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Candidates Declaration

I hereby certify that the work which is being presented in the thesis, entitled

Experimental and Numerical Investigation of Solar Powered Solid

Desiccant Dehumidifier for the award of the degree of Doctor of Philosophy

submitted in the department of Mechanical Engineering, National Institute of

Technology, Kurukshetra, is an authentic record of my own work carried out under the

supervision of Dr. V. K. Bajpai, Associate Professor, Department of Mechanical

Engineering, National Institute of Technology, Kurukshetra, India.

The matter presented in this thesis has not been submitted in part or in full for the award

of degree/diploma of this or any other University/Institute.

Date: Avadhesh Yadav


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Certificate

This is to certify that the thesis entitled Experimental and Numerical Investigation of

Solar Powered Solid Desiccant Dehumidifierbeing submitted by Avadhesh Yadav

(Registration number 2K08-NITK-PhD-1201-M) for the award of the degree of Doctor

of Philosophyis a record of bona fide research work carried out by him.

Mr. Avadhesh Yadav worked under my guidance and supervision and has fulfilled the

requirements for the submission of this thesis, which to my knowledge has reached the

requisite standard.

The results contained herein have not been submitted in part or in full, to any other

University or Institute for the award of any degree.

Date: Dr. V.K. Bajpai

Associate professor,

Department of Mechanical EngineeringNational Institute of Technology,

Kurukshetra-136119,

Haryana,

INDIA.


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Acknowledgements

First and foremost, a great deal of gratitude goes to my thesis supervisor Dr. V. K. Bajpai

Associate Professor, Mechanical Engineering Department, National Institute of

Technology, Kurukshetra for his continuous support, encouragement and keenness which

made this thesis possible. His painstaking efforts, methodical approach and individual

help made it possible for me to complete this work in time.

I express my feeling of thanks to Dr. Sudhir Saxena, Professor and Head of Mechanical

Engineering Department, for providing me all possible help to carry out my experiments.

Thanks are also due to Dr. S. S. Rattan, Dr. Dinesh Khanduja, Dr. Puneet Kumar, Dr. P.

K. Saini and Dr. Gulshan Sachdeva and all other faculty members of Mechanical

Engineering Department, National Institute of Technology, Kurukshetra for their help,

inspiration and moral support which went a long way in successful completion of my

thesis.

I also thank Mr. B. S. Saini, lab supervisor and all technical staff from workshop for

extending their help in the fabrication and successful installation of experimental setup.

I would also like to express my sincere gratitude to Mr. Deepak Pahwa (Chairman),

Desiccant Rotors International (DRI) India which provided desiccant wheel and Mr. H.S.

Chadha, (M.D.) Sunson Energy Devices (P) LTD, New Delhi, India which provided me

evacuated tube solar system that were among main parts of my experimental setup.

My ultimate gratefulness is for my parents and the rest of my family who have

enthusiastically supported all of my academic undertakings. I share this accomplishment

with them all. I am also thankful to my friends for their co-operation and valuable

suggestions.

Last but not least, I would like to thank almighty God for helping and guiding me during

my life and throughout my study.

Avadhesh Yadav


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better adsorbent as compared to activated alumina and activated charcoal for humid

climatic conditions of India.

Experimental studies have been carried out on solar powered desiccant wheel for

producing the dry air in Indian climatic conditions. The regeneration and adsorption

processes have been taken into account for this setup. The regeneration performance and

adsorption performance are affected by the regeneration temperature, rotational speed of

desiccant wheel, air flow rate and ambient conditions. Regeneration temperature directly

affects the effectiveness of the desiccant wheel.

Experimental results obtained from solar powered desiccant wheel for the moisture

removal process have been compared with simulation results of mathematical model at

same operating and design parameters. Simulation results are also validated with the

experimental data of Kodama PhD thesis (1995).

A mathematical model has been used to estimate the optimum design parameters of a

desiccant wheel for reducing its weight and size. This model has been used to conduct a

comparative performance analysis in both the directions of rotation (clockwise and

anticlockwise) of desiccant wheel with purge sector and it has been found that the

anticlockwise direction gave better results than clockwise direction for all the cases.

This model has also been used to compare the performances of two sector and four sector

desiccant wheels. It has been found that the maximum relative moisture removal

efficiency of both, the two sector and four sector is same but in two sector, it has been

obtained at twice the rph of a four sector. This model has also been used to analyze the

performance of two sector desiccant wheel with heated and cooled purge at different

regeneration temperatures and it has been found that at low rph, the desiccant system with

a higher purge angle and lower regeneration angle performed better as compared to a

lower purge angle and higher regeneration angle.

It has been concluded from the present work that the solar powered solid desiccant

dehumidifier operated well in hot and humid climatic conditions of India and can be a

viable alternative to the conventional heater. This system can also be effectively used for

industrial and domestic purposes.


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ContentsPage No.

Candidates Declaration ii

Certificate iii

Acknowledgements iv

Abstract v

Contents vii

L ist of F igures x

L ist of Tables xxi i

Nomenclature xxiv

CHAPTER 1 Introduction..1-5

CHAPTER 2 Literature Review 6-24

2.1 Solar Assisted Solid Desiccant Dehumidification System.. 7

2.2 Evacuated Tube Solar Collector.. 14

2.3 Mathematical Modeling of Desiccant Wheel.. 17

2.4 Research Gaps from Literature Survey 22

2.5 Objectives of the Present Work 232.6 Methodology Adopted . 23

CHAPTER 3 Experimental Studies on Evacuated Tube Solar Air Collector 25-51

3.1 Introduction. 25

3.2 Experimental Setup. 25

3.2.1 Evacuated tubes. 27

3.2.2 Header (heat exchanger) 28

3.2.3 Copper coil. 29

3.2.4 Reflectors 30

3.2.5 Working fluid. 30

3.3 Measuring Devices and Instruments 30

3.4 System Operation 31

3.4.1 Ordinary collector.. 31

3.4.2 Ordinary collector with reflectors.. 32

3.4.3 Ordinary collector with reflectors and copper coil. 32

3.5 Collector Performance Theory.. 32


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3.6 Experimental Results and Discussion 33

3.7 Conclusions 51

CHAPTER4 Experimental Comparison of Various Solid Desiccants for

Regeneration by Evacuated Tube Solar Air Collector and Air

Dehumidification52-71

4.1 Introduction 52

4.2 Experimental Setup 52

4.2.1 Evacuated tube solar air collector.. 55

4.2.2 Container55

4.3 Measuring Devices and Instruments.. 56

4.4 System Operation57

4.5 Analysis of Experimental Data. 58

4.6 Experimental Results and Discussion 58

4.7 Conclusions 70

CHAPTER 5 Experimental Studies on Solar Powered Desiccant Wheel...72-125

5.1 Introduction 72

5.2 Experimental Setup.... 72

5.2.1 Evacuated tube solar air collector...... 73

5.2.2 Desiccant wheel. 73

5.3 Measuring Devices and Instruments.. 76

5.4 System Operation76

5.5 Analysis of Experimental Data.. 78

5.6 Experimental Results and Discussion.79

5.6.1 Effect of different air flow rates 79

5.6.2 Effect of different rotational speeds.. 99

5.7 Conclusions 124

CHAPTER 6 Mathematical Modeling of Desiccant Wheel126-177

6.1 Introduction 126

6.2 Mathematical Model.. 126

6.2.1 Model assumptions 127

6.2.2 Mass conservation in control volume of air.. 129

6.2.3 Mass conservation in control volume of desiccant 131

6.2.4 Energy conservation in control volume of air134

6.2.5 Energy conservation in control volume of desiccant.... 136


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6.2.6 Auxiliary conditions.. 138

6.3 Solution Method. 141

6.4 Performance Index. 141

6.5 Mathematical Validation 142

6.5.1 Validation of the model by experimental results ..... 142

6.5.2 Validation of the model by experimental data given in

literature . 144

6.6 Analysis of Design Parameters to Improve the Performance of

Desiccant Wheel 146

6.6.1 Boundary and initial conditions 147

6.6.2 Solution method 147

6.6.3 Numerical results and discussion 147

6.7 Analysis of Desiccant Wheel with Purge Sector for Improving Its

Performance 154

6.7.1 Boundary and initial conditions 155

6.7.2 Solution method 156

6.7.3Numerical results and discussion. 156

6.8 Comparative Study between Four Sector and Two Sector of

Desiccant Wheel .. 164

6.8.1 Boundary and initial conditions.. 164

6.8.2 Solution method.. 165

6.8.3Numerical results and discussion 165

6.9 Analysis of Heated and Cooled Purge Sectors of a Desiccant

Wheel for Improving the Performance... 169

6.9.1 Boundary and initial conditions.. 170

6.9.2 Solution method.. 171

6.9.3 Numerical results and discussion 171

6.10 Conclusions.. 175

CHAPTER 7 Overall Conclusions and Recommendations 178-180

LIST OF PUBLICATIONS.. 181-182

REFERENCES..183-194

Appendix: Program flow charts ...195-199


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List of Figures

Figure Description Page No.

