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Exergy Based Analysis of LiCl-H2O Absorption Cooling System
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 109 (2017) 261 – 269
International Conference on Recent Advancement in Air Conditioning and Refrigeration, RAAR 2016, 10-12 November 2016, Bhubaneswar, India
Exergy Based Analysis of LiCl-H2O Absorption Cooling System Jatin Patela * ,Bhargav Pandyab , Anurag Mudgalc 0F0F
a,b,c
Department of Mechanical Engineering, School of Technology, Pandit Deendayal Petroleum University, Gandhinagar, Gujarat 382007, India
Abstract
A thermodynamic analysis of single effect LiCl-H2O vapour absorption cooling system of 1 TR capacity is conducted based on first and second laws. Mathematical models derived from thermodynamics theory, are employed in engineering equation solver to perform the calculations. It is found that maximum exergy destruction in system occurs in the absorber and generator while the pump and the expansion valve have the lowest. A performance comparison between LiCl-H2O and LiBr-H2O absorption system is also evaluated under identical operating conditions. It is found that LiCl-H2O working pair performs thermodynamically better compared to LiBr-H2O in vapour absorption cooling system. 2016The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license ©©2017 Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review by the scientific conference committee of RAAR-2016. Peer-review under responsibility of the organizing committee of RAAR 2016. Keywords: VARS, LiCl-H2O, LiBr-H2O, Exergy, Exergetic efficiency
1. Introduction Absorption system is becoming more popular nowadays because it is heat driven system instead of conventional compressing chillers which are work driven. Vapour absorption refrigeration system (VARS) is the optimum alternative for vapour compression chiller. VARS can also utilize non-conventional energy i.e. solar energy, biomass, geothermal energy etc. VARS performance is affected by the type of absorbent-refrigerant pair that has been chosen. There are limited pairs available for VARS, in which generally used pairs are LiBr-H2O and NH3-H2O [1]. NH3-H2O
* Corresponding author. Tel.: +919428415422; fax: +917923275030. E-mail address: [email protected]
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of RAAR 2016. doi:10.1016/j.egypro.2017.03.061
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Jatin Patel et al. / Energy Procedia 109 (2017) 261 – 269
absorption system is more complicated as it requires separate rectifier mechanism to remove water vapour from refrigerant vapour, whereas main problem occurs in LiBr-H2O system is the crystallization [1]. Current research work focuses the potential refrigerant-absorbent pair for VARS. LiCl-H2O pair is one of the good options for VARS working pair as it has advantages of triple state point, long term stability and cost, compared to LiBr-H2O [2]. There is a study available for thermodynamic design data for double effect LiCl-H2O absorption system and comparison for the performance of LiCl-H2O and LiBr-H2O absorption system and it is found that COP with LiCl-H2O is better than that of LiBr-H2O system [3]. Feasible range for each operating parameters and possible combination of operating temperature for LiCl-H2O system has been also evaluated [4]. Experimental investigation of intermittent LiCl-H2O absorption cycle has been performed and it is found that that LiCl-H2O system can operate with low temperature heat source [5]. There has been recently developed novel absorption system with LiCl-H2O pair in the high pressure cycle and LiBr-H2O pair in the low pressure cycle [6]. In this study three different heat source utilization mode has been considered i.e. two parallel modes and one series mode and it is concluded that intermediate pressure has a large influence upon system performance. Many literatures are available for second law analysis for LiBr-H2O absorption system [7-11] and NH3-H2O [12-13] VARS. Very few literatures are available on second law analysis of LiCl-H2O absorption system [14-15]. Crystallization is the major issue in any salt based aqueous solution, so to prevent this phenomenon; LiCl-H2O pair operator has to choose operating parameters in optimistic range. The detailed exergy analysis is carried out in the present work and the optimized operating parameters are suggested for LiCL-H2O VARS. Nomenclature COP e
coefficient of performance Specific exergy (kJ/kg) specific enthalpy (kJ/kg)
E G
evaporator Generator inlet stream
mass flow rate (kg/s)
in out
Q
heat load (kW)
o
dead state (ambient)
s T
specific entropy (kJ/kg K) temperature (K)
P
r
pump refrigerant
W
pump work (kW)
REXP
refrigerant expansion valve
efficiency
SEXP
solution expansion valve
SHX ss sys ws
solution heat exchanger
h x
m x
x
K
x
exergy destruction (kW) '\ Subscripts Absorber A C Condenser
outlet stream
strong solution system weak solution
2. System Description The schematic diagram of the LiCl-Water vapour absorption cooling system is shown in Fig. 1. Main components of the system are generator, evaporator, condenser, absorber, solution pump, solution heat exchanger and expansion valve. It uses water as refrigerant and LiCl solution as absorbent. As shown in Fig. 1, at absorber outlet (1), the solution is rich in refrigerant and pump forces the liquid through a solution heat exchanger to the generator (3). The temperature of the solution in the heat exchanger increases. In the generator thermal energy is added and refrigerant escape from the solution. The refrigerant vapour (7) flows to the condenser, where the refrigerant gets condensed. The condensed refrigerant (8) flows through expansion valve to the evaporator (9). In the evaporator, the heat from the load evaporates the refrigerant, which flows back to the absorber (10). At the generator exit (4), the steam consists of absorbentrefrigerant solution, which is cooled in the heat exchanger. From points (6) to (1), the solution absorbs refrigerant vapour from the evaporator and rejects heat through a heat exchanger.
