Comprehensive exergy analysis and optimization of operating parameters for double effect parallel flow absorption refrigeration Cycle

Journal Pre-proofs Comprehensive Exergy Analysis and Optimization of Operating Parameters for Double Effect Parallel Flow Absorption Refrigeration Cycles Md. Azhar, M. Altamush Siddiqui PII: DOI: Reference:

S2451-9049(19)30116-7 https://doi.org/10.1016/j.tsep.2019.100464 TSEP 100464

To appear in:

Thermal Science and Engineering Progress

Received Date: Revised Date: Accepted Date:

18 March 2019 7 December 2019 15 December 2019

Please cite this article as: Md. Azhar, M. Altamush Siddiqui, Comprehensive Exergy Analysis and Optimization of Operating Parameters for Double Effect Parallel Flow Absorption Refrigeration Cycles, Thermal Science and Engineering Progress (2019), doi: https://doi.org/10.1016/j.tsep.2019.100464

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Title: Comprehensive Exergy Analysis and Optimization of Operating Parameters for Double Effect Parallel Flow Absorption Refrigeration Cycles Md. Azhar* and M. Altamush Siddiqui Affiliation: Computational and Experimental Heat Transfer Research Laboratory Department of Mechanical Engineering, Z. H. College of Engineering and Technology Aligarh Muslim University, Aligarh-202002, Uttar Pradesh, India. *Corresponding author’s contact: Telephone: +91-7895698621, Email: [email protected] & [email protected]

Abstract: In this communication, comprehensive exergy analysis has been discussed for double effect parallel flow direct as well as indirect fired vapour absorption refrigeration systems using Lithium bromide-water as working fluid. The temperatures in the main generator and intermediate generator and condenser are optimized parametrically. The solution distribution ratio is also optimized considering exergy coefficient of performance (ECOP) and exergy destruction rate (EDR) as the objective functions. Further, comparison of parallel flow cycle with series flow configuration are presented for same operating parameters. Moreover, the effect of the temperature difference between intermediate generator and intermediate condenser of both the double effect parallel and series flow cycles are discussed. Results indicate that the maximum ECOP of the parallel flow cycle are 3-6% higher than the series flow cycle and minimum EDR is about 4% lower as compared to the series flow configuration. Furthermore, optimum temperature of intermediate generator for parallel flow cycle is significantly lower than the series flow cycle while main generator temperature is higher. Optimum parameters are tabulated for both series and parallel flow cycles, which will help to the design engineers.

1

Keywords: Exergy Analysis; Parallel Flow cycle; Series flow cycle; Direct Fired System; Indirect Fired System; Optimization NOMENCLATURE

PH1, PH2

preheaters

A

absorber

Q

heat transfer rate [kJ s-1]

C

main condenser

s

specific entropy [kW K-1]

C2

condenser at intermediate

T

temperature [oC or K]

Tcold

cooled space temperature

pressure COP

coefficient of performance [-]

TV

throttle valve

Cp

specific heat [kJ kg-1 K-1]

Wp

pump work [kW]

E

evaporator

Greek Symbols:

ECOP

exergy coefficient of

 

specific exergy

performance [-]

ɛ

effectiveness of heat

EDR

exergy destruction rate [kW]

exchanger

G

main generator

ɳp

G2

generator at intermediate

Subscripts:

pressure

pump efficiency

a

absorber

h

specific enthalpy [kJ kg-1]

c

main condenser

I

irreversibility induced [kW]

c2

condenser C2

LiBr

lithium bromide salt

e

evaporator

m

mass flow rate [kg s-1]

g

main generator

P

Pressure [kPa]

g2

generator G2

1. Introduction In the recent era, vapour absorption refrigeration systems have become very popular due to use of low-grade energy. Apart from this, the absorption system uses natural working fluid which save the environment. However, the performance of the system is low as compared to 2

the vapour compression system. Due to this, researchers in this area are very interested to improve the performance of vapour absorption systems by exposing different methods. Some of the them have developed multi effect cycles such as double effect, triple effect and fourth effect cycles. The fourth effect cycle will operate at very high generator temperature, usually above 315°C [1], which may increase the corrosion rate in the main generator. This is reason that the fourth effect cycle could not been commercialized yet. The double effect cycle has one generator-condenser set, while the triple and fourth effect cycles have two and three generator-condenser sets, respectively. The higher effect cycles require significantly high main generator temperature and also yield a high coefficient of performance as compared to the basic cycle [2]. But, such high temperature waste are generally not available due to this reason double effect cycles become advantageous. Azhar and Siddiqui [3] performed optimization of operating parameters of double effect series flow absorption cycle for maximum coefficient of performance (COP) and maximum ECOP. They have tabulate the optimum values such as main generator temperature, intermediate generator/condenser temperature and LiBr salt concentration of both the generators for maximum COP and ECOP. Bouaziz and Lounissi [4] carried out first and second law analyses of double effect hybrid absorption system and compared performance of this cycle with the conventional double effect cycle. They obtained that the proposed cycle operated at low value of Tg and about 25–32% higher COP with reduced exergy loss. Gebreslassie et al. [5] performed the both first and second law analyses of the half to triple effect cycles and concluded that the COP of the cycles increases from half to triple effect. While the exergetic efficiency is higher in the double effect cycle. Mohtaram et al. [6] carried out exergy analysis of single effect absorption cycle and reported that the largest exergy destruction occurred in the absorber. Yang et al. [7] carried out design analysis of cascade LiBr/H2O absorption refrigeration/transcritical CO2 process for 3

low-grade waste heat recovery (90–150°C) and reported the better performance than simple LiBr/H2O absorption. Some people have focused their work on the working fluids used in the absorption system and concluded that absorbent-refrigerant pairs are very important in the operation of the vapour absorption refrigeration system. Sun et al. [8] discussed different working fluids for absorption cycles and concluded that water is generally used as a refrigerant with different absorbents. Merkel et al. [9] performed an experimental analysis of absorption transformer using ionic liquid 1-ethyl-3-methyl-imidazolium methanesulfonate (EMIM OMs) as an absorbent with water being a refrigerant. Takalka et al. [10] performed thermodynamic analysis of the vapour absorption refrigeration system using a new absorbent as 1 Ethyl-3methylimidazolium ethyl sulphate (EMISE) with water as the refrigerant. Siddiqui [11] has carried out the performance of a single effect absorption system for heating and cooling purposes for two modes of operation: air conditioning mode and refrigeration mode. In the air conditioning mode, he considered Lithium Bromide-Water (LiBr-H2O) as the working fluid, while in refrigeration mode he has taken Ammonia-Water (NH3-H2O), Lithium NitrateAmmonia (LiNO3-NH3) and Sodium Thiocyanate-Ammonia (NaSCN-NH3 pairs) as the working fluids. He had shown that the LiBr-H2O pair has the highest COP and lowest fuel requirement as compared to the other pairs, while the LiNO3-NH3 pair has low cost and better performance in the refrigeration mode. Aman et al. [12] analyzed bubble-pump-driven diffusion absorption refrigeration system using LiBr-H2O and Lithium Chloride-Water (LiCl-H2O) as the working pair and found that the LiCl-H2O system has significantly higher performance and higher cooling load as compared to the LiBr-H2O system under the same operating conditions. Na et al. [13] presented the application of CaCl2–LiBr–LiNO3/H2O working pair in single effect absorption cycle. They have reported that this new working pair has 7.7 °C lower operating temperature

