SUBMITED BY NARENDRA.R.JADHAV

T.Y.B.Sc [PHYSICS] 2009-2010

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COLLEGE

ROLL NO: 2706

EXAM

SEAT NO:___________

“Education through self – help is our Motto” –

KARMAVEER

RAYAT SHIKSHAN SANSTHA`S

Mahatma Phule Mahavidyalya,

Pimpri, Waghere, Pune -17.

CERTIFICATE

Department of Physics

This is to certify that Mr.Narendra.Ramesh.Jadhav has satisfactorily carried out

the Project named “ SOLAR DRYER ” according to syllabus of PUNE UNIVERSITY

prescribed for the T.Y.B.Sc. Course in the year 2009-2010.

Date:

Teacher Head Examiner

In Charge (Department of physics)

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ACKNOWLEDGEMENT

With the deep sense of regard and being thankful for, I take this opportunity of

expressing my gratitude and sincere thanks Dr. R.S. Nimbalkar (H.O.D. of Physics) for

encouraging me and providing well guidance and supervision during the entire period of

my project work.

Most respectively I am thankful to respected professor’s staff Prof.

Dr.R.S.Nimbalkar, Prof. M.A.Patil & Miss Ashwini Narkhede for their co-operation

and excellent guidance during the entire period of my project.

I am special thankful to them as they have given their valuable time for my

project. My special thanks to Miss.Nirmala Pawar and our college lab assistance of

physics department Mr.Dasia (kaka) for their kind support & co-operation during course

of my project.

(NARENDRA.R.JADHAV)

T.Y.B.Sc

(PHYSICS)

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INDEX

CONTENTS PAGE NO.

INTRODUCTION TO SOLAR ENERGY 05

TYPES OF SOLAR ENERGY 07

SOLAR RADIATIONS 08

ADVANTAGES & DISADVANTAGES OF SOLAR

ENERGY

11

APPLICATION OF SOLAR ENERGY 12

BASIC THEORY OF SOLAR DRYER 13

NEED OF SOLAR DRYER 13

MOISTURE CONTENTS 14

TYPES OF SOLAR DRYER 19

CABINET SOLAR DRYER 21

COMPONENTS USED IN SOLAR DRYER 22

PRINCIPLE & CONSTRUCTION OF SOLAR DRYER 23

WORKING OF SOLAR DRYER 23

ADVANTAGE & DISADVANTAGES OF CABINET

SOLAR DRYER

24

LIMITATIONS OF CABNET OF SOLAR DRYER 25

THERMAL ANALYSIS OF CABNET SOLAR DRYER 26

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REFERENCES BOOKS 27

INTRODUCTION TO SOLAR ENERGY

All forms of energy on the earth are derived from sun. however, the more

conventional forms of energy, the fossil fuel receives their solar energy and posses the

energy in greatly concentrated form. These highly concentrated solar energy sources are

being used as such at a rapidly rate they will be depleted in not too distant future.

There are four primary sources of energy-petroleum, natural gas, coal and wood.

Excepting wood, all this common sources have finite supplies. The life time is estimated

to range from 15 years for natural gas to nearly 300 years for coal. Therefore, as this non-

renewable sources are consumed, the mankind must turn its attention to longer term,

permanent type of solar energy sources. The two most significant such sources are

nuclear and solar energy.

Out of these two energies, solar energy is good option because solar energy is

depended energy source without any requirement, no polluting effect from its use and

easily available with free cost.

Modern scientific research in the utilization of solar energy commenced in 1885

when C.Gunter, an Austrian invented a solar boiler using mirrors. In 1876, an American

inventor, John Ericsoon who invented several types of hot air engines. The waning solar

energy research was received in 1940 when God fray Cabot left a large sum of money for

research project at the Massachusetts Institute of Technology. The near feature will

certainly show some major break through in solar energy technology.

