Principles, Classification and Selection of Solar Dryers

G. L. Visavale - Principles, Classification and Selection of Solar Dryers

Chapter 1

Principles, Classification and Selection of Solar Dryers
G. L. Visavale
Computational Fluid Dynamics Division, Centre for Computational Technologies Pvt. Ltd.
Pune-411007, India. email: ganesh@cctech.co.in

Table of Contents
Table of Contents .................................................................................................................................... 1
1.1.

INTRODUCTION ............................................................................................................................. 2

1.2

DRYING PRINCIPLES ...................................................................................................................... 3

1.2.1
1.3

PSYCHROMETRY IN DRYING ...................................................................................................... 5
MECHANISM OF DRYING .............................................................................................................. 7

1.3.1

Internal Mechanism of Liquid Flow........................................................................................... 7

1.3.2

PERIODS OF DRYING.................................................................................................................. 8

1.3.2.1

CONSTANT-RATE PERIOD ...................................................................................................... 9

1.3.2.2

FALLING-RATE PERIOD .......................................................................................................... 9

1.3.2.3

ESTIMATIONS FOR TOTAL DRYING TIME................................................................................. 10

1.4

WATER ACTIVITY, {αw} ................................................................................................................. 10

1.5

EQUILIBRIUM MOISTURE CONTENT ........................................................................................... 10

1.6

SORPTION ISOTHERMS ............................................................................................................... 10

1.6.1

SORPTION EQUATIONS ....................................................................................................... 11

1.6.2

THE BET EQUATION............................................................................................................. 12

1.6.3

GAB EQUATION ................................................................................................................... 12

1.7

THIN LAYER DRYING AND DEEP BED DRYING ............................................................................. 12

1.7.1

THIN LAYER SOLAR DRYING..................................................................................................... 13

1.7.2

DEEP BED SOLAR DRYING........................................................................................................ 13

1.8 SOLAR DRYING TECHNOLOGY ....................................................................................................... 13
1.8.1
1.8.2

CLASSIFICATION OF SOLAR DRYERS ........................................................................................ 14
APPLICATIONS OF SOLAR DRYERS ....................................................................................... 17
1


SOLAR DRYING OF AGRICULTURAL PRODUCTS .................................................................................. 17
SOLAR DRYING OF MARINE PRODUCTS .............................................................................................. 19
1.9 WORKING PRINCIPLE .................................................................................................................... 19
1.9.1

OPEN SUN DRYING (OSD).................................................................................................... 20

1.9.2

DIRECT SOLAR DRYING (DSD) .............................................................................................. 20

1.9.3

INDIRECT SOLAR DRYING (ISD) ............................................................................................ 21

1.9.4

HYBRID SOLAR DRYING (HBD) ............................................................................................. 22

1.10 TYPES OF SOLAR DRYER ................................................................................................................ 22
1.10.1

PASSIVE SOLAR DRYING SYSTEMS........................................................................................... 22

1.10.2

INDIRECT-TYPE PASSIVE SOLAR-ENERGY DRYERS ............................................................... 22

1.10.3

DIRECT-TYPE PASSIVE SOLAR-ENERGY DRYERS .................................................................. 23

1.10.4

HYBRID-TYPE PASSIVE SOLAR-ENERGY DRYERS .................................................................. 26

1.11 ACTIVE SOLAR DRYING SYSTEMS .................................................................................................. 28
1.11.1
1.11.2
1.11.3

INDIRECT-TYPE ACTIVE SOLAR DRYING SYSTEMS ............................................................... 29
DIRECT-TYPE ACTIVE SOLAR-ENERGY DRYING SYSTEMS ........................................................ 30
HYBRID-TYPE ACTIVE SOLAR-ENERGY DRYERS ................................................................... 31

1.12 SELECTION OF SOLAR DRYERS ...................................................................................................... 32
Test case of solar drying at ICT Mumbai ............................................................................................. 35
1.12.1

ECONOMICS OF SOLAR DRYERS ............................................................................................ 114

1.12.2

DYNAMIC METHODS OF ECONOMIC EVALUATION .............................................................. 114

1.13 CONCLUSIONS ............................................................................................................................. 116
NOMENCLATURE................................................................................................................................. 117
REFERENCES ........................................................................................................................................ 119

1.1. INTRODUCTION
Preservation of fruits, vegetables, and food are essential for keeping them for a long time without
further deterioration in the quality of the product. Several process technologies have been
employed on an industrial scale to preserve food products; the major ones are canning, freezing,
and dehydration. Among these, drying is especially suited for developing countries with poorly
established low-temperature and thermal processing facilities. It offers a highly effective and
practical means of preservation to reduce postharvest losses and offset the shortages in supply.
Drying is a simple process of moisture removal from a product in order to reach the desired
moisture content and is an energy intensive operation. The prime objective of drying apart from
extended storage life can also be quality enhancement, ease of handling, further processing and
2


sanitation and is probably the oldest method of food preservation practiced by humankind
(Mujumdar, 2007). Drying involves the application of heat to vaporize moisture and some means of
removing water vapor after its separation from the food products. It is thus a combined and
simultaneous heat and mass transfer operation for which energy must be supplied. The removal of
moisture prevents the growth and reproduction of microorganisms like bacteria, yeasts and molds
causing decay and minimizes many of the moisture-mediated deteriorative reactions. It brings
about substantial reduction in weight and volume, minimizing packing, storage, and transportation
costs and enables storability of the product under ambient temperatures. These features are
especially important for developing countries, in military feeding and space food formulations
(vanArsdel, 1965).
Drying in earlier times was done primarily in the sun, now many types of sophisticated equipments
and methods are used to dehydrate foods. During the past few decades, considerable efforts have
been made to understand some of the chemical and biochemical changes that occur during
dehydration and to develop methods for preventing undesirable quality losses[5]. The widest among
drying methods is convective drying, i.e. drying by ﬂowing heated air circulating either over the
upper side, bottom side or both, or across the products. Hot air heats up the product and conveys
released moisture to atmosphere. In direct solar drying called ``sun drying'' the product is heated
directly by the sun's rays and moisture is removed by natural circulation of air due to density
differences.
Solar radiation in the form of solar thermal energy, is an alternative source of energy for drying
especially to dry fruits, vegetables, agricultural grains and other kinds of material, such as wood.
This procedure is especially applicable in the so-called “sunny belt” world-wide, i.e. in the regions
where the intensity of solar radiation is high and sunshine duration is long. It is estimated that in
developing countries there exist significant post-harvest losses of agricultural products, due to lack
of other preservation means. Drying by solar energy is a rather economical procedure for
agricultural products, especially for medium to small amounts of products. It is still used from
domestic upto small commercial size drying of crops, agricultural products and foodstuff, such as
fruits, vegetables, aromatic herbs, wood, etc., contributing thus significantly to the economy of small
agricultural communities and farms.

1.2 DRYING PRINCIPLES
Drying is basically a phenomena of removal of liquid by evaporation from a solid. Mechanical
methods for separating a liquid from a solid are not generally considered drying. In the following
section an attempt is made to provide a concise overview of the fundamental principles of drying
process for agricultural products. These principles are applied, in general, to mechanical
conventional drying and here concerned mainly with solar drying. However in general, must be
noted that conventional drying principles and phenomena are independent of the type of energy
used. Ekechukwu and Norton (1999) and Mujumdar, 2007 gives a comprehensive review of
fundamental principles and theories governing the drying process. A major part of energy
consumption during drying is for the evaporation of liquid water in to its vapour (2258 kJ/kg at
101.3 kPa). The water may be contained in the solid in various forms like free moisture or bound
form which directly affects the drying rate. The commonly encountered terminologies in
psychrometry and drying are briefly tabulated in table 1.1. Moisture content is expressed either on
3


dry or wet basis, e.g. moisture content in wet (
material:

) basis is the weight of moisture per unit of wet

kg per kg of mixture

and on dry basis

(1.1)

, is expressed as the ratio of water content to the weight of dry material:
(1.2)

Figure 1. 1 Relationship between wet-weight and dry-weight basis (Perry 2007).

Although the most convenient way to express moisture for mathematical calculations is on dry
basis but for agricultural products moisture content normally is expressed in wet basis. Fig. 1.1
shows the plot of relationship between the dry and wet-weight basis.
Table 1.1 Commonly encountered terms in psychrometry and drying (Perry 2007).
Sr
no.

