Table Of Content

Solar Drying Systems Solar Drying Systems Om Prakash Anil Kumar First edition published 2020 by CRC Press 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 and by CRC Press 2 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN © 2021 Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, LLC Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. 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Library of Congress Cataloging-in-Publication Data ISBN: 978-0-367-28043-7 (hbk) ISBN: 978-0-429-29935-3 (ebk) Typeset in Palatino by SPi Global, India Contents Foreword by Rebecca R. Milczarek .....................................................................ix Preface ......................................................................................................................xi Acknowledgments ..............................................................................................xiii Authors ...................................................................................................................xv 1. Fundamentals of Solar Drying Systems .....................................................1 1.1 Introduction ...........................................................................................1 1.2 Importance of Drying ...........................................................................3 1.3 Crop Drying Characteristics ................................................................6 1.4 Safe Moisture Content ..........................................................................8 1.5 Dried Product Quality Parameters .....................................................8 1.6 Classification of Drying ........................................................................8 1.7 Introduction to Solar Drying .............................................................10 Problems ..........................................................................................................11 References .......................................................................................................11 2. Drying Methodology ....................................................................................13 2.1 Introduction .........................................................................................13 2.2 Moisture Content ................................................................................14 2.3 Moisture Movement Mechanism ......................................................15 2.4 Equilibrium Moisture Content (EMC) .............................................17 2.4.1 EMC Models ...........................................................................17 2.4.1.1 Henderson Equation .............................................18 2.4.1.2 Chung–Pfost Equation ..........................................19 2.4.1.3 Modified Halsey Equation ...................................19 2.4.1.4 Modified Oswin Equation ....................................19 2.5 Drying Theory .....................................................................................19 2.6 Drying Rate Equation .........................................................................20 2.6.1 Constant Drying Rate Period ...............................................21 2.6.2 Falling Drying Rate Period ...................................................21 2.7 Shrinkage ..............................................................................................22 2.7.1 Shrinkage Model ....................................................................23 2.7.2 Volume Shrinkage ..................................................................23 2.7.3 Bulk Density ...........................................................................23 2.7.4 Particle Density ......................................................................23 2.7.5 Dry Solids Density .................................................................24 2.7.6 Equilibrium Density ..............................................................24 2.7.7 Porosity ....................................................................................24 v vi Contents 2.8 Pretreatments for Drying ...................................................................24 2.8.1 Pretreatment of Vegetables ...................................................24 2.8.2 Pretreatment of Fruit .............................................................26 Problems ..........................................................................................................27 References .......................................................................................................27 3. Various Designs of Solar Drying Systems ...............................................29 3.1 Introduction .........................................................................................29 3.1.1 Open Sun Drying (OSD) .......................................................29 3.1.2 Direct Solar Drying (DSD) ....................................................31 3.1.3 Indirect Solar Drying (ISD) ...................................................31 3.2 Classification of Solar Dryers ............................................................34 3.3 Passive Solar Dryers ...........................................................................35 3.3.1 Direct Passive Solar Dryers ..................................................35 3.3.2 Cabinet Dryers .......................................................................35 3.3.2.1 Normal Absorber Cabinet Dryer .........................38 3.3.2.2 Reverse Absorber Cabinet Dryer (RACD) ....................................................................38 3.3.3 Greenhouse Dryers ................................................................39 3.3.3.1 Definition of Greenhouse Effect ..........................40 3.3.4 Indirect Passive Solar Dryers ...............................................42 3.3.5 Mixed Mode Passive Solar Dryers ......................................43 3.4 Active Solar Dryers .............................................................................44 3.4.1 Direct Active Solar Dryers ....................................................45 3.4.2 Indirect Active Solar Dryers .................................................45 3.4.3 Mixed Mode Active Solar Dryers ........................................45 3.5 Hybrid Solar Dryers ...........................................................................47 3.6 Innovations in Solar Dryers ...............................................................48 3.7 Miscellaneous Design Solar Dryer ...................................................49 Problems ..........................................................................................................51 References .......................................................................................................52 4. Performance Analysis of Solar Drying Systems .....................................55 4.1 Introduction .........................................................................................55 4.2 Drying Efficiency .................................................................................56 4.3 Coefficient of Performance ................................................................57 4.4 Thermal Efficiency ..............................................................................57 4.5 Overall Daily Thermal Efficiency .....................................................58 4.6 Exergy Analysis ...................................................................................58 4.7 Exergy Analysis of Various Solar Dryers .........................................59 4.8 Testing Procedure of Solar Dryers in Load and No-Load Conditions ............................................................................................63 4.8.1 Load Condition ......................................................................63 4.8.2 No-Load Condition ...............................................................63 Contents vii 4.9 Discussion ............................................................................................65 4.10 Performance Characteristics of Various Solar Dryers ....................70 Problems ..........................................................................................................70 References .......................................................................................................71 5. Thermal Modeling of Solar Drying Systems ..........................................73 5.1 Introduction .........................................................................................73 