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.
(Project Term January-May 2015)
PROJECT ON SOLAR DRYER
SARVASINDHU MISHRA : 11107851
RISHABH YADAV : 11102399
ANUGRAH SOY : 11103846
MD. ISMIL : 11101103
PROJECT GROUP NUMBER……….
Under The Guidance Of
Mr. RAHUL WANDRA
LOVELY SCHOOL OF SCIENCE AND TECHNOLOGY
CAPSTONE PROJECT REPORT
We hereby declare that the project work entitled SOLAR CABINET DRYER is an authentic
record of our own work carried out as requirements of Capstone Project for the award of degree
of Bachelor Of Technology in MECHANICAL ENGINEERING from Lovely Professional
University, Phagwara, under the guidance of Mr.RAHUL WANDRA, during January to May,
PROJECT GROUP MEMBER:
This is to certify that the declaration statement made by this group of students is correct to the
best of my knowledge and belief. The Capstone Project Proposal based on the technology / tool
learnt is fit for the submission and partial fulfillment of the conditions for the award of Bachelor
Of Technology in MECHANICAL ENGINEERING from Lovely Professional University,
Signature of Faculty Mentor
We would like to express our sincere gratitude to the H.O.D “Mr. GurpreetPhool” of School of
Mechanical Engineering, Lovely Professional University Punjab for his help and support which
was vital in the completion of this report. We also want to express our sincere gratitude &
respect to the people at LOVELY PROFESSIONAL UNIVERSITY who always helped &
guided us in understanding various concepts, which were unknown to us. We are also thankful to
“RAHUL WANDRA” under whose visionary enlightenment we were able to complete this
TABLE OF CONTENTS
1.1 Literature Review…………………………………………………………….8
1.2 Problem Statement …………………………………………………………..11
1.3 Problem Statement Objectives ………………………………………………12
1.4 Problem Justification and Outcomes…………………………………………12
2. DESIGN APPROACH AND METHODOLOGY
2.1 Design Approach
2.1.1 Drying Mechanism…………………………………………………………13
2.1.2 Air Properties ……………………………………………………………..16
2.1.3 Classification Of Drying Systems ………………………………………….18
2.2 Design Methodology
2.2.1 Types Of Solar Dryers………………………………………………………19
(i) Open Sun Drying ………………………………………………………20
(ii) Direct type Solar Drying ………………………………………………21
(iii) Indirect Type Solar Drying …………………………………………...22
2.2 Applications Of Solar Dryers …………………………………….23
3. THEORETICAL BACKGROUND
Design Specifications And Assumptions
3.1.1 Introduction …………………………………………………………….25
3.1.2 Solar Dryer Components ……………………………………………….25
3.1.3 The Orientation Of Solar Collector ………………………………………26
3.1.4 Materials Required For Making Solar Dryer …………………………….27
3.2Mathematical Models And Formulations
3.2.1 Operation Of Dryer ………………………………………………………..28
3.2.2 Drying Mechanism……………………………………………………… 28
3.2.3 Basic Theory (Formulations)……………………………………………….29
4. Design Procedure And Implementations
4.1 Design Procedure ……………………………………………………………..30
4.2 The Experimental Set Up ……………………………………………………..31
4.3 Design Implementation ……………………………………………………….31
4.3.1 Object Of The Observation ……………………………………………….32
4.3.2 Graphical Representation Of Drying Rate ………………………………..34
4.3.3 Result And Discussions …………………………………………………...34
5. Feasibility Studies And Market Needs………………………...36
6. Conclusion And Recommendation……………………………....37
LIST OF FIGRUES
1) Figure 2.1 Moisture In The Drying Materials………………………………….14
2) Figure 2.2 Rate of moisture loss……………………………………………….15
3) Figure 2.3 Rate Vs Moisture Content………………………………………….15
4) Figure 2.4 Representation Of Drying Process…………………………………17
5) Figure 2.5 Open Sun Drying ………………………………………………….20
6) Figure 2.6 Direct Type Solar Drying………………………………………….21
7) Figure 2.7 Indirect Type Solar Drying ………………………………………..22
8) Figure 3.1 View Of Solar Cabinet Dryer……………………………………...27
9) Figure 3.2 Tray Arrangement Of Solar Dryer ………………………………..27
10) Figure 3.3 Front View Of Solar Dryer ………………………………………. 27
11) Figure 4.1 Solar Dryer Making ……………………………………………....31
12) Figure 4.2 Complete Setup Of Solar Dryer …………………………………..31
13) Figure 4.3 Chili Before Drying……………………………………………….33
14) Figure 4.4 Chili Inside Dryer …………………………………………………33
15) Figure 4.5 Chili After Drying ………………………………………………...34
16) Figure 4.6 Graphical Representation Of Drying Rate ………………………..34
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 years.