1.1 Increase in microbial growth in stored grain as a function

of relative humidity 2

3.1(a, b) Schematic diagrams of evacuated tube solar air collector based

on air heating system . 26

3.2(a) Experimental setup of evacuated tube solar air collector

with parallel flow .. 26

3.2(b) Experimental setup of evacuated tube solar air collector

with counter flow .. 273.3(a-c) Illustration of glass evacuated tube 28

3.4 Schematic diagram of the header (heat exchanger) 29

3.5 Schematic diagram of a copper coil in circular pipe of the header 29

3.6(a) Schematic diagram of ordinary collector 32

3.6(b) Schematic diagram of ordinary collector with reflectors... 32

3.6(c) Schematic diagram of ordinary collector with reflectors and

copper coil.. 32

3.7 Variation of temperature difference and solar radiation intensity

during the day at an air flow rate of 96.48 kg/hr in case of

parallel flow for ordinary

collector .. 33

3.8 Variation of thermal efficiency and solar radiation intensity



collector .. 34




collector . 35



parallel flow for ordinarycollector ..... 35



counter flow for ordinary

collector .. 36


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counter flow for ordinarycollector. 37









parallel flow for ordinarycollector with reflectors. 39



parallel flow for ordinarycollector with reflectors. 40



parallel flow for ordinarycollector with reflectors 41



parallel flow for ordinarycollector with reflectors 41



counter flow for ordinarycollector with reflectors. 42









counter flow for ordinary

collector with reflectors. 44


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parallel flow for ordinarycollector with reflectors & copper coil 45



parallel flow for ordinarycollector with reflectors & copper coil.46



parallel flow for ordinarycollector with reflectors & copper coil.47



parallel flow for ordinarycollector with reflectors & copper coil47



counter flow for ordinarycollector with reflectors & copper coil.48



counter flow for ordinarycollector with reflectors & copper coil49







4.1(a) Schematic diagram of experimental setup for regeneration of

silica gel, activated alumina or activated charcoal53

4.1(b) Experimental setup for regeneration of silica gel, activated

alumina or activated charcoal53

4.1(c) Schematic diagram of experimental setup for moisture adsorption

onto silica gel, activated alumina or activated charcoal54

4.1(d) Experimental setup for moisture adsorption onto silica gel,

activated alumina or activated charcoal54

4.2(a) Schematic diagram of the container 55

4.2(b) Photograph of the container.. 55


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4.3 Photograph of various solid desiccants 56

4.4(a) Psychrometric processes during sensible heating and regeneration.. 57

4.4(b) Psychrometric process during adsorption.. 57

4.5 Variation of temperature difference of air in evacuated tube solar

air collector and solar intensity for silica gel during the day time

at an air flow rate of 88 kg/hr.. 59

4.6 Variation of regeneration rate and regeneration temperature in

the regeneration process for silica gel during the day time at the

air flow rate of 88 kg/hr 59


air collector and solar intensity for activated alumina during the

day time at an air flow rate of 88 kg/hr 60

4.8 Variation of regeneration rate and regeneration temperature in the

regeneration process for activated alumina during the day time



air collector and solar intensity for activated charcoal during the

day time at an air flow rate of 88 kg/hr.. 61

4.10 Variation of regeneration rate and regeneration temperature in

the regeneration process for activated charcoal during the day time



air collector and solar intensity for silica gel during the day time

at an air flow rate of 138 kg/hr 62


regeneration process for silica gel during the day time at the air

flow rate of 138 kg/hr. 63


air collector and solar intensity for activated alumina during the

day time at an air flow rate of 138 kg/hr. 64


regeneration process for activated alumina during the day time

at an air flow rate of 138 kg/hr. 64


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air collector and solar intensity for activated charcoal during the

day time at an air flow rate of 138 kg/hr 65


regeneration process for activated charcoal during the day time

at an air flow rate of 138 kg/hr 66

4.17 Variation of the adsorption rate in the adsorption process for silica

gel during the evening time at an air flow rate of 88 kg/hr. 66

4.18 Variation of adsorption rate in the adsorption process for activated

alumina during the evening time at an air flow rate of 88 kg/hr. 67


charcoal during the evening time at an air flow rate of 88 kg/hr..67

4.20 Variation of adsorption rate in the adsorption process for silica gel

during the evening time at an air flow rate of 138 kg/hr68


alumina during the evening time at an air flow rate of 138 kg/hr..69


charcoal during the evening time at an air flow rate of 138 kg/hr... 69

5.1(a) Schematic diagram of the experimental setup (side view)72

5.1(b) Experimental setup of solar powered desiccant wheel ............ 73

5.2(a, b) Schematic diagrams of (a) rotary desiccant wheel, (b) cross section

of channels. 74

5.3(a) Photograph of the desiccant wheel... 75

5.3(b) Photograph of the driving system. 75

5.3(c) Schematic diagram of the desiccant wheel box.75

5.4 Psychrometric processes during sensible heating, regeneration and

adsorption. 77

5.5 Sketch of the desiccant dehumidification unit..77

5.6 Variation of temperature difference of air in evacuated tube solar air

collector and solar intensity during the day at an air flow rate of

105.394 kg/hr (m p = m r = 105.394 kg/hr).............. 81


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5.7 Variation of regeneration rate and regeneration temperature during

the day at an air flow rate of 105.394 kg/hr

(m p = m r = 105.394 kg/hr)..81

5.8 Variation of wheel effectiveness in regeneration sector duringthe day at an air flow rate of 105.394 kg/hr

(m p = m r = 105.394 kg/hr). 82

5.9 Variation of adsorption rate and regeneration temperature during


(m p = m r = 105.394 kg/hr). 83

5.10 Variation of wheel effectiveness in adsorption sector during

the day at an air flow rate of 105.394 kg/hr(m p = m r = 105.394 kg/hr). 84


collector and solar intensity during the day at an air flow rate

of 210.789 kg/hr (m p = m r = 210.789 kg/hr). 85



(m p = m r = 210.789 kg/hr) 865.13 Variation of wheel effectiveness in regeneration sector during the

day at an air flow rate of 210.789 kg/hr

(m p = m r = 210.789 kg/hr) 87



(m p = m r = 210.789 kg/hr). 88

5.15 Variation of wheel effectiveness in adsorption sector during theday at an air flow rate of 210.789 kg/hr

(m p = m r = 210.789 kg/hr). 89



210.789 kg/hr (m p = 105.394 kg/hr, m r = 210.789 kg/hr). 90


the day at an air flow rate of 210.789 kg/hr(m p = 105.394 kg/hr, m r = 210.789 kg/hr). 91


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5.18 Variation of wheel effectiveness in regeneration sector during the


