1

CHAPTER 1

INTRODUCTION

Drying is one of the methods used to preserve food products for longer periods.

The heat from the sun coupled with the wind has been used to dry food for preservation

for several thousand years. 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. Solar air heaters are simple devices to heat

air by utilizing solar energy and it is employed in many applications requiring low to

moderate temperature below 80°C, such as crop drying and space heating. Drying is

the oldest preservation technique of agricultural products and it is an energy intensive

process. High prices and shortages of fossil fuels have increased the emphasis on

using alternative renewable energy resources. Drying of agricultural products using

renewable energy such as solar energy is environmental friendly and has less

environmental impact.

Different types of solar dryers have been designed, developed and tested in the

different regions of the tropics and subtropics. The major two categories of the dryers

are natural convection solar dryers and forced convection solar dryers. In the natural

convection solar dryers the airflow is established by buoyancy induced airflow while in

forced convection solar dryers the airflow is provided by using fan operated either by

electricity/solar module or fossil fuel. Now the solar dryer designed and developed for

and used in tropics and subtropics are discussed under two headings.

2

CHAPTER 2

LITERATURE REVIEW

2.1 STUDIES RELATED TO DESIGN AND CONSTRUCTION OF SOLAR DRYING

SYSTEM

Diemuodeke E.OGHENERUONA, Momoh O.L. YUSUF(2011)., Designed and

fabricated direct natural convection solar dryer to dry tapioca in rural areas. A minimum

of 7.56 m2 solar collector area is required to dry a batch of 100 kg tapioca in 20 hours

(two days drying period). The initial and final moisture content considered were 79 %

and 10 % wet basis, respectively. The average ambient conditions are 32ºC air

temperatures and 74 % relative humidity with daily global solar radiation incident on

horizontal surface of 13 MJ/m2/day. The weather conditions considered are of Warri

(lat. 5°30’, long. 5°41’), Nigeria.A prototype of dryer was fabricated with minimum

collector area of 1.08 m2 .

M.MOHANRAJ,P.CHANDRASEKAR (2009).,The performance of an indirect

forced convection solar drier integrated with heat storage material was designed,

fabricated and investigated for chili drying. The drier with heat storage material enables

to maintain consistent air temperature inside the drier. The inclusion of heat storage

material also increases the drying time by about 4 h per day. The chili was dried from

initial moisture content 72.8% to the final moisture content about 9.2% and 9.7% (wet

basis) in the bottom and top trays respectively. They concluded that, forced convection

solar drier is more suitable for producing high quality dried chilli for small holders.

Thermal efficiency of the solar drier was estimated to be about 21% with specific

moisture extraction rate of about 0.87 kg/kW h.

Bukola O. Bolaji and Ayoola P. Olalusi (2008)., Built a simple and inexpensive

mixed mode solar dry locally source materials. The temperature rise inside the drying

3

cabinet was up to 24o C (74%) for a hours immediately after 12.00h(noon). The drying

rate, collector efficiency and percentage of moist removed (dry basis) for drying yam

chips were 0.62 kgh-1, 57.5 and 85.4% respectively.

Bukola O. Bolaji.et.al (2011), Designed, constructed and tested the solar

windventilated cabinet dryer in Nigeria on latitude 7.5o N.Comparatively, drying with the

solar cabinet dryer showed better results than open air-drying. During the period of test,

the average air velocity through the solar dryer was 1.62 m/s and the average daylight

efficiency of the system was 46.7%. The maximum drying air temperatures was found to

be 64oC inside the dryer. The average drying air temperature in the drying cabinet was

higher than the ambient temperature in the range of 5oC in the early hours of the day to

31oC at midday.80% and 55% weight losses were obtained in the drying of pepper and

yam chips, respectively, in the dryer.

Ahmed Abed Gatea (2009), Designed and developed solar drying system for

maize with V-groove collector of 2.04 m² area, drying chamber and blower. The thermal

energy and heat losses from solar collector were calculated for each three tilt angles

(30º,45º, 60º). The results obtained during the test period denoted that the maximum

gained energy occurred at 11 o'clock hour and then gradually declined since the

maximum solar radiation occurred at this time. Other many important results found are

The theoretical thermal energy, the experimentally actual heats gain increase by

increasing radiation intensity, the maximum values occurred at the 11 am and then

gradually declined. The energy gained obtained at the angle tilt 45º is higher than the

corresponding values obtained at 60º, 30º tilt.