4

than simple than LiBr-H2O system. Kim et al. [14] carried out the performance analysis of double effect series flow absorption chiller using new working pairs such as LiBr+(HOCH2CH2)2NH+H2O, LiBr+H2N(CH2)2OH+H2O, and LiBr+HO(CH2)3OH+H2O. They calculated the cooling capacity and crystallization for these pairs and found that the pairs are suitable for air-cooled absorption chillers. Saravanan and Maiya [15] investigated the thermodynamic performance of different working pairs (including four binary mixtures, five ternary mixtures and seven quaternary mixtures) used in the absorption systems. They concluded that the LiCl-H2O pair is efficient in the view of cut-off temperature and circulation ratio while H2O-LiBr+LiCl +ZnCl2 pair is efficient in the view of energy COP and exergy COP. Moreover, the flow configuration of the solution in the absorption cycle also has an important role in the performance of the system, which has been investigated by several researchers. It has been shown that the parallel flow configuration has significantly higher COP as compared to the other flow configurations. Arun et al. [16] compared series and parallel flow double effect cycles and reported that the maximum COP of the parallel flow cycle is higher than that of the series flow cycle. Also, the parallel flow cycle is more sensitive to changes in the evaporator temperature and less sensitive to the condenser/absorber temperature. Xu and Dai [17] carried a performance analysis of the double effect parallel flow LiBr-H2O cycle in terms of efficiency of preheaters, circulation ratio and solution distribution ratio. They presented optimum conditions for maximum COP and found that the COP of the system increases with an increase in the efficiency of the heat exchanger, solution distribution ratio and decrease in solution circulation ratio. Oh et al. [18] developed simulation model for double effect parallel flow heat pump to analyze the characteristics of the system such as absorber inlet source temperature, solution concentrations, solution distribution ratio and exit temperature differences of the heat-

5

exchanging components. They presented the optimum operating conditions for the design of LiBr-H2O absorption heat pump. Riffat and Shankland [19] performed the first law analysis of the absorption cycle and that of an integrated absorption and compression cycle. The operating conditions of the continuous absorption cycles such as single, double effect series flow and double effect parallel flow cycles were discussed and it was reported that the parallel flow cycle has higher performance in both refrigeration and heating modes. Arora et al. [20] carried out energy and exergy analyses of the double effect parallel flow absorption cycle and presented the optimum solution distribution ratio for maximum energy coefficient of performance and maximum exergetic efficiency. Azhar and Siddiqui [21] also carried out energy optimization of the double effect parallel flow cycle. Konwar and Gogoi [22] carried out a performance analysis of the double effect cycle using LiCl-H2O pair and compared it with the LiBr-H2O solution. They considered double effect LiCl-H2O cycle with three different configurations: series, parallel and reverse, and obtained optimum temperatures at which the system performance was maximum. The studies carried out up to now in the literature on the double effect parallel flow cycle deals first law optimization of solution distribution ratio and main generator temperature. To the best of author knowledge the effect of intermediate generator and condenser temperatures have not yet been optimized in the literature from the view of second law of thermodynamics. Therefore, the present study aims at optimization of temperatures in both the generators as well as high pressure condenser for maximum ECOP and minimum EDR of the cycle. Also, the solution distribution ratio of parallel flow cycle is optimized for different evaporator and condenser/absorber temperatures. Furthermore, comparison of double effect-parallel flow cycle with series flow cycle is presented in detail. Moreover, most of the works in open literature have considered indirect fired heat sources. These sources are cheaper but their availability at the place where the absorption

6

systems are to be installed is very important. Otherwise, there will be a problem in its transportation and also fall in the energy level of the heat source. In such cases, the best option is to choose direct fired system. Therefore, the present study focusing the use of direct fired absorption system. Additionally, the present study highlights the optimum operating parameters for both the series and parallel flow cycles. 2. Description of the Cycles 2.1 Double Effect Parallel Flow Cycle Figure 1 shows the schematic arrangement of the double effect-parallel flow direct fired vapour absorption refrigeration cycle. The main components of the cycle are main generator, main condenser, evaporator, absorber and intermediate condenser-generator set. The main generator is operated by burning of either gases. While the intermediate generator is operated using the heat of condensation of the intermediate condenser. Initially, the diluted solution (shown in yellow colour) from the absorber is pushed to the preheater (PH1) through the solution pump and then divided to both in the main generator and in the intermediate generator, simultaneously. The ratio of the solution entering the main generator to the solution leaving the absorber is known as solution distribution ratio, represented by ‘Z’ in the present study. The remaining part of the solution that is (1-Z) flows to the intermediate generator which is controlled by the valve TV1. In both the generators, the solution boils and produces refrigerant vapour. The refrigerant from the main generator flows to the intermediate condenser while from the intermediate generator it goes to the main condenser. In the intermediate condenser, the refrigerant vapour condenses and the heat of condensation are used to operate the intermediate generator. The condensate refrigerant is then flows to the main condenser through a throttle valve (TV2) that lowers its temperature and pressure. The refrigerant form both the generators collected in the main condenser, where it get cooled by rejecting heat to the surrounding. 7

After condensation, the liquid refrigerant passes to the evaporator via the throttle valve (TV3) in order to produce the cooling effect. In contrast, the strong solution (violet and blue colours) from both the generators come back to the absorber for recycling of the working fluid. The throttle valves are used for pressure drop according to the requirement, while the preheaters are used to exchange the heat between the working fluid to improve the performance of the absorption system.

