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Optimum Performance Analysis of a Hybrid Cascade Refrigeration System Using Alternative Refrigerants
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 28374–28383
www.materialstoday.com/proceedings
ICCMMEMS_2018
Optimum Performance Analysis of a Hybrid Cascade Refrigeration System Using Alternative Refrigerants Naushad Ahmad Ansaria*, Akhilesh Arorab, Samsherc, Manjunath Kd a
Department of Mechanical Engineering,Delhi Technologial University, Shahbad Daulatpur,Main Bawana Road, Delhi-42, India, Department of Mechanical Engineering,Delhi Technologial University, Shahbad Daulatpur,Main Bawana Road, Delhi-42, India, c Department of Mechanical Engineering,Delhi Technologial University, Shahbad Daulatpur,Main Bawana Road, Delhi-42, India, d Department of Allied Engineering,Ch. B. P. Govt. Engineering College, Jaffarpur, Delhi-73, India,
b
Abstract
This research article deals with a Hybrid cascade refrigeration system (HCRS) comprising of vapor absorption refrigeration system (VARS) at the top or high temperature stage and vapor compression refrigeration system (VCRS) at the bottom or low temperature stage in a two-stage cascade refrigeration system using a mathematical model for energy and exergy analysis. The theoretical analysis is carried out taking H2O-LiBr solution in VARS and R134a, R32, R1234yf in VCRS as refrigerants. The three refrigerants (R134a, R32 and R1234yf) in this study are selected because these refrigerants have been recommended as future alternative refrigerants due to their zero ODP and low GWP properties. It has been found that the optimum generator temperature of HCRS is same for all the three eco-friendly refrigerants at which maximum coefficient of performance (COP) and maximum exergetic efficiency occurs. Further it is observed that the performance of the system considered in the study is better with R134a in comparison to R32 and R1234yf as refrigerants in the VCRS and R32 exhibits better result when compared with R1234yf. Keywords: Hybrid Cascade Refrigeration; Generator; Coefficient of Performance; Exergetic Efficiency; Eco-friendly Refrigerants
© 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International Conference on Composite Materials: Manufacturing, Experimental Techniques, Modeling and Simulation (ICCMMEMS-2018).
*Corresponding author, Tel.: +91-9818243128 E-mail address: [email protected]
2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International Conference on Composite Materials: Manufacturing, Experimental Techniques, Modeling and Simulation (ICCMMEMS-2018).
Naushad Ahmad Ansari / Materials Today: Proceedings 5 (2018) 28374–28383
28375
Nomenclature ODP To ηex or ηexer h EP ED
Ozone depletion potential Ambient temperature Exergetic efficiency Enthalpy Exergy product Exergy destruction
GWP COPc Wcomp s Tr E
Global warming potential COP of cascade system Compressor work Entropy Temperature of space to be cooled Exergy
1. Introduction The drawback of the VCRS systems for low temperature applications is their high electricity expenditure. This drawback could be prevailed over by a hybrid cascade refrigeration system comprising of VCRS in low temperature stage and a VARS in high temperature stage. The VARS can be operated by waste heat assumed to be supplied in the generator of absorption cycle as suggested by Cimsit and Ozturk [1] and therefore electricity expenditure can be reduced. A cascade refrigeration system is appropriate for low temperature applications where the temperature ranges from -30°C to -100°C in different industries such as food, pharmaceutical, blast freezing, chemical and liquefaction of gases. For applications where temperature below -40°C is required, a simple vapor compression system or a single stage absorption system is too expensive to achieve the desired result as reported by Chakravarthy et al. [2]. Meng et al. [3] suggested a hybrid system utilising low grade energy from solar energy. The solar energy is used to run the absorption stage saving conventional energy used NH3-H2O in absorption cycle with R134a in vapour compression cycle. Colorado and Velazquez (4) done exergy based thermodynamic analysis of compressionabsorption cascade refrigeration system by using R134a, NH3 and CO2 as refrigerants in VCRS cycle and H2O-LiBr solution in VARS cycle to discover best working substance of the three and also suitable operating parameters. They predicted that highest irreversibility occurred in cascade condenser. Jain et al. [5] performed the multi-objective optimization using NSGA-II technique to evaluate the performance of a 170 kW vapour-absorption cascade cycle and predicted that multi-objective design criteria is superior compared to two single-objective thermodynamic criteria and also suggested the total product cost optimized design. Dixit et al. [6] approved a study on analogous cascade refrigeration system with NH3, CO2, and R-134a in VCRS and H2O-LiBr in VARS and suggested that combination of R-134a with H2O-LiBr is most proper of the three refrigerants based on comparing various parameters of the system. Yingjie Xu et al. [7] analyzed and compared two low grade heat driven compressionabsorption cycles. Their study was based on energy, exergy, environmental and economic analysis. They compared a novel absorption-compression cycle with evaporator sub-cooler and a conventional absorption-compression cycle with an evaporator-condenser. They concluded that novel absorption-compression system was more cost-effective when waste heat was used whereas the conventional absorption-compression system was more economical when solar heat was used. Agnew and Ameli [8] studied the optimization of a two-stage cascade refrigeration system using finite time thermodynamic approach with R717 in high temperature stage and R508b in low temperature stage. Riffat and Shankland [9] investigated energy analysis of