In addition to the thousand of ways in which both nature and man through out the

time to grow food, to see by, to dry cloths, have used the sun energy. Solar energy is used

to heat & cool the building, to heat water and swimming pools, to power refrigeration,

and to operate engines, pumps and sewage treatment plants. Wind is used to generate

electrically and mechanical power and solar converted electricity is used both on earth

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and in space. Stoves and cars run on solar made methane gas, power plants operate on

organic trash and sewage plants produce methane gas.

NEED OF SOLAR ENERGY

Need of these uses however, can be comprehended without a knowledge of the

basic principle of solar energy. Most of the energy we receive from sun comes in the

form of light, a short wave radiation, not all of which is visible to human eye. When this

radiation strikes a solid or liquid, it is observed and transformed into heat energy ; the

material becomes warm and store heat ; conducts it to surrounding material or Re-

radiated is large wave radiation.

Man has needed and used energy at an increasing rate since he came on the earth

a few million years ago. At the beginning man required energy preliminary in the food.

He derived this by plants or animal which he hunted. Then he discovered fire and energy

needs increased the energy needs for cooking as well as for keeping himself warm. With

the passage of time. Man started to cultivate land for agriculture. With the further demand

for energy man being to use the wind for sailing shills and for driving wind mills and the

force of falling water to turn water wheels. Till this time, it would not be wrong to say

that sun was supplying all the energy needs of man either directly or indirectly and that

man was using only renewable sources of energy.

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TYPES OF SOLAR ENERGY

The energy sources can be divide into two types :-

( 1 ) Conventional sources of energy :-

( 2 ) Non- Conventional sources of energy :-

( 1 ) CONVENTIONAL SOURCES OF ENERGY :-

Conventional sources are those which we have may using since past. The

conventional sources generally include wood , coal , petrol and gas. But due to use of this

sources lot of carbon monoxide and carbon dioxide gases are evolved which is major

cause of air pollution.

The production of oil have appeared to have touched maximum around 1980 and

is now slowly declining on the other hand the production of natural gas is increasing

present indication that most of the reserves will fall on coal it is likely that production of

coal will touch a maximum between the year 2030 and 2060 eventually entire storage of

coal will be completely exhausted.

( 2 ) NON CONVENTIONAL SOURCES OF ENERGY : -

This type of energy sources are very important in today’s life due to the above

reason they generally include solar energy , wind and tidal energy the above sources of

energy of some of the internal sources of energy and are called non-conventional sources

of energy. Out of these three solar energy would be the best alternative because –

1. Wind does not flow continuously

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2. The waves are not constant

SOLAR RADIATION

All substances , solids , liquids and gases at temperature above absolute zero, emit

energy in the form of electromagnetic waves, this energy is called “ Radiation ”. The

radiation which are important to us, is that from the sun, the earth and the atmosphere

lying within the ultraviolet , visible and infrared spectral region.

The earth and solar constant

SUN EARTH

32° Solar constant Isc = 1367 w/m2

Distance = 1.495 × 107 ± 1.7%

SUN-EARTH RELATIONSHIP

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D =1.36 × 109m

d =1.36×109

m

The earth is shaped as a spheroid. The earth takes rotation about its axis every 24 hours and completes a revolution about the sun in a period of approximately 365.25 days.The earth revolves an elliptical orbit round the sun, with the later at one of the foci of the ellipse. The apparent path of the sun from the earth is known as the elliptic.

The distance between the sun and the earth varies by 1.7% at a distance of one astronomical unit, 1.495 × 1011 m , the mean earth sun distance, the sun sustain an angle of 32° on earth.

The solar constant ( Isc ) is defined as the total energy received from the sun per unit time, on a surface of unit area kept perpendicular to the radiation in space, just outside the earth’s atmospheric when the earth is at its mean distance from the sun.

SOLAR RADIATION OUTSIDE THE EARTH’S ATMOSPHERE

The characteristic of the sun’s energy available outside the earth’s atmosphere are first consideration.

The sun is large sphere of very hot gases , the heat being generated by various kinds of fusion reactions. Its diameter is 1.36 ×109 while that of the earth is 1.27 × 107 m. the mean distance between the two is 1.4496 × 1011 mm . Although the sun is large ,it subtends an angle of the 32° at a very large distance. Thus , the beam radiation received from the sun on the earth is almost parallel. The brightness of the sun varies from its center to its edge. The measurements indicate that the energy flux received from the sun outside the earth’s atmosphere is essentially constant.

