PARAMETRIC BEHAVIOR OF VAPOR ABSORPTION COOLING SYSTEM …€¦ · Key words: Solar, Vapour...

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International Journal of Mechanical Engineering and Technology (IJMET)

Volume 9, Issue 13, December 2018, pp. 1537–1548, Article ID: IJMET_09_13_155

Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=9&IType=13

ISSN Print: 0976-6340 and ISSN Online: 0976-6359

© IAEME Publication Scopus Indexed

PARAMETRIC BEHAVIOR OF VAPOR

ABSORPTION COOLING SYSTEM USING

H2O-LiBr AS WORKING FLUID FOR SOLAR

ENERGY APPLICATIONS

Arshi Banu P.S., Balaji Dhanapal, Mathevan Pillai T, Shaik Mohammed Shafee

Department of Mechanical Engineering,

Hindustan Institute of Technology and Science, Chennai, India

ABSTRACT

Vapor absorption refrigeration system is widely recognized as a prospective eco-

friendly technology for efficient and economic use of solar heat energy for cooling

applications. An intensive search for working fluid combinations(refrigerant-

absorbent) suitable for operating at temperature levels attainable with solar energy is

underway. H2O-LiBr as working fluid pair is found to be most viable option at

moderate temperatures for cooling applications using solar energy. However,

thermodynamic analysis needs to be performed to find the suitability of a working

fluid combination for a specific application. In the present work, detailed

thermodynamic analysis is performed on single-effect H2O-LiBr vapor absorption

refrigeration system of one ton capacity. Analysis is performed based on mass,

concentration and energy balance equations of each component of the system. The

influences of operating variables like generator, evaporator, condenser, absorber

temperatures and solution heat exchanger on performance parameters like Carnot

coefficients of performance (COPmax), Enthalpy based coefficients of performance

(COP), circulation ratio(CR) and efficiency ratio (η) are discussed using performance

plots. The effect of operating temperatures on thermal loads of each component also

discussed. Possible combinations of temperatures for optimum operation of the system

are identified. Operational limits are obtained for the system. This analysis can be

used as source of reference for comparison with new working fluid pairs.

Key words: Solar, Vapour absorption refrigeration system, Thermodynamic analysis.

Cite this Article: Arshi Banu P.S., Balaji Dhanapal, Mathevan Pillai T, Shaik

Mohammed Shafee, Parametric Behavior of Vapor Absorption Cooling System Using

H2O-LiBr as Working Fluid for Solar Energy Applications, International Journal of

Mechanical Engineering and Technology 9(13), 2018, pp. 1537–1548.

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=13



1. INTRODUCTION

Vapor absorption systems runs on low grade energy such as solar energy or waste heat from

industrial processes. Working fluid pairs used in the cycles are natural materials, which have

no ozone depleting and global warming potential. This system is more attractive in energy and

environment perspective. Even though it has the above advantages, performance of the vapor

absorption system is lesser compared to vapor compression system. Hence improving the

performance of absorption systems becomes the high research priority at present. Selecting

the suitable working fluids is the only important factor to improve the performance of

absorption systems. Thermodynamic analysis is the deciding tool to predict the performance

behavior of any absorption system. Thermodynamic analyses used to find the suitability of

working fluids for a specific application for specific range of temperatures [1]. Among the

Enormous working fluid pairs (refrigerant-absorbent combinations) available in literature [2],

H2O-LiBr and NH3-H2O are extensively used combinations. H2O-LiBr it is used in the

applications requiring refrigeration at temperatures above 00C. Hence these systems can be

used for air conditioning applications. However, ammonia-water systems are equally widely

used where subzero temperatures are required. Compared with refrigerant using NH3,

absorption cycles adopting H2O- LiBr show higher efficiency, lower pressure. The analysis of

H2O-LiBr system is relatively easy as the vapor generated in the generator is almost pure

refrigerant (water), unlike NH3-H2O systems where both ammonia and water vapor are

generated in the generator; therefore, no rectifying column is required [2]. Hence in the

present work H2O- LiBr chosen as working fluid for thermodynamic analysis for space

cooling applications.