Term

1.

Bound moisture

2.

Capillary flow

3.

Constant-rate period

4.

Critical moisture content

5.

Dry-weight basis

6.

Equilibrium moisture
content

Meaning/ Definition
is that in a solid liquid, which exerts a vapor pressure less than that of the
pure liquid at the given temperature.
is the flow of liquid through the interstices and over the surface of a solid,
caused by liquid-solid molecular attraction.
is that drying period during which the rate of water removal per unit of
drying surface is constant.
is the average moisture content when the constant-rate period ends.
expresses the moisture content of wet solid as kilograms of water per
kilogram of bone-dry solid.
is the limiting moisture to which a given material can be dried under
specific conditions of air temperature and humidity.

4


7.

Falling-rate period

8.

Free-moisture content

9.

Funicular state

10.

Hygroscopic material
Initial moisture
distribution

11.
12.

Internal diffusion

13.

Moisture content

14.
15.
16.
17.
18.
19.

1.2.1

Moisture gradient
Non-hygroscopic
material
Pendular state
Unaccomplished
moisture change
Unbound moisture
Wet-weight basis

is a drying period during which the instantaneous drying rate continually
decreases.
is that liquid which is removable at a given temperature and humidity,
may include bound/unbound moisture.
is that condition in drying a porous body when capillary suction results in
air being sucked into the pores.
is material that may contain bound moisture.
refers to the moisture distribution throughout a solid at the start of
drying.
may be defined as the movement of liquid or vapor through a solid as the
result of a concentration difference.
of a solid is usually expressed as moisture quantity per unit weight of the
dry or wet solid.
refers to the distribution of water in a solid at a given moment in the
drying process.
is material that can contain no bound moisture.
is that state of a liquid in a porous solid when a continuous film of liquid
no longer exists around and between discrete particles so that flow by
capillary cannot occur.
is the ratio of the free moisture present at any time to that initially
present.
in a hygroscopic material, is that moisture in excess of the equilibrium
moisture content corresponding to saturation humidity.
expresses the moisture in a material as a percentage of the weight of the
wet solid.

PSYCHROMETRY IN DRYING

In drying phenomena the psychrometry is of importance as it refers to the properties of air-vapor
mixture that controls the rate of drying. When an adequate supply of heat is provided for drying,
the temperature and rate at which the liquid vaporization occurs will depend on the vapour
concentration in the surrounding atmosphere. When a free liquid or wetted surface is present,
drying will occur at the saturation temperature, just as free water at 101.325 kPa vaporizes in a 100
percent steam atmosphere at 100°C. On the other hand, when evolved vapor is purged from the
dryer environment by using a second (inert) gas, the temperature at which vaporization occurs will
depend on the concentration of vapor in the surrounding gas.

5


Figure 1. 2 Psychrometric chart: properties of air and water-vapor mixtures

In effect, the liquid must be heated to a temperature at which its vapor pressure equals or exceeds
the partial pressure of vapor in the purge gas, while in the reverse situation condensation occurs. In
most drying operations, water is the liquid evaporated and air is the normally employed purge gas.
For drying purposes, a psychrometric chart found very useful is that reproduced in Fig. 1.2 and can
be explained as follows (Perry 2007 and Treybal 1980).:
a. In the psychrometric chart the wet-bulb or saturation temperature line gives the maximum
weight of water vapor that 1 kg of dry air can carry at the intersecting dry-bulb temperature
shown on the abscissa at saturation humidity. The partial pressure of water in air equals the
water vapor pressure at that temperature and the saturation humidity is defined by
(1.3)
b. The percent relative humidity is defined by
( )

(1.4)

c. Humid volumes are given by the curves entitled “volume m3/kg dry air.” The difference between
dry-air specific volume and humid-air volume at a given temperature is the volume of water
vapor.
d. Enthalpy data are given on the basis of kilojoules per kilogram of dry air. Enthalpy-at-saturation
data are accurate only at the saturation temperature and humidity.
e. There are no lines for humid heats on Fig. 1.2. and can be calculated by
(1.5)
f. The wet-bulb-temperature lines also represent the adiabatic saturation lines for air and water
vapor only, and are based on the relationship
6


( )

(1.6)

The slope of the adiabatic saturation curve is
. These lines show the relationship between the
temperature and humidity of air passing through a continuous dryer operating adiabatically. The
wet-bulb temperature is established by a dynamic equilibrium between heat and mass transfer
when liquid evaporates from a small mass, such as the wet bulb of a thermometer, into a very large
mass of gas such that the latter undergoes no temperature or humidity change, and is expressed by
the relationship
(1.7)
A given humidity chart is precise only at the pressure for which it is evaluated and most air–watervapor charts are based on a pressure of 1 atm. Humidities read from these charts for given values of
wet-bulb and dry-bulb temperature apply only at an atmospheric pressure of 760 mmHg. If the
total pressure is different, the humidity at a given wet-bulb and dry-bulb temperature must be
corrected according to the following relationship.
(

)

(1.8)

1.3 MECHANISM OF DRYING
Drying basically comprises of two fundamental and simultaneous processes: (i) heat is transferred
to evaporate liquid, and (ii) mass is transferred as a liquid or vapor within the solid and as a vapor
from the surface. The factors governing the rates of these processes determine the drying rate. The
different dryers may utilize heat transfer by convection, conduction, radiation, or a combination of
these. However in almost all solar dryers and other conventional dryers heat must flow to the outer
surface first and then into the interior of the solid, with exception for dielectric and microwave
drying.

1.3.1

Internal Mechanism of Liquid Flow

The movement of moisture within the solid result from a concentration gradient which is
dependent on the characteristics of the solid, that may be porous or nonporous. Thus the structure
of the solid determines the mechanism for which internal liquid flow may occur and these
mechanisms can include
(a) diffusion in continuous, homogeneous solids,
(b) capillary flow in granular and porous solids,
(c) flow caused by shrinkage and pressure gradients,
(d) flow caused by gravity, and
(e) flow caused by a vaporization-condensation sequence.
In general, one mechanism predominates at any given time in a solid during drying, but it is not
uncommon to find different mechanisms predominating at different times during the drying cycle.
In the plots shown in figure 1.3, the curves indicate that capillary flow is typified by a moisture
gradient involving a double curvature and point of inflection (Fig. 1.3a) while diffusional flow is a
smooth curve, concave downwards (Fig. 1.3b). They also showed that the liquid-diffusion
coefficient is usually a function of moisture content which decreases with decreasing moisture
(Keey, 1978 and Treybal 1980).
7


Figure 1. 3 Two types of internal moisture gradients in solid drying (Perry 2007).

1.3.2

PERIODS OF DRYING

Figure 1.4 shows a plot of moisture content ( ) versus time ( ) generally obtained by
experimentally drying a solid. This curve represents a typical case when a wet solid loses moisture
initially by evaporation from a saturated surface on a solid, followed by a period of evaporation
from a saturated surface on a solid, followed by a period of evaporation from a saturated surface of
gradually decreasing area and finally when the latter evaporated in the interior of the solid. Figure
1.4a indicates that the drying rate is subject to variation with time or moisture content, further
better illustrated by graphically or numerically differentiating the curve and plotting dX/dθ versus
W, as shown in Fig. 1.4b, or as dX/dθ versus θ, as shown in Fig. 1.4c. These rate curves illustrate that
the drying process is not a smooth, continuous one in which a single mechanism controls
throughout. Figure 1.4c has the advantage of showing how long each drying period lasts.
The section AB on each curve represents a warming-up period of the solids. Section BC on each
curve represents the constant-rate period. Point C, where the constant rate ends and the drying rate
begins falling, is termed the critical-moisture content. The curved portion CD on Fig. 1.4a is termed
the falling-rate period and, as shown in Fig. 1.4b and c, is typified by a continuously changing rate
throughout the remainder of the drying cycle. Point E (Fig. 1.4b) represents the point at which all
the exposed surface becomes completely unsaturated and marks the start of that portion of the
drying cycle during which the rate of internal moisture movement controls the drying rate. Portion
CE in Fig. 1.4b is usually defined as the first falling-rate drying period; and portion DE, as the second
falling-rate period (Perry 2007).