SOLAR RADIATION AT EARTH’S SURFACE

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Solar radiation scattered radiation

Absorption upper limit of atmosphere Scattering Earth atmosphere Diffused radiation ( scattered radiation Earth surface reaching earth surface )

BEAM AND DIFFUSE SOLAR RADIATION

Solar radiation receives at the earth’s surface without change of direction i.e. in the line with the sun is called beam or direction radiation. Solar radiation has been changed by reflection and scattering by the atmosphere is called as diffuse radiation.

Solar radiation received at earth’s surface which depends on absorption and scattering as it passes through the earth’s atmosphere. Absorption occurs primarily because of presence of ozone and water vapour in the atmosphere and particular matter. On the other hand scattering occur due to all gaseous molecules and as well as particulate matter in the atmosphere. The scattered radiation is redistributed in all directions, some going back into space and some reaching the earth’s surface.

The intensity of the diffused radiation seen by an observer on a clear day is not isotropic but varies as function of latitude ,time of the year, time of the day, atmosphere contents and other factor.

ADVANTAGES AND DIS-ADVANTAGES OF SOLAR ENERGY

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ADVANTAGES:

( 1 ) Solar energy is hygienic , clean and unpolluted source of energy.

( 2 ) By solar energy consuming environment can be protected by avoiding fossil fuel burning like CO2, CO, SO2.

( 3 ) Solar energy is absolutely free of coast.

( 4 ) Solar energy is easily available and almost in all parts of the world.

( 5 ) Solar energy is available throughout the year.

DISADVANTAGES:

( 1 ) Solar energy is very dilute source of energy.

( 2 ) Solar energy is not available during night time.

( 3 ) It is useless in rainy season.

APPLICATIONS OF SOLAR ENERGY

The application of solar energy most success today are : -

1. Heating of building.

2. Cooling of building.

3. Solar drying of agriculture product.

4. Food refrigeration .

5. In direction source of solar energy conversion i.e. in the form of wind energy.

The heat from solar energy can be used for heating buildings. When the building needs heat, the air or water from its heating system passes through the storage is warm and is then fed through the conventional heater to another space. For sunless or cloudy days an auxiliary system as a back-up is always required. The heat from solar energy can be used to cool buildings , using the absorption cooling principle. A great deal of current research is being devoted to developing system requiring lower operating temperatures. A traditional and wide-special use of solar energy is for drying the agricultural products. These are special interest in the case of soft

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fruits ; these are attacked by insects as the sugar concentration increases during drying. Fruit dryer in which fruit is placed , in carefully designed rack to provide control exposure to solar radiation often improves product quality and saves considerable time. Large drying system like grain, paddy, maize like ginner, cash rapper etc. have been developed.

Solar refrigeration is intended for food preservation and deserves top priority in our country. Solar air-conditioning can be utilized for space-cooling. Solar refrigeration provides an effective solution in sub-tropical areas of our country. Solar refrigeration with an absorption system is a better way to direct utilization of energy.

Wind energy which is an indirect source of energy conversion can be utilize to run windmills, which in turn drives a generator power , such as for water pumping. In India, generally wind speeds obtainable are in the lower range. Therefore, on the development of low cost speed mills for providing water in rural areas. In India high wind speed are obtainable in coastal areas Saurashtra, Rajasthan and some parts of central India. In these areas, development of large size wind mills for generating electricity.

BASIC THEORY OF SOLAR DRYER

It has been estimated that the world’s population will become 7 billion by the year 2000. It is also estimated that about 600 to 900 millions people do not have enough food to eat now and this number is likely to increase with the increase in population. There are obviously two direct ways of solving the problem.

( 1 ) Increase food production by bringing mare area under cultivation, better irrigation and by using newer and mechanized methods of agricultures practices.

( 2 ) Reduce the food demand by reducing the population growth.

