Energy and Exergy Analyses for Optimization of the Operating Temperatures in Double Effect Absorption Cycle

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 109 (2017) 211 – 218

International Conference on Recent Advancement in Air Conditioning and Refrigeration, RAAR 2016, 10-12 November 2016, Bhubaneswar, India

Energy and Exergy Analyses for Optimization of the Operating Temperatures in Double Effect Absorption Cycle Md Azhara* and M Altamush Siddiquia a

Department of Mechanical Engineering, Aligarh Muslim University, Aligarh, India

Abstract

Energy and exergy analyses of double effect lithium bromide-water vapour absorption cycle has been carried out to optimize the operating temperatures in the main generator and the secondary condenser/ generator for maximum coefficient of performance (COP) and exergetic efficiency. There exists maximum COP and also maximum exergetic efficiency as the temperature in the secondary generator and the main generator are varied, thus leading to the optimum temperatures. Because of two generators in the double effect cycle, the optimization has been done in two steps. Since the secondary generator operates by using the heat of condensation release in the secondary condenser, the temperature in the secondary condenser will be nearly same as in the secondary generator. The analysis has been done for fixed temperatures in the evaporator, main condenser and the absorber. Results show that the COP increases while the exergetic efficiency decreases with increases in the evaporator temperature. The optimum parameters such as temperatures in the main generator, secondary generator and condenser, including the LiBr-Salt concentrations in the two generators, are presented for different values of the evaporator, main condenser and the absorber temperatures. A computer program has been developed for simulating the cycle. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-reviewunder under responsibility of organizing the organizing committee of RAAR Peer-review responsibility of the committee of RAAR 2016. 2016. Keywords: LiBr-water; Double effect; Coefficient of performance; Optimum temperature; Exergetic efficiency

* Corresponding author. Tel.: +91-7895698621; fax: +91-571-2701454. 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.2017.03.043

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1. Introduction Recently the absorption cooling technology has become very popular. The attractive feature of this technology is, it uses low grade energy to drive the system. The huge amount of heat that go wasted from the industries, can thus be utilized by this system. Moreover, this system is environmentally safe due to the use of natural working fluid, such as NH3-H2O and LiBr-H2O etc. The single-effect absorption system has relatively low COP due to which it is not competing economically with the conventional vapour compression system, except in case of waste heat applications where the input energy is virtually free of cost. To reduce size of the equipment and operating cost of the absorption cycle, it is desirable to increase the COP. Therefore, multi-effect absorption systems with high temperature heat sources have now been developed [1]. The double effect system was first patented by Loweth [2] and commercialized by TRANE in 1970 which was later modified and improved by Saito and Inoue [3] and by Alefed [4]. Siddiqui [5], Saghiruddin and Siddiqui [6,7], Malik and Siddiqui [8] have carried thermodynamic and economic analysis of single effect cycle with LiBr-H2O, NH3-H2O, NH3-LiNO3 and NH3-NaSCN combinations using different sources of energy like, Biogas, LPG and Solar Collectors. Arora and Kaushik [9] and Marcos et al. [10] performed energy and exergy analysis of single effect and double effect absorption LiBr-H2O system. Lee and Sherif [11] and Sencan et al. [12] carried thermodynamic analysis of single effect absorption system for cooling and heating applications using pressurised hot water as the source of energy. Samanta and Basu [13] have performed the first and second law optimization of single effect absorption system. So far the studies carried on the double effect cycle, discuss optimization of the main generator temperature to some extent without any focus on the secondary condenser / generator temperature. The present analysis is therefore carried to optimize the secondary condenser / generator temperatures along with the main generator temperature for maximum COP and exergetic efficiency. Nomenclature A Absorber Subscripts C Main condenser at Pressure P2 a Absorber Cs Secondary condenser at pressure P3 c Main condenser COP Coefficient of performance [-] cs Secondary condenser E Evaporator e Evaporator G Main generator at Pressure P1 g Main generator Gs Secondary generator at pressure P2 gs Secondary generator h Specific enthalpy [kJ kg-1] i Inlet LiBr Lithium bromide salt o Outlet m Mass flow rate [kg s-1] P Pressure [kPa] PH1, PH2 Preheaters Greek Symbols Q Rate of heat transfer [kJ s-1] ɛ Effectiveness of heat exchanger T Temperature [oC or K] η Exergetic Efficiency TV Throttle valve Wp Work of pump [kW] X, Xc Mass concentration and Crystallization value of lithium bromide salt [%] 2. Description of The Double Effect Absorption Cycle The schematic diagram of a double effect vapour absorption cycle is shown in Fig. 1; the absorption process of which is shown on P-T-X diagram in Fig. 2. It consists of two generators: a main generator (G) and a secondary generator (Gs), and two condensers: a secondary condenser (Cs) and a main condenser (C). The heat is to be rejected to the surrounding from the main condenser C. The main generator (G) and the secondary condenser (Cs) operate at high pressure (P3 = Pg= Pcs) while the secondary generator (Gs) and the main condenser (C) operate at medium pressure (P 2

