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

7/25/2019 DESIGN DEVELOPMENT AND PERFORMANCE EVALUATION OF SOLAR DRYER FOR DRYING OF TOMATO AND ONION SLICES.pdf

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DESIGN DEVELOPMENT AND PERFORMANCE

EVALUATION OF SOLAR DRYER FOR DRYING OFTOMATO AND ONION SLICES

M. Sc. Thesis


2/98



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


3/98

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 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 Signature Date


4/98

DEDICATION

I dedicate this thesis manuscript to my father UMAR AHMED , and my motherASHA ABDULAHI , for nursing me with affection and love and for theirdedicated 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 solemnlydeclare 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.


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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


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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 Technologystaff for providing me materials and services.


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STATEMENT OF THE AUTHOR iv

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 xiv

1. INTRODUCTION 1

2. LITERATURE REVIEW 4

2.1. Drying 4 2.1.1. Purpose of drying 4 2.1.2. Application of drying 5 2.1.3. Drying methods 5

2.2. Theory of Drying 6


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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 32 3.2.2. The collecting chamber 35

3.3. Performance Evaluation of Solar Dryer 39 3.3.1. Measuring instruments 39

3.3.2. Preliminary test of the solar dryer 40 3.3.3 Efficiency of solar dryer 40 3.3.4. Sample preparation 42 3.3.5. Moisture content determination of samples 42 3.3.6. Testing the solar dryer using tomato with natural convection current 44

3.3.8. Performance evaluation of solar dryer using tomato and onion inforced ventilation 46 3.3.9. Kinetics of drying 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 ConvectionCurrent 52


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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 controldevice .......................................................................................................................... 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 anddrying rate on dry basis on Tray1, Tray2, Tray 3, Tray 4, Tray 5 and open airsun trays during tomato drying using natural convection current and open-airsun drying .................................................................................................................... 54

Table 5. Weight of onion, percentage moisture contents on wet basis, moisture contentson dry basis and drying rate on dry basis on Tray1,Tray2, Tray 3, Tray 4, Tray5 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 percentagedrying rate on dry basis on Tray1, Appendix Tray2, Tray 3, Tray 4 and Tray 5and 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 inthe solar dryer and open-air sun drying. ........ ....... ....... ....... ....... ....... ....... ........ ...... ...... 66


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LIST OF TABLES IN APPENDIX

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


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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 1m 2 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 20C to 41C to a warm air of 28C to 64C between the morning


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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 maximumdrying 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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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


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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 performanceevaluation 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.


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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.


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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

f h f d


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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 ( N w) to the

amount the air will hold when saturated at the same temperature ( N ws).The partial pressure


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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 ( t wb) 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 therate 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


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Psychrometric Charts

Properties shown on most psychrometric charts are dry bulb, wet-bulb, and dew pointtemperatures; 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

airs 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 psychrometriccharts. 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,

t t i d b t i t t t d d i t h d


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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 thegenerally 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


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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 = M e) 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.


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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 Newtons 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) indicatingnon 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


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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 Ficks 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.

kt et Ae Mei M

M M (2)

If the constant A in the above equation is equal to unity, the equation is reduced to the

same form as Newtons 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:

nqt e Me Mt (3)


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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


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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 heatmay 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).


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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 solardrying 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


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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 androdents 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).


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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


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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), glass-fiber 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


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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 incidentsolar 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 worlds 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 andthe 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


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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 infresh 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 65C 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


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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 dueto their high organo-sulphur compounds (Mazza and LeMaguer, 1980; Corzo-Martnez 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.


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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, andtested 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/m 2 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 60C 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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Drying is the application of heat under controlled condition to remove the majority of thewater normally present in a food by evaporation and extend the shelf life of food byreduction of water activity. The decrease in weight and volume reduce transport andstorage costs. Design of drying equipment and operation is aimed at minimizing thesenegative 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 moisture-

free 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:

d l d i


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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 = M e) 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.

D i i d ith i thi l d i i d l d i I thi l d i


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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 thedrying 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 ruralfarmers 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:


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Solar drying

Solar drying is often differentiated from sun drying by the use of equipment to collect

the suns 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. Suchis 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 open-

air 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


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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.


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A direct passive dryer is one in which the food is directly exposed to the suns 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 suns 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


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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 fordrying and mostly becomes 4-5 kg when dried. Maximum permissible drying air

temperature is 65C 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/m 2 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 60C 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.


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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, relativehumidity and air velocity.


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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


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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.


