ENERGY AND EXERGY ANALYSES OF COMPRESSION, ABSORPTION AND COMBINED

CYCLE COOLING SYSTEMS

AKHILESH ARORA

Centre for Energy studies

Submitted in fulfillment of the requirements of the degree of DOCTOR OF PHILOSOPHY

to the

INDIAN INSTITUTE OF TECHNOLOGY, DELHI NEW DELHI-110016

OCTOBER, 2010

Dedicated to my wife Reena and

children Ksheerja & Tanishka

CERTIFICATE

This is to certify that the thesis entitled "ENERGY AND EXERGY ANALYSES OF

COMPRESSION, ABSORPTION AND COMBINED CYCLE COOLING SYSTEMS"

being submitted by Mr. Akhilesh Arora, to the Indian Institute of Technology, Delhi for the

award of degree of "Doctor of Philosophy" is a record of bonafide work carried by him under my

guidance and supervision. The research material and results contained in the present form of the

thesis have not been submitted in part or fully to any other University or Institute for the award

of any other degree or diploma to the best of my knowledge. He has carried out his research

work during the period July 2003 to June 2009 at Centre for Energy Studies, IIT Delhi.

Date: 24 June 2009 Prof. (Dr.) S. C. Kaushik Professor & Head Centre for Energy Studies Indian Institute of Technology New Delhi - 110016

Email : kaushik@ces. iitd. emet. in

ACKNOWLEDGEMENT

It is my privilege to express my deep gratitude and sincere thanks to thesis supervisor Dr. S. C.

Kaushik, Professor and Head, Centre for Energy Studies, IIT Delhi for his invaluable

guidance, painstaking efforts, dynamic supervision and deep interest in thesis without which this

thesis would not have reached the present state. I am highly indebted for his critical discussion

and continuous encouragement. His critics and suggestions have always guided me towards

perfection.

I would also like to express my deep gratitude to Dr. B. B. Arora, Assistant Professor,

Mechanical Engineering Department, Delhi college of Engineering, Delhi for his moral

support and help whenever needed. I thank from the bottom of my heart to my research

colleagues Mr. Paramjit Singh Bilga, Mr. Manoj Gupta, Amit Sharma and Dr. Vineet Tyagi for

making various technical discussions during the thesis writing.

My sincere thanks are due to my other research colleagues Mr. Manjunath, Miss Meenakshi and

Mr. Ram Naresh Tripathi, and Mr. Isidore Budding, Senior Technical Assistant, of the Solar

Thermal Science Laboratory at Centre for Energy Studies, I.I.T. Delhi for their motivation

during the period of Ph.D. Last but not the least, I must thank to my wife Dr. Reena Arora for her

timely support, inspiration, encouragement and help. Finally it is the Omnipresent Almighty

who has given me strength and courage to finish this task timely.

Date: 24 June 2009 Akhilesh Arora

ABSTRACT

Ozone layer depletion, global warming and prevailing energy crisis have significant

effect on refrigeration sector. This has necessitated the search for alternative refrigerants

and technologies which are not only energy efficient but also environmental friendly in

comparison to the existing ones. This thesis is an effort in this direction and hence the

energy and exergy analyses of various Vapour Compression Refrigeration systems,

Absorption Refrigeration systems and combined Compression Absorption systems have

been carried out to assess the alternative refrigeration and air-conditioning technologies.

The analysis of a Vapour Compression Refrigeration (VCR) system is

accomplished to identify alternative refrigerant for R502 among HCFC22, R404A, R717

and R507A. Based on the system performance under various operating conditions, it is

established that R717 is the best alternative refrigerant to R502. In a two stage VCR

system, the optimum inter-stage saturation temperature (pressure) computed

corresponding to maximum Coefficient of Performance (COP) and maximum exergetic

efficiency is found to be the same. Moreover, it is in the vicinity of geometric mean of

condensation and evaporation temperatures for an actual cycle for the refrigerants

HCFC22, R410A and R717.