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Fig. 1. Schematic of lithium chloride-water absorption cooling cycle
3. Thermodynamic Formulation The COP of the absorption system is defined as the heat load in the evaporator per heat load in the generator and work required for solution pump and it can be written as: x
COP
QE x
(1)
x
QG W P
Exergy analysis is combination of first and second law of thermodynamics and it indicates the maximum work potential of the system with respect to its surroundings [16]. Specific exergy of pure substance is given by [16]
h ho To s so
e
(2)
Rate of exergy destruction in any component undergoing a steady flow process can be given by [16] x
'\ destyoyed
x x ª x§ T ¦ m in ein ¦ m out eout «¦ Q¨ 1 0 T © ¬
x ·º ¸» r W P ¹¼
(3)
Owing to the fact that exergy balances the dead state values get cancelled [17] therefore, with reference of Eq.3 exergy destruction in each component can be written as: x
'\ G
x
x
x
x
m ws h3 To s 3 m ss h4 To s 4 m r h7 To s7 m G h11 To s 11 h12 To s 12
x
m r h10 To s10 m ss h6 To s6 m ws h1 To s1 m A h13 To s13 h14 To s14
x
m r h9 To s 9 m r h10 To s10 m E h17 To s17 h18 To s18
'\ A
'\ E
x
x
x
x
x
x
(4) (5)
x
(6)
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Jatin Patel et al. / Energy Procedia 109 (2017) 261 – 269 x
x
x
x
x
(7)
x
m ws h2 h3 To s 2 s 3 m ws h4 h5 To s 4 s 5
'\ SHX x
x
m r h7 To s7 m r h8 To s8 m C h15 To s15 h16 To s16
'\ C
x
(8)
x
'\ P
m ws h1 h2 To s1 s2 WP
x
mr h8 h9 To s8 s9
(9)
x
'\ REXP x
(10)
x
m ss h5 h6 To s5 s6
'\ SEXP
(11)
The total rate of exergy destruction of absorption system is the sum of exergy destruction in each component and can be written as: x
'\ sys
x
x
x
x
x
x
x
x
'\ G '\ A '\ E '\ C '\ SHX '\ REXP '\ SEXP '\ P
(12)
For this system exergetic efficiency can be determined as:
K exergetic
x ª T º Q E «1 o » T E ¼ ¬ x ª To º x Q G «1 » W ¬ TG ¼
(13) p
4. Model Validation In order to validate the second law based present model of LiCl-H2O, the results from this model have been compared with simulation data available from recent literature of Gogoi and Konwar [15]. From this comparison it is observed that agreement between two model is satisfactory. Data of comparison has been presented in Table 1. Table 1. Comparison of present model with model of Gogoi and Konwar [15] Total exergy destruction (kW)
Temperature (°C)
TE TE TE TE TE
4, 5, 6, 7, 4,
TC
34 , T A
TC
28 , T A
TC
41 , T A
TC
39 , T A
TC
37 , T A
31 , TG 73 31 , TG 66 31 , TG 79 36 , TG 77 32 , TG 76
Error (%)
Gogoi and Konwar [15]
Present study
40.634
39.75
-2.17
36.60
35.99
-1.66
41.352
40.99
-0.87
37.123
37.9
2.09
39.5
39.99
1.22
5. Simulation Model In this study, the software used for modelling is engineering equation solver (EES) [18]. To simplify the modelling of system, several assumptions are made as follows: (1) the system is simulated under steady state conditions, (2) pressure drop along the fluid flow and heat losses/gains are negligible, (3) refrigerant condensed in the condenser to
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a saturated liquid while in evaporator the refrigerant evaporates to a saturated vapour. The initial conditions considered for 1 TR cooling capacity are generator temperature as 78 °C, absorber temperature as 40 °C, condenser temperature as 36 °C, evaporator temperature as 10 °C and effectiveness of solution heat exchanger as 0.7. VARS thermodynamic data and performance parameters are listed in Table 2 and 3 respectively. Based on these conditions, COP of system is calculated as 0.7877 and exergetic efficiency is obtained as 27.65 %. It is observed from Table 3 that under given operating condition, most of the exergy destruction occurred in absorber and generator. Maximum exergy destruction takes place in absorber followed by generator, evaporator, condenser, solution heat exchanger, and refrigerant expansion valve. It is also found that exergy destruction in pump and expansion valve are very small and usually ignored. Table 2. Absorption system data obtained from the thermodynamic analysis State Temperature Concentration Mass flow rate (°C) (% of LiCl) point (kg/s) 1 40 42.33 0.01164 2 40 42.33 0.01164 3 61.39 42.33 0.01164 4 78 48.52 0.01016 5 51.4 48.52 0.01016 6 51.4 48.52 0.01016 7 78 0.00148 8 36 0.00148 9 36 0.00148 10 10 0.00148 11 92 0.1181 12 83 0.1181 13 25 0.1276 14 33 0.1276 15 33 0.4425 16 35 0.4425 17 25 0.1051 18 17 0.1051 Table 3. Performance parameters of system Component Energy Flow (kW) Generator Absorber Condenser Evaporator Solution heat exchanger Pump Refrigerant expansion valve Solution expansion valve Total
4.465 4.268 3.705 3.517 0.6492 0.00004579 16.60
Enthalpy (kJ/kg) 177.1 177.1 232.9 319.8 255 255 2646 150.8 150.8 2519 385.4 347.6 104.8 138.3 138.3 146.7 104.8 71.38
Entropy (kJ/kg K) 0.3483 0.3483 0.5206 0.6135 0.4218 0.4218 8.753 0.5185 0.5352 8.899 1.216 1.11 0.367 0.477 0.477 0.505 0.367 0.2532
Exergy destruction (kW)
Non dimensional exergy loss (%)
0.1558 0.2037 0.03055 0.1379 0.02567 0.0000457 0.007365 0.5612
27.77 36.31 5.44 24.58 4.575 0.0081 1.312 100
6. Result and Discussion Variation of exergy destruction in whole system and non-dimensional exergy destruction in various components with operating temperature is shown in Fig. 2 to 4.