The third alternative which is equally important but not given adequate attention is reducing the loss of food during and after harvest. The actual estimate of food loss during post harvest period, due to spillage, contamination, attack by birds, rodents and insects and during storage is difficult to know due to technical and practical reason but it is so enormous that most conservative estimates were enough in 1975 to result in a resolution by the seventh special session of United Nation’s General Assembly to seek 50% reduction in post harvest food losses within a decade.

A very conservative estimate showed that a minimum of 17 million tones of food were lost in 1976 out of 750 million tones produced including durable and perishable. In developing countries where cold storage facilities are not adequately available, post harvest losses between 30-50 % in perishable are reported. This food loss can be reducing

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in developing countries if in rural areas and efficient policy and administrative infrastructure. The post harvest losses very considerable and depend on crop variety, climate, the system of harvesting, processing, storage, marketing and the social and cultural setting.

The developing countries produce about 15% of the world crop. Many commercial crop like tea, coffee, coca, tobacco, nuts etc. are grown only in developing countries. Crops like Wheat, paddy, potatoes, barley etc. contribute about 40% of the world’s harvest about 36% of the vegetables and 50% of the food of the world are produced in developing countries. Fish is another food product whose estimate is not available but it is produced in large quantities in developing countries even if the 50 % of the post harvest food losses are reduced then many developing countries will be self-sufficient in food.

MANUFACTURING OF SOLAR DRYER

In manufacturing there are many countries like Australia, China, Germany, USA, India etc. are some countries where we find solar dryer. In these Australia is now independent company, but still conducts all its research and manufacture in conjunction with Rose, gum, timbers at their east. Solar dryer are used in different product drying purpose,

1. China dryer – Chinese professional manufacturers of dryer.2. Solar dryer called “Hohenheimer tunnel dryer” developed by German company.3. In India mostly found in Bangalore.

MOISTURE CONTENTS AND MEASUREMENTS

The moisture contents of a substance is expressed in percentage by weight on the wet basis. But the moisture content on dry basis is more simple to use in calculation as the quantity of moisture present at any time is directly proportional to the moisture contents m , percent wet basis is as follows,

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M = Wm × 100 Wm+ Wd

Where, Wm = weight of moisture.Wd = weight of dry material.

The moisture constant M, dry basis percent is ,

M = Wm × 100 = m × 100Wd 100 – m

It can also be seen that,

Wm = m1 – m2 = M1 – M2 W1 100 – m2 100 + M1

Wm = m1 – m2 = M1 – M2 W2 100 – m1 100 + M2

Where,W1 = initial wt. of the dry material = ( Wm + Wd ) kgWm = wt. of the evaporated moisture W2 = final wt. of the dried material M1,m2 = initial and final moisture components, dry basis.

The moisture measurement can be done by ,( a ) Direct method :-( b ) Indirect method :-

The direct method include the air-oven drying method. The indirect method in which the electrical resistance of a measured amount of grain sample at a given compaction and temperature is measured. The electrical resistance varies with moisture contents, temperature and degree of compaction and temperature is measured. The electrical resistance varies with moisture contents, temperature and degree of compaction by properly calibrating the instrument, the moisture contents can be measured. The universal moisture contents of the grain directly.

When a solid is exposed to a continuous supply of air at constant temperature and humidity, having a fixed partial pressure of the vapour, the solid will either lose moisture by evaporation or grain moisture from the air until the vapour pressure of the moisture of the solid equal P, the solid and the gas are then in equilibrium with the surrounding condition is known as equilibrium moisture content EMC. The EMC is useful to determine whether the product will gain or lose moisture under a given set of temperature and relative humidity condition. Thus humidity is directly related to drying and storage different material have different equilibrium moisture contents. The EMC is dependant upon the temperature and the relative humidity of the environment and on the variety & maturity of the grain.

The EMC is determine by two method (1) The static method and (2) The dynamic method. In static method, the grain is allowed to come to equilibrium with the

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surrounding still air without any agitation whereas in dynamic method the air is generally forced. The EMC is to be determined under constant relative humidity and temperature condition of air.