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.
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.
1.1 Literature Review
Crop drying is the most energy consuming process in all processes on the farm. The purpose of drying is
to remove moisture from the agricultural produce so that it can be processed safely and stored for
increased periods of time. Crops are also dried before storage or, during storage, by forced circulation of
air, to prevent spontaneous combustion by inhibiting fermentation. It is estimated that 20% of the world‘s
grain production is lost after harvest because of inefficient handling and poor implementation of post-
harvest technology, says Hartman‘s (1991). Grains and seeds are normally harvested at a moisture level
between 18% and 40% depending on the nature of crop. These must be dried to a level of 7% to 11%
depending on application and market need. Once a cereal crop is harvested, it may have to be stored for a
period of time before it can be marketed or used as feed. The length of time a cereal can be safely stored
will depend on the condition it was harvested and the type of storage facility being utilized. Grains stored
at low temperature and moisture contents can be kept in storage for longer period of time before its
quality will deteriorate. Some of the cereals which are normally stored include maize, rice, beans.
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
Energy is important for the existence and development of human kind and is a key issue in international
politics, the economy, military preparedness, and diplomacy. To reduce the impact of conventional
energy sources on the environment, much attention should be paid to the development of new energy and
renewable energy resources. Solar energy, which is environment friendly, is renewable and can serve as a
sustainable energy source. Hence, it will certainly become an important part of the future energy structure
with the increasingly drying up of the terrestrial fossil fuel. However, the lower energy density and
seasonal doing with geographical dependence are the major challenges in identifying suitable applications
using solar energy as the heat source. Consequently, exploring high efficiency solar energy concentration
technology is necessary and realistic.
Solar energy is free, environmentally clean, and therefore is recognized as one of the most promising
alternative energy recourses options. In near future, the large-scale introduction of solar energy systems,
directly converting solar radiation into heat, can be looked forward. However, solar energy is intermittent
by its nature; there is no sun at night. Its total available value is seasonal and is dependent on the
meteorological conditions of the location. Unreliability is the biggest retarding factor for extensive solar
energy utilization. Of course, reliability of solar energy can be increased by storing its portion when it is
in excess of the load and using the stored energy whenever needed.
Solar drying is a potential decentralized thermal application of solar energy particularly in developing
countries. However, so far, there has been very little field penetration of solar drying technology. In the
initial phase of dissemination, identification of suitable areas for using solar dryers would be extremely
helpful towards their market penetration.
Solar drying is often differentiated from ―sun drying‖ by the use of equipment to collect the sun‘s
radiation in order to harness the radiative energy for drying applications. Sun drying is a common farming
and agricultural process in many countries, particularly where the outdoor temperature reaches 30 o
higher. In many parts of South East Asia, spice s and herbs are routinely dried. However, weather
conditions often preclude the use of sun drying because of spoilage due to rehydration during unexpected
rainy days. Furthermore, any direct exposure to the sun during high temperature days might cause case
hardening, where a hard shell develops on the outside of the agricultural products, trapping moisture
inside. Therefore, the employment of solar dryer taps on the freely available sun energy while ensuring
good product quality via judicious control of the radiative heat. Solar energy has been used throughout the
world to dry products. Such is the diversity of solar dryers that commonly solar-dried products include
grains, fruits, meat, vegetables and fish. A typical solar dryer improves upon the traditional open-air sun
system in five important ways.
It is more efficient. Since materials can be dried more quickly, less will be lost to spoilage
immediately after harvest. This is especially true of products that require immediate drying such as
freshly harvested grain with high moisture content. In this way, a larger percentage of products will be
available for human consumption. Also, less of the harvest will be lost to marauding animals and insects
since the products are in safely enclosed compartments. It is hygienic. Since materials are dried in a
controlled environment, they are less likely to be contaminated by pests, and can be stored with less
likelihood of the growth of toxic fungi. It is healthier. Drying materials at optimum temperatures and in a
shorter amount of time enables them to retain more of their nutritional value such as vitamin C. An added
bonus is that products will look better, which enhances their marketability and hence provides better
financial returns for the farmers. It is cheap. Using freely available solar energy instead of conventional
fuels to dry products, or using a cheap supplementary supply of solar heat, so reducing conventional fuel
demand can result in significant cost savings.
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.
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. 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.
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 countries are not connected,
the use of these dryers is limited to electrified urban areas.