The analysis of a Cascade Refrigeration system has been carried out to compute

the optimum cascade condenser coupling temperature for R717/R744 and R1270/R744

pairs. It is observed that maximum values of COP and exergetic efficiency occur

corresponding to identical values of cascade condenser coupling temperature. The effects

of various parameters have been investigated on system performance. It is established

that sub-cooling has maximum effect in improving the system performance in

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comparison to other parameters. The R717/R744 pair of refrigerants performs better than

R1270/R744.

The parametric analysis of water-lithium bromide Single Effect and Series Flow

Double Effect Generation Vapour Absorption Refrigeration (VAR) systems is carried out

and the effects of various parameters are investigated on system performance. It is

observed that the COP of Double Effect Generation VAR system is approximately double

in comparison to Single Effect system. The exergetic efficiency of a Double Effect

system is higher in comparison to Single Effect, if the same heat source is used. The

analysis of a Parallel Flow Double Effect VAR system has been carried out to compute

the optimum solution distribution ratio (corresponding to maximum COP and maximum

exergetic efficiency). It is observed that optimum solution distribution ratio (R0 ) lies

between 0.19 and 0.28 depending upon the HP generator temperature corresponding to

absorber and condenser temperatures equal to 29.4.°C. Increasing the absorber and

condenser temperatures increases the optimum solution distribution ratio (R0t).

The analysis of Series Flow Triple Effect VAR system is carried out to investigate

the effects of generator, absorber and evaporator temperatures. The COP and exergetic

efficiency of the Triple Effect VAR system increases with increase in HP generator

temperature at absorber temperatures between 25°C and 30°C. However, at absorber

temperatures above 35°C and up to 40°C, the COP and exergetic efficiency curves show

a reducing trend with increase in HP generator temperature.

The Half Effect VAR system has been investigated to find out the optimum

intermediate pressure. It is observed that maximum values of COP and exergetic

efficiency occur corresponding to identical optimum intermediate pressure. The effects of

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various parameters were also investigated to understand the variations in optimum

intermediate pressure and corresponding values of COP and exergetic efficiency.

In Absorption Recompression Refrigeration system, the effects of various

parameters such as generator, absorber and evaporator temperatures are investigated on

system performance. The performance of a Compression Absorption Refrigeration

(CAR) system is investigated and it is observed that COP varies between 5.69 and 7.5.

The parametric study of Compression Absorption Cascade Refrigeration (CACR)

system is carried out with VCR system in low temperature stage and single effect VAR

system in high temperature stage. The results reveal that there exist different generator

temperatures corresponding to which COP and exergetic efficiency are maximum. It is

observed that the performance of CACR system is better when ammonia is used in VCR

system in comparison to carbon dioxide.

The results provided in the thesis will facilitate the engineers and researchers in

improving the performance of refrigeration systems.

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CONTENTS

Certificate i

Acknowledgements ii

Abstract

Contents vi

List of Figures xiv

List of Tables xxvi

Nomenclature xxix

CHAPTER I INTRODUCTION

1.1 Objectives and Scope of the Thesis 4

1.2 Chapter-wise Presentation of the Thesis 6

CHAPTER II LITERATURE REVIEW

2.1 Vapour Compression Refrigeration Systems and Alternate 15 Refrigerants

2.1.1 Single Stage Vapour Compression Refrigeration 15 Systems

2.1.2 Multi-Stage Vapour Compression Refrigeration 22 Systems

2.1.3 Cascade Vapour Compression Refrigeration Systems 25

2.2 Vapour Absorption Refrigeration (VAR)Systems 28

2.2.1 Single and Double Effect Generation VAR Systems 28

2.2.2 Triple Effect Generation VAR System 31

2.2.3 Half Effect Generation VAR System 32

2.3 Compression Absorption Refrigeration (CAR) Systems 33

2.4 Conclusions of the Literature Review 33

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2.5 Objectives and Investigations of the Research