different configurations of integrated vapor and absorption refrigeration systems. Fernandez-Seara et al. [10] carried out the first law analysis of similar combined system with ammonia-water in absorption stage and CO2 and NH3 refrigerants in compression stage. Seyfouri and Ameri [11] suggested a configuration of integrated refrigeration system of compression chiller and absorption chiller using a micro turbine to run the compression chiller in low temperature stage and waste heat of micro turbine is used to run the absorption chiller thus saving energy. Ansari et al. [12] carried out the optimum performance analysis on similar system with water-lithium bromide solution in absorption stage and NH3 and CO2 as refrigerants in compression stage while predicting NH3 to be a better refrigerant in low temperature stage. Kaushik et al. [13] examined a similar system and computed the optimum generator temperature as 83.50C and 72.250C for maximum COP and for maximum exergetic efficiency respectively taking NH3 as a refrigerant in the low temperature cycle. Patel et al. [14] in their analysis on HCRS powered by solar-biomass organic Rankine cycle with working fluid Toluene and R245fa suggested that Thermo-economic performance of the system is affected by the type of working fluid used in organic Rankine cycle, electricity and heating costs, biomass type, discount rate, solar collector field
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and location of installation. Chen et al [15] proposed and analyzed a new heat driven absorption compression system to attain a temperature as low as -600C. The proposed system had three sub-systems explicitly a power cycle, an ammonia water absorption cycle and a CO2 vapor compression refrigeration cycle. They suggested that their results were adaptable and suitable for practical applications. On the basis of literature survey, it is evident that scant work is carried out on HCRS using the new alternative refrigerants R32 and R1234yf. Hence, the objective of the present work is to analyse the performance of ecofriendly alternative refrigerants i.e. R32 and R1234yf in a HCRS and also comparing their performance with refrigerant, R134a. 2. Thermodynamic Modelling of a Hybrid Cascade Refrigeration System In the present study a hybrid cascade refrigeration system consisting of two stages has been examined (Refer Fig. 1) in which vapor absorption cycle is considered to be operating at high temperature (i.e. HT stage) and vapor compression cycle is assumed to be operating at low temperature (i.e. LT stage). It is analysed with three different eco-friendly refrigerants namely R134a, R32, R1234yf in the VCRS and H2O-LiBr solution in the VARS stage. 2.1. System Description The HCRS considered for study is schematically shown in Figure 1. It is a two-stage refrigeration system having high temperature stage as vapor absorption cycle and low temperature stage as vapor compression cycle. In figure 1, Qa is heat rejected from absorber to surrounding, Wp is work of pump during process 1-2, ‘she’ is the solution heat exchanger between absorber and generator, Qg is the heat supplied in the generator, Qc is the heat rejected by condenser to the surrounding, STV is the solution thermostatic expansion valve, RTV1 is the refrigerant thermostatic expansion valve of high temperature stage, RTV2 is the refrigerant thermostatic valve of low temperature stage, Wcomp is the compressor work (Comp is compressor) and Qe is the heat absorbed in the evaporator. Qc
Qg 7
Generator 4
3
8 High temperature stage
she WP
Condenser
5 STV Pump 6 1 10 Absorber 2
Qa Low temperature stage
RTV1 9
Cascade Condenser 13
12
RTV2 Comp 14 11
Wcomp
Evaporator Qe
Fig. 1. Schematic diagram of Hybrid cascade refrigeration system 2.2. Thermodynamic Analysis The thermodynamic analysis of HCRS involves the principles of mass conservation, energy conservation and exergy balance. A computational model has been formulated and the system analysis has been carried out using Engineering Equation Solver (EES) software [16].
Naushad Ahmad Ansari / Materials Today: Proceedings 5 (2018) 28374–28383
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2.2.1. Mass Balance .
The mass flow rate of refrigerant through each component of low temperature circuit is m r _ vcr (kg/s). It is calculated using equation (1). .
.
Q e m r _ vcr ( h11 h14 )
(1)
.
Where, Q e is heat absorbed in evaporator (kW) and h is enthalpy (kW/kg) Mass balance equations in high temperature circuit are specified below. 2.2.2. Mass Balance at Absorber or Generator .
.
.
ms mr m w
(2)
.
Where, m s is the mass of solution (kg/s) flowing through absorber and generator. .
m r is the mass flow rate (kg/s) through the condenser and cascade condenser. .
m w is the mass (kg/s) of absorbent flowing through solution heat exchanger. 2.2.3. Energy Balance The energy balance equations for the system shown in Figure 1(a) are given below: Energy balance in high temperature stage .
.
.
.
Q a m r h10 m w h6 m s h1 .
.
.
.
Q g m r h7 m w h4 m s h3 .
(3) (4)
.
Q c m r ( h7 h8 ) .
(5)
.
.
Q she m s (h3 h2 ) m w (h4 h5 ) .
(6)
.
W p m s (h2 h1 )
(7)
2.2.4. Energy Balance in low temperature stage .
.
W comp m r _ vcr (h12 h11 ) .
(8)
.
Q e m r _ vcr ( h11 h14 )
(9)
.
.
Power input W comp W p
(10)
Energy Input .
.
.
.
.
E in Q g Q e W p W comp
(11)
Energy Output .
.
.
E out Q a Q c
(12)
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Naushad Ahmad Ansari / Materials Today: Proceedings 5 (2018) 28374–28383
Coefficient of Performance (COP) .
Qe
COP
.
.
(13)
.
Q g W p W comp 2.2.5. Exergy Balance Exergy destruction in components of the cascade system is furnished below: 2.2.6. Exergy Analysis in high temperature stage .
.
.
.
ED a m r ( h10 To s10 ) m w ( h6 To s6 ) m s ( h1 To s1 )
(14)
. . . . . T ED g m s (h3 To s3 ) m w (h4 To s4 ) mr (h7 To s7 ) Q g 1 0 Tg
(15)
.
.
ED c m r (( h7 h8 ) To ( s7 s8 )) .
(16)
.
.
