COMPUTER BASED ANALYSIS OF HEAT AND MASS TRANSFER IN SOLAR DISTILLATION

SYSTEMS AND ITS EXPERIMENTAL VALIDATION

by

SANJAY KUMAR

THESIS SUBMITTED IN FULFILLMENT OF THE REQUIREMENTS FOR

THE DEGREE OF

DOCTOR OF PHILOSOPHY

Centre for Energy Studies INDIAN INSTITUTE OF TECHNOLOGY, DELHI

HAUZ KHAS, NEW DELHI-110016 INDIA

AUGUST, 1996

4.. r. oz1.41-11. LiePARY Aft. No Al-

Dedicated to

My mother

Mrs. Savitri Devi

My father

Prof. Ti ej Narayan Kumar

and

My uncle

Shri Shyain Narayan Kumar

CERTIFICATE

It is certified that the thesis entitled, "COMPUTER BASED ANALYSIS

OF HEAT AND MASS TRANSFER IN SOLAR DISTILLATION SYSTEMS

AND ITS EXPERIMENTAL VALIDATION" submitted by Sanjay Kumar is

worthy of consideration for the award of the degree of 'Doctor of Philosophy'

and is a record of the original bona-fide research work carried out by him

under my guidance and supervision. The results contained in the thesis have

not been submitted in part or full to any other university or institute for the

award of any degree or diploma.

Date: ;1 g Aug 96 Dr. . Tiwari Assistant Professor Centre for Energy Studies Indian Institute of Technology Hauz Khas, New Delhi-110 016

()Cl/Aug 1996

vranCA

Sanjay Kumar

ACKNOWLEDGEMENTS

I feel immense pleasure to express my heart felt gratitude to Dr. G.N. Tiwari for his constant and consistent inspiring guidance and utmost co-operation at every stage which culminated in successful completion of my research work.

My wife joins me to extend our deep sense of obligation to Mrs. Tiwari and her children for their high degree of hospitality and encouragement throughout our stay in Delhi.

I am very much thankful to Prof. D.P. Kothari, Head, CES, Prof. H.P. Garg, Ex-Head, CES and my M.Tech Dissertation Supervisor Prof. Bharathi Bhat, Ex-Head of Centre for Applied Research in Electronics, for their academic discussion and ecouragement to pursue the research work. Sincere thanks are due to Dr. T.C.Kandpal and other faculty members of CES and my colleagues for their valuable suggestions and help rendered during my research period.

I take an opportunity to thank the staff of Main Computer Centre, CES Computer Centre and Distillation Laboratory during the research period and students of B.Tech & M.Tech, namely, Mr.Raghu, Rohit, Narendra and Sanjeev for their help during the experimentation. I am also

thankful to Major Anand Prakash and Mr. R.K Goyal for their friendly support.

I am deeply indebted to Air Commodore M. Singh VSM, Air Commodore S.P. Singh, Air Vice Marshal D.C. Nigam, Gp Capt R.K.Srivastava, Maj Gen P.S. Modak and Maj Gen L. Tahiliani AVSM, for their encouragement and all possible helps extended to me to complete this daunting task of research work.

I owe my profound gratitude to my parents, brothers Suman, Manglam, Ratnesh and Sambhavam who have been the constant source of inspiration without which I would not have completed this work. Also, I am deeply indebted to my wife Indu, son Anand and daughter Apoorva for their whole hearted support and patience they have shown during the long hours spent at Computer Centre and Central Library.

Last but not the least, I convey my sincere thanks to Sunil and Finny for their untiring effort to type the manuscript in efficient manner.

