ABSTRACT
The research presents the design and development of indirect passive solar dryers, equipped with paraffin and used engine oil as thermal storage material embedded in a heat exchanger copper pipe, which runs from the collector to the drying chamber. The dryers were operated and tested under ambient conditions of temperature and relative humidity ranges of 29.1-40.2oC and 27-90% respectively. Paraffin and used engine oil thermal storage material has a temperature of 10.8°C each above ambient temperature at noon time which kept the product drying. Collector of dryer-a has the highest recorded efficiency of 26.67% as a function of the presence of the paraffin oil available as the thermal storage material in the heat exchanger pipe. Pre-treated ginger slices with 30seconds, 60 seconds, and 90seconds boiling water were used to test, compare and select dryer A as the best dryer based on standard dryer performance parameters. Dryer A equipped with paraffin oil produced 0.016MJ of energy with total useful energy of 1.30MJ reduced moisture content of ginger from 82.14%wb to 7.14%wb within 15 hours, while Dryer B with used engine oil produced 0.009MJ of energy with total useful energy of 1.29MJ dried similar product for 16 hours. Dryer A shows shorter drying time while Dryer B and C which carries air showed greater average exergy efficiency of 44.9% and 55.6% respectively. Pre-treatment and drying time showed statistical significant effect on moisture content at 5% probability level. Finite difference method was used to solve and simulate the heat and mass transfer equations of the selected dryer. The numerically simulation under natural convection using COMSOL multi physics software showed a perfect modelling of the system. Drying temperature, relative humidity and moisture distribution of the ginger slices where predicted and compared with experimented results with R2 values of 0.67, 0.61 and 0.95 respectively.
TABLE OF CONTENTS
Title Page i
Declaration ii
Certification iii
Dedication iv
Acknowledgements v
Table of Contents vi
List of Tables xi
List of Figures xii
List of Plates xiv
Abstract xv
CHAPTER 1
1.0 INTRODUCTION 1
1.1 Background of the study 1
1.2 Statement of Problem 3
1.3 Objective of the Study 4
1.3.1 General Objective 4
1.3.2 Specific Objectives 4
1.4 Justification 5
CHAPTER 2
2.0 LITERATURE REVIEW 6
2.1 Drying 6
2.2 Agricultural Product Drying Mechanism 6
2.3 Methods of drying 7
2.3.1 Sun drying 7
2.3.2 Shade drying 8
2.3.3 Oven drying 9
2.3.4 Heat pump drying 10
2.3.5 Vacuum drying 10
2.3.6 Freeze drying 11
2.3.7 Drum drying 12
2.3.8 Hot air drying 12
2.3.8.1 Spray drying: 12
2.3.9 Microwave drying 13
2.3.10 Infrared drying 14
2.3.11 Fluidized bed drying 14
2.3.12 Hybrid drying method 14
2.3.13 Solar drying 14
2.4 Solar Dryers 15
2.4.1 Classification of Solar dryers 15
2.4.1.1 Direct Solar Dryer 15
2.4.1.2 Indirect Solar Dryer 16
2.4.1.3 Solar dryers with forced convection and natural convection 16
2.4.1.4 Mixed mode 16
2.5 Previous studies on solar dryers 17
2.5.1 Solar dryers under natural convection 17
2.5.2 Solar dryers for greenhouse 18
2.5.3 Indirect type solar dryer 19
2.5.4 Cabinet type solar dryers 22
2.5.5 Tunnel type solar dryers 23
2.5.6 Integral solar dryers 24
2.5.7 Mixed-mode natural convective solar dryers 25
2.5.8 Solar chimney dryer 28
2.5.9 Back-pass and multi-pass solar dryer 29
2.5.10 Domestic and industrial economical solar dryers 30
2.5.11 Solar timber kilns 31
2.6 Heat Exchanger 31
2.6.1 Types of heat exchangers 33
2.6.1.1 Parallel and counter flow 33
2.6.1.2 Cross flow 33
2.6.1.3 Shell and tube 34
2.6.1.4 Compact heat exchanger 34
2.6.2 Heat exchanger for solar drying 35
2.6.2.1 Solar dryer with geothermal water heat exchanger 35
2.6.2.2 Solar Air Heat Exchanger in Phase Change Material 36
2.7 Thermal energy storage 37
2.8 Thermal storage Materials 39
2.9 Previous work with thermal application in solar dryers 41
2.10 Energy based analysis and Exergy based analysis 44
2.11 Drying and Dryer efficiency 46
2.12 Numerical Simulation 48
CHAPTER 3