35

CHAPTER III

ENERGY AND EXERGY ANALYSES OF SINGLE AND TWO STAGE VAPOUR COMPRESSION REFRIGERATION SYSTEMS

3.1 Description of Vapour Compression Refrigeration (VCR) 38 System

3.2 Thermodynamic Analysis of the VCR System 40

3.2.1 Exergy Balance and Exergetic Efficiency 42

3.2.2 Non Dimensional Exergy Destruction 44

3.2.3 Efficiency Defect 45

3.2.4 Exergy Destruction Ratio (EDR) 45

3.3 Simulation Model 46

3.4 Results and Discussion 46

3.4.1 Influence of Operational Parameters on the System 47 Performance

3.4.2 Effect of Condenser Temperature 48

3.4.3 Effect of Variation in Evaporation Temperature 50

3.4.4 Efficiency Defects in System Components 53

3.4.5 Effect of Sub-Cooling in Condenser 56

3.4.6 Effect of Superheating in Evaporator 56

3.4.7 Effect of Effectiveness of Liquid Vapour Heat 56 Exchanger

3.4.8 Effect of Variation in Isentropic Efficiency of 59 Compressor

3.4.9 Effect of Pressure Drops in Evaporator and 61 Condenser

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CHAPTER IV

3.4.10 Effect of Performance Parameters on Efficiency 62 Defects in VCR System Components

3.5 Description of Two Stage VCR System 66

3.6 Thermodynamic Analysis of the Two Stage VCR Cycle 66

3.6.1 Mass Balance 66

3.6.2 Energy Balance 68

3.7 Simulation Model and Validation 69

3.8 Results and Discussion 69

3.8.1 Effect of Variation in Reduced Inter-Stage 71 Saturation Temperature (0)

3.8.2 Effect of Sub-Cooling, Superheating and 76 Compressor Efficiency on Optimum Inter-Stage Saturation Temperature

3.9 Conclusions 83

3.9.1 Single Stage VCR System 84

3.9.2 Two Stage VCR system 85

ENERGY AND EXERGY ANALYSES OF CASCADE VAPOUR COMPRESSION REFRIGERATION SYSTEM

4.1 Description of Cascade Refrigeration System 87

4.2 Thermodynamic Analysis of Cascade Refrigeration System 90

4.2.1 Mass Balance 89

4.2.2 Energy Balance 89

4.2.3 Exergy Balance 91

4.2.4 Exergetic Efficiency 92

4.2.5 Exergy Destruction Ratio (EDR) 93

4.3 Results and Discussion 93

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CHAPTER V

4.3.1 Effect of Cascade Condenser Temperature 96

4.3.2 Effect of Evaporator Temperature 97

4.3.3 Effect of Condenser Temperature 103

4.3.4 Effect of Approach (A) 107

4.3.5 Effect of Sub-Cooling of Refrigerant in Condenser 109 in 'htc'(ATsb) and Superheating of Refrigerant in Evaporator in `ltc'(ATsh)

4.4 Conclusions 116

ENERGY AND EXERGY ANALYSES OF SINGLE AND DOUBLE EFFECT GENERATION WATER- LiBr VAPOUR ABSORPTION COOLING SYSTEMS

5.1 Description of Single Effect and Series Flow Double Effect 120 Generation Water Lithium Bromide Vapour Absorption Refrigeration (VAR) Systems

