DESIGN, FABRICATION AND TESTING OF A SOLAR-BIOMASS INTEGRATED DRYER

A PROJECT REPORT

BY

OJO, ADESHINA JONATHAN (053241)

BISIRIYU, ISAIKA OLAJIDE(053715)

OKELEYE, SAMUEL ADEOLA(053547)

SUBMITTED TO

THE DEPARTMENT OF MECHANICAL ENGINEERING

FACAULTY OF ENGINEERING AND TECHNOLOGY

LADOKE AKINTOLA UNIVERSITY OF TECHNOLOGY,

OGBOMOSO, OYO STATE.

IN PARTIAL FULFILMENT OF THE AWARD OF DEGREE OF BACHELOR OF

TECHNOLOGY,(B.TECH) IN MECHANICAL ENGINEERING

MARCH, 2011

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CERTIFICATION

This is to certify that this project was carried out by:

OJO, Adeshina Jonathan (053241)

BISIRIYU, Isiaka Olajide (053715)

OKELEYE, Samuel Adeola (053547)

The project report has been read through, approved and certified having met with the

requirement for the award of Bachelor of technology (B.Tech) from the department of

Mechanical Engineering, Ladoke Akintola University of Technology, (LAUTECH) Ogbomoso.

______________________ ________________________

ENGR. O.S OLAOYE DATE

PROJECT SUPERVISOR

_____________________ _________________________

DR. A.S. ONAWUMI DATE

HEAD OF DEPARTMENT

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DEDICATION

We dedicate this project to the Almighty God, the one and only true GOD for his high

protection, guidance and provision all through our academic years. We cannot Thank you

enough but we are using this medium to say thank you Lord. ABBA FATHER, Glory be to your

name.

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ACKNOWLEDGEMENT

Our sincere gratitude goes to our supervisor Engr. O.S Olaoye for his dedicated supervision to

successful completion of this project.

A special thanks to our parents for their understanding and moral supports for our

accomplishments, we will also remember our uncles, aunties and all our benefactors for their

supports.

Also, to all our lecturers in the department, thank you all, we are grateful.

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ABSTRACT

This project reports the design, fabrication and testing of a solar-biomass dryer. It is designed

to dry agricultural crops. In this project, ginger of 55g is being dried to test the effectiveness of

the dryer. The design of the dryer was made from locally available materials and works in such a

way that solar radiation is incident directly on the glazing which in turn heats up the solar

chamber. The heat is retained by the black body located at the bottom of the solar chamber.

The dryer also has a back-up heater which uses biomass as its source of fuel, this serves as the

heating source beyond sunshine hours. This makes the integrated dryer very effective in drying

of crops. The test results gave temperature as high as 560C in the drying chambers during

sunshine hours and 55g of fresh ginger was reduced to 18g, during two days of drying in the

chamber.

Drying processes play an important role in the preservation of agricultural products. They are

defined as a process of moisture removal due to simultaneous heat and mass transfer (Ertekin

and Yaldiz, 2004).According to Ikejiofor (1985) two types of water are present in food items; the

chemically bound water and the physically held water. In drying, it is only the physically held

water that is removed. The most important reasons for the popularity of dried products are longer

shelf-life, product diversity as well as substantial volume reduction. This could be expanded

further with improvements in product quality and process applications.

The materials used for the construction of the solar-biomass dryer are cheap and easily

obtainable in the local market. The essential features of the dryer consist of the solar collector

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(air heater), the drying cabinet and drying trays. Test was carried-out on the solar chamber at

12pm noon and readings of the internal temperature of the solar chamber was recorded at

different points, i.e. temperature at first tray, second tray and also that around the black body.

Moisture of 15g was removed on the first day, and moisture content removed on the second day

was 22g. The drying time was two (2) days and a drying efficiency of 82.4% was achieved when

drying 55g of wet ginger using 100g of charcoal, which have a calorific value of 29.6 MJ/Kg.