14 4a

Qg = mf Qgas

Condenser C2

15

3

8

Qg2

Main Condenser (C)

16

Generator G2

4c

PH2

2d

5

TV1

2c TV4

8a

2a

8d

TV3

PH1

Weak solution stream Main generator solution stream Generator G2 solution stream Mixture of G and G2 solution stream Refrigerant stream

6

Evaporator (E)

Qe

2b

8b 8c

17

13

Main Generator (G)

4b

Qc

Reactant

Combustion Chamber

4

TV2

Product

9

10 Absorber (A)

7 18

2

TV5

11

Qa

1

Wp

12

Figure 1. Schematic diagram of double effect-parallel flow vapour absorption refrigeration system 2.2 Double Effect Series Flow Cycle Figure 2 shows the schematic arrangement of the double effect-series flow direct fired vapour absorption refrigeration cycle. The operation of the series flow cycle is same as the parallel flow cycle except that the weak solution (shown in yellow colour) in the absorber is directly pumped to the main generator where the refrigerant vapour is formed. The remaining 8

strong solution (shown in blue colour) then goes to the intermediate generator, again some more refrigerant vapour is generated. The vapour refrigerant generated in both the generators is then goes to the main condenser where condensation proceeds and the refrigerant vapour becomes liquid. The liquid refrigerant passes through the throttle valve to reduce its pressure. Therefore low-pressure low-temperature refrigerant liquid flows to the evaporator for producing an efficient cooling effect. The strong solution (shown in violet colour) from the intermediate generator and the refrigerant vapour leaving from the evaporator, mix in the absorber. In this manner, the series flow cycle gets completed.

14 4a

Qg = mf Qgas

TV1

4c

3

8

Qg2

Main Condenser (C)

16

13

Main Generator (G)

4b

Qc

Reactant

Combustion Chamber

4 Condenser C2

15

Product

Generator G2

8b

TV3

8a

PH2

5

2a PH1

TV2

8c 9

Evaporator (E)

17

Qe

10 Absorber (A)

7 18

2

TV4

Weak solution stream Main generator solution stream Generator G2 solution stream Refrigerant stream

6

11

Qa

1

Wp

12

Figure 2. Schematic diagram of double effect-series flow vapour absorption refrigeration system

3. Mathematical Modeling 3.1 Thermodynamic modelling of Absorption Cycle The analyses of the double effect parallel and series flow direct fired vapour absorption cycles are based on the energy and exergy concepts. These analysis consists of mass, species, 9

energy and exergy balance at each component of the absorption cycles, which are shown below:

 m   m 

Mass balance:

in

Species balance:

out

Where,

(1)

dt

   mX    mX in

Energy balance:

dm system

d ( mX ) system

out

(2)

dt

d E system E in  E o u t  dt

(3)

 E  Q  W  mh

Exergy balance:

 in   out   destroyed 

d system

(4)

dt

Where  represent the total rate of exergy of the stream. Assuming no kinetic and potential energy effects in the streams then  is define as follows:   m   ( h  ho )  To ( s  s o ) 

For the case of steady state condition, the terms appear in the right-hand side of the equations (1) to (4) will be equal to zero. The mass and energy balance including the irreversibility induced in every component of the two cycles shown in the Figs. 1 and 2 are written as follows: Absorber:

7  m 10  m 11  m 1  m 12  0 m

(5)

 7 h7  m 10 h10  m 11h11  m 1h1  m 12 h12  0 m

(6)

m 7 7  m 1010  m 1111  m 11  m 1212  I gen , A  0

(7)

Solution pump:

Evaporator:

1  m 2  0 m

(8)

m 1h1  m 2 h2  W pump  0

(9)

m 11  m 2 2  I gen, P  0

(10)

6  m 17  m 7  m 18  0 m

(11)

 6 h6  m 17 h17  m  7 h7  m 18 h18  0 m

(12) 10

m 6 6  m 1717  m 7 7  m 1818  I gen , E  0

Main generator: m 3  m 13  m 4  m 8  m 14  0

(13) (14)

 3h3  m 13h13  m  4 h4  m  8h8  m 14 h14  0 m

(15)

m 3 3  m 1313  m 4 4  m 8 8  m 1414  I gen ,G  0

(16)

Main condenser:  4b  m  4c  m 15  m 5  m 16  0 m

(17)

 4b h4b  m  4c h4c  m 15h15  m  5h5  m 16 h16  0 m

(18)

m 4b 4b  m 4 c 4 c  m 1515  m 5 5  m 1616  I gen ,C  0

(19)

The equations which will be different for the parallel and the series flow cycles are written as follows: Parallel Flow Cycle (derived equations from Fig. 1): Intermediate Generator-Condenser set (G2-C2): 4  m  2d  m  4a  m  4c  m  8c  0 m

(20)

 4 h4  m  2d h2d  m  4a h4a  m  4c h4c  m  8c h8c  0 m

(21)

m 4 4  m 2 d 2 d  m 4 a 4 a  m 4 c 4 c  m 8c 8c  I gen ,Gs -Cs  0

(22)

Effectiveness of Preheaters: ε PH1 

T8 d  T9 T8 d  T 2

and

ε PH2 

T8  T8 a T8  T 2 b

(23) Solution distribution ratio: Z 

solution entering the primary generator m2b m2b   solution leaving the absorber m2 a m1

(24)

After mas and species balance the concentration of LiBr-salt solution at state point 8d: X 8d 

1.0 ( Z / X g )  (1.0  Z ) / X gs

; X8d  X9  X10

(25)

11

Mass flow rate: m1 

m5 X 8 d m X mX (1.0  Z ) ; m8c  2 c a ; m8  3 a ; m2 c  m1 X gs Xg ( X 8d  X a )

1  m 2  m  2a ; m  2b  m  3; m  2c  m  2d ; m 8  m  8a  m  8b m

(26) (27)

Series Flow Cycle (derived equations from Fig. 2): Intermediate Generator-Condenser set (G2-C2): 4  m  8b  m  4a  m  4c  m  8c  0 m

(28)

 4 h4  m  8b h8b  m  4a h4a  m  4c h4c  m  8c h8c  0 m

(29)

m 4 4  m 8b 8b  m 4 a 4 a  m 4 c 4 c  m 8c 8c  I gen ,Gs -Cs  0

(30)

Effectiveness of Preheaters: T8 c  T9 T8 c  T2

ε PH1 

Mass flow rate: m1  1  m 2  m  2a  m 3 ; m

;

ε PH2 

m5 X gs ( X gs  X a )

T8  T8 a T8  T 2 a

; m8c 

m8 X g X gs

(31)

; m8 

m3 X a Xg

8  m  8a  m  8b ; m  8c  m 9  m 10 m

(32) (33)

The second law performance of the systems are measured in terms of ECOP and EDR. ECOP 

 17  18   17  m Desired Exergy out   Required Exergy in m 13 ( 13   14 )  W p

(34)

EDR   destroyed  I gen

(35)

3.3 Assumption, solution technique and validation of the present work 3.3.1 Assumptions for present work: For simplification, simulation of the double effect-parallel and series flow direct fired cycles are made after several assumptions [3], which are as follows: 

All the processes in the systems are considered to be steady state.