0.6
40
0.5 30 0.4 20 0.3 10
0 72
0.2
73
74
75
76
77
78
79
0.1 80
50
0.6
45 0.5
40 35
0.4
30 25
0.3
20 0.2
15 10
0.1
5 0 30
32
Generator temperature (°C) Absorber
SHX
Condensor
Evaporator
34
36
0 40
38
Absorber temperature (°C) Generator system
Fig. 2. Variation of non-dimensional exergy destruction and total exergy destruction in various component with generator temperature
0.7 40 0.6 0.5
30
0.4 20
0.3 0.2
10
SHX
Condensor
Evaporator
Generator system
0.35
0.8
Exergetic efficiency (%)
50
Absorber
Fig. 3. Variation of non-dimensional exergy destruction and total exergy destruction in various component with absorber temperature
Total exergy destruction (kW)
Non dimensional exergy destruction (%)
Total exergy destruction (kW)
0.7
Total exergy destruction (kW)
50
Non dimensional exergy destruction (%)
Jatin Patel et al. / Energy Procedia 109 (2017) 261 – 269
Non dimensional exergy destruction (%)
266
0.3
0.25
0.1 0 5
7
9
11
13
0 15
0.2 0
10
Evaporator temperature (°C) Absorber
SHX
Condensor
Evaporator
20
30
40
50
60
70
80
90
Temperature (°C) Generator system
Fig. 4. Variation of non-dimensional exergy destruction and total exergy destruction in various component with evaporator temperature
Absorber
Condensor
Evaporator
Generator
Fig. 5. Variation of exergetic efficiency of system with operating temperature
Fig. 4 illustrate the variation of non-dimensional exergy destruction and total exergy destruction in various components with evaporator temperature. It is found that non dimensional exergy destruction in evaporator, SHX and total exergy destruction of system decreases with evaporator temperature under given operating condition. Non dimensional exergy destruction in generator, absorber and condenser increases with evaporator temperature. From Fig. 5 it is found that exergetic efficiency of the system first increases and then start decreasing with evaporator and generator temperature while exergetic efficiency decreases with absorber and condenser temperature. 7. Performance Comparison Between LiCl-H2O and LiBr-H2O Absorption System In this study detailed comparison between LiCl-H2O and LiBr-H2O VARS has been evaluated. Comparison of these two system has provided in terms of thermodynamic first and second law of i.e. heat load, exergy destruction,
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COP and exergetic efficiency. Various performance parameters comparison of LiC-H2O and LiBr-H2O VARS is illustrated in Table 4. Table 4. Various performance parameters comparison of LiCl-H2O and LiBr-H2O VARS LiCl-H2O VARS x Temperature (°C) COP '\ sys ( kW ) Kexergetic
TE
8 , TG
TE
10 , TG
TE
7 , TG
68 , TC
30 , T A
TE
8 , TG
85 , TC
42 , T A
75 , TC
90 , TC
35 , T A
45 , T A
40
LiBr-H2O VARS x
COP
'\ sys ( kW ) Kexergetic
0.74
0.7182
0.313
0.70
0.7646
0.295
0.76
0.6967
0.22
0.74
0.7196
0.21
32
0.81
0.645
0.42
0.80
0.6527
0.41
40
0.73
0.7243
0.2664
0.70
0.7625
0.254
40
Table 5. Energetic and Exergetic performance comparison of LiC-H2O and LiBr-H2O VARS LiBr-H2O VARS LiCl-H2O VARS Component
Energy Flow (kW)
Exergy destruction (kW)
Energy Flow (kW)
Exergy destruction (kW)
Generator Absorber Evaporator Condenser SHX Pump REXP Total
4.64 4.472 3.517 3.712 1.217 0.00008169 17.55
0.2119 0.1946 0.1318 0.1144 0.04056 0.00008169 0.008534 0.7019
4.513 4.309 3.517 3.712 0.7121 0.00005433 16.76
0.2185 0.1779 0.1318 0.1144 0.02939 0.00005433 0.008534 0.6806
Energetic and exergetic comparison for this two VARS is shown in Table 5, for which the operating parameters are taken as generator temperature as 80 °C, evaporator temperature as 10 °C, absorber temperature as 40 °C and condenser temperature as 38 °C. From Table.4, it is found that at various operating condition COP and exergetic efficiency of LiCl-H2O VARS is better compare to that of LiBr-H2O VARS. Therefore, it is obvious that exergy destruction of LiCl-H2O VARS is lower compared to that of LiBr-H2O VARS. It is found that total heat load and exergy destruction of LiCl-H2O VARS is lower compare to LiBr-H2O VARS. It is observed that exergy destruction in generator of LiCl-H2O VARS is 3% higher compare to that of the LiBr-H2O VARS while exergy destruction in absorber of LiCl-H2O VARS is 13% and exergy destruction in SHX is 4 % lower compared to that of the LiBr-H2O VARS. This arises mainly due to improved thermodynamic properties of LiCl-H2O aqueous solution. The difference in exergy destruction of this two system mainly arises due to the difference in exergy destruction of absorber and SHX. Exergy destruction in condenser, evaporator and expansion valve remain same for both VARS system. Graphical presentation of exergy destruction in each component for both of this VARS is shown in Fig. 6.