Eisa [3] found thermodynamic design data for the H2O-LiBr absorption system for

cooling. Tabulated the various range of operating temperatures and corresponding

concentrations, flow ratio, Carnot coefficient of performance and enthalpy-based coefficient

of performance. Effect of operating temperatures on concentrations, flow ratio, Carnot

coefficient of performance and enthalpy-based coefficient of performance illustrated

graphically. Grossman [4] developed computer simulation code to simulate the performance

analysis of single-stage H2O-LiBr vapor absorption systems. He discussed the effect of

generator temperatures on coefficient of performance of above systems and showed them

graphically. H2O-LiBr thermodynamic potential compared with other fluid pairs of absorption

system. Da-Wen Sun [5] compiled thermodynamic properties for LiBr-H2O solutions and

used them in cycle simulation. Detailed thermodynamic design data and optimum design

maps are presented. They can be used in selecting operating conditions for existing systems

and achieving automatic control for maintaining optimum operation of the systems.

Saravanan [6] did thermodynamic analysis of H2O-LiBr vapor absorption systems along with

fifteen water based fluids and find the effect of generator, evaporator, condenser and absorber

temperatures on performance parameters like cut-off temperature, circulation ratio, and

coefficient of performance, efficiency ratio and heat exchanger effectiveness. He correlated

the performance parameters in terms of operating temperatures from the regression analysis.

Omer Kaynkali [7] performed detailed thermodynamic analysis of the H2O-LiBr absorption

refrigeration cycle. The influences of operating temperature and effectiveness of heat

exchanger on the thermal loads of components, coefficients of performance and efficiency

ratio are investigated. It is concluded that the COP values increase with increasing generator

and evaporator temperatures but decrease with increasing condenser and absorber

temperatures. Also, the effects of solution heat exchanger and refrigerant heat exchanger on

the performance and efficiency ratio of the system are compared. As a result, it is found that

the solution heat exchanger has more effect on the investigated parameters than the refrigerant

heat exchanger.

Parametric Behavior of Vapor Absorption Cooling System Using H2O-LiBr as Working Fluid for

Solar Energy Applications


2. WORKING OF H2O-LIBR VAPOUR ABSORPTION SYSTEM [8]

Figure.1 shows a vapor absorption refrigeration system. In this system, low temperature and

low pressure refrigerant with low quality enters the evaporator and vaporizes by producing

useful refrigeration Qe. From the evaporator, the low temperature, low pressure refrigerant

vapor enters the absorber where it comes in contact with a solution that is weak in refrigerant.

The weak solution absorbs the refrigerant and becomes strong in refrigerant. The heat of

absorption (Qa) is rejected to the external heat sink at Ta. The solution that is now rich in

refrigerant is pumped to high pressure using a solution pump and fed to the generator. In the

generator heat (Qg) at high temperature tg is supplied, as a result refrigerant vapor is generated

at high pressure. This high pressure vapor is then condensed in the condenser by rejecting

heat of condensation (Qc) to the external heat sink at Tc. The condensed refrigerant liquid is

then throttled in the expansion device and is then fed to the evaporator to complete the

refrigerant cycle. On the solution side, the hot, high-pressure solution that is weak in

refrigerant is throttled to the absorber pressure in the solution expansion valve and fed to the

absorber where it comes in contact with the refrigerant vapor from evaporator. Thus

continuous refrigeration is produced at evaporator at Te, while heat at high temperature (Tg) is

continuously supplied to the generator. Heat rejection to the external heat sink takes place at

absorber and condenser (Tc = Ta). The heat exchanger allows the solution from the absorber to

be preheated before entering the generator by using the heat from the hot solution leaving the

generator. A small amount of mechanical energy is required to run the solution pump. If we

neglect pressure drops, then the absorption system operates between the condenser and

evaporator pressures. Pressure in absorber is same as the pressure in evaporator and pressure

in generator is same as the pressure in condenser

Figure 1 Basic/Single effects vapour absorption refrigeration system

3. THERMODYNAMIC ANALYSIS

3.1. Importance of thermodynamic analysis [1][3]

Thermodynamic Analysis/First Law Analysis is performed due to following reasons:

To find the suitability of a refrigerant-absorbent combination for a specific application.

Thermodynamic property data obtained from analysis can be used to find the parameter range

of interest for particular application such as solar energy use.



