DESIGN DEVELOPMENT AND PERFORMANCE EVALUATION OF SOLAR DRYER FOR DRYING OF TOMATO AND ONION SLICES.pdf

DESIGN DEVELOPMENT AND PERFORMANCE EVALUATION OF SOLAR DRYER FOR DRYING OF TOMATO AND ONION SLICES

M. Sc. Thesis

ABDULAHI UMAR

April 2011 HARAMAYA UNIVERSITY i

DESIGN DEVELOPMENT AND PERFORMANCE EVALUATION OF SOLAR DRYER FOR DRYING OF TOMATO AND ONION SLICES

A Thesis Submitted to the School of Graduate Studies through Department of Food Science and Post Harvest Technology HARAMAYA UNIVERSITY

In Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE IN FOOD ENGINEERING

By Abdulahi Umar

April 2010 Haramaya University

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SCHOOL OF GRADUATE STUDIES HARAMAYA UIVERSITY

As Thesis Research advisor, I hereby certify that I have read and evaluated this thesis prepared, under my guidance, by Abdulahi Umar entitled “Design Development and Performance Evaluation of Solar Dryer for Drying of Tomato and Onion Slices”. I recommend that it be submitted as fulfilling the thesis requirement. Solomon Abera (D. Eng.) Major Advisor

_________________

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Signature

Date

As member of the Board of Examiners of the M.Sc. Thesis Open Defense Examination, We certify that we have read, evaluated the Thesis prepared by Abdulahi Umar and examined the candidate. We recommended that the Thesis be accepted as fulfilling the Thesis requirement for the Degree of Master of Science in Food Engineering.

______________________ Chairperson

______________________ Internal Examiner

______________________ External Examiner

_________________ Signature

_________________ Signature

_________________ Signature

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____________ Date

_____________ Date

____________ Date

DEDICATION

I dedicate this thesis manuscript to my father UMAR AHMED, and my mother ASHA ABDULAHI, for nursing me with affection and love and for their dedicated partnership in the success of my life.

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STATEMENT OF THE AUTHOR

First, I declare that this thesis is my bonafide work and that all sources of materials used for this thesis have been duly acknowledged. This thesis has been submitted in partial fulfillment for the requirements for M.Sc. degree in Food Engineering at the Haramaya University and is deposited at the University Library to be made available to borrowers under rules of the library. I solemnly declare that this thesis is not submitted to any other institution anywhere for the award of any academic degree, diploma, or certificate.

Brief quotations from this thesis are allowable without special permission provided that accurate acknowledgement of source is made. Requests for permission for external quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the School of Graduate Studies when in his or her judgment the proposed use of the material is in the interest of scholarship. In all other instances, however, permission must be obtained from the author.

Name: Abdulahi Umar

Signature:

Place: Haramaya University, Haramaya Date of Submission:

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LIST OF ABBREVIATIONS

ANUB

Annual Net Undiscounted Benefits

DM

Dry matter

FARC

Fadis Agricultural Research Center

GPS

Global Positioning Satellite

II

Initial Investment

MMSCD

Mixed mode solar cabinet dryer

NCSD

Natural convention solar drying

OARI

Oromia Agricultural Research Institute

OASD

Open- air sun drying

PC

Polycarbonate

PP

Payback Period

PVSD

Photo voltaic ventilated solar drying

UV

Ultra violet

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BIOGRAPHY

The author was born in September 1967 in Haramaya town, Ethiopia. He attended his elementary and secondary school education at Bate Junior and Senior Secondary School, and Harar Junior and Secondary High School, Harar, respectively. He joined the then Alemaya University of Agriculture (AUA) and received B.Sc. in Agricultural Engineering in 1988. Soon after leaving Alemaya University, he was employed by Ministry of Agriculture (1989-1995), Haramaya University (1996-2006) and Oromia Agricultural Research Institute (OARI) until joining the School of Graduate Studies of Haramaya University for his graduate studies since Oct. 2008.

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ACKNOWLEDGEMENTS

Praise to God, the Almighty who sustain my life in this world and in the hereafter. The Author is highly indebted to his advisor D. Eng. Solomon Abera without his encouragement, insight, guidance and professional suggestions, the completion of this work would not have been possible. I also thank Dr. Geramew Bultesa, for my successes and who has encouraged me in this field. His advice and guidance for my research and contribution to my education has been invaluable. I thank Dr. Eng. Solomon Worku, for the inspiration and encouragements to complete this research work.

Great deal of thanks must be given to the sponsor, OARI and its staff for providing the funds for this research. Special thanks go to FARC and its staff for providing workshop services and sincere cooperation. Special thanks go to the FARC workshop staff in manufacturing the solar dryer and for their technical support and friendly assistance during the manufacturing work at FARC. Special thanks go to Haramaya University Food Science and Post-harvest Technology staff for providing me materials and services.

A very deep admiration and special thanks also go to my parents, family and friends for encouragement, financial support, and affection during my stay at SGS and immeasurable sacrifices they made to bring me to this stage.

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STATEMENT OF THE AUTHOR

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LIST OF ABBREVIATIONS

v

BIOGRAPHY

vi

ACKNOWLEDGEMENTS

vii

TABLE OF CONTENTS

ix

LIST OF FIGURES

xi

LIST OF TABLES

xii

LIST OF TABLES IN APPENDIX

xiii

ABSTRACT

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1. INTRODUCTION

1

2. LITERATURE REVIEW

4

2.1. Drying

4

2.1.1. Purpose of drying 2.1.2. Application of drying 2.1.3. Drying methods

4 5 5

2.2. Theory of Drying

6

2.2.1. Air properties

6

2.2.2. Drying mechanism

9

2.3. Thin Layer Drying Models

11

2.4. Sun and Solar Drying

13

2.3.1. Classification of solar drying 2.3.2. Types of solar dryers 2.3.3. Major components of solar dryers

15 16 18

2.4. Drying Efficiencies

19

2.5. Drying of Tomato and Onion

20

2.5.1. Solar drying of tomato 2.5.2. Solar drying of onions

20 21

3. MATERIALS AND METHODS viii

31

TABLE OF CONTENTS(Contd) 3.1. Description of the Study Site

31

3.2. The Design of the Solar Dryer

31

3.2.1 Drying chamber 3.2.2. The collecting chamber

32 35

3.3. Performance Evaluation of Solar Dryer 3.3.1. Measuring instruments

39 39

3.3.2. Preliminary test of the solar dryer 3.3.3 Efficiency of solar dryer 3.3.4. Sample preparation 3.3.5. Moisture content determination of samples 3.3.6. Testing the solar dryer using tomato with natural convection current 3.3.8. Performance evaluation of solar dryer using tomato and onion in forced ventilation 3.3.9. Kinetics of drying

40 40 42 42 44 46 46

3.4. Statistical Analysis

47

4. RESULTS AND DISCUSSION

49

4.1. Preliminary Test Data of the Solar Dryer

49

4.2. Collector Efficiency

51

4.3. Test of Solar Dryer Using Tomato Slice in Natural Convection Current

52

4.4. Test of Solar Dryer Using Onion Slice in Natural Convection Current

56

4.5. Characteristics of the Solar Dryer under Forced Ventilation

59

4.6. Testing the Solar Dryer in Forced Air Circulation Using Tomato

61

4.7. Testing the Solar Dryer in Forced Air Circulation Using Onion

63

4.9. Economic Feasibility and Pay Back Analysis of the Solar Dryer

69

5. SUMMARY, CONCLUSION AND RECOMMENDATION

71

5.1. Summary

71

5.2. Conclusions

73

5.3. Recommendations

74 ix

TABLE OF CONTENTS(Contd) 6. REFERENCES

75

7. APPENDIX

82

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LIST OF FIGURES Page Figure 1 Framework of the solar Dryer ......................................................................... 32 Figure 3. Drying chamber frame of the solar dryer ........................................................ 33 Figure 4. Drying chamber wall frame ........................................................................... 34 Figure 5. The roof frame of drying chamber ................................................................. 34 Figure 6. The position of the shelves in the drying chamber .......................................... 35 Figure 7. The collector plate of the solar dryer .............................................................. 37 Figure 8. The roof frame structure of the collecting chamber ........................................ 38 Figure 9. Photo of solar dryer ....................................................................................... 39 Figure 10. Schematic diagram of solar dryer ................................................................ 40 Figure 11. The solar radiation, collector outlet & ambient air temperature..................... 50 Figure 13. The profile of relative humidity in the drying chamber ................................ 61

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LIST OF TABLES

Table 1. Treatment combination, replication and randomization.................................................. 48 Table 2. Preliminary test data at no load of the dryer at half open position of control device .......................................................................................................................... 49 Table 3. Raw data of the collector efficiency analysis for solar dryer .......................................... 51 Table 4. Weight of tomato, percentage moisture contents on wet basis, dry basis and drying rate on dry basis on Tray1, Tray2, Tray 3, Tray 4, Tray 5 and open air sun trays during tomato drying using natural convection current and open-air sun drying .................................................................................................................... 54 Table 5. Weight of onion, percentage moisture contents on wet basis, moisture contents on dry basis and drying rate on dry basis on Tray1,Tray2, Tray 3, Tray 4, Tray 5 and open air sun tray during onion drying using natural convection current and open-air sun drying tests ........................................................................................ 58 Table 6. Weight of tomato, percentage, moisture content on wet basis and percentage drying rate on dry basis on Tray1, Appendix Tray2, Tray 3, Tray 4 and Tray 5 and open air sun Tray4 and Tray 5 (Ventilated tomato drying) ...................................... 62 Table 7. Weight, percentages of moisture content on wet basis and drying rate on dry basis of onion samples in the dryer on trays 1,2.3.4 and 5 and open air sun (Ventilated onion drying) ............................................................................................. 64 Table 8. Values of drying rate coefficients ‘k’(h-1) for tomato and onion slices dried in the solar dryer and open-air sun drying. ...................................................................... 66 Table 9. ANOVA of the drying rate coefficient........................................................................... 66 Table 10. Payback period of the solar dryer used for drying tomato and onion ........................... 70

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LIST OF TABLES IN APPENDIX

Appendix Table 1: Whether parameters of Haramaya University ................................................ 83

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DESIGN DEVELOPMENT AND PERFORMANCE EVALUATION OF SOLAR DRYER FOR DRYING OF TOMATO AND ONION SLICES

ABSTRACT

A solar dryer was designed and manufactured at Fadis Agricultural Research Center workshop of Oromia Agricultural Research Institute. The framework of all the parts of the dryer were built by joining perforated angle irons of 40 mm  40 mm  4 mm and 20 mm  20 mm  4 mm by means of bolts and nuts. The dryer covers 3.0 m  3.0 m area of the ground of which the 1m2 was used for drying chamber while the rest was saved for collecting solar radiation. The drying chamber surrounded by the collector from three sides , had five shelves positioned one on the top of another with 10 cm clearance in between. The roofs and walls of the dryer were covered with the flexible transparent plastic leaving the three sides of the solar collector open to allow air in. Preliminary tests with no load to the dryer showed that the solar collector raised the ambient air temperature of 20°C to 41°C to a warm air of 28°C to 64°C between the morning and midday. This lowered the relative humidity of air from average 26% in the morning to 5% at midday. The dryer, loaded at 5 kg/m2, dried tomato slices of 8 mm thickness from initial moisture content of 93.3% (w.b) to final moisture content of 12% (w.b) in 13 hours and11hours when operated under natural convection current. Similarly, onion slices of 3 mm thickness, loaded at a rate of 4 kg/m2, dried from 87.10% (w.b) initial moisture content to 9.1% (w.b) final moisture content in 10 hours. Using forced ventilation, the slices of tomato and onion took 11 hours and 9 hours to reach their final moisture contents of 12% and 9.1% (w.b), respectively. The open air-sun drying tests conducted side by side with solar drying needed an average of 20 hours to reach the same final moisture contents for both tomato and onion slices. The maximum drying rate of tomato slices attained under natural convection and forced circulation were 3.1 and 2.8 xiv

kg of water per kg of dry matter-hr, while those of the onion slices 2.6 and 1.5 kg of water per kg of dry matter-hr. For the open-air sun drying, the maximum drying rates for tomato and onion slices were 1.5 and 0.82 kg of water per kg of dry matter-hr. Drying tomato and onion slices to their final moisture contents took one-half, two & four days and one, two and three days in PVSD, NCSD and OASD, respectively. Drying rate coefficients ‘k’(-1hr) of Lewis model were statistically significantly different and could be used to describing solar and open-air sun drying characteristics of solar and open-sun dryings of tomato and onion slices. From economic feasibility and payback analysis of the solar dryer, the payback period was determined and was very small (1.20 months) compared to the life of the dryer, so the dryer will dry product free of cost for almost its life period of 15 years.

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1. INTRODUCTION

Vegetables and their products are of great nutritional importance since they make a significant contribution in supplying wealth of essential vitamins, minerals, antioxidants, fibers and carbohydrates that improve the quality of the diet. Vegetable production is seasonal in nature and during peak, harvest there is often a glut to the market and at unsafe storage moisture levels. That leads to drastic drop in the price of the produce as there are no facilities for long-term storage and that the commodity has to be sold out before it perishes. Ethiopia has different agro-climates and soil types that enable to produce various types of vegetable and fruit crops for both local consumption and export markets. However, growing and marketing fresh produce in Ethiopia is complicated by high postharvest loss, which reaches about 30% (EARO, 2000). Naturally, fresh produce needs low temperature and high relative humidity environment during storage and transportation.

However, the means of achieving these for long-term purpose is beyond the reach of the economy of the majority of the producers and local traders. Established system of cold chain consisting of packinghouses, cold storage and refrigerated transportation is needed to reduce this loss to acceptable level.

Drying is a common method for preservation of food products. The main purpose of drying is the reduction of moisture content to a safe level for extending the shelf life of products. The removal of water from fruit and vegetables provides microbiological stability and reduces deteriorative bio-chemical reactions. In addition, the process allows a substantial reduction in terms of mass, volume and packaging requirement, which reflects on handling, storage and transportation costs with more convenience (Okos et al., 1992). It ensures their availability at all times of the year.

Drying kinetics is generally affected by air temperature, relative humidity of the air, air velocity and material size (Kiranoudis et al., 1992). Generally, the drying phenomena can be described using thin layer drying models mainly to estimate the drying times and 1

moisture content of the food materials at any time after they are subjected to a known temperature and relative humidity (Torgul and Pehlivan, 2004). Many research studies have been reported on mathematical modeling and experimental studies conducted on thin layer drying process of various food products such as onion and pepper (Kiranoudis et al., 1992), chilli (Hussain and Bala, 2002), carrot (Doymaz, 2004) and tomato (Sacilik et al., 2006).

Use of dehydrated vegetables in various convenience foods is a common phenomenon all over the world. The application of dried potatoes, tomatoes, garlic, onion, carrot, mushrooms and sweet potatoes in various food products including bread, doughnuts, soups, stews, etc. is a practice of long history.

The introduction of solar drying system seems to be one of the most promising alternatives to reduce postharvest losses. Solar dried products have much better colour and texture as compared to open sun dried products. The justification for solar dryers is that they dry products rapidly, uniformly and hygienically. Since, they are more effective than open sun drying and have lower operating costs than mechanized dryers (Diamente and Munro, 1993; Condori et al., 2001); more importance is given now a day to the use of solar dryers. The open-air sun drying process is not very hygienic. It depends on weather conditions and there is a risk of deterioration (Bala et al., 2003). Some of the problems associated with open-air sun drying can be solved with a solar dryer, which can reduce crop losses and improve the quality of dried product significantly compared to traditional drying methods (Madhlopa and Ngwalo, 2007).