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Jatin Patel et al. / Energy Procedia 109 (2017) 261 – 269
0.25 Li-Cl Li-Br
Total exergy distruction (kW)
0.2
0.15
0.1
0.05
0 Generator
Absorber
Evaporator
Condenser
SHX
Fig. 6. Exergy destruction in various component of LiBr-H2O and LiCl-H2O VARS
Exergetic efficiency
0.35
0.3
0.25
0.2
0.15 0
10
20
30
40
50
60
70
80
90
100
T emperature (°C) Absorber (LiCl-Water)
Condenser (LiCl-Water)
Absorber (LiBr-Water)
Condenser (LiBr-Water)
Generator (LiCl-Water)
Evaporator (LiCl-Water)
Generator (LiBr-Water)
Evaporator (LiBr-Water) Fig. 7. Variation of exergetic efficiency in LiBr-H2O and LiCl-H2O VARS with various operating temperature
It is observed that exergetic efficiency of LiCl-H2O VARS is better compare to that of LiBr-H2O VARS as mentioned in Table 4. Graphical representation of this observation has been illustrated in Fig.7. As condenser and absorber temperature increases, temperature difference between refrigerant and outer circuit cooling water increases hence system subjected to higher entropy generation and higher exergy destruction causes to decrease exergetic efficiency of the system. The important result observed from this analysis is that exergetic efficiency of the system reduces at higher evaporator and generator temperature for both LiCl-H2O and LiBr-H2O VARS. These observations are compatible with the numerical predictions of [7].
Jatin Patel et al. / Energy Procedia 109 (2017) 261 – 269
8. Conclusion The detailed thermodynamic second law analysis of LiCl-H2O absorption system has been evaluated. The simulation shows that most of exergy destruction occurred in absorber and generator under given operating condition. Exergy destruction in pump and expansion valve is found lowest. Exergetic efficiency of the system reaches maximum at 73 °C of generator temperature and 6 °C of evaporator temperature. It is observed that COP of LiCl-H2O system is 4-9% higher than that for LiBr-H2O system while exergetic efficiency founds 3-6% more in LiCl-H2O system. It is interesting to observe that the exergy destruction in generator in LiCl-H2O system is higher which is compensated by lower exergy destruction in absorber and SHX and leads the total exergy destruction to 3 % lower in case for LiClH2O system compared to that of LiBr-H2O system under similar operating conditions. This arises mainly due to improved thermodynamic properties of LiCl-H2O aqueous solution. References [1] Srikhirin P, Aphornratana S, Chungpaibulpatana S. A review of absorption refrigeration technologies. Renewable and sustainable energy reviews 2001; 5:343-72. [2] Gommed K, Grossman G, Ziegler F. Experimental investigation of a LiCl-water open absorption system for cooling and dehumidification. Journal of solar energy engineering 2004; 126:710-5. [3] Won SH, Lee WY. Thermodynamic design data for double effect absorption heat pump systems using waterlithium chloride—cooling. Heat Recovery Systems and CHP 1991; 11:41-8. [4] Grover GS, Eisa MA, Holland FA. Thermodynamic design data for absorption heat pump systems operating on water-lithium chloride—part one. Cooling. Heat Recovery Systems and CHP 1988; 8:33-41. [5] El-Ghalban AR. Operational results of an intermittent absorption cooling unit. International journal of energy research, 2002;26:825-35. [6] She X, Yin Y, Xu M, Zhang X. A novel low-grade heat-driven absorption refrigeration system with LiCl–H2O and LiBr–H2O working pairs. International Journal of Refrigeration 2015; 58:219-34. [7] Arora A, Kaushik SC. Theoretical analysis of LiBr/H2O absorption refrigeration systems. International Journal of Energy Research 2009; 33:1321-40. [8] Kilic M, Kaynakli O. Second law-based thermodynamic analysis of water-lithium bromide absorption refrigeration system. Energy 2007; 32:1505-12. [9] Talbi MM, Agnew B. Exergy analysis: an absorption refrigerator using lithium bromide and water as the working fluids. Applied Thermal Engineering 2000; 20:619-30. [10] Sencan A, Yakut KA, Kalogirou SA. Exergy analysis of lithium bromide/water absorption systems. Renewable energy, 2005;30:645-57. [11] Gogoi TK, Talukdar K. Thermodynamic analysis of a combined reheat regenerative thermal power plant and water–LiBr vapour absorption refrigeration system. Energy Conversion and Management 2014; 78:595-610. [12] Adewusi SA, Zubair SM. Second law based thermodynamic analysis of ammonia–water absorption systems. Energy conversion and management 2004; 45:2355-69. [13] Wang J, Yan Z, Wang M, Dai Y. Thermodynamic analysis and optimization of an ammonia-water power system with LNG (liquefied natural gas) as its heat sink. Energy 2013; 50:513-22. [14] Gunhan T, Ekren O, Demir V, Hepbasli A, Erek A, Sahin AS. Experimental exergetic performance evaluation of a novel solar assisted LiCl–H2O absorption cooling system. Energy and Buildings 2014; 68:138-46. [15] Gogoi TK, Konwar D. Exergy analysis of a H2O–LiCl absorption refrigeration system with operating temperatures estimated through inverse analysis. Energy Conversion and Management 2016; 110:436-47. [16] Cengel YA, Boles MA. An engineering approach. ENERGY 2002; 1:51. [17] Palacios-Bereche R, Gonzales R, Nebra SA. Exergy calculation of lithium bromide-water solution and its application in the exergetic evaluation of absorption refrigeration systems LiBr-H2O. International Journal of Energy Research, 2012;36:166-81. [18] Klein SA, Alvarado FL. Engineering equation solver. F-Chart Software, Madison, WI. 2002;1.