8


1.3.3

Figure 1. 4 Drying period curves (Perry 2007).

1.3.2.1 CONSTANT-RATE PERIOD
In the constant-rate period moisture movement within the solid is rapid enough to maintain
a saturated condition at the surface, and the rate of drying is controlled by the rate of heat
transferred to the evaporating surface. Drying proceeds by diffusion of vapor from the saturated
surface of the material across a stagnant air film into the environment and as the rate of mass
transfer balances heat transfer, the temperature of saturated surface remains constant. If the heat
supplied for drying is solely by convection, the surface temperature approaches the boiling point
temperature rather than the wet bulb temperature. This mode of heat transfer is typically seen in
indirect dryers. Radiation is an effective mode of heat transfer as it increases the constant rate by
augmenting the convection heat transfer and raising the surface temperature above the wet bulb
temperature.
The magnitude of the constant rate depends upon three factors:
 The heat or mass transfer coefficient
 The area exposed to the drying medium
 The difference in temperature or humidity between the gas stream and the wet surface of
the solid.
All these factors are the external variables. The internal mechanism of liquid flow does not affect
the constant rate (Keey, 1980).

1.3.2.2 FALLING-RATE PERIOD
The falling-rate period begins at the critical moisture content when the constant-rate period ends.
This is generally divided into two zones viz., (i) the zone of unsaturated surface drying and (ii) the
zone where internal moisture movement controls. In the first zone, the entire evaporating surface
can no longer be maintained and saturated by moisture movement within the solid. The drying rate
decreases from the unsaturated portion, and hence the rate for the total surface decreases.
Generally, the drying rate depends on factors affecting the diffusion of moisture away from the
evaporating surface and those affecting the rate of internal moisture movement. As drying
proceeds, the point is reached where the evaporating surface is unsaturated. The point of
evaporation moves into the solid, and the dry process enters the second falling-rate period. The
drying rate is now governed by the rate of internal moisture movement; the influence of external
9


variables diminishes. This period usually predominates in determining the overall drying time to
lower moisture content (Treybal 1980).

1.3.2.3 ESTIMATIONS FOR TOTAL DRYING TIME
Estimates of both the constant-rate ( ) and the falling-rate periods (
total drying time (
for a given drying operation as

are needed to estimate the
(1.9)

1.4 WATER ACTIVITY, {αw}
The ratio of the equilibrium vapor pressure to the saturation vapor pressure is known as the
equilibrium relative humidity (ERH), or water activity. Water activity αw, is of great importance
for food preservation as it is a measure and a criterion of microorganism growth and probably toxin
release, of enzymatic and non-enzymatic browning development.
( )

(1.10)

For every food or agricultural product there exists an activity limit below which microorganisms
stop growing. Beuchat (1981) refers that the vast majority of bacteria will grow at about α w = 0.85,
mold and yeast about αw = 0.61, fungi at αw < 0.70, etc. In these cases water activity is regulated in
detail, after drying by the addition of some solutions of sugars, starch, etc. Water in food and
agricultural crops, is in the form of a solution which contains salts, sugars, carbohydrates, proteins,
etc., which at constant temperature are in thermodynamic equilibrium. The water activity is then
given by the following equation:

1.5 EQUILIBRIUM MOISTURE CONTENT
A wet solid exposed to a continuous supply of fresh gas continues to lose moisture until the vapor
pressure of the moisture in the solid is equal to the partial pressure of the vapor in the gas. The
solid and gas are then said to be in equilibrium, and the moisture content of the solid is called the
equilibrium moisture content under the prevailing conditions. Further exposure to this air for
indefinitely long periods will not bring about any additional loss of moisture. The moisture content
in the solid could be reduced further by exposing it to air of lower relative humidity. This means
that moisture desorption from the product is in dynamic equilibrium with the absorption of the
environmental air moisture contain. Relative humidity at this point is known as the “equilibrium
relative humidity”, and is characterized by the curves of moisture content plots against equilibrium
humidity known as moisture equilibrium isotherms.

1.6 SORPTION ISOTHERMS
Various changes in physical, chemical and biological characteristics of foodstuffs occur during
processing, storage and distribution therefore the knowledge of water sorption isotherm is
necessary to evaluate their stability (Roca et al., 2006). Moisture sorption isotherms, an important
tool for predicting the interactions of water and food components, describes the relationship
between water activity and the equilibrium moisture content of a foodstuff. These are graphical
10


representations of the relationship between moisture content at the corresponding water activity
αw, over a range of values at constant temperature. Fig. 1.5 presents the absorption and desorption
isotherms. It is obvious that it has a slight hysteresis in re-absorbing water when the product has
been dried. There exist numerous mathematical models, theoretical, empirical and semi-empirical
relationships, developed for various agricultural products, crops, grains, etc. The three models most
widely used in food drying are described here.

Figure 1. 5 Sorption Isotherm (A) Absorption curve (B) Desorption curve (Belessiotis
and Delyannis, 2011)

1.6.1

SORPTION EQUATIONS

A number of mathematical equations have been proposed to describe the sorption phenomena but
only the BET and GAB equations are widely accepted especially for crops. These are based on the
theory of water molecular layers absorbed onto the materials surface, a modification for multi-layer
absorption, of the Langmuir’s (1918) water molecular mono-layer equation:
(

)

(1.11)

where are: , the moisture content at the observation time period,
, the mono-layer moisture
content and is the Langmuir’s constant. As shown in the schematic model of Kei on Fig. 1.6 (Toei,
1996), it is assumed that, with respect to the externally exposed molecules on each layer within the
multi-layer, dynamic equilibrium between Langmuir’s adsorption– desorption rates still holds for
the exposed molecules on each layer.
The isotherm models used to fit the data were BET (Brunauer et al., 1938) (Eq. 1) and GAB models
(Van den Berg, 1984) (Eq. 1.12):
Xe 

X m C GAB K GAB a w
[(1  K GAB a w )(1  K GAB a w  C GAB K GAB a w )]

(1.12)

where Xe is the equilibrium moisture content (g water/g solids); Xm is the monolayer moisture
content (g water/g solids); n is the number of layers; CBET, CGAB and KGAB are the constants.

11


Figure 1. 6 Schematic model of BET theory (Belessiotis and Delyannis, 2011)

1.6.2

THE BET EQUATION

The BET (Brunauer et al., 1938), equation is a modification of the previous Lagmuir equation,
widely accepted for foods. The BET equation is:
(1.13)
where Mw is the water content (kg water/kg dry solids), Mb is the BET monolayer water content
(kg water/kg dry solids) and B is a constant related to the net heat of sorption. The BET isotherm
holds well between water activities of 0.05 and 0.45, an adequate range for the calculation of
parameters Mb and B (Rahman, 1995).

1.6.3 GAB EQUATION
The GAB (Guggenheim, 1966; Anderson, 1946; de Boer, 1953) equation is the most widely accepted
isotherm model as it fits isotherms of products in a range of water activities from aw = 0 up to aw =
0.99 (Van der Berg, 1981).
The GAB equation is:
(1.14)
where
is the GAB monolayer water content (dry basis). is a constant related to the monolayer
heat of sorption and is a factor related to the heat of sorption of the multilayer. BET and GAB
models are the most commonly used models to fit sorption data of food materials. The GAB
isotherm equation is an extension of the BET model taking into account the modified properties of
the sorbate in the multilayer region and the bulk liquid water properties through the introduction
of a third constant . Estimation of three parameters in GAB using two variables (i.e. water content
and water activity) leads to a non-linear optimization. The BET monolayer value is more acceptable
than that of GAB monolayer value, although the GAB model provides accurate prediction over the
water activity range up to 0.90 (Rahman, 1995).

1.7 THIN LAYER DRYING AND DEEP BED DRYING
Drying rate is controlled by the external process factors of the process and the internal diffusion
mechanisms. Apart from the external factors, the type and size of the product also affect the drying
rate. Thus many products, i.e. fruits, vegetables, sliced fruits, are better to be dried in thin layers,
but grains, i.e. corn, beans, etc., can be dried in deep beds, in dryers, or into bins.