In conclusion, the need for the construction of solar-biomass dryer arose as an alternative for

ordinary sun drying techniques. The solar-biomass hybrid dryer is very efficient in operation

because most crops are being sun-dried on the ground, this is however a slow ineffective method

which takes days to achieve tangible results. We recommend that during future construction of

the dryer, rock slab should be in between the biomass chamber and the solar chamber, in order to

avoid direct heat flow from the biomass to the solar chamber, also, the efficiency of the solar-

biomass integrated dryer can be improved upon by reducing the various heat losses in and

around the solar chamber.

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TABLE OF CONTENT

TITLE PAGE

CERTIFICATION II

DEDICATION III

ACKNOWLEDGMENT IV

ABSTRACT V

TABLE OF CONTENTS VII

LIST OF TABLES IX

LIST OF FIGURES X

CHAPTER ONE

1.0 INTRODUCTION 1

1.1 Preamble 1

1.2 Aim and Objectives 2

1.3 Justification 2

1.4 Scope of Study 3

CHAPTER TWO

2.0 LITERATURE REVIEW 4

2.1 General 4

2.1.1 The Old Traditional Way, the Open Air Drying 6

2.1.2 Fire wood / fuel Drying 6

2.1.3 Electric Drying 6

2.1.4 Photovoltaic (Pv) Power or Simple Solar Drying 6

2.2 The Solar Dryer Technology and Utilization 7

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2.2.1 Low cost Solar Dryer 7

2.2.2 Electrical Solar dryer AC powered 7

2.2.3 Solar Dryer – DC powered 8

2.3 Theory, Materials and Methods Basic Theory 11

2.4 Energy Balance Equation for the Drying Process 13

CHAPTER THREE

3.0 METHODOLOGY 15

3.1 Construction of the Solar-Biomass Dryer 15

3.1.1 Collector (Air Heater) 15

3.1.2 The Drying Cabinet 16

3.1.3 Drying Tray 17

3.1.4 The orientation of the Solar Collector 18

3.1.5 Biomass Stove 18

3.2 Operation of the Dryer 20

3.3 Reason for alternating the arrangement of Trays 22

3.4 Effect of Increase In Trays of Dryer 24

3.5 Reason for Using A Flat Plate Collector 24

3.5.1 Advantages of Flat Plate 25

3.5.2 Principle of A Flat plate collector 25

3.6 Reason for Material Selection 26

3.6.1 Uses of Stainless Steel 26

3.6.2 Uses of Aluminum 26

3.7 Merit of Solar Biomass Integrated Dryer 27

3.8 Fuel used in Biomass Chamber 27

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3.9 Design Modification and Calculation 27

3.10 Determination of Heat Required for Water Removal 31

3.11 Determination of Drying Time 33

3.12 Determination of Dryer Capacity 35

CHAPTER FOUR

4.0 RESULT AND DISCUSSION 38

4.1 Result 38

4.1.1 Laboratory Test 38

4.1.2 Calculation to Determine Percentage Moisture 39

4.2 No Load Test of the Solar Dryer 40

4.3 Load Test of the solar dryer 41

4.4 Determination of Dryer’s efficiency 42

4.5 Discussion of Results 44

CHAPTER FIVE

5.0 CONCLUSION AND RECOMMENDATION 45

5.1 Conclusion 45

5.2 Recommendations 46

REFERENCES

APPENDIX

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LIST OF TABLES

TABLE PAGE

2.1 Advantages and Disadvantages of Different Dryers. 2

4.1 Moisture Content Analysis of Ginger. 38

4.2 Variation of Temperature with Time on the First Day. 41

4.3 Variation of Temperature with Time On The Second Day. 42

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LIST OF FIGURES

FIGURE PAGE

3.1 The Glazing 16

3.2 The Drying Cabinet 17

3.3 The Drying Tray 17

3.4 The Biomass Stove 19

3.5 The Drier’s Chimney. 19

3.6 The Solar Biomass Integrated Dryer 21

3.7 Section of the Biomass Chamber of the Solar-Biomass Integrated Drier 23

3.8 Section of the Solar Chamber of the Solar-Biomass Integrated Drier 23

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CHAPTER ONE

1.0 INTRODUCTION

1.1 PREAMBLE

Drying is the removal of solvent from product; this term is used interchangeably with heating

or curing. Heating is simply raising the temperature of a product while curing is holding a

product at a given temperature for a given time to complete a reaction (Theo and holly, 1997).