The state of the condenser outlet is saturated liquid while the evaporator outlet is saturated vapour.

12



The state of the absorber outlet is called a weak solution at temperature Ta, which is equal to the main condenser temperature that is Ta=Tc.



Pressure losses will be considered as negligible during the flow of the working fluids.



No extra heat loss to and from the surrounding, other than the streams passing in the main components of the system.



The reference environmental state for the system is considered as temperature To=25oC and pressure Po=1 atm.



The system produces chilled water in the evaporator while heat rejects to cooling water at the main condenser/absorber.

3.3.2 Solution technique and validation for present work: Computer programs have been developed for the simulation of double effect-parallel and series flow cycles. The fixed operating parameters at which the systems operates are given in Table 1. The properties of pure refrigerant and refrigerant-absorbent mixture required to define the thermodynamic state points are evaluated from the available correlations [23-25] as subroutines in the main computer programs. The programs are written in FORTRAN 90. Figures 3 and 4 show flow chart for the simulation procedure of both series and parallel flow double effect cycles. The results of the present analysis were at first validated with the results of [19-20] at the similar operating conditions listed in Table 2, which are in good agreement with the work carried in the present analysis. This validates the present code developed for the double effect-parallel flow cycle. Also, The thermodynamic properties at the fixed operating condition for the parallel flow cycle shown in Table 3. Table 1. Fixed operating parameter of the present work [3] Load of the evaporator, Qe 300 kW Evaporator temperature, Te 4 to 10oC Absorber/main condenser temperature, Ta/Tc 30 to 39oC Temperature in the intermediate condenser 50 to 110oC with the increment of 1oC 13

Cooled space temperature, Tcold Heat exchangers efficiency, ɛ Pump efficiency, ɳp

Te+5.0 70% 85%

Table 2. Validation of the present work with Riffat & Shankland [1993] and Arora & Kaushik [2016] Rate of Heat transfer in the Operating Parameters (Double Effect-Parallel Flow) main components ( in kW) Qe=100kW,Tg=115oC Te=5oC, Tc=Ta=30oC, ɛ=0.7, ɳp=0.95, Tgs=70oC, Z=0.5 Ref. [19] Ref. [20] Present work Main Generator 71.2 70.81 71.38 Main Condenser 51.20 49.32 51.04 Evaporator 100.0 100.0 100.0 Absorber (A) 119.99 121.5 120.35 Pump work 0.0 0.0083 0.0108 COP 1.404 1.41 1.401

Table 3. Properties at different states points and performance parameters of parallel flow direct fired absorption refrigeration cycle (for Z=0.36) State Points 1 2 2a 2b 2c 2d 3 4 4a 4b 4c 5 6 7 8 8a 8b 8c 8d 9 10

T [oC] 33.0 33.017 60.11 60.11 60.11 60.11 102.94 149.99 79.0 33.0 75.92 33.0 4.0 4.0 149.99 87.77 87.77 75.92 81.33 47.51 47.51

Qg=223.59 kW Qc=153.89 kW

Enthalpy (kJkg-1) 0.008117 54.74 79.39 0.45424 54.74 79.43 0.45424 54.74 134.08 0.45424 54.74 134.08 0.45424 54.74 134.08 0.05034 54.74 134.08 0.45424 54.74 224.70 0.45424 ------ 2780.47 0.45424 ------ 331.08 0.05034 ------ 331.08 0.05034 ------ 2642.71 0.05034 ------ 138.40 0.008117 ------ 138.40 0.008117 ------ 2508.63 0.45424 66.74 359.25 0.45424 66.74 248.76 0.05034 66.74 248.76 0.05034 59.54 184.34 0.05034 61.93 205.86 0.05034 61.93 144.03 0.05034 61.93 144.03 Performance Parameters EDRg=200.90 kW QPH1=59.56 kW EDRPH1=2.854 kW QPH2=35.55 kW P [bar]

X (%)

14

Entropy (kJkg-1K-1) 0.02013 0.02014 0.375 0.375 0.375 0.375 0.6291 7.98 1.063 1.108 8.64 0.478 0.50 9.052 0.735 0.453 0.453 0.433 0.415 0.261 0.261

EDRc=2.039 kW, EDRPH2=2.579 kW EDRa=13.21 kW EDRC3-G2=3.551 kW EDRe=3.909 kW, EDRp=0.035 kW Total EDR of the system= 229.077kW

Qa=369.73 kW QC3=QG2=172.79kW Qe=300.00 kW Wp=0.0366 kW COP=1.3415, ECOP=7.32%

15

START

START

Initialize Te=4oC, Tc=30oC, Tc2=50oC, Qe=300 kW Considering Ta=Tc

Initialize Te=4oC, Tc=30oC, Tc2=50oC, Qe=300 kW Taking Ta=Tc

Is Te ≤ 13oC Tc ≤ 40oC Tc2 ≤ 110oC

No

Is Te ≤ 13oC Tc ≤ 40oC Tc2 ≤ 110oC

STOP

Yes

Yes

Assume Xg1=Xa+0.1 , and increases Xg1 by 10%

Xg2=Xg2+0.1

Assume Xg2=Xg1+0.1, and increases Xg2 by 10%

Calculate Tg=f(Tc2,Xg) and Tg2=f(Tc , Xg2) using concentration equation and then calculate specific enthalpy, density, specific heat of weak and strong solutions

Calculate Tg=f(Tc2,Xg) and Tg2=f(Tc , Xg2) using concentration equation Also calculate Sp. Enthalpy, Density, specific heat of weak/strong solutions

Calculate heat load at each components

Check Energy balance (Qc2 – Qg2)≤ 0.001 kW & Xg3 < Xc

No

No

STOP

Calculate Xa=f(Te,Ta) using concentration equation Calculate Sat. Pressure, density, Specific Enthalpy of refrigerant by using subroutines

Calculate Xa=f(Te,Ta) using concentration equation. Initialize Xg1=Xa+0.1 and Xg2=Xa+0.1 Calculate Saturation Pressure, density, Specific Enthalpy of refrigerant by using subroutines

Xg1=Xg1+0.1

No

Check Energy balance (Qc3 – Qg2)≤ 0.001 kW (Tc2–Tg2)<2oC and Xg3 < Xc

No Yes

Check Tg2
Yes

No

STOP

Again calculate heat load at each components Also calculate system performance Calculate heat load at each components Calculate system performance and all others parameters