12


1.7.1

THIN LAYER SOLAR DRYING

Thin layer drying models fall into the three categories, namely theoretical, semi-theoretical and
empirical, contributing to the understanding of the drying characteristics of agricultural materials.
The theoretical approach concerns either the diffusion equation or simultaneous heat and mass
transfer equation, the semi-theoretical approach concerns approximated theoretical equations and
the empirical equations are easily applied to drying simulation as they depend on experimental
data. Among these models, the theoretical approaches account for only the internal resistance to
moisture transfer, while the semi-theoretical and empirical approaches consider only the external
resistance to moisture transfer between the product and air (Usub et al., 2010). In general they are
based on the assumption that the ratio of air volume to the crop volume is infinitely large.
Considering this assumption drying rate depends only on the properties of the material to be dried,
its size, the drying air temperature and the moisture content. Recently, there have been many
researches on the mathematical modeling and experimental studies of the thin layer solar drying
process of various vegetables and fruits, such as grapes [10], apricots [11,12], green pepper, green
bean and squash [13], Eucalyptus globulus [14,15], mint, verbena, and sage [15], pistachio [8,9],
and prickly pear fruit [16]. As an example, an empirical equation predicting drying time is given
which is valid for temperatures from 60 to 80 for solar drying, up to 140 for conventional and
hybrid solar drying (Eissen, 1985):
[
The parameters

1.7.2

]

(

[

])

(1.15)

and are given by the following numerical equations:
-0.0301T , where
is in degrees Celsius ( ).

DEEP BED SOLAR DRYING

In deep bed drying solid phase is stationary and remains in the dryer for a certain time while gas
flows through it continuously. Drying begins at the inlet end of gas and progresses through the
entire bed (Ekechukwu, 1999). It is obvious that the lower zone dries rapidly. Air moves from the
lower to thr upper zone and increases its moisture content and cools due to evaporation. Thus a
gradient of temperature and relative humidity is formed between the lower and the upper zone,
which is a measure of the drying rate. Final moisture content is the mean moisture of these zones.
The critical drying factors are: air flow rate, drying air temperature and the bed’s depth. By
adjusting these parameters, a moderate drying operation can be achieved without over-drying in
the lower material zone.

1.8 SOLAR DRYING TECHNOLOGY
Solar drying has been used since time immemorial to dry plants, seeds, fruits, meat, fish, wood, and
other agricultural, forest products. In order to benefit from the free and renewable energy source
provided by the sun several attempts have been made in recent years to develop solar drying
mainly for preserving agricultural and forest products. However, for large-scale production the
limitations of open-air drying are well known. Among these are high labour costs, large area
requirement, lack of ability to control the drying process, possible degradation due to biochemical
or microbiological reactions, insect infestation, and so on. The drying time required for a given
13


commodity can be quite long and result in post-harvest losses(more than 30%) . Solar drying of
agricultural products in enclosed structures by forced convection is an attractive way of reducing
post-harvest losses and low quality of dried products associated with traditional open sun-drying
methods (Jain and Tiwari, 2003). In many rural locations in most developing countries, gridconnected electricity and supplies of other non-renewable sources of energy are either unavailable,
unreliable or, too expensive. In such conditions, solar dryers appear increasingly to be attractive as
commercial propositions (Mekhilefa et al., 2011; Xingxing et al., 2012).
During the last decades, several developing countries have started to change their energy policies
toward further reduction of petroleum import and to alter their energy use toward the utilization of
renewable energies. With very few exceptions, the developing countries are situated in climatic
zones of the world where the insolation is considerably higher than the world average of 3.82
kWh/m2 day. In Figure 1.7 daily average horizontal insolation data and sunshine hours of some
developing countries are given. An alternative to traditional drying techniques and a contribution
toward the solution of the open air drying problems is the use of solar dryers. Accordingly, the
availability of solar energy and the operational marketing and economy reasons offer a good
opportunity for using solar drying all over the world.

Figure 1. 7 Total horizontal solar insolation for some developing countries (Visavale, 2009)

1.8.1

CLASSIFICATION OF SOLAR DRYERS

Drying equipment may be classified in several ways. The two most useful classifications are based
on (1) the method of transferring heat to the wet solids or (2) the handling characteristics and
physical properties of the wet material. The first method of classification reveals differences in
dryer design and operation, while the second method is most useful in the selection of a group of
dryers for preliminary consideration in a given drying problem. A classification chart of drying
equipment on the basis of heat transfer is shown in Fig. 1.8 (Ekechukwu,1999; Sharma et al., 2009).
This chart classifies dryers as direct or indirect, with subclasses of continuous or batchwise
14


operation. Solar-energy drying systems are classified primarily according to their heating modes
and the manner in which the solar heat is utilized (Belessiotis et al., 2011). In broad terms, they can
be classified into two major groups, namely:
 passive solar-energy drying systems (conventionally termed natural-circulation solar
drying systems) and,
 active solar-energy drying systems (most types of which are often termed hybrid solar
dryers).

15


SOLAR DRYERS

Conventional
drying

Bulk or
storage (low
temperatur
e) dryers

Solar energy drying

Batch and
continuous flow
(high
temperature)
dryers

Solar energy
dryers

Natural dryers

Crops dried
in-situ
Active
dryers

Distributed
type dryers

Drying on
ground
mats
concrete
and floor

Horizontal
trays

Mixed
mode
dryers

Passive
dryers

Integral
type dryers

Distributed
type dryers

Drying on
trays

Vertical or
inclined racks

Cabinet
type dryers

Mixed
mode
dryers

Integral
type
dryers

Green house
dryers

Figure 1. 8 Classifications of dryers and drying modes (Ekechukwu,1999)
Three distinct sub-classes of either the active or passive solar drying systems can be identified
namely:
 direct-type solar dryers;
 indirect-type solar dryers; and
 hybrid solar dryers.
16


The main features of typical designs of the various classes of solar-energy dryers are
illustrated in Fig. 1.9, showing three main groups for solar dryers on the basis of the energy
sources used (Leon et al., 2002). The design of solar dryers is adjusted to the quantity, character,
and designation of the material to be dried as well as to the energy sources used and
accordingly, various types of solar dryers have been developed and are in use to date.

Figure 1. 9 Typical solar energy dryer designs(Ekechukwu and Norton, 1999)

1.8.2

APPLICATIONS OF SOLAR DRYERS

SOLAR DRYING OF AGRICULTURAL PRODUCTS
Solar drying methods as applied to foods have earlier been reviewed by Bolin and Salunkhe
(1980, 1982). The former surveyed methods of solar drying having varying levels of technological
sophistication and compared the thermodynamics of conventional dehydration with those of solar
drying. It has been exhaustively reviewed the drying methods using only solar energy, as well as
those using solar plus and auxiliary energy source, besides discussing the quality and economic
17