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Pressure

= Pc=Pgs). The evaporator and the absorber work at low pressure (P 1 = Pe = Pa). In this system, the weak solution at state 1 is pumped from the absorber to the main generator (G) through two heat exchangers (i.e. PH1 and PH2). The LiBr-water solution in the main generator is heated at high temperature to boil out the refrigerant vapour from the solution. Thus, the primary vapour, from G goes to the secondary condenser (Cs). The heat of condensation from this Cs is utilized by the secondary generator (Gs). The strong solution which is released by G at state 8 flows to Gs through PH2 where the solution is cooled to some extent by exchanging heat with the weak solution. At the outlet of the Gs secondary vapour is produced, which together with the condensed water vapour from the secondary condenser flows into the main condenser. Thus, the total amount of liquid refrigerant leaving the main condenser is the sum of refrigerant coming from the generators G and Gs. The refrigerant liquid from this condenser flows to the evaporator through a throttle valve. After extracting heat from the medium to be cooled, the refrigerant evaporates and then passes to the absorber where it gets absorbed by the strong solution coming from the secondary generator (G s) through a preheater PH1. The resulting weak solution in the absorber is then pumped to the main generator and the cycle completes.

Qg

Refrigeration Side Solution Side Qg

CS 4a TV1

MAIN CONDENSER C

4c

SECONDARY GENERATOR GS

P3=Pg=Pcs

MAIN GENERATOR G

4a

TV3 8a

PH2

PH2

C

Qc

P2=Pgs=Pc

Qgs 4c

10

2a Xa

5 PH1

10

PH1

11

6

8a

GS 9

2a

5

TV2

8 Xg

3

4b 9

G

4 X=0

3

8

QGS

4b

Qc

4

SECONDARY CONDENSER CS

2

Xgs

TV4 PH=Preheater TV=Throttle Valve Q=Heat Transfer Rate WP =Pump Work

2

Qe

12

E

EVAPORATOR

ABSORBER

Wp

1

7

Qe

P1=Pe=Pa

6 Pump

1 7 X=0

Te

Qa

Fig 1. Schematic Diagram of double effect absorption cycle

A Qa Tc

12

11

Tgs

Tg Temperature

Fig 2. P-T-X diagram of double effect absorption cycle

3. Mathematical Modelling The thermodynamic analysis of the system involves the first and second laws. The first law analysis consist of mass, concentration and energy balance at each component of the system, whereas the second law analysis deals with exergy destruction of the system components and Exergetic efficiency of the whole system.

Energy conservation

¦m = ¦m ¦ (mX) = ¦ (mX) ¦ (mh)  ¦ (mh)  ª¬¦Q ¦Q º¼  W

Exergy Destruction

EDi

Mass conservation Concentration conservation

i

(1)

o

i

(2)

o

i

i

o

¦ (me)  ¦ (me) i

o

o

0

ª § T · § T · º  « ¦ Q ¨1  o ¸ ¦ Q ¨1  o ¸ » r ¦ W © T ¹i © T ¹o ¼ ¬

(3)

0

(4)

The mathematical equations obtained by assuming each component in the Fig.1 as control volume are as follows: m1 = m12 + m7 (5) Absorber:

m1X1 = m12 X12 + m7 X7 Qa m7 h 7  m12 h12  m1h1 EDa m7 (h 7  To s7 )  m12 (h12  To s12 )  m1 (h1  To s1 )  Qa (1 To / Ta )

(6) (7) (8)

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m 3 = m8 + m 4

Main Generator:

(9)

m3 X3 = m8 X8 + m4 X4

Qg

m 4 h 4  m8 h 8  m 3 h 3

Evaporator:

(12)

m5 = m 4b + m 4c

(13)

Qc m4c h 4c  m4b h 4b  m5 h 5 EDc m4b (h 4b  To s4b )  m4c (h 4c  To s4c )  m5 (h 5  To s5 )  Qc (1  To / Tc ) Qe m7 h 7  m6 h 6 EDe m7 (h 7  To s7 )  m6 (h 6  Tos6 ) + Qe (1  To / Te )

