STUDY OF HEAT AND MASS TRANSFER AND THERMAL MODELING FOR SOLAR
DISTILLATION SYSTEMS
by
RAJESH TRIPATHI
Centre for Energy Studies
Submitted in fulfilment of the requirements of the degree of
Doctor of Philosophy
to the
Indian Institute of Technology, Delhi April, 2005
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CERTIFICATE
It is certified that the thesis entitled, "Study of heat and mass transfer and thermal modeling for solar distillation systems", submitted by Rajesh Tripathi to Indian Institute of Technology, Delhi is worthy of consideration for the award of the degree of 'Doctor of Philosophy' and is a record of the original bonafide 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: April 25, 2005 (D .N.Tiwari)
Professor
Centre for Energy Studies Indian Institute of Technology Hauz Khas, New Delhi- 110016
ACKNOWLEDGEMENTS
I feel immense pleasure to express my heart felt gratitude to my supervisor, Prof.
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.
I am very much thankful to Prof. M.K.G. Babu, Head, Prof. A. Chandra, Prof.
S.C. Kaushik, Dr. J.C. Joshi and Dr. Subodh Kumar of Centre for Energy Studies for their kind advice and help from time to time. My sincere thanks go to Prof. V.K.
Srivastava, Department of Chemical Engineering for his academic discussion and encouragement.
I am deeply indebted to Dr. Sanjeev Kumar for his encouragement and all possible helps extended to me to complete this daunting task of research work.
I express my utmost obligation to Mr. Arvind Tiwari for providing me great moral support by taking keen interest in my work and helping me from time to time.
My special thanks go to my colleagues and to my friends Dr. Amita Gupta, Dr.
M. K. Ghosal, Dr. H. N. Singh, Dr. 0. P. Chaurasia, Dr. Ravi Gupta, Dr. Lopa Ghosh, Mr. Anil Kumar, Mr. Anil Kumar Tiwari, Mr. Bikash Sarkar, Mr. Vimal Dimri, Mr.
Anand Joshi, Mr. Ashish Shukla, Mr. Mansoor, Mrs. Nisha Dahiya, Ms. Tribeni Das and Ms. Sonia Bansal.
I owe my profound gratitude to my parents, brother Brajesh Tripathi, sister Suman Tiwari and brother-in-law Sushil Tiwari who have been a constant source of inspiration without which I would not have completed this work. I have no befitting words to express my deep sentiments to them for their wholehearted support and patience during the period of study.
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I along with my family members extend our deep sense of obligation to Mrs.
Tiwari (aunty) and her children for their high degree of hospitality and encouragement throughout my study.
Last but not the least, I convey my sincere thanks to Mr. Lakhmi Chand, Mr.
Dhaney Singh, Mr. Shankar Lal, Mr. Umesh Kumar Mishra and staff members of IIT, Delhi for their kind support and help in completing this research work.
Date: April 25, 2005 (RAJESH TRIPATHI)
iii
ABSTRACT
It is a well known fact that, at present, one of the foremost challenges in front of the entire world is to obtain fresh water from the natural resources of water so as to meet the increasing demand of fresh water for domestic as well as industrial use. The water available from rivers, lakes and underground reservoirs contain a large number of micro- organisms which may cause health hazard to human beings. However, the available water, after distillation, may be used for domestic and industrial use. Although conventional methods for distillation exist, they are energy intensive techniques and require fast depleting sources of energy. In such circumstances, solar energy, which is the oldest form of energy available to mankind and is abundant in nature, provides the best alternative to obtain fresh water by the use of solar still.
The rate of distillate mainly depends on operating temperature and shape and material of condensing cover. Further, it is important to study the behavior of heat and mass transfer relation as a function of operating temperature, shape and material of condensing cover. It is to be noted that the primary aim of most of the research work done in the field of solar distillation is to increase the yield of the distiller unit, which can be attained by maximizing the temperature difference between water and the condensing cover. Hence the design parameters should be employed efficiently to attain the above- mentioned aim. Intensity and temperature are interrelated to each other. Higher intensity leads to high temperature inside the solar still and hence results in higher temperature of water in it. It is essential that the solar radiation falling on solar still should contribute towards the enhancement of temperature of water, especially during winter months.