aspects, suggesting that to produce a high quality product economically it must be dried fast
without excessive heat (Singh et al., 2011; Jangam et al., 2011).
Imre (2004) described in detail the construction principles of solar dryers and flat plate
collectors along with their economics and performance evaluation. Several authors have studied
the usability and performance of various types of solar dryers, energy storage systems (Kalra et al.,
1981; Miller, 1985) and pretreatment techniques (Islam et al., 1982; Moyls, 1986; Vaghani, 1986).
Cheema and Ribero (1978) studied the comparative performance of three dryers for the drying of
cashew, banana and pineapple and found that optimum combination of solar and conventional
drying is more suitable. To shorten the drying time Wagner et al. (1979) utilized the principles of
parabolic reflector to increase the radiation on the product.
Bolin et al. (1980) discussed the relative merits of five experimental methods for the solar
dehydration of fruits, namely: black wooden tray, solar troughs of various materials designed to
reflect radiant energy onto bottom of black metal drying trays, cabinet dryers with slanted plate
heat collectors with natural convection, utilizing inflated polyethylene (PE) tubes as solar collectors
with and without partial air recirculation; and PE semicylinder with a fan blower to be used in
inflated hemispheres or as a solar collector, to blow air over the fruit in a cabinet dryer. They
reported that utilizing inflated PE tubes method was cheap, 38% faster than sun drying for apricots
and could be used as supplementary heat source for conventional dryer.
Kalra and Bhardwaj (1981) described two simple models of solar dehydrator with the
functions of direct and indirect dryers for mango products and vegetables which are well suited to
rural conditions and small scale industries. Sun drying of grapes requires longer time to dry and
also there is a browning, contamination and spoilage of the product when exposed to the open
atmosphere (Pangavhane et al.,1999, 2002; El-Sebaii et al., 2002).
A number of investigations are reported (Bolin et al., 1980; Eisen et al., 1985; Lutz et al.,
1986; Raouzeous et al., 1986) on the solar dehydration of grapes to reduce drying time and
improve quality of raisin which include development of low cost dryer optimized through data
obtained from laboratory tests and a solar tunnel dryer with integrated collector and a small radial
fan. The effect of various surface dip treatments aimed for removing the waxy layer to increase
drying rates and reduce drying times of grapes have been studied. The dips generally found
effective include methyl oleate/K2CO3, alkaline ethyl oleate emulsion, boiling 0.25 % NaOH or hot
(80°C) NaOH-ethyl oleate. Treatment with 4% methyl oleate was also found most effective for
prunes (Pangavhane et al., 2002).
Natural convection solar dryers have been the subject of investigation by many workers for
studying the drying behavior of several fruits and vegetables. Islam and Flink (1982), conducted
experiments on potato at low air velocities as encountered in solar dryers and found that due to
extensive external mass transport resistance in deep bed drying, the air flow behavior of the bed
was more important than drying behavior of the pieces. Drying time increased less rapidly than
increase in bed depth. On the basis of simulated solar drying experiments, Shakya and Flink (1986),
concluded that drying rate of potato increased with increased inlet air temperature and/or air flow
potential and overall productivity increased with increasing bed depth.
Sharma (1987), described the design details and performance of two types of low cost solar
crop dryers, conventional cabinet dryer with direct heating mode and an integrated solar collectorcum-drying system based on the principle of natural convection. Results obtained with several
18


vegetables viz. cauliflower, green peas and potato showed satisfactory overall efficiency and
performance of both dryers. Banout et al. (2011) recently had studied the design and performance
of a double pass solar dryer for drying of red chilli and Montero et al. (2010) has studied the design
and performance for drying of agro-industrial by-products.
Several attempts at developing forced convection solar dryers (both of the cabinet-type and
the tunnel-type) have been investigated and experimented on, over the years, and are described in
the literature. The forced convection solar crop dryer design include: direct, indirect and mixedmode solar-dryers. Comparative studies on these three dryer designs suggested that the
performance of the mixed-mode forced convection solar dryer is potentially most effective and it
appears to be particularly promising in tropical humid regions. The forced convection type solar
cabinet dryer, basically a cabinet-type of solar dryer with attached solar air heater and forced
airflow, has been considered favorable since such a dryer utilizes solar energy directly, as well as
the convective energy of the heated air. Bennamoun (2003), presented the effect of surface of the
collector, the air temperature and the product characteristics during solar drying of onion.
Nandwani (2007), has discussed the design and development of a multipurpose fruit and vegetable
dryer for domestic use.

SOLAR DRYING OF MARINE PRODUCTS
Marine products i.e. fish being highly perishable are processed by freezing, canning, salting and
dehydration (Posomboon, 1998). In most of the developing countries, it has been estimated that
about 40% of fish landing is still preserved by either sun-drying or salt curing or drying. Prawns are
treated with boiling water prior to drying to reduce the microbial flora to an acceptable level and to
improve the flavor of dried prawns. Jain (2006) studied the drying of Bombay duck to determine
the heat and mass transfer coefficients by solar drying. Bala and Mondol (2001) described the use
of solar tunnel drying as an effective method for drying of silver jew fish (Johnius argentatus) along
with salting pretreatment and Reza et al. (2009) have optimized marine fish drying using solar
tunnel drying. Jain and Pathare (2007) studied the drying behaviour of prawn and chelwa fish
(Indian minor carp), by open sun drying. Sankat and Mujaffar (2004) studied the sun and solar
cabinet drying of salted shark fillets . Baird et al. (1981) have extensively reviewed the solar drying
of seafood by forced air convection and by direct insolation, as well as hot smoking fish with a solar
assisted fish smoker. Kittu et al., (2010) had simulated the thin layer drying model of tilapia fish
using a solar tunnel dryer. As a number of solar dryers exist for a large variety of products to be
dried, the preference of one over the rests depends on a number of factors like ease of material
handling, its drying characteristics, final product quality and sometimes the available resources.
Table 1.2 (section 1.12) presents a general checklist to be considered in the preliminary selection of
a solar dryer, and the steps involved in consideration are described (Perry, 2007).

1.9 WORKING PRINCIPLE
Solar energy dryers can broadly be classified into direct, indirect and hybrid solar dryers. The
working principle of these dryers mainly depends upon the method of solar-energy collection and
its conversion to useful thermal energy for drying.

19


1.9.1 OPEN SUN DRYING (OSD)
Fig. 1.10 shows the working principle of open sun drying by using only the solar energy. The crops
are generally spread on the ground, mat, cement floor where they receive short wavelength solar
energy during a major part of the day and also natural air circulation. A part of the energy is
reflected back and the remaining is absorbed by the surface depending upon the colour of the
crops. The absorbed radiation is converted into thermal energy and the temperature of the material
starts to increase. However there are losses like the long wavelength radiation loss from

Figure 1. 10 Working principle of open sun drying (Sharma et al. 2009)
the surface of crop to ambient air through moist air and also convective heat loss due to the
blowing wind through moist air over the crop surface. The process is independent of any other
source of energy except sunlight and hence the cheapest method however has a number of
limitations as discussed in section 2.1.1. In general, the open sun drying method does not fulfill the
required quality standards and sometimes the products cannot be sold in the international market.
With the awareness of inadequacies involved in open sun drying, a more scientific method of solarenergy utilization for crop drying has emerged termed as solar drying.

1.9.2

DIRECT SOLAR DRYING (DSD)

The working principle of direct solar crop drying is shown in Fig. 1.11, also known as a solar
cabinet dryer. Here the moisture is taken away by the air entering into the cabinet from below and
escaping through at the top exit as shown in Figure. In the cabinet dryer, of the total solar radiation
impinging on the glass cover, a part is reflected back to atmosphere and the remaining is
transmitted inside the cabinet. A part of the transmitted radiation is then reflected back from the
crop surface and the rest is absorbed by the surface of the crop which causes its temperature to
increase and thereby emit long wavelength radiations which are not allowed to escape to
atmosphere due to the glass cover. The overall phenomena causes the temperature above the crop
inside the cabinet to be higher. The glass cover in the cabinet dryer thus serves in reducing direct
convective losses to the ambient which plays an important role in increasing the crop and cabinet
temperature.
20


Figure 1. 11 Working principle of direct solar drying (Sharma et al. 2009)
The advantages of solar drying over open sun drying are as follows:
a) Simpler and cheaper to construct than the indirect-type for the same loading capacity.
b) Offer protection from rains, dews, debris etc.
A cabinet dryer has the following limitation:
a) Liability to over-heat locally, causing crop damage.
b) Poor vapour removal rates leading to relatively slow overall drying rates
c) Small capacity limits it to small scale applications.
d) Discolouration of crop due to direct exposure to solar radiation.
e) Moisture condensation inside glass covers reduces its transmittivity.
f) Insufficient rise in crop temperature affects moisture removes.
g) Limited use of selective coatings on the absorber plate and inside the cabinet dryer.

1.9.3

INDIRECT SOLAR DRYING (ISD)

These differ from direct dryers with respect to heat transfer and vapor removal. Fig. 1.12 describes
the working principle of indirect solar drying. The crops in these indirect solar dryers are located in
trays or shelves inside an opaque drying cabinet and a separate unit termed as solar collector is
used for heating of the entering air into the cabinet. The heated air is allowed to flow through/over
the wet crop that provides the heat for moisture evaporation by convective heat transfer between
the hot air and the wet crop. Drying takes place due to the difference in moisture concentration
between the drying air and the air in the vicinity of crop surface. The advantages of indirect solar
drying are:
a) Offers a better control over drying and the product obtained is of better quality than sun
drying.
b) Caramelization and localized heat damage do not occur as the crops are protected and
opaque to direct radiation.
c) Can be operated at higher temperature, recommended for deep layer drying.
d) Highly recommended for photo-sensitive crops.
e) Have inherent tendency towards greater efficiency than direct solar drying.
They are, however, relatively elaborate structures requiring more capital investment in equipment
and incur larger maintenance costs than the direct drying units.