(m p = 105.394 kg/hr, m r = 210.789 kg/hr).. 92

5.19 Variation of adsorption rate and regeneration temperature duringthe day at an air flow rate of 105.394 kg/hr

(m p = 105.394 kg/hr, m r = 210.789 kg/hr)...93

5.20 Variation of wheel effectiveness in adsorption sector during the


(m p = 105.394 kg/hr, m r = 210.789 kg/hr)...94


collector and solar intensity during the day at an air flow rate of105.394 kg/hr (m p = 210.789 kg/hr, m r = 105.394 kg/hr)... 95



(m p = 210.789 kg/hr, m r = 105.394 kg/hr). 96

5.23 Variation of wheel effectiveness in regeneration sector during the


(m p = 210.789 kg/hr, m r = 105.394 kg/hr). 975.24 Variation of adsorption rate and regeneration temperature during


(m p = 210.789 kg/hr, m r = 105.394 kg/hr).. 98

5.25 Variation of wheel effectiveness in adsorption sector during the day

at an air flow rate of 210.789 kg/hr

(m p = 210.789 kg/hr, m r = 105.394 kg/hr).. 98

5.26 Variation of temperature difference of air in evacuated tube solar aircollector and solar intensity during the day at an air flow rate of

210.789 kg/hr at13 rph 101


the day at an air flow rate of 210.789 kg/hr at 13 rph...102

5.28 Variation of wheel effectiveness and regeneration temperature in

regeneration sector during the day at an air flow rate of

210.789 kg/hr at 13 rph..103


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5.29 Variation of adsorption rate and regeneration temperature during the

day at an air flow rate of 210.789 kg/hr at 13 rph. 104


adsorption sector during the day at an air flow rate

of 210.789 kg/hr at 13 rph..105



of 210.789 kg/hr at 16 rph. 107


the day at an air flow rate of 210.789 kg/hr at 16 rph107



210.789 kg/hr at 16 rph..108

5.34 Variation of adsorption rate and regeneration temperature during the

day at an air flow rate of 210.789 kg/hr at 16 rph... 109


adsorption sector during the day at an air flow rate of

210.789 kg/hr at 16 rph.. 110



210.789 kg/hr at 19 rph.............. 111


the day at an air flow rate of 210.789 kg/hr at 19 rph........... 112



210.789 kg/hr at 19 rph.113


the day at an air flow rate of 210.789 kg/hr at 19 rph113



of 210.789 kg/hr at 19 rph.114



210.789 kg/hr at 22 rph 116


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the day at an air flow rate of 210.789 kg/hr at 22 rph.. 116



210.789 kg/hr at 22 rph.117


the day at an air flow rate of 210.789 kg/hr at 22 rph.. 118


adsorption sector during the day at an air flow rate of

210.789 kg/hr at 22 rph.................................................................119



of 210.789 kg/hr at 9 rph.. 120


the day at an air flow rate of 210.789 kg/hr at 9 rph. 121


regeneration sector during the day at an air flow rate

of 210.789 kg/hr at 9 rph.. 122


the day at an air flow rate of 210.789 kg/hr at 9 rph.123



of 210.789 kg/hr at 9 rph.. 123

6.1(a) Schematic diagram of rotary desiccant wheel. 126

6.1(b) Schematic diagram of cross section of channels. 127

6.1(c) Schematic diagram of differential control volume.. 127

6.2 Control volume of air for mass conservation... 130

6.3 Control volume of desiccant for mass conservation 131

6.4 Control volume of air for energy conservation 134

6.5 Control volume of desiccant for energy conservation.. 136

6.6 Moisture removal during the day: comparison between experimental

results and simulation results for different working conditions (up,in =

4 m/s, ur,in = 4 m/s, p/r = 1, Lw = 0.1 m, N = 22 rph). 142


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6.7 Moisture removal during the day: comparison between experimental

results and simulation results for different working conditions (up,in =

2 m/s, ur,in = 4 m/s , p/r = 1, Lw = 0.1 m, N = 22 rph)143

6.8 Variation of Yp,out/Yp,in with rotational speed of desiccant wheel:

comparison between experimental data and simulation results for

different working conditions(up,in = ur,in = 1 m/s, Lw = 0.2 m)..144

6.9(a, b) Schematic diagrams of (a) rotary desiccant wheel, (b) front view of

desiccant wheel. 146

6.10 Effect of length of wheel on moisture removal and pressure drop149

6.11 Effect of area ratio on moisture removal.....149

6.12 Effect of aspect ratio on moisture removal and pressure drop.. 1506.13 Effect of pitch of flow passage on moisture removal and

pressure drop 151

6.14 Effect of height of flow passage on moisture removal and

pressure drop.... 152

6.15 Effect of porosity on moisture removal........ 152

6.16 Effect of volume ratio in desiccant layer of channel on

moisture removal.. 153

6.17(a) Schematic diagram of rotary desiccant wheel with purge sector. 154

6.17(b) Schematic diagram of desiccant wheel configuration

with purge sector . 154

6.17(c) Schematic diagram of front view of desiccant wheel with purge

sector......................................................... 154

6.18(a, b) Schematic diagrams of (a) desiccant wheel with purge sector for

clockwise rotation, (b) desiccant wheel with purge sector for

anticlockwise rotation.. 155

6.19 Variation of relative moisture removal efficiency with rotational

speed of desiccant wheel for clockwise and anticlockwise

direction 157

6.20 Variation of temperature difference of process air with rotational

speed of desiccant wheel for clockwise and anticlockwise

direction 158


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6.21 Variation of relative moisture removal efficiency with regeneration

temperature for clockwise and anticlockwise direction... 159

6.22 Variation of temperature difference of process air with regeneration

temperature for clockwise and anticlockwise direction... 160

6.23 Variation of relative moisture removal efficiency with velocity

for clockwise and anticlockwise direction....... 161

6.24 Variation of temperature difference of process air with velocity

for clockwise and anticlockwise direction 161

6.25 Variation of relative moisture removal efficiency with humidity

ratio for clockwise and anticlockwise direction... 162

6.26 Variation of temperature difference of process air with humidity

ratio for clockwise and anticlockwise direction163

6.27(a, b) Schematic diagrams of (a) desiccant wheel with two sectors,

(b) desiccant wheel with four sectors... 164

6.28 Variation of relative moisture removal efficiency and temperature

difference of process air with rotational speed of desiccant wheel

for two sector (PS: RS=180:180) and four sector

(PS1:RS1:PS2:RS2= 90:90:90:90). 166




(PS1:RS1:PS2:RS2= 100:80:100:80)..167




(PS1:RS1:PS2:RS2= 110:70:110:70). 169

6.31(a, b) Schematic diagrams of (a) rotary desiccant wheel with cooled purge

and heated purge for clockwise direction, (b) desiccant wheel

configuration with cooled purge and heated purge ..170

6.32 Variation of relative moisture removal efficiency with rotational speed

of desiccant wheel with two sectors having low purge angle at

different regeneration temperatures.. 172


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6.33 Variation of temperature difference of process air with rotational

speed of desiccant wheel with two sectors having low purge angle at


6.34 Variation of relative moisture removal efficiency with rotational speed

of desiccant wheel with two sectors having high purge angle at


6.35 Variation of temperature difference of process air with rotational speed

of desiccant wheel with two sectors having high purge angle at

different regeneration temperatures...175


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6.2 Obtained ambient humidity, ambient temperature, regeneration

temperature (outlet temperature of air from evacuated tube solar air

collector) and solar intensityobtained on 02/10/2011

(up,in

= 2 m/s, ur,in

= 4 m/s, N = 22 rph).. 143

6.3 Input data used for comparison between simulation and experimental

results.144

6.4 Pressure drop: comparison between experimental data and simulation

results 145

6.5 Design parameters of the desiccant wheel 148

6.6 Operating and structural parameters for optimization of design

parameters. 148

6.7 Operating and structural parameters for desiccant wheel with purge

sector 156

6.8 Operating parametersfor the case of two sector and four sector

desiccant wheel 165

6.9 Operating parametersfor the case of desiccant wheel with heated

and cooled purge 172


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Nomenclature

Symbol Description Units

Notation

A cross sectional area (m2)