The correlations are obtained between the operating temperatures, together with the

coefficients of performance and the flow ratios. This will help the process design engineer to

choose the equipment and their size, especially for the economizer heat exchanger.

For the same value of the coefficient of performance, the flow ratio will be different from one

working pair to another. Correlations can be developed for any working pair for which the

appropriate thermodynamic and thermo physical data are available.

It will provide the information on influence on performance parameters such as maximum

coefficients of performance, enthalpy based coefficients of performance, circulation ratio and

efficiency ratio due to changes in of operating conditions (temperature).

It will provide the information on possible combinations of temperatures and operational

limits of temperatures for the system.

It will be useful in comparison of the performance of various working fluids.

3.2. Method of Performing Thermodynamic Analysis: [9] [10]

Figure.1 shows the schematic of the system indicating various state points. A steady flow

analysis of the system is carried out with the following assumptions:

Steady state and steady flow

Changes in potential and kinetic energies across each component are negligible

No pressure drops due to friction

Only pure refrigerant boils in the generator in the condenser, the refrigerant condenses to a

saturated liquid, while in the evaporator, the refrigerant evaporates to a saturated vapour.

The solutions leaving the generator and absorber are at the same temperature and

concentration as in the generator and absorber, and they are in thermodynamic equilibrium.

Thermodynamic analysis can be performed using following procedure:

Finding the solution concentrations(X) of weak and strong solutions.

Finding the circulation ratio (CR) using solution concentrations.

Finding the state point enthalpies of pure refrigerant and solution.

Performing mass and energy balance of each component.

Finding the performance parameters.

Finding the solution concentrations(X) of weak and strong solutions:

Empirical equation of Dühring plot or p-T-X plots are used to find concentration(X) at

solution temperature (Ts) and refrigerant temperatures (Tr). [11]

∑ ∑

(1)

Finding the circulation ratio (CR) using solution concentrations

The circulation ratio is an essential design and optimization parameter. It can be defined in

two ways.

i) The circulation ratio (CR) is defined as the ratio of weak solution flow rate to

refrigerant flow rate.

a.

(2)

ii) The circulation ratio can also be calculated using weak and strong solution

concentrations.

(3)




Present analysis circulation ratio is calculated based on concentration.

Finding the state point enthalpies of pure refrigerant and solution

Enthalpies of refrigerant (water):

Water circuit (1-2-3-4) enthalpies can be found from property equations or in-built functions

of software of pure water.

State 1: Water is at super-heated steam state from generator at temperature (T1). Super-heated

steam enthalpy of water is given by equation:

h1=2052+1.667 (T1+273) (4)

State 2: Super-heated steam condensed to water at temperature (T2). Liquid water enthalpy

found from following equation:

h2=4.187 T2 (5)

State 3: State 2 to state 3 is throttling process or isenthalpic process in expansion valve.

h3 =h2 (6)

State 4: Liquid water extracts heat in evaporator and water vapor is formed.

h4 =Water vapor enthalpy at temperature (Te) and pressure (pe) from steam tables.

Enthalpies of solution:

Empirical equation of h-T-X diagram gives specific enthalpy values at different

concentrations(X) and solution temperatures (Ts) for water-lithium bromide solutions. [11]

∑ ∑

∑

(7)

Solution circuit (5-6-7-8) enthalpies h5, h8, h9, h10can be found from above equation in the

analysis.

State 5: T = Ta and X = Xw

∑

∑

∑

(8)

State 6: Neglecting pump work h6 =h5.

State 8: T = T8 and X = Xs

∑

∑

∑

(9)

State 9: T = T9 and X = Xs

∑

∑

∑

(10)

State 10: In solution expansion valve process is isenthalpic.

h10 = h9 (11)

Performing mass and energy balance of each component:

Condenser

Mass Balance:

(12)

Energy Balance:

( ) (13)

Refrigerant Expansion Valve

Mass Balance:

(14)

Energy Balance:



h2 = h3 (15)

(Isenthalpic process)

Evaporator

Mass Balance:

(16)

Energy Balance:

( ) (17)

Absorber:

Total mass balance:

(18)

(19)

( ) (20)

From mass balance for LiBr solution:

( ) ( ) (21)

(22)

Energy Balance:

[( ) ( )] (23)

Solution Pump

Mass balance:

(24)

Heat Balance:

h5 = h6 (25)

(Neglecting Pump work)

Solution Heat Exchanger

Mass Balance:

(26)

(27)

Energy Balance:

( )( ) ( )(28)

h7 can found from above energy balance equation.