F.K.Forson.et.al (2007), Designed A mixed-mode natural convection solar crop

dryer (MNCSCD) for dryingcassava and other crops. A batch of cassava 160 kg by

mass, having an initial moisture content of 67% wet basis from which 100 kg of water is

required to be removed to have it dried to a desired moisture content of 17% wet basis,

is used as the drying load in designing the dryer. A drying time of 30–36 h is assumed

4

for the anticipated test location (Kumasi; 6.71N,1.61W) with an expected average solar

irradiance of 400W/m2 and ambient conditions of 25 1C and 77.8% relative

humidity.They concluded that A minimum of 42.4m2 of solar collection area, according

to the design, is required for an expected drying efficiency of 12.5%. Under average

ambient conditions of 28.2 1C and 72.1% relative humidity with solar irradiance of

340.4W/m2, a drying time of 35.5 h was realised and the drying efficiency was evaluated

as 12.3% when tested under full designed load signifying that the design procedure

proposed is sufficiently.

EL- Amin Omda Mohamed Akoy.et.al (2009), A natural convection solar

dryer(Cabinet Type) was designed and constructed to dry mango slices. They

concluded that the designed dryer with a collector area of 16.8m2 is expected to dry

195.2kg fresh mango (100kg of sliced mango) from 81.4% to 10% wet basis in two days

under ambient conditions during harvesting period from April to June. A prototype of the

dryer is designed and constructed that has a maximum collector area of 1.03m2.

M.A. Hossaina and B.K. Bala (2006), Designed and developed A Mixed mode

type forced convection solar tunnel drier to dry hot red and green chillies under the

tropical weather conditions of Bangladesh. Moisture content of red chilli was reduced

from 2.85to 0.05 kg/kg(db) in 20 h in solar tunnel drier and it took 32 h to reduce the

moisture content to 0.09 and 0.40 kg/kg (db) in improved and conventional sun drying

methods, respectively.

J. Banout et.al (2010), Doubled Pass Solar Dryer (DPSD) was designed for

drying red chilli in central Vietnam and DPSD is compared with cabinet dryer (CD) and

traditional open sun drying. They found that average drying temperatures were 60ºC,

52ºC and 35.8ºC and corresponding relative humidity 34%, 45% and 62% for DPSD,

CD and open air sun drying, respectively. The overall drying efficiency of DPSD is 20%

which is typical for forced convection solar dryer. The moisture content of fresh red chilli

was almost similar during all drying tests where as the initial values were

5

9.18kg/kg,9.17kg/kg and 9.30kg/kg (db) for DPSD, CD and open-air sun drying,

respectively. Where the final moisture content in case of DPSD 0.05kg/kg was reached

after 23 h, 0.09kg/kg after 29h for CD and 0.18kg/kg after 36 h in case of open sun

drying (excluding nights).The performances of a new designed DPSD have been

compared with those of a typical CD and a traditional open-air sun drying for drying of

red chilli. The DPSD resulted in the shortest drying time to meet desired moisture

content of chilli (10% w.b.), which corresponds to the highest drying rate comparing to

other methods. Although the construction cost of DPSD was higher than CD the overall

drying efficiency was more than two times higher in case of DPSD compared to CD.

Hence, Double pass solar drier was found to be technically and economically suitable

for drying of red chillies under the specific conditions in central Vietnam.

6

CHAPTER 3

PROBLEM DEFINITION

3.1 PROBLEM STATEMENT

Food scientists have found that by reducing the moisture content of food to

between 10 and 20%, bacteria, yeast, mold and enzymes are prevented from spoiling it.

The flavor and most of the nutritional value is preserved and concentrated. Wherever

possible, it is traditional to harvest most grain crops during a dry period or season and

simple drying methods such as sun drying are adequate. However, maturity of the crop

does not always coincide with a suitably dry period. Furthermore, the introduction of

high-yielding varieties, irrigation, and improved farming practices have led to the need

for alternative drying practices to cope with the increased production and grain

harvested during the wet season as a result of multi-cropping.