5.2 Convective Heat and Mass Transfer .................................................74 5.2.1 Convective Heat Transfer .....................................................74 5.2.2 Convective Mass Transfer .....................................................77 5.3 Modeling Procedure for Convective Heat Transfer (CHT) ...........82 5.3.1 Case I: Natural Convection ..................................................84 5.3.2 Case II: Forced Convection ...................................................84 5.4 Thermal Modeling of Various Types of Dryers ..............................85 5.4.1 Thermal Modeling of Open Sun Drying ............................85 5.4.1.1 Crop Surface ...........................................................85 5.4.1.2 Moist Air above the Crop .....................................86 5.4.1.3 Analysis for Steady State Condition ...................88 5.4.2 Thermal Modeling of Greenhouse Drying Systems (TMGDS) .................................................................................89 5.4.2.1 Greenhouse Dryers under Natural Convection Mode ..................................................89 5.4.2.2 Solution of the Thermal Model for Greenhouse Dryers under Passive Mode ..........91 5.4.2.3 Greenhouse Dryers under Forced Convection Mode ..................................................92 5.4.2.4 Solution of the Thermal Model ............................93 5.4.3 Thermal Modeling of Indirect Solar Drying (ISD) Systems ....................................................................................94 5.4.3.1 Solar Air Heater .....................................................94 5.4.3.2 Drying Chamber ....................................................95 5.5 Statistical Parameters ..........................................................................95 5.5.1 Root Mean Square Deviation/Root Mean Square Error .................................................................................95 5.5.2 Coefficient of Correlation (r) ................................................96 5.5.3 Coefficient of Determination ................................................96 5.5.4 Sum of Squared Errors ..........................................................96 5.5.5 Root Mean Square Error (RMSE) .........................................96 5.5.6 Mean Percentage Error (MPE) .............................................97 5.5.7 Mean Square Error (MSE) .....................................................97 5.5.8 Adjusted R2 (R2) ....................................................................97 5.5.8.1 Case Study ..............................................................97 Problems ........................................................................................................101 References .....................................................................................................102 viii Contents 6. Energy Analysis of Solar Drying Systems .............................................103 6.1 Introduction .......................................................................................103 6.2 Embodied Energy of Different Prominent Materials ...................106 6.3 Energy Payback Time .......................................................................107 6.4 CO Emissions ...................................................................................107 2 6.5 Earned Carbon Credit.......................................................................108 6.6 Energy Analysis of Various Solar Dryers ......................................108 6.7 Energy Efficiency of Solar Drying Systems ...................................109 6.7.1 Case Study 1: Direct Solar Dryer .......................................111 6.7.2 Case Study 2: Indirect Solar Dryer ....................................112 6.7.3 Case Study 3: Mixed Solar Dryer .......................................112 Problems ........................................................................................................119 Nomenclature ...............................................................................................119 References .....................................................................................................119 7. Economic Analysis of Solar Drying Systems ........................................123 7.1 Introduction .......................................................................................123 7.2 Cost Analysis .....................................................................................123 7.2.1 Capital Recovery Factor (CRF) ..........................................124 7.2.2 Uniform Annualized Cost (UAC) ......................................124 7.2.3 Sinking Fund Method (SFM) ..............................................124 7.3 Cash Flow ...........................................................................................124 7.4 Payback Time .....................................................................................125 7.5 Benefit–Cost (B/C) Ratio..................................................................125 7.5.1 Advantages and Limitations of the B/C Ratio ................126 7.5.2 Shortcomings of the B/C Ratio ..........................................126 7.6 Effect of Depreciation .......................................................................126 7.7 Annual Cost Method ........................................................................127 7.8 Economic Analysis of Various Solar Dryers ..................................127 7.8.1 Direct Solar Dryers ..............................................................127 7.8.2 Mixed Mode Solar Dryers...................................................129 7.8.3 Indirect Solar Dryers ...........................................................130 Problems ........................................................................................................132 References .....................................................................................................133 Appendix: The Microsoft Excel Templates ...................................................135 Index .....................................................................................................................137 Foreword by Rebecca R. Milczarek Much regarding the solar drying of food materials has remained unchanged in the 14,000 years that cultures around the globe have practiced it. Today, mil- lions of entities—ranging from smallholder farmers in developing regions to multinational companies operating on thousands of hectares—still employ open sun drying to preserve fruit, vegetables, grains, and pulses. The contin- ued popularity of open sun drying is because it is perhaps the most widely accessible and low-cost food preservation method available. While much in this field has remained the same, since the 1990s there have been rapid technological advancements in solar drying that promise to enhance product quality, shorten drying time, and (further) improve the environmental sustainability of the process. While open sun drying still has a strong presence, indirect, mixed-mode, and active solar dryer designs are becoming increasingly popular. A deeper understanding of solar drying’s mass and energy transport phenomena is now possible through computa- tional multiphysics modeling programs. New dryer designs can be dissemi- nated across the globe in mere seconds. The drive to reduce greenhouse gas and carbon emissions in the food processing industry has also renewed inter- est in exploring and optimizing solar drying technologies. Solar Drying Systems is a text that meets this moment. Drs Prakash and Kumar have brought their collective 15-plus years of solar drying research and teaching experience to bear on this comprehensive reference and instruc- tional work. The authors have studied the solar drying of a diverse array of crops and food products—apple, bitter gourd, gooseberry, jaggery, onion, tomato, and watermelon, to name a few. The Agricultural Engineering com- munity widely cites their physics-driven approach to designing and ana- lyzing solar drying systems’ performance, and this approach forms the framework of their book. Readers will gain an understanding of both the fundamental principles of solar drying, which have remained constant over millennia, as well as recent advances in dryer design, analysis, and optimiza- tion, which are enabling a modern reimagining of the humble solar dryer. Looking forward to a bright future, Dr. Rebecca R. Milczarek Research Agricultural Engineer September 2020 ix