Generator

Mass Balance:

=

(29)

Energy Balance:

[( ) ( )] (30)

Solution Expansion Valve:

Mass Balance:

(31)




Energy Balance:

h10 = h9 (32)

(Isenthalpic Process)

Hence heat interactions , , , in each component are calculated.

Finding the performance parameters:

Coefficient of performance (COP):

(33)

Enthalpy Based Coefficient of performance (COP):

( )

[( ) ( )] (34)

Maximum COP/ Carnot COP:

(

) (

) (35)

Efficiency ratio (or) Second law efficiency (η):

(

) (

) (

) (36)

4, VALIDATION

In order to validate the present model, the simulation results of present work have been

compared with the available numerical data in the literature. The comparative variation of the

COP value with generator temperature is given in Figure. 2. In this simulation, the following

values have been used.

5. FLOW CHART



Te = 60C, Tc = Ta = 30

0C without heat exchanger (HXe = 0). It can be seen that, as

expected, the COP value increases with increasing generator temperature, and the results

obtained from the present simulation are in good agreement with the results of Eisa [3],

Romero [13] and Kayankali [7]. Furthermore, at different operating conditions (Te =50C, Tc =

Ta = 350C, HXe = 0.7), the variations of the COP and CR values with generator temperatures

are given in Figure. 3. The results obtained from the present model (both COP and CR) are

also in good agreement with the results of Saravanan[6] and Kayankali [7].

6. RESULTS AND DISCUSSIONS

Various parametric plots have been drawn with the results obtained from analysis to predict

the behavior of the vapour absorption refrigeration system. In vapour absorption systems, the

coefficient of performance is a measure of the system's efficiency. The flow ratio determines

the size of the various items of equipment. With the increase in flow ratio, the load on the

solution heat exchanger, heat losses from the system and power required for the solution

pump will increase. Figure4-5 shows the variations of the coefficients of performance and

flow ratio with generator temperature. In the plotting = 50C and Ta =25, 30, 35, 40, 45and

50°C respectively considered. The coefficients of performance increase while the flow ratio

decreases as the generator temperature is increased. The values of the coefficients of

performance are higher at the lower absorber temperature. The rate of decrease in flow ratio is

much higher at higher absorber temperatures.

The influences of operating temperature (Ta, Tc, Te, Tg) on maximum coefficients of

performance (COPmax), enthalpy based coefficients of performance ( )and efficiency

ratio(η)are given in Figures. 6–9. For the simulation1ton capacity system is considered. In

these calculations, Te =5 0C, Tc = Ta = 35

0C, Tg = 90

0C were assumed apart from the

independent variable. The COPmax and COP value increases with generator and evaporator

temperatures (Figures. 6-7). As is seen from the Equation. (35), COPmax directly depends on T

e, Tg temperatures. Since the increase in COPmax is faster than that in COP, the η value

gradually decreases after reaching maximum value at 640C on increasing Tg. It is seen from

Figure. 8, when the temperatures of the condenser and absorber increase, the thermal load of

the generator rises, and the performance (COPmax and COP) of the system gets reduced. While

the η value increases till 470C then decreases due to the relatively rapid decrease of COP.

It is seen from Figure.9.COP value gradually increases from HXe = 0 to the best case

condition HXe = 1 where strong solution outlet temperature equals weak solution inlet

temperature. While the COPmax value does not change with the effectiveness and remains

constant. The effects of the generator, evaporator, condenser and absorber temperatures on the

thermal loads of the components are shown in Figures. 10–13. In these calculations, Te = 5 0C, Tc = Ta = 35

0C, Tg = 90

0C, and HXe = 0.7 were assumed. As it can be seen from

Figure.10, when the generator temperature (Tg) increases, the thermal loads of generator and

absorber (Qg and Qa) decrease. If the generator temperature gets higher, the concentration of

the solution leaving the generator increases, and hence, the CR decreases, as can be seen from

Eqs. (3). Moreover, the weak solution temperature and, hence, the enthalpy (h7) is increased

by the strong solution in the SHE. The generator thermal load is decreased both by decreasing

the CR and increasing h7. The enthalpy of the superheated water vapor (h1) leaving the

generator increases with increasing generator temperature. Condenser thermal load (Qc)

increases.