SUMMARY

In the present thesis, a model has been developed for heat and mass transfer

relations by using linear regression analysis for outdoor conditions unlike the models

developed earlier under simulated conditions. Also, theoretical models have been developed

for passive and active stills on the basis of energy balance equation of each component of

still. The developed model for convective and evaporative loss has been used in theoretical

models of various types of solar stills. The results obtained are experimentally validated

for the following designs of solar stills :-

(a) Single slope passive still

(b) Double slope passive still

(c) Double slope multiwick still

(d) Active still with water flow and

(e) Active still without water flow

It has been observed that there is a fair agreement between theoretical results

obtained by using the modified expression for convective and evaporative loss and

experimental results. Effects of various design and climatic parameters e.g water depth,

insulation thickness, water flow rate, wind velocity etc on daily yield by using the proposed

model have been studied. The results are found in accordance with the work reported

earlier. The proposed method for evaluating the convective mass transfer relation can be

used for any design of solar still without any limitation of operating temperature,

inclination of solar still and shape of condensing surface. The model developed can also

be effectively utilized for precise evaluation of product of absorptivity and transmittivity of

water mass and basin liner.

PREFACE

Fresh water is one of the basic necessities of the mankind and is essential for human

survival. The entire world has been dependent on the natural resources of water such as

sea, rivers, lakes ponds and underground reservoirs for domestic, agricultural and

industrial use. The quality of the water particularly for the use of domestic and chemical

industries is of paramount importance. The water available from rivers, lakes and

underground reservoirs contain a large number of micro-organisms some of which may

cause health hazard to human beings. Direct 'consumption of the water from natural

resources is not advisable at all.

A considerable chunk of our world population lives in such regions where either the

water resources are located at remote places or the water available to them is brackish or

saline. Transportation of fresh water to these areas is either through roadways or pipelines

which is quite expensive keeping in view of manifold rise in petroleum prices. Hence,

distillation of brackish or saline water is a suitable and attractive alternative to obtain

fresh water.

Conventional desalination processes such as multi-effect evaporation, thin film

distillation, reverse osmosis and electrodialysis are energy intensive techniques and they

utilize now renewable energy. However, utilization of solar energy for distillation systems

is the best possible solution due to its simple technology and being economical. Low cost

solar stills [Kudish, 1982] have been designed and proved very economical.

Many researchers have worked on the various designs of solar stills [Rajkamal et

al, 1992] discussed about the solar distillation system in Indian scenario [Gomakle,

1988]. Theoretical models to predict the performance of solar stills have been developed

by various authors viz. [Malik and Tran, 1973] ; [Garg and Mann, 1976] ; [Hirschmann

and Roefler, 1970] ; [Thanvi and Pandey, 1987] ; [Wibulswas, 1984] ; [Zaki et al, 1993].

Periodic as well as transient analysis of various solar stills have been presented by Nayak

et at [1980], Sodha et at [1980, 1981], Malik et at [1982], Tiwari and Yadav [1987].

In solar distillation, there are basically three modes of heat transfer namely

radiation (q.,), convection (i) and evaporation (q.,)from the water surface to the glass

cover. The rate of heat transfer from the water surface to the glass cover by convection

(c,) in u pward direction through humid fluid can be estimated by

cw ( Tw -Tg )

V-1,„ can be determined from the following relation

Nu- h cwd f) -C (Gr131- )n f

(P-1)

h cw =C ( GrPr )

I f (P-1)

For the normal operating temperature range (5 50"C), Dunkle [1961] derived the

following expression for h„ using C =0.075 and n=1/3

h

884 [ -Tg ) +( Pw -Pg ) Tw +273 ) ( 268 9 x10 3 -Pw )

(P-3)

where,

Pw=exp [ 25 317 - 8144 Tw +273 . 15

and

Pg=exp [ 25 317 - 5144 Tg- +273 . 15

The expression given below is for the rate of heat transfer per unit area from the

water surface to the glass cover which has been given by many researchers viz. [Dunkle,

1961] ; [Cooper, 1970] ; [Tiwari et al, 1982].