269
ScienceDirect Energy Procedia 109 (2017) 261 – 269
International Conference on Recent Advancement in Air Conditioning and Refrigeration, RAAR 2016, 10-12 November 2016, Bhubaneswar, India
Exergy Based Analysis of LiCl-H2O Absorption Cooling System Jatin Patela * ,Bhargav Pandyab , Anurag Mudgalc 0F0F
a,b,c
Department of Mechanical Engineering, School of Technology, Pandit Deendayal Petroleum University, Gandhinagar, Gujarat 382007, India
Abstract
A thermodynamic analysis of single effect LiCl-H2O vapour absorption cooling system of 1 TR capacity is conducted based on first and second laws. Mathematical models derived from thermodynamics theory, are employed in engineering equation solver to perform the calculations. It is found that maximum exergy destruction in system occurs in the absorber and generator while the pump and the expansion valve have the lowest. A performance comparison between LiCl-H2O and LiBr-H2O absorption system is also evaluated under identical operating conditions. It is found that LiCl-H2O working pair performs thermodynamically better compared to LiBr-H2O in vapour absorption cooling system. 2016The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license ©©2017 Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review by the scientific conference committee of RAAR-2016. Peer-review under responsibility of the organizing committee of RAAR 2016. Keywords: VARS, LiCl-H2O, LiBr-H2O, Exergy, Exergetic efficiency
1. Introduction Absorption system is becoming more popular nowadays because it is heat driven system instead of conventional compressing chillers which are work driven. Vapour absorption refrigeration system (VARS) is the optimum alternative for vapour compression chiller. VARS can also utilize non-conventional energy i.e. solar energy, biomass, geothermal energy etc. VARS performance is affected by the type of absorbent-refrigerant pair that has been chosen. There are limited pairs available for VARS, in which generally used pairs are LiBr-H2O and NH3-H2O [1]. NH3-H2O
* Corresponding author. Tel.: +919428415422; fax: +917923275030. E-mail address: [email protected]
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of RAAR 2016. doi:10.1016/j.egypro.2017.03.061
262
Jatin Patel et al. / Energy Procedia 109 (2017) 261 – 269
absorption system is more complicated as it requires separate rectifier mechanism to remove water vapour from refrigerant vapour, whereas main problem occurs in LiBr-H2O system is the crystallization [1]. Current research work focuses the potential refrigerant-absorbent pair for VARS. LiCl-H2O pair is one of the good options for VARS working pair as it has advantages of triple state point, long term stability and cost, compared to LiBr-H2O [2]. There is a study available for thermodynamic design data for double effect LiCl-H2O absorption system and comparison for the performance of LiCl-H2O and LiBr-H2O absorption system and it is found that COP with LiCl-H2O is better than that of LiBr-H2O system [3]. Feasible range for each operating parameters and possible combination of operating temperature for LiCl-H2O system has been also evaluated [4]. Experimental investigation of intermittent LiCl-H2O absorption cycle has been performed and it is found that that LiCl-H2O system can operate with low temperature heat source [5]. There has been recently developed novel absorption system with LiCl-H2O pair in the high pressure cycle and LiBr-H2O pair in the low pressure cycle [6]. In this study three different heat source utilization mode has been considered i.e. two parallel modes and one series mode and it is concluded that intermediate pressure has a large influence upon system performance. Many literatures are available for second law analysis for LiBr-H2O absorption system [7-11] and NH3-H2O [12-13] VARS. Very few literatures are available on second law analysis of LiCl-H2O absorption system [14-15]. Crystallization is the major issue in any salt based aqueous solution, so to prevent this phenomenon; LiCl-H2O pair operator has to choose operating parameters in optimistic range. The detailed exergy analysis is carried out in the present work and the optimized operating parameters are suggested for LiCL-H2O VARS. Nomenclature COP e
coefficient of performance Specific exergy (kJ/kg) specific enthalpy (kJ/kg)
E G
evaporator Generator inlet stream
mass flow rate (kg/s)
in out
Q
heat load (kW)
o
dead state (ambient)
s T
specific entropy (kJ/kg K) temperature (K)
P
r
pump refrigerant
W
pump work (kW)
REXP
refrigerant expansion valve
efficiency
SEXP
solution expansion valve
SHX ss sys ws
solution heat exchanger
h x
m x
x
K
x
exergy destruction (kW) '\ Subscripts Absorber A C Condenser
outlet stream
strong solution system weak solution
2. System Description The schematic diagram of the LiCl-Water vapour absorption cooling system is shown in Fig. 1. Main components of the system are generator, evaporator, condenser, absorber, solution pump, solution heat exchanger and expansion valve. It uses water as refrigerant and LiCl solution as absorbent. As shown in Fig. 1, at absorber outlet (1), the solution is rich in refrigerant and pump forces the liquid through a solution heat exchanger to the generator (3). The temperature of the solution in the heat exchanger increases. In the generator thermal energy is added and refrigerant escape from the solution. The refrigerant vapour (7) flows to the condenser, where the refrigerant gets condensed. The condensed refrigerant (8) flows through expansion valve to the evaporator (9). In the evaporator, the heat from the load evaporates the refrigerant, which flows back to the absorber (10). At the generator exit (4), the steam consists of absorbentrefrigerant solution, which is cooled in the heat exchanger. From points (6) to (1), the solution absorbs refrigerant vapour from the evaporator and rejects heat through a heat exchanger.