TYPES OF THE MOISTURE

Bound moisture :

This refers to the moisture contained by a substance which exerts equilibrium vapour present equal to that of the pure liquid at the same temperature.

Free moisture :

Free moisture is the moisture contained by a substance in excess of equilibrium moisture M-M0 only moisture can be evaporated.

Equilibrium moisture level 1.0

Relative Unbound Humidity moisture of air Bound moisture RH

Free moisture

equm moisture m0 m

MOISTURE IN THE DRYING MATERIAL

The above relations are shown in fig.( 1 ) for solid of the moisture content M exposed to air of relative humidity RH.

A typical drying curve is shown in fig.( 3 ) clearly shows that there are two major periods of drying, namely the constant rate period and the falling rate period.

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Moisture Content M equm moisture level 0 time t RATE OF MOISTURE LOSS

Constant rate drying phase First falling rate

Drying rate dM/dt second falling rate equm moisture level 0 time t

The plots of the moisture contents versus drying time or drying rate versus drying time or drying rate versus moisture contents are known as drying curves. Fig.(2), Fig.(3)

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Constant rate

Falling rate Critical point

Drying rate dM/dt Non hygroscopic material

hygroscopic material

Ms Moisture contents

A cure is plotted between drying rate dM / dt versus moisture content M as shown in the fig.(4). As seen from this figure for both hygroscopic and non-hygroscopic material, there is a constant drying rate terminating at the critical moisture contents followed by falling drying rate. The constant drying rate for both non-hygroscopic and hygroscopic material is the same while the period of falling rate is little different. For non-hygroscopic material in the period of falling rate, the drying rate goes on the decreasing till the moisture contents becomes zero. In the hygroscopic materials, the periods of falling rate is similar unit the unbound moisture is completely removed, then it further decreases and some bound moisture is removed, this is continuous till the vapour pressure of material becomes equal to the vapour pressure of the drying air. When this equilibrium reaches in the drying rate becomes zero.

The constant drying rate is referred as adiabatic drying since the total heat content of the air remains constant and depends on external factor such as air flow rate, thermodynamic state and transport properties of the air, and the state of aggregation of material. Under dynamic equilibrium conditions, rate of the water evaporated is equal to the rate of heat transfer to the surface.

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Constant rate period :

Some crops including cereal grains at high moisture constant are dried under a constant rate period at the initial period of drying. Falling rate periods follows subsequently. When is dried under constant rate period. When its moisture contents exceeds 72 % . In the constant rate period the rate of evaporation under any given set of air conditions is independent of the solid and is essentially same as the rate of evaporation from a free liquid surface under the same condition.

Falling rate period :

Cereal grains are usually dried entirely under falling rate period. The falling rate period enters after the constant drying rate period and corresponds to the drying cycle where all surface is no longer wetted and the wetted surface continuingly decreases, until at the end of this period the surface is dried. The cause of falling off in the rate of drying is due to the inability of the moisture to be conveyed from the center of the body to the surface at the rate comparable with the moisture evaporation from its surface to the surrounding.

The falling rate period is characterized by increasing temperature both at the surface and within the solid. Furthermore, changes in air velocity have a much smaller effect than during the constant rate period. The falling rate period of drying is controlled largely by the product and is dependent on the movement of the moisture within the material from the center to the surface by liquid diffusion and the removable of the moisture from the surface of the product.

The falling rate period of drying often can be divided into two stages,

( 1 ) Unsaturated surface drying.( 2 ) Drying where the rate of the water diffusion within the product is show and is the controlling factor.

Practically all central grains are dried under falling rate period if the moisture content is not very high.

The liquid movement may be due to,(a) Moisture concentration difference.(b) Surfaces forces.(c) Moisture diffusion in pores.(d) Differences in the vapour pressure.(e) Differences in the temperature.(f) Differences in the total pressure.