If the evaporator temperature rises, the concentration of the weak solution and the CR

decrease. They cause a decrease in the absorber thermal load; on the other hand, the

decreasing of CR decreases the generator thermal load (Figure. 11).




The high pressure of the system increases and the concentration of the strong solution

decreases when the condenser temperature increases. With decreasing strong solution

concentration, the CR increases, and in this case, the thermal loads of both the generator and

absorber increase (Figure. 12). The enthalpy of the saturated liquid (h2) leaving the condenser

increases with increasing condenser temperature. Thus, it causes a small amount of decrease

in the condenser and evaporator thermal loads.

By increasing the absorber temperature, the concentration of the weak solution approaches

the concentration of the strong solution, and the CR increases. Therefore, the thermal loads of

the generator and absorber increase (Figure. 13). However, the thermal load of the condenser

is not affected by the absorber temperature, and thermal loads remain unchanged.

Figure 2 Comparison of COP plots without solution HX Figure 3. Comparison of COP and CR plots with solution HX

Figure 4. Generator temperature (Tg) Vs COP

Figure 5.Generator temperature (Tg )Vs Circulation

ratio(CR)

0.50

0.55

0.60

0.65

0.70

0.75

55 60 65 70 75 80 85 90

CO

P

Generator Temerature (OC)

Kayankali

Romero

Present work

Eisa

5

10

15

20

25

30

35

40

45

0.64

0.66

0.68

0.7

0.72

0.74

0.76

0.78

0.8

70 75 80 85 90 95

CO

P


Kayankali-COP

Saravanan-COP

Present work-COP

Kayankali-CR

Saravanan-CR

Presnt work-CR



Figure 6. Variation of performance parameters with the

generator temperature


evaporator temperature

Figure 7. Variation of performance parameters with

the evaporator temperature


condenser/Absorber temperature

Figure 9. Variation of performance parameters with the Heat

exchanger effectiveness

Figure 9. Variation of performance parameters with

the Heat exchanger effectiveness

Figure10. Variation of thermal loads with the generator

temperature

Figure11. Variation of thermal loads with the evaporator

temperature

Figure12.Variation of thermal loads with the condenser

temperature

Figure13. Variation of thermal loads with the absorber

temperature

0.4

0.6

0.8

1

1.2

1.4

1.6

70 75 80 85 90 95 100


cop

copmax

𝛈𝐈𝐈

Te = 5oC Tc = 35oC Ta = 35oC

0

0.5

1

1.5

2

2.5

0 5 10 15 20 Evaorator Temerature (OC)

COP

COPmax

n

Tg = 90oC Tc = 35oC Ta = 35oC

0

0.5

1

1.5

2

2.5

0 2 4 6 8 10 12 14 16 18 20 Evaorator Temerature (OC)

COP COPmax n

Tg = 90oC Tc = 35oC Ta = 35oC

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

20 25 30 35 40 45 50 55

Condenser/ Absorber Temerature (OC)

COP

COPmax

n

Tg = 90oC

0.45

0.55

0.65

0.75

0.85

0.95

1.05

1.15

1.25

1.35

1.45

0 0.2 0.4 0.6 0.8 1HXe

COP

COPmax

n

0.45

0.55

0.65

0.75

0.85

0.95

1.05

1.15

1.25

1.35

1.45

0 0.2 0.4 0.6 0.8 1HXe

COP

COPmax

n

3

3.5

4

4.5

5

5.5

6

6.5

7

80 85 90 95 100 105 110

Ther

mal

Lo

ad


3

3.2

3.4

3.6

3.8

4

4.2

4.4

4.6

4.8

0 2 4 6 8 10 12

Ther

mal

Lo

ad

Evaorator Temerature (OC)

3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

25 30 35 40 45 50 55

Ther

mal

Lo

ad

Condenser Temerature (OC)

3

3.5

4

4.5

5

5.5

6

6.5

20 25 30 35 40 45 50 55

Ther

mal

Lo

ad

Absorber Temerature (OC)

qa

qc

qe

qg




7. CONCLUSIONS

Detailed thermodynamic analysis is performed on H2O-LiBr vapor absorption refrigeration

system of one-ton capacity.