Af cross sectional area of flow passage of one channel (m2)

Ar area ratio of air flow passage to the total area of

one channel Dimensionless

At total cross-sectional area of one channel (m2)

AP area of evacuated tube solar air collector (m2)

cd specific heat of silica gel (J/kg K)

cm specific heat of matrix material (J/kg K)

cp specific heat at constant pressure (J/kg K)

dE diameter of absorber tube (m)

dpipe diameter of pipe (m)

D diameter of wheel (m)

Dcomb combined diffusivity including ordinary

and Knudsen diffusivity (m2/s)

Dh hydraulic diameter of flow passage of one channel (m)

Dk Knudsen diffusivity (m2/s)

Dm mass diffusion coefficient of vapour in the air Dimensionless

Do ordinary diffusivity (molecular diffusivity) (m2/s)

DS surface diffusivity (m2/s)

EA effectiveness of wheel in adsorption sector Dimensionless

ER effectiveness of wheel in regeneration sector Dimensionless

f friction factor Dimensionless

GA adsorption rate (kg/hr)

GR regeneration rate (kg/hr)

Gz Graetz number Dimensionless

h convective heat transfer coefficient (W/m2K)

hads heat of adsorption (J/kgadsorbate )


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hfg latent heat of water vapour (J/kg)

hm convective mass transfer coefficient (kg/m2s)

I0 solar intensity (W/m2)

k thermal conductivity (W/m K)Le Lewis number Dimensionless

LE length of evacuated tube (m)

Lw wheel length (m)

m mass (kg)

m mass flow rate (kg/s)

m c air flow rate of evacuated tube solar collector (kg/hr)

m p air flow rate of process air (kg/hr)M molecular weight of water (kg/mol)

Mr moisture removal (kgwater vapour /kgdry air )

m r air flow rate of regeneration air (kg/hr)

N rotational speed (rph)

N rate of mass transfer (kg/s)

Nu Nusselt number Dimensionless

NuFd Nusselt number for fully developed region Dimensionless

P pressure (Pa)

Pa atmospheric pressure (Pa)

P pressure drop (Pa)

Pe perimeter of air flow passage of one channel (m)

Pr Prandtl number Dimensionless

rate of flow energy due to advection (J/s)

q rate of energy transfer (J/s)

r pore radius (m)

RH relative humidity Dimensionless

Re Reynold number Dimensionless

Sh Sherwood number Dimensionless

t time (s)

T temperature ()

Tin inlet temperature of air at evacuated tubesolar air collector ()


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Tout outlet temperature of air from evacuated tube

solar air collector ()

u velocity (m/s)

V volume (m

3

)W water content of desiccant (kgadsorbate /kgadsorbent )

x axial direction

Y humidity ratio (kgwater vapour /kgdry air )

Greek symbols

etc solar collector efficiency Dimensionless

p sector angle of process air (degree)

r sector angle of regeneration air (degree)

density (kg/m3)

porosity Dimensionless

tortuosity factor Dimensionless

aspect ratio (ratio of height to pitch) for one channel Dimensionless

volume ratio of desiccant material in layer Dimensionless

dynamic viscosity (Pa s)

sector angle (degree)

purge sector angle of purge air (degree)

thickness of channel wall (m)

relative moisture removal efficiency Dimensionless

Subscripts

a air

comb combined

cp cooled purge

d desiccant

da dry air

hp heated purge


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in inlet

m matrix material

0 initial state

out outlet

p process air

r regeneration air

sur surface

v water vapour

w water

Abbreviations

COP coefficient of performance

ETC evacuated tube collector

EXP experimental

GI galvanized iron

MIN minimum

NTU number of transfer unit

OPT optimum

PDE partial differential equation

PS process sector

RS regeneration sector

SIM simulation

VC vapour compression


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1

CHAPTER 1

Introduction

Dry air plays a vital role for improving the process, product or conditions in many

industries such as food production, pharmaceutical production, industrial chemicals

production etc. It is also required in warehouse storage, packaging equipment rooms,

hygroscopic raw materials storage, organic plant dehydration and inorganic products.

Typical conditions for different applications have been defined by Bry Air Asia Pvt Ltd.

India as shown in Table 1.1.

Table 1.1Typical application standards

Typical conditions

Applications Temperature

()Relative humidity

(%)

Humidity ratio

(kgwater vapour/kgdry air)

Sugar storage 26.66 35 0.0076

Cookie drying 18.33 20 0.0026

Potato chips 23-26 20 0.0034-0.0041

Chocolates 32 13 0.0038

Instant coffee packing 26.66 20 0.0043

Capsule storage 23.89 35-40 0.0064-0.0073

Cough syrups 26.66 40 0.0087

Grain storage 15.55 40 0.0043

Electronic appliances 22.22 15 0.0024

Some examples of industrial processes/manufacturing units along with their effects of

humidity control as elaborated by Arundel et al. (1992)were:

1. To prevent corrosion and improve production of lithium batteries.

2. To prevent condensation and corrosion on metal surface in computer and

electronic equipments.

3. To prevent deterioration of products in confectionary and pharmaceutical packing.


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4.

To optimize seed moisture level and minimize microbial deterioration in seed and

grain storage houses.

5. To improve the product finish by preventing condensation on the mould surfaces

in plastic moulding.

Humidity control is also related with the growth of fungi and bacteria which causes

spoilage of products and affects the health of living beings. The range of the growth of

fungi and bacteria with respect to relative humidity is shown in Figure 1.1.

Figure 1.1 Increase in microbial growth in stored grain as a function of

relative humidity (Arundel et al. 1992)

The most common methods for producing dry air are cooling based dehumidification,

compression based dehumidification and chemical dehumidification. In the past, methods

of cooling based dehumidification and compression based dehumidification have been

used. In the cooling based dehumidification method (vapour compression system), the

dry air is produced by cooling the atmospheric air below the dew point temperature. In

other words below the dew point temperature, water vapour gets condensed and separated

from the air. This method has the following advantages:

1. Light weight and compact size.

2. Independent of weather conditions.

3.

Suitable for low quantity of dry air.

4. Easy handling of operations and installation.


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But this method has some drawbacks like it cools the air below the dew point temperature

so it consumes more electricity which is high grade energy.Dai et al. (2001) suggested

that nowadays air conditioner is becoming the basic need in human life and in future, it is

expected to play a vital role in our lives. Conventional air conditioner based on vapour

compression system utilizes HFC and HCFC which are harmful to the environment.

Another common method of producing dry air is compression based dehumidification.

When air is compressed, the dew point temperature of moist air is raised to a point where

moisture can be condensed from the air at a higher temperature. This method has the

following advantages:

1. Compact size and light weight.

2. Independent of weather conditions.

3. It is very beneficial where small amount of dry air is needed for humidity control.

4. It is suitable for using in space because of the availability of compressed air.

This method has some drawbacks like initial cost and running cost are very high. The

amount of cooling water required for after cooling makes it very impractical for large

volume of air and it is very difficult to handle the high range of pressure required with

proper safety.

The simple and effective way of producing the dry air is by using chemical

dehumidification (using solid desiccant). Solid desiccant attracts moisture due to vapour

pressure difference without any change in their physical and chemical composition. The

amount of vapour adsorbed is proportional to the surface area of desiccant due to its

enormous affinity to adsorb moisture and considerable ability to hold water. The saturated

desiccant is regenerated by passing hot air through it so that desiccant can be used again.

Various solid desiccants like silica gel, activated charcoal, activated alumina and zeolite

can be used.

Sheridan et al. (1985)described a desiccant cooling system as more attractive alternative

than conventional vapour compression systems due to its advantages of utilizing low

temperature energy and providing an environment conscious operation. The method of

chemical dehumidification has the following advantages:


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1.

Driven by renewable energy i.e. solar energy.

2. Simple operation and easy to understand.

3. Low operating costs (Economical method).

4.

Low maintenance costs.

5. Long life.

6. Low noise.

7.

Easy availability.