(j- ew=0 . 016 h ( Pw -Pg )

(P-4)

Substituting the value of h, from Eq.(P-2) in Eq.(P-4) we get,

ew 016 C( GrPr ) 11 ( ) ( Pw -Pg )

(P-5)

Now, hourly distillate output from a distiller unit can be obtained as

d'pr 0 016 ) C ( GrPr ) 12 ( ( Pw -Pg ) L u f

(P-6)

Here, MW = hourly distillate output in Kg/sec

Clark [1990] in his model for distillate output, also derived an equation for qe„

under simulated condition i.e

(j- ew=0 008 /2,( Pw -P9 )

(P-7)

Apparently, Eq.(P-7) is similar to the Eq.(P-4) except the change in the coefficient

which appears as if it has reduced to half. [Clark, 1990, 1985]. As a matter of fact, the

coefficient of Eq.(P-7) will remain same as in case of Eq.(P-4), since it has been derived

[Dunkle, 1961]; [Malik et al., 1982]. So, the only possibility of change remains with the

coefficients C and n of Eq.(P-2). To verify these observations, Adhikari et al., [1990] made

an attempt to obtain the values of C and n under simulated conditions and found that the

values of coefficients C and n are not in accordance with the values as proposed by Clark

[1990] and he proposed the different values of C and n.

In the present study, extensive experimentation has been carried out to analyse the

heat and mass transfer coefficients to obtain the values of C and n through the thermal and

computer models using the experimental values of water and glass temperature and

(iv)

distillate output for single and double slope conventional solar stills, multiwick solar still

and active solar still. When hot water is fed into the basin of passive solar still through

Flat Plate Collector [Norton, 1992] it is referred as active solar still. Further, thermal and

computer models for each type of solar still have been developed and using new values of

coefficients C and n, their performance parameters have been predicted. Also, parametric

studies on various types of stills have been carried out using new values of C and n and

values proposed by Dunkle. Percentage deviation between experimental and theoretical

results also has been evaluated.

Based on the details of work reported above the thesis has been divided into seven

chapters. Chapter-wise summary has been presented below.

In the first chapter, work carried out by various authors in solar distillation systems

has been reviewed. The review includes the historical background of solar distillation,

working principle of solar distillation, fraction of energy absorbed at different components

of distiller unit, introduction of heat and mass transfer coefficients and instantaneous and

overall efficiency [Tamami, 1987, Tiwari et al, 1996] of solar stills. Internal and external

heat transfer coefficients, associated top and bottom loss coefficients and method for

evaluation of distillate output have also been discussed. Limitations of coefficients C and

n proposed by Dunkle [1961] have been brought out and a new approach for evaluation of

coefficients C and n using linear regression analysis has been suggested.

(v)

In the second chapter, various designs of solzr stills have been discussed. The

details of the following experimental set-ups mounted on roof top have been described.

(a) Passive solar still

(i) Single slope still

(ii) Double slope still

(iii) Multiwick still [Tanaka, 1983]

(b) Active solar still

(i) With water flow arrangement.

(ii) Without water flow arrangement.

Also, measurement techniques of different parameters such as water and glass

temperature, yield , solar intensity, wind velocity, ambient temperature, water mass etc have

been narrated. A brief description on various measuring instruments like Suryamapi,

digital multimeter, anemometer, thermometer and experimental precautions has also been

incorporated. Also, sample experimental observations of each type of still for five days have

been tabulated and experimental uncertainties have been evaluated.

Third chapter deals with methodology formulated for evaluation of coefficients C and

using regression analysis. Flow chart of a computer model for evaluation of new

:oefficients using experimental parameters of different solar stills has been discussed. A

:omparative study of newly developed coefficients with that of Dunkle has been carried out.

(vi)

Effect on distillate output using new values of C and n and values proposed by Dunkle has

been analyzed. Also, methods for evaluation of physical properties of water-vapour inside

the distiller unit and their variations due to different operating temperature range has also

been studied.

In the fourth chapter, analytical expressions have been derived for water and glass

temperature, yield and instantaneous efficiency factor of passive and active solar distillation

systems. The derivation is based on basic energy balance equations for each component of

the systems. Based on these energy balance equations, a computer model has been

developed to predict the performance parameters of stills. This model uses the new values

of C and n, as discussed in Chapter 3 by using the experimental results and compares the

predicted and experimental parameters of stills. A reasonable agreement between the

experimental and predicted theoretical results has been achieved.