3.0 MATERIALS AND METHOD 50
3.1 Materials for Construction 50
3.2 Design consideration 51
3.3 Design Calculation 51
3.3.1 Amount of water to be removed from the Product 51
3.3.2 Collector design 52
3.3.2.1 Determination of useful heat for drying 52
3.3.2.2 Sizing of the collector 53
3.3.2.3 The Collector Orientation and tilt angle (β) of the solar collector 53
3.3.3 Mass of air required for evaporation of water 54
3.3.4 Air Flow Requirement 54
3.4 Study Area 55
3.5 Experimental procedure 55
3.5.1 Sample Preparation 55
3.5.2 Data collection 55
3.5.3 Pre-treatment of Ginger 56
3.6 Experimental Design 56
3.7 Dryer Performance evaluation parameters 58
3.7. 1 Percentage Moisture Loss 58
3.7.2 Average Drying Rate 59
3.7.3 Solar Collector Efficiency 59
3.7.4 Dryer Efficiency 60
3.8 Energy Analysis 61
3.9 Exergy Analysis 62
3.10 The exergy efficiency 63
3.11 Statistical Analysis 63
3.12 Numerical simulation 64
3.12.1 Modeling of the dryer 64
3.12.2 Solar collector Modelling 65
3.12.3 Modeling of the drying chamber 74
3.12.4 Energy balance 74
3.12.5 Mass balance 83
3.12.6 Thermo physical properties used 86
3.12.7 Determination of geometrical factors 89
3.12.8 Determination of air masses 89
3.12.9 Solution procedure 89
3.12.10 Analytical procedure 90
3.12.11 Model Validation 91
CHAPTER 4
4.0 RESULTS AND DISCUSSION 93
4.1 Description of the developed solar dryers 93
4.2 Performance of the dryers 96
4.2.1 Performance indicators 96
4.2.2 Drying kinetics of ginger 103
4.2.3 Drying kinetics of pre-treated ginger product using the best selected Dryer (Dryer A) 105
4.3 Energy and Exergy Analysis 107
4.3.1 Energy Analysis 103
4.3.2 Exergy analysis on the Dryers 109
4.4 Statistical Analysis of the drying conditions 111
4.4.1 Effect of Solar Drying Conditions on the Performance of the Solar Dryers 111
4.5 Numerical simulation of the Best dryer 112
CHAPTER 5
5.0 CONCLUSION AND RECOMMENDATIONS 122
5.1 Conclusion 122
5.2 Recommendation 122
REFERENCES 124
APPENDICES 138
LIST OF TABLES
2.1: Thermal properties of some commonly used thermal storage materials 39
3.1: Experimental design 57
3.2: Heat mass 87
3.3: Optical properties 88
4.1: Design parameters of the solar dryers 96
4.2: Performance parameter of the developed solar dryers 109
4.3: Analysis of Variance for Dryer A: Effect of Pre-treatment and drying time on moisture content level of the sample in Dryer A 111
4.4: Coefficients for Dryer A 112
LIST OF FIGURES
2.1: Spray drying 13
2.2: Schematic of semi-cylindrical solar tunnel drying system 24
2.3: Mixed-mode natural convection solar dryer developed by 26
2. 4: Mixed mode dryer 28
2.5: Functional architecture of the chimney-dependent direct-mode solar crop dryer 29
2.6: Solar kiln dryer with energy storage 32
2.7: Heat exchanger showing Co-current, Cross flow, Counter-current and cross counter flow 33
2.8: Showing shell and tube heat exchanger (a) and Compact heat exchanger (b) 35
2.9: Experimental set up for solar air heat exchanger 37
2.10: Different types of thermal storage of solar energy 38
3.1: Schematic diagram showing the heat transfer on the solar collector 64
3.2: Schematic diagram showing the heat transfer in the drying chamber 74
3.3: Flowchart of the simulation 91
4.1: The solar dryers 95
4.2: Solar collector temperatures for different dryers 98
4.3: Temperatures of thermal storage material 98
4.4: Solar collector Relative humidity for different dryers 98
4.5: Solar radiation intensity for the three days of drying 99
4.6 Drying chamber temperatures 100
4.7 Drying chamber Relative Humidity 101
4.8: Collector Efficiency of the Solar Dryers 101
4.9 Dryer Efficiency for different solar dryers 102
4.10: Moisture loss with respect to time for different dryers 104
4.11: Drying rate of ginger for different dryers 104
4.12 Moisture Ratio of the product 105
4.13 Drying rate of the product 106
4.14: Average Effective moisture diffusivity 107
4.15 Exergy in and Exergy out of the dryers 109
4.16: Exergy Loss 110
4.17: Exergy Efficiency 110
4.18: Experimented and Predicted Drying chamber temperatures 114