Use of solar dryers is a much-preferred alternative in view of its low initial capital and running costs, and free and ample supply of solar energy in the country. However, no information is available on solar drying of fruit and vegetables under Ethiopian climatic conditions in general and particularly under the local conditions of the eastern part of the country. 2

Although a number of designs of solar dryers exist in various countries, there are no such dryers with proper design with adequate information on drying performance available on the market in Ethiopia. The very few attempts done in some places ended up in solar dryers that are not affordable by the farming communities, difficult to transport from place to place, and have no scientific information at all on the capacity, drying performance and utilization.

Those which are imported from elsewhere are expensive, cumbersome,

complicated and unavailable to the users.

One can clearly see the need for easily available and affordable appropriate drying technology as a means of tackling the unacceptably high postharvest loss of fruits and vegetables in Ethiopia. Development of solar dryer with all the necessary information on its performance and operation can be one aspect of the solution for the problems. Therefore, this research was initiated to design, develop and conduct performance evaluation of a solar dryer for drying of vegetables and fruits. Tomato and onion were considered as study crops, based on ease of supply during the test period. The dryer was intended for use with mainly natural convection air movement but also tested with photovoltaic powered fans for use (in the event) when the need arises to increase the drying efficiency.

The general objective of this work, therefore, is the development of economically affordable and simple to construct solar operated dryer for drying fruit and vegetables. The specific objectives include: o To design, construct and evaluate the performance of solar dryer for drying fruits and vegetables. o To compare the characteristics and performance of solar dryings to open air sun drying.

3

2. LITERATURE REVIEW

This chapter deals with the review of research works carried out on drying and its theory, classification, types of dryers and general information about tomatoes and onion drying.

2.1. Drying Drying is one of the oldest food preservation methods and it is defined as the application of heat under controlled condition to remove the majority of the water normally present in a food by evaporation. (Fellows, 2000).

2.1.1. Purpose of drying The main purpose of drying is to extend the shelf life of food by reduction of water activity. This inhibits microbial growth and enzyme activity, but the drying air temperature is usually insufficient to cause their inactivation. Furthermore drying causes decrease in weight and volume of vegetables thereby reducing transport and storage costs. Since drying can lead to deterioration of both the eating quality and the nutritive value of the food, design of drying equipment and operation is aimed at minimizing these negative effects by selection of appropriate drying conditions for the food.

The basic essence of drying is to reduce the moisture content of the product within a certain period, to a level that prevents deterioration, normally regarded as the “safe storage moisture”. It was described by Ife and Bas (2003), that the moisture level of most vegetables is 10-15% so that the microorganisms present cannot thrive and the enzymes become inactive, that dehydration is usually not desired, because the products often become brittle and stored in a moisture-free environment, ,

4

2.1.2. Application of drying Drying operation is used for dehydration of various types of foods. The drying of fruit and vegetables is a subject of great importance. Dried fruit and vegetables have gained commercial importance and their growth on a commercial scale has become an important sector of agricultural industry (Karim and Hawlader, 2005). Examples of commercially important dried foods are coffee, milk, raisins, sultanas, and other fruits, vegetables, pasta, flours (including bakery mixes), beans, pulses, nuts, breakfast cereals, tea and spices. Important dried ingredients used by food manufacturers include egg powders, flavorings, colorings and lactose, sucrose, or fructose powder, enzymes and yeasts. The advantages of dried foods were listed as follows:  Extended shelf life because of inhibition of microbial and enzymatic reactions.  Providing consistent product and the seasonal variations are diminished.  Substantially lower cost of handling, transportation and storage.  The dried products size, shape and form are modified and the price is constant throughout the year.  Dried foods can be packed in recyclable packages; this is not always done with fresh foods.  The dried foods can be used as snacks and other processed foods. 2.1.3. Drying methods Several drying methods are commercially available and the selection of the optimal method is determined by quality requirements, raw material characteristics, and economic factors. There are three types of drying processes: sun and solar drying; atmospheric dehydration including stationary or batch processes (kiln, tower, and cabinet driers) and continuous processes (tunnel, continuous belt, belt-trough, fluidized-bed, explosion puffing, foam-mat, spray, drum, and microwave-heated driers); and sub-atmospheric dehydration (vacuum shelf, vacuum belt, vacuum drum, and freeze driers) (Chua, 2003). 5

2.2. Theory of Drying Dehydration involves the simultaneous application of heat and removal of moisture from food; the factors that control the rate of transfer are summarized and categorized as those related to the processing conditions, nature of the food and the drier design.

2.2.1. Air properties The properties of the air flowing around the product are major factors in determining the rate of moisture removal. The capacity of air to remove moisture is principally dependent upon its initial temperature and humidity; the greater the temperature and lower the humidity is the higher the moisture removal capacity of the air. The relationship between temperature, humidity and other thermodynamic properties is represented by the psychrometric chart. The absolute humidity is the moisture content of the air (mass of water per unit mass of air) whereas the relative humidity is the ratio, expressed as a percentage, of the moisture content of the air at a specified temperature to the moisture content of air if it were saturated at that temperature.

Relative humidity is defined as the ratio of the amount of water vapor in the air (Nw) to the amount the air will hold when saturated at the same temperature (Nws).The partial pressure of water vapor at saturation (Pws) is a function only of temperature. However, this relationship is complex, involving multiple exponential and logarithmic terms (Wilhelm, 1976).

Another psychrometric parameter of interest is the humidity ratio (W). The humidity ratio is the ratio of the mass of water vapor in the air to the mass of the dry air. The partial pressure of the dry air (Pa) is the difference between atmospheric pressure and the partial pressure of the water vapor, also called vapor pressure.

The degree of

saturation is another psychrometric parameter that is sometimes used. It is the humidity ratio divided by the humidity ratio at saturation.

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A final parameter that can be determined from the perfect gas law is the specific volume of the moist air. The specific volume is defined in terms of a unit mass of dry air.

The dew point temperature (tdp) is the temperature at which moisture begins to condense if air is cooled at constant pressure. The dew point temperature is directly related to partial pressure of the water vapor (Pw); however, that relationship is complex, involving several logarithmic terms (ASHRAE, 1997). Since Pw is also related to the humidity ratio W, this means that specifying any one of the three parameters tdp, Pw, and W specifies all three. The wet-bulb temperature (twb) is the temperature measured by a sensor (originally the bulb of a thermometer) that has been wetted with water and exposed to air movement that removes the evaporating moisture. The evaporating water creates a cooling effect. When equilibrium is reached, the wet-bulb temperature will be lower than the ambient temperature. The difference between the two (the wet bulb depression) depends upon the rate at which moisture evaporates from the wet bulb. The evaporation rate, in turn, depends upon the moisture content of the air. The evaporation rate decreases as the air moisture content increases. Thus, a small wet bulb depression indicates high relative humidity, while a large wet bulb depression is indicative of low relative humidity.

The enthalpy (h) of moist air is one of the most frequently used psychrometric parameters. It is a measure of the energy content of the air and depends upon both the temperature and the moisture content of the air. It is determined by adding the enthalpy of the moisture in the air (W hw) to the enthalpy of the dry air (ha): h = ha +W hw = Cpa t +W(hfg + Cpw t)

(Wilhelm, 1976).

h = 1.006t +W(2501+1.805t), based upon (Cpa) a specific heat for air of 1.006 kJ/kg K° and a zero value of h at t = 0°C. The enthalpy of water is based upon: a zero value of h at 0°C (liquid state); hfg = 2501 J/kg at 0°C; and an average specific heat for water vapor of 1.805 kJ/kg K. The above equation provides a good approximation for the enthalpy of moist air over a wide range of temperatures; however, the error increases rapidly at temperatures above 100°C. Empirical relationships, charts, or tables must then be used to determine.

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Psychrometric Charts Properties shown on most psychrometric charts are dry bulb, wet-bulb, and dew point temperatures; relative humidity; humidity ratio; enthalpy; and specific volume.

Processes on the Psychrometric Chart The psychrometric chart is used in many applications both within and outside the food industry. Drying with air is an extremely cost-effective method to reduce the moisture of a biological material, and the addition of a small amount of heat significantly improves the air’s drying potential. .

By a process, it means moving from one state point to another state point on the chart. Few simple processes, the paths of these processes can be displayed on small psychrometric charts. These are ideal processes assuming no heat transfer from the surroundings. In actual processes, there will be always some heat gain or loss. These processes are: Heating or cooling

These processes follow a constant moisture line (constant humidity ratio). Thus, temperature increases or decreases but moisture content and dew point are unchanged. Further cooling follows the saturation (100% relative humidity) line until the final temperature is reached. Moisture is condensed during the part of the process that follows the saturation line.

Moisture addition

While using only energy from the air, both drying and evaporative cooling follow this process. A constant wet-bulb line represents it. Temperature and moisture content change but the wet-bulb temperature remains constant. This can be verified by an energy balance analysis. Note that enthalpy increases slightly in this process. This is due to energy present in the water before it is evaporated. 8

Heating and drying

This process is common in drying applications. Air is heated and passed over the material to be dried. A second stage of heating and drying is sometimes included.

Adiabatic mixing (no heat transfer) of air

Moist air from two sources and at different state points is mixed to produce air at a third state point. Relationships among the properties at the three state points are established from mass and energy balances for the air and water components.

Adiabatic saturation

The drying process was identified earlier as a constant wet bulb process. While this is the generally accepted approach, a review of the adiabatic saturation process is provided here for added clarification. An adiabatic saturation process occurs when the humidity of the air is increased as it flows through an insulated chamber. Water evaporates into the air as it passes through the chamber. If the chamber is long enough for equilibrium to occur, then the exit air will be saturated at an equilibrium temperature, t. 2.2.2. Drying mechanism In the process of drying, heat is necessary to evaporate moisture from the material and a flow of air helps in carrying away the evaporated moisture. There are two basic mechanisms involved in the drying process: the migration of moisture from the interior of an individual material to the surface, and the evaporation of moisture from the surface to the surrounding air. The drying of a product is a complex heat and mass transfer process which depends on external variables such as temperature, humidity and velocity of the air stream and internal variables which depend on parameters like surface characteristics (rough or smooth surface), chemical composition (sugars, starches.), physical structure (porosity, density), and size and shape of products. The rate of moisture movement from the product inside to the air outside differs from one product to another and depends very much, on whether the material is hygroscopic or non-hygroscopic. Non-hygroscopic materials can be dried to zero moisture level while the hygroscopic materials like most of 9

the food products will always have residual moisture content. This moisture, in hygroscopic material, may be bound moisture, which remained in the material due to closed capillaries or due to surface forces and unbound moisture, which remained in the material due to the surface tension of water.

When the hygroscopic material is exposed to air, it will absorb either moisture or desorbs moisture depending on the relative humidity of the air. The equilibrium moisture content (EMC = Me) will soon be reached when the vapor pressure of water in the material becomes equal to the partial pressure of water in the surrounding air (Garg, 1987). The equilibrium moisture content in drying is therefore important since this is the minimum moisture to which the material can be dried under a given set of drying conditions. A series of drying characteristic curves can be plotted. The best is if the average moisture content, “M” of the material is plotted versus time. Another curve can be plotted between drying rate i.e. “dM/dt” versus time. But more information can be obtained if a curve is plotted between drying rate “dM/dt” versus moisture content.

For both non-hygroscopic and hygroscopic materials, there is a constant drying rate terminating at the critical moisture content followed by falling drying rate. The constant drying rate for both non-hygroscopic and hygroscopic materials is the same while the period of falling rate is little different. For non-hygroscopic materials, in the period of falling rate, the drying rate goes on decreasing until the moisture content become zero. While in the hygroscopic materials, the period of falling rate is similar until the unbound moisture content is completely removed, then the drying rate further decreases and some bound moisture is removed and continues till the vapor pressure of the material becomes equal to the vapour pressure of the drying air. When this equilibrium reaches then the drying rate becomes zero (Garg, 1987).

The period of constant drying for most of the organic materials like fruits, vegetables, timber and the like is short and it is the falling rate period in which is of more interest and which depends on the rate at which the moisture is removed. In the falling rate regime moisture is migrated by diffusion and in the products with high moisture content, the diffusion of moisture is comparatively slower due to turgid cells and filled interstices. In most agricultural products, there is sugar and minerals of water in the liquid phase which 10

also migrates to the surfaces, increase the viscosity hence reduce the surface vapour pressure and hence reduce the moisture evaporation rate. Drying is done either in thin layer drying or in deep layer drying. In thin layer drying, which is done in case of most of fruits and vegetables, the product is spread in thin layers with entire surface exposed to the air moving through the product and the Newton’s law of cooling is applicable in the falling rate region (Garg, 1987).

There were many research reports, where the drying took place only in the falling rate period and constant stage was not observed during the drying experiments. These characteristics for tomato slices were reported by Hawlader et al. (1991), Akanbi et al. (2006) and Sacilik et al. (2006) . Krokida et al. (2003) reported similar characteristics for some different vegetables. For thin carrot, mulberry fruits and figs (Cui et al., 2004 and Doymaz ,2005) indicating non exist ant water film at the surface of the crop and transfer of moisture could be effectuated by liquid diffusion or vapor diffusion or capillary forces which complicated mechanism that could change during the drying process. Most probable mechanism controlling the mass transfer in agricultural products are diffusion (Diamente and Munro, 1993). Such similar observations were also reported by (Togrul and Pehlivan, 2004)

2.3. Thin Layer Drying Models Thin layer drying models have gained wide acceptance to design new or simulate the existing system or for the analytical drying solutions. Many researchers have used the exponential drying model in describing the drying behavior of the food materials. The solution of the Fick’s equation, with the assumptions of diffusion based moisture migration, negligible shrinkage, constant diffusion coefficients and temperature, is simplified to get the simple exponential model (Lewis, 1921) as: Moisture Ratio (MR) =

Mt  Me  e  kt Mi  Me

Where, Mi = initial moisture content, dry basis (decimal) Mt = Moisture content, dry basis (decimal) at time ‘t’ 11

(1)

Me = equilibrium moisture content, dry basis (decimal) k = drying rate constant (min-1) t = drying time, min The Henderson and Pabis (1961) model is also the general series solution of Fick’s second law. The following thin layer drying equation (Henderson and Pabis model) was successfully used by Doymaz, (2004); Sacilik et al., (2006) for the prediction of drying time and for generalization of drying curves. Mt  Me  Ae  kt M  Me i

(2)

If the constant ‘A’ in the above equation is equal to unity, the equation is reduced to the same form as Newton’s law of cooling for highly conductive materials.

Another model which has been widely used to fit the thin layer drying data is the Page equation (Hossain and Bala, 2002; Wang, 2002). It is a simple modification of the exponential law using moisture ratio with additional drying parameter. Page (1949) proposed a thin layer drying equation:

n Mt  Me  e  qt Mi  Me

(3)

Where, ‘q’ and ‘n’ are drying constants that depend on the air temperature and type of material.

The empirical equation used to describe the thin layer drying characteristics of food materials (Akpinar et al., 2003; Doymaz, 2004):

Mt  Me  1  at  bt 2 Mi  Me

(4)

Where, ‘a’ and ‘b’ are drying constants.