21


Figure 1. 12 Working principle of indirect solar drying system (Sharma et al.
2009)

1.9.4

HYBRID SOLAR DRYING (HBD)

The hybrid solar dryers combine the features of the direct and indirect type solar-energy dryers.
Here the combined action of incident direct solar radiation on the product to be dried and air preheated in a solar collector heater produces the necessary heat required for the drying process

1.10

TYPES OF SOLAR DRYER

On the basis of the mode of drying, e.g. direct or indirect, solar dryers may be classified as passive
and active ones: (a) Passive dryers, where crops are dried by direct impingement from the sun’s
radiation with or without natural air circulation, and (b) Active solar dryers, where hot drying air is
circulated by means of a ventilator (forced convection).

1.10.1

PASSIVE SOLAR DRYING SYSTEMS

In a passive solar dryer, air is heated and circulated naturally by buoyancy force or as a result of
wind pressure or in combination of both. Normal and reverse absorber cabinet dryer and
greenhouse dryer operates in passive mode. Passive drying of crops is still in common practice in
many Mediterranean, tropical and subtropical regions especially in Africa and Asia or in small
agricultural communities. These are primitive, inexpensive in construction with locally available
materials, easy to install and to operate especially at sites far off from electrical grid. The passive
dryers are best suited for drying small batches of fruits and vegetables such as banana, pineapple,
mango, potato, carrots etc (Hughes et al., 2011).

1.10.2

INDIRECT-TYPE PASSIVE SOLAR-ENERGY DRYERS

These are indirect-type dryers with natural convection of air for drying. In order to increase the
capacity of a dryer i.e. operate with more than one layer of trays with crops within the available
area, the trays are generally placed in vertical racks with some space in between consecutive trays.
The additional resistance generated for the air movement due to this arrangement of the trays is
achieved by the “chimney effect”. The chimney effect increases the vertical flow of air as a result of
the density difference of the air in the cabinet and atmosphere. A typical indirect passive solar22


energy dryer used for crop drying are shown in Fig. 1.13. Figure 1.14 shows the design by Ortho
Grainger & Twidel (1981) for maize drying. The designs generally comprise of an air-heating solarenergy collector, an insulated ducting, a drying chamber and a chimney.

Figure 1. 13 Typical distributed-type
(indirect) natural-circulation solar-energy
(Ekechukwu and Norton, 1999)

1.10.3

Figure 1. 14. A distributed-type naturalcirculation solar maize dryer (Ekechukwu and
Norton, 1999)

DIRECT-TYPE PASSIVE SOLAR-ENERGY DRYERS

The features of a typical direct-type passive solar dryer are illustrated in Fig. 1.15. In these type of
solar dryers the direct exposure of the crop to the sunlight enhances the color ripening desired in
certain varieties of grapes, dates, coffee and development of full flavour in roasted beans. Two basic
types of dryers in this category can be identified as the cabinet and greenhouse dryers.

23


Figure 1. 16. A typical natural-circulation
solar-energy cabinet dryer (Ekechukwu and
Norton, 1999)

Figure 1. 15 Features of a typical integral-type
(direct) natural-circulation solar-energy dryer

1.10.3.1 SOLAR CABINET DRYERS
The passive solar cabinet dryers are generally simple and inexpensive units having high
applications for domestic purposes. They are suitable for drying of agricultural products, spices and
herbs etc., constructed normally with a drying area of 1-2 m2 and capacities of 10-20 kg. Figure 1.16
shows a typical passive solar cabinet dryer. The heat for drying is transmitted through the glass
cover and is absorbed on the blackened interior and crops as well. The required air circulation is
maintained by the warm moist air leaving via the upper vent under the action of buoyancy forces
and generating suction of fresh air from the base inlet. Pioneering works on solar cabinet dryers
were reported by the Brace Research Institute, (1980).

24


Figure 1. 17 A modified natural-circulation Figure 1. 18 A natural-circulation solar-energy
solar-energy cabinet dryer(Ekechukwu cabinet dryer with chimney (Ekechukwu and
Norton, 1999)
and Norton, 1999)
A number of other designs of passive solar cabinet dryer in configuration to that developed by
Brace Research Institute have been built and tested for a variety of crops and locations. Ezekwe
(1981) reported a modification of the typical design shown in Fig 1.17 equipped with a wooden
plenum guiding the air inlet and a long plywood chimney to enhance natural circulation,
accelerating the drying rate by about 5 times over open sun drying. Figure 1.18 shows the design by
Henriksson and Gustafsson (1986) with mesh work floor and a chimney with black PVC foil facing
the southwards (sunlight). The passive solar cabinet dryer have an advantage of being cheap and
easy in construction from locally available material, however their major drawback is poor moist
removal rates and very high temperature (70-100 ) causing overheating of the product.

1.10.3.2

NATURAL-CIRCULATION GREENHOUSE DRYERS

These are also called as tent dryers and are basically modified greenhouses. They are designed with
vents of appropriate size and position to have a controlled air flow. They are characterized by
extensive glazing by the transparent cover of polyethylene sheet. Figure 1.19 shows the earliest
form of passive solar greenhouse dryer by the Brace Research Institute, with slanted glass roof,
allowing direct solar radiation over the product. The length-wise north-south alignment of the
dryer had black coated internals for improved absorption of solar radiation with the ridge-cap over
the roof for exit vent. Doe et al. (1977) later designed the widely reported poly-ethylene tent dryer,
illustrated in figure 1.20 consisting of a ridged bamboo framework clad with a clear polythene sheet
over it. A black poly-ethylene sheet was also spread on the floor inside the tent to enhance the
absorption of solar radiation. The air flow into the tent was controlled by rolling/ unrolling of the
cladding at the bottom edge of front side and the vents at the top served as the exit for the moist
exhaust air.
Sachithananthan et al. (1983) reported a horticultural greenhouse of clear plastic sheet cladding
over a semi-cylindrical metal frame (Fig 1.21). The modification were with a black galvanized iron
25


sheet absorber at floor, inlet vents along the full length of both sides of base and exit with plastic
nets at the top to protect from insects and dust.
Fleming et al. (1986) reported a typical greenhouse type solar dryer with a transparent semicylindrical chamber with a cylindrical solar chimney posted vertically at one end and a door for air
inlet and access to the chamber at other end as shown in figure 1.22. Rathore et al. (2010) has
conducted various experimental studies on a modified design of hemi-cylindrical solar tunnel dryer
for drying of grapes also few researchers (Jaijai et al., 2011) have used a polycarbonate cover for its
construction. Afriyie et al. (2011) has reported the study of simulation and optimization of a
chimney ventilated solar crop dryer.

Figure 1. 19 Natural-circulation glass-roof solarenergy dryer (Ekechukwu and Norton, 1999)

Figure 1. 20 Natural-circulation polythenetent dryer (Ekechukwu and Norton, 1999)

Figure 1. 21 Natural-circulation solar
dome dryer (Ekechukwu and Norton, Figure 1. 22 A greenhouse type naturalcirculation
solar–energy
dryer
1999)

1.10.4

HYBRID-TYPE PASSIVE SOLAR-ENERGY DRYERS

A hybrid type passive solar-energy dryer would have the same typical structural features as the
indirect-type and direct-type (i.e. a solar air heater, a separate drying chamber and a chimney), and
in addition has glazed walls inside the drying chamber so that the solar radiation impinges directly
on the product as in the direct-type dryers as shown in figure 1.23. Exell et. al. (1980), Sodha et al.
26


(1987), at Asian Institute of Technology developed the widely reported solar rice dryer (Fig. 1.24).
Ayensu and Asiedu (1986) designed the hybrid dryer consisting of an air heater with a pile of
granite functioning as an absorber cum heat storage (Fig 1.25). Sauliner first reported a multi
stacked hybrid design different from that of Exell, followed by Lawand at Brace Research Institute
also designed and tested by Sharma et al. (1986, 1987), as shown in figure 1.26. The multi-stacked
design enables the simultaneous drying of a variety of crops. Several other designs of the hybridtype passive solar energy have also been reported in (Amer et al., 2010; Hughes et al., 2011).