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Fig. 1. Schematic of lithium chloride-water absorption cooling cycle
3. Thermodynamic Formulation The COP of the absorption system is defined as the heat load in the evaporator per heat load in the generator and work required for solution pump and it can be written as: x
COP
QE x
(1)
x
QG W P
Exergy analysis is combination of first and second law of thermodynamics and it indicates the maximum work potential of the system with respect to its surroundings [16]. Specific exergy of pure substance is given by [16]
h ho To s so
e
(2)
Rate of exergy destruction in any component undergoing a steady flow process can be given by [16] x
'\ destyoyed
x x ª x§ T ¦ m in ein ¦ m out eout «¦ Q¨ 1 0 T © ¬
x ·º ¸» r W P ¹¼
(3)
Owing to the fact that exergy balances the dead state values get cancelled [17] therefore, with reference of Eq.3 exergy destruction in each component can be written as: x
'\ G
x
x
x
x
m ws h3 To s 3 m ss h4 To s 4 m r h7 To s7 m G h11 To s 11 h12 To s 12
x
m r h10 To s10 m ss h6 To s6 m ws h1 To s1 m A h13 To s13 h14 To s14
x
m r h9 To s 9 m r h10 To s10 m E h17 To s17 h18 To s18
'\ A
'\ E
x
x
x
x
x
x
(4) (5)
x
(6)
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Jatin Patel et al. / Energy Procedia 109 (2017) 261 – 269 x
x
x
x
x
(7)
x
m ws h2 h3 To s 2 s 3 m ws h4 h5 To s 4 s 5
'\ SHX x
x
m r h7 To s7 m r h8 To s8 m C h15 To s15 h16 To s16
'\ C
x
(8)
x
'\ P
m ws h1 h2 To s1 s2 WP
x
mr h8 h9 To s8 s9
(9)
x
'\ REXP x
(10)
x
m ss h5 h6 To s5 s6
'\ SEXP
(11)
The total rate of exergy destruction of absorption system is the sum of exergy destruction in each component and can be written as: x
'\ sys
x
x
x
x
x
x
x
x
'\ G '\ A '\ E '\ C '\ SHX '\ REXP '\ SEXP '\ P
(12)
For this system exergetic efficiency can be determined as:
K exergetic
x ª T º Q E «1 o » T E ¼ ¬ x ª To º x Q G «1 » W ¬ TG ¼
(13) p
4. Model Validation In order to validate the second law based present model of LiCl-H2O, the results from this model have been compared with simulation data available from recent literature of Gogoi and Konwar [15]. From this comparison it is observed that agreement between two model is satisfactory. Data of comparison has been presented in Table 1. Table 1. Comparison of present model with model of Gogoi and Konwar [15] Total exergy destruction (kW)
Temperature (°C)
TE TE TE TE TE
4, 5, 6, 7, 4,
TC
34 , T A
TC
28 , T A
TC
41 , T A
TC
39 , T A
TC
37 , T A
31 , TG 73 31 , TG 66 31 , TG 79 36 , TG 77 32 , TG 76
Error (%)
Gogoi and Konwar [15]
Present study
40.634
39.75
-2.17
36.60
35.99
-1.66
41.352
40.99
-0.87
37.123
37.9
2.09
39.5
39.99
1.22
5. Simulation Model In this study, the software used for modelling is engineering equation solver (EES) [18]. To simplify the modelling of system, several assumptions are made as follows: (1) the system is simulated under steady state conditions, (2) pressure drop along the fluid flow and heat losses/gains are negligible, (3) refrigerant condensed in the condenser to
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a saturated liquid while in evaporator the refrigerant evaporates to a saturated vapour. The initial conditions considered for 1 TR cooling capacity are generator temperature as 78 °C, absorber temperature as 40 °C, condenser temperature as 36 °C, evaporator temperature as 10 °C and effectiveness of solution heat exchanger as 0.7. VARS thermodynamic data and performance parameters are listed in Table 2 and 3 respectively. Based on these conditions, COP of system is calculated as 0.7877 and exergetic efficiency is obtained as 27.65 %. It is observed from Table 3 that under given operating condition, most of the exergy destruction occurred in absorber and generator. Maximum exergy destruction takes place in absorber followed by generator, evaporator, condenser, solution heat exchanger, and refrigerant expansion valve. It is also found that exergy destruction in pump and expansion valve are very small and usually ignored. Table 2. Absorption system data obtained from the thermodynamic analysis State Temperature Concentration Mass flow rate (°C) (% of LiCl) point (kg/s) 1 40 42.33 0.01164 2 40 42.33 0.01164 3 61.39 42.33 0.01164 4 78 48.52 0.01016 5 51.4 48.52 0.01016 6 51.4 48.52 0.01016 7 78 0.00148 8 36 0.00148 9 36 0.00148 10 10 0.00148 11 92 0.1181 12 83 0.1181 13 25 0.1276 14 33 0.1276 15 33 0.4425 16 35 0.4425 17 25 0.1051 18 17 0.1051 Table 3. Performance parameters of system Component Energy Flow (kW) Generator Absorber Condenser Evaporator Solution heat exchanger Pump Refrigerant expansion valve Solution expansion valve Total
4.465 4.268 3.705 3.517 0.6492 0.00004579 16.60
Enthalpy (kJ/kg) 177.1 177.1 232.9 319.8 255 255 2646 150.8 150.8 2519 385.4 347.6 104.8 138.3 138.3 146.7 104.8 71.38
Entropy (kJ/kg K) 0.3483 0.3483 0.5206 0.6135 0.4218 0.4218 8.753 0.5185 0.5352 8.899 1.216 1.11 0.367 0.477 0.477 0.505 0.367 0.2532
Exergy destruction (kW)
Non dimensional exergy loss (%)
0.1558 0.2037 0.03055 0.1379 0.02567 0.0000457 0.007365 0.5612
27.77 36.31 5.44 24.58 4.575 0.0081 1.312 100
6. Result and Discussion Variation of exergy destruction in whole system and non-dimensional exergy destruction in various components with operating temperature is shown in Fig. 2 to 4.