The rapid rate of drying in the dryer reveals its ability to dry food items reasonably rapidly to a

safe moisture level.

Various drying techniques are employed to dry different food products. Each technique has its

own advantages and limitations. Choosing the right drying techniques is thus important in the

process of drying of these perishable products. The Brace type solar drier is one of the few

designs that have achieved some level of acceptance. One significant disadvantage of this drier is

that it is normally not used without any form of back-up heating. To reduce its dependence on

solar radiation for operation and to improve the quality of drying, a biomass stove was

incorporated with solar drier. It has extended the period of drying beyond sunshine hours, and

even at night, while drying high-value products.

1.2 AIM AND OBJECTIVES.

The principal aim of this project is to design, fabricate and test a solar-biomass integrated

dryer.

The main objectives are:

-to show the advantages of solar-biomass integrated dryer over separate individual dryer.

-to show that the solar-biomass drier is more efficient as compared to separate individual dryers.

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1.3 JUSTIFICATION

A very large proportion of food product in developing countries like Nigeria is destroyed

before they reach the market. This waste may be reduced considerably if the products are dried

before they are transported to the market.

Most crops are being sun-dried on the ground, this is however a slow ineffective method

because it takes days to achieve tangible results. Also, crops are exposed to attack by bacteria

and the like. Ordinary sun drying method was found to be very tedious, time wasting, wastage, in

terms of produce and consequently having a very low hygienic level.

The direct exposure to sunlight, or more precisely ultraviolet radiation, can greatly reduce the

level of nutrients such as vitamins in the dried product. Zingiber officinale (ginger), Solanum

lycopersium (tomatoes), Abelmoschus esculentus (okro), Zea mays(maize), which are used

extensively in Nigeria.

1.4 SCOPE OF STUDY.

The scope of this work is to fabricate and test the solar –biomass integrated dryer to dry crops

like mainly Ginger, Tomatoes, Okro and Maize, which covers all classes of food crops. This

project also evaluates the efficiency of the solar-biomass hybrid drier against ‘solar only’,

‘biomass only’ and open sun condition.

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CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 General.

In many parts of the world there is a growing awareness that renewable energy have an

important role to play in extending technology to the farmer in developing countries to increase

their productivity (Waewsak, et al. 2006). Solar thermal technology is a technology that is

rapidly gaining acceptance as an energy saving measure in agriculture application. It is preferred

to other alternative sources of energy such as wind and shale, because it is abundant,

inexhaustible, and non-polluting (Akinola 1999; Fapetu 2006; Akinola, et al. 2006). Solar air

heaters are simple devices to heat air by utilizing solar energy and employed in many

applications requiring low to moderate temperature below 80 o C, such as crop drying and space

heating (Kurtbas and Turgut 2006).

Drying process play an important role in the preservation of agricultural products. They are

defined as a process of moisture removal due to simultaneous heat and mass transfer (Ertekin

and Yaldiz 2004). According to Ikejiofor (1985) - two types of water are present in food items;

the chemically bound water and the physically held water. In drying, it is only the physically

held water that is removed. The most important reasons for the popularity of dried products are

longer shelf-life, product diversity as well as substantial volume reduction. This could be

expanded further with improvements in product quality and process applications. The application

of dryers in developing countries can reduce post harvest losses and significantly contribute to

the availability of food in these countries. Estimations of these losses are generally cited to be of

the order of 40% but they can, under very adverse conditions, be nearly as high as 80%. A

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significant percentage of these losses are related to improper and/or untimely drying of

foodstuffs like ginger, tomatoes, okro and maize. (Bassey 1989; Togrul and Pehlivan 2004)

Traditional drying, which is frequently done on the ground in the open air, is the most

widespread method used in developing countries because it is the simplest and cheapest method

of conserving foodstuffs. Some disadvantages of open air drying are: exposure of the foodstuff to

rain and dust; uncontrolled drying; exposure to direct sunlight which is undesirable for some

foodstuffs; infestation by insects; attack by animals; etc (Madhlopa, et al. 2002).