12

A mathematical model for drying kinetics is normally based on the physical mechanisms of internal heat and mass transfer and on the heat transfer conditions external to the material being dried that control the process resistance, as well as on the structural and thermodynamic assumptions. Modeling of drying is usually complicated by the fact that more than one mechanism may contribute to the total mass transfer rate and the contribution from the different mechanisms may change during the drying process (Cui et al., 2003). The effect of air conditions (air temperature, air humidity and air velocity) and characteristic sample size on drying kinetics of various food materials such as tomato, potato, carrot, pepper, garlic, mushroom, onion, leek, pea, corn, celery, pumpkin during air drying was examined by Krokida et al., (2003). They found that the parameters of the model considered were greatly affected by the air conditions and sample size during drying and in particular, the temperature increment increased the drying constant and decreased the equilibrium moisture content of the dehydrated products. 2.4. Sun and Solar Drying Open-air sun drying (without drying equipment) is the most widely practiced agricultural processing operation in the world; in some countries, food is simply laid out on roofs or flat surfaces and turned regularly until dry. More sophisticated methods of solar drying collect solar energy and heat air, which in turn is used for drying the food.

The term ‘sun drying’ is used to describe the process whereby some or all of the energy for drying of foods is supplied by direct radiation from the sun. The term ‘solar drying’ is used to describe the process whereby solar collectors are used to heat air, which then supplies heat to the food by convection. For centuries, fruit, vegetables, meat and fish have been dried by direct exposure to the sun. The fruit or vegetable pieces were spread on the ground, on leaves or mats while strips of meat and fish were hung on racks. While drying in this way, the foods were exposed to the variability of the weather and to contamination by dust, insects, birds and animals. Drying times were long and spoilage of the food could occur before a stable moisture content was attained. Covering the food with glass or a transparent plastic material can reduce these problems. A higher temperature can be attained in such an enclosure compared to those reached by direct exposure to the sun. Most of the incident radiation from the sun will pass through such transparent materials. However, most radiation from hot surfaces within the enclosure will be of longer 13

wavelength and so will not readily pass outwards through the transparent cover. This is known as the ‘greenhouse effect’ and it can result in shorter drying times as compared with those attained in uncovered food exposed to sunlight. A transparent plastic tent placed over the food, which is spread on a perforated shelf raised above the ground, is the simplest form of covered sun-drier. Warm air moves by natural convection through the layer of food and contributes to the drying. The capacity of such a drier may be increased by incorporating a solar collector. The warm air from the collector passes up through a number of perforated shelves supporting layers of food and is exhausted near the top of the chamber. A chimney may be fitted to the air outlet to increase the rate of flow of the air. The taller the chimney, the faster the air will flow. If a power supply is available, a fan may be incorporated to improve the airflow still further. Heating by gas or oil flames may be used in conjunction with solar drying. This enables heating to continue when sunlight is not available. A facility for storing heat may also be incorporated into solar driers. Tanks of water and beds of pebbles or rocks may be heated via a solar collector. The stored heat may then be used to heat the air entering the drying chamber. Drying can proceed when sunlight is not available. Heat storing salt solutions or adsorbents may be used instead, water, or stones. Quite sophisticated solar drying systems, incorporating heat pumps, are also available (Brennan 1994, Barbosa-Canovas and Vega-Mercado, 1996, Salunkhe, 1982, Imrie, 1997).

In solar drying, solar-energy is used either as the sole source of the required heat or as a supplemental source. The airflow can be generated by either natural or forced convection. The heating procedure could involve the passage of preheated air through the product or by directly exposing the product to solar radiation or a combination of both (Ekechukwu and Norton, 1998). The major requirement is the transfer of heat to the moist product by convection and conduction from the surrounding air mass at temperatures above that of the product or by radiation, mainly from the sun and to a little extent from surrounding hot surfaces (McLean, 1980). In a direct radiation drying, part of the solar radiation may penetrate the material and be absorbed within the product itself, thereby generating heat in the interior of the product as well as at its surface, and thereby enhancing heat transfer (Basunia and Abe, 2001). During drying, there is a tendency of the food to form dry surface layers which are impervious to subsequent moisture transfer, if the drying rate is 14

very rapid. To avoid this effect, the heat transfer and evaporation rates must be closely controlled to guarantee optimum drying rates (Arinze et al., 1979).

2.3.1. Classification of solar drying

2.3.1.1. Natural convection and other solar dryings All drying systems can be classified primarily according to their operating temperature ranges into two main groups of high temperature dryers and low temperature dryers. However, dryers are more commonly classified broadly according to their heating sources into fossil fuel dryers (more commonly known as conventional dryers), electric powered and solar energy dryers. Further, solar-energy drying systems are classified primarily according to their heating modes and the manner in which the solar heat is utilized (El-Sebaii et al., 2002).  passive solar-energy drying systems (conventionally termed natural-convection solar drying systems); and  active solar-energy drying systems (most types of which are often termed hybrid solar dryers).

Although, for commercial production of dried agricultural products, forced convection solar dryer might provide a better control of drying air; natural convection solar dryer does not require any other energy during drying operation. Hence, natural convection solar dryer is highly preferred for drying food products especially when in thin layers of drying (Pangavhane et al., 2002).

Natural convection solar drying depends for its operation entirely on solar-energy in which, solar-heated air is circulated through the product by buoyancy forces or as a result of wind pressure, acting either singly or in combination. It is reported that the dryer is superior operationally and competitive economically to natural open sun drying. The advantages of natural convection solar drying over open sun drying are reported by Ekechukwu and Norton (1998) as follows:

15

 It requires a smaller area of land in order to dry similar quantities of product.  It yields a relatively high quality of dry food because fungi; insects and rodents are unlikely to infest the food during drying.  The drying period is shortened compared with open-air sun drying, thus attaining higher rates of product throughput.  Protection from sudden down pours of rain. 2.3.2. Types of solar dryers Solar dryers are also classified into direct natural circulation driers (a combined collectors and drying chamber), direct driers with a separate collectors and indirect forced convection driers (separate collectors and drying chamber) Ekechukwu and Norton (1998).

2.3.2.1. Direct natural convection solar dryers These dryers do not use any fans and/or any blower; low cost and easy to operate. In the simple design, they consist of some kind of enclosure and a transparent cover. The food product gets heated due to direct sunlight, due to high temperature in the enclosure and therefore moisture from the product evaporates, and goes out by natural circulation of air. These dryers are mostly on use in developing countries (1982, Imrie, 1997).

a) Solar cabinet dryer

The main characteristic of simplest solar cabinet dryers is that the heat needed for drying gets into the material through direct radiation. The drying material is spread in a thin layer on a bottom perforated tray through which air flows by natural convection and finally leaves through the upper part of the cabinet. Its design is simple, low in cost, suitable for drying small quantities (10-20 kg) of granular materials (e.g., for individual farmers). Drying of the material can be made more even by periodic turning over of the material. The usual size of the drying area is 1-2 m2 (Imrie, 1997 and Garg, 1987).

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b) Green house type solar dryer

This dryer appears to look like a small greenhouse where there are two parallel long drying platforms made of wire mesh and are covered with slanted long glass roof with long axis along the north-south direction. There is a metallic cap at the top of the glass roof does not allow rain. The inside of the dryer as well as the trays are painted black. Solar radiation penetrates through the glass roof, heats the product directly and absorbed within the dryer increasing the inside temperature (Garg, 1987, Bala et al., 2002).

2.3.2.2. Indirect type solar dryers a) Shelf type solar dryer

In a shelf dryer, the material to be dried is placed on perforated shelves (trays) built one above the other. Shelf type solar dryer was tested by Best (1979) in which the movement of air around produce was further facilitated by drying on perforated trays rather than on solid platforms. The front wall of the case faces south, its top and sides, are covered by transparent walls (glass or sheet), and the back wall is heat insulated and painted black. A flat-plate collector, which is, situated below and besides the drying chamber heats the ambient air that flows up to the space under the lowest shelf. Moist air exits to the open through the upper opening of the casing. The chimney effect is ensured by the increased height of the dryer. The experiments indicated that separation of the collector is only justified with a high efficiency collector. The suitability of such dryer for drying fruits and vegetables were described by Imrie, (1997).

Exell and Kornsakoo (1978) developed a simple mixed mode solar dryer consisting of a separate solar collector and a drying unit, both having a transparent cover on the top. Solar radiation is received in the collector as well as in the dryer. The dryer was initially designed with a bed of burnt rice husk as the absorber and clear UV stabilized polyethylene plastic sheet as transparent cover.

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2.3.2.3. Indirect forced convection driers The indirect forced drier consists of a separate flat plate air-heating collector, a tunneldrying unit and a small fan to force or to provide the required airflow over the product to be dried. These are connected in series. Both the collector and the drying unit are covered with ultraviolet (UV) stabilized plastic sheet. Black paint is used as an absorber on the collector. The products to be dried are placed in a thin layer in the tunnel.

Different types of solar dryers such as solar tunnel, roof-integrated and greenhouse type solar dryers have been demonstrated for drying fruits, vegetables, spices, medicinal plants and fish in the tropics and subtropics (Lutz et al., 1987 and Schrimer et al., 1996).

2.3.3. Major components of solar dryers Many designs of solar dryers have the following major components: solar collector, drying unit or chamber where the materials to be dried comes in direct contact with the hot air from the collector, connecting ducts, the transparent cover and ventilations.

2.3.3.1. Solar collector The solar collector plays the part of primary energy source for a solar dryer. Essentially, it has functions of energy conversion and energy transfer. Use of blackened surface as a collector is required because matt black surfaces absorb solar radiation more efficiently than others, and so the improvements can be enhanced by use of such surfaces. It has been demonstrated for example by Thanh et al., (1978) that the time required to dry cassava chips on a concrete floor is reduced by about 15% if the floor is painted black.

2.3.3.2. Transparent cover The most common use of plastics in solar collectors and dryers is as a transparent cover allowing incident radiation to pass through and impinge on an absorber surface - or on the materials being dried and must be able to withstand elevated temperatures, high levels of 18

insolation, high humidities, wind loading and the effects of heavy rain over long periods of time. Low cost, low density and good optical properties make some plastics very suitable for use in solar collectors and dryers. The physical effects of photo-degradation vary from loss of transmissivity and discoloration to crazing of the surface and embrittlement of the plastics resulting in a lowering of the efficiency of a collector or drier will render the plastic more prone to damage by wind and rain. Degradation of plastics occurs more rapidly at higher temperatures and thus deterioration is often worst at hot-spots such as points where the plastic is supported or attached to the framework (White, 1977).

A wide range of clear plastic sheet and film with properties suitable for use in solar energy applications, which also have good resistance to weathering, is now available. Plastics commonly used for glazing in solar collectors include PMMA, polycarbonate (PC), glassfiber reinforced polyester (GRP), polyvinyl fluoride (PVF), fluorinated ethylene propylene copolymer and polyester film (FEP) (White, 1977).

Specifying the polymer will not always be sufficient. In order to achieve the length of service of which UV resistant plastics are capable, methods of attaching the plastic to the framework, commonly used in simple agricultural systems, such as stapling or nailing are unsatisfactory as they create point of stress where the material is likely to fail. When attaching plastic sheet to the framework a method should be chosen which will distribute any stresses on the sheet as evenly as possible over its whole length or width.

2.4. Drying Efficiencies The efficiency of solar drying can be studied under two contexts: Collection efficiency (  c) and the system efficiency (  s).

Collection efficiency (  c) measures how effectively the incident energy on the solar collector is transferred to the air flowing through the collector and is given as the ratio of the useful energy output (over a specified time period), to the total solar radiation energy, G, available during the same period. 19

The thermal performance of the solar collector is determined by obtaining values of instantaneous efficiency using the measured values of incident radiation, ambient temperature, and inlet air temperature. This requires continuous measurement of incident solar radiation on the solar collector as well as the rate of energy addition to the air as it passes through the collector, all under steady state or quasi-steady state conditions (Imrie, 1997).

2.5. Drying of Tomato and Onion 2.5.1. Solar drying of tomato Tomatoes are the world’s most commercially produced and used vegetable crop (Ensminger, 1988). The annual worldwide production of tomatoes has been estimated at 125 million tons in an area of about 4.2 million hectares. The global production of tomatoes (fresh and processed) has been increased by 300% in the last four decades and the leading tomato producers are in both tropical and temperate regions (Dhaliwal et al., 2003). Ethiopian climate is suitable for the production of tomato and with an annual production of about 338,380.91 quintals only on small-scale farms in Maher season and mostly used for fresh fruit consumption (CSA, 2008).

Over the last few years, tomato products have aroused new scientific interest due to their antioxidant activity. Tomatoes and tomato products are rich in health-related food components as they are good sources of carotenoids (in particular, lycopene), ascorbic acid (vitamin C), vitamin E, folate and flavanoids (Davies and Hobson, 1981). They also provide potassium, iron, phosphorus and some B vitamins and are a good source of dietary fiber. They have around 90% water and the large amount of water also makes the fruit perishable. In a ripe fruit, solids form about 5-7% of the fruit, mainly sugars in the form of glucose and a small portion of acid in the form of citric acid (Wills, 1998). The chemical composition of the tomato fruit depends on factors such as cultivar, maturity and environmental conditions, in which they are grown (Davies and Hobson, 1981). It is a short duration crop, giving high yield. However, the excess production results in a glut in the market and reduction in tomato prices. In addition, it is highly perishable in the fresh state leading to wastage and losses during the peak harvesting period. The prevention of 20

these losses and wastage during peak harvest is very much important to avoid imbalance in supply and demand during off-season and for economic consideration (Karim and Hawlader, 2005). Therefore, there is a need to increase the shelf life of tomatoes either in fresh or in processed form using food preservation techniques such as drying.

In the guidelines of preparation, drying conditions and information given by Ife and Bas (2003), tomatoes are washed in water and sliced 7-10 mm thick with a loading rate of 5 kg per square meter of a tray. A 100 kg fresh tomato yields 70- 90 kg when prepared for drying and mostly becomes 4-5 kg when dried. Maximum permissible drying air temperature is 65°C and a 5% moisture content of final product, which is tough and brittle, was given in the literature.

Sacilik et al., (2006) reported on the thin layer solar drying experiments of organic tomato using multi-purpose solar tunnel dryer under the ecological conditions of Ankara, Turkey. They reported that organic tomatoes could be dried to the final wet basis moisture content of 11.5% from 93.3% in four days of drying in the solar tunnel dryer as compared to five days of drying in the open sun drying.

2.5.2. Solar drying of onions Onion, Allium cepa L., is considered as one of the most important crops in all countries. Domestic onion is the round, edible bulb of Allium cepa, a species of the lily family, and one of the world’s oldest cultivated vegetable crops. Red, white and gold onions represent the most known varieties of this species, but growers distinguish them also between freshly consumer onions and onions for industrial transformation, on the basis of sowing time and technique, harvesting time, bulb size, among others characteristics (Bonaccorsi et al., 2008). Onion has a universal appeal in the Ethiopian diet and dehydrated onion is well accepted by consumers. The technique for sun drying onion is a simple one and the dry product has good storage life. There is a good export market for dehydrated onion. 21

In the chemical composition of onions, carbohydrates are source of food energy reserves and make up much of the structure framework of cells. Shallot contain higher levels of fats and soluble solids, including sugars, than bulb onion with 16-33% dry weight vs. 7-15% dry weight, respectively (Currah and Proctor, 1990; Messiaen, 1992) which, together with sulphur-containing compounds, make shallot an essential component in cooking.

Onion is a strong-flavored vegetable used in a wide variety of ways, and its characteristic flavor (pungency) or aroma, biological compounds and medical functions are mainly due to their high organo-sulphur compounds (Mazza and LeMaguer, 1980; Corzo-Martínez et al., 2007).

In the manufacture of processed foods such as soups, sauces, salad dressings, sausage and meat products, packet food and many other convenience foods, dehydrated onion is normally used as flavor additive, being preferred to the fresh product, because it has better storage properties and is easy to use (Rapusas and Driscoll, 1995; Kaymak-Ertekin and Gedik, 2005). In addition, the preservation of vegetables, such as onion, in the dried form is commonly practiced to reduce the bulk handling, to facilitate transportation and to allow their use during the off-season. However, in the drying process of shelf-stable vegetables it is essential to preserve their desired quality attributes.

