: Figure 7 shows the effect of condenser temperature on COP of the system. Condenser inlet temperature varies in the range of K (T 4 ). As condenser temperature increases the COP of ISSN Page 102

5 the system with CFC12 was reduced by about 48%. Similar trend was presented by HFC134a, and HFO1234yf. Highest COP was calculated with HFO1234yf at condenser temperature 308K, whereas CFC12 shows minimum COP about 3.76 at condenser temperature 328K. Amongst all selectedrefrigerant, HFO1234yf shows better COP. International Journal of Technical Research (IJTR) Figure 8: Condenser temperature versus exergetic efficiency Figure 7: Condenser temperature versus COP Effect of condenser temperature on exergetic efficiency is shown in Figure8,Exergetic efficiency was increased with corresponding temperatures. Exergetic efficiency of CFC12, HFC134a and HFO1234yf was increased by 2.83, 3.66 and 3.93%, respectively. CFC12 shows the least variation in exergetic efficiency, whereas HFO1234yf shows maximum fluctuation. HFO1234yf shows highest exergetic efficiency at all corresponding condenser temperature. Effect of Liquidvapour heat exchanger effectiveness: Figure 9 shows the effect of liquidvapour heat exchanger effectiveness on COP. The effectiveness of liquidvapour heat exchanger varies in the range of COP of the refrigerant was increasing corresponding to increase in evaporator effectiveness. COP of HFO1234yf was at 0.5 liquidvapour heat exchanger (LV HE) effectiveness and it was increased by at 1.0 LVHE effectiveness. It was clear from Figure9that the HFO1234yf shows better COP than other refrigerants. Figure 9: Heat exchanger effectiveness versus COP ISSN Page 103

6 Figure 10shows the effect of VLHE effectiveness on exergetic efficiency. As effectiveness increases, the exergetic efficiency was also increased. Overall HFO1234yf shows better exergetic efficiency as compared to other refrigerant. Fig 11: Evaporator Temperature versus COP Figure 12 shows the effect of evaporator temperature on exergetic efficiency. The value of exergetic efficiency decreases with increasing the evaporator temperature. The highest exergetic efficiency occurs at the optimum evaporation temperature. Exergeticefficiency was decreased by 9.41, 10.5 and 12.58% in CFC12, HFC134a and HFO1234yf respectively. Figure 10: Heat exchanger effectiveness versus Exergetic efficiency Effect of evaporator temperature: Figure 11 shows the effect of evaporator temperature on COP. The evaporator temperature was varied in the range of K. As evaporator temperature increases, COP was also increasing. HFO1234yf shows better COP performance as compared to other refrigerant. COP of HFO1234yf at 233K was about and it attained at 293K, its COP was increased by %. Figure 12: Evaporator Temperature versus Exergetic efficiency Effect of sub-cooling of condenser out let: Figure 13shows the performance of refrigeration cycle with sub-cooling. The operating condition under sub-cooling was kept between to K (T 5 ). HFO1234yf shows maximum value of COP (5.457) at K and it was higher than CFC12 and HFC134a by 1.09 and 2.95%, respectively. Hence, subcooling is desirable to improve the cycle performance. ISSN Page 104

7 Effect of superheating of evaporator out let: In vapour compression refrigeration cycle, during the evaporation process the refrigerant is completely vaporised in evaporator. If the superheating takes place in the evaporator, the enthalpy of the refrigerant is increased, extracting additional heat and increasing the refrigeration effect of the evaporator. The operating condition under superheating was kept between to K. Figure 15 reveals that as temperature increases, the increasing trend of COP was found with all selected refrigerants. COP of HFO1234yf increased by 23.40%. Similarly for CFC12, andhfc134a, it was increased by and 20.59% respectively. Figure 13: Subcooling temperature versus COP Figure 14 shows the effect of sub-cooling on exergetic efficiency. The similar trend was found as found in case of COP. At higher subcooling, high-exergetic efficiency was found. HFO1234yf shows better exergetic efficiency as compared to other selected refrigerants. Figure 15: Superheating temperature versus COP Figure 16 shows the effect of super-heating temperature on exergetic efficiency. The value of exergetic efficiency decreases with increasing the super-heating temperature. Exergetic efficiency was decreased by 1.16, 1.34 and 1.45% in CFC12, HFC134a and HFO1234yf respectively. Figure 14: Subcooling temperature versus exergetic efficiency ISSN Page 105

8 replaced as per Environmental issues HFO1234yf is very long time alternative eco friendly refrigerant. Figure 16: Superheating Temperature versus Exergetic efficiency CONCLUSION HFO1234yf is very good alternative to HFC134a and CFC12. HFO1234yf have high value of coefficient of performance and Exergetic efficiency than HFC134a.The value of Global warming potential is very low that is only 4 of HFO1234yf and ozone layer depletion zero. Work consumed for HFO1234yf is low as compared to HFC134a and high as compared to CFC12 So according to work done required CFC12 is better than two selected refrigerants but it is to be REFERENCES [1]. PardeepKumar,Maman Singh, Anil jaglan, Energy &Exergy analysis of HFO1234yf for air conditioner applications IJERSTE, Volume 4, Issue 4, April [2]. 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) 14:47 53 [3]. Agrawal K Mukesh,Matani Ashok, Evaluation of Vapour Compression Refrigeration System Using Different Refrigerants-A Review International Journal of Engineering and Innovative Technology (IJEIT) Volume 2, Issue 4, October [4]. Ahamed J.U, Saidur R, Masjuki H.H, A review on exergy analysis of Vapour compression refrigeration system Renewable and Sustainable Energy Reviews 15 (2011) [5]. Apera C, Greco A, An exergetic analysis of R22 substitution. Applied thermal engineering (2002) [6]. Arora A. Kaushik SC (2008) Theoretical analysis of a vapour compression refrigeration system with R502,R404A,R507A.International Journal of Refrigeration 31: [7]. Arora CP, Refrigeration and air conditioning ;Tata McGraw- Hill;2007. [8]. Ballaney PL, Refrigeration and Air conditioning; khanna publishers;2012. [9]. CengelYonus, Boles Michael, thermodynamics an engineering approach 7 th edition Tata McGraw-Hill, [10]. Gong Guangcai, Zeng Wei, Chang Shijun, He Jun, Li kongqing, Scheme selection optimization of cooling and heating sources based on exergy analysis. Applied thermal engineering (2007). [11]. Mani and Selladurai, Experimental analysis of a new refrigerant mixture as drop-in replacement for CFC12 and HFC134a, International Journal of Thermal Sciences 47, pp , ISSN Page 106