Te increases by 2oC Tc increases by 3oC Tc2 increases by 1oC

Te increases by 2oC Tc increases by 3oC Tc2 increases by 1oC

Figure 3. Algorithm used in double effect-parallel flow cycle

Figure 4. Algorithm used in double effect-series flow cycle

16

4. Result and Discussion With the use of operating parameters given in Table 1, the calculations have been performed for both parallel and series flow direct as well as indirect fired double effect cycles. Then optimization of the decision variables (that is Tg, Tc2, Tg2 and Z) are done for the mentioned objective functions. 4.1 Exergy coefficient of performance Figure 5(a & b) shows the variation of energy exergy coefficient of performance (ECOP) with Tg for various temperatures in the evaporator and fixed temperatures in the main condenser or absorber (i.e. Tc=Ta=30oC) and condenser C2 as Tc2=70oC and 80oC. Here, the solution distribution ratio is kept as Z=0.36. The ECOP of the cycle is calculated with the consideration of inlet and outlet exergy of gases. Due to consideration of the combustion effect of the gases, the exergy COP is significantly lower as compared to the ECOP obtained at the fixed inlet and outlet generator temperature. Again, with an increase in the value of Tc2 from 70oC to 80oC, the ECOP decreases also requiring high Tg as seen in Fig. 5(b). The maximum ECOP at each Te is marked with circle. Same conclusions are obtained, that there is a need to optimize Tc2 (≈Tg2) for maximum ECOP. 0.08

Z=0.36, Tc=Ta=30oC, Tc2=70oC

0.075 0.07 0.065 0.06 0.055 0.05 Max. ECOP Te=4°C Te=6°C Te= 8°C Te=10°C

0.045 0.04 0.035 0.03 95

105 Main 115 Generator 125 135 Temperature, Tg in oC

Exergy Coefficient of Performance

Exergy Coefficient of Performance

0.08

Z=0.36, Tc=Ta=30oC, Tc2=80oC

0.075 0.07 0.065 0.06 0.055 0.05 0.045

Te=4°C Te=6°C Te= 8°C Te=10°C

0.04 0.035 0.03

145

95

Main Generator 105 115 125 135 145 155 165 Temperature, Tg in oC

(a) (b) Figure 5. Variation in ECOP of double effect parallel flow cycle with Tg at different Te

17

4.2 Optimization of operating temperatures for maximum ECOP As we know that the double effect absorption cycle having two generators, main and the intermediate generator, the temperature of both the generators will affect the performance of the absorption refrigeration system. The main generator is operated by means of some external heat source while the intermediate generator is operated by means of the heat of condensation from the intermediate condenser. Therefore, the temperature of the intermediate generator is related to the intermediate condenser. This is why optimization of temperatures for maximum ECOP is carried out for both, the main generator as well as the intermediate condenser and intermediate generator. The optimized temperatures at maximum ECOPs are thus called as optimum temperatures. The required optimization is performed in two steps for both the double effect-parallel as well as the series flow cycles. In the present work, the optimization process is presented only for the double effect-parallel flow cycle because optimization of the series flow cycle follows the same procedure. In the first step, a variation of ECOP with Tg are presented in Fig. 6 for different temperatures of Tc2 keeping the temperatures in the evaporator and main condenser/absorber (i.e. Te=4oC and Tc=Ta=30oC) as fixed at Z=0.36. From Fig. 6 it is observed that the ECOP increases with Tg and then either terminates due to certain limitations or become constant at higher values of Tc2. Also, with an increase in Tc2, the ECOP first increases reach to a maximum value and then decreases.

18

Exergy Coefficient of Performance

0.08

Z=0.36, Tc=Ta=30oC, Te=4oC

0.075 0.07 0.065 Max. ECOP Tc2=65°C Tc2=70°C Tc2=75°C Tc2=80°C Tc2=85°C Tc2=90°C

0.06 0.055 0.05 0.045 100

110

120

130

140

150

Main Generator Temperature, Tg in oC

160

170

Figure 6. Variation in ECOP of parallel flow cycle with Tg at different temperatures in the intermediate condenser

Similarly, in the 2nd step of optimization, the maximum ECOP from each plot for Tc2 in Figs. 7 (a & b) and the corresponding temperature of the main generator are noted. Now, these maximum values of ECOP are re-plotted with intermediate generator temperature in Fig. 7(a) and with main generator temperature in Fig. 7(b) for different values of evaporator temperature and fixed value of Tc=Ta=30oC at Z=0.36. It is seen that the maximum ECOP increases with increase in Tc2 as well as Tg reaches maxima and then decrease. Therefore, the temperature in the intermediate condenser ‘Tc2’ and main generator ‘Tg’ is optimized for optimum ECOP at each value of Te. The optimum points are marked with circles in Figs. 7(a & b) giving an optimum temperature for Tc2 and Tg. Since the intermediate generator drives through the intermediate condenser, the optimum temperature in the intermediate generator is also known which would be slightly lower than Tc2. Similar curves can also be drawn for other values of Tc=Ta.

19

Optimum ECOP Te=4°C Te=6°C Te=8°C Te=10°C

0.085 0.08 0.075

Z=0.36, Tc=Ta=30oC

0.07 0.065 0.06 0.055 0.05 0.045

Maximum Exergy Coefficient of Performance

Maximum Exergy Coefficient of Performance

0.09

0.09

Optimum ECOP Te=4°C Te=6°C Te=8°C Te=10°C

0.085 0.08 0.075

Z=0.36, Tc=Ta=30oC

0.07 0.065 0.06 0.055 0.05 0.045 0.04

0.04

80 100 120 140 Main Generator Temperature, Tg in oC

50 60 70 80 90 Intermediate Condenser Temperature, Tc2 in oC

160

(a) (b) Figure 7. Variation in maximum ECOP of parallel flow cycle with temperature Tc2 and Tg at different Te

The double effect-series flow cycle is then also optimized following the same procedure and the variation of maximum ECOP with Tc2 and Tg are shown in Fig. 8. The maximum of the maximum ECOP at each Te are marked with circles and the corresponding values of Tc2 and Tg, which are then called as optimum temperatures, are thus known. It is attention-grabbing that the maximum ECOP of the series flow cycle lie at a significantly higher value of Tc2 but at somewhat lower values of Tg as compared to the parallel flow cycle. Moreover, the optimum values of ECOP at which the system operates for the parallel flow cycle is almost 3-6% higher than the series flow

Tc=Ta=30oC

Maximum Exergy Coefficient of Performance

Maximum Exergy Coefficient of Performance

cycle.