27


Figure 1. 24 A mixed-mode natural-circulation
solar rice dryer (Ekechukwu and Norton, 1999)
Figure 1. 23 Features of a typical mixedmode natural-circulation solar-energy dryer

Figure 1. 25 A mixed-mode naturalcirculation solar-energy dryer with thermal
storage (Ekechukwu and Norton, 1999)

1.11

Figure 1. 26 A multi-stacked mixed-mode natural
circulation solar-energy dryer (Ekechukwu and
Norton, 1999)

ACTIVE SOLAR DRYING SYSTEMS

28


Active solar drying systems are designed incorporating external means, like fans or pumps, for
moving the solar energy in the form of heated air from the collector area to the drying beds. Thus all
active solar dryer are, by their application, forced convection dryer. A typical active solar dryer
depends on solar-energy only for the heat source, while for air circulation uses motorized fans or
ventilator. These dryers find major applications in large-scale commercial drying operations in
combination with conventional fossil-fuel to have a better control over drying by consolidating the
effect of fluctuations of the solar insolation on the drying air temperature. Active solar dryers are
known to be suitable for drying higher moisture content foodstuffs such as papaya, kiwi fruits,
brinjal, cabbage and cauliflower slices. A variety of active solar-energy dryers exist which could be
classified into either the direct -type, indirect-type or hybrid dryers.

1.11.1

INDIRECT-TYPE ACTIVE SOLAR DRYING SYSTEMS

These active dryers as discussed for indirect dryer in section 2.3 have a separate collector and
drying unit. They are generally comprised of four basic components viz., a solar air heater, drying
chamber, a fan for air circulation and ductings. Due to the separate air heating unit higher
temperatures can easily be obtained with a control on air flow rate. However as the efficiency of
collector decreases at higher temperature operation, an optimum temperature and airflow rate has
to be determined to have a cost effective design. While most solar collectors are madeup of metal or
wood absorbers with appropriate coatings, materials like black polythene are also used as they
form an economic substitute. Fig. 1.27 shows a typical indirect-type active solar dryer. A few
designs also employ the recirculation of drying air, that ensures low exhaust air temperature and
thereby efficient use of energy. Fig 1.28 illustrates a system employing partial air-recirculation, in a
polyethylene-tube solar collector. The efficiency of the indirect-type active solar dryer also depends
on the location of the fan, though not so significantly in small batches. The prime objective of the fan
is to maintain a desired flow-rate in the drying cabinet causing uniform evaporation of moisture
from the wet material and in the collector is the collection of heat maintaining a negative pressure,
reducing the heat losses. Fig. 1.29 shows the role of the fan in a typical continuous flow in an active
solar dryer, in vertical bin for grain drying. Slama et al. (2011) has recently developed and
evaluated a forced convection solar dryer for drying of orange peels.

Figure 1. 27 Features of a typical distributedtype active solar energy dryer (Ekechukwu and

29

Norton, 1999)

Figure 1. 28 A distributed-type active solar
dehydrator with partial air re-circulation

Figure 1. 29 A continuous-flow active grain dryer (Ekechukwu and Norton, 1999)

1.11.2

DIRECT-TYPE ACTIVE SOLAR-ENERGY DRYING SYSTEMS

The direct-type active solar dryers are designed with an integrated solar energy collection unit.
Generally, three distinct designs of direct-type active solar dryers can be identified viz., the
absorption type, storage type and greenhouse dryers.

1.11.2.1 ABSORPTION DRYERS

The direct absorption dryer as the name suggests are designs of the direct-type active dryers
wherein the crops absorb the solar radiation directly. Typical practical design for large-scale
commercial forced-convection greenhouse dryer, are of the type of solar kilns for timber drying
(Fig 1.30), transparent roof solar brans as shown in Figure 1.31 and small scale force-convection
dryers equipped with auxiliary heating in Figure 1.32. Typical designs include with the roof or wall
of the dryer functioning as a collector of the drying chamber. Figure 1.33 shows the greenhouse
dryer designed by Huang et. al (1981) with a semi-cylindrical structure made of a Tedlar coated
clear corrugated fibre-glass and an internal dry chamber of rotary or stationary drum with a blackpainted outer surface to effect solar absorbtion. Trim and Ko (1982) developed another design with
a clear plastic outer cover and a black plastic interior and Akachukwu (1986) described the dryer
30


with a solar kiln design for timber drying with a single glazing of horticultural-grade polythene with
an internal internal black-painted corrugated metal absorber over the timber stack.

Figure 1. 30 A forced-convection greenhouse
dryer (Ekechukwu and Norton, 1999)
Figure 1. 31 A forced-convection transparent-roof
solar barn (Ekechukwu and Norton, 1999)

Figure 1. 33 Interior-drum-absorber greenhouse
active solar dryer (Ekechukwu and Norton, 1999)
Figure 1. 32 Features of a typical active solarenergy cabinet dryer (Ekechukwu and Norton,
1999)

1.11.3

HYBRID-TYPE ACTIVE SOLAR-ENERGY DRYERS

The hybrid solar dryers combine the features of a solar energy with a conventional or some
auxiliary source of energy and can be operated either in combination or in single mode with either
source of energy. These dryers generally are medium to large installations operating in the range of
50-60%, and compensate the temperature fluctuations induced by the climatic uncertainties.
Figure 1.34 shows the features of a typical active-type hybrid solar dryer. Bena and Fuller (2002)
31


described a direct-type natural convection solar dryer combined with a simple biomass burner
suitable for drying fruits and vegetables in regions without electricity. Fig. 34 shows a hybrid solar
dryer, a modification of the dryer in Fig. 35 with auxiliary source of heating. The ambient air flows
through a heat exchanger where it is heated to the desired temperature, by combustion gas. Part of
the used air is exhausted from the north wall of the dryer and the rest is recycled through the
recycling tube and the cooled gas exits through the chimney to the ambient (Condori et al., 2001).
Amer et al. (2011) has recently designed and evaluated a hybrid solar dryer for drying of banana,
consisting of a heat exchanger and heat storage facility.

Figure 1. 35 Solar tunnel dryer with an
additional auxiliary heat source (Ekechukwu
and Norton, 1999)
Figure 1. 34 Features of a typical mixed-mode active
solar-energy dryer (Ekechukwu and Norton, 1999)

1.12

SELECTION OF SOLAR DRYERS

The diversity of food products has introduced many types and combinations of solar dryers to the
food industry. The methods of supplying heat and transporting the moisture and the drying product
are the basic variations among different types of solar dryers. Table 1.2 enumerates the typical
checklist for evaluation and selection for solar dryers.
Table 1.2 Typical checklist for preliminary evaluation and selection of solar dryers.
Sr. no

Parameters

1.

Physical features of dryer

2.

Thermal performance

Features
 Type, size and shape
 Collector area
 Drying capacity/loading density (kg/unit tray
area)
 Tray area and number of trays
 Loading/unloading convenience
 Solar insolation
 Drying time/drying rate
 Dryer/drying efficiency
32


3.

Properties of the material
being handled

4.

Drying characteristics of the
material

5.

Flow of material to and from
the dryer

6.

7.

Product qualities

Recovery problems

8.

Facilities available at site of
proposed installation

9.

Economics

10.