3.1.1 Space Problems

Drying and preservation of agricultural products have been one of the oldest

uses of solar energy. The traditional method, still widely used throughout the world, is

open sun drying where diverse crops, such as fruits, vegetables, cereals, grains,

tobacco, etc. are spread on the ground and turned regularly until sufficiently dried so

that they can be stored safely. However, there exist many problems associated with

open sun drying. It has been seen that open sun drying has the following

disadvantages. It requires both large amount of space and long drying time.

3.1.2 Weather Problems

The crop is damaged because of the hostile weather conditions; contamination

of crops from the foreign materials, degradation by overheating, and the crop is subject

to insect infestation, the crop is susceptible to reabsorption of moisture if it is left on the

ground during periods of no sun, and there is no control on the drying process. This

7

could lead to slow drying rate, contamination and poor quality of dried products, and

loss in production.

Although the spreading of the crop on the ground or on a platform and drying it

directly by the sun is cheap and successfully employed for many products throughout

the world, where solar radiation and climatic conditions are favorable, because of the

above mentioned factors of open sun drying process and a better understanding of the

method of utilizing solar energy to advantage, have given rise to a scientific method

called solar drying. Solar drying of farm crops offers the following advantages by

permitting: early harvest which reduces the field loss of products from storm and natural

shattering.

3.1.3 Technology Problems

The disadvantages of open sun drying need an appropriate technology that can

help in improving the quality of the dried products and in reducing the wastage. This led

to the application of various types of drying devices like solar dryer, electric dryers,

wood fuel driers and oil-burned driers. However, the high cost of oil and electricity and

their scarcity in the rural areas of most third world countries have made some of these

driers very unattractive. Therefore interest has been focused mainly on the development

of solar driers .Solar dryers are usually classified according to the mode of air flow into

natural convection and forced convection dryers. Natural convection dryers do not

require a fan to pump the air through the dryer. The low air flow rate and the long drying

time, however, result in low drying capacity. Thus, this system is restricted to the

processing of small quantities of agricultural surplus for family consumption. Where

large quantities of fresh produce are to be processed for the commercial market, forced

convection dryers should be used.

3.1.4 Power Problems

One basic disadvantage of forced convection dryers lies in their requirement of

electrical power to run the fan. Since the rural or remote areas of many developing

8

countries are not connected to the national electric grids, the use of these dryers is

limited to electrified urban areas. Even in the urban areas with grid-connected

electricity, the service is unreliable. In view of the prevailing economic difficulties in most

of these countries, this situation is not expected to change in the foreseen able future.

The use of natural convection solar dryer could boost the dissemination of solar dryers

in the developing countries. Therefore, experimental performance of solar dryer has

been evaluated in this project.

3.2 PROBLEM STATEMENT OBJECTIVES

The objective of this study is to develop a mixed-mode solar drying system in

which the vegetables are dried simultaneously by both direct radiation through the

transparent walls and roof of the cabinet and by the heated air from the solar collector.

The problems of low and medium scale processor could be alleviated, if the solar dryer

is designed and constructed with the consideration of overcoming the limitations of

direct and indirect type of solar dryer. So therefore, this work will be based on the

importance of a mixed mode solar dryer which is reliable and economically, design and

construct a mixed mode solar dryer using locally available materials and to evaluate the

performance of this solar dryer.

3.3 PROBLEM CONSTRAINS

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. The purpose of this project is to present the developments and

potentials of solar drying technologies for drying grains, fruits, vegetables, spices,

medicinal plants. The traditional method of drying, known as sun drying, involves simply

laying the product in the sun on mats, roofs or drying floors. Major disadvantage of this

method is contamination of the products by dust, birds and insects – Some percentage

will usually be lost or damaged, it is labour intensive, nutrients loss, such as vitamin A

and the method totally depends on good weather conditions.Because the energy

9

requirements - sun and wind - are readily available in the ambient environment, little

capital is required. This type of drying is frequently the only commercially used and

viable methods in which to dry agricultural products in developing countries. The safer

alternative to open sun drying is solar dryer.This is a more efficient method of drying

that produces better quality products, but it also requires initial investments. If drying

conditions such as weather and food supply are good, natural circulation solar energy,

solar dryers appear to be increasingly attractive as commercial proposition.