1.3 Problem Statement Objectives
The objective of this study is to develop a solar dryer in which the grains are dried
simultaneously 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 solar dryer which is reliable and
economically, design and construct a solar dryer using locally available materials and to
evaluate the performance of this solar dryer.
1.4Problem Justification and Outcomes
Drying is one of the methods used to preserve food products for longer periods. It has been
established as the most efficient preservation technique for most tropical crops.
This project presents the design, construction and performance of a solar dryer for food
preservation. In the dryer, the heated air from a separate solar collector is passed through a glass,
and at the same time, the drying cabinet absorbs solar energy directly through glass arrangement.
The results obtained during the test period revealed that the temperatures inside the dryer and
solar collector were much higher than the ambient temperature during most hours of the day-
light. The temperature rise inside the drying cabinet was up to 74% for about three hours
immediately after 12.00h (noon). The dryer exhibited sufficient ability to dry food items
reasonably rapidly to a safe moisture level and simultaneously it ensures a superior quality of the
DESIGN APPROACH AND METHODOLOGY
Solar drying refers to a technique that utilizes incident solar radiation to convert it into thermal
energy required for drying purposes. Most solar dryers use solar air heaters and the heated air is
then passed through the drying chamber (containing material) to be dried. The air transfers its
energy to the material causing evaporation of moisture of the material.
2.1 Design approach
There are two basic mechanisms involved in the drying process:
The migration of moisture from the interior of an individual material to the surface, and the
evaporation of moisture from the surface to the surrounding air.
The drying of a product is a complex heat and mass transfer process which depends on external
variables such as temperature, humidity and velocity of the air stream and internal variables
which depend on parameters like surface characteristics (rough or smooth surface), chemical
composition (sugars, starches, etc.), physical structure(porosity, density, etc.), and size and shape
of products. The rate of moisture movement from the product inside to the air outside differs
from one product to another and depends very much on whether the material is hygroscopic or
non-hygroscopic. Non-hygroscopic materials can be dried to zero moisture level while the
hygroscopic materials like most of the food products will always have residual moisture content.
This moisture, in hygroscopic material, may be bound moisture which remained in the material
due to closed capillaries or due to surface forces and unbound moisture which remained in the
material due to the surface tension of water.
Figure 2.1 Moisture in the drying material
When the hygroscopic material is exposed to air, it will absorb either moisture or desorbs
moisture depending on the relative humidity of the air. The equilibrium moisture content (EMC
= Me) will soon reach when the vapour pressure of water in the material becomes equal to the
partial pressure of water in the surrounding air . The equilibrium moisture content in drying
is therefore important since this is the minimum moisture to which the material can be dried
under a given set of drying conditions. A series of drying characteristic curves can be plotted.
The best is if the average moisture content M of the material is plotted versus time.
Figure 2.2 Rate of moisture loss
Figure 2.3 rate dM/dt versus moisture content M
As is seen from Figure 2.3 for both non-hygroscopic and hygroscopic materials, there is a
constant drying rate terminating at the critical moisture content followed by falling drying rate.
The constant drying rate for both non-hygroscopic and hygroscopic materials is the same while
the period of falling rate is little different. For nonhygroscopic materials, in the period of falling
rate, the drying rate goes on decreasing till the moisture content become zero. While in the
hygroscopic materials, the period of falling rate is similar until the unbound moisture content is
completely removed, then the drying rate further decreases and some bound moisture is removed
and continues till the vapour pressure of the material becomes equal to the vapour pressure of the
drying air. When this equilibrium reaches then the drying rate becomes zero.
The period of constant drying for most of the organic materials like fruits, vegetables, timber,
etc. is short and it is the falling rate period in which is of more interest and which depends on the
rate at which the moisture is removed. In the falling rate regime moisture is migrated by
diffusion and in the products with high moisture content, the diffusion of moisture is
comparatively slower due to turgid cells and filled interstices. In most agricultural products,
there is sugar and minerals of water in the liquid phase which also migrates to the surfaces,
increase the viscosity hence reduce the surface vapour pressure and hence reduce the moisture
2.1.2 Air Properties
The properties of the air flowing around the product are major factors in determining the rate of
removal of moisture. The capacity of air to remove moisture is principally dependent upon its
initial temperature and humidity; the greater the temperature and lower the humidity the greater
the moisture removal capacity of the air. The relationship between temperature, humidity and
other thermodynamic properties is represented by the psychometric chart. It is important to
appreciate the difference between the absolute humidity and relative humidity of air. The
absolute humidity is the moisture content of the air (mass of water per unit mass of air) whereas
the relative humidity is the ratio, expressed as a percentage, of the moisture content of the air at a
specified temperature to the moisture content of air if it were saturated at that temperature.