(14)

Secondary Generator:

Qcs m4 h 4  m4a h 4a Qgs m 4c h 4c  m10 h10  m 9 h 9

Solution pump:

Wp

Coefficient of performance:

COP

Secondary Condenser:

(11)

m 4 (h 4  To s 4 )  m8 (h 8  To s8 )  m3 (h 3  To s3 )  Q g (1  To / Tg )

EDg Main Condenser:

(10)

(15) (16) (17) (18) (19)

m 2 h 2  m1h1

(20)

Qe Q g  Wp

(21)

Exergetic Efficiency: η = Exergy of product 1  Total Exergy Destruction ex Exergy Supplied

Exergy Supplied

T Qe §¨1  o ·¸ Te ¹ © § T · Qg ¨1  o ¸  Wp Tg ¹ ©

The crystallization values of LiBr salt in the system are obtained using the following equation [14]: Xc=9.8459E-02(T-273.15) + 59.7995; in the range: 320 ≤ T ≤ 375 K

(22)

(23)

4. Calculation Procedure For simulation following assumptions are made: x The process in the cycle is under steady condition. x Temperature of absorber and condenser is consider as same (T a=Tc). x Expansion in throttling valve is isenthalpic. x Refrigerant at the outlet of the condenser is saturated liquid & outlet of evaporator is saturated vapour. The thermodynamic properties of the refrigerant (water) and LiBr-H2O mixture, such as specific enthalpy, specific heat, density, concentration, saturation pressure and temperature are calculated for each state point in the cycle using the available correlations [14, 15] as subroutines in the main computer program. The present results were at first validated with the simulation data of Arora and Kaushik [9] for the double effect cycle at same operating conditions in Table 1. This is in close agreement with the present data analysis. Table 1. Comparison of the present results with Arora and Kaushik [2009] Double Effect Heat Transfer Rate [in kW] and COP Absorber Generator Condenser Evaporator Pump work COP

Operating condition: Tg=140.6oC, Te=7.2oC, ɛ=0.7 Tc=Ta=37.8 oC, mass flow rate of refrigerant = 1 kg s-1 Ref [9]

Present work

2942.175 1868.711 1282.052 2355.450 0.3598 1.26

2942.319 1869.100 1283.171 2355.995 0.3960 1.260

Some fixed data used for simulation of the double effect vapour absorption cycle is given in the Table 2. In order to

Md Azhar and M. Altamush Siddiqui / Energy Procedia 109 (2017) 211 – 218

compute the analysis, a computer programme in FORTRAN language has been made. Fig. 3 shows flow chart of the programme. In the present study, analysis is done for fixed pressure in the generator while the concentration and temperature of the LiBr-H2O solution in it are varied. Since the condenser and the generator are at same pressure, the saturation temperature of the refrigerant in them, get fixed. The variation of the concentration and temperature in each generator in this analysis proceeds systematically which simplifies the calculation. Table 2. Fixed data used in the simulation double effect absorption system Evaporator cooling capacity

300 kW

Evaporator temperature, Te

4oC, 6oC, 8oC, 10oC

Absorber temperature, Ta

33oC, 36oC, 39oC

Main Condenser (C) temperature, Tc

33oC, 36oC, 39oC

Secondary Condenser (CS) temperature, Tcs

50oC to 110oC in steps of 5oC

Heat exchanger efficiency

70%

Pump efficiency

85%

Calculate Tg=f(Tc3,Xg) and Tg2=f(Tc , Xg2) using concentration equation and then calculate Sp. Enthalpy, Density, specific heat of weak and strong solutions

START

Initialize Te=275 K, Tc=303 K, Tc3=323 K, Ta=Tc Qe=300 kW Check Energy balance Qc3 – Qg2 ≤ 0.001 k W

No Is Te ≤ 283 K Tc ≤ 313 K Tc3 ≤ 393 K

Yes Check Tg2
No

STOP

No STOP

Yes

Calculate Xa=f(Te,Ta) using concentration equation Calculate Sat. Pressure, density, Specific Enthalpy of refrigerant by using subroutines Calculate heat load at different components, COP and Exergetic efficiency

Te increases by 2 K Tc increases by 3 K Tc3 increases by 5 K

Assume X a=Xg , and increases Xg by 10% Similarly assume Xg2=Xg, and increase Xg2

Fig. 3 Flow chart of the computed programme for the double effect absorption cycle