In order to improve traditional drying, solar dryers which have the potential of substantially

reducing the above-mentioned disadvantages of open air drying; have received considerable

attention over the past 20 years (Bassey 1989). Solar dryers of the forced convection type can be

effectively used. They however need electricity, which unfortunately is non-existent in many

rural areas, to operate the fans. Even when electricity exists, the potential users of the dryers are

unable to pay for it due to their very low income. Forced convection dryers are for this reason

not going to be readily applicable on a wide scale in many developing countries. Natural

convection dryers circulate the drying air without the aid of a fan. They are therefore, the most

applicable to the rural areas in developing countries.

Solar drying may be classified into direct, indirect and mixed-modes. In direct solar dryers

the air heater contains the grains and solar energy passes through a transparent cover and is

absorbed by the grains. Essentially, the heat required for drying is provided by radiation to the

upper layers and subsequent conduction into the grain bed. In indirect dryers, solar energy is

collected in a separate solar collector (air heater) and the heated air then passes through the grain

bed, while in the mixed-mode type of dryer, the heated air from a separate solar collector is

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passed through a grain bed, and at the same time, the drying cabinet absorbs solar energy directly

through the transparent walls or roof.

There are four common drying methods/techniques, which are utilized for drying agricultural

products. They are;

2.1.1 The Old Traditional Way, the Open Air Drying

This is the present drying techniques used by urban and rural farmers. It is the old fashion

drying techniques, spreading the crops on a carpet or directly on the soil in open air and exposed

to the sun for drying.

2.1.2 Firewood/Fuel Drying

This technique is using fuel source to create the required heat for drying. It is used for tobacco

drying, and is common in Uganda, and is more popular in West Nile for the tobacco business.

2.1.3 Electrical Drying

This technique is similar to fuel drying only that apart from heat from fuel, energy source is

created by electricity.

2.1.4 Photovoltaic (PV) Powered or Simple Solar Drying

The difference of this technique from the above mentioned, is that the heat is created by the

sun radiation. The heat could be circulated by a ventilator powered by PV array.

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The three first mentioned techniques can be replaced by the environmental friendly drying

techniques.

2.2. The Solar Dryer Technology and Utilization

The description of the solar drying technologies will be limited to few types, which are very

common and well known.

2.2.1 Low Cost Solar Dryer

The low cost dryer has different types of its kind, example are the cabinet solar dryer, the flat

solar dryer, etc. The low cost dryer is the most suitable even for the small farmer, who have a

very limited income. It will be possible for them to cover the investment cost. This is produced

form locally available materials, and by local carpenter.

2.2.2 Electrical Solar Dryer- AC powered

This type is quite different from the low cost solar dryer, because this has bigger size and is

usually made for heavy duty operations; such as the tobacco, timber or coffee drying. This type

is supplied by fan to circulate the hot air inside the dryer. The heating energy is provided by the

sun collector placed on the roof.

During the utilization of this type in rural areas, the air circulation fan can be DC powered.

This type of dryer can also be supplied by an electrical heating element for providing the heat.

The dryer is usually equipped with steering system to control the drying temperature, which

varies according to the type of crops.

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2.2.3 Solar Dryer - DC Powered

This type is similar to the above mentioned AC powered, but a small size and developed for

drying only food crops, cash crops and fruits. The dryer consist of:

1. Sun collector panel for air heating

2. Drying chamber

3. Fan for the hot air circulation

4. Moisture and temperature sensor

5. PV- panel for powering the fan

Table 2.1 Advantages and Disadvantages of Different Dryers.