The changes in condition of air when it is heated using the solar energy and then passed through
a bed of moist product are shown in Figure 2.4. The heating of air from temperature TA to TB is
represented by the line AB. During heating the absolute humidity remains constant at A
whereas the relative humidity falls. As air moves through the material to be dried, it absorbs
Under (hypothetical) adiabatic drying; sensible heat in the air is converted to latent heat and the
change in the condition of air is represented along a line of constant enthalpy, BC. Both absolute
humidity and relative humidity increase from B and C and from to C, respectively, but
air temperature decreases to, TC. The absorption of moisture by the air would be the difference
between the absolute humidities at C and B. ( C - A). If unheated air is passed through the bed,
the drying process would be represented by the line AD. Assuming that the air at D to be at the
same relative humidity, C , as the heated air at C, then the absorbed moisture would be(( D -
A), considerably less than that absorbed by the heated air (( C - A).
Figure 2.4 Representation of drying process
2.1.3 Classification of drying systems
All drying systems can be classified primarily according to their operating temperature ranges
into two main groups of high temperature dryers and low temperature dryers. However; dryers
are more commonly classified broadly according to their heating sources into fossil fuel dryers
(more commonly known as conventional dryers) and solar-energy dryers. Strictly, all practically-
realized designs of high temperature dryers are fossil fuel powered, while the low temperature
dryers are either fossil fuel or solar-energy based systems.
1. High temperature dryers
High temperature dryers are necessary when very fast drying is desired. They are usually
employed when the products require a short exposure to the drying air. Their operating
temperatures are such that, if the drying air remains in contact with the product until equilibrium
moisture content is reached, serious over drying will occur. Thus, the products are only dried to
the required moisture contents and later cooled. High temperature dryers are usually classified
into batch dryers and continuous-flow dryers. In batch dryers, the products are dried in a bin and
subsequently moved to storage. Thus, they are usually known as batch-in-bin dryers.
Continuous-flow dryers are heated columns through which the product flows under gravity and
is exposed to heated air while descending. Because of the temperature ranges prevalent in high
temperature dryers, most known designs are electricity or fossil-fuel powered. Only a very few
practically-realized designs of high temperature drying systems are solar energy heated.
2. Low temperature dryers
In low temperature drying systems, the moisture content of the product is usually brought in
equilibrium with the drying air by constant ventilation. Thus, they do tolerate intermittent or
variable heat input. Low temperature drying enables products to be dried in bulk and is most
suited also for long term storage systems. Thus, they are usually known as bulk or storage dryers.
Thus, some conventional dryers and most practically-realized designs of solar-energy dryers are
of the low temperature type.
2.2 Design Methodology
2.2.1 Types of solar driers
Solar-energy drying systems are classified primarily according to their heating modes and the
manner in which the solar heat is utilized. In broad terms; they can be classified into two major
• Direct (integral) type solar dryers.
• Indirect (distributed) type solar dryers.
• Direct solar dryers have the material to be dried placed in an enclosure, with a transparent
cover on it. Heat is generated by absorption of solar radiation on the product itself as well as
on the internal surfaces of the drying chamber. In indirect solar dryers, solar radiation is not
directly incident on the material to be dried. Air is heated in a solar collector and then ducted
to the drying chamber to dry the product. Specialized dryers are normally designed with a
specific product in mind and may include hybrid systems where other forms of energy are
also used. Although indirect dryers are less compact when compared to direct solar dryers,
they are generally more efficient. Hybrid solar systems allow for faster rate of drying by
using other sources of heat energy to supplement solar heat.
• The three modes of drying are: (i) open sun, (ii) direct and (iii) indirect in the presence of
solar energy. The working principle of these modes mainly depends upon the method of
solar-energy collection and its conversion to useful thermal energy.
(i)Open sun drying (OSD)
Figure shows the working principle of open sun drying by using solar energy.
Figure 2.5 : Open Sun Drying
solar energy falls on the uneven product surface. A part of this energy is reflected back and the
remaining part is absorbed by the surface. The absorbed radiation is converted into thermal
energy and the temperature of product stars increasing. This results in long wavelength radiation
loss from the surface of product to ambient air through moist air. In addition to long wave length
radiation loss there is convective heat loss too due to the blowing wind through moist air over the
material surface. Evaporation of moisture takes place in the form of evaporative losses and so the
material is dried. Further apart of absorbed thermal energy is conducted into the interior of the
product. This causes a rise in temperature and formation of water vapor inside the material and
then diffuses towards the surface of the and finally losses thermal energy in the end then diffuses
towards the surface of the and finally losses the thermal energy in the form of evaporation. In the
initial stages, the moisture removal is rapid since the excess moisture on the surface of the
product presents a wet surface to the drying air. Subsequently, drying depends upon the rate at
which the moisture within the product moves to the surface by a diffusion process depending
upon the type of the product.