The moisture removal during drying processes is greatly affected by the drying air conditions as well as the characteristic dimension of the material, whereas all other process factors have a practically negligible influence (Kiranoudis et al., 1997). The effect of drying parameters on moisture removal, expressed by kinetic models have been studied for different varieties of onion (Krokida et al., 2003; Sarsavadia et al., 1999; Kiranoudis et al., 1992; Yald ´yz and Ertek ´ yn, 2001; Wang, 2002). However, the drying conditions, such as temperature and moisture content, have a great influence on the food properties, such as flavor or colour and nutritional composition during processing or storage (Kaymak-Ertekin and Gedik, 2005).

Kaymak-Ertekin and Gedik (2005) studied the kinetics of non-enzymatic browning and thiolsulphinate loss in onion slices during drying at different temperatures and air 22

velocities and the corresponding quality losses. Kumar et al. (2007) dried onion slices under different processing conditions applying infrared radiation assisted by hot air, varying the drying temperature, slice thickness, inlet air temperature and air velocity, and tested different thin layer models. Kumar and Tiwari (2007) studied the open sun and greenhouse drying of onion flakes to evaluate the effect of mass on convective mass transfer coefficient. Sarsavadia (2007) developed a solar-assisted forced convection dryer for the drying of onion slices and studied the effect of airflow rate, air temperature, and fraction of air recycled on the total energy requirement. Sharma et al. (2005) developed an infrared dryer and studied the infrared radiation thin layer drying of onion slices at different infrared power levels, different air temperatures and air velocities.

It is not uncommon to preserve onion by drying. Various studies have been made on different aspects of onion. All these studies aimed at facilitating the onion drying and improving the quality of the dried product. Thus drying of onion is a widely used preservation techniques.

In the guidelines of preparation, given by Ife and Bas (2003), onion is cleaned, washed, peeled and sliced 3 mm thick for drying at a loading rate of 4 kg/m2 of a drying tray. A 100 kg fresh onion yields 90 kg when prepared for drying and mostly becomes 9 kg dried product at a 60°C maximum permissible drying air temperature and 5-7% moisture content of final product which is brittle that could be ground to powder.

Summary

The eastern region of the country is capable of producing large quantities of fruits and vegetables for local consumption and export. Many of these fruits and vegetables contain a large quantity of initial moisture content and are therefore highly susceptible to rapid quality degradation, even to the extent of spoilage, if not kept in thermally controlled storage facilities. Therefore, it is imperative that, besides employing reliable storage systems, post harvest methods such as drying can be implemented hand-in-hand to convert these perishable products into more stabilized products that can be kept under a minimal controlled environment for an extended period. 23

Drying is the application of heat under controlled condition to remove the majority of the water normally present in a food by evaporation and extend the shelf life of food by reduction of water activity. The decrease in weight and volume reduce transport and storage costs. Design of drying equipment and operation is aimed at minimizing these negative effects by selection of appropriate drying conditions for the food.

“Safe storage moisture” the moisture level of most vegetables is 10-15% so that the microorganisms present cannot thrive and the enzymes become inactive, that dehydration is usually not desired, because the products often become brittle and stored in a moisturefree environment, , Commercially important dried foods are coffee, milk, raisins, sultanas, and other fruits, vegetables, pasta, flours (including bakery mixes), beans, pulses, nuts, breakfast cereals, tea and spices. Drying methods

Several drying methods are commercially available and the selection of the optimal method is determined by quality requirements, raw material characteristics, and economic factors. Types of drying processes: 

sun and solar drying;



atmospheric dehydration including stationary or batch processes (kiln, tower, and cabinet driers) and



continuous processes (tunnel, continuous belt, belt-trough, fluidized-bed, explosion puffing, foam-mat, spray, drum, and microwave-heated driers); and subatmospheric dehydration (vacuum shelf, vacuum belt, vacuum drum, and freeze driers) (Chua, 2003).

The factors that control the rate of heat transfer and removal of moisture are related to the processing conditions, nature of the food and the drier design.

24

Properties air the major factors in determining the rate of moisture removal. Air capacity depend

upon its initial temperature and humidity; thermodynamic properties is

represented by the psychrometric chart.

In the drying process are the migration of moisture from the interior of an individual material to the surface, and the evaporation of moisture from the surface to the surrounding air depends on external variables such as temperature, humidity and velocity of the air stream and internal variables. These in turn influenced by parameters like:  surface characteristics (rough or smooth surface),  chemical composition (sugars, starches.),  physical structure (porosity, density), and  size and shape of products. The equilibrium moisture content (EMC = Me) will soon be reached when the vapor pressure of water in the material becomes equal to the partial pressure of water in the surrounding air (Garg, 1987). The equilibrium moisture content in drying is therefore important since this is the minimum moisture to which the material can be dried under a given set of drying conditions.

Drying is done either in thin layer drying or in deep layer drying. In thin layer drying, which is done in case of most of fruits and vegetables, the product is spread in thin layers with entire surface exposed to the air moving through the product and the Newton’s law of cooling is applicable in the falling rate region (Garg, 1987).

A mathematical model for drying kinetics is normally based on the physical mechanisms of internal heat and mass transfer and on the heat transfer conditions external to the material being dried that control the process resistance, as well as on the structural and thermodynamic assumptions. The effect of air conditions (air temperature, air humidity and air velocity) and characteristic sample size on drying kinetics of various food materials such as tomato, potato, carrot, pepper, garlic, mushroom, onion, leek, pea, corn, 25

celery, pumpkin during air drying was examined by Krokida et al., (2003). They found that the parameters of the model considered were greatly affected by the air conditions and sample size during drying and in particular, the temperature increment increased the drying constant and decreased the equilibrium moisture content of the dehydrated products.

Many food industries dealing with commercial products employ state-of-the-art drying equipment such as freeze dryers, spray dryers, drum dryers and steam dryers. The prices of such dryers are significantly high and only commercial companies generating substantial revenues can afford them. Therefore, because of the high initial capital costs, most of the small-scale companies are not able to afford the price of employing such high-end drying technologies that are known to produce high quality products. Instead cheaper, easy-to-use and practical drying systems become appealing to such companies or even to the rural farmers themselves. It is also useful to note that in many remote-farming areas in Ethiopa, a large quantity of natural building material and bio-fuel such as wood are abundant but literacy in science and technology is limited. In this Thesis, literatures on different types of dryers for agricultural foodstuffs, are reviewed and low cost dryer for application in farming areas where raw materials and labor are readily available was proposed to be designed, constructed and evaluate its performance. The proposed dryer

possess the

following characteristics:

1. low initial capital costs; 2. easy to construct and fabricate with available natural materials; 3. easy-to-operate with no complicated electronic/ mechanical protocol; 4. effective in promoting better drying kinetics and product quality than the sun-drying method; 5. easy to maintain all parts and components; and 6. simple replacement of parts during breakdowns, 26

Solar drying

Solar drying is often differentiated from ‘‘sun drying’’ by the use of equipment to collect the sun’s radiation in order to harness the radiative energy for drying applications. Sun drying is a common farming and agricultural process in many countries, particularly where the outdoor temperature reaches 30 °C or higher. In many parts of South East Asia, spice crops and herbs are routinely dried. However, weather conditions often preclude the use of sun drying because of spoilage due to rehydration during unexpected rainy days. Furthermore, any direct exposure to the sun during high temperature days might cause case hardening, where a hard shell develops on the outside of the agricultural products, trapping moisture inside. Therefore, the employment of solar dryer taps on the freely available sun energy while ensuring good product quality via judicious control of the radiative heat. Solar energy has been used throughout the world to dry food products. Such is the diversity of solar dryers that commonly solar-dried products include grains, fruits, meat, vegetables and fish. A typical solar food dryer improves upon the traditional openair sun system in five important ways:

1. It is faster. Foods can be dried in a shorter period of time. Solar food dryers enhance drying times in two ways. Firstly, the translucent, or transparent, glazing over the collection area traps heat inside the dryer, raising the temperature of the air. Secondly, the flexibility of enlarging the solar collection area allows for greater collection of the sun’s energy. 2. It is more efficient. Since foodstuffs can be dried more quickly, less will be lost to spoilage immediately after harvest. This is especially true of products that require immediate drying such as freshly harvested grain with a high moisture content. In this way, a larger percentage of 27

food will be available for human consumption. Also, less of the harvest will be lost to marauding animals and insects since the food products are in safely enclosed compartments. 3. It is hygienic. Since foodstuffs are dried in a controlled environment, they are less likely to be contaminated by pests, and can be stored with less likelihood of the growth of toxic fungi. 4. It is healthier. Drying foods at optimum temperatures and in a shorter amount of time enables them to retain more of their nutritional value such as vitamin C. An added bonus is that foods will look and taste better, which enhances their marketability and hence provides better financial returns for the farmers. 5. It is cheap. Using freely available solar energy instead of conventional fuels to dry products, or using a cheap supplementary supply of solar heat, so reducing conventional fuel demand can result in significant cost savings.

Types of solar dryer

Solar dryers can generally be classified into two broad categories: active and passive. Passive dryers use only the natural movement of heated air. They can be constructed easily with inexpensive, locally available materials which make them appropriate for small farms where raw construction material such as wood is readily available.

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A direct passive dryer is one in which the food is directly exposed to the sun’s rays. Direct passive dryers are best for drying small batches of fruits and vegetables such as banana, pineapple, mango, potato, carrots and French beans (Jayaraman, Das Gupta & Babu Rao, 2000). This type of dryer comprises of a drying chamber that is covered by a transparent cover made of glass or plastic. The drying chamber is usually a shallow, insulated box with air-holes in it to allow air to enter and exit the box. The food samples are placed on a perforated tray that allows the air to flow through it and the food. Solar radiation passes through the transparent cover and is converted to low-grade heat when it strikes an opaque wall. This low-grade heat is then trapped inside the box by what is known as the ‘‘greenhouse effect.’’ Simply stated, the short wavelength solar radiation can penetrate the transparent cover. Once converted to low-grade heat, the energy radiates as a long wavelength that cannot pass back through the cover. Active solar dryers are designed incorporating external means, like fans or pumps, for moving the solar energy in the form of heated air from the collector area to the drying beds . The collectors should be positioned at an appropriate angle to optimize solar energy collection.

Tilting the collectors is more effective than placing them horizontally, for two reasons. Firstly, more solar energy can be collected when the collector surface is nearly perpendicular to the sun’s rays. Secondly, by tilting the collectors, the warmer, less dense air rises naturally into the drying chamber. In an active dryer, the solar-heated air flows through the solar drying chamber in such a manner as to contact as much surface area of the food as possible. Thinly sliced foods are placed on drying racks, or trays, made of a screen or other material that allows drying air to flow to all sides of the food. Once inside the drying chamber, the warmed air will flow up through the stacked food trays.

As the warm air flows through several layers of food on trays, it becomes moisture laden. This moist air is vented out through the outlet port. Fresh air is then taken in to replace the exhaust air. Active solar dryers are known to be suitable for drying higher moisture content foodstuffs such as papaya, kiwi fruits, brinjal, cabbage, cauliflower slices, tomato and onion . 29

In the guidelines of preparation, drying conditions and information given by Ife and Bas (2003), tomatoes are washed in water and sliced 7-10 mm thick with a loading rate of 5 kg per square meter of a tray. A 100 kg fresh tomato yields 70- 90 kg when prepared for drying and mostly becomes 4-5 kg when dried. Maximum permissible drying air temperature is 65°C and a 5% moisture content of final product, which is tough and brittle, was given in the literature.

Sacilik et al., (2006) reported on the thin layer solar drying experiments of organic tomato using multi-purpose solar tunnel dryer under the ecological conditions of Ankara, Turkey. They reported that organic tomatoes could be dried to the final wet basis moisture content of 11.5% from 93.3% in four days of drying in the solar tunnel dryer as compared to five days of drying in the open sun drying.

In the guidelines of preparation, given by Ife and Bas (2003), onion is cleaned, washed, peeled and sliced 3 mm thick for drying at a loading rate of 4 kg/m2 of a drying tray. A 100 kg fresh onion yields 90 kg when prepared for drying and mostly becomes 9 kg dried product at a 60°C maximum permissible drying air temperature and 5-7% moisture content of final product which is brittle that could be ground to powder.

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3. MATERIALS AND METHODS 3.1. Description of the Study Site The dryer was designed and manufactured at the Fadis Agricultural Research Center Workshop, Oromia Agricultural Research Institute, Ethiopia. The drying experiment was conducted at Bate Peasant Association located at 09° 25` 03``N and 42° 02`58``E as determined by GPS. The site has an altitude 2051meters above sea level. It is located 1.50 km to the east of main campus of the Haramaya University, which is located in eastern Ethiopia. 3.2. The Design of the Solar Dryer The solar dryer consists of heat collector area and drying chamber, the former surrounding the latter. Fig.1 shows the general framework of the dryer, which is built using perforated steel angle irons of 20 mm  20 mm  4 mm and 40 mm  40 mm  4.0 mm thick joined by bolts and nuts. All the sides and top surfaces, except the chimney, are covered with transparent plastic (PE), 0.2 mm thick in order to allow the solar radiation in to the unit covering an area of 3.0 m  3.0 m. The lower side of the floor is off the ground by 0.3 m supported on eleven legs. The designs of various parts are presented in the following sections.

31

Figure 1 Framework of the solar Dryer (A) collector support; (B) collector; (C) plastic cover; (D) support for plastic cover; (E) saturated air out let (chimney); (F) drying chamber (cabinet); (G) drying cabinet layer (shelves); (H) Drying chamber air inlet; (I) Tray wire mesh; (J) Doors (product out let and inlet) I, H and E are some of the respective measuring points of temperature, relative humidity and air velocity. 3.2.1 Drying chamber The drying chamber (Fig.2) is of a square cross-section having dimensions of 1.0 m width x 1.0 m length x 1.65 m height. The structures, formed of the angle irons, are partitioned into right and left compartments each having five shelves at 10 cm height from each other. All of the four walls and the roof were transparent to light. It accommodated 10 drying trays, placed on 5 shelves in each of compartments. The warm air coming from the surrounding collector flows into a plenum, empty space at the bottom section of the drying chamber, where the warm air flows upwards through the perforated trays loaded with the material to be dried. The air, after passing through the trays, leaves the chamber through a chimney, located at the center of the drying chamber roof. The side of the drying chamber where the collector 32

chamber does not extend was provided with a twin door for the access into the interior. The door was transparent, hinged to the frames of the drying chamber, and fitted with a door lock.

Figure 2. Drying chamber frame of the solar dryer Drying chamber frame was made of the perforated angle irons of 40 mm x 40 mm x 4.0 thick mm bolted to each other. This steel angle iron, commonly known in the market as Dixon angle iron is provided with slots to join and adjustment with bolts and nuts and prefabricated for shelves and other purposes. All the frames of the four walls and the roof (Fig.3 and Fig.4, respectively) were formed by joining the angle irons using bolts and nuts, and were assembled to form the unit. Four pieces of angle irons two on the sidewall where the doors were located and the remaining two on the side opposite to them were fixed vertically at the mid points of the wall frames for mounting the rails in the partitions of the chamber.

33

Figure 3. Drying chamber wall frame The roof of the drying chamber was made by joining angle irons such that they form the slopping trusses. These slopping trusses were made from 20 mm x 20 mm x 4.0 thick mm angle irons as shown in Fig. 4 below. The chimney being located at the center, the four quarters of the roof have dimension of 0.70 m and a slope of 16° to the horizontal.