0.075

0.065

0.055

0.045

0.035

Tc=Ta=30oC 0.075

0.065

0.055

0.045

0.035 50 60 70 80 90 100 110 Intermediate Condenser Temperatue, Tc2 in oC

70

(a)

80 90 100 110 120 130 140 150 160 Main GeneratorTemperatue, Tg in oC

(b)

20

Figure 8. Variation in maximum ECOP of series flow cycle with temperature Tc2 and Tg at different Te

4.3 Optimization of ‘Z’ for maximum ECOP in double effect parallel flow cycle Unlike the double effect-series flow cycle, the performance of the double effect-parallel flow cycle is also affected by the solution distribution ratio. In order to have an appropriate value of Z, optimization for maximum ECOP is again required. The ECOP is calculated for different values of Z and the optimization processes repeated as done earlier for a fixed value of Z=0.36. These maximum values of ECOP so obtained are plotted with Z in Fig. 9 for different Te and Tc=Ta. It is seen that with an increase in the solution distribution ratio ‘Z’ ECOP increases, reaches to a maximum and then decreases. Thus, Z gets optimized corresponding to maximum ECOP for the double effect-parallel flow cycle. It is also observed that with a change in the condenser or evaporator temperatures the optimum Z changes. However, the optimum value of Z lies between 0.32-0.40 in the entire temperature range under consideration. Therefore for simplicity, the value of Z is selected as 0.36 at which the results have been presented and discussed. Moreover, the system operator may choose the exact value of Z from Fig. 10 for the required temperatures in the

Exergy Coefficient of Performance

evaporator and condenser/absorber. 0.075

Tc=Ta=30oC

0.07 0.065 0.06 0.055 0.05 0.045

Te=4

Te=6

Te=8

Te=10

0.04 0

0.1

0.2 0.3 0.4 Solution Distribution Ratio, Z

0.5

0.6

Figure 9. Variation in maximum ECOP of the double effect parallel flow cycle with solution distribution ratio at different Te (fixed value of Tc=Ta=30oC)

21

Solution distribution ratio, Z

0.44

Tc=30°C Tc=33°C

0.42

Tc=36°C

0.4

Tc=39°C

0.38 0.36 0.34 0.32 0.3 4

6

8

Evaporator Temperature (in oC)

10

Figure 10. Optimum values of Z for maximum ECOP at different Te and Tc=Ta

4.4 Input power to the main generator Figure 11(a & b) show variation in the heat load (Qg) of the main generator with temperature Tg for both parallel and series flow cycles for different Te and fixed Tc=Ta=30oC. It is observed that with an increase in the main generator temperature the heat input Qg decreases, reaches to a same minimum value and then increases. The heat load is found to be minimum where the COP of both the cycles is maximum. Moreover, with increase in Te the minimum heat load Qg move to low values of Tg. Also, the heat input required in series flow cycle is approximately 5% higher as compared to the parallel flow cycle for the same temperatures in the main condenser and evaporator. But, the main generator temperature in the series flow cycle is lower than in the parallel flow cycle. Similar plots can also be shown for other values of Tc=Ta.

22

290 280

Te=4°C Te=6°C Te=8°C Te=10°C

270 260 250

Tc=Ta=30oC

350

Inlet power, Qg in kW

Inlet power, Qg in kW

370

Z=0.36,Tc=Ta=30oC

240 230 220 210

Te=4°C

330

Te=6°C Te=8°C

310

Te=10°C

290 270 250 230 210

200 190

190 80

90

100 110 120 130 140 150 160

Main Generator Temperature, Tg in

75

oC

85

95

105 115 125 135 145 155

Main Generator Temperature, Tg in oC

(a) Parallel Flow (b) Series Flow Figure 11. Variation in inlet power of double effect-parallel and series flow cycles with Tg at different Te

4.5 LiBr salt concentration Figure 12(a) shows LiBr-salt concentrations in the parallel flow double effect cycle for different evaporators temperature and fixed temperature of Tc=Ta=30oC and Tc2=80oC. There are four different concentrations in the parallel flow system, which is Xa for the absorber, Xg for the main generator, Xgs for the intermediate generator and X8d for the mixture of Xg and Xgs. It is observed that the concentration Xa in the absorber remains constant with increase in Tg. But with increase in Te, Xa decreases. Since Xa is a function of Te and Ta; at a fixed value of Ta, Xa will decrease with increase in Te as it is evident from the P-T-X diagram available in standard reference. However, with increase in Ta, it will increase if Te is fixed. It is also seen in Fig. 12(a) that with increase in Tg the concentration Xg increases and with increase in Te it overlaps extending towards high Tg. The concentration of LiBr-salt in the intermediate generator Xgs and that of the mixture that is X8d increase with Tg. However, with increase in evaporator temperature, they decrease in the magnitude following a similar trend. The crystallization limit for LiBr-salt concentration (Xc) is also plotted in Fig. 12(a) which far away from the concentration in the system. Similarly, the concentration of LiBr-salt in the series flow double effect cycle for different components are shown in Fig. 12(b). In the series flow cycle there are only three concentration 23

limits. One in the absorber Xa, another in the main generator Xg and the third in the intermediate generator Xgs. Almost, similar behaviour in the variation of Xg and Xgs, is observed as it was in the parallel flow cycle. However, in the series flow cycle, Xg is lower than Xgs as it is expected since the solution first goes to the main generator and then to the intermediate generator releasing refrigerant subsequently from the same solution. In the series flow cycle, the crystallization concentration Xc lies very close to Xgs; which needs special attention in the operation of the system. 80

Ta=Tc=30oC, Tc2=80oC, Z=0.36

75

Concentration of LiBr salt, X (%)

Concentration of LiBr salt, X (%)

80

Xc

70

Xg Te=4°C Te=6°C Te= 8°C Te=10°C

65 60

X8d Xgs

55 Xa

50

Ta=Tc=30oC Tc2= 80oC

75 70

Xc

65

Xgs

60

Xg

55

Xa

50 45

45 105

115

125

135

145

155

Main Generator Temperature, Tg in oC

105

165

115

125

MAin Generator Temperature, Tg in oC

(a) Parallel Flow (b) Series Flow Figure 12. Variation in LiBr salt concentration of double effect-parallel and series flow cycles with Tg at different Te

4.6 Exergy destruction rate The variation of EDR of the parallel and series flow cycles are plotted with the optimized value of Tg and shown in Fig. 13 for various Te and fixed Tc=Ta=30oC; the parallel flow cycle at Z=0.36. It is perceived that exergy destruction rate of both cycles decrease with increase in Tg, reach to a minimum value and then increase slightly. It is to be noted that the minimum value of exergy destruction rate lies nearly at the same value of Tg where the ECOP is maximum. With the increase in the evaporator temperature, the minimum EDR decrease and move towards low values of Tg. Similar curves can also be drawn for other values of Tc=Ta. It is found that the EDR of the parallel flow cycle is around 4% lower than that of the series flow cycle. But, with higher generator

24

temperature Tg in the parallel flow cycle. Also, the EDR of both cycles has relatively higher due to combustion effect considered in the main generator.