10

CHAPTER 4

THEORETICAL BACKGROUND

4.1 DESIGN SPECIFICATION AND ASSUMPTION

Solar drying may be classified into direct and indirect solar dryer. In direct solar

dryers the air heater contains the grains and solar energy which 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. However, 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

mixedmode type of dryer, the heated air from a separate solar collector is passed

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

directly through the transparent walls or the roof. The objective of this study is to design

a mixed-mode solar dryer in which the grains are dried simultaneously by both direct

radiation through the transparent walls and roof of the cabinet and by the heated air

from the solar collector. The materials used for the construction of the mixed-mode solar

dryer are cheap and easily obtainable in the local market.

4.1.1 Solar Dryer Components

The solar dryer consists of the solar collector (air heater), the drying cabinet and

drying trays

1.Collector(Air Heater)

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 W/mK . 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

11

provides effective air heating because solar radiation that passes through the

transparent cover is then absorbed by both the mesh and back-plate. The glazing is a

single layer of 4 mm 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 rage 0.2 – 2.0 μm and

opaque to wave lengths greater than 4.5 μm. One end of the solar collector has an air

inlet vent of area 0.0888 m2, which is covered by a galvanized wire mesh to prevent

entrance of rodents, the other end opens to the plenum chamber.

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 13 mm 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.

3.Drying Trays

The drying trays are 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.

4.1.2 The Orientation of 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 used. 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

45 to the horizontal. This inclination is also to allow easy run off of water and enhance

air circulation.

12

CHAPTER 5

DESIGN PROCEDURE AND IMPLEMENTATION

5.1 DESIGN PROCEDURE

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. 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. Solar air heaters are simple devices to heat

air by utilizing solar energy and employed in many applications requiring low to

moderate temperature below 80O C, such as crop drying and space heating. 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. According to 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 significant percentage of these losses are

related to improper and/or untimely drying of foodstuffs such as cereal grains, pulses,

tubers, meat, fish, etc.

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:

13

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 .

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. 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.

5.1.1 The Experimental Setup

The mixed-mode solar dryer with box-type absorber collector was constructed

using the materials that are easily obtainable from the local market. Figure 5.1 shows

the sectional view of mixed mode dryer. The dryer has four main features namely: the

box-type absorber solar air collector, the drying chamber and the drying rack.

Figure 5.1 Sectional view of mixed mode dryer

14

5.1.2 Collector

The heat absorber (inner box) of the solar air heater was constructed using 1

mm thick galvanized plate, painted black, the surface facing sunlight was painted with

black paint containing (5%)black chromium powder to increase its absorbing capability.

Figure 5.2 shows the typical solar collector for air heating. The solar collector was

insulated with rock wool of about 5 cm thickness.

Figure 5.2 Typical solar collector for air heating

The solar collector assembly consists of air flow channel enclosed by transparent cover

(glazing). The glazing is a single layer of 4 mm thick transparent glass sheet. It has a

surface area of 0.82 by 1.20 cm and of transmittance above 0.86.

5.1.3 Drying Cabinet

The designing of the drying chamber depends on many factors such as the

product to be dried, the required temperature and velocity of the air to dry food material,

the quantity of the dried product and the relative humidity of the air passing over the

food material. The drying chamber houses four drying racks, between a tray and

another tray is 10 cm. Four trays of dimension (0.75 x 0.30 x 0.08 m) were fabricated

and stacked uniformly/evenly at distances (0.02 m) apart, for placing of material to be

15

dried. The tray was made from an aluminum wire mesh (0.003 x 0.003 m in size)

attached to it. Metal handles (0.076 m) were attached on each tray for ease of handling

and sliding the trays inside the chamber through the produce to be dried. The drying

chamber was also lined with foam insulation material 5 cm thick to prevent loss of heat.

5.1.4 Drying Mechanism

In the process of drying, heat is necessary to evaporate moisture from the

material and a flow of air helps in carrying away the evaporated moisture. There are two

basic mechanisms involved in the drying process:

1. The migration of moisture from the interior of an individual material to the

surface.The evaporation of moisture from the surface to the surrounding air.

2. The drying of a product is a complex heat and mass transfer process which

depends on external variables such as temperature, humidity and velocity of the

air stream and internal variables which depend on parameters like surface

characteristics (rough or smooth surface), chemical composition (sugars,

starches, etc.), physical structure (porosity, density, etc.), and size and shape of

product.