Possible combinations and operational limits of temperatures for the H2O-LiBr system have

been identified as

20C<Te<18

0C

260C<Ta<50

0C

26 0C <Tc< 50

0C

70 0C <Tg<100

0C

The correlation between the generator temperatures with performance parameters help to

define the suitability of vapor absorption system for solar applications and also useful to

choose the suitable solar collector.

The correlation between the generator temperatures with the coefficients of performance and

the flow ratio presented in this work will help in the choice of equipment and their sizing,

especially for the solution heat exchanger.

The influences of operating temperature and effectiveness of heat exchanger on the

coefficients of performance and efficiency ratio are investigated. It is concluded that the COP

values increase with increasing generator and evaporator temperatures but decrease with

increasing condenser and absorber temperatures.

The influences of operating temperature on the thermal loads of components are investigated.

It is concluded that the generator thermal load and absorber thermal load values increase with

increasing condenser and absorber temperatures but decreases with increasing generator and

evaporator temperatures.

This analysis can be used as source of reference for comparison with new absorbent-

refrigerant pairs.

REFERENCES

[1] K Badarinarayana, S Srinivasa Murthy, M.V Krishna Murthy, Thermodynamic analysis of

R 21-DMF vapour absorption refrigeration systems for solar energy applications.

International Journal of Refrigeration, Volume 5, Issue 2, 1982, pp.115-119

[2] Jian Sun, Lin Fu, Shigang Zhang, A review of working fluids of absorption cycles.

Renewable and Sustainable Energy Reviews, 16, 2012, pp. 1899-1906

[3] M. A. R. Eisa, S. Devotta and F. A. Holland, Thermodynamic design data for absorption

heat pump systems operating on water-lithium bromide: Part I, Cooling. Journal of

Applied Energy, 24,1986 pp.287-301

[4] K. Gommed and G. Grossman, Performance analysis of staged absorption heat pump:

water-lithium bromide systems. ASHRAE Trans. 96, 1990, pp.1590-1598

[5] Da-Wen Sun, Thermodynamic Design Data and Optimum Design Maps for Absorption

Refrigeration Systems. Applied Thermal EngineeringVol. 17, No. 3,1997, pp. 211 -221

[6] R. Saravanan and M. P. Maiya, Thermodynamic comparison of water-based working fluid

combinations for a vapour absorption refrigeration system. Applied Thermal

Engineering.18,1998, pp.553-568



[7] Omer Kaynakli, Muhsin Kilic , Theoretical study on the effect of operating conditions on

performance of absorption refrigeration system. Energy Conversion and Management, 48,

2007, pp. 599-607

[8] http://nptel.ac.in/courses/Webcourse-contents/IIT Kharagpur/Ref and Air Cond/pdf/RAC

Lecture 14.pdf

[9] E. Keith, Herold, R. Radermacher, S.A. Klein, Absorption Chillers and Heat Pumps, CRC

Press, Boca Raton, FL, USA (1996)

[10] http://nptel.ac.in/courses/Webcourse-contents/IIT Kharagpur/Ref and Air Cond/pdf/RAC

Lecture 15.pdf

[11] https://www.ashrae.org/resources--publications/handbook

[12] http://www.fchart.com/ees/

[13] R.J. Romero, W. Rivera, J. Gracia, R. Best , Theoretical comparison of the performance of

an absorption heat pump system cooling and heating operating with an aqueous ternary

hydroxide and water/lithium bromide. Applied Thermal Engineering, 21,2007, pp. 1137-

1147

http://nptel.ac.in/courses/Webcourse-contents/IIT%20Kharagpur/Ref%20and%20Air%20Cond/pdf/RAC%20Lecture%2014.pdf




https://www.ashrae.org/resources--publications/handbook

http://www.fchart.com/ees/

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