300 275

325

Z=0.36, Tc=Ta=30oC Te=4°C Te=6°C Te=8°C Te=10°C

250 225

Total exergy destruction rate ( kW)

Total exergy destruction rate,(kW)

325

200

Tc=Ta=30oC

300

275

250

225

200

80 90 100 110 120 130 140 150 Main Generator Temperature, Tg in oC

80

90

100

110

120

130

140

Main Generator Temperature, Tg in oC

150

(a) Parallel Flow (b) Series Flow Figure 13. Variation in EDR of the cycle with Tg for different Te and fixed value of Tc=Ta=30oC

The exergy destruction rate in each component of the parallel and the series flow cycles are also shown in Fig.14 for Te=4oC and Tc=Ta=30oC. The destruction in the main generator is very high as compared to the other components. This is because combustion of gases occurred inside the main generator. The present analysis considered the effect of exergy destruction rate when the combustion process executed. Therefore, the EDR of the main generator has been shown on the other axis (left side of Fig. 14). The EDR of the remaining components such as absorber, evaporator, condenser and throttle valve etc. are shown on the same axis that is the right side of the Fig. 14. It is observed from Fig. 14, the exergy destruction rate at each component of the parallel flow cycle is lower as compared to the series flow cycle except intermediate condenser and generator set. The EDR in the evaporator shows the same value in both cycles, due to same operating conditions. The EDR in throttle valves is low as compared to the other components. Also, the total EDR of parallel flow cycle is significantly lower as compared to the series flow cycle.

25

Tc=Ta

Z=0.36

Parallel

236

16

234

14

Series

232

12

Exergy Destruction Rate, kW

=30oC,

Exergy Destruction Rate of generator, kW

Te=4oC,

10 230 8 228 6 226

Main Generator

TV3

TV2

Double

PH2 PH1 Cs-Gs Effect Set Systems Components

Cond. (C)

Evap. (E)

4

224

2

222

0

Abs. (A)

Figure 14. Comparison of EDR for double-effect parallel and series flow cycle components

4.7 Effect of temperature difference of the condenser-generator set on ECOP and EDR The double effect cycle having two generators referring to Figs. 1 and 2. The main generator is operated through the outside heat source while the intermediate generator operated by the heat of condensation of the intermediate condenser. Therefore, it is necessary to keep the temperature difference between intermediate generator and intermediate condenser so that heat can be transferred smoothly. Figure 15(a) shows the effect of the temperature difference between Tc2 and Tg2 with ECOP of the system. With increase in Tc2 Tg2, maximum ECOP decrease keeping Te=4oC, Tc=Ta=30oC and Z=0.36 as fixed. Also, the parallel flow cycle has higher ECOP. Moreover, the reverse trend is observed in EDR, which are depicted in Fig. 15(b). With the increase in Tc2 Tg2 EDR increase. The performance parameters presented in Figs. 15(a & b) are the optimum values at fixed Te and Tc=Ta after optimization of all the decision variables. Therefore, the system should be operated with a minimum difference of temperature between Tc2 and Tg2. However, the temperature difference is necessary for proper heat exchange between them.

26

0.077

236

Te=4oC, Tc=Ta=30oC, Z=0.36 Parallel Series

Te=4oC, Tc=Ta=30oC, Z=0.36

Parallel Series

234

0.076

232

Minimum EDR

Maximum ECOP

230 0.075

0.074

228 226 224 222

0.073

220 0.072

1˚C

2˚C

3˚C

Tc2 Tg2

4˚C

218

5˚C

1˚C

2˚C

3˚C

Tc2 Tg2

4˚C

5˚C

Figure 15. Maximum ECOP and minimum EDR with difference in temperature between Tc2 and Tg2 for parallel and series flow cycles

4.7 ECOP and EDR of indirect fired system The double effect parallel flow indirect fired cycle is optimized in a similar manner as described above for the direct fired parallel flow cycle. The source temperature for indirect fired cycle is taken as Ts=Tg+13, which is generally adopted by most of the researchers. It is also discussed earlier in the present work that the source temperature plays a vital role in exergy performance (either ECOP or EDR) of the system. Therefore, after optimization of the intermediate condenser/generator temperatures, the variation of ECOP with Tg is shown in Fig. 16 (a & b) for double effect parallel flow cycle at different evaporator temperature. Similar trend is observed in Fig. 16(a) that ECOP initially increases, reach to maxima and then decrease. It is also noticed that the maximum ECOP obtained is at significantly lower Tg as obtained in case of direct fired cycle. The reason has been discussed by Azhar and Siddiqui [3]. It is also interesting to see that the ECOP of indirect fired parallel flow cycle is very high as compared to the direct fired parallel flow cycle, it is because direct fired cycle also accounts for the exergy loss which is due to the combustion of gaseous fuel. The maximum ECOP of both the direct and indirect fired double effect parallel flow cycles for different Te and Tc=Ta are also shown in Table 4. 27

Similarly, the variation of exergy destruction rate with Tg for double effect parallel flow cycles are shown in Fig. 16(b). The source temperature is again taken as Ts=Tg+13, the evaporator temperature varies from 4 to 10oC and the condenser/absorber temperature is 30oC. It is seen from Fig. 16(b), the EDR of the double effect parallel flow cycles show same trend as obtained for the series flow cycles. However, the operating generator temperature of parallel flow cycles are significantly higher as compared to the series flow cycles. Figure 16(b) shows the EDR of double effect parallel flow cycles which decrease with Tg, reach to minima and then increase. Moreover, the EDR of indirect fired cycles are very low as compared to the direct fired cycles; the reason is explained by Azhar and Siddiqui [3]. Also, the minimum EDR in case of indirect fired cycles are obtained at significantly lower generator temperature as compared to the direct fired cycles. The trend of EDR is almost reverse of ECOP and minimum EDR obtained in Fig. 16(b) correspond to maximum ECOP for both direct and indirect fired double effect vapour absorption refrigeration cycles, respectively. The minimum EDR of double and triple effect direct as well as indirect fired cycles are also shown in Table 6 for different values of Te and Tc=Ta. 52

Tc=Ta=30oC, Ts=Tg+13

0.34

Exergy destruction rate, kW

Exergy Coefficient of Performance

0.36

0.32 0.3 0.28 0.26 Te=4°C Te=6°C Te=8°C Te=10°C

0.24 0.22 80

85

90

95

100

105

110

115

47 42 37 32 27 22

120

Tc=Ta=30oC, Ts=Tg+13

Te=4°C Te=6°C Te=8°C Te=10°C

75Main 80 Generator 85 90 95 100 105 110 Main Generator Temperature (Tg in Temperature (Tg115 in 120 oC) oC) Figure 16 Variation of ECOP of double effect parallel flow indirect fired cycle with Tg for different Te