8. Environment friendly.

A detailed literature review on solar assisted desiccant dehumidification system has been

described in chapter 2 of the thesis.

India is a tropical country and surrounded by oceans from three sides, where the climatic

conditions are hot and humid for 6 to 8 months during a year; humidity ranges from 0.018

to 0.024 kgwater vapour/kgdry air, solar intensity ranges from 700 to 900 W/m2during day time

(10:00 hr - 16:00 hr). It means solar powered desiccant dehumidifier can be operated well

under these conditions.

Experimental setup of evacuated tube solar air collector has been used for analysis of

thermal performance of one ended evacuated tube solar air collector at different air flow

rates. The details of evacuated tube solar air collector along with measuring devices and

instruments for India [2958' (latitude) North and 7653'(longitude) East] are described

in the thesis. The experiments have been carried out during some selected clear sky days

in the month of June, 2011. The experimental data is recorded at intervals of 1hr during

the daytime. The experimental results and discussion are helpful to find the performance

of the evacuated tube solar air collector described in chapter 3 of the thesis.

The experimental comparison of various solid desiccants for regeneration by an

evacuated tube solar air collector and air dehumidification has been studied. In this study,

the main concern is the regeneration of desiccant by the evacuated tube solar air collector

and then the adsorption process at different air flow rates. The experimental data has been

collected in the month of July, 2011during which the ambient temperature varied from

31.5 to 43.5 in most of the clear sky days (12:00 hr - 20:00 hr). The experimentswere performed in noon for regeneration and in evening for adsorption. The results are

shown in chapter 4.


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The feasibility of solar powered desiccant wheel has been checked for Indian climatic

conditions to investigate the adsorption performance of the desiccant wheel at different

air flow rates and rph. The desiccant wheel has been regenerated by an evacuated tube

solar air collector. The experiments have been performed at NIT Kurukshetra, India. The

experimental data is recorded during the day time in the month of September to

November, 2011. The experimental results of this study of solar powered desiccant wheel

at different operating parameters are evaluated in chapter 5.

Desiccant wheel is the most important part of a solar powered desiccant wheel which can

be analyzed in detail by mathematical modeling of desiccant wheel. This model takes into

account both gas and solid side resistance. Also, in this model the four governing

equations of heat and mass transfer are non-linear and coupled. These equations are

solved using a PDE solver which is based on the finite element method (FEM). The

programming is done in the script language of solver. The two programs (process &

regeneration) are coupled in the solver to simulate it with real conditions.

This mathematical model is validated with the results obtained from the experimental test

rig on solar powered desiccant wheel performed at NIT Kurukshetra and also validated

with the experimental data(Kodama PhD thesis 1995).

This model is used to evaluate the optimum value of operating and design parameters so

as to increase the performance of desiccant wheel at low regeneration temperatures and

new wheel designs are developed which are easily regenerated at low regeneration

temperatures by using solar energy. These results are presentedin chapter 6 of the thesis.

The main conclusions arising from this research work have been presented in chapter 7 of

the thesis.


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CHAPTER 2

Literature Review

For many industrial and domestic applications, dry air is produced by using solid

desiccant. This desiccant can be used once or many times. When it is used once, there is

wastage of desiccant. For using it again, it is regenerated by using conventional heater

which consumes high grade energy.

The regeneration of desiccant wheel is also done by using low grade energy such as solar

energy which will help in producing the dry air in adsorption process. This process also

saves a lot of energy and is environment friendly. In the past, Dunkle (1965) presented

the alternative method where regeneration of desiccant material was done by using solar

energy.

Kodama et al. (2005) carried out experiments on desiccant cooling process where

regeneration of desiccant wheel was done at low temperature i.e. 60 and heat wasobtained from low grade energy such as waste heat or solar heat instead of electricity.

Various solid desiccants like silica gel, activated charcoal, activated alumina and zeolite

etc. can be regenerated at low temperature by using renewable energy (i.e. solar energy)

which can be easily collected by simple flat plate and evacuated tube solar air collectors

etc.

For better utilization of low temperature to regenerate the desiccant wheel, a

mathematical model of desiccant wheel has been proposed to find out its best suited

operating and design parameters and different wheel designs according to low

regeneration temperature which is easily available from those collectors.

The work reported in the literature on solar assisted desiccant system can be divided into

three categories:

Solar assisted solid desiccant dehumidification system

Evacuated tube solar collector

Mathematical modeling of desiccant wheel


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2.1 Solar Assisted Solid Desiccant Dehumidification System

Rush et al. (1975) carried out an experimental work in which desiccant wheel was

regenerated by using solar energy and natural gas in Los Angeles and found that the COP

was 0.53 at one operating point.

Nelson et al. (1978) analyzed an open cycle air conditioning system by using solid

desiccant and solar energy. They developed computer models of the various components

of the desiccant cooling system and evaluated the potential of solar energy system for the

regeneration of dehumidifier under typical weather conditions and also suggested that the

solar energy was a better match for cooling and dehumidification system.

Collier et al. (1981) and Worek (1982) described that solid desiccants could be

regenerated using low grade energy at different levels of temperature. This temperature

was found to be depended on the desiccant materials which were being used for the

regeneration. For example silica gel was one of the most extensively investigated and

promising solid desiccant material which required a regeneration temperature of about

65.Monnier et al. (1982) and Barlow (1983)combined a vapour compression (VC) unit with

the solid desiccant dehumidifier and the result was an energy efficient air conditioning

system. This system relieved VC unit from the latent heat portion of the load because theload was taken by desiccant. They also suggested that such energy could be supplied in

various forms like direct fuel firing, waste heat recovery and solar energy. Solar energy

could be used by a flat plate collector.

Kettleborough (1983)described the basic solar assisted comfort conditioning systems and

stated that vapour compression system was more economical for cooling and

dehumidification than other systems at that time but suggested that more research and

development was required due to limitation of availability of fossil fuels.

Jurinak et al. (1984)evaluated the performance of open cycle desiccant air conditioners

for residential application and compared it with vapour compression air conditioning

system on the basis of cost and energy. They also suggested that when these systems were

coupled with solar energy to regenerate the desiccant wheel, they performed better than

the conventional air conditioners.

Maclaine (1987) studied the feasibility of gas fired hybrid desiccant cooling systems for

medium to large unit of air conditioning applications and suggested that an engine drive


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performance was simulated in a residential building. Three locations were considered and

its performance was found for all the locations. He found that solar energy available in

the southwestern US well matched with desiccant cooling system and air conditioners

showed better performance than expected but required more auxiliary energy and its COP

was found to be 0.506 at the design operating point.

San and Jiang (1994) tested the regeneration of a silica gel packed bed. The optimum

operating time, after which the maximum amount of moisture had been removed, was

determined at three regeneration temperatures, 65 , 75 and 85 . Higher theregeneration temperature, shorter was the optimum operating time.

BabusHaq et al. (1996) used the waste heat of a natural gas fired combined heat and

power (CHP) system to regenerate a desiccant wheel which was used for the

dehumidification of moist air in a swimming pool. A payback period of 4 years was

calculated taking zero resale value at the end of 4 years.

Lazzarin and Gasparella (1997) studied a two stage system for air conditioning against

outdoor conditions using rotary heat exchanger having efficiency of 0.85 to 0.89 and heat

pipe heat exchanger having efficiency of 0.66 to 0.70. The COP of this system was about

0.85 and highest regeneration temperature was 85and the system could be driven byheat recovered from internal combustion engine.

Thorpe (1998) developed and analyzed a mathematical model for a solar regenerated

open cycle grain cooling system where the regeneration of desiccant was done by using

solar energy. It was found that the grain cooler worked effectively in subtropical climate

but did not work effectively in humid tropics.

Singh and Singh (1998)fabricated and tested a multi shelf dehumidifier for regeneration

of solid desiccant (silica gel). The effect of regeneration temperature (42-72 ), airvelocity (0.175-0.550 m/s) and number of shelves (1 to 4) on regeneration time of silica

gel was performed and it was found that regeneration time got reduced with an increase in

regeneration temperature, air velocity and number of shelves but the advantage of

increasing air velocity was reduced with an increase in regeneration temperature. They

also discussed the effect of rest period on the drying time of silica gel and found that

drying time reduced with an increase in rest period.