0.6
40
0.5 30 0.4 20 0.3 10
0 72
0.2
73
74
75
76
77
78
79
0.1 80
50
0.6
45 0.5
40 35
0.4
30 25
0.3
20 0.2
15 10
0.1
5 0 30
32
Generator temperature (°C) Absorber
SHX
Condensor
Evaporator
34
36
0 40
38
Absorber temperature (°C) Generator system
Fig. 2. Variation of non-dimensional exergy destruction and total exergy destruction in various component with generator temperature
0.7 40 0.6 0.5
30
0.4 20
0.3 0.2
10
SHX
Condensor
Evaporator
Generator system
0.35
0.8
Exergetic efficiency (%)
50
Absorber
Fig. 3. Variation of non-dimensional exergy destruction and total exergy destruction in various component with absorber temperature
Total exergy destruction (kW)
Non dimensional exergy destruction (%)
Total exergy destruction (kW)
0.7
Total exergy destruction (kW)
50
Non dimensional exergy destruction (%)
Jatin Patel et al. / Energy Procedia 109 (2017) 261 – 269
Non dimensional exergy destruction (%)
266
0.3
0.25
0.1 0 5
7
9
11
13
0 15
0.2 0
10
Evaporator temperature (°C) Absorber
SHX
Condensor
Evaporator
20
30
40
50
60
70
80
90
Temperature (°C) Generator system
Fig. 4. Variation of non-dimensional exergy destruction and total exergy destruction in various component with evaporator temperature
Absorber
Condensor
Evaporator
Generator
Fig. 5. Variation of exergetic efficiency of system with operating temperature
Fig. 4 illustrate the variation of non-dimensional exergy destruction and total exergy destruction in various components with evaporator temperature. It is found that non dimensional exergy destruction in evaporator, SHX and total exergy destruction of system decreases with evaporator temperature under given operating condition. Non dimensional exergy destruction in generator, absorber and condenser increases with evaporator temperature. From Fig. 5 it is found that exergetic efficiency of the system first increases and then start decreasing with evaporator and generator temperature while exergetic efficiency decreases with absorber and condenser temperature. 7. Performance Comparison Between LiCl-H2O and LiBr-H2O Absorption System In this study detailed comparison between LiCl-H2O and LiBr-H2O VARS has been evaluated. Comparison of these two system has provided in terms of thermodynamic first and second law of i.e. heat load, exergy destruction,
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COP and exergetic efficiency. Various performance parameters comparison of LiC-H2O and LiBr-H2O VARS is illustrated in Table 4. Table 4. Various performance parameters comparison of LiCl-H2O and LiBr-H2O VARS LiCl-H2O VARS x Temperature (°C) COP '\ sys ( kW ) Kexergetic
TE
8 , TG
TE
10 , TG
TE
7 , TG
68 , TC
30 , T A
TE
8 , TG
85 , TC
42 , T A
75 , TC
90 , TC
35 , T A
45 , T A
40
LiBr-H2O VARS x
COP
'\ sys ( kW ) Kexergetic
0.74
0.7182
0.313
0.70
0.7646
0.295
0.76
0.6967
0.22
0.74
0.7196
0.21
32
0.81
0.645
0.42
0.80
0.6527
0.41
40
0.73
0.7243
0.2664
0.70
0.7625
0.254
40
Table 5. Energetic and Exergetic performance comparison of LiC-H2O and LiBr-H2O VARS LiBr-H2O VARS LiCl-H2O VARS Component
Energy Flow (kW)
Exergy destruction (kW)
Energy Flow (kW)
Exergy destruction (kW)
Generator Absorber Evaporator Condenser SHX Pump REXP Total
4.64 4.472 3.517 3.712 1.217 0.00008169 17.55
0.2119 0.1946 0.1318 0.1144 0.04056 0.00008169 0.008534 0.7019
4.513 4.309 3.517 3.712 0.7121 0.00005433 16.76
0.2185 0.1779 0.1318 0.1144 0.02939 0.00005433 0.008534 0.6806
Energetic and exergetic comparison for this two VARS is shown in Table 5, for which the operating parameters are taken as generator temperature as 80 °C, evaporator temperature as 10 °C, absorber temperature as 40 °C and condenser temperature as 38 °C. From Table.4, it is found that at various operating condition COP and exergetic efficiency of LiCl-H2O VARS is better compare to that of LiBr-H2O VARS. Therefore, it is obvious that exergy destruction of LiCl-H2O VARS is lower compared to that of LiBr-H2O VARS. It is found that total heat load and exergy destruction of LiCl-H2O VARS is lower compare to LiBr-H2O VARS. It is observed that exergy destruction in generator of LiCl-H2O VARS is 3% higher compare to that of the LiBr-H2O VARS while exergy destruction in absorber of LiCl-H2O VARS is 13% and exergy destruction in SHX is 4 % lower compared to that of the LiBr-H2O VARS. This arises mainly due to improved thermodynamic properties of LiCl-H2O aqueous solution. The difference in exergy destruction of this two system mainly arises due to the difference in exergy destruction of absorber and SHX. Exergy destruction in condenser, evaporator and expansion valve remain same for both VARS system. Graphical presentation of exergy destruction in each component for both of this VARS is shown in Fig. 6.