5.2 DESIGN IMPLEMENTATION

A transparent layer is over top side of the collector which allows the solar

radiation inside the collector. The other sides of the collector are insulated from

atmosphere to decrease heat loss. A heat absorber has been placed inside the collector

with the projections on it to increase the intensity of radiation. A air blower has been

placed at the inlet of collector to increase heat transfer to the air from the absorber

plate. Two thermometers have been positioned to measure the air temperature at the

inlet and outlet portion of the air heater. Ambient temperature was also recorded during

the course of experiments with the help of digital sensor.

16

CHAPTER 6

FEASIBIILITY STUDIES AND MARKET NEEDS

6.1 FEASIBILITY STUDY FOR FOOD SOLAR DRYER SYSTEM

Cost Economics, of Food Solar dryer System enterprises are worked out for fruits

and vegetables. 1 Million For one unit of 10 dryers. It can transact 10 tons of fruits or

fruit bars in dehydrated form. This is an excellent income and profitable venture in rural

Saudi Arabia. The cost benefit analysis of our dryers indicates that a commercial

venture of a project with 10 solar dryers will give the payback period of 2 - 2½ years.The

profitability of the technology in terms of employment potential and income generation is

established and acceptability of the product in the market is evaluated from the proven

market demand. Our expectation about the feasibility of the technology for rural

employment has been realized.

The reasons for the success are

1.The grass root level Non Government and voluntary organizations have

devotion for service to rural people and have the ability to capacity building and skill

development among rural women.

2. ood Solar drying process is the integration of food science and technology and

solar drying technology disciplines. So the practice followed in solar food processing is

based on these two techniques. To make the solar food processing products, one

needs rigorous training in this technology by well qualified persons, close monitoring

and supervision of the operations and following the food safety, clean & hygienic

practices, quality consciousness and assurance in day to day production. The social

entrepreneurs have proved very successful in this respect .

17

CHAPTER 7

CONCLUSION

This project aims to present the design, construction and performance evaluation

of a solar drying system for food preservation. A prototype of the solar drying system is

to be designed and its performance is to be evaluated for different air velocities

The solar dryer can raise the ambient air temperature to a considerable high

value for increasing the drying rate of vegetables. The product inside the dryer requires

less attentions, like attack of the product by rain or pest, compared with those in the

open sun drying. Although the dryer was used to dry vegetables, it can be used to dry

other crops like yams, cassava, maize and plantain etc. There is ease in monitoring

when compared to the natural sun drying technique. The capital cost involved in the

construction of a solar dryer is much lower to that of a mechanical dryer.

18

CHAPTER 8

REFERENCES

1. Diemuodeke E. Ogheneruona, Momoh O.L. Yusuf., "Design and Fabrication of a Direct Natural Convection Solar Dryer for Tapioca",Leonardo Electronic Journal of Practices and Technologies, Issue 18, (2011) p. 95-104.

2. M.Mohanraj, P.Chandrasekar., "Performance of a Forced Convection Solar Drier Integrated With Gravel As Heat Storage Material For Chili Drying", Journal of Engineering Science and Technology, Vol. 4, (2009) p. 305 – 314.

3. Bukola O. Bolaji and Ayoola P. Olalusi.,"Performance Evaluation of a Mixed-Mode Solar Dryer",Renewable Energy, (2008) p. 225-231.

4. Bukola O. Bolaji , Tajudeen M.A. Olayanju and Taiwo O. Falade., "Performance Evaluation of a Solar Wind Ventilated Cabinet Dryer", The West Indian Journal of Engineering, (2011) p.12-18.

5. Ahmed Abed Gatea., "Design, construction and performance evaluation of solar maize dryer", Journal of Agricultural Biotechnology and Sustainable Development,(2010) p. 039-046.

6. F.K. Forson,M.A.A. Nazha, F.O. Akuffo,H.Rajakaruna., "Design of mixedmode natural convection solar crop dryers", Renewable Energy (2007) p. 2306–2319.

7. M.A. Hossaina and B.K. Bala.,"Drying of hot chilli using solar tunnel drier" Bangladesh Agricultural University, (2006) p. 23- 31.

8. J. Banout, P. Ehl, J. Havlik, B. Lojka, Z. Polesny, V. Verner., "Design and performance evaluation of a Doublepasssolar drier for drying of red chilli", (2010) p. 835– 844.

9. Ahmed Abed Gatea. "Design and construction of a solar drying system, a cylindrical section and analysis of the performance of the thermal drying system", University of Baghdad, (2010) p. 201- 211.