Techajunta et al. (1999) carried out experimental investigations on the regeneration of

silica gel bed with simulated solar energy in which incandescent electric bulbs were used

to simulate solar irradiations. The regeneration rate was found to be strongly dependent


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on irradiation, but was slightly affected by air flow rate. In air dehumidification process,

the dehumidification rate decreased with decrease in irradiation but slightly increased

with air flow rate. They suggested that this system worked better in tropical humid

climate while performing regeneration process during the day and dehumidification

during the night.

Henning et al. (2001)studied a system in which solar air collector was integrated with the

desiccant cooling cycle as the only heat source and compared its performance for

different climaticconditions and configurations. This system performed better under high

value of ambient humidity and the combination of air conditioning system with solar

thermal collector saved the primary energy up to 50%.

Dai et al. (2002)proposed a solar powered hybrid system for cooling grains which was a

combination of rotary desiccant dehumidification and solid adsorption refrigeration

system and compared it with a solid adsorption refrigeration system alone. It was found

that performance of hybrid system was better than solid adsorption refrigeration system

and the COP of hybrid system was more than 0.4 under typical condition which was

higher than single solid adsorption refrigeration system.

Florides et al. (2002) presented a brief review of various cooling systems with solar

energy and low energy technology such as solar sorption cooling, solar mechanical

systems, solar related air conditioning and other low energy cooling technologies. These

technologies reduced energy consumption and the impact on environment. They also

pointed out that solar energy was more suitable for desiccant cooling.

Mavroudaki et al. (2002)presented a model in which solar desiccant cooling was used to

evaluate the potential of using solar energy to drive a single stage desiccant cooling

system with condition of low latent heat gain. They also suggested that this system was

less efficient in the higher relative humidity environment because the temperature

required for regeneration was too high.

Ahmed et al. (2005) fabricated an experimental set up of a desiccant wheel, regenerated

by solar and electric heater together. Experimental results were used to validate the

numerical results and evaluated the performance of solar system and desiccant wheel

under the climatic conditions of Cairo [30 latitude (North)]. They also discussed the

effect of operating and design parameters on the performance of desiccant wheel

numerically.

Jalalzadeh-Azar et al. (2005) fabricated and tested a cooling system in a combined heat

and power (CHP) application incorporating a reciprocating internal combustion engine,


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heat exchanger, desiccant dehumidifier and direct/indirect dehumidifier. Regeneration of

desiccant wheel was performed by using the heat recovered from internal combustion

engine.

Kodama et al. (2005) experimentally tested a solar desiccant cooling system and

investigated its actual performance with the configuration of one desiccant wheel, one

sensible heat exchanger and two water evaporative cooler. The effects of various

operating conditions like regeneration temperature, ambient air and solar irradiation were

studied on the performance of cooling system. It was found that stable solar irradiation

(600 W/m2) and high regeneration temperature (over 50) were required to producesufficient cool air.

Ando et al. (2005) experimentally investigated and proposed 4-rotor desiccant cooling

process equipped with a double stage dehumidification. It was found that this system

produced sufficient dehumidifying performance at regeneration temperature of around

70and at high ambient humidity which was not produced by 2-rotor desiccant coolingprocess.

Daou et al. (2006) studied a desiccant cooling system with evaporative cooling and

chilled ceiling radiant cooling in different climates and pointed out its advantages. One of

its salient features was regeneration of desiccant wheel done by the free energy (waste

heat and solar energy) without any prior conversion.

Zhuo et al. (2006) designed and manufactured a desiccant air conditioning system. The

desiccant wheel was made of composite silica gel and was regenerated by solar air heater

to maintain the indoor air temperature in the range of 24to 28and relative humidityin the range of 50% to 70%. It also showed the feasibility of using low grade energy

(solar energy) in the air conditioning system using a desiccant wheel.

Kabeel (2007) studied a solar assisted desiccant wheel made up of iron wire and cloth

layer (cloth layer was the layer of cloth wrapped on iron wire) impregnated with calcium

chloride solution. In this system a solar air heater containing a porous material was used

for regeneration purpose and the effect of the air flow rate and the solar radiation intensity

on the system for regeneration and absorption process was analyzed. It was found that

this system was highly effective in regeneration process and maximum efficiency (0.6)

was found at wheel effectiveness of 0.92 for regeneration process and 0.65 for absorption

process at a flow rate of 90 kg/hr.


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Jia et al. (2007) developed a novel compound desiccant wheel made up of more

hygroscopic composite material which worked under low regeneration temperature and

had higher dehumidification capacity. The performance of this system was analyzed by a

mathematical model and it was pointed out that this system could work under very low

regeneration temperature having high COP. Hence, low grade thermal energy resources

like solar energy, waste heat etc could be used to operate the system efficiently.

Pramuang and Exell (2007) used a compound parabolic concentrator collector to

regenerate the silica gel for an air conditioning system. The regeneration rate and

regeneration efficiency were greatly dependant on solar radiation but slightly dependant

on different initial moisture content of silica gel and number of silica gel beds. It was also

found that the silica gel could be regenerated at 40

by high air flow rate (0.03 kg/s) and

at 50by low flow rate (0.003 kg/s).Ge et al. (2008) investigated a one-rotor two stage rotary desiccant cooling system

(OTSDC) experimentally and evaluated its performance under various operating

conditions. Results were compared with two stage rotary desiccant cooling system

(TSDC) with two desiccant wheels at same operating conditions and found that OTSDC

had high thermal COP and compact size (half of TSDC). It also had the advantage of low

regeneration temperature as compared to TSDC.

Bourdoukan et al. (2008) used a heat pipe vacuum tube (HPVT) collector in a solar

desiccant cooling system to overcome the problem of flat plate collectors and air

collectors. The efficiency of HPVT collector was between 0.6 and 0.7 for one operating

day and same efficiency was obtained by flat plate collector but the area had to be

increased by 20-25%. Hence, HPVT was a better option for regeneration of desiccant

wheel.

Ge et al. (2009) conducted an experimental analysis of two-stage rotary desiccant cooling

system (TSRDC) using newly compound desiccant (silica gel-haloids) and evaluated its

performance under three typical environmental conditions. It was found that the

temperature required for regeneration of TSRDC was much lower than one stage system.

Hence, low grade energy like solar energy and waste heat were a better option for

TSRDC and it provided high thermal COP under low regeneration temperature.

Khalid et al. (2009) carried out experimental and simulation study on a solar assisted pre-

cooled hybrid desiccant cooling system and found that for pre-cooling of air, better COP

was achieved using indirect evaporative cooling (IEC) and for post cooling of air, better

COP was achieved using direct evaporative cooling (DEC). They also suggested that by


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replacing DEC with an IEC the regeneration temperature could be reduced by 15% and

the decrease in dehumidification was 6%.

White et al. (2009) modeled a solar desiccant cooling system using TRNSYS computer

simulation software. The study was focused on designing and operation of desiccant

cooling system without any thermal backup provided to overcome the effect of

intermittent solar energy. It was found that ventilation desiccant cooling system was not

good for humid climate. At low regeneration temperature, regeneration of desiccant

wheel improved its efficiency but it required more air to achieve satisfactory comfort

conditions which encouraged the use of low temperature solar collectors.

Ge et al. (2010) compared a solar driven two stage rotary desiccant cooling system with a

vapour compression system (VCS) in two cities namely Berlin and Shanghai with

different climatic conditions and evaluated its thermodynamic and economic

performances. They obtained useful data for practical applications and it was found that

desiccant cooling system had advantages like better supply of air quality and less

electricity consumption than VCS. The required regeneration temperatures for Berlin and

Shanghai were 55and 85respectively.La et al. (2010)proposed an innovative thermally driven air conditioning system by

combining the technology of desiccant dehumidification and regenerative evaporative

cooling. It was found that the system could achieve thermal COP higher than 1.