(ii)Direct type solar drying (DSD)
Direct solar drying is also called natural convection cabinet dryer. Direct solar dryers use only
the natural movement of heated air. A part of incidence solar radiation on the glass cover is
reflected back to atmosphere and remaining is transmitted inside cabin dryer. . A direct solar
dryer is one in which the material is directly exposed to the sun‘s rays. This dryer comprises of a
drying chamber that is covered by a transparent cover made of glass or plastic. The drying
chamber is usually a shallow, insulated box with air-holes in it to allow air to enter and exit the
box. The product samples are placed on a perforated tray that allows the air to flow through it
and the material. Fig. 2.6 shows a schematic of a simple direct dryer . Solar radiation passes
through the transparent cover and is converted to low-grade heat when it strikes an opaque wall.
This low-grade heat is then trapped inside the box by what is known as the ‗greenhouse effect.‘‘
Simply stated, the short wavelength solar radiation can penetrate the transparent cover. Once
converted to low-grade heat, the energy radiates.
Figure 2.6: Direct Type Solar Drying
(iii)Indirect type solar drying (ISD)
This type is not directly exposed to solar radiation to minimize discolorations and cracking. The
drying chamber is used for keeping the in wire mesh tray. A downward facing absorber is fixed
below the drying chamber at a sufficient distance from the bottom of the drying chamber. A
cylindrical reflector is placed under the absorber fitted with the glass cover on its aperture to
minimize convective heat losses from the absorber. The absorber can be selectively coated. The
inclination of the glass cover is taken as 45o
from horizontal to receive maximum radiation. The
area of absorber and glass cover are taken equal to the area of bottom of drying chamber. Solar
radiation after passing through the glass cover is reflected by cylindrical reflector toward an
absorber. After absorber, a part of this is lost to ambient through a glass cover and remaining is
transferred to the flowing air above it by convection. The flowing air is thus heated and passes
through the placed in the drying chamber. The exhaust air and moisture is removed through a
vent provided at the top of drying chamber.
Figure 2.7: Indirect Type Solar Drying
2.3 Applications of solar driers
The change of main variables such as moisture content along the drying tunnel is considered
unlike in previous works where uniform distribution is assumed .This is a study of tunnel green
house drier which is continuous type. The conditions for improvement of efficiency are
evaluated. A linear relationship between the tunnel output temperature and incident solar
radiation is obtained. The drier production is presented by a performance parameter which is
defined as the ratio between the energy actually used in the evaporation and the total available
energy for the drying process.
A non-dimensional variable is also defined which has all the meteorological information. It is
found that, the average moisture content value of the tunnel can be considered to be constant.
The construction and working of solar tunnel drier is explained in detail. Three fans run by a
solar module are used to create forced convection. The drying procedure and the instrumentation
are also described. The major advantage of solar tunnel drier is that the regulation of the drying
temperature is possible. During high insulation periods, more energy is received by the collector,
which tends to increase the drying temperature and is compensated by the increase of the air flow
rate. The variation of voltage with respect to radiation in a given day and variation of radiation
with respect to time of the day are presented. The comparative curves using the tunnel dryer and
natural sun drying are presented to show that, the tunnel drying time is less . A substantial
increase in the average sugar content is observed. The economics of the drier is worked out to
show that, the payback period is 3 years.
The dependence of the drying on the characteristics of product remains still as a problem, for
comparison of drying efficiencies of various driers. Author presented a comprehensive review of
the various designs, details of construction and operational principles of the wide variety of
practically realized designs of solar-energy drying systems. Two broad groups of solar energy
dryers can be identified, viz., passive or natural-circulation solar-energy dryers and active or
forced-convection solar-energy dryers (often called hybrid solar dryers). Three sub-groups of
these, which differ mainly on their structural arrangement, can also be identified, via integral or
direct mode solar dryers, distributed or indirect-modes. This classification illustrates clearly how
these solar dryer designs can be grouped systematically according to their operating temperature
ranges, heating sources and heating modes, operational modes or structural modes. Though
properly, designed forced-convection (active) solar dryers are agreed generally to be more
effective and more controllable than the natural-circulation (passive) types. This chapter also
presents some easy-to-fabricate and easy-to-operate dryers that can be suitably employed at
small-scale factories. Such low-cost drying technologies can be readily introduced in rural areas
to reduce spoilage, improve product quality and overall processing hygiene.