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TYPES OF THE SOLAR DRYER

The traditional way of the drying agricultural products in developing countries is to spread the material in the thin layer on a platform in open sun. The natural drying does not involve money except some labour, but result in poor quality of products due to no control over drying rate, unhygienic condition and spoilage. Solar energy can be effectively employed for controlled drying resulting in good quality products. Actually controlled drying means controlling the drying parameters like air temperature humidity, drying rate, moisture contents and air flow rate. Therefore a solar dryer is to be designed carefully keeping all the above drying parameters in mind and the appropriateness of the dryer. Since there are many option in the design of the solar dryer, hence there are large variety of solar dryers. These solar dryers has been classified in many ways. But practicability of dryers, this can be classified basically into three types,

(1) Direct type or natural convention type dryer.(2) Mixed mode type dryer.(3) Forced circulation type dryer.

( 1 ) Natural convention solar dryer :

These dryers appears to be more attractive for use in developing countries since these do not use fan or blower to be operated by electrical energy. Moreover, they are low in cost and easy to operate. However the problems with these dryers are slowly drying, not much control on temperature and humidity, small quantity can be dried and some products change colour and flavour due to direct exposure to sun. in its simplest form, they consist of some kind of enclosure and a transparent cover. The food product gets heated evaporate and goes out by the natural circulation of the air. There are several design of direct type dryers and these are developed keeping in mind either the available of local material required for its fabrication or for drying a particular products. Several dryers are fabricated, tested and analyzed in many countries. A simple cabinet dryer is discussed here.

( 2 ) Mixed mode solar dryer :

In this the solar air heater without any fan along with the drying bin is used. The flow of air is generally by the natural circulation. This dryer named as “ rice dryer ” was developed by exell in Bangkok, Thailand and consist of a simple air heater, drying chamber and a tall chimney used to increase the convention effect. This dryer is designed to dry rice only but other food products or grain which requires slow and low temperature drying may also be dried. The drying rate will dependant on the depth of the bed, initial moisture content of the material, solar insulation, ambient temperature and the design of the dryer. It is experienced that if the material in the dryer remained un-touch then the material in the lower layers gets overheated and over-dried while at the top remained under-dried. Therefore stirring of material and drying in thin layer is recommended.e.g. Chimney type dryer.

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( 3 ) Forced circulation type dryer :

As the name implies, in these dryer some kind of blower is used for the circulation of air which is either operated electrically or mechanically. Such dryer are more efficient, faster and can be used for drying large quantities of agricultural products. These forced circulation type dryer are also categorized as direct mode, forced mode, circulation type solar dryer. The direct mode forced circulation dryer are similar to the indirect type natural circulation dryer except that here circulation of air is made by forced circulation and therefore are not very efficient and not preferred. While incident type or forced circulation dryer are very efficient can be used at low as well as high temperature and for drying large quantities of agricultural products. This dryer are of bin type, tunnel type, belt type, column type or rotary type. A forced circulation dryer which uses some kind of thermal storage unit, heat recovery wheel and auxiliary heating arrangement is described here. The storage is charged when the solar radiation is at the peak or when the drying is not required. Storage unit is put in series with the solar collector.

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CABINET SOLAR DRYER

Cabinet solar dryer are of two types,(1) Deep bed drying (2) Thin layer drying

( 1 ) Deep bed drying :

In this all the grains in the dryer are not fully exposed in the same condition of drying air. The condition of drying air at any point in the grain mass changes with the time and with the depth of the grain bed. Over and above the rate of the air flow per unit mass of the grain is small compared to the thin layered drying of the grain.

The drying of grain in the deep bed can be taken as the sum of the several thin layers. The humidity and temperature of air entering and leaving each layer vary with time depending upon the stage of drying, moisture removed from dry layer until the equilibrium moisture content is reached. Little moisture is removed rather a small amount may be added to the wet zone varies with the temperature and humidity of entering air, the moisture content of green and velocity of air movement.

Drying will be used as soon as the product comes in equilibrium with the air.

( 2 ) Thin layer drying :

Thin layer dryer refers to the grain drying process in which all the grains are fully exposed to the drying air under constant drying condition i.e. at constant air temp and humidity. Generally upto the 20 cm thickness of grain bed is taken as thin bed.