Jeong et al. (2010) developed and analyzed the concept of utilizing the exhaust heat

(50) from fuel cell or air conditioning system as the heat source in a four partitiondesiccant dehumidification system which led to considerable saving of energy. Results

showed that an optimal rotational speed existed which maximized the dehumidification

performance and improved its COP by 94% as compared to conventional vapour

compression refrigerator.

Fong et al. (2010) designed a solar-assisted desiccant cooling system (SADCS) to control

the cooling load of typical office environment in Hong Kong and optimized its

performance through simulation. Since this system used auxiliary heater for regeneration

of desiccant wheel, so it was important to minimize its usage by optimal design and

control schemes of SADCS. This system was more feasible and had the advantages like

energy efficiency and improved indoor air quality because of sufficient ventilation at

same outdoor conditions.

La et al. (2011) carried out an experimental investigation on solar heating and

humidification using rotary desiccant humidification and evacuated tube solar air


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collector. The theoretical analysis was done by creating a performance model in

TRNSYS. It was found that system could convert 50% of solar radiation in space heating

and increased indoor air temperature by about 10. The model was validated with theexperimental data and it was pointed that to improve the indoor thermal comfort, solar

heating with desiccant dehumidification should be preferred.

Goldsworthy and White (2011) analyzed the performance of a combined solid desiccant

indirect evaporative cooling system by solving the heat and mass transfer equations for

both the components simultaneously. Analysis was focused on supply/regeneration air

flow ratio and indirect cooler secondary/primary air flow ratio. Results were obtained and

it was found that the electrical coefficient of performance (COPe)was greater than 20

when the regeneration temperature was 70

with supply/regeneration air flow ratio of

0.67 at ambient conditions. Hence this system had potential to achieve substantial energy

saving and reduced green house gas emission.

Ge et al. (2012) developed and simulated a solar power desiccant coated heat exchanger

cooling system and evaluated its performance in Shanghai during summer conditions with

high temperature as well as high humidity ratio. It was found that this system supplied

sufficient air to be conditioned for indoor space in the month of June and July during day

time and its cooling powers were 2.9 kW and 3.5 kW and corresponding solar COP were

0.22 and 0.24 respectively. They also calculated and discussed the effect of main design

parameters on system performance.

2.2 Evacuated Tube Solar Collector

Garg and Chakravertty (1988) developed an empirical relation of evacuated tubular

collector to find out an overall heat loss coefficient for all possible variables. They

compared the efficiency of selectively coated evacuated collector with normal black

painted collectors and found that efficiency decreased in series combinations and

remained constant for parallel combinations in both type of collectors.

Gaa et al. (1996) developed an experimental set-up and investigated the flow inside an

inclined cylindrical open thermosyphon. The cylinder walls were heated by uniform wall

temperature and differential wall heating method and it was found that differential heating

was more efficient than uniform heating.


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Schnieders (1997) compared one stationary and five dynamic models to predict the

thermal behavior of solar collectors in different ways and found that additional error

might occur in stationary model.

Kumar et al. (2001) studied the thermal performance of community type solar pressure

cooker based on evacuated tube solar collector and mathematical model was validated

against experimental results. It was found that such a system based on evacuated tube

collector supplied heat at higher temperature (~120) as compared to normal flat platecollector.

Behnia and Morrison (2003) investigated a free convective flow in an open ended

inclined evacuated tube solar collector using two heating methods. In steady state uniform

heating, stagnant region was found near the close end of tube. In differential heating,

stagnant region was found when top half of the tube was having lower temperature than

bottom half of the tube which was near close end of the tube. Stagnant region decreased

the effectiveness of heat transfer through open end of the tube.

Morrison et al. (2004) investigated the performance of water-in-glass single ended

evacuated tube solar heater using numerical study of water circulation by thermosyphon.

It was found that there was possible presence of a stagnation region in the bottom of very

long tube which influenced the operation of tube.

Shah and Furbo (2004) carried out an experiment on a prototype collector of parallel

connected evacuated double glass tubes and measured its performance at outside

conditions. In theoretical model they divided the tube into small slices and each slice was

treated as if it was a flat plate collector and integrated the flat plate collector equation

over the whole absorber circumference and determined the shading of tubes as a function

of solar azimuth and compared with measured results. The values of these results had

good degree of similarity. Also, this model was used for theoretical investigation on

vertical placed pipes in Copenhagen (Denmark) and Uummannaq (Greenland) and found

that their high thermal performance were obtained if the distance between tubes was

about 0.2 m and collector azimuth must be 45-60towards the west.Morrison et al. (2005) evaluated the characteristics of water-in-glass evacuated tube solar

water heater including assessment of the circulation rate through the single ended tube

and developed a numerical model of heat transfer and fluid flow inside the tube. It was

found that natural convection flow rate in the tube was high enough to disturb the tank

stratification and the tank temperature strongly affected the circulation flow rate through

the tubes.


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Sharma et al. (2005) investigated the thermal performance of a solar cooker based on

evacuated tube solar collector with a phase change material (PCM) storage unit. Cooking

experiments and PCM storage unit worked simultaneously and it was found that evening

cooking using PCM heat storage was faster than noon cooking and it was not even

affected by noon cooking.

Li and Wang (2006) studied two different fluids (H2O & N2) in two different solar

evacuated tubes to measure their heating efficiency and temperature using parabolic

trough concentrator. It was found that the water easily boiled when liquid rate was less

than 0.0046 kg/s and gave better efficiency (70-80%) at 90-100and above 100, N2worked well but when temperature reached 320-420, its efficiency was less than 40%.Budihardjo et al. (2007) developed a correlation in terms of solar input, tank temperature,

collector inclination and tube aspect ratio for natural circulation flow rate through single

ended water-in-glass evacuated tubes mounted over a diffuse reflector using experimental

and numerical investigation. The developed correlation could be used to determine the

flow rate at any time of the day.

Kim and Seo (2007) studied the thermal performance of different arrangements of a glass

evacuated tube solar collector with different shapes of absorber tube to find the best shape

of the absorber tube for solar collector. Beam irradiation, diffused irradiation and shade

due to adjacent tubes were also considered to obtain realistic estimation of collector

model.

Shah and Furbo (2007) investigated heat transfer and flow structures inside all glass

evacuated tubular collectors for three different tube lengths with five different inlet mass

flow rates at a constant temperature using computational fluid dynamics and found that

the collector with the shortest tube length had highest efficiency. The optimal inlet flow

rate was around 0.4-1.0 kg/min in all the tubes and flow structure in the glass tubes was

relatively not affected by inlet flow rate.

Zhang and Yamaguchi (2008) studied the basic solar collector characteristics using

supercritical CO2 as working fluid and found that temperature, pressure and mass flow

rate of working fluid (CO2) increased with the solar radiation which was different from

those of traditional solar collector using liquid as working fluid and its efficiency was also

higher (above 60%) than that of water based solar collector.

Budihardjo and Morrison (2009) evaluated the performance of water-in-glass evacuated

tube solar water heater and compared it with flat plate solar collector for domestic

purpose in Sydney. The results showed that the performance of 30 evacuated tubes array


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was lower than a typical two panel flat plate (3.7 m2) array but was less sensitive to tank

size.

Tang et al. (2009) developed a two dimensional mathematical procedure to estimate daily

collectible radiation on a single tube of all-glass evacuated tube solar collectors based on

solar geometry. It was found that the annual collectible radiation on unit length of a single

tube was affected by many factors such as central distance between tubes, collector type,

size of evacuated tube, tilt and azimuth angle and use of reflector.

Ma et al. (2010) investigated the thermal performance of single glass evacuated tube solar

collector using one dimensional analytical method and studied the influence of air layer

and solar radiation intensity on heat efficiency. It was found that influence of thermal

resistance of air layer on the heat efficiency was higher. Initially, the efficiency increased

with an increase in solar radiation intensity and finally achieved a constant value.