ED cc m r {( h9 h10 ) To ( s 9 s10 )) m r _ vcr (( h12 h13 ) To ( s12 s13 )} .
.
ED she m s {( h2 h3 ) To ( s 2 s 3 )) m w (( h4 h5 ) To ( s 4 s 5 )} .
(18)
.
ED rtv1 m r To ( s 9 s8 ) .
(17)
.
(19)
.
ED stv m w To ( s6 s5 )
(20)
2.2.6. Exergy Analysis in low temperature stage
. . EDcomp = mr _ vcr T0(s12-s11 )
(21)
. . . T . ED e m r _ vcr (h14 T0 s14 ) Q e 1 0 m r _ vcr (h11 T0 s11 ) Tr . . . . ED rtv 2 = m r _ vcr (h13 -T0 s13 ) - m r _ vcr (h14 -T0 s14 ) m r _ vcr T0(s14 - s13 ) .
.
.
.
.
.
.
.
(23)
.
ED total ED a ED g ED c ED cc ED she ED rtv _ htc ED stv ED rtv _ ltc .
(22)
.
(24)
ED comp ED e 2.2.7. Exergetic Efficiency
ηex
Exergy in product Exergy of fuel
.
EP .
EF
.
.
EF ED total .
EF
.
1
ED total .
EF
(25)
Naushad Ahmad Ansari / Materials Today: Proceedings 5 (2018) 28374–28383
. . . . T EF W comp W p Q g 1 0 Tg . . T EP Q e 1 0 Tr
ηex
(26)
(27)
. T Q e 1 0 Tr
(28)
T W comp W p Q g 1 0 Tg .
.
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.
3. Results and Discussion In the present analysis, using EES software a code has been developed. The properties of water-lithium bromide solution have been referred from Pa´tek and Klomfar [17]. The code has been validated by comparing the results with Cimsit and Ozturk [1]. The values of computed parameters by Cimsit and Ozturk [1] viz. COP, Wcomp and Qg for refrigerant R134a in the low temperature stage were given as 0.590, 8.25kW and 76.45kW respectively whereas the corresponding values with refrigerant R134a in the low temperature stage computed in this study are 0.5793, 8.387kW and 77.7kW respectively. The error falls within the acceptable range. Also, Kaushik et al. [13] predicted the optimum generator temperature to be 83.50C and 72.250C for maximum COP and maximum exergetic efficiency respectively with NH3 in the low temperature stage for the similar system whereas in the present study the optimum generator temperature has been computed as 84.210C and 72.590C for the maximum COP and maximum exergetic efficiency for all the three refrigerants in low temperature stage which is also within the acceptable range of error. The results have been computed and represented through graphs (figures) in the article. The parameters considered for computing the results are given below: .
1.
Cooling capacity ( Q e )
: 100 kW
2. 3.
Isentropic efficiency of compressor, (η_comp) Evaporator temperature, ( Te _ vcr )
: 80 % : -5°C to -45°C
4.
Cascade Condenser temperature, ( Tcc )
: 2-14°C
5.
Generator temperature ( T g )
: 55-115°C
6.
Absorber temperature ( Ta )
: 28-40°C
7.
Effectiveness of solution heat exchanger, (ε_she)
: 0.7
8.
Condenser temperature ( Tc )
: 28-40°C
9.
Approach in cascade condenser (A)
: 0°C
The minimum cascade condenser temperature is assumed to be 2°C because the lowest temperature in water lithium bromide system depends upon the freezing point of water (00C).
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Naushad Ahmad Ansari / Materials Today: Proceedings 5 (2018) 28374–28383
3.1. Results 0.54
0.45 COPc (R134a) COPc (R32) COPc (R1234yf)
hexer(R134a) hexer(R32) hexer(R1234yf)
0.4
0.46
0.35
COPc
hexer
0.5
0.42 60
70
80
90
100
110
Generator Temperature (°C)
0.3 120
Fig. 2. Variation of COP and ηexer versus generator temperature for R134a, R32 and R1234yf (Ta = Tc = 35°C, Te_vcr = -40°C, ε_she = 0.7, η_comp =0.8, A = 0°C, To= 298.15 K)
0.5
0.45
COPc
0.4
0.4
0.35
0.35
0.3
0.3 hexer(R134a) hexer(R32) hexer(R1234yf)
0.25
0.2 0
hexer
0.45
0.5 COP(R134a) COP(R32) COP(R1234yf)
2
4
6
8
10
Cascade condenser temperature(°C)
12
14
0.25
0.2 16
Fig. 3. Effect of cascade condenser temperature on COP and ηexer for R134a, R32 and R1234yf (Ta = Tc = 40°C, Te_vcr = -40°C, Tg = 84.5°C, ε_she = 0.7, η_comp =0.8, A = 0 °C, To = 298.15 K)
Naushad Ahmad Ansari / Materials Today: Proceedings 5 (2018) 28374–28383 0.72
28381 0.4
0.68 0.36
0.6
COPc(R134a) COPc(R32) COPc (R1234yf)
0.32
hexer
COPc
0.64
0.56 0.52
hexer(R1234yf)
0.48 -50
0.28
hexer(R134a) hexer(R32)
-40
-30
-20
Evaporator temperature(°C)
-10
0.24 0
Fig. 4. Effect of evaporator temperature on COP and ηexer for R134a, R32 and R1234yf (Ta = Tc = 35°C, Tg = 84.5°C, ε_she = 0.7, η_comp =0.8, A = 0 °C, To = 298.15 K) 0.58
0.4 COPc(R134a) COPc (R32) COPc (R1234yf)
0.38
0.54
0.36
hexer
COPc
0.56
hexer(R134a) hexer(R32)
0.52
0.5 25
hexer(R1234yf)
30
0.34
35
Condenser temperature(°C)
40
0.32 45
Fig. 5. Effect of condenser temperature on COP and ηexer for R134a, R32 and R1234yf ( Te_vcr = -30°C, Tg = 84.5°C, ε_she = 0.7, η_comp =0.8, A = 0 °C , To = 298.15 K) 3.2. Discussion Figure 2 shows variation of COP and exergetic efficiency corresponding to generator temperature for given set of parameters viz. absorber temperature, condenser temperature, evaporator temperature, approach (A), and cascade condenser temperature. For the refrigerants (R134a, R32 and R1234yf) considered in the study, with increase in generator temperature COP increases becomes maximum at 84.210C and then decreases. Similar trend can be observed for the variation in exergetic efficiency but the maximum exergetic efficiency occurs at a generator temperature of 72.590C which is same for all the three refrigerants. So, there is a trade-off between maximum COP and maximum exergetic efficiency for the optimum generator temperature.