Chapter 5 deals with thermal analysis of double slope conventional and multiwick

solar stills which has been carried out by considering modified values of C and n for

convective mass transfer. Analytical expressions for water and glass temperature, distillate

output and instantaneous efficiency have been derived. To predict the performance of stills,

operating in actual climatic conditions, computer model has been developed. A fair

agreement between the theoretical and experimental data has been achieved.

Chapter 6 is devoted to the parametric studies of stills. The effects of different

iesign parameters namely water depth, absorptivity of basin liner and water mass, bottom

insulation thickness for passive solar stills and collector area, flow rate over glass cover for

active solar still have been studied by using modified values of C and n for convective mass

transfer relations. The studies have been based on energy balance equations of different

:omponents of passive and active solar distillation systems taking into account of modified

values of C and n for convective mass transfer. The results obtained in the present studies

are in accordance with the results reported earlier.

Based on the results of the present studies conclusions and recommendations have

been made in Chapter 7.

The above mentioned work has been reported in the form of following

publications/communications.

1. Kumar, Sanjay and Tiwari, G.N., Performance evaluation of an active

solar still, Energy, 21, 0, (1996).

2. Kumar, Sanjay and Tiwari, G.N.,Estimation of convective mass transfer in

solar distillation system, Solar Energy, Accepted for publication, (1996).

3. Kumar, Sanjay and Tiwari, G.N.,Estimation of convective mass transfer in

double slope solar distillation systems, International Communication in Heat

and Mass Transfer, Communicated, (1996).

(viii)

4. Kumar, Sanjay and Tiwari,G.N.,Computer modelling of thermal analysis in

solar distillation systems, International Journal of Solar Energy,

Communicated, (1996).

5. Kumar, Sanjay and Tiwari,G.N.,Computer modelling of thermal analysis in

double slope solar distillation systems, Energy Conversion and Management,

Communicated, (1996).

6. Kumar, Sanjay and Tiwari, G.N.,Parametric studies of solar distillation

systems by using modified convective mass transfer relations, Ambient Energy,

Communicated, (1996).

In addition to the above publications reported in the thesis, other publication is as

follows :-

I. Tiwari, G.N.,Kumar, Sanjay, Sharma, P.B. and Khan, I., Instantaneous

thermal efficiency of an active thermal still, Applied Thermal Engineering,

16, 2, 189-192, (1996).

CONTENTS

Page

PREFACE

(i)