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DIAGRAMS OF THE CABINET SOLAR DREYER

Trays Insulation

AS BACK VIEW

holes AS TOP VIEW

Glass cover Ventilation holes

Trays

insulation pipes for bottom ventilation holes

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PRINCIPLE AND CONSTRUCTION OF SOLAR DRYER

PRINCIPLE :

Mostly cabinet solar dryer has main principle of controlled drying the food by using solar energy.

CONSTRUCTION :

For construction of cabinet solar dryer, we require plywood, transparent glass cover etc. It is 3-D diagrammatic model which contains length of 2.6 foot and breadth 1 foot while width 1 foot. The length is generally kept as 3 times of its width. Make holes on upper side of cabinet solar dryer and again on downward direction for ventilation purpose. From bottom place plywood at a same distance which is used for insulation purpose. For insulating, place wooden dust in it. Insert rubber pin in the bottom hole for ventilation. The diameter of hole must be 1 inch.

Make door on one side of cabinet solar dryer at last point. The cabinet and tray with black cabinet.

WORKING PRINCIPLE :

The working principle of these models mainly depend on method of solar energy collection and its conservation to useful thermal energy.

Solar cabinet dryer working :

The dryer which has attracted the attention of many scientist is the cabinet solar dryer which is also natural convection type. The solar cabinet of a wooden or of any material box certain width and length is generally kept three times its width insulation at its base and also preferable at the side and covered with the transparent roof. The detail of the cabinet dryer is given in the figure. The inside surface of the box are coated with the black point and the product to be dried is kept in the trays made of wires mesh bottom these made loaded trays are kept through on open able door provided on the rear side of the drier ventilation holes are made in the bottom through which fresh air is sucked automatically. Holes are also provided on the upper side of the dryer through which moist warm air escapes. When the food product is placed in the trays and exposed to solar radiation, the temperature of the product rises resulting in evaporation of moisture this warm moist air passes through upper ventilation holes by natural convection, creating a partial vacuum and drawing fresh air up through the holes provides in the base of the dryers. Temperature high as 90° C has been recorded in this type dryer when it is empty.Observation made on this dryer in many countries is quite encouraging and concluded that it can reduce the drying time from one half to one third compared to open sun drying. Recently its performance has been compared with other dryer by Sharma etal. And found that for drying small products cabinet dryer is better than other dryers.

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ADVANTAGES AND DISADVANTAGES OF CABINET SOLAR DRYER

ADVANTAGES :

(a) Facilities early harvest.(b) Permits planning the harvest season.(c) Helps in long beam storage.(d) Helps to better return.(e) Helps to better quality product.(f) Reduce the requirement of storage.(g) Helps in humbling transport of crops.(h) Permits maintaining viability of seeds.

DISADVANTAGES :

(a) It requires more space.(b) During night time it cannot be used.(c) It can not be used in winter season.(d) It is more expensive to plot in areas.

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A CABINET DRYER HAS THE FOLLOWING LIMITATIONS

(A) Due to its small capacity its use is limited to small scale application.

(B) Discolourization of crops due to direct exposure to solar radiation.(C) Moisture condensations inside glass cover reducing its transitivity.(D) Sometime insufficient rise in cop temperature affecting moisture

removable.

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THERMAL ANALYSIS OF CABINET DRYER

In solar to write the energy balance for different components of cabinet dryer, the following additional assumptions have been made,

(A) The heat capacity of glass, crop, trays, drying chamber wall air has been neglected.

(B) Volume shrinkage is negligible during the drying process.(C) Particle to particle conduction is negligible.(D) The heat flow is negligible.(E) There is no conduction of water vapour in drying chamber.(F) There is no satisfaction along the depth of the crop due to small

depth of crop.

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REFERENCE BOOKS

1. SOLAR ENERGY UTILIZATION : BY G.D.RAI

2. SOLAR ENERGY FUNDAMENTALS, DESIGN, MODELLING & APPLICATION : BY G.N.TIWARI

3. SOLAR ENERGY : BY H.P.GERGE

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