4.19: Experimented and predicted drying air RH in the drying chamber 115
4.20: Experimented and predicted moisture content of the drying product (ginger slices) 115
4.21: Temperature distribution between 1-4hours drying 116
4.22: Temperature Distribution 5-8hours of drying 117
4.23: Drying chamber relative humidity 1-4hours 118
4.24: Relative humidity distribution 5-8hour 119
4.25: Moisture content distribution of drying ginger slice 1-4hours 120
4.26: Moisture content distribution of ginger slice (5-8hours) 121
LIST OF PLATES
2.1: Showing sun drying and it limitations (Source: Google search) 8
2.2: Shows an Oven dryer (Naseer et al., 2013). 9
2.3 Shows a Freeze dryer (Naseer et al., 2013) 11
2.4: Portable form solar dryer developed by (Singh et al., 2004) 22
2.5: Mixed mode solar dryer (Ndukwu et al., 2020d) 27
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND OF THE STUDY
Solar energy which is an alternative energy source is a cheap, clean and a very safe renewable energy source that has been used for drying agricultural products (Alonge et al., 2012a). Since ancient times, solar drying has been used in the processing of agricultural products to extend the shelf life of food, using different types of dryers to generate abundant solar energy (Fakayode 2013; Azaizia et al., 2017). The operation of solar dryers depends entirely on solar energy and ambient air affecting pressure gradients that naturally promote vertical airflow within a drying chamber. Solar drying has shown significant potential in elongating product shelf life while consequently reducing both the product weight and volume. This has however reduced the cost of packaging of the product, storage, and even transportation (Chaudhri et al., 2009). On the general, solar dryers exhibit faster drying rates than sun drying methods by achieving higher temperatures, lower humidity, and increased air flow movement.
Research has also shown that solar drying can improve some aspects of a product in terms of quality, flavour, colour and appearance which projects the product’s marketability and allows for improved financial opportunities for the farmers (Eke 1991, Chen et al., 2007 and Alonge et al., 2007). Solar dryer is preferred over the traditional method of open-air sun-drying as presented by various researchers (El-Sabaii and Shalaby, 2012; Jairaj et al., 2009; Alonge et al., 2012a 2012b). While sun-drying in an open space causes loss of product quality and quantity. This is as a result of contamination by dust, dirt, unexpected rain, attacks from birds, rodents and various kinds of animals. Solar drying system in the other hand addresses this challenges using a solar collector and drying cabinet covered with a transparent glass, which provides a clean and healthy environment for the product being dried and perfectly harnesses the solar radiant energy for improved quality of dried products.
According to Ricci et al. (2012) using solar energy for the purpose of food production has provided substantial non-polluting tendencies in the subtropical and tropical regions of the world. Utilization of solar dryer allows for the processing of the products at a low initial capital investment and requires only basic training necessary to equip the operational staff. Based on their unique mode of operation, solar dryers are typical classified into three groups: Solar dryers, indirect solar dryers, and hybrid solar dryers (or hybrid operations) (Kumar et al., 2016). For the direct solar dryers, the collector and the drying chamber forms a single unit. While for the indirect systems, the sun irradiation is trapped by a fluid (air) reservoir that provides hot moving fluid into the drying chamber which the produce to be dried are kept. Mixed mode solar dryers combines the drying mechanism of both the direct and indirect drying system, where drying is done concurrently by both direct exposure of the food produce to solar irradiation and passing heated air from the solar collector to the produce in the drying chamber (Fudholi et al., 2010).