Figure 4. The roof frame of drying chamber The drying chamber, having two vertical partitions the right and the left, was provided with shelves for holding the drying trays. The shelves were made, in each partition by fixing, parallel angle irons of 20 mm x 20 mm x 4 mm to the main vertical stands, at 10 cm optimum vertical height interval between trays in order to obtain uniform air circulation as shown in Fig. 5.

34

0.60 m

1.0 m

0.30m

0.40m

1.30 m

Figure 5. The position of the shelves in the drying chamber Drying Trays The trays for drying tomato were made of a 5.0 mm x 5.0 mm chicken wire mesh of approximately 0.20 mm diameter and wooden frame of 2.0 cm x 3.0 cm cross section. The trays for drying onion were made of a 1.0 mm x 1.0 mm chicken mesh wire of approximately 0.20 mm diameter and wooden frame of 2.0 cm x 3.0 cm cross section. The effective surface area of a single tray was 0.41 m x 0.96 m = 0.3936 m2, giving a total drying surface area of 10 x 0.369 m2 = 3.936 m2.

Chimney Chimney was required for ventilation of the dryer. Incorporating chimney regulates the residency period and rate of ventilation of the drying chamber. The air carrying moisture from the materials was exhausted through the chimney, made of 1 mm thick galvanized iron sheet metal rolled into cylinder of 20 cm diameter and 28 cm height, positioned on the top of the drying chamber. 3.2.2. The collecting chamber The frame of the collecting chamber is made of the same type of angle irons used for other parts. The floor of the collecting chamber of the dryer, shown in (Fig. 6) is made of galvanized metal sheet of thickness 0.20 mm and riveted to 12 mm thick plywood having 35

dimension of 3.0 m x 3.0 m and a net area of 8.0 m2. Having its own roof structure, covered with transparent plastic shown in (Fig.7), it forms the heat collecting chamber by absorbing sun`s radiation striking the floor area.

The front sides and the roof of the collecting chamber are covered with the plastic with the roof inclined by 10° upward towards the drying chamber. The inclination causes the warm air to flow into the plenum of the drying chamber from three directions. The floor of the collecting chamber was painted matt black to reduce reflection of solar radiation. The collector surface is placed at a height of 0.30 m above the ground to level it with the floor of the drying chamber, and to protect the entrance of the brimming animals and crawling insects.

The air enters in from all three sides of the collector area and is directed to the plenum of the drying chamber owing to the slope of the plastic roof. The warm air passes through the product taking up the moisture is exhausted through the chimney situated on the roof of the dryer.

Collector Plate The collector frame was made from same type of perforated angles irons 20.0 mm x 20.0 mm x 4.0 thick mm having the profile and dimensions shown in Fig.6. The frame supported the collector plate, the ventilating fans and the transparent cover. Besides, it was considered to withstand the loads that might come from the wind, rain and birds (hens).

The collector surface was made from matt black painted galvanized metal sheet of 0.20 mm thickness for its excellent heat absorption and conduction properties. A lining of plywood of 12 mm thickness was used on its bottom side for the purpose of insulation. Plywood was selected due to its low heat conduction property, low cost, ease of availability and simplicity of construction. The standard size (2400 mm x 1200 mm) plywood currently available on the local markets was cut in to one square meter piece and riveted to galvanized tin sheet. Thus, the total collector surface was formed by assembling these pieces on the floor frame of the collector, giving a total collector area of 8.0 meter 36

square. The perforated angle irons were bolted together to form the frames for accommodating the collector plates, which were lifted off the ground by 0.30 m with the help of eleven stands.

Figure 6. The collector plate of the solar dryer

The roof and roof support structure of the collecting chamber The roof of the collector chamber (Fig.7) was made using the same angle irons such that they form the slopping trusses. The drying chamber being located in the middle front side of the collector plate, the six roofs have different dimensions but similar slope in order to cause movement of the warm air towards the drying chamber. There are vertical walls to divide the collecting chamber into three parts directing the drying air towards the plenum of the drying chamber. The roof has 10° slop uphill towards the drying chamber to induce the flow of the warm air into the plenum.

37

Figure 7. The roof frame structure of the collecting chamber

Plastic Cover Transparent plastic of type polyethylene sheet of 0.2 mm thick was used to cover both the top and sides of the drying chamber as well as the collector chamber roof. The chimney, being an opaque and painted black, was an exception. The transparent plastic cover allows incident radiation to pass through and impinge on an absorber surface and/ or on the food to be dried. The plastic cover can withstand the elevated temperatures, high levels of insolation, high humidities, and the effects of heavy rain over long periods with high resistance to degradation.

The plastic sheet was attached to the frames by means of screws running along the length of angle irons. Sharp edges on the screws and framework was avoided. Wood as weatherproof seals and washer materials between the screws and the plastic was used for even stress distribution, especially in cases where the framework has uneven surfaces (Fig.8).

38

Figure 8. Photo of solar dryer

3.3. Performance Evaluation of Solar Dryer

3.3.1. Measuring instruments Thermo-hygrometer (CompuFlow 8612), temperature and humidity meter, with accuracy level of ±0.10°C and ±2.0 %RH, was used to measure temperatures and humidity at various points inside the collector and drying chamber of the solar dryer. The locations of the sensors are shown in Fig.8 at points “a”s. Both the temperature and humidity of air were measured at these points. The temperature and humidity data were recorded at onehour interval. The air speeds (ms-1) inside the dryer and, at the exit of the moist air (chimney), were measured with a vane type digital anemometer (Testo model 21-63, accuracy ±0.03 m s-1).

Weight measurement was done with a digital balance DHAUS of model – CT 6000-s, accuracy (±0.0 g) it was done by removing trays from the drying cabinet for few seconds. The dryer door was opened and closed during the time required to remove each tray, 39

weigh it, record it, and return it to the appropriate location in the shelves of the drying chamber. The design solar dryer is presented in Fig. 9.

Figure 9. Schematic diagram of solar dryer

3.3.2. Preliminary test of the solar dryer The dryer was placed on a raised platform, far from the shade of trees and buildings during the whole duration of the experiment (Fig.9). Preliminary tests were conducted to evaluate the performances of the dryer at no-load (empty) conditions. The degrees of opening of the vent (chimney) were calibrated and marked for three levels (quarter, half and fully open) positions of inside air temperature, relative humidity and velocities were measured and recorded.

3.3.3 Efficiency of solar dryer The study of the solar dryer efficiency provides a means of assessing just how well (or poorly) a dryer operates under certain conditions. Collector efficiency of solar energy absorption and conversion to heat is defined as the ratio of energy output of the collector to energy input to the collector and is calculated as: 40

C 

Qu AcG

(1)

Where,  C is collector efficiency (%) .

C 

mC P (Tc ,out  Tc ,in )

(2)

Ac G

QU  mCp (Tc ,out  Tc ,in )

(3)

Qu is useful heat flow rate (J/s) .

m  q ,

(4)

m is air mass flow rate (kg/s)

 is density of air (kg/m3) q = AV,

(5)

q is volume flow rate of air (m3/s) A is the collector exit area (m2) V is air velocity (m/s) Cp is specific heat of air (1007 J kg-1 °K-1 for air), Tc.out is output collector temperatures (°C), Tc,in is input collector temperatures (°C), GAc is solar energy input on the collector (J) Ac is collector area (m2) G is global solar radiation (W/m2)

41

3.3.4. Sample preparation Tomato Freshly harvested and known varieties of tomato, melka shola , which were grown in Fadis Agricultural Research Center and by local farmers, were procured from local market. First, the tomato was thoroughly cleaned so that all dirt, soils, and mud or insecticide residues were removed. Cleaning was made by simply washing with a tap water.

After cleaning, the tomato was sliced into circular discs (thin slices) of 8 mm thickness (Ife and Bas, 2003; Wang, 2002), using an electrical operated mechanical slicer. The sliced tomato was carefully loaded on wire mesh trays without overlapping the slices or in single layer, at the rate of 5 kg/m2.

Onion Freshly harvested and known variety of onion Adama Red, which were grown in Fadis Agricultural Research Center and by local farmers, were procured from local market. First, the onion was thoroughly cleaned so that all dirt, soils, and mud or insecticide residues were removed. After cutting the top and root of the onion, it was peeled using sharp stainless steel knife. Cleaning was made by simply washing with a tap water.

After cleaning, the onion was sliced into circular discs (thin slices) of 3 mm thickness (Ife and Bas, 2003; Wang, 2002), using an electrical operated mechanical slicer. The sliced onion was carefully loaded on the trays without overlapping the slices or in single layer, wire mesh trays at the rate of 4 kg/m2.

3.3.5. Moisture content determination of samples The initial moisture content on wet and dry basis of the samples used in the experimental work was determined by electric heated air-dry box (AMB 310) moisture balance). The dryer box was set at Mode 1, with drying temperature 70°C. Then, slices of samples were placed in the dryer box. Samples of initial weight (Wo) were dried in the moisture balance at 70°C for 24 hrs until the weight (Wd ) of the dried sample became stable. 42

The initial moisture content on dry basis, Mto(d.b) (%) of the sample was expressed as

Mt o ( d .b ) 

Wo  Wd  100 % (Karl and Hall, 1996). Wd

(6)

Where, Wo (g) is the initial weight of sample; Wd(g) is the final dry weight of sample; For the determination of the moisture content, dry basis, Mti (d.b ), (%), of the samples at any time (ti) during the drying process, the following equation was used:

Mti ( d .b ) 

Wti  W d Wd

 100 %

(7)

Where, Mti(d.bis moisture content on dry basis of samples; Wti(g) is the weight of the samples at time, (ti); or moisture content , wet basis, of the samples at any time (ti) during the drying process

Mti ( w .b ) 

Wti  W d Wo

 100 %

(8)

Where, Wti is the weight of the samples at time, ti; The determination of the samples weight was done by weighing the drying tray with its load of samples at 2 hours interval in the drying process.

For the determination of the instantaneous drying rate (Rti) (dry basis), equation (9) was applied:

Rti 

=

 Wti  ti  W d

(9)

Wi 1  Wi  100 (kgw/kg.DM.h.) Wd * (t i 1  t i )

43

Where ti-1 and ti are successive times corresponding to when two successive measurements of weights during drying samples was made. Another equation can be used for the determination of the drying rate, (dry basis):

Rti 

MCti 1  MCti ti

Rti 

Mt i 1  Mti ( t i 1  ti )

(10) (kgW/kgDM.h.)

Where, Rti (kgW/kgDM.h.) is the instantaneous drying rate, The final dry mass was determined as follows: Final mass = Initial mass  (1- initial moisture content on wet basis) 3.3.6. Testing the solar dryer using tomato with natural convection current The sliced tomato was uniformly loaded over pairs of trays, T1, T2, T3, T4, and T5 positioned in shelves 1, 2, 3, 4 and 5 respectively, of right and left compartments of the drying chamber. During the drying, the weight of the trays with the tomato was recorded at the interval of 2 hours. Drying began at 8:30 o’clock in the morning, proceeded throughout the day and ended at 5:30 o’clock. The slices on every tray were manually stirred randomly after recording the weights to facilitate the drying process. This was to help the exposure of the slice to the hot air in all direction to ensure the uniform drying. The drying process continued until the moisture content reached the target value or until the safe moisture content and tomatoes were dried to the final moisture content of 11.5% (w.b) (Doymaz, 2007).

The initial weight of the sample used in this experiment for tomato was 2.0 kg per tray. The material holding in single batch for drying tomato was estimated to be 20 kg. There is optimum vertical distance of 10 cm between trays in order to obtain good air circulation. The door of the dryer was properly closed to prevent air leakage.

44

3.3.7. Testing the solar dryer using onion with natural convection current The dryer was placed on a raised ground, far from the shade of trees and buildings during the whole duration of the experiment (Fig.9). Preliminary tests were conducted to evaluate the performances of the dryer at no-load (empty) conditions. The degrees of opening of the vent (chimney) were calibrated and marked for various levels of inside temperature and air velocity, weights of drying trays were measured and recorded. The sliced onion was uniformly loaded over pairs of trays, T1, T2, T3, T4, and T5 positioned in shelves 1, 2, 3, 4 and 5 respectively, of right and left compartments of the drying chamber. During the drying, the weight of the trays with the tomato was recorded at the interval of 2 hours. Drying began at 8:30 o’clock in the morning, proceeded throughout the day and ended at 5:30 o’clock. The slices on every tray were manually stirred randomly after recording the weights to facilitate the drying process. This was to help the exposure of the slice to the hot air in all direction to ensure the uniform drying. The drying process continued until the moisture content reached the target value or until the safe moisture content and onion were dried to the final moisture content of 5-7% (w.b) (Ife and Bas, 2003).

The initial weight of the sample used in this experiment was 1.57 kg per tray. The material holding in single batch for drying was 16 kg.. The door of the dryer was properly closed to prevent air leakage.

Simultaneously, similar samples were dried in open air under the direct sunlight. The trays and the loading rates were the same and were placed on the platform to lift them off the ground. Weights of samples on trays were recorded every two hours in the way done for samples in the solar dryer. The dried products on each tray were packed in the labeled airtight plastic bags to be used for further laboratory analysis and experiments. Ambient weather data including local air temperature and relative humidity were measured. Other weather data such as solar insolation and wind velocity were obtained from weather station in the area.

45

3.3.8. Performance evaluation of solar dryer using tomato and onion in forced ventilation During performance evaluation of solar dryer using tomato and onion in forced ventilation, the procedures for samples preparation, moisture content determination and testing of the solar dryer were similar as those procedures used in section (3.4), . The ventilating fan of 20 cm diameter (model MSF-5503, power input 53 W, running at 800 rpm was installed for the dryer powered photovoltaic cell module, allowing the choice of the desired air mass flow. The fan was fixed below product trays at the bottom of the dryer to ensure an even distribution of air and evacuate the humidity of the product to the surrounding.

3.3.9. Kinetics of drying Drying rate equation

During the drying tests the comparisons of moisture contents as a function of the drying time were made. A drying characteristic data were calculated (periodical data of the moisture contents and drying rate).

An appropriate thin layer drying equation can express the rate of change of moisture content of a thin layer product inside the dryer. The Newton equation in differential form is given by Lewis (1921). dM   k ( M  Me ) dt

(11)

The solution of (1), assuming k is independent of Mti and Me is: MRNewton = exp (-kt)

(12)

Where, MR is a moisture ratio given by:

Mti  Me  exp (-kt) Mo  Me

46

Where, Mti= moisture content, % (db), t = time, hour, Mo = initial moisture content (% db),

Me = equilibrium moisture content, % or ratio (db) k = drying constant (hr-1), The nonlinear regression, the least square was employed to evaluate the parameters of the model chosen with the process of Levenberg–Marquardt using SPSS 16.0 software package.

3.4. Statistical Analysis All observations were recorded as means of three replications. The data pertaining moisture contents and drying rate coefficients were statistically analyzed to determine the significant difference, if any between solar drying methods of photovoltaic (PV) ventilated forced drying, natural convection solar drying and open-air sun drying, for dried tomato and onion slices. ANOVA under factorial experimental design and the mean separation by LSD (P < 0.05) method was carried out for the drying data. Experimental design The factorial experimental design where the main plot treatment is the two types of vegetables tomato and onion (T, O) and the treatment as three types of drying methods, natural convection solar drying and Photovoltaic (PV) ventilated forced solar drying with the open-air sun drying as a control were used.