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Naushad Ahmad Ansari / Materials Today: Proceedings 5 (2018) 28374–28383
Figure 3 shows the variation of COP and exergetic efficiency corresponding to cascade condenser temperature. COP of R134a, R32 and R1234yf is found to attain maximum at a cascade condenser temperature of 120C, 120C and 110C respectively and it decreases after attaining the maximum value at higher cascade condenser temperature. Exergetic efficiency of R134a, R32 and R1234yf is observed to reach maximum at a cascade temperature of 70C, 70C and 60C respectively and it decreases thereafter for the given set of parameters. Figure 4 shows that with the decrease in evaporator temperature both the COP and exergetic efficiency decrease for the three refrigerants considered in the study. COP and exergetic of efficiency of R134a is higher compared to R32 and R1234yf whereas R32 gives better performance than R1234yf. Further, it can be observed that the performance of R1234yf cannot be computed below -300C of evaporator temperature for the given condition. Figure 5 shows that with the increase in condenser temperature both the COP and exergetic efficiency of the system decrease significantly for the three refrigerants and the performance order of these refrigerants are R134a, R32 and R1234yf in the descending order respectively. 4. Conclusion The two-stage hybrid cascade refrigeration system having single effect vapor absorption cycle as high temperature stage and vapor compression cycle as low temperature stage has been presented in the research. H2OLiBr solution is the working fluid in the vapor absorption cycle and R134a in the vapor compression cycle. The performance of the HCRS has been evaluated and compared with R32 and R1234yf in VCRS. The results show that the value of optimum generator temperature for maximum COP is same for all the three refrigerants i.e. 84.210C. Similarly, maximum exergetic efficiency occurs at optimum generator temperature of 72.590C for the three refrigerants considered in this study. However, based on performance R134a is best due to higher COP and higher exergetic efficiency among the three refrigerants at the given generator temperature. Further, R32 shows better performance in comparison to R1234yf. It is also observed that COP and exergetic efficiency increase with increase in cascade condenser temperature, become maximum and then start decreasing when cascade condenser temperature is further increased. The difference in COP of R134a with R32 is 0.71% and exergetic efficiency is 1.31% which is not very significant whereas this difference between R32 and R1234yf is found to be 1.2% and 2.23% respectively. The three refrigerants are eco-friendly with zero ODP but GWP of R32 is almost half compared to R134a and atmospheric lifetime of R32 is also lower than R134a. Although, R1234yf scores over both the refrigerants from the environment view point (GWP <4 and lowest atmospheric lifetime of the three) but it cannot be used for very low temperature applications. So, from environment aspect R32 is a better choice over R134a for very low temperature application. References [1] C. Cimsit, I.T. Ozturk, Appl Therm Eng 40 (2012) 311-317. [2] V.S. Chakravarthy, R.K. Shah, G. Venkatarathnam, J. Therm. Sci. Eng. Appl. 3 (2011) 020801-020819. [3] X. Meng, D. Zheng, J. Wang, X. Li, Renewable Energy 57 (2013) 43-50. [4] D. Colorado, V.M. Velazquez, Int. J. Energy Res. 37 (2013) 1851-65. [5] V. Jain, G. Sachdeva, S.S. Kachhwaha, B. Patel, Energy Conversion and Management. 113 (2016) 230–242. [6] M. Dixit, S.C. Kaushik, A. Arora, Journal of Thermal Engineering. 1 (2015) 1-12. [7] Xu. Yingjie, N. Jiang, F. Pan, Q. Wang, Z. Gao, G. Chen, Energy Conversion and Management. 133 (2017) 535347. [8] B. Angel, S.M. Ameli, Applied Thermal Engineering. 24 ((2004) 2557-2565. [9] S.B. Riffat, N. Shankland, Applied Energy. 46 (1993) 303-316. [10] J. Fernandez-Seara, J. Sieres, M. Vazquez, Applied Thermal Engineering. 26 (2006) 502-512. [11] Z. Seyfouri, M. Ameri, International journal of Refrigeration. 35 (2012) 1639-1646. [12] Naushad Ahmad Ansari, Akhilesh Arora, Samsher, K. Manjunath, Journal of Thermal Engineering. (2018) in press.
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[13] S.C. Kaushik, A. Arora, Bilga Parmajit Singh, Alternatives in Refrigeration and Air Conditioning, I.K. International House Publishing,1/e. India, 2016. [14] B. Patel, N.B. Desai, S.S. Kachhwaha, Solar energy. 157 (2017) 920-933. [15] Y. Chen, W. Han, H. Jin, Applied Energy. 185 (2017) 2106-2116. [16] S.A. Klein, F. Alvarado, Engineering Equation Solver Version 7.441, F-Chart software Middleton, WI, 2005. [17] J. Pa,´tek, J. Klomfar, International Journal of Refrigeration. 29 (2006) 566–578.