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Jatin Patel et al. / Energy Procedia 109 (2017) 261 – 269
0.25 Li-Cl Li-Br
Total exergy distruction (kW)
0.2
0.15
0.1
0.05
0 Generator
Absorber
Evaporator
Condenser
SHX
Fig. 6. Exergy destruction in various component of LiBr-H2O and LiCl-H2O VARS
Exergetic efficiency
0.35
0.3
0.25
0.2
0.15 0
10
20
30
40
50
60
70
80
90
100
T emperature (°C) Absorber (LiCl-Water)
Condenser (LiCl-Water)
Absorber (LiBr-Water)
Condenser (LiBr-Water)
Generator (LiCl-Water)
Evaporator (LiCl-Water)
Generator (LiBr-Water)
Evaporator (LiBr-Water) Fig. 7. Variation of exergetic efficiency in LiBr-H2O and LiCl-H2O VARS with various operating temperature
It is observed that exergetic efficiency of LiCl-H2O VARS is better compare to that of LiBr-H2O VARS as mentioned in Table 4. Graphical representation of this observation has been illustrated in Fig.7. As condenser and absorber temperature increases, temperature difference between refrigerant and outer circuit cooling water increases hence system subjected to higher entropy generation and higher exergy destruction causes to decrease exergetic efficiency of the system. The important result observed from this analysis is that exergetic efficiency of the system reduces at higher evaporator and generator temperature for both LiCl-H2O and LiBr-H2O VARS. These observations are compatible with the numerical predictions of [7].
Jatin Patel et al. / Energy Procedia 109 (2017) 261 – 269
8. Conclusion The detailed thermodynamic second law analysis of LiCl-H2O absorption system has been evaluated. The simulation shows that most of exergy destruction occurred in absorber and generator under given operating condition. Exergy destruction in pump and expansion valve is found lowest. Exergetic efficiency of the system reaches maximum at 73 °C of generator temperature and 6 °C of evaporator temperature. It is observed that COP of LiCl-H2O system is 4-9% higher than that for LiBr-H2O system while exergetic efficiency founds 3-6% more in LiCl-H2O system. It is interesting to observe that the exergy destruction in generator in LiCl-H2O system is higher which is compensated by lower exergy destruction in absorber and SHX and leads the total exergy destruction to 3 % lower in case for LiClH2O system compared to that of LiBr-H2O system under similar operating conditions. This arises mainly due to improved thermodynamic properties of LiCl-H2O aqueous solution. References [1] Srikhirin P, Aphornratana S, Chungpaibulpatana S. A review of absorption refrigeration technologies. Renewable and sustainable energy reviews 2001; 5:343-72. [2] Gommed K, Grossman G, Ziegler F. Experimental investigation of a LiCl-water open absorption system for cooling and dehumidification. Journal of solar energy engineering 2004; 126:710-5. [3] Won SH, Lee WY. Thermodynamic design data for double effect absorption heat pump systems using waterlithium chloride—cooling. Heat Recovery Systems and CHP 1991; 11:41-8. [4] Grover GS, Eisa MA, Holland FA. Thermodynamic design data for absorption heat pump systems operating on water-lithium chloride—part one. Cooling. Heat Recovery Systems and CHP 1988; 8:33-41. [5] El-Ghalban AR. Operational results of an intermittent absorption cooling unit. International journal of energy research, 2002;26:825-35. [6] She X, Yin Y, Xu M, Zhang X. A novel low-grade heat-driven absorption refrigeration system with LiCl–H2O and LiBr–H2O working pairs. International Journal of Refrigeration 2015; 58:219-34. [7] Arora A, Kaushik SC. Theoretical analysis of LiBr/H2O absorption refrigeration systems. International Journal of Energy Research 2009; 33:1321-40. [8] Kilic M, Kaynakli O. Second law-based thermodynamic analysis of water-lithium bromide absorption refrigeration system. Energy 2007; 32:1505-12. [9] Talbi MM, Agnew B. Exergy analysis: an absorption refrigerator using lithium bromide and water as the working fluids. Applied Thermal Engineering 2000; 20:619-30. [10] Sencan A, Yakut KA, Kalogirou SA. Exergy analysis of lithium bromide/water absorption systems. Renewable energy, 2005;30:645-57. [11] Gogoi TK, Talukdar K. Thermodynamic analysis of a combined reheat regenerative thermal power plant and water–LiBr vapour absorption refrigeration system. Energy Conversion and Management 2014; 78:595-610. [12] Adewusi SA, Zubair SM. Second law based thermodynamic analysis of ammonia–water absorption systems. Energy conversion and management 2004; 45:2355-69. [13] Wang J, Yan Z, Wang M, Dai Y. Thermodynamic analysis and optimization of an ammonia-water power system with LNG (liquefied natural gas) as its heat sink. Energy 2013; 50:513-22. [14] Gunhan T, Ekren O, Demir V, Hepbasli A, Erek A, Sahin AS. Experimental exergetic performance evaluation of a novel solar assisted LiCl–H2O absorption cooling system. Energy and Buildings 2014; 68:138-46. [15] Gogoi TK, Konwar D. Exergy analysis of a H2O–LiCl absorption refrigeration system with operating temperatures estimated through inverse analysis. Energy Conversion and Management 2016; 110:436-47. [16] Cengel YA, Boles MA. An engineering approach. ENERGY 2002; 1:51. [17] Palacios-Bereche R, Gonzales R, Nebra SA. Exergy calculation of lithium bromide-water solution and its application in the exergetic evaluation of absorption refrigeration systems LiBr-H2O. International Journal of Energy Research, 2012;36:166-81. [18] Klein SA, Alvarado FL. Engineering equation solver. F-Chart Software, Madison, WI. 2002;1.
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