The applied drying technique Advantages Disadvantages

Old traditional way - Open air drying

-No investment is required-No fuel is required

The product is exposed to various risks, such as dust, animals, ants, rain, etc..It takes time to dryIt requires labour to survey the cropsThe quality of the drying is not good (exposed to dust)

Firewood/fuel drying -Quicker than the open airtype

Requires investment in fuelRequires investment in building ordryer boxPollution and forest degradationThe quality of the dried product,can be affected with smokeNot easy to control the

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dryingRequires labour for operation and maintenance

Electrical drying -The drying is quicker thanboth the above mentioned-The drying of the crops iswell controlled-The quality of the driedproduct is excellent, depending on the type of the dryer

The dryer is costlyIt is expensive to run and operate the dryerThe dryers require energy, which is not available in rural areasIt is not affordable for small farmer and even co-operativesThe dryer has to be installed in building - further investmentThe drying is not environmentallyfriendly – if powered by a generatorRequire labour for operation and maintenanceRequires introduction to theTechnology

Solar Drying -Very cheap investment-Very easy to maintain and operate-Does not require a specialized manpower-Well controlled drying-Quicker drying-The crops are well protected during the drying process-does not require fuel.-Affordable to every body-Environmentally friendly

Requires introduction to the technology- The cost of is dependent on the loan and the various expensesThe dryer should be operated properly to reach the good quality ofthe dried productsRequires a small investment

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From table above, there is a clear indication, that the solar-biomass hybrid dryer is the most

suitable for the small scale farmers to cover their needs, during the processing of cash crops. It is

also suitable for drying the available fruits and vegetables.

2.3 Theory, Materials and Methods Basic Theory

The energy balance on the absorber is obtained by equating the total heat gained to the total

heat lost by the heat absorber of the solar collector. Therefore,

IAc = Qu + Qcond + Qconv + QR + Qρ ,………..(1)

Where:

I = rate of total radiation incident on the absorber’s surface (Wm –2 );

Ac = collector area (m 2 );

Qu = rate of useful energy collected by the air (W);

Qcond = rate of conduction losses from the absorber (W);

Qconv = rate of convective losses from the absorber (W);

QR = rate of long wave re-radiation from the absorber (W);

Qρ= rate of reflection losses from the absorber (W).

The three heat loss terms Qcond, Qconv and QR are usually combined into one-term (QL), i.e.,

QL = Qcond + Qconv + QR……………. (2)

If Ʈ is the transmittance of the top glazing and IT is the total solar radiation incident on the top

Surface, therefore,

IAc = Ʈ ITAc . (3)

The reflected energy from the absorber is given by the expression:

Qρ = Ʈ ITAc, (4)

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Where ρ, is the reflection coefficient of the absorber. Substitution of Eqns. (2), (3) and (4) in

Eq. (1) yields:

Ʈ I TAc = Qu + QL + ρƮ ITAc, or

Q = ƮITA(1 – ρ) – QL.

For an absorber (1 – ρ) = α and hence,

Qu = (αƮ ) ITAc – QL,………….. (5)

Where α is solar absorptance.

QL composed of different convection and radiation parts. It is presented in the following

form (Bansal et al. 1990):

QL = ULAc(Tc – Ta),…………… (6)

Where:

UL = overall heat transfer coefficient of the absorber (Wm –2 K –1 );

Tc = temperature of the collector’s absorber (K);

Ta = ambient air temperature (K).

From Eqns. (5) and (6) the useful energy gained by the collector is expressed as:

Qu = (αƮ )ITAc – ULAc(Tc – Ta)……………… (7)

Therefore, the energy per unit area (qu) of the collector is

qu = (αƮ)IT – UL(Tc – Ta)……………….. (8)

If the heated air leaving the collector is at collector temperature, the heat gained by the air Qg

is:

Qg = maCpa(Tc – Ta),…………………… (9)

Where:

ma = mass of air leaving the dryer per unit time (kgs – 1 );

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Cpa = specific heat capacity of air (kJkg – 1 K – 1 ).

The collector heat removal factor, FR, is the quantity that relates the actual useful energy

gained of a collector, Eq. (7), to the useful gained by the air, Eq. (9). Therefore,

….(10)

or

Qg = AcFR[(αƮ )IT – ULAc(Tc – Ta)]……… (11)

The thermal efficiency of the collector is defined as (Itodo et al. 2002):

……..(12)

2.4 Energy Balance Equation for the Drying Process

The total energy required for drying a given quantity of food items can be estimated using the

basic energy balance equation for the evaporation of water (Youcef-Ali, et al. 2001; Bolaji

2005):

mwLv = maCp(T1 – T2),…………. (13)

where:

mw = mass of water evaporated from the food item (kg);

ma = mass of drying air (kg);

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T1 and T2 = initial and final temperatures of the drying air respectively (K);

Cp = Specific heat at constant pressure

(kJkg –1 K –1 ).