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1.0 m

1 . 3 0 m

0 . 6

0 m

0 . 4

0 m

0 . 3 0 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 m 2, giving a total

drying surface area of 10 x 0.369 m 2 = 3.936 m 2.

Chi


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dimension of 3.0 m x 3.0 m and a net area of 8.0 m 2. 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. Thecollector 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 ofthe 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


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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


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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


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Figure 8. Photo of solar dryer

3.3. Performance Evaluation of Solar Dryer

3.3.1. Measuring instruments

Th h (C Fl 8612) d h idi i h


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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


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(1)

Where, C is collector efficiency (%)

(2)

)( ,, incout cU T T mCpQ (3)

Qu is useful heat flow rate (J/s)

qm .

, (4)

m is air mass flow rate (kg/s)

is density of air (kg/m 3)

q = AV, (5)

q is volume flow rate of air (m 3/s)

A is the collector exit area (m 2)

G AQ

c

uC

G A

T T mC

c

incout cPC

.

,, )(


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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/m 2.

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.


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

%100).( d

d obd o W

W W Mt (Karl and Hall, 1996). (6)

Where, W o (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 (t i) during the drying process, the following equation was used:

%100).( d

d bd W

W Wti Mti (7)

Where, Mti (d.b is 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

%100).(

o

d

bw W

W Wti Mti

(8)

Where Wti is the weight of the samples at time t i;


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Where t i-1 and t i 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):

(10)

)( 11

tit Mti Mt Rti

i

i (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, T 1, T2, T3, T4, and T 5

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 oclock in the morning, proceededthroughout the day and ended at 5:30 oclock. The slices on every tray were manually

stirred randomly after recording the weights to facilitate the drying process This was to

ti

MCti MCt Rti i 1


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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 duringthe 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, T 1, T 2, T3, T 4, and T 5 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 oclock in the morning, proceeded throughout the day and ended at

5:30 oclock. 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 themoisture 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.


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3.3.8. Performance evaluation of solar dryer using tomato and onion in forcedventilation

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


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Where, Mti= moisture content, % (db),

t = time, hour,

M o = 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 LevenbergMarquardt 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

Th f t i l i t l d i h th i l t t t t i th t t f


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Table 1. Treatment combination, replication and randomization

Crops:

(TomatoOnion)

Drying Methods (DM)

Trays

I II III

Tomato

Natural convection solar Drying (NCSD) TNCSD TNCSD TNCSD

Photovoltaic ventilated solar drying(PVSD)

TPVSD TPVSD TPVSD

Open-air sun drying (OASD)TOASD TOASD TOASD

Onion Natural convection solar Drying (NCSD)

ONCSD OONCSD ONCSD

Photovoltaic ventilated solar drying(PVSD)

OPVSD OPVSD OPVSD


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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, measurementswere 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 fully-

open positions using manually operated control valve fitted in the chimney, a maximum

temperature rise of 41C 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


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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 18C

at 8:00 oclock in the morning to about 37C at midday. The period starting from 10:00am 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/m 2 in the morning to

965W/m 2 at midday the temperature of the air in the collector rose from 36C to 60C.

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 64C, 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 41C (varying between 15C and 41C). Theaverage air velocity was 0.04 m s -1 at the drying chamber outlet.


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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

Time drying velocity Airflow Air Temp. ( C) Solar Energy Collectorof day time (m/s) rate radiation Total Useful efficiency

(hr) (hr) V(kg/s) Tam Tco (Tco-Tam) (W/m ) (W) (W) (%)

71 0.01 0.0065 15 28 13 50 400 84 21

8 2 0.02 0.0259 18 36 18 175 1400 468 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

980


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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 threemeasurements 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 openair

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 recordingsduring 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


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54

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 Moisture content Moisture content on dry basis Drying rate on dry basisDate Record Drying time Mass of tomato(gm) on wet basis (%) (kg of water/kg.dry matter) (kg of water/kg.DM.hr.)