5.2 Thermodynamic Analysis of Single and Series Flow Double 125 Effect Generation Water Lithium Bromide VAR Systems

5.2.1 Principle of Species Conservation 125

5.2.2 Mass and Species Conservation 125

5.2.3 Energy Balance of Single Effect Generation VAR 126 system

5.2.4 Exergy Balance of Single Effect Generation VAR 126 System

5.2.5 Energy Balance of Series Flow Double Effect VAR 127 System

5.2.6 Exergy Analysis of Series Flow Double Effect 128 Generation VAR System

5.2.7 Exergetic Efficiency 128

5.2.8 Efficiency Defect 129

5.3 Assumptions 129

5.4 Results and Discussion 130

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5.5

5.6

5.7

CHAPTER VI

6.1

6.2

5.4.1 Validation of the Simulation 132

5.4.2 Effect of Variation in Generator Temperature 135

5.4.3 Effect of Variation in Absorber Temperature 138

5.4.4 Effect of Difference in Absorber and Condenser 140 Temperatures

5.4.5 Effect of Variation in Evaporator Temperature 142

5.4.6 Effect of Effectiveness of Solution Heat 143 Exchangers

5.4.7 Effect of Pressure Drop Between Evaporator and 144 Absorber

5.4.8 Effect of Temperature Difference in Heat Source 146 and Generator on Exergetic Efficiency

5.4.9 Effect of Temperature Difference in Cold Room 147 and Evaporator on Exergetic Efficiency

5.4.10 Effect of Generator Temperature on Efficiency 148 Defects

Description of Parallel Flow Double Effect Generation VAR 151 System

Thermodynamic Analysis and Results 153

Conclusions 162

5.7.1 Single Effect and Series Flow Double Effect 162 Generation VAR Systems

5.7.2 Parallel Flow Double Effect Generation VAR 163 system

ENERGY AND EXERGY ANALYSES OF TRIPLE AND HALF EFFECT GENERATION WATER- LiBr VAPOUR ABSORPTION COOLING SYSTEMS

Description of Triple Effect Generation VAR System 166

Thermodynamic Analysis of Triple Effect Generation VAR 167 system

6.2.1 Mass Balance 168

x

6.2.2 Energy Balance 169

6.2.3 Exergy Balance 169

6.2.4 Exergetic Efficiency 170

6.3 Assumptions 170

6.4 Results And Discussion 171

6.4.1 Effect of HP Generator Temperature 174

6.4.2 Effect of Absorber Temperature 176

6.4.3 Effect of Evaporator Temperature 179

6.5 Description of Half Effect Generation VAR System 185

6.6 Thermodynamic Analysis of Half Effect Generation VAR 187 System

6.6.1 Mass Balance 187

6.6.2 Energy Balance 188

6.6.3 Exergy Balance 189

6.6.4 Exergetic Efficiency 190

6.7 Results and Discussion 190

6.8 Conclusions 204

6.8.1 Triple Effect Generation VAR System 204

6.8.2 Half Effect Generation VAR System 205

CHAPTER VII ENERGY AND EXERGY ANALYSES OF WATER- LITHIUM BROMIDE COMPRESSION ABSORPTION COOLING SYSTEMS

7.1 Description of the Absorption Recompression Refrigeration 209 System

7.2 Thermodynamic Analysis of the Absorption Recompression 209 System

7.2.1 Mass Balance 209

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7.2.2 Energy Balance 210

7.2.3 Exergy Balance 210

7.3 Results and Discussion 211

7.4 Description of Compression Absorption Refrigeration System 217

7.5 Thermodynamic Analysis of Compression Absorption 218 Refrigeration System

7.5.1 Mass Balance 218

7.5.2 Energy Balance 218

7.5.3 Exergy Balance 220

7.6 Results and Discussion 220

7.6.1 Effect of Variation in Generator Temperature 221

7.6.2 Effect of Variation in Absorber Temperature 227

7.6.3 Effect of Variation in Effectiveness of Solution 229 Heat Exchanger

7.7 Conclusions 230

7.7.1 Absorption Recompression Refrigeration System 230

7.7.2 Compression Absorption Refrigeration System 230

CHAPTER VIII ENERGY AND EXERGY ANALYSES OF COMPRESSION ABSORPTION CASCADE REFRIGERATION SYSTEM

8.1 Description of the Compression Absorption Cascade 233 Refrigeration (CACR) System

8.2 Thermodynamic Analysis of the CACR System 233

8.2.1 Mass Balance 234

8.2.2 Energy Balance 235

8.2.3 Exergy Balance 236

8.2.4 Exergetic Efficiency 236

xii

8.3 Results and Discussion 237

8.3.1 Effect of Cascade Condenser Temperature on COP 238 and Exergetic Efficiency

8.3.2 Effect of Generator Temperature 242

8.3.3 Effect of Effectiveness of Solution Heat 247 Exchanger, Approach in Cascade Condenser and Efficiency of Compressor in Low Temperature Stage

8.4 Conclusions 250

CHAPTER IX CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK

9.1 Overall Conclusions 252

9.2 Recommendations for Future Work 257

9.3 Relevant Research Publications 258

REFERENCES 261

APPENDIX A Engineering Equation Solver 270

APPENDIX B Thermodynamic Properties of LiBr-H20 Solutions 278

APPENDIX C Errors and uncertainties 282

APPENDIX D Exergy and its significance 290

BIODATA OF 295 THE AUTHOR