3.1 Design specifications and assumptions
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 mixed mode 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
The materials used for the construction of the mixed-mode solar dryer are cheap and easily
obtainable in the local market. Figure3.1 shows the main components of the dryer, consisting of
the solar collector (air heater), the drying cabinet and drying trays.
3.1.2 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 well seasoned
woods painted black. 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.
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. 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.
3.1.3 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 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.
3.1.4 Materials required for making the solar dryer:
The materials which are used to make the solar dryer are used in our everyday life .And
they are found easily near our locality.
3) Nail And Glue
4) Wired Mess
7) Black Paint
Figue:(3.1): view of solar cabinet dryer,(3.2): trey arrangement of solar dryer
(3.3): front view of solar dryer
3.2 Mathematical models and formulations
3.2.1 Operation of the Dryer
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 greenhouse 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 temperature enters through the lower vent in the
3.2.2 Drying mechanism
In the process of drying, heat is necessary to evaporate moisture from the material and a flow of
air helps in carrying away the evaporated moisture. There are two basic mechanisms involved in
the drying process:
1) The migration of moisture from the interior of an individual material to the surface.
2) The evaporation of moisture from the surface to the surrounding air.
The drying 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.
3.2.3 Basic Theory (Formulations)
Some important formulae used are given as follows:
1. Dryer efficiency(η d) :
Dryer efficiency is the ratio of collection efficiency (ηc) and the system efficiency (ηs).
(ηc) = Qu/ AcIs
Where, Qu= mCp∆t
Ac = collector surface area
Is = Insulation on tilted surface
Efficiency (ηs) =WL / AcIs
Where, W= mass of moisture evapourated.
L= latent heat of evapouration in the dryer temperature.
2. Determination of moisture content :
Mwb = (Mi – Md)/ Mi× 100
Where, Mwb = moisture on wet basis
Mi= initial mass of the sample
Md= final mass of the sample
DESIGN PROCEDURE AND IMPLEMENTATION
4.1 Design Procedures
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 80 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.
4.2 The Experimental Set-Up
Figure 4.1: View Of Solar Dryer
Figure 4.2: Complete Setup Of Solar Dryer
The mixed-mode solar dryer with box-type absorber collector was constructed using the
materials that are easily obtainable from the local market.
4.3 Design Implementation
Ambient temperature was recorded during the course of experiments with the help of digital
sensor. This project presents the design, construction and performance of a mixed-mode solar
dryer for food preservation. The dryer exhibited sufficient ability to dry food items reasonably
rapidly to a safe moisture level and simultaneously it ensures a superior quality of the dried
4.3.1 Object Of The Observation
Details of moisture removed during drying (in the month off fab-march) both in outsideand the
inside chamber are as shown below. Room temperature during drying periodwas 310
comparing the percentage of moisture removed from the solar dryer and the ordinary air(fruit
present in the atmosphere) the following table is experimental based data.
Also we take different fruit for calculation of experiment average dryer efficiency for one day was
found to be 13% while the moisture content for various samples like chilli, grapes and apple were found
64% ,58% and 60% respectively. All the readings were on a day basis i,e for one day.
The following data is given bello:
TABLE 1 Temperature, weight and %moisture removed in different
No Time Upper Tray Lower Tray Outside
1. 10:00(AM) 31 250 0.00% 31 250 0.00% 250 0.00%
2. 11:00(AM) 58 225.00 10.00 56 228 8.8 247 1.20
3. 12:00(AM) 63 202.01 19.20 63 206.45 17.42 236.03 5.58
4. 01:00(PM) 66 177.02 29.20 65 184.66 26.13 226.54 9.38
5. 02:00(PM) 71 125.00 50.00 68 128.20 48.72 199.04 20.38
6. 03:00(PM) 75 90.03 64.00 73.97 93.44 62.66 150.00 40.00
physical appearance of (chili) before and after 6 hours of drying in a full sunny
Figure 4.3: chili before dry
Figure 4.4: chili after dry
Figure 4.5: chilli inside dryer
4.3.2 Graphical Representation of Drying Rate:
The following graph represents total moisture % removed per every hour inside and the outside
of the chamber. The lower most and the middle graphical line represent moisture content
removed in % at inside the chamber. Lower most graphical lines represent the MC removed in %
outside the drying chamber. The following represents the MC removed in % with respect to time
and the temperature at that point. Since the solar drying does not give constant temperature
because of climatic condition; so the moisture % removed varies un-uniformly with time and the
Figure 4.6: Graphical representation of drying rate
1 2 3 4 5 6
outside the chamber
CALCULAION OF SOLAR DRYER EFFICIENCY
1 Dryer efficiency
One day Dryer efficiency (η d) for green chilli = 13.6%
One day Dryer efficiency (η d) for grapes = 14.19 %
One day Dryer efficiency (η d) for apples = 13.78%
The average dryer efficiency is found out to be 13% for one day.