The mass of water evaporated is calculated from Eq. 14:

………………(14)

where:

mi = initial mass of the food item (kg);

Me = equilibrium moisture content (% dry basis);

Mi = initial moisture content (% dry basis).

During drying, water at the surface of the substance evaporates and water in the inner part

migrates to the surface to get evaporated. The ease of this migration depends on the porosity of

the substance and the surface area available. Other factors that may enhance quick drying of food

items are: high temperature, high wind speed and low relative humidity. In drying grains for

future planting, care must be taken not to kill the embryo. Excessive heating must also be

avoided, as it spoils the texture and quality of the item.

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CHAPTER THREE

3.0 METHODOLOGY

3.1 Construction of the Solar-Biomass Dryer.

The materials used for the construction of the solar-biomass dryer are cheap and easily

obtainable in the local market. The essential features of the dryer consist of the solar collector

(air heater), the drying cabinet and drying trays.

3.1.1 Collector (Air Heater): Flat plate collector is used; it consists of thin box with a

transparent cover called the glazing which is mounted on the top of the dryer facing the Sun. The

Sun heats a blackened metal plate inside the box, called an absorber plate, which in turn heats the

air in the box. Microsoft Encarta (2009) viewed on 01/06/2010.

The heat absorber (inner box) of the solar air heater was constructed using 2 mm thick

aluminum plate, painted black, is mounted in an outer box built from well-seasoned woods. The

space between the inner box and outer box is filled with foam material of about 40 mm thickness

and thermal conductivity of 0.043 Wm –1 K –1 . The solar collector assembly consists of air flow

channel enclosed by transparent cover (glazing). An absorber mesh screen midway between the

glass cover and the absorber back plate provides effective air heating because solar radiation that

passes through the transparent cover is then absorbed by both the mesh and back-plate. The 24

glazing is a single layer of 4mm thick transparent glass sheet; it has a surface area of 820 mm by

1020 mm and of transmittance above 0.7 for wave lengths in the range 0.2 – 2.0 ì m and opaque

to wave lengths greater than 4.5 ì m. The effective area of the collector glazing is 0.8 m 2 . One

end of the solar collector has an air inlet vent of area 0.0888m 2 , which is covered by a

galvanized wire mesh to prevent entrance of rodents, the other end opens to the plenum

chamber. All dimensions are in centimeters.

FIG3.1.1 THE GLAZING

3.1.2 The Drying Cabinet: The drying cabinet together with the structural frame of the dryer

was built from well-seasoned woods which could withstand termite and atmospheric attacks. An

outlet vent was provided toward the upper end at the back of the cabinet to facilitate and control

the convection flow of air through the dryer. Access door to the drying chamber was also

provided at the back of the cabinet. This consists of three removable wooden panels made of

13mm plywood, which overlapped each other to prevent air leakages when closed. The roof and

the two opposite side walls of the cabinet are covered with transparent glass sheets of 4 mm

thick, which provided additional heating. All dimensions are in centimeters.25

FIG 3.1.2 THE DRYING CABINET

3.1.3 Drying Trays: The drying trays are made of aluminum material contained inside the

drying chamber and were constructed from a double layer of fine chicken wire mesh with a fairly

open structure to allow drying air to pass through the food items. All its dimensions are in

centimeters.

FIG 3.1.3 THE DRYING TRAY

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3.1.4 The Orientation of the Solar Collector: The flat-plate solar collector is always tilted and

oriented in such a way that it receives maximum solar radiation during the desired season of use.

The best stationary orientation is due south in the northern hemisphere and due north in southern

hemisphere. Therefore, solar collector in this work is oriented facing south and tilted at 17.5 o to

the horizontal. This is approximately 10 o more than the local geographical latitude, which

according to Adegoke and Bolaji (2000) is the best recommended orientation for stationary

absorber. This inclination is also to allow easy run off of water and enhance air circulation.