(hr) (hr) T1 T2 T3 T4 T5 TOS T1 T2 T3 T4 T5 TOS T1 T2 T3 T4 T5 TOS T1 T2 T3 T4 T5 TOS8:30 0 109 120 110 122 110 125 93.3 93.3 93.3 93.3 93.3 93.3 13.9 13.9 13.9 13.9 13.9 13.9

4/11/2010 10:30 2 74 82 80 84 68 100 61.2 61.6 66.0 62.2 55.1 73.3 9.1 9.2 9.9 9.3 8.2 10.9 2.4 2.4 2.0 2.3 2.8 1.512:30 4.5 61 70 66 71 52 85 49.3 51.6 53.3 51.5 40.6 61.3 7.4 7.7 8.0 7.7 6.1 9.1 0.9 0.7 0.9 0.8 1.1 0.914:30 6.5 46 60 54 58 40 60 35.5 43.3 42.4 40.8 29.7 41.3 5.3 6.5 6.3 6.1 4.4 6.2 1.0 0.6 0.8 0.8 0.8 1.516:30 8.5 36 51 45 46 32 52 26.3 35.8 34.2 31.0 22.4 34.9 3.9 5.3 5.1 4.6 3.3 5.2 0.7 0.6 0.6 0.7 0.5 0.517:30 10.5 30 43 38 38 27 49 20.8 29.1 27.8 24.5 17.8 32.5 3.1 4.3 4.2 3.6 2.7 4.9 0.4 0.5 0.5 0.5 0.3 0.2

8:30 10.5 25 39 33 38 20 40 16.2 25.4 22.8 20.0 11.5 25.3 3.1 4.3 4.2 3.6 2.7 3.810:30 12.5 20 34 26 20 20 36 11.5 21.6 16.9 11.8 11.5 22.1 1.2 3.2 2.5 1.4 0.6 3.2 1.0 0.6 0.8 1.1 1.0 0.3

5/11/2010 12:30 14.5 20 26 20 20 20 35 11.5 15.0 11.5 11.8 11.5 21.3 0.9 2.2 1.7 0.8 0.4 2.7 0.1 0.5 0.4 0.3 0.1 0.214:30 16.5 20 20 20 20 20 33 11.5 10.0 11.5 11.8 11.5 19.7 0.6 1.5 1.2 0.6 0.2 2.3 0.1 0.4 0.3 0.1 0.1 0.216:30 18.5 20 20 20 20 20 30 11.5 10.0 11.5 11.8 11.5 17.3 0.4 0.9 0.8 0.5 0.2 2.0 0.1 0.3 0.2 0.1 0.0 0.217:30 20.5 20 20 20 20 20 28 11.5 10.0 11.5 11.8 11.5 15.7 0.4 0.4 0.5 0.5 0.2 1.7 0.0 0.2 0.1 0.0 0.0 0.1

Finaldry mass 7.3 8.04 7.37 8.2 7.4 8.4


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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 20hrs, 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 dryingcharacteristics 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


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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 duringthe drying process (Cui et al., 2004).


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and T4, located in the mid height of the chamber, had the slowest fall of the moisture

content extending to the 14 th 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 tothose of the solar dried slices even after 25 thhours. 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


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58

Table 5. Weight of onion, percentage moisture contents on wet basis, moisture contents on dry basis and drying rate on dry basis onTray1,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 Mass of onion (g) Moisture content on dry basis Drying rate on dry basisof day time on trays in dryer Moisture content on wet basis (%) (kg of water/kg of dry matter) (kg of water/kg of dry matter)

Date (hr) (hr) T1 T2 T3 T4 T5 TOS T1 T2 T3 T4 T5 TOS T1 T2 T3 T4 T5 TOS T1 T2 T3 T4 T5 TOS8:30 0 240 271 283 282 272 286 87.0 87.0 87.0 87.0 87.0 87.0 6.7 6.7 6.7 6.7 6.7 6.7

10:30 2 144 185 197 180 176 224 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

12:30 4 99 148 158 139 134 174 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 0.715/10/2010 14:30 6 90 138 148 130 125 146 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

16:30 8 70 116 128 107 100 120 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 0.317:30 9.5 49 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

8:30 9.5 49 80 89 76 65 108 7.4 16.5 18.4 13.9 10.9 24.8 0.6 1.3 1.4 1.1 0.8 1.910:30 11.5 35 63 72 61 40 96 1.6 10.3 12.4 8.62 1.69 20.6 0.1 0.8 1.0 0.7 0.10 1.6 0.2 0.2 0.2 0.2 0.2 0.2

12:30 13.5 35 47 54 48 40 89 1.6 4.35 6.08 4.01 1.69 18.1 0.1 0.3 0.5 0.3 0.10 1.4 0.1 0.2 0.2 0.2 0.2 0.116/10/2010 14:30 15.5 35 44 49 39 40 76 1.6 3.25 4.31 0.82 1.69 13.6 0.1 0.2 0.3 0.1 0.10 1.0 0.0 0.0 0.7 0.1 0.0 0.2

16:30 16.5 35 37 41

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