47

Table 1. Treatment combination, replication and randomization

Crops: (Tomato Onion)

Tomato

Trays Drying Methods (DM)

I

II

III

Natural convection solar Drying (NCSD)

TNCSD

TNCSD

TNCSD

Photovoltaic ventilated solar drying (PVSD)

TPVSD

TPVSD

TPVSD

TOASD

TOASD

TOASD

Open-air sun drying (OASD)

ONCSD OONCSD ONCSD Natural convection solar Drying (NCSD) Onion Photovoltaic ventilated solar drying (PVSD)

OPVSD

OPVSD

OPVSD

Open-air sun drying (OASD)

OOASD OOASD

OOASD

48

4. RESULTS AND DISCUSSION

4.1. Preliminary Test Data of the Solar Dryer In order to characterize the solar dryer, temperature and relative humidity of the air in solar collector and the corresponding data of the ambient air need to be examined. Information on the temperature rise of air is important when evaluating a solar collector especially for drying purposes. During the preliminary tests of the dryer, measurements were taken for few days at no-load. The outlet air temperature of the flat plate collector, which is also the temperature of the drying air at the inlet of the drying chamber, is important parameter for evaluating the collector performance.

The collector performance could be seen from the difference in air temperature at the exit and inlet of the solar collector. During the preliminary tests with quarter, half and fullyopen positions using manually operated control valve fitted in the chimney, a maximum temperature rise of 41°C above the ambient air were recorded. Due to better temperature rise and optimum air velocity, half- open position was decided and selected to operate the dryer exit in the chimney (Table 2).

. Table 2. Preliminary test data at no load of the dryer at half open position of control device Time of the day (hour) 7 8 9 10 11 12 13 14 15 16 17 18

Ambient air Tc,in (°C) 15 18 20 21 22 23 23 22 21 20 20 19

Collector outlet Tc,o (°C) 28 36 42 49 53 60 64 61 50 42 31 27

RH Tc,out(%) 36 34 30 28 18 8 5 10 25 35 46 53 49

Tc,out-Tc,in (°C) 13 18 22 28 31 37 41 39 29 22 11 8

Air velocity (m/s) 0.01 0.02 0.02 0.04 0.05 0.06 0.06 0.05 0.05 0.03 0.04 0.03

Solar radiation (W/m²) 50 175 450 650 866.53 965 1035.7 980 870 570 350 160

Table 2 presents the variation of the ambient air temperature and that of the air leaving the collector. The rise, in air temperature after passing through the collector varied from 18°C at 8:00 o’clock in the morning to about 37°C at midday. The period starting from 10:00 am in the morning to 4:00 pm in the afternoon was where the significant rise in temperature occurred. The one-hour interval data recorded indicated that the collector absorbed the solar radiation striking its surface, converted it to heat and transferred it to the air inside it. As the solar radiation increased from 175 W/m2 in the morning to 965W/m2 at midday the temperature of the air in the collector rose from 36°C to 60°C.

The data presented in Fig.10 varied with the daily radiance incident on the collector. It can be noted, in the experiment, the absorbed solar energy raised the collector outlet air temperature up to 64°C, just at 1:00 pm. The experiments during these months showed that during the peak afternoon hours, the average rise of air temperature (between the input and output of the collector) was equal to 41°C (varying between 15°C and 41°C). The average air velocity was 0.04 m s-1 at the drying chamber outlet.

Figure 10. The solar radiation, collector outlet & ambient air temperature 50

4.2. Collector Efficiency The instantaneous efficiency of the solar collector shown in Table 3, started to rise in the morning period, was relatively constant at 77% from 12:00 hours to 13:30 hours, and dropped down in late afternoon. The variation obtained is typical for a flat plate collector and indicates strong dependence of efficiency on the meteorological data. The daily efficiency, averaged over 11 hours (7:00 to 18:00) comes out to be 51%.

Table 3. Raw data of the collector efficiency analysis for solar dryer Air Temp. (°C) Time drying velocity Airflow Solar Energy Collector of day time (m/s) rate radiation Total Useful efficiency T (W/m²) (W) (W) T (hr) (hr) (T -T ) (%) V(kg/s) am co co am 50 1 0.01 0.0065 15 28 13 400 84 21 7 175 1400 468 8 2 0.02 0.0259 18 36 18 33 9

3

0.07

0.0905

20

42

22

450

3600 2001

56

10

4

0.09

0.1164

21

49

28

650

5200 3275

63

11

5

0.11

0.1422

22

53

31

867

6932 4431

64

12

6

0.12

0.1552

23

61

38

965

7720 5926

77

13

7

0.12

0.1552

23

64

41

1036

8286 6393

77

14

8

0.11

0.1422

22

61

39

980

7840 5575

71

15

9

0.11

0.1422

21

50

29

870

6960 4145

60

16

10

0.09

0.1099

20

42

22

570

4560 2430

53

17

11

0.04

0.0517

20

31

11

350

2800 572

20

18

12

0.02

0.0259

19

27

8

160

1280 208

16

51

4.3. Test of Solar Dryer Using Tomato Slice in Natural Convection Current

The initial moisture content of the tomato slices was determined

by taking three

measurements and mean relative moisture is 93.3%±0.9 (w.b.Table 2) shows the change in moisture contents of tomato slices with drying time in solar dryer (SD) and in open–air sun drying. During the experiments, tomato slices were dried to the final moisture content of 12% (Sacilk et al., 2006). All the recordings exhibited similar moisture reduction in that the moisture dropped drastically in early periods two hours of the drying process, with that of open-air sun dried slices showing the highest moisture content.

As drying continued slices on the top most trays T5, showed the lowest moisture content in all the recordings until the final stage of drying. This can be explained by the fact that the tray received direct sunlight in addition to the warm air coming up through the drying chamber. The slice showing the next lowest moisture content, in almost all the recordings during the drying period, was that of the slices on the bottom tray T1 (Table 2.). This tray got the warmest air coming from the collector chamber, which is also the driest or lowest relative humidity, thus transferring much heat to the slices while picking up the evaporated water. The moisture reductions of the slices on the rest of the trays were almost similar or very close to each other for the most of the periods. These were the trays situated in the middle of the drying chamber, where the drying air was considered to have low temperature and high relative humidity, since it already had picked up moisture from the slices at the bottom of the drying chamber .

Slices on trays 5 & 1 attained their final moisture content after 10hrs (the shortest drying period of all) and 12 hours, respectively. The vertical order of the trays influenced the rate of drying and the duration needed to lower the moisture contents to a given target. The top and bottom position resulted in the fastest reduction of moisture content and shortest drying time.

52

Table 4. Weight of tomato, percentage moisture contents on wet basis, dry basis and drying rate on dry basis on Tray1, Tray2, Tray 3, Tray 4, Tray 5 and open air sun trays during tomato drying using natural convection current and open-air sun drying Time of Date Record Drying time (hr) (hr) 8:30 0 4/11/2010 10:30 2 12:30 4.5 14:30 6.5 16:30 8.5 17:30 10.5 8:30 10:30 5/11/2010 12:30 14:30 16:30 17:30 Final dry mass

10.5 12.5 14.5 16.5 18.5 20.5

Mass of tomato(gm) T1 T2 T3 T4 T5 TOS 109 120 110 122 110 125 74 82 80 84 68 100 61 70 66 71 52 85 46 60 54 58 40 60 36 51 45 46 32 52 30 43 38 38 27 49

T1 93.3 61.2 49.3 35.5 26.3 20.8

Moisture content on wet basis (%) T2 T3 T4 T5 93.3 93.3 93.3 93.3 61.6 66.0 62.2 55.1 51.6 53.3 51.5 40.6 43.3 42.4 40.8 29.7 35.8 34.2 31.0 22.4 29.1 27.8 24.5 17.8

25 20 20 20 20 20

16.2 11.5 11.5 11.5 11.5 11.5

25.4 21.6 15.0 10.0 10.0 10.0

39 34 26 20 20 20

33 26 20 20 20 20

38 20 20 20 20 20

20 20 20 20 20 20

40 36 35 33 30 28

22.8 16.9 11.5 11.5 11.5 11.5

7.3 8.04 7.37 8.2 7.4 8.4

54

20.0 11.8 11.8 11.8 11.8 11.8

11.5 11.5 11.5 11.5 11.5 11.5

TOS 93.3 73.3 61.3 41.3 34.9 32.5

Moisture content on dry basis (kg of water/kg.dry matter) T1 T2 T3 T4 T5 TOS 13.9 13.9 13.9 13.9 13.9 13.9 9.1 9.2 9.9 9.3 8.2 10.9 7.4 7.7 8.0 7.7 6.1 9.1 5.3 6.5 6.3 6.1 4.4 6.2 3.9 5.3 5.1 4.6 3.3 5.2 3.1 4.3 4.2 3.6 2.7 4.9

25.3 22.1 21.3 19.7 17.3 15.7

3.1 1.2 0.9 0.6 0.4 0.4

4.3 3.2 2.2 1.5 0.9 0.4

4.2 2.5 1.7 1.2 0.8 0.5

3.6 1.4 0.8 0.6 0.5 0.5

2.7 0.6 0.4 0.2 0.2 0.2

3.8 3.2 2.7 2.3 2.0 1.7

Drying rate on dry basis (kg of water/kg.DM.hr.) T1 T2 T3 T4 T5 TOS 2.4 0.9 1.0 0.7 0.4

2.4 0.7 0.6 0.6 0.5

2.0 0.9 0.8 0.6 0.5

2.3 0.8 0.8 0.7 0.5

2.8 1.1 0.8 0.5 0.3

1.5 0.9 1.5 0.5 0.2

1.0 0.1 0.1 0.1 0.0

0.6 0.5 0.4 0.3 0.2

0.8 0.4 0.3 0.2 0.1

1.1 0.3 0.1 0.1 0.0

1.0 0.1 0.1 0.0 0.0

0.3 0.2 0.2 0.2 0.1

The slices dried in the open-air sun had recordings showing the highest moisture content all along the drying periods. Not only that, the final moisture content attained was also well above that of the other slices dried by the solar dryer and that it was attained after 20 hrs, the longest period of drying.

The drying time reduced as per the position of drying trays in the drying chamber of solar dryer and trays of open-air sun drying, because the resistance to moisture movement is relatively higher in slices dried in open-air sun drying trays and those trays in middle of drying chamber than those slices dried in on trays in the bottom and top of the SD. This resistance is known to decrease the drying rate, which resulted in increased drying time of slices in the middle of drying chamber and in open-air sun drying. Generally, it is observed that the time required to reduce the moisture content of tomato slices to any required moisture level was dependent on the drying conditions that are influenced by weather parameters. Similarly, Sacilik et al. (2006) also observed that the drying characteristics of tomato slices in solar tunnel and open sun drying methods were highly influenced by weather parameters.

The drying rate data in Table 2 of tomato slices dried in the solar dryer and by open-air sun drying. Expressed as kilogram of evaporated water per kilogram of dry matter-hr, all the curves indicated that the initial drying rate was very high. Values of drying rate of tomato slices in the solar dryer varied from 2.8 kg of water per kg of dry matter-hr on tray5, located on the top in the drying chamber to 2.0 kg of water per kg of dry matter-hr on tray3, located in the middle of the drying chamber. As drying, proceeded the rate of loss of moisture decreased continuously due to reduced moisture content and latter in the afternoon due to the fall of the incoming air temperature. Values of drying rate at some points are close to each other indicating existence of only minor differences among themselves.

The drying rate exhibited a sudden leap in value in the following day showing a rise in the drying rate. This was attributed to the rise in the air temperature coming from the collector as the solar radiation increased towards the middle of the day. However, that leap

55

gradually subsided in the afternoon as the moisture content reduced and with the fall of air temperature due to less solar radiation.

The drying rate values of the slices dried in the open-air sun drying remained lower in the recordings for most part of the drying time, exhibiting lower rate of drying. This is in harmony with the drying data (Tab.2) showing higher moisture contents than similar recordings of slices dried in the solar dryer.

In this experimental condition, the samples show that drying took place only in the falling rate and no constant rate of drying was observed (Tab.2) (Akpinar et.,al 2003). The mechanisms of mass transfer in food are complex in nature. However, the main mechanism of moisture movement is assumed to be by diffusion that may have both liquid and vapour diffusion components. Similar drying characteristics were reported by Hawlader et al. (1991), Akanbi et al. (2006) and Sacilik et al. (2006) for tomato slices. Like wise Krokida et al. (2003) for different vegetables, Doymaz (2004) for thin carrot, mulberry fruits and for figs have shown drying rate data of similar characteristics. This implied that a film of water did not exist at the surface of the slices and moisture transfer from the interior of the product to its surface is effected by several complicated mechanisms (liquid diffusion or vapor diffusion or capillary forces) which change during the drying process (Cui et al., 2004).

4.4. Test of Solar Dryer Using Onion Slice in Natural Convection Current The initial moisture content of the onion slices was found to be 87.10% (w.b.) and dried to the final moisture content of 9.1% (w.b Table 3) (Sacilk et al., 2006) drying data of the onion slices dried in the solar dryer under the natural convection current and that of the slices dried in the open-air sun drying. The onion slices of different trays placed in the solar dryer and that of slices dried in the open-air sun exhibited similar trends of a rapidly falling moisture content. However, slice on tray1 (the bottom tray) showed the highest moisture reduction, indicating rapid fall of the moisture content. For this tray, the slices reached the lowest level moisture content in 10 hours. Slices on tray5 the next rapid fall of moisture content reaching the final moisture content after 12th hour. Slices of trays, T2, T3 56

and T4, located in the mid height of the chamber, had the slowest fall of the moisture content extending to the 14th hour to reach a final moisture content value. The position of the drying trays in the chamber had undoubtedly great influence on the speed of moisture reduction, the bottom and top position of trays favoring fast removal of moisture.

The slices dried in open-air sun exhibited the least removal of the moisture throughout the drying time. Furthermore, the moisture content could not be lowered to a level equal to those of the solar dried slices even after 25thhours. Thus, the solar dryer resulted in a drying time reduced by at least half (12hrs) as compared to open-air sun drying, which is required over 24, hours. A constant rate-drying period was not observed in both the drying methods but only a long falling rate-drying period.

The drying rate data of onion slices dried in the solar dryer and open-air sun drying expressed as kilogram of evaporated water per kilogram of dry matter- hour, all the records (Table 3) indicated that the initial drying rate was very high. Values of drying rate of onion slices in the solar dryer varied from 1.50 kilogram of water per kilogram of dry matter-hr on tray1 located at the bottom of chamber to 1.2 kilogram of water per kilogram of dry matter-hr of trays 2 & 3 located in the middle of the chamber. As drying, proceeded the rate of loss of moisture decreased continuously due to reduced moisture content and latter in the afternoon due to the fall of the incoming air temperature. The values of drying rate are equal at some points and close to each other at other points indicating existence of only minor differences among themselves.

The drying rate data exhibited a small increase at the start of the drying process in the following day showing a rise in the drying rate. This was attributed to the rise in the air temperature coming from the collector as the solar radiation increased towards the middle of the day. However, that rise gradually subsided in the afternoon as the moisture content reduced and with the fall of air temperature due to less solar radiation.