CHAPTER-1 INTRODUCTION

1.1 Solar Irradiance 01

1.2 Importance and Brief History of 03 Distillation

1.3 Economic Viability of Solar Stills 10

1.4 Working Principle of Solar Stills 11

1.5 Basic Elements of Heat Transfer 17

1.5.1 External Heat Transfer 19

1.5.1a Top Loss Coefficient 19

1.5.1b Bottom and Side Loss Coefficient 21

1.5.2 Internal Heat Transfer 23

1.5.2a Convective Loss Coefficient 23

1.5.2b Evaporative Loss Coefficient 25

1.5.2c Radiative Loss Coefficient 27

1.5.3 Overall Heat Transfer 29

1.5.3a Top Loss Coefficient 29

1.5.3b Bottom Loss Coefficient 30

1.6 Evaluation of Distillate Output 31

1.7 Thermal Efficiency of Distiller Unit 32

1.7.1 Instantaneous Efficiency

1.7.2 Overall Thermal Efficiency

1.8 Limitations of Heat and Mass Transfer Coefficients Proposed by Dunkle

Page

32

33

34

1.9 Scope of Present Studies and Methodology 35 Adopted to Propose New Coefficients

IHAPTER-2 EXPERIMENTAL SET-UP AND OBSERVATIONS

2.1 Introduction 38

2.2 Design of Solar Stills 39

2.2.1 Single Slope FRP Conventional Solar Still 39

2.2.2 Double Slope FRP Conventional Solar Still 43

2.2.3 Multiwick Double Slope FRP Solar Still 45

2.2.4 Active Single Slope FRP Solar Still 50

2.2.4a Flat Plate Collector 54

2.2.4b Glazing Material of FPC 56

2.2.4c Absorber Plates of FPC 57

2.2.5 Fabrication Materials of Solar Still 60

2.3 Measurements of Parameters 63

2.3.1 Temperature Measurement 63

2.3.2 Calibration of Thermocouples 64

2.3.2a Calibration Procedure 66

2.3.3 Solar Radiation Measurement 67

Page

2.3.4 Yield Measurement 68

2.3.5 Wind Speed Measurement 68

2.4 Measuring Instruments 68

2.4.1 Digital Electronic Meter 68

2.4.2 Solarimeter 69

2.4.3 Anemometer 69

2.5 Experimental Observations 71

2.6 Experimental Uncertainties 84

2.7 Experimental Precautions 87

CHAPTER-3 ANALYSIS OF CONVECTIVEMASS TRANSFER IN SOLAR STILLS

3.1 Introduction 91

3.2 Thermal Model 93

3.2.1 Identification of Problem 93

3.2.2 Methodology for Evaluation of C and n 97

3.3 Computer Model 98

3.4 Results and Discussion on Analysis of 102 Convective Mass Transfer

3.4.1 Single Slope Passive Solar Still 103

3.4.2 Single Slope Active Solar Still With and 104 Without Water Flow

3.4.3 Double Slope Passive Solar Still 104

3.4.4 Double Slope Multiwick Solar Still 105

Page

3.5 Conclusions 115

:HAPTER-4 COMPUTER MODELLING OF THERMAL ANALYSIS IN SOLAR DISTILLATION SYSTEMS

4.1 Introduction 116

4.2 Mathematical Models 117

4.2.1 Passive Solar Still 117

4.2.2 Active Solar Still 126

4.3 Computer Model 130

4.4 Numerical Results and Discussions 131

4.4.1 Design Parameters 133

4.4.2 Climatic Parameters 133

4.5 Conclusions 141

'HATTER-5 COMPUTER MODELLING OF THERMAL ANALYSIS IN DOUBLE SLOPE SOLAR DISTILLATION SYSTEMS

5.1 Introduction 142

5.2 Mathematical Models 143

5.2.1 Double Slope Conventional Solar Still 143

5.2.2 Multiwick Solar Still 149

5.3 Computer Model 152

5.4 Numerical Results and Discussion 154

5.4.1 Design Parameters 154

5.4.2 Climatic Parameters 155

Page

5.5 Conclusions 161

CHAPTER-6 PARAMETRIC STUDIES OF SOLAR DISTILLATION SYSTEMS BY USING MODIFIED CONVECTIVEMASS TRANSFER RELATIONS

6.1 Introduction 162

6.2 Effect on Heat Transfer Coefficients 163

6.3 Effect of Modified C and n on Daily Yield 164

6.4 Effect of Design Parameters of Solar Still 170

6.4.1 Effect of Water Depth 170

6.4.2 Effect of Thickness of Insulation 170

6.4.3 Effect of Absorptivity 172

6.4.4 Effect of Water Flow Rate 172

6.4.5 Effect of Collector Area 174

6.5 Effect of Meteorological and Other Parameters 174

6.5.1 Effect of Wind Velocity 174

6.5.2 Effect of Solar Intensity and Ambient 176 Temperature

6.5.3 Effect of Salt Deposition 176

6.5.4 Effect of Dye 178

6.6 Conclusions 178

CHAPTER-7 CONCLUSIONS AND RECOMMENDATIONS 180

Page

NOMENCLATURE 183

REFERENCES 191

BIO-DATA 202

REPRINTS/PREPRINTS 203

APPENDIX - I