2 Moisture content
Moisture content for green chilli = 64%
Moisture content for grapes = 58%
Moisture content for apple = 60%
4.3.3 Result And Discussion
After study we have found that the solar dryer gives more than three-four times heat inside the
chamber than that of the outside atm temperature. In 6 hours continuous drying under the same
climatic condition and same time it removed 28.73 % (upper tray) and 27.28 % (lower tray) moisture
content from inside chamber chili whereas at outside only 12.75 % moisture content was removed. our
experiment of average dryer efficiency for one day was found to be 13% while the moisture content for
various samples like chili, grapes and apple were found 64% ,58% and 60% respectively. All the readings
were on a day basis i,e for one day.
FEASIBIILITY STUDIES AND MARKET NEEDS
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
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
2. Food 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.
CONCLUSION AND RECOMMENDATIONS
From the test carried out, the following conclusions were made. The solar dryer can raise the
ambient air temperature to a considerable high value for increasing the drying rate of agricultural
crops. The product inside the dryer requires less attentions, like attack of the product by rain or
pest (both human and animals), compared with those in the open sun drying. Although the dryer
was used to dry Potato, 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.
Also from the test carried out, the simple and inexpensive solar dryer was designed and
constructed using locally sourced materials. . In this experiment we find that how much moisture
removed from the sample which is present in solar dryer and the sample which is present in
ordinary air and we compare both of them by mathematical calculation. In this paper we took
green chili, some of the chili we put inside the dryer and some in the ordinary air and then
compare their moisture removed with respect to time and temperature. We find that temperature
inside the dryer is two times outside the temperature. As per our experiment the maximum peak
temperature inside the drying chamber is 75°C during mid-day(3pm) and in an average
C-62°C in a full sunny day(10:00AM to 03:00PM). In 6 hours continuous
drying in one full sunny day under the same climatic condition and in same time the solar dryer
removed a maximum of 30- 40% moisture content from drying chamber for drying of low
moisture content food products.
experimental observation shows that the solar dryer can be used as an alternative in case of food
preservation and the efficiency is also acceptable . The people can make it in their homes
,especially in the developing countries where the energy demand is skyrocketing. It can be handy
in times of recession .The food stuffs can be stored in this dryer and used for days without
wasting it .The data concluded while performing this experiment is shown in the following table
for different samples:
Green chilli 13.6% 64%
Grapes 14.19% 58%
Apple 13.78% 60%
The performance of existing solar food dryers can still be improved upon especially in the aspect
of reducing the drying time, and probably storage of heat energy within the system by increasing
the size of the solar collector. Also, meteorological data should be readily available to users of
solar products to ensure maximum efficiency and effectiveness of the system. Such information
will probably guide a local farmer on when to dry his agricultural produce and when not to dry
1 Ajayi, C., Sunil, K.S., and Deepak, D. 2009. “Design of Solar Dryer with Turbo ventilator
and Fireplace”. International Solar Food Processing Conference 2009.
2 Brenidorfer B, Kennedy L, Bateman C O (1995). Solar dryer; their role in post harvest
processing, Commonwealth Secretariat Marlborough house, London,Swly 5hx.
3.A.A. El-Sebaii; S.M. Shalaby (2012): Solar drying of agricultural products: A review,
Renewable and Sustainable Energy Reviews 16, 37– 43.
4. Fadhel; S. Kooli; A. Farhat; A. Bellghith (2005): Study of the solar drying of grapes by three
different processes, Desalination 185, 535–541.
5. GuttiBabagana; Kiman Silas and Mustafa B. G. (2012): Design and Construction of
Forced/Natural Convection Solar Vegetable Dryer with Heat Storage, ARPN Journal of
Engineering and Applied Sciences, VOL. 7, NO. 10.
6. B.K. Bala; M.R.A. Mondol; B.K. Biswas; B.L. Das Chowdury; S. Janjai (2003): Solar drying
of pineapple using solar tunnel drier, Renewable Energy 28, 183–190.