28

Table 4. Maximum ECOP and minimum EDR for double and triple effect parallel flow cycles Te (oC)

Ta (oC)

Tc (oC)

4 6 8 10 4 6 8 10 4 6 8 10 4 6 8 10

30

30

33

33

36

36

39

39

Exergy Coefficient of Performance (%) Direct Fired Indirect Fired

7.30 6.63 5.92 5.18 6.98 6.31 5.64 4.93 6.65 6.04 5.38 4.69 6.34 5.77 5.14 4.47

33.36 31.68 30.44 26.57 29.56 27.80 26.47 24.33 26.08 24.06 23.71 21.7 22.79 21.47 20.06 18.03

Exergy Destruction Rate (kW) Direct Fired Indirect Fired

220.40 215.21 210.10 205.10 226.22 220.92 215.97 210.94 232.09 226.77 221.93 216.90 239.54 233.05 227.61 222.73

33.75 32.14 30.39 28.07 35.34 33.90 32.97 30.77 38.50 37.79 34.12 31.22 43.76 40.51 36.82 34.98

4.8 Comparison between Series and Parallel Flow Direct as well as Indirect Fired Cycles The optimum operating parameters which influence the system performance in both the series and parallel flow cycles are optimized in the present work for direct as well as indirect fired systems. Figure 17 shows comparison of the double effect series and double effect parallel direct fired cycles for Te=4°C and 10°C, with Tc=Ta=30°C is fixed. Also, the plots in Fig. 17 are drawn after optimization of all the temperatures and SDR(s) in all the cycles. The ECOP of double effect parallel flow cycles are around 5% and 8% higher from its respective series flow cycles. Also, the maximum ECOP for each Te lie at significantly higher Tg as compared to the series flow cycle. It is therefore inferred from the present work that parallel flow cycles have significantly higher performance as compared to the series flow cycle, but they need relatively high heat source temperature.

29

Exergy coefficient of performance

0.09

Tc=Ta=30oC Double Te=4oC

0.08 0.07 0.06 0.05 0.04

Double Te=10oC

0.03 60

80

100

120

140

Main Generator Temperature (Tg in oC)

Figure 17 Variation of maximum ECOP of parallel and series flow double and triple effect direct fired cycles

Similarly, comparison of double effect indirect fired cycle is shown in Fig. 18 for both parallel and series flow configurations. The source temperature for all the four indirect fired cycles are considered as Ts= Tg+13. It is seen that in case of indirect fired cycle, the ECOP of double effect is slightly higher than the triple effect cycle, both for series and parallel flow configuration, which is unlike the result obtained in the direct fired cycle (refer Fig. 17). The ECOP of double effect cycle is around 4% higher than the triple effect cycle. Also, the ECOP of double effect parallel flow cycle is up to 11% higher than the double effect series flow cycle.

30

Exergy coefficient of performance

0.36

Tc=Ta=30oC

0.34

0.32

Double Te=4oC

0.3

0.28

0.26

Double Te=10oC

0.24

Parallel

0.22

Series6 0.2

75

85

95

105

115

125

Main Generator Temperature (Tg in oC)

Figure 18 Variation of maximum ECOP of parallel and series flow double and triple effect indirect fired cycles

5. Conclusion In the present work, the double effect-parallel flow and series flow direct fired absorption cycles have been simulated and investigated the optimum operating parameters. Optimization is performed considering maximum ECOP and minimum EDR as the objective functions. The comparison of both cycles are shown for different Te and Tc=Ta. Some following conclusion obtained from the results of the present work are:  The ECOP of the direct fired system is significantly lower due to the consideration of the combustion effect of the gases.  The variation in ECOP of the double effect-parallel flow cycle and series flow cycles with Tg show maxima, yielding optimum value of Tg.  Similarly, with the increase in the intermediate condenser/generator temperature, the ECOP again shows maxima giving the optimum value of Tg2 and Tc2.  With the increase in the solution distribution ratio, ECOP shows maxima resulting in optimum values of Z. 31

 The optimum value of Z for the range of the temperatures in the evaporator and condenser considered lie between 0.32 to 0.40. The results have been therefore presented and discussed only for Z=0.36. Also, the bar chart for each Te and Tc (under consideration of present work) is shown for the design engineers.  The variation of the generator heat load and total exergy destruction rate follow reverse trend that of the ECOP, all lying nearly at the same value of Tg.  The double effect direct fired parallel flow cycle has 3-6% higher ECOP, 3% lower generator heat load, 4% lower EDR as compared to the series flow cycle.  With increase in temperature difference (Tc2−Tg2), both the maximum ECOP and minimum EDR decreases. However, the parallel flow cycle is more affected than the series flow cycle.  In case of indirect fired (at Ts=Tg+13), the ECOP of double effect parallel flow cycle is up to 11% higher than the double effect series flow cycle.  The optimum operating parameters in case of indirect fired system lies relatively lower generator temperature than direct fired system for both series and parallel flow cycles. Finally, it is inferred from the present simulation that the double effect-parallel flow cycle yields higher ECOP, lower generator heat load, less exergy destruction rate as compared to the series flow cycle. Also, the direct fired system can be installed anywhere. Hence, double-effect parallel flow cycle is more efficient and economical in operation. It can be concluded from the present work for availability for high Tg the parallel flow configuration would be better.

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 

Energy and Exergy analysis of parallel flow double effect cycle is shown



35

Optimization is performed for best performance of the system. Compared the parallel flow with series flow cycle.



fact that all Authors listed on the title page

Effect of temperature difference between secondary generator & condenser is discussed

have contributed significantly to the work, have read the manuscript, attest to the

Conflicts of Interest Statement

validity and legitimacy of the data and its interpretation, and agree to its submission to

Manuscript title: “Comprehensive Energy

the

Journal

of

Thermal

Science

and

and Exergy Analyses of

Engineering Progress. Finally, we have no

Double Effect-Parallel and

conflict of interest.

Series Flow Direct Fired Absorption Cycles

Refrigeration

for

Optimum

Thanking You, Md. Azhar (Corresponding Author)

Operating Parameters”

Heat Transfer Research Laboratory Department of Mechanical Engineering, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh-202002 (India) (O) +91-571-2720335, (M) +917895698621 Email: [email protected]

This statement is to certify that all Authors have seen and approved the manuscript being submitted. We warrant that the article is the Authors' original work. We warrant that the article has not received prior publication and is not under consideration for publication elsewhere. On

behalf

corresponding

of

Author

Co-Author,

the

shall

full

bear

responsibility for the submission. This research

has

not

been

submitted

for

publication nor has it been published in whole or in part elsewhere. We attest to the

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