ScienceDirect Materials Today: Proceedings 5 (2018) 28374–28383
www.materialstoday.com/proceedings
ICCMMEMS_2018
Optimum Performance Analysis of a Hybrid Cascade Refrigeration System Using Alternative Refrigerants Naushad Ahmad Ansaria*, Akhilesh Arorab, Samsherc, Manjunath Kd a
Department of Mechanical Engineering,Delhi Technologial University, Shahbad Daulatpur,Main Bawana Road, Delhi-42, India, Department of Mechanical Engineering,Delhi Technologial University, Shahbad Daulatpur,Main Bawana Road, Delhi-42, India, c Department of Mechanical Engineering,Delhi Technologial University, Shahbad Daulatpur,Main Bawana Road, Delhi-42, India, d Department of Allied Engineering,Ch. B. P. Govt. Engineering College, Jaffarpur, Delhi-73, India,
b
Abstract
This research article deals with a Hybrid cascade refrigeration system (HCRS) comprising of vapor absorption refrigeration system (VARS) at the top or high temperature stage and vapor compression refrigeration system (VCRS) at the bottom or low temperature stage in a two-stage cascade refrigeration system using a mathematical model for energy and exergy analysis. The theoretical analysis is carried out taking H2O-LiBr solution in VARS and R134a, R32, R1234yf in VCRS as refrigerants. The three refrigerants (R134a, R32 and R1234yf) in this study are selected because these refrigerants have been recommended as future alternative refrigerants due to their zero ODP and low GWP properties. It has been found that the optimum generator temperature of HCRS is same for all the three eco-friendly refrigerants at which maximum coefficient of performance (COP) and maximum exergetic efficiency occurs. Further it is observed that the performance of the system considered in the study is better with R134a in comparison to R32 and R1234yf as refrigerants in the VCRS and R32 exhibits better result when compared with R1234yf. Keywords: Hybrid Cascade Refrigeration; Generator; Coefficient of Performance; Exergetic Efficiency; Eco-friendly Refrigerants
© 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International Conference on Composite Materials: Manufacturing, Experimental Techniques, Modeling and Simulation (ICCMMEMS-2018).
*Corresponding author, Tel.: +91-9818243128 E-mail address: [email protected]
2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International Conference on Composite Materials: Manufacturing, Experimental Techniques, Modeling and Simulation (ICCMMEMS-2018).
Naushad Ahmad Ansari / Materials Today: Proceedings 5 (2018) 28374–28383
28375
Nomenclature ODP To ηex or ηexer h EP ED
Ozone depletion potential Ambient temperature Exergetic efficiency Enthalpy Exergy product Exergy destruction
GWP COPc Wcomp s Tr E
Global warming potential COP of cascade system Compressor work Entropy Temperature of space to be cooled Exergy
1. Introduction The drawback of the VCRS systems for low temperature applications is their high electricity expenditure. This drawback could be prevailed over by a hybrid cascade refrigeration system comprising of VCRS in low temperature stage and a VARS in high temperature stage. The VARS can be operated by waste heat assumed to be supplied in the generator of absorption cycle as suggested by Cimsit and Ozturk [1] and therefore electricity expenditure can be reduced. A cascade refrigeration system is appropriate for low temperature applications where the temperature ranges from -30°C to -100°C in different industries such as food, pharmaceutical, blast freezing, chemical and liquefaction of gases. For applications where temperature below -40°C is required, a simple vapor compression system or a single stage absorption system is too expensive to achieve the desired result as reported by Chakravarthy et al. [2]. Meng et al. [3] suggested a hybrid system utilising low grade energy from solar energy. The solar energy is used to run the absorption stage saving conventional energy used NH3-H2O in absorption cycle with R134a in vapour compression cycle. Colorado and Velazquez (4) done exergy based thermodynamic analysis of compressionabsorption cascade refrigeration system by using R134a, NH3 and CO2 as refrigerants in VCRS cycle and H2O-LiBr solution in VARS cycle to discover best working substance of the three and also suitable operating parameters. They predicted that highest irreversibility occurred in cascade condenser. Jain et al. [5] performed the multi-objective optimization using NSGA-II technique to evaluate the performance of a 170 kW vapour-absorption cascade cycle and predicted that multi-objective design criteria is superior compared to two single-objective thermodynamic criteria and also suggested the total product cost optimized design. Dixit et al. [6] approved a study on analogous cascade refrigeration system with NH3, CO2, and R-134a in VCRS and H2O-LiBr in VARS and suggested that combination of R-134a with H2O-LiBr is most proper of the three refrigerants based on comparing various parameters of the system. Yingjie Xu et al. [7] analyzed and compared two low grade heat driven compressionabsorption cycles. Their study was based on energy, exergy, environmental and economic analysis. They compared a novel absorption-compression cycle with evaporator sub-cooler and a conventional absorption-compression cycle with an evaporator-condenser. They concluded that novel absorption-compression system was more cost-effective when waste heat was used whereas the conventional absorption-compression system was more economical when solar heat was used. Agnew and Ameli [8] studied the optimization of a two-stage cascade refrigeration system using finite time thermodynamic approach with R717 in high temperature stage and R508b in low temperature stage. Riffat and Shankland [9] investigated energy analysis of different