5. Result and Discussion 5.1 Optimization of secondary condenser temperature and main generator temperature: Generator is the most important component in the absorption system, where refrigerant generates in the form of vapour when heated by burning gases. It is found that on increasing the main generator temperature, to which heat is supplied, the COP of the double effect cycle increases and becomes constant or terminates at last due to crystallization of LiBr salt, as shown in Fig. 4 for different temperatures in the secondary condenser at fixed temperatures in the evaporator and the main condenser. Thus, the maximum COP and the corresponding values of the generator temperature T g are identified for each value of the temperature Tcs for the given values of Te and Tc. The maximum values of COP, so identified from the Fig. 4, are plotted against the secondary condenser temperature (T cs) in Fig. 5 for different temperatures in the evaporator and at T c=35oC. The maximum COP of the cycle increases with increase in the secondary condenser temperature, reaches to maximum value and then decreases. Thus, the temperatures in the secondary condenser get optimized corresponding to maximum of the maximum COP. Since the secondary condenser and the main generator operate at the same pressure P3, temperature Tg is the superheated temperature of the refrigerant leaving the main generator G and T cs is the saturation temperature of this refrigerant condensing in the secondary condenser. Also, Tg is temperature of the LiBr-water solution leaving the main generator. Therefore, change in the

215

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value of Tcs results in subsequent change in the value of T g as well. Similar plots of maximum COP with Tg are also shown in Fig. 6 resulting in the same values of the maximum COP as shown in the Fig. 5 against T cs. Thus the secondary condenser temperature T cs and main generator temperature Tg (both at pressure P3) obtained corresponding to maximum of the maximum COP (in the Figs. 5 and 6) are termed as the optimum temperatures that are listed in Table 3 for different values of T e and Tc. It is seen that on increasing the main condenser temperature, the optimum temperatures of the secondary condenser and main generator also increase. The corresponding values of Tgs at pressure P2 and concentration at both generators are also given in the Table 3. 1.5

Tcs=75 Tcs=85 Tcs=95 Tcs=105

Te=5oC Tc=35oC

1.3

1.5

Tcs=80 Tcs=90 Tcs=100 Tcs=110

1.3

COP

COP

1.1 0.9 0.7

Tcs=85 Tcs=95 Tcs=105

Te=5oC Tc=40oC

Tcs=90 Tcs=100 Tcs=110

1.1 0.9 0.7

0.5

0.5 110

120

130

140

150

160

170

130

Main Generator Temperature, Tg in oC

140

150

160

170

180

Main Generator Temperature, Tg in oC

1.7

1.7

1.5

1.5

1.3 1.1 Te=5

0.9

Te=10 Te=15

0.7 Tc=35 oC

Maximum COP

Maximum COP

Fig. 4. Variation in COP of double effect cycle with main generator temperature at different temperatures in the secondary condenser

1.3 1.1 0.9 0.7 Tc=35oC

Te=20

0.5

0.5 55 65 75 85 95 105 115 Secondary Condenser Temperature, Tcs in oC Figure 5. Variation in COP of double effect cycle with Secondary Condenser temperature at different temperature in the evaporator

Te=5 Te=10 Te=15 Te=20

80 90 100 110 120 130 140 150 160 170 Main Generator Temperature, Tg in oC Figure 6. Variation in COP of double effect cycle with main generator temperature at different temperature in the evaporator

Table 3. Optimum operating conditions for maximum COP at different values of evaporator and condenser temperatures Corresponding Values Optimum Values Tc at P2 Te at P1 At P2 At P3 Maximum COP (oC) (oC) Xcs (%) Tgs (oC) Tcs (oC) Tg (oC) Xg (%) 5 56.41 74.47 75 117.48 60.46 1.377 10 53.36 69.20 70 105.26 58.01 1.488 30 15 48.84 59.33 60 85.44 53.02 1.614 20 45.90 59.52 60 80.38 52.41 1.715 5 59.81 89.41 90 143.03 64.34 1.283 10 56.04 79.52 80 122.37 60.09 1.379 35 15 51.99 69.91 70 102.53 55.56 1.491 20 48.51 64.37 65 90.39 52.69 1.615 5 61.44 96.96 100 159.18 64.94 1.182 10 59.43 94.54 95 147.87 63.97 1.286 40 15 55.67 84.55 85 127.26 59.74 1.382 20 51.63 74.92 75 107.45 55.21 1.494