3.1.5 Biomass Stove.

The design of the biomass system is based on the following points:

(1) The heating is indirect, i.e. the flue gases from the chimney and the drying air could not be

mixed. This is protecting the product from contamination by smoke, soot and ash of the flue

gases.

(2) Temperature of the drying air could be controlled, by maintaining the combustion in the

stove, with the opening or closing of the primary air supply gate.

(3) Biomass burning could be carried out for extended periods of time, unattended. The biomass

stove is having a dimension of 0.65 m by 0.60 m by 0.55 m surrounded by brick walls (1.45 m

by 1.17 m by 0.9 m). A burner grate with a perforated tray is provided inside the stove. The

exhaust gases exit via a 35 mm diameter and 65 cm long chimney located at one side of the

stove. In order to lengthen the flow path of exhaust gases and maximize the transfer of heat to

the stove walls, three metal baffle plates are inserted at s distance of 0.1 m above the grate and

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below the chimney in the burning chamber feeding hole having dimension of 0.018 and 0.11

meters squares, respectively.

FIG 3.1.4 THE DRIER’S CHIMNEY.

3.2 Operation of the Dryer

This solar-biomass hybrid drier is designed to use solar energy as main heat source and

biomass stove is used only when solar is not available, during early morning, late evening,

cloudy weather conditions and at night. The dryer is a passive system in the sense that it has no

moving parts. It is energized by the sun’s rays entering through the collector glazing. The

trapping of the rays is enhanced by the inside surfaces of the collector that were painted black

and the trapped energy heats the air inside the collector. The green house effect achieved within

the collector drives the air current through the drying chamber. If the vents are open, the hot air

rises and escapes through the upper vent in the drying chamber while cooler air at ambient

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temperature enters through the lower vent in the collector. Therefore, an air current is

maintained, as cooler air at a has relative humidity ‘Ha’ and the out-going air at a temperature

‘Te’, has a relative humidity ‘He’. Because Te > Ta and the dryer contains no item, Ha > He.

Thus there is tendency for the out-going hot air to pick more moisture within the dryer as a result

of the difference between Ha and He. Therefore, insulation received is principally used in

increasing the affinity of the air in the dryer to pick moisture.

During periods of low or zero solar radiation, biomass stove is used for back-up heating. The

combustion gases heat up the stove wall surface, which in turn warm the air as it moves over the

outer surface. The warm air rises up into the drying chamber, evaporating and picking up

moisture from the product as it passes through the trays, and then escapes through the top vents

as before. Temperature inside the drier is controlled by regulation of entering and burning rate of

the biomass.

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FIG 3.2 THE BIOMASS STOVE (All dimensions in centimeters)

3.3 Reasons for alternating the arrangement of trays.

During drying, it is necessary to remove free moisture from the surface and also from the

interior of the material. When hot air is blown over the grain, heat is transferred to its surface and

the latent heat of vaporization causes water to evaporate. Water vapour diffuses through a

boundary film of air. This creates a region of lower vapour pressure at the surface of the grain

and a water vapour gradient is established from the most interior part of the grain to the dry air.

The gradient provides the driving force for removal of water from the food.

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Water moves to the surface by the following mechanisms:

Liquid movement by capillary force

Diffusion of liquids caused by difference in the concentration of solutes in different regions of

the grain

Diffused liquids are absorbed in a layer at the surface of solid components of the grains

Drying takes place at the surface of the grain and is similar to evaporation of moisture from

free water surface. The rate of evaporation depends largely on the surroundings and only little on

the type of grain. The three characteristics of air that are necessary for successful drying in the

constant rate period are [3]:

A moderately high dry bulb temperature

A low relative humidity

A high air velocity

The trays in the heating chamber are so arrange to keep the temperature constant at the dry

bulb temperature value, to keep the humidity relatively low and make the air move conveniently

with high velocity.

31

Solar-Biomass Integrated Dryer.

32

Section of the Biomass Chamber of the Solar-Biomass integrated Dryer .

33

Section of the Solar Chamber of the Solar-Biomass Integrated Drier

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REFERENCES

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