configurations of integrated vapor and absorption refrigeration systems. Fernandez-Seara et al. [10] carried out the first law analysis of similar combined system with ammonia-water in absorption stage and CO2 and NH3 refrigerants in compression stage. Seyfouri and Ameri [11] suggested a configuration of integrated refrigeration system of compression chiller and absorption chiller using a micro turbine to run the compression chiller in low temperature stage and waste heat of micro turbine is used to run the absorption chiller thus saving energy. Ansari et al. [12] carried out the optimum performance analysis on similar system with water-lithium bromide solution in absorption stage and NH3 and CO2 as refrigerants in compression stage while predicting NH3 to be a better refrigerant in low temperature stage. Kaushik et al. [13] examined a similar system and computed the optimum generator temperature as 83.50C and 72.250C for maximum COP and for maximum exergetic efficiency respectively taking NH3 as a refrigerant in the low temperature cycle. Patel et al. [14] in their analysis on HCRS powered by solar-biomass organic Rankine cycle with working fluid Toluene and R245fa suggested that Thermo-economic performance of the system is affected by the type of working fluid used in organic Rankine cycle, electricity and heating costs, biomass type, discount rate, solar collector field
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and location of installation. Chen et al [15] proposed and analyzed a new heat driven absorption compression system to attain a temperature as low as -600C. The proposed system had three sub-systems explicitly a power cycle, an ammonia water absorption cycle and a CO2 vapor compression refrigeration cycle. They suggested that their results were adaptable and suitable for practical applications. On the basis of literature survey, it is evident that scant work is carried out on HCRS using the new alternative refrigerants R32 and R1234yf. Hence, the objective of the present work is to analyse the performance of ecofriendly alternative refrigerants i.e. R32 and R1234yf in a HCRS and also comparing their performance with refrigerant, R134a. 2. Thermodynamic Modelling of a Hybrid Cascade Refrigeration System In the present study a hybrid cascade refrigeration system consisting of two stages has been examined (Refer Fig. 1) in which vapor absorption cycle is considered to be operating at high temperature (i.e. HT stage) and vapor compression cycle is assumed to be operating at low temperature (i.e. LT stage). It is analysed with three different eco-friendly refrigerants namely R134a, R32, R1234yf in the VCRS and H2O-LiBr solution in the VARS stage. 2.1. System Description The HCRS considered for study is schematically shown in Figure 1. It is a two-stage refrigeration system having high temperature stage as vapor absorption cycle and low temperature stage as vapor compression cycle. In figure 1, Qa is heat rejected from absorber to surrounding, Wp is work of pump during process 1-2, ‘she’ is the solution heat exchanger between absorber and generator, Qg is the heat supplied in the generator, Qc is the heat rejected by condenser to the surrounding, STV is the solution thermostatic expansion valve, RTV1 is the refrigerant thermostatic expansion valve of high temperature stage, RTV2 is the refrigerant thermostatic valve of low temperature stage, Wcomp is the compressor work (Comp is compressor) and Qe is the heat absorbed in the evaporator. Qc
Qg 7
Generator 4
3
8 High temperature stage
she WP
Condenser
5 STV Pump 6 1 10 Absorber 2
Qa Low temperature stage
RTV1 9
Cascade Condenser 13
12
RTV2 Comp 14 11
Wcomp
Evaporator Qe
Fig. 1. Schematic diagram of Hybrid cascade refrigeration system 2.2. Thermodynamic Analysis The thermodynamic analysis of HCRS involves the principles of mass conservation, energy conservation and exergy balance. A computational model has been formulated and the system analysis has been carried out using Engineering Equation Solver (EES) software [16].
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2.2.1. Mass Balance .
The mass flow rate of refrigerant through each component of low temperature circuit is m r _ vcr (kg/s). It is calculated using equation (1). .
.
Q e m r _ vcr ( h11 h14 )
(1)
.
Where, Q e is heat absorbed in evaporator (kW) and h is enthalpy (kW/kg) Mass balance equations in high temperature circuit are specified below. 2.2.2. Mass Balance at Absorber or Generator .
.
.
ms mr m w
(2)
.
Where, m s is the mass of solution (kg/s) flowing through absorber and generator. .
m r is the mass flow rate (kg/s) through the condenser and cascade condenser. .
m w is the mass (kg/s) of absorbent flowing through solution heat exchanger. 2.2.3. Energy Balance The energy balance equations for the system shown in Figure 1(a) are given below: Energy balance in high temperature stage .
.
.
.
Q a m r h10 m w h6 m s h1 .
.
.
.
Q g m r h7 m w h4 m s h3 .
(3) (4)
.
Q c m r ( h7 h8 ) .
(5)
.
.
Q she m s (h3 h2 ) m w (h4 h5 ) .
(6)
.
W p m s (h2 h1 )
(7)
2.2.4. Energy Balance in low temperature stage .
.
W comp m r _ vcr (h12 h11 ) .
(8)
.
Q e m r _ vcr ( h11 h14 )
(9)
.
.
Power input W comp W p
(10)
Energy Input .
.
.
.
.
E in Q g Q e W p W comp
(11)
Energy Output .
.
.
E out Q a Q c
(12)
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Coefficient of Performance (COP) .
Qe
COP
.
.
(13)
.
Q g W p W comp 2.2.5. Exergy Balance Exergy destruction in components of the cascade system is furnished below: 2.2.6. Exergy Analysis in high temperature stage .
.
.
.
ED a m r ( h10 To s10 ) m w ( h6 To s6 ) m s ( h1 To s1 )
(14)
. . . . . T ED g m s (h3 To s3 ) m w (h4 To s4 ) mr (h7 To s7 ) Q g 1 0 Tg
(15)
.
.
ED c m r (( h7 h8 ) To ( s7 s8 )) .
(16)
.
.