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Temperature is the most critical climatic factor during winter. In the winter season the day length is short and also the altitude angle of sun is very low. Some part of the solar radiation transmitted inside the solar still falls on the north wall of the solar still.
The north wall of the solar still absorbs a part of this radiation while a large part of it is reflected back to the water in the basin of the solar still. Till now the solar radiation falling on north wall of the solar still has not been considered while doing the thermal modeling of the solar still. In the present work, a term 'solar fraction' is used so as to include the solar radiation falling on north wall for the thermal modeling of the solar still.
Solar fraction for a particular wall of the solar still is defined as the ratio of the solar radiation available on the wall of a still at a given time to the solar radiation measured on the wall and floor of the still for the same time. Solar fraction is more during winter months due to low altitude angle of the sun.
It is observed that the concept of solar fraction is very important in thermal analysis of a solar still. For a south directed solar still particularly in northern hemisphere, solar radiation that falls on the north wall can prove to be a major factor while designing an efficient solar distillation system.
In the present work, extensive experimentation has been carried out to analyze the heat and mass transfer coefficients to obtain the values of C and n through the thermal and computer models using the experimental observations for indoor as well as outdoor distillation. Heat and mass transfer relations have been studied by considering the different sizes and materials of the condensing cover by using the constant temperature bath for indoor distillation. Further, thermal and computer models for passive and active solar stills have been developed by using the concept of solar fraction for outdoor distillation.
CONTENTS
Certificate
Acknowledgements Abstract
Contents List of figures List of tables Nomenclature
Page i ii iv vi ix xiv xvii
Chapter -I General introduction 1-13
1.1 Introduction 1
1.2 Historical review 3
1.3 Principle of solar distillation: A state of art 4
1.4 Hourly yield 5
1.5 Classification of solar distillation systems 6
1.6 Performance of solar still 9
1.7 Global status of solar distillation 10
Chapter -II Effect of size and material of condensing cover on heat and
mass transfer for distillation 14-40
2.1 Introduction 14
2.2 Design of the condensing cover 17
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2.3 Experimental set-up, procedure and observations 18
2.4 Numerical computation 31
2.5 Results and discussion 33
Chapter -III Performance evaluation of a conventional single slope solar
still by using the concept of solar fraction (Fe) 41-60
3.1 Introduction 41
3.2 Experimental set-up, procedure and observations 42
3.3 Evaluation of solar fraction 44
3.4 Energy balance 47
3.5 Statistical analysis 49
3.6 Results and discussion 50
Chapter -IV Effect of water depth on heat and mass transfer of a passive solar still: An experimental validation
4.1 Introduction
4.2 Experimental set-up, procedure and observations 4.3 Thermal modeling
4.4 Statistical analysis 4.5 Results and discussion
61-77 61 62 68 69 70
Chapter -V Effect of water depth on heat and mass transfer of an active
solar still: A n experimental validation 78-94
5.1 Introduction 78
5.2 Experimental set-up, procedure and observations 79 vii
5.3 Thermal modeling 85
5.4 Statistical analysis 86
5.5 Results and discussion 87
Chapter -VI Comparison between the performance of passive and active
solar distillation system 95-103
6.1 Introduction 95
6.2 Effect of water depth 96
6.3 Effect of absorptivity 100
6.4 Effect of thickness of insulation 101
6.5 Effect of absorptivity-transmittivity product (at)e of collector 102
Chapter -VII Conclusions and recommendations 104-106
7.1 Conclusions 104
7.2 Recommendations 106
References 107-114
Appendix 115
Reprints/ preprints 116
Brief bio-data of the author 117
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