7. Wang, Y., Zhang, M., Mujumdar, A.S., Mothibe, K.J., RoknulAzam, S.M. Effect of blanching
on microwave freeze drying of stem lettuce cubes in a circular conduit drying chamber, (2012)
Journal of Food Engineering, 113 (2), pp. 177-185.
8. Zhonghua Dr., W., Long, W., Zhanyong, L., Mujumdar, A.S. Atomization and Drying
Characteristics of Sewage Sludge inside a Helmholtz Pulse Combustor (2012) Drying
Technology, 30 (10), pp. 1105-1112.
9. Jiang, Y., Xu, P., Mujumdar, A.S., Qiu, S., Jiang, Z. A Numerical Study on the Convective
Heat Transfer Characteristics of Pulsed Impingement Drying (2012) Drying Technology, 30
(10), pp. 1056-1061.
10. J. Kaewkiew; S. Nabnean; S. Janjai (2012): Experimental investigation of the performance
of a large-scale greenhouse type solar dryer for drying chilli in Thailand. Procedia Engineering
32, 433 – 439.
11. J.K. Afriyie; M.A.A. Nazha; H. Rajakaruna; F.K. Forson (2009): Experimental
investigations of a chimney dependent solar crop dryer, Renewable Energy 34, 217– 222
12.Sharma, A., Chen, C. R., Vu Lan, N., 2009. Solar- energy drying systems: A review.
Renewable and Sustainable Energy Reviews, Vol.13, pp. 1185-1210.
13.Sodha, M.S., Dang, A., Bansal, P.K., Sharma, S.B., 1985. An analytical and experimental
study of open sun drying and a cabinet type drier. Energy Conversion &Management,, Vol.25(3),
14.Thoruwa, T.F.N., Johnstone, M.C., Grant, A.D., Smith, J.E., 2000. Novel, low cost
CaCl2based desiccants for solar crop drying applications. Renewable Energy, Vol.19,
15. Xie, W.T., Dai Y.J., Wang, R.Z., Sumathy, K., 2011. Concentrated solar energy applications
using Fresnel lenses: A review Renewable & Sustainable Energy Revıews, Vol. 15(6),pp. 2588 –
16.Torres-Reyes, E., Gonzalez, N.J.J., Ibarra-Salazar, B.A., 2002. Thermodynamic method for
designing dryers operated by flat plate solar collectors. Renewable Energy, Vol.26.pp. 649–660.
17.Youcef-Ali, S., Desmons, J.Y., 2004. The turbulence effect of the airflow on the calorific
losses in foodstuff dryers. Renewable Energy, Vol.29, pp.661-674.
18.Adegoke, C.O.; and Bolaji, B.O. 2000. Performance evaluation of solar-operated
thermosyphon hot water system in Akure. Int. J. Engin. Engin. Technol. 2(1): 35-40.
19.Akinola, O.A.; Akinyemi, A.A.; and Bolaji, B.O. 2006. Evaluation of traditional and solar
fish drying systems towards enhancing fish storage and preservation in Nigeria. J. Fish. Int.,
Pakistan 1(3-4): 44-9.
20.Bassey, M.W. 1989, Development and use of solar drying technologies, Nigerian Journal of
Solar Energy 89: 133-64.
21.Bolaji, B.O. 2005. Performance evaluation of a simple solar dryer for food preservation.
Proc. 6 Ann. Engin. Conf. of School of Engineering and Engineering Technology, Minna,
Nigeria, pp. 8-13.
22.Ertekin, C.; and Yaldiz, O. 2004. Drying of eggplant and selection of a suitable thin layer
drying model, J. Food Engin. 63: 349-59.
23.Ikejiofor, I.D. 1985. Passive solar cabinet dryer for drying agricultural products. In: O. Awe
(Editor), African Union of Physics. Proc. Workshop Phys. Tech. Solar Energy Convers, Univ. of
Ibadan, Nigeria, pp. 157-65.
24.Itodo, I.N.; Obetta, S.E.; and Satimehin, A.A. 2002. Evaluation of a solar crop dryer for rural
applications in Nigeria. Botswana J. Technol. 11(2): 58-62.
25.Kurtbas, I.; and Turgut, E. 2006. Experimental investigation of solar air heater with free and
fixed fins: efficiency and energy loss. Int. J. Sci. Technol. 1(1): 75-82.
26.Madhlopa, A.; Jones, S.A.; and Kalenga-Saka, J.D. 2002. A solar air heater with composite
absorber systems for food dehydration. Renewable Energy 27: 27–37.
27.Togrul, I.T.; and Pehlivan, D. 2004. Modelling of thin layer drying kinetics of some fruits
under open-air sun drying process. J. Food Engin. 65: 413-25.