ED cc m r {( h9 h10 ) To ( s 9 s10 )) m r _ vcr (( h12 h13 ) To ( s12 s13 )} .
.
ED she m s {( h2 h3 ) To ( s 2 s 3 )) m w (( h4 h5 ) To ( s 4 s 5 )} .
(18)
.
ED rtv1 m r To ( s 9 s8 ) .
(17)
.
(19)
.
ED stv m w To ( s6 s5 )
(20)
2.2.6. Exergy Analysis in low temperature stage
. . EDcomp = mr _ vcr T0(s12-s11 )
(21)
. . . T . ED e m r _ vcr (h14 T0 s14 ) Q e 1 0 m r _ vcr (h11 T0 s11 ) Tr . . . . ED rtv 2 = m r _ vcr (h13 -T0 s13 ) - m r _ vcr (h14 -T0 s14 ) m r _ vcr T0(s14 - s13 ) .
.
.
.
.
.
.
.
(23)
.
ED total ED a ED g ED c ED cc ED she ED rtv _ htc ED stv ED rtv _ ltc .
(22)
.
(24)
ED comp ED e 2.2.7. Exergetic Efficiency
ηex
Exergy in product Exergy of fuel
.
EP .
EF
.
.
EF ED total .
EF
.
1
ED total .
EF
(25)
Naushad Ahmad Ansari / Materials Today: Proceedings 5 (2018) 28374–28383
. . . . T EF W comp W p Q g 1 0 Tg . . T EP Q e 1 0 Tr
ηex
(26)
(27)
. T Q e 1 0 Tr
(28)
T W comp W p Q g 1 0 Tg .
.
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.
3. Results and Discussion In the present analysis, using EES software a code has been developed. The properties of water-lithium bromide solution have been referred from Pa´tek and Klomfar [17]. The code has been validated by comparing the results with Cimsit and Ozturk [1]. The values of computed parameters by Cimsit and Ozturk [1] viz. COP, Wcomp and Qg for refrigerant R134a in the low temperature stage were given as 0.590, 8.25kW and 76.45kW respectively whereas the corresponding values with refrigerant R134a in the low temperature stage computed in this study are 0.5793, 8.387kW and 77.7kW respectively. The error falls within the acceptable range. Also, Kaushik et al. [13] predicted the optimum generator temperature to be 83.50C and 72.250C for maximum COP and maximum exergetic efficiency respectively with NH3 in the low temperature stage for the similar system whereas in the present study the optimum generator temperature has been computed as 84.210C and 72.590C for the maximum COP and maximum exergetic efficiency for all the three refrigerants in low temperature stage which is also within the acceptable range of error. The results have been computed and represented through graphs (figures) in the article. The parameters considered for computing the results are given below: .
1.
Cooling capacity ( Q e )
: 100 kW
2. 3.
Isentropic efficiency of compressor, (η_comp) Evaporator temperature, ( Te _ vcr )
: 80 % : -5°C to -45°C
4.
Cascade Condenser temperature, ( Tcc )
: 2-14°C
5.
Generator temperature ( T g )
: 55-115°C
6.
Absorber temperature ( Ta )
: 28-40°C
7.
Effectiveness of solution heat exchanger, (ε_she)
: 0.7
8.
Condenser temperature ( Tc )
: 28-40°C
9.
Approach in cascade condenser (A)
: 0°C
The minimum cascade condenser temperature is assumed to be 2°C because the lowest temperature in water lithium bromide system depends upon the freezing point of water (00C).
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3.1. Results 0.54
0.45 COPc (R134a) COPc (R32) COPc (R1234yf)
hexer(R134a) hexer(R32) hexer(R1234yf)
0.4
0.46
0.35
COPc
hexer
0.5
0.42 60
70
80
90
100
110
Generator Temperature (°C)
0.3 120
Fig. 2. Variation of COP and ηexer versus generator temperature for R134a, R32 and R1234yf (Ta = Tc = 35°C, Te_vcr = -40°C, ε_she = 0.7, η_comp =0.8, A = 0°C, To= 298.15 K)
0.5
0.45
COPc
0.4
0.4
0.35
0.35
0.3
0.3 hexer(R134a) hexer(R32) hexer(R1234yf)
0.25
0.2 0
hexer
0.45
0.5 COP(R134a) COP(R32) COP(R1234yf)
2
4
6
8
10
Cascade condenser temperature(°C)
12
14
0.25
0.2 16
Fig. 3. Effect of cascade condenser temperature on COP and ηexer for R134a, R32 and R1234yf (Ta = Tc = 40°C, Te_vcr = -40°C, Tg = 84.5°C, ε_she = 0.7, η_comp =0.8, A = 0 °C, To = 298.15 K)
Naushad Ahmad Ansari / Materials Today: Proceedings 5 (2018) 28374–28383 0.72
28381 0.4
0.68 0.36
0.6
COPc(R134a) COPc(R32) COPc (R1234yf)
0.32
hexer
COPc
0.64
0.56 0.52
hexer(R1234yf)
0.48 -50
0.28
hexer(R134a) hexer(R32)
-40
-30
-20
Evaporator temperature(°C)
-10
0.24 0
Fig. 4. Effect of evaporator temperature on COP and ηexer for R134a, R32 and R1234yf (Ta = Tc = 35°C, Tg = 84.5°C, ε_she = 0.7, η_comp =0.8, A = 0 °C, To = 298.15 K) 0.58
0.4 COPc(R134a) COPc (R32) COPc (R1234yf)
0.38
0.54
0.36
hexer
COPc
0.56
hexer(R134a) hexer(R32)
0.52
0.5 25
hexer(R1234yf)
30
0.34
35
Condenser temperature(°C)
40
0.32 45
Fig. 5. Effect of condenser temperature on COP and ηexer for R134a, R32 and R1234yf ( Te_vcr = -30°C, Tg = 84.5°C, ε_she = 0.7, η_comp =0.8, A = 0 °C , To = 298.15 K) 3.2. Discussion Figure 2 shows variation of COP and exergetic efficiency corresponding to generator temperature for given set of parameters viz. absorber temperature, condenser temperature, evaporator temperature, approach (A), and cascade condenser temperature. For the refrigerants (R134a, R32 and R1234yf) considered in the study, with increase in generator temperature COP increases becomes maximum at 84.210C and then decreases. Similar trend can be observed for the variation in exergetic efficiency but the maximum exergetic efficiency occurs at a generator temperature of 72.590C which is same for all the three refrigerants. So, there is a trade-off between maximum COP and maximum exergetic efficiency for the optimum generator temperature.