57

Table 5. Weight of onion, percentage moisture contents on wet basis, moisture contents on dry basis and drying rate on dry basis on Tray1,Tray2, Tray 3, Tray 4, Tray 5 and open air sun tray during onion drying using natural convection current and open-air sun drying tests Time Drying of day time Date (hr) (hr) T1 8:30 0 240 10:30 2 144 12:30 4 99 15/10/2010 14:30 6 90 16:30 8 70 17:30 9.5 49 8:30 10:30 12:30 16/10/2010 14:30 16:30 17:30 Final dry mass

9.5 11.5 13.5 15.5 16.5 17.5

49 35 35 35 35 35

Mass of onion (g) Moisture content on dry basis Drying rate on dry basis on trays in dryer Moisture content on wet basis (%) (kg of water/kg of dry matter) (kg of water/kg of dry matter) T2 T3 T4 T5 TOS T1 T2 T3 T4 T5 TOS T1 T2 T3 T4 T5 TOS T1 T2 T3 T4 T5 TOS 271 283 282 272 286 87.0 87.0 87.0 87.0 87.0 87.0 6.7 6.7 6.7 6.7 6.7 6.7 185 197 180 176 224 47.0 55.3 56.6 50.8 51.7 65.3 3.6 4.3 4.4 3.9 4.0 5.0 1.5 1.2 1.2 1.4 1.4 0.8 148 158 139 134 174 28.3 41.6 42.8 36.3 36.3 47.8 2.2 3.2 3.3 2.8 2.8 3.7 0.7 0.5 0.5 0.6 0.6 0.7 138 148 130 125 146 24.5 37.9 39.3 33.1 32.9 38.0 1.9 2.9 3.0 2.5 2.5 2.9 0.1 0.1 0.1 0.1 0.1 0.4 116 128 107 100 120 16.2 29.8 32.2 24.9 23.8 29.0 1.2 2.3 2.5 1.9 1.8 2.2 0.3 0.3 0.3 0.3 0.4 0.3 90 99 81 77 108 7.4 20.2 22 15.7 15.3 24.8 0.6 1.6 1.7 1.2 1.2 1.9 0.3 0.4 0.4 0.4 0.3 0.2 80 63 47 44 37 37

89 72 54 49 41 41

76 61 48 39 38 38

65 40 40 40 40 40

108 96 89 76 69 66

7.4 1.6 1.6 1.6 1.6 1.6

16.5 10.3 4.35 3.25 0.66 0.66

18.4 12.4 6.08 4.31 1.48 1.48

13.9 8.62 4.01 0.82 0.46 0.46

31 35 37 37 35 37.2

58

10.9 1.69 1.69 1.69 1.69 1.69

24.8 20.6 18.1 13.6 11.1 10.1

0.6 0.1 0.1 0.1 0.1 0.1

1.3 0.8 0.3 0.2 0.1 0.1

1.4 1.0 0.5 0.3 0.1 0.1

1.1 0.7 0.3 0.1 0.10 0.10

0.8 0.10 0.10 0.10 0.10 0.10

1.9 1.6 1.4 1.0 0.9 0.8

0.2 0.1 0.0 0.0 0.0

0.2 0.2 0.0 0.1 0.1

0.2 0.2 0.7 0.1 0.2

0.2 0.2 0.1 0.0 0.2

0.2 0.2 0.2 0.1 0.0 0.2 0.0 0.1 0.0 0.04

The drying rate data of the slices dried in the open-air sun drying remained lower in the record for most part of the drying time, exhibiting lower rate of drying. This is in harmony with the moisture content data (Tab.3) which showed higher moisture contents than similar records of slices dried in the solar dryer.

In this experimental condition, the samples show that drying took place only in the falling rate and no constant rate of drying was observed (Tab.3). 4.5. Characteristics of the Solar Dryer under Forced Ventilation In the solar collector with forced ventilation, the increase in temperatures between the ambient and collector outlet air temperatures was observed ranging from 5°C to 25°C. This gave heated air temperature of up to 50 °C, which is more than adequate to dry fruit and vegetables. Such a rise in the incoming air temperature into the drying chamber lowers the relative humidity of air. Lowering relative humidity of air increase the capacity of air to carry more moisture. The photovoltaic powered ventilation system increases the air velocity flowing into the drying chamber. High velocity of the drying air improves the rate of drying as it reduces the thickness of the film of the moist air around the food decreasing the resistance to release of moisture into the air. The average air velocity recorded due to the ventilation was 0.60 m-1s. The temperature profile of the drying chamber under forced ventilation is shown in Fig.18. The difference in temperature between the incoming dry warm air and the discharged moist air ranged from about 6°C in the early morning and /or in the late afternoon to 14°C at midday. As the incoming warm air, passes the heat to the moist drying food, its temperature drops to wet bulb temperature. Towards the end of the drying period of the food the temperature of air remains high, close the that of the incoming air.

59

Figure 11. Temperature profile of the drying chamber. The relative humidity of the air coming from the collector was highly reduced due to the warming effect as compared to the relative humidity of the ambient air. As the air, picks up the moisture on its way up the drying chamber the relative humidity increases as can be seen in relative humidity curves of belonging to the middle chamber and that of dryer outlet air.

60

Figure 12. The profile of relative humidity in the drying chamber

4.6. Testing the Solar Dryer in Forced Air Circulation Using Tomato The drying data of tomato slices of various trays in the dryer and that of the slices dried in open-air sun are shown in table 4. Slices on tray T5 (upper most tray) and tray T1 (bottom tray) and tray T4 exhibited the lowest moisture content and shortest drying times of 11.5 hrs. The drying data of slices on trays T4, T3 and T2 had drying periods of 13.5 and 14.5 hrs, respectively. The moisture contents of slices on trays 1, 2, 3, 4 and 5 have attained the target moisture contents of 11.7-11.5 % (w.b) within the indicated periods. The drying data of the slices dried on trays in the open-air sun showed moisture content levels very much higher than those of slices dried on trays of the solar dryer.

61

Table 6. weight of tomato, percentage, moisture content on wet basis and percentage drying rate on dry basis on Tray1, Appendix Tray2, Tray 3, Tray 4 and Tray 5 and open air sun Tray4 and Tray 5 (Ventilated tomato drying) Time Drying Moisture content on the day time wet basis (%) Date (hr) (hr) T1 T2 T3 T4 T5 12:00 0 93.3 93.3 93.3 93.3 93.3 12/12/2010 14:00 2 68.3 71.6 72.7 72.7 68.6 16:00 4 51.6 58.5 56.4 56.4 52.1 17:30 5.5 39.1 46.8 43.3 43.3 39.8

13/12/2010

14/12/2010

TOS 93.3 90.4 74.7 71.3

Moisture content on dry basis (%) T1 T2 T3 T4 T5 13.9 13.9 13.9 14.0 13.9 10.2 10.7 11.2 10.9 10.2 7.7 8.7 8.8 8.4 7.8 5.8 7.0 6.9 6.5 5.9

TOS 13.9 13.5 11.1 10.6

3.1 2.7 2.3 2.5 3.1 2.1 1.6 2.0 2.0 2.0 1.6 1.5 1.5 1.6 1.5

0.2 1.2 0.2

T1

Drying rate (kg W/kg DM.hr) T2 T3 T4 T5 TOS

8:30 10:30 12;30 14:30 16:30 17:30

5.5 7.5 9.5 11.5 13.5 14.5

30.8 20.0 12.0 11.5 11.5 11.5

43.3 33.3 25.5 19.0 14.2 11.6

37.7 28.3 20.9 15.7 11.5 11.5

37.7 28.3 20.9 15.7 11.5 11.5

34.0 23.8 15.5 11.7 11.7 11.7

68.5 67.3 57.0 45.1 38.2 32.0

4.6 3.0 1.8 0.9 0.9 0.9

6.5 5.0 3.8 2.8 2.1 1.6

6.3 4.9 3.8 2.8 2.2 2.2

5.6 4.2 3.1 2.4 2.4 2.4

5.1 3.5 2.3 1.4 1.4 1.4

10.0 8.5 6.7 5.7 4.8 4.2

1.3 1.0 0.8 0.5 0.1

1.2 1.0 0.8 0.6 0.4

1.2 0.9 0.8 0.5 0.4

1.2 0.9 0.6 0.5 0.4

1.3 1.0 0.7 0.5 0.4

0.8 0.9 0.5 0.5 0.3

8:30 10:30 12:30 14:30 16:30 17:30

14.5 16.5 18.5 20.5 22.5 23.5

11.5 11.5 11.5 11.5 11.5 11.5

11.5 11.5 11.5 11.5 11.5 11.5

11.5 11.5 11.5 11.5 11.5 11.5

11.5 11.5 11.5 11.5 11.5 11.5

11.7 11.7 11.7 11.7 11.7 11.7

28.4 21.9 16.0 11.2 11.2 11.2

0.9 0.9 0.9 0.9 0.9 0.9

1.6 1.6 1.6 1.6 1.6 1.6

2.2 2.2 2.2 2.2 2.2 2.2

2.4 2.4 2.4 2.4 2.4 2.4

1.4 1.4 1.4 1.4 1.4 1.4

3.3 2.4 1.7 0.9 0.9 0.9

0.1 0.1 0.1 0.1 0.1

0.3 0.3 0.1 0.1 0.1

0.4 0.3 0.2 0.2 0.1

0.3 0.1 0.0 0.0 0.0

0.2 0.1 0.0 0.0 0.0

0.4 0.4 0.4 0.0 0.0

62

The lowest content attained after 23.5 hours of drying was 11.2% that was the final moisture content. As it can be seen, the 12% moisture content was attained after nine hrs of drying by incorporating power-operated fans. As the drying period of tomato slices in natural circulation of the air had been determined to be 12hours as shown in Tab.3, an advantage has been noticed in drying time, attributed to use of fan for tomato drying.

The drying rate data (Tab.4) of the tomato slices dried in the solar drier and in open-air sun drying. Slices on trays 1&5 started with the highest rate of drying followed by slices on trays 2, 4 & 3. However, after 6 hours of drying the slices on majority of the trays inside the dryer exhibited similar rate of drying. This condition persisted to the end of drying. This can be explained by the fact in the falling rate period of drying; the rate of drying is governed by the rate of internal diffusion of moisture to the surface of slices. Once the moisture on the surface of the slices is removed, which is governed by the air temperature and rate of heat transfer to the moisture on the surface, the drying rate is influenced by the rate of replacement of moisture from the interior of the food. This replacement is the same for all the slices thus making the rate of drying more or less the same. The drying rate in all cases reduced as drying time increased.

4.7. Testing the Solar Dryer in Forced Air Circulation Using Onion

The drying data of slices of various trays in the dryer and that of the slices dried in open air sun are shown in Table 5. Slices on tray, T5 (upper most tray) exhibited the lowest moisture content and shortest drying time of 10 hrs. While tray1 (bottom tray), tray2, and tray3 were the next lowest moisture content and shorter drying period of 12 hrs. The moisture of slices on tray4 had lowest moisture content after drying period of 14 hours. The moisture contents of slices on all the trays have attained the range of target moisture contents (%) on wet basis of 7.6, 6.8 and 5.9 within the indicated periods, which was considered the final moisture content (Sacilk, 2006). The drying data of the slices dried in the open-air sun showed moisture content levels higher than those of slices dried in the

63

Table 7. Weight, percentages of moisture content on wet basis and drying rate on dry basis of onion samples in the dryer on trays 1,2.3.4 and 5 and open air sun (Ventilated onion drying) Time Reco d Drying Date (hr) time,h T1 8:30 0 128 10:30 2 98 12:30 4.5 73 19/12/2010 15:30 6.5 53 16:30 8.5 43 17:30 9.5 35

Mass of onion (gm) T2 T3 T4 T5 TOS 127 128 126 127 127 102 106 102 92 99 82 86 80 62 83 65 68 62 42 69 50 53 50 32 53 36 41 42 28 48

T1 87.1 63.7 44.1 28.5 20.7 14.5

Moisture content on wet baasis (%) T2 T3 T4 T5 87.1 87.1 87.1 87.1 67.4 69.9 68.0 59.5 51.7 54.3 50.6 35.9 38.3 40.2 36.3 20.2 26.5 28.5 26.7 12.3 15.4 19.1 20.4 9.1

Moisture content on dry basis (kg of water/kg of DM) TOS T1 T2 T3 T4 T5 TOS 87.1 6.76 6.75 6.75 6.73 6.74 6.74 65.0 1.75 2.07 2.32 2.13 1.47 1.86 52.4 0.79 1.07 1.19 1.02 0.56 1.10 41.4 0.40 0.62 0.67 0.57 0.25 0.71 28.8 0.26 0.36 0.40 0.37 0.14 0.40 24.9 0.17 0.18 0.24 0.26 0.10 0.33

Drying rate (kg of water/kg of DM.hr) T1 T2 T3 T4 T5 TOS 2.5 0.48 1.0 0.10 0.05

2.3 0.50 0.22 0.13 0.09

2.2 0.60 0.26 0.14 0.08

2.3 0.60 0.23 0.10 0.05

2.6 1.2 0.50 0.38 0.15 1.0 0.06 0.15 0.02 0.04

0.04 0.02 0.01 0.0 0.0

0.034 0.013 0.004 0.0 0.0

0.04 0.02 0.01 0.004 0.0

0.04 0.03 0.01 0.009 0.004

0.01 0.009 0.004 0.004 0.004

0.0 0.0 0.0

0.0 0.0 0.0

0.0 0.0 0.0

8:30 10:30 12:30 20/12/2010 14:30 16:30 17:30

9.5 11.5 13.5 15.5 17.5 18.5

33 24 24 24 24 24

32 25 25 25 25 25

35 28 24 24 24 24

40 33 27 24 24 24

26 26 26 26 26 26

46 12.9 12.3 42 5.9 6.8 39 5.9 6.8 37 5.9 6.8 35 5.9 6.8 33 5.9 6.8

14.4 9.0 5.9 5.9 5.9 5.9

18.8 13.3 8.5 6.1 6.1 6.1

7.6 7.6 7.6 7.6 7.6 7.6

23.3 20.2 17.8 16.2 14.6 13.1

0.15 0.06 0.03 0.03 0.03 0.03

0.14 0.07 0.07 0.07 0.07 0.07

0.17 0.10 0.06 0.06 0.06 0.06

0.23 0.15 0.09 0.07 0.07 0.07

0.08 0.08 0.08 0.08 0.08 0.08

0.30 0.25 0.22 0.19 0.17 0.15

8:30 21/12/2010 10:30 12:30 14:30 Final dry mass

18.5 20.5 22.5 23.5

24 24 24 24

25 25 25 25

24 24 24 24

24 24 24 24

26 26 26 26

30 29 28 27

5.9 5.9 5.9 5.9

6.1 6.1 6.1 6.1

7.6 7.6 7.6 7.6

10.7 9.9 9.1 8.3

0.03 0.03 0.03 0.03

0.07 0.07 0.07 0.07

0.06 0.06 0.06 0.06

0.07 0.07 0.07 0.07

0.08 0.08 0.08 0.08

0.12 0.11 0.0 0.10 0.0 0.09 0.0

17

16 17 16 16

16

5.9 5.9 5.9 5.9

6.8 6.8 6.8 6.8

64

0.03 0.02 0.01 0.01 0.01

0.0 0.005 0.0 0.005 0.0 0.005

solar dryer. The lowest moisture content attained after 26 hours of drying was 8.3% on wet basis, which was the equilibrium moisture content relative to the relative humidity of the ambient air. As it can be seen, shortest drying period attained was 10 hours by incorporating poweroperated fans to accelerate the airflow through the collector.

From the drying rate data shown (Tab.7) of the slices dried in the solar drier and in openair sun drying, slices on trays 5&1 started with the highest rate of drying followed by slices on trays 2, 4 & 3. However, after approximately 6 hours of drying the slices on all the trays inside the dryer exhibited similar values indicating similar rate drying. This condition persisted to the end of drying. This can be explained by the fact in the falling rate period of drying, the rate of drying governed by the rate of internal diffusion of moisture to the surface of slices. Once the moisture on the surface of the slices is removed, which is governed by the air temperature and rate of heat transfer to the moisture on the surface, the drying rate is influenced by the rate of replacement of moisture from the interior of the food. This replacement is the same for all the slices thus making the rate of drying more or less the same. The drying rate in all cases reduced as drying time increased.