Zambolin and Del Col (2010) tested on a standard glazed flat plate collector and

evacuated tube collector in same working conditions using steady state and quasi-

dynamic method and compared their daily energy performance. It was found that the

optical efficiency of flat plate collector decreased in morning and afternoon hours due to

more reflection losses whereas evacuated collector tube had higher efficiency for all

range of operating conditions.

Hayek et al. (2011) investigated the overall performance of solar collector using two

types of evacuated tube solar collectors, namely, the water-in-glass and the heat-pipe

designs. It was found that heat-pipe based collector had higher efficiency (about 15-20%)

than the water-in-glass designs.

Tang et al. (2011) studied the comparative performance of two sets of water in glass

evacuated tube solar water heater with different collector tilt angle from the horizon and

found that the heat removal from solar tube to storage tank was not influenced by

collector tilt angle. The results also depicted that the daily collectible radiations and daily

solar heat gains of system were very much affected by collector tilt angle.

2.3 Mathematical Modeling of Desiccant Wheel

Farooq and Ruthven (1991) identified that the main component of the solid desiccant

system was the dehumidifier wheel and its COP could be significantly improved by

improving its performance. So, the analysis of design and operating parameters ofdesiccant wheel was necessary.


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San and Hsiau (1993) developed a one-dimensional transient heat and mass transfer

model to analyze the effect of axial heat conduction and mass diffusion on the

performance of a desiccant wheel and discussed that Biot number (Bi) and number of

transfer unit (NTU) were two important parameters which affected the dehumidification

ability.

Zheng and Worek (1995) investigated the effects of desiccant sorption properties, heat

and mass transfer characteristics and size of the wheel on dehumidification performance.

They also discussed the isotherm shape of desiccant and it was found that to obtain

maximum dehumidification, separation factor should be 0.07.

Majumdar (1998) investigated the performance of adsorption and desorption process

during a single blow operation for a dehumidifier made of composite mixture of silica gel

particles and inert particles. They also discussed the effect of different compositions of

inert material and thermo physical properties of composite desiccant on adsorption and

regeneration performance.

Dai et al. (2001) evaluated the dehumidification performance of desiccant wheel on the

basis of wave shape through wave analysis using psychrometric chart and discussed the

effects of some important parameters, such as heat capacity, adsorption heat, rotational

speed, regeneration temperature, thickness of the desiccant matrix and desiccant isotherm

on the performance.

Zhang and Niu (2002) developed a two dimensional (axial direction and thickness

direction) transient heat and mass transfer model for desiccant wheel and took into

account both gas side and solid side resistance. They compared the performance of a

desiccant wheel used in air dehumidification and enthalpy recovery on the basis of rotary

speed, NTU and specific area.

Niu and Zhang (2002) developed a two dimensional (axial direction and thickness

direction) transient heat and mass transfer model for desiccant wheel to calculate the

optimum rotary speed for sensible heat recovery, latent heat recovery and air

dehumidification which takes into account both gas side and solid side resistance .They

also analyzed the effect of channel wall thickness on the optimum rotary speed used in air

dehumidification and enthalpy recovery.

Zhang et al. (2003)developed a one-dimensional coupled heat and mass transfer model

allowing lumped parameter method to analyze the temperature and humidity profile in

honeycombed rotary desiccant wheel during both dehumidification and the regeneration

process. They also investigated the effects of velocity of regeneration air, regeneration


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temperature and velocity of process air on the hump curve and it was found important to

accelerate the hump curve moving from duct entrance to duct exit so as to improve the

performance of desiccant wheel.

Gao et al. (2005)described a one dimensional mathematical model considering the heat

and mass transfer within moist air as well as desiccant material to predict the transient

and steady state transport in a desiccant wheel. The effect of desiccant thickness (felt

thickness) and passage shape on the performance of a desiccant wheel was also

investigated.

Xuan and Radermacher (2005) developed a one-dimensional transient heat and mass

transfer model to investigate the performance of the desiccant wheel. Their simulation

results revealed a significant effect of different regeneration temperatures, air flow rates

and wheel speeds on the performance of wheel.

Harshe et al. (2005) presented a two-dimensional steady-state model pertaining to a rotary

desiccant wheel which included the mass and energy balance equations for the air streams

and the desiccant wheel. The model was capable of predicting the steady-state behavior of

a desiccant wheel for process, purge and regeneration sector.

Nia et al. (2006) developed a one dimensional transient heat and mass transfer model.

They determined the optimum rotational speed by examining the outlet adsorption side

humidity ratio to improve the performance of an adiabatic rotary dehumidifier.

Sphaier and Worek (2006)compared one dimensional and two dimensional mathematical

models for both solid side and gas side resistance and found that one dimensional

formulation could be used in desiccant wheel applications whereas a two dimensional

model was needed for an enthalpy exchanger when thermal resistance in desiccant

material was high.

Ruivo et al. (2006) described a one dimensional transient numerical solution of the

conservation equations for heat, water vapour and adsorbed water inside the porous

medium. They found that surface diffusion was the most important mechanism of water

transport within the porous medium by assuming a lumped heat capacitance model in the

cross directions of the channel wall.

Ruivo et al. (2007) developed two mathematical formulations (detailed model and

simplified model) for different lengths of channel of hygroscopic desiccant wheel. In the

detailed model, air flow field was obtained after the solution of two dimensional

conservation equations for the momentum, mass and energy but in simplified model

hypothesis of bulk flow was adopted in air flow domain and the conservation equations


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were solved as one dimensional. Result showed that use of simplified model for

simulation of real hygroscopic rotor for channel length was greater than 0.1 m.

Ruivo et al. (2007) developed a simplified model (described in part-1) for the behavior of

channel with parallel desiccant walls to analyze the heat and mass transfer phenomenon

in desiccant wheel. The characterization of the corrugate matrix and an inspection of the

effect of the corrugated curvature was presented. They also investigated the influence of

the dimensions of matrix cells, thickness of channel wall, rotation speed and the influence

of air flow conditions on the performance of desiccant wheel.

Golubovic et al. (2007)evaluated the performance of a rotary dehumidifier on the basis of

three sectors namely: purge, process and regeneration. The performance of a desiccant

wheel with a heated effective purge angle was compared with the performance of the

same wheel without a purge angle. It was found that heated effective purge angle had an

overall positive effect on the performance of a rotary dehumidifier.

Ge et al. (2008)presented a review of various efforts that researchers have made to

mathematically model the coupled heat and mass transfer processes occurring within the

wheel. They explained the fundamental principle of heat and mass transfer mechanisms

taking into account both gas side and solid side resistance. They showed that gas and

solid side resistance models were higher in precision and more complex compared to gas

solid resistance.

Bourdoukan et al. (2008) performed a sensitivity analysis of a desiccant wheel

dehumidification using the design of experiments and also studied the effect of operating

parameters on the dehumidification rate of the wheel by experimental and numerical

results.

Zhai et al. (2008) developed a one-dimensional transient heat and mass transfer equations

for a desiccant wheel allowinglumped formulation. The performance model related the

wheels design parameters (wheel dimension, channel size and desiccant properties) and

operating parameters (rotary speed, condition of process and regeneration air and

regeneration air flow rate) to its operating performance. They also discussed the effect of

some practical issues such as wheel purge, residual water in the desiccant and the wheel

supporting structure on the wheel performance.

Ruivo et al. (2008)developed one dimensional transient heat and mass transfer model for

desiccant wheel and presented two approaches. In the first approach, the model was valid

for thickness lower than 0.1 mm while neglecting the transversal heat and mass transfer


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resistance in micro porous desiccant and for the second approach, the model was valid for

thickness lower than 5 mm while neglecting only thermal resistance.

Chung and Lee (2009) developed a one dimensional transient model to examine the

operating and design parameters (rotational speed and area ratio of regeneration to

adsorption) of desiccant wheel for a range of regeneration temperature (50-150).Performance evaluation was based on MRC (moistu

avdesh yadav 2012

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