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Figure 3 shows the variation of COP and exergetic efficiency corresponding to cascade condenser temperature. COP of R134a, R32 and R1234yf is found to attain maximum at a cascade condenser temperature of 120C, 120C and 110C respectively and it decreases after attaining the maximum value at higher cascade condenser temperature. Exergetic efficiency of R134a, R32 and R1234yf is observed to reach maximum at a cascade temperature of 70C, 70C and 60C respectively and it decreases thereafter for the given set of parameters. Figure 4 shows that with the decrease in evaporator temperature both the COP and exergetic efficiency decrease for the three refrigerants considered in the study. COP and exergetic of efficiency of R134a is higher compared to R32 and R1234yf whereas R32 gives better performance than R1234yf. Further, it can be observed that the performance of R1234yf cannot be computed below -300C of evaporator temperature for the given condition. Figure 5 shows that with the increase in condenser temperature both the COP and exergetic efficiency of the system decrease significantly for the three refrigerants and the performance order of these refrigerants are R134a, R32 and R1234yf in the descending order respectively. 4. Conclusion The two-stage hybrid cascade refrigeration system having single effect vapor absorption cycle as high temperature stage and vapor compression cycle as low temperature stage has been presented in the research. H2OLiBr solution is the working fluid in the vapor absorption cycle and R134a in the vapor compression cycle. The performance of the HCRS has been evaluated and compared with R32 and R1234yf in VCRS. The results show that the value of optimum generator temperature for maximum COP is same for all the three refrigerants i.e. 84.210C. Similarly, maximum exergetic efficiency occurs at optimum generator temperature of 72.590C for the three refrigerants considered in this study. However, based on performance R134a is best due to higher COP and higher exergetic efficiency among the three refrigerants at the given generator temperature. Further, R32 shows better performance in comparison to R1234yf. It is also observed that COP and exergetic efficiency increase with increase in cascade condenser temperature, become maximum and then start decreasing when cascade condenser temperature is further increased. The difference in COP of R134a with R32 is 0.71% and exergetic efficiency is 1.31% which is not very significant whereas this difference between R32 and R1234yf is found to be 1.2% and 2.23% respectively. The three refrigerants are eco-friendly with zero ODP but GWP of R32 is almost half compared to R134a and atmospheric lifetime of R32 is also lower than R134a. Although, R1234yf scores over both the refrigerants from the environment view point (GWP <4 and lowest atmospheric lifetime of the three) but it cannot be used for very low temperature applications. So, from environment aspect R32 is a better choice over R134a for very low temperature application. References [1] C. Cimsit, I.T. Ozturk, Appl Therm Eng 40 (2012) 311-317. [2] V.S. Chakravarthy, R.K. Shah, G. Venkatarathnam, J. Therm. Sci. Eng. Appl. 3 (2011) 020801-020819. [3] X. Meng, D. Zheng, J. Wang, X. Li, Renewable Energy 57 (2013) 43-50. [4] D. Colorado, V.M. Velazquez, Int. J. Energy Res. 37 (2013) 1851-65. [5] V. Jain, G. Sachdeva, S.S. Kachhwaha, B. Patel, Energy Conversion and Management. 113 (2016) 230–242. [6] M. Dixit, S.C. Kaushik, A. Arora, Journal of Thermal Engineering. 1 (2015) 1-12. [7] Xu. Yingjie, N. Jiang, F. Pan, Q. Wang, Z. Gao, G. Chen, Energy Conversion and Management. 133 (2017) 535347. [8] B. Angel, S.M. Ameli, Applied Thermal Engineering. 24 ((2004) 2557-2565. [9] S.B. Riffat, N. Shankland, Applied Energy. 46 (1993) 303-316. [10] J. Fernandez-Seara, J. Sieres, M. Vazquez, Applied Thermal Engineering. 26 (2006) 502-512. [11] Z. Seyfouri, M. Ameri, International journal of Refrigeration. 35 (2012) 1639-1646. [12] Naushad Ahmad Ansari, Akhilesh Arora, Samsher, K. Manjunath, Journal of Thermal Engineering. (2018) in press.
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[13] S.C. Kaushik, A. Arora, Bilga Parmajit Singh, Alternatives in Refrigeration and Air Conditioning, I.K. International House Publishing,1/e. India, 2016. [14] B. Patel, N.B. Desai, S.S. Kachhwaha, Solar energy. 157 (2017) 920-933. [15] Y. Chen, W. Han, H. Jin, Applied Energy. 185 (2017) 2106-2116. [16] S.A. Klein, F. Alvarado, Engineering Equation Solver Version 7.441, F-Chart software Middleton, WI, 2005. [17] J. Pa,´tek, J. Klomfar, International Journal of Refrigeration. 29 (2006) 566–578.