From the experimental drying data of the shortest drying times and the highest drying rate were exhibited by tomato and onion slices dried in the bottom T1 and top tray T5 in the solar dryer. Whereas slices dried on trays in the middle of drying chamber were exhibited the longer drying times and lower drying rates. But slices dried on the open-air sun had the longest and lowest drying times and drying rates, respectively.

For the comparisons of moisture contents as a function of the drying time Lewis model of thin layer drying processes was used to determine the drying rate coefficient (k) was. The ‘k’ values of the experimental drying data of natural convection solar drying (NCSD), photovoltaic ventilation solar drying (PVSD) and open-air sun drying (OASD) methods 65

for tomato and onion slices were statistical analyzed, for the slices on the bottom, middle and top trays (T1, T3 and T5) of the solar dryer and the slices dried on three trays in openair sun drying for both crops, tomato and onion.

Table 8. Values of drying rate coefficients ‘k’(h-1) for tomato and onion slices dried in the solar dryer and open-air sun drying. Crops

Drying Methods (DM)

Tomato

Onion

1 0.35 0.595 0.04 0.389 0.58 0.06

NCSD PVSD OASD NCSD PVSD OASD

Drying Trays 3 0.28 0.477 0.02 0.273 0.533 0.02

5 0.36 0.543 0.03 0.362 0.912 0.08

The was statistical analyses using factorial experimental design and the analysis of variance (ANOVA) table shown below.

Table 9. ANOVA of the drying rate coefficient Source of variation

SS

Df

MS

Fcal.

Fcrit.

Trays

0.036

2

0.018

2.43

4.1 ns

Treatment total cells

0.98

5

0.2

27

3.33

Crops

0.074

1

0.074

10.3

4.96

Drying methods

0.95

2

0.475

64

4.1

Crops-Drying methods

0.98

2

0.49

66.2

4.1

Error

0.074

10

0.0074

Total

1.09

17

LSD, (P < 0.05) = 0.13

From the ANOVA table and the list significant difference (LSD, P < 0.05), a measure for comparing the means, all the expected variations except the trays are statistically significantly different. This is in agreement with the experimental results, because the drying trays used were similar accordingly for both the crops throughout the experimental 66

periods. In the treatments, deferent drying methods and crops (tomato and onion) were used during the performance evaluations of the newly designed and constructed solar dryer. Natural convection, photovoltaic forced and open-air sun drying methods having different layers of vertical positions with loaded with the materials to be dried trays in the open-air sun placed on the raised ground. From the experimental data the slices on trays in the bottom and top of the drying chamber exhibited shortest drying times and higher drying rates than slices dried on trays in the middle of drying chamber and open-air sun drying. The crops are statistically significantly different from the ANOVA table and LSD, P < 0.05, mean separation methods shown bellow. Crops

Means

Onion 1.068 ± 0.12 a* Tomato 0.9 ± 0.12b* a* b* Means of different letter are statistically different, This is in agreement with the experimental data of the drying in that most of drying processes onion drying had shorter drying times and drying rates. Drying methods The drying methods from the ANOVA table and LSD, P < 0.05, mean separation methods are statistically significantly different shown bellow. Drying methods

Means

Photovoltaic ventilated forced solar drying (PVSD)

1.215 a*

Natural convection solar drying (NCSD)

0.67b*

Open-air sun drying (OASD)

0.083 c*

a* b* c*

Means of different letter are statistically different,

The statistical results obtained are in agreement with the experimental results (Tab.4, 5, 6 and 7) in that the onion and tomato slices dried in the Photovoltaic ventilated forced solar dryer exhibited the shortest drying times and highest drying rates whereas longest times and lowest drying rates were exhibited by slices dried on open-air sun drying.

67

Incorporating ventilation systems to the natural convection solar drying improves drying processes of fruit and vegetables. Drying methods-crops interrelation Treatment combination of drying methods and crops their interrelation is statistically significantly different.

Drying methods

Means

Photovoltaic ventilated forced solar drying of onion (OPVSD)

0.68 a**

Photovoltaic ventilated forced solar drying of tomato (TPVSD)

0.54b*

Natural convection solar drying of onion (ONCSD)

0.34c*

Natural convection solar drying of tomato (TNCSD)

0.33c*

Open-air sun drying of onion (OOASD)

0.05d*

Open-air sun drying of tomato (TOASD)

0.03d*

a**, b*, c* and d* a* b* c*

Means of different letter are statistically different

Photovoltaic ventilated forced solar drying of onion, from the ANOVA table and LSD, P < 0.05, mean separation methods is highly statistically significantly different from all other drying methods and crops treatment combinations. Photovoltaic ventilated forced solar drying of tomato (TPVSD) is the next treatment combinations seen to be statistically significantly different from the rest drying methods and crops treatment combinations

Natural convection solar dryings are statistically significantly different from open-air sun dryings of onion and tomato. However, both methods are not statistically significantly different within themselves from each other. The statistical results are in agreement with the experimental results obtained during the drying processes. The mean values calculated for the model coefficients k (-1 hr.) were statistically analyzed to describing drying characteristics of solar and open-sun dryings of tomato and onion slices.

MR 

Mti  Me  exp (-kt) Mo  Me

68

OPVSD,

MR  exp (-0.68t)

TPVSD,

MR  exp (-0.54t)

ONCSD,

MR  exp (-0.34t)

TNCSD,

MR  exp (-0.33t)

OOASD,

MR  exp (-0.05t)

TOASD,

MR  exp (-0.03t)

Natural convection solar dryers has the advantage of cheap, easy construction from locally available materials and do not require any other energy during operation. Its major drawbacks are the decrease drying rates, important drying time and the very high internal temperature with the likelihood of overheating the product; all these behaviors are due to the extremely low buoyancy conduced air flow inside the dryer as reported by Bala and Woods (1994). In natural convection solar dryer prototype, I have noticed the poor moist air removal and some samples of tomato and onion in the circumference of trays were roasted.

During drying processes the dried tomato slices dried by photovoltaic ventilated forced solar drying had more red color and lighter as compared to those dried in natural convection solar and open- air sun dryings. The forced convection (active) solar dryer is more effective, faster and more controllable than the natural convection current solar dryer.

This conclusion is in agreement with previously found results (Bala and Janjai,

2005).

4.9. Economic Feasibility and Pay Back Analysis of the Solar Dryer The climatic conditions in the Eastern Hararghe allow using the solar dryer for almost the whole year (250 days). The capacity of the dryer 20 kg and 16 kg of fresh tomato and onion, respectively. It can uniformly dry the products within one to two days either in forced or natural convection solar dryer. The expected service life of the dryer is estimated to be 15 years. Assuming the capacity of the dryer per day for tomato and onion at the same time the costs and the main economic parameters based on the local market price 69

situation in the area shown in Table.1. Using this data, the payback period was calculated using the formula below (Neufville, 1990). Payback period (PP) =

II = 6000.00 = 0.098 year ANUB 61200.00

Where, II is initial investment

ANUB is annual net undiscounted benefits

The payback period is determined as the time required for the investment cost to equal the return. In this case the payback period is very small (1.2 months) compared to the life of the dryer, 15 years, so the dryer will dry product free of cost for almost its life period.

Table 10. Payback period of the solar dryer used for drying tomato and onion Item description Item Description 1

Item description Item Description Cost of the dryer

Birr 6000.00

2.

Capacity of the dryer

20kg

3.

Life of dryer

15 years

4.

Depreciation (10%)

Birr 600.00

5.

Cost of maintenance

Birr 300.00

6.

Labor cost 50 x 250

Birr 12500.00

7.

Cost of raw tomato 4 x 20 x 250

Birr 20,000.00

8.

Total cost

Birr 38800.00

9.

Total income 20 x 20 x 250

Birr 100000.00

10.

Net income

Birr 61200.00

70

5. SUMMARY, CONCLUSION AND RECOMMENDATION 5.1. Summary A solar dryer was designed and manufactured at Fadis Agricultural Research Center workshop of Oromia Agricultural Institute. The framework of all the parts of the dryer was built by joining perforated angle irons of 20 mm x 20 mm x 4 mm by means of bolts and nuts. The dryer covers 3.0 m x 3.0 m area of the ground of which the 1m2 was used for drying chamber while the rest saved for collecting solar radiation.

The dryer radiation collector floor, made from thin sheet metal lined with plywood underneath, was painted mat black to increase light absorption capacity. Its roof was covered with transparent flexible plastic to allow sun light in and strike its floor.

The solar dryer was tested at a site where houses, trees and other objects could not block the sun`s light and air flow. In the preliminary tests, the performance of the collector was evaluated at no-load condition i.e. without materials to be dried in the dying chamber. Actual performances were evaluated using tomato and onion slices loaded on to the drying trays and placed in drying chamber.

The collector data showed average temperature rise varying from 13°C at 7:00 o`clock in the morning to 37°C at midday with average solar radiation varying from 175 W/m2 to 965W/m2. Thus, the temperature of the air leaving the collector and entering the bottom of the drying chamber ranged from 28°C to 64°C. Simultaneously the relative humidity of the air decreased from 62% to 5% before it gets into the drying chamber, thus raising its water holding capacity. The daily efficiency, averaged over 11 hours (7:00 to 18:00) comes out to be 51%. As the increases of collector efficiency starting from early morning, reaching its maximum in the midday the dryer performance was highly improved in

71

increasing the drying rates and thus lowering the drying times of the materials being dried in the solar dryer.

Tomato variety, melka shola, sliced in 8 mm thickness and loaded on drying trays in single layer at a rate of 5 kg/m2 was used for the first test. All the five shelves were loaded with tomato slices and drying proceeded keeping records of air temperatures and relative humidity at collector chamber inlet and outlet, and inside the drying chamber (bottom, middle and outlet). The temperature in the drying chamber varied from 25°C to 34°C and the relative humidity from 26% to 34%. Simultaneously samples were dried in the open air in the sun with the same loading rate on similar trays raised 70 cm off the ground.

The tomato slices on the bottom and top trays in the natural convection dryer exhibited the faster drying by lowering the moisture content from 93.3% to 12% in 13 hours, while trays in the center were delayed by 2 to 4 hours. The drying rate was also higher for the bottom and top trays ranging from 2.8 to 0.10 kg of water per kg of dry matter-hr at the start and end of the drying, respectively. To completely and uniformly dry tomato slices in all the trays in the dryer it took two days. Similarly, the 3 mm thick onion slices on the bottom and top trays reduced in moisture content from 87.10 % to 7% in 10 hours with the slices in the remaining trays adding 2 to 4 more hours. The drying rates of the two trays varied from 1.5 to 0.02 kg of water per kg of dry matter-hr from the start to the end of drying. To completely and uniformly dry tomato slices in all the trays in the dryer, it took two days.

In the forced ventilation test the tomato slices of the top tray attained 11.5% moisture content after 12 hours, while the others added 2 to 4 hours. The drying rate of slices on the top tray ranged from 3.1 to 0.10 kg of water per kg of dry matter-hr. it took one and half day to dry the slices in all the trays in the dryer completely and uniformly. Similarly, the onion slices on the top tray in the forced ventilation needed 9 hours to reach 9.1 % moisture content and slices on other trays added 2 more hours. The drying rates of the fastest drying ranged between 2.6 to 0.004 kg of water per kg of dry matter-hr from the start and end of drying, respectively. Drying slices in all the trays in the dryer to the target 72

final moisture content took only one day, the shortest drying time recorded during the drying experiment.

The open-air sun drying of tomato slices took 20 hours to bring down the moisture content to 18% and drying rate was ranged from 0.83 to 0.3 kg of water per kg of dry matter-hr taking four days to the moisture content down to the target moisture content. Similarly, drying times for onion slices dried in the open-air sun drying was 20 hrs, 1 kg of water per kg of dry matter-hr and took three days to dry to the final moisture content.

The results of statistical analyses drying rates coefficients k (-1 hr.) are in harmony of the experimental data during the drying experiment. From the ANOVA table and the list significant difference (LSD, P < 0.05) the means were found to be statistically significantly and compared in the order of OPVSD, TPVSD, ONCSD, TNCSD, OOASD and TOASD for ‘k’ values of 0.68, 0.54, 0.34, 0.33, 0.05 and 0.03. From economic feasibility viewpoints, when the dyer is used to dry tomato and onion, the payback period of the dryer was estimated to be (5 months).

5.2. Conclusions

From the data collected during the performance evaluation of the solar dryer and statistical analyses of the experimental data undertaken, the following conclusions can be drawn. 1. The solar dryer is capable of raising the drying air temperature many times higher than ambient air temperature thereby lowering its relative humidity. This increases considerably the drying potential of the air. 2. The solar dryer can give a higher drying rate than open air-sun drying, thus can considerably decrease the drying time needed for any given product. 3. Use of forced circulation in solar dryer can increase the drying rate and thus may reduce the drying time.

73

4. Onion can be dried from initial moisture of 87.10% (w.b) to final moisture content of 9.1% within one, two and three days using PVSD, NCSD and OASD. 5. Tomato can be dried from initial moisture of 93.3% (w.b) to final moisture content of 12% (w.b) within one & half, two and four days using PVSD, NCSD and OASD. 6. The drying process of solar and open-air sun drying can be represented by Lewis model for tomato and onion samples respectively. 7. It can also be concluded that the designed and manufactured solar dryer can be used to dry other fruits and vegetables sliced in to pieces very much faster than the open-air sun drying.

5.3. Recommendations

1. The manufactured solar dryer has been evaluated using tomato and onion, It is important that it be tested using other fruits and vegetables of different moisture content and structural make up to establish their drying pattern and generate additional information to complete its characterization. 2. The solar dryer was evaluated under ideal environment of long sunshine period and low humidity of the ambient air. It would be of much use to know the drying performance in seasons of low solar radiation and higher ambient air humidity. This will help to predict drying times of various products and accordingly plan their drying operations when the need arises. 3. The solar dryer was evaluated at a loading rate of 5 kg/m2 and 4 kg/m2 for tomato and onion respectively. That is 20 kg of tomato and 16 kg of onion were dried to 12% and 9.1% (w.b) moisture content levels respectively. It appears that the capacity could be even higher and that higher loading rates must be investigated to assess its potential in favorable weather conditions.

74

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7. APPENDIX

82

Appendix Table 1: Whether parameters of Haramaya University Months

October

Max. AvG. MIN.

Temprature Max. Min. 27.5 11.5 25.75 6.8 23 3.0

Nov.

Max. AvG. MIN.

26 23.61 20.5

14.9 5.503 0

9.6 0.53 0

62 37.8 15

1.5 0.396 0.05

11.3 8.95 1.5

1037.7 798.028 160.45

Max. AvG. MIN.

26 20.2 22.88

24 3.34 -4

0

Dec.

55 32.5 10

2.12 1.5 0.62

10.8 9.34 1.5

1037.7 844.62 160.45

Oct.- Dec.

Max. AvG. MIN.

28 24 -4

9.6 2.3 0

62 36.2 10

2.12 0.781 0.05

11.3 7.2 1.5

1038 668.88 160

Sunrise Sunset Annual max. rain fall

Rainfall (mm) 5.6 0.2 0.0

Relative humidity(RH, %) 59 32.1 22

Wimd speed(m/s) 0.67 0.126 0.05

Hours of Sunshine 11.2 8.423 1.8

Solar Radiation (w/m²) 1038 783 160

12:35 12:00

43 ml

83