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5.2 Second law optimization: Exergy is the best way to optimize the system. It is the combination of first and second laws. It gives the maximum useful work that can be obtained from the system when brought to the dead state. Second law performance can be measured in terms of exergetic efficiency. It is the ratio of exergy of the product to exergy supplied. The double effect system is therefore also analyzed for the optimum temperatures under second law. Figure 7 shows the variation of exergetic efficiency with main generator temperature at different values of the secondary condenser temperature for fixed values of Te and Tc. It is seen that exergetic efficiency of the double effect cycle follows the same trend as the first law COP. Tcs=75 Tcs=85 Tcs=95 Tcs=105

Te=5oC Tc=35o

32

28

Tcs=80 Tcs=90 Tcs=100 Tcs=110

Exergetic efficiency (%)

Exergetic efficiency (%)

36

28 24 20 16 12

Te=5oC Tc=40o

Tcs=85 Tcs=95 Tcs=105

Tcs=90 Tcs=100 Tcs=110

24

20

16

12 110

120

130

140

150

160

130

170

140

150

160

170

Main Generator Temperature, Tg in oC

Main Generator Temperature, Tg in oC

Figure 7. Variation in exergetic efficiency of double effect cycle with main generator temperature at different temperatures in secondary condenser

The maximum values of the exergetic efficiency taken from Fig. 7 are plotted against the secondary condenser temperature in Figs. 8 and 9. Similar procedure is used to optimize the secondary condenser / generator and main generator temperatures as done for the COP. Table 4 shows the optimum temperatures corresponding to the maximum exergetic efficiency. From Table 4, it is seen that optimum temperatures in the secondary condenser and main generator, corresponding to maximum exergetic efficiency are relatively lower than those obtained corresponding to the maximum COP, as is evident from the Table 3. Interestingly one can also see some peculiar result during the second law analysis. That is, the exergetic efficiency decreases with increase in the evaporator and main condenser temperatures. 35.5

Tc=35oC

30.5

Maximum Exergetic Efficiency (%)

Maximum Exergetic Efficiency (%)

35.5

25.5 20.5 15.5 10.5 5.5

Te=5 Te=15

0.5 55

65

75

Te=10 Te=20 85

95

105

30.5 25.5 20.5 15.5 10.5 5.5 0.5

115

Secondary Condenser Temperature, Tcs in oC

Fig 8. Variation in maximum exergetic efficiency with secondary condenser temperature at different values of Te.

Tc=35oC

Te=5 Te=15

Te=10 Te=20

80 90 100 110 120 130 140 150 160 170 Main Generator Temperature, Tg in oC Fig 9. Variation in maximum exergetic efficiency with main generator temperature at different values of Te

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Table 4. Optimum operating conditions for maximum exergetic efficiency at fixed values of evaporator and condenser temperatures Corresponding Values Optimum Values Tc at P2 Te at P1 Maximum Exergetic At P2 At P3 o o ( C) ( C) Efficiency (%) o o o Xcs (%) Tgs ( C) Tcs ( C) Tg ( C) Xg (%) 5 54.01 64.42 65 100.92 55.68 40.3 10 50.96 59.40 60 89.29 53.06 35.6 30 15 47.64 54.73 55 77.8 50.44 29.4 20 44.90 54.06 55 73.30 50.05 12.6 5 57.61 79.31 80 125.98 60.01 31.3 10 54.84 74.28 75 114.03 57.69 26.9 35 15 50.59 64.38 65 94.22 52.70 21.2 20 45.71 54.36 55 74.59 46.79 10.3 5 60.84 94.0 95 151.62 63.76 25.2 10 58.43 89.72 90 139.52 61.99 20.8 40 15 53.27 74.38 75 110.74 54.95 15.8 20 50.23 69.36 70 99.14 52.35 7.4

6. CONCLUSIONS x x x x

With fixed temperatures in the secondary condenser / generator, both the COP and the exergetic efficiency increase almost in a similar manner with rise in the main generator temperature (Tg) and show maxima at a certain value of Tg. With fixed temperatures in the evaporator, the COP and the exergetic efficiency increase with rise in the secondary condenser / generator temperature, reach to some maximum value and then decrease, again showing maxima at certain values of Tg in each case. With increase in temperature ‘Te’, COP of the cycle increases while the exergetic efficiency decreases. The maximum COP obtained come out to be 1.715 at Tcs= 60oC, Tgs=59.52oC, Tg=80.38oC, Te=20oC and Tc=30oC, while the maximum exergetic efficiency will be 40.3% at Tcs=65oC, Tgs=64.42oC, Tg=100.92oC Te=5oC and Tc=30oC.

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