However due to the challenges posed by weather, solar irradiation is intermittent. In the absence of solar irradiation, there is the possibility of re-wetting of the product; which will prolong the drying time and leads to product decay (Ndukwu et al., 2016, 2017a, 2017b, 2018, 2020b, 2020c and 2020d). Therefore hybrid dryers are developed to assist the drying during off-sunshine period. The design includes the use of electric heater, biomass gassifier, or thermal storage material as supplementary heat (Ndukwu et al., 2020a, 2020b). Hybrid solar dryer are found to produce better quality product at a faster drying rate in comparison with other solar dryer types due to combined heat sources (Simate, 2003; Singh and Kumar, 2012).
Thermal storage material captures the solar energy and stores them. This stored energy is released as the product cools during off-sunshine period. Most of the thermal storage materials are phase change material with very high latent heat which it gives out as it changes from one phase to another. Existing literature has shown that materials like paraffin, glycerine, hydrated salts etc, were among phase change material used. However, used engine oil has been found to possess a high latent heat too, since this oil is a waste material from engine, it can be used as a cheap source of thermal storage material in solar dryer designs. Another challenge is using phase changing liquid with constant repositioning of the material from the collector to the drying chamber. Therefore integrating them in a tubular heat exchanger as a compact composite from the collector to the drying chamber will be an innovative design (Ndukwu et al., 2020d).
Although different designs of hybrid solar dryers exist in literature; only a few employed numerically designed models. Numerical modelling of systems is important to determine the drying conditions necessary to achieve the best drying point (Yadav and Bhagoria, 2013).
1.2 STATEMENT OF PROBLEM
Open sun drying mostly adopted by farmers exhibits slow drying rate; there is possibility of dirt or fungal contamination of product due to birds and rodents attack. Thus leading to deterioration of quality, especially colour or flavour degradation (Hossain, 2003). One of the purposes of drying agricultural products is to ensure their good quality during storage, as the reduction in the moisture content level retards the biological activity. This will be shown in the physical and chemical changes that occur during the period of storage. The study of drying mechanism provides adequate information on the mass and heat transfer that occur between the biological material and that of the drying element (mostly heated or non-heated atmospheric air), which is important for the design, operation and simulation of drying systems as well as solar dryers (Correa et al., 2003). Knowledge of the drying parameters involved in the drying of agricultural products aids engineers to design and develop better drying equipment, calculate energy requirements for the process, study the properties of water adsorbed, evaluate the microstructure of food and study the physical phenomena that takes place at the material surface (Corrêa et al., 2010). Furthermore, adopting solar dryers in drying of agricultural product raises the challenge of intermittent drying due to weather instability. Hence there is need to develop a solar dryer with different thermal storage material to achieve better and all round drying during the day and night hour for better performance. Again developing the solar dryer is one but scaling it up to industrial application requires development of appropriate models for optimization purposes.
1.3 OBJECTIVE OF THE STUDY
1.3.1 General objective
The general objective of this work was to develop and test various solar dryers integrated with tubular heat exchanger filled with liquid paraffin, used engine oil and air.
1.3.2 Specific objectives
In order to achieve the main objective of this study, the specific objectives are to:
1. Design and fabrication of four solar dryers equipped with heat exchanger.
2. Determine the drying efficiencies of the dryers.
3. Carryout comparative analysis on the four dryers to select the best dryer.
4. Carry out energy and exergy analysis on the dryers.
5. Study the drying kinetics (drying rates, moisture ratio, diffusivity etc.) of the dried product using the best dryer.
6. Carry out statistical analysis of the drying conditions.
7. Numerically simulate the coupled heat and mass transfer of the best solar dryer to predict the temperature, moisture distribution of ginger slices and relative humidity of the drying process.
1.4 JUSTIFICATION
Solar energy offers a good alternative and environmental friendly energy source for drying of crops with regards to the environmental demerits faced by the use of fossil fuel based dryers. However, solar dryers are faced with the challenge of crop rewetting when the condition is not favourable especially during off sun shine period (Ndukwu et al., 2020a, 2020c). Therefore integrating heat exchanger with thermal storage material will solve this problem and assist in drying during off sun period hence justifies the project. Developing a novel model for the dryer will assist in the optimization of the process indicators of solar dryers at different environmental condition. The model will also help in the scaling up of the developed dryer at any chosen environment.
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