Other parameters





























Drying air temperature and relative humidity
Airflow rate
Physical characteristics (wet/dry)
Acidity
Corrosiveness
Toxicity
Flammability
Particle size
Abrasiveness
Type of moisture (bound, unbound, or both)
Initial moisture content
Final moisture content (maximum)
Permissible drying temperature
Probable drying time for different dryers
Quantity to be handled per hour
Continuous or batch operation
Process prior to drying
Process subsequent to drying
Shrinkage
Contamination
Uniformity of final moisture content
Decomposition of product
Over-drying
State of subdivision
Appearance
Flavour
Bulk density

 Dust recovery
 Solvent recovery












Space
Temperature, humidity, and cleanliness of air
Available fuels
Available electric power
Permissible noise, vibration, dust, or heat losses
Source of wet feed
Exhaust-gas outlets
Cost of dryer
Cost of drying
Payback
Skilled technician and operator requirements,
33


 Safety and reliability
 Maintenance
1. Initial selection of solar dryers.
The initial selection can be of the type of solar dryer that appears best suited for handling the
wet material and dry product, that fits well into the continuity of the process as a whole, and
that produces the product of desired properties.
2. Initial comparison of solar dryers.
The solar dryers selected in step one should then be evaluated from the available cost and
performance data selecting a few. The rests should then be eliminated from further
considerations.
3. Solar Drying tests.
Drying tests should then be conducted in small-scale units based on the solar dryers selected in
step two. These tests are expected to establish the optimum operating conditions, the ability of
the dryer to handle the material physically, product quality and characteristics, and dryer size.
Occasionally, simple laboratory experiments can serve to reduce further the number of dryers
under consideration, as there is rare scope for changes to be made once the dryer is installed.
Based on the results of these drying tests that establish size and operating characteristics
formal quotations and guarantees should be obtained from dryer manufacturers. Initial costs,
installation costs, operating costs, product quality, dryer operability, and dryer flexibility can
then be given proper weight in final evaluation and selection.
4. Final selection of solar dryer.
From the results of the drying tests and quotations, the final selection of the most suitable solar
dryer can be made.
The physical nature of the material to be handled is one of the primary features for consideration as
these are different for coarse solid, slurry and sheet material. After the preliminary selection of the
suitable type of solar dryer, a further analysis fo revaluation of its size and cost should be done to
select the most economical one. After analysing adequate data on the cost and performance
obtained from the equipment manufacturers the factors governing their performance should be
due considered. This may involve at times compromising certain steps that precede or follow
drying process, like sorting , conveying or packaging and hence must be considered carefully.

34


Test case of solar drying at ICT Mumbai

Solar cabinet drying of fish
A comparative study was carried out at ICT, Mumbai to compare various aspects of drying kinetics
and quality of fish and vegetable dried by open sun, solar cabinet, hot air and freeze drying.

Figure 1.36 Pictorial view of Solar Cabinet Dryer at ICT-Mumbai (Visavale, 2009)
The solar cabinet dryer consisted of an indirect forced convection solar dryer with a solar
air collector, a blower for air circulation and a drying cabinet The solar air collector had dimensions
of 4 × 5 m. A plain copper sheet painted black was used as an absorber plate for incident solar
radiation. It was oriented southward under the collector angle of 31°. The collector and blower
system was well insulated to prevent heat losses. The drying cabinet dimensions were length 1.20
m, breadth 0.76 m and height 0.40 m constructed with insulated walls and had 16 shelves. A
centrifugal blower (capacity 1000 m3/h) connected to the drying cabinet provides a maximum air
velocity of 1.0 m/s. The circulation fan that supplies fresh air has a power of 1.5 kW. During
experiments the air velocity, temperature and relative humidity in the cabinet were in the range
0.9-1.0 m/s, 40-60°C and 50-65%, respectively. The sample data of solar intensity and respective
inlet air temperature in the cabinet were recorded during one of the drying experiments and shown
graphically in Figure 1.37. Geographical location of Mumbai is at 18.96º N and 72.82º E on the sea
coast at the average height between 10 to 15 m over the sea level. Summer is hot and humid, while
winter is cool and dry. The tray loading density was 4 kg/m2. To collect the drying data, tray at the
centre was selected and weight loss was recorded at predetermined time intervals. The product
was uniformly spread on 16 mesh trays which were placed in the drying cabinet. In solar drying
processes, the drying air temperature can vary based on the magnitude of the solar radiation.
However, the recycle damper was used for controlling the drying air temperature to some extent.
The amount of solar radiation was measured with a pyranometer. Temperature and relative
humidity were measured by thermocouple and RH-meter, respectively (Jadhav et al., 2010; Mathew
et al., 2010).

35


1.12 × 0.76 × 0.40 m
0.025 m
Al/ SS-316
0.56 x 0.76 x 0.025
0.025 m
mesh tray
16
0.002 m
0.20 m
Forced convection
1000 m3/h
~1m/s
Mumbai, India. (18° 53”N & 72°50"E)
20 m2
10
Black painted copper sheet
0.05 m
Plexifoam
Air
25°, Facing south
Glass
1mx2m
20 G
Rectangular aperture of 0.125 m2
70
Forced convection
Insolation

700

Temperature

600

60

50

500
Insolation,
(W m-2)

40

400
30

300

20

200

10

100
0
8

Temperature at the dryer inlet,°C

Table 1.3. Solar cabinet dryer configuration
Drying Cabinet
Gross dimensions
Spacing between two trays
Tray material
Tray Dimensions
Space between two trays
Tray type
Number of trays
Tray thickness
Outlet pipe diameter
Mode of air flow
Air flow rate
Air velocity in cabinet
Geographical Location
Collector
Area of absorbing surface
Number of absorbing surface
Absorbing material
Insulation thickness
Insulation material
Transfer fluid
Collector tilt angle
Cover plate material
Dimensions of each panel
Absorber plate thickness
Air passage
800
Mode of air flow

0
9

10

11

12

13

14

15

16

17

18

Time of the day, (h)
36


Figure 1.37 Variation of solar radiation throughout the day during day of drying
experiment (March 23, 2008) (Visavale, 2009)

The plot for drying rate versus average moisture content for six types of fish using different
dryers are shown in fig 1.38-1.43. The specific energy consumption was estimated for the three
types of dryers viz., SCD, FD and HAD (Visavale et al., 2011, Visavale 2009), considering the total
energy supplied to dry the fish from initial moisture content of about 74-87% to a moisture content
of 5-6 % (approx.). Open sun drying was considered as a non-energy intensive process as only solar
radiation was used to dry the samples. The specific energy consumption, H, is expressed as follows
(Sharma and Prasad, 2004).

H

Total energy supplied in drying process
, kJ/kg water evaporated
Amount of water removed during drying

(1.16)

Enthalpy of the drying air was estimated from psychometric properties. Specific energy
consumption in drying process, H is defined as follows

H

h1  h 2  h 3  h 4
, kJ/kg water evaporated
Amount of water removed during drying

(1.17)

where, h1 is energy required to heat the air, kJ; h2 is energy requirement of the air blower, kJ; h3 is
energy requirement of the vacuum pump, kJ; h4 is energy requirement for freezing the fish sample
before freeze drying, kJ. In case of SCD only h2 was estimated whereas for HAD, both the h1 and h2
values were calculated and for FD only h3 and h4 were considered.
All drying methods were compared using one way ANOVA at 5% level of significance. The
values of critical difference were calculated for all the quality parameters and specific energy
consumption and pair wise comparison between drying methods was done to find the significant
difference. The best drying method was selected on the basis of selected quality attributes like
rehydration ratio, change in color, hardness, sensory quality (overall acceptability) and specific
energy consumption.
Specific energy consumption for selected fish (bombay duck, prawn, pomfret, mandeli,
jawala, ribbon fish) dried using four methods is given in Table 1.4-1.5. There was no significant
difference (p>0.05) in specific energy consumption during drying of fish by OSD and SCD. It can be
observed that, OSD is least energy intensive process as solar radiation is freely available whereas,
FD was the most expensive drying process with 21,200 and 13,507 kJ/kg water removal Esp for
Bombay duck and Prawn, respectively as high energy is required for prefreezing at (-10°C for 10 h)
and to maintain vacuum. After FD, HAD was energy intensive process with 18,950 and 13,507 kJ/kg
water removal Esp for Bombay duck and Prawn, respectively. SCD was found to be significantly low
energy intensive process saving 88 to 93% (approx.) energy compared to FD and HAD. In SCD only

37


electric energy was used for air recirculation which was quite negligible whereas, free solar
radiation was used to heat the drying air.

38


0.050

-1

Average drying rate, kg water kg dm min

-1

OSD
SCD
FD
HAD

0.040

0.030

0.020

0.010

0.000
0

1

2
3
4
5
6
7
Average moisture content, kg water kg-1dm

8

-1


-1

Figure 1.38 Effect of drying method on variation of drying rate with average
moisture content for Bombay duck

OSD
SCD
FD
HAD

0.060
0.050
0.040
0.030
0.020
0.010
0.000
0

1
2
3
4
-1
Average moisture content, kg water kg dm

5

Figure 1.39 Effect of drying method on variation of drying rate with average
moisture content for Prawn


Average drying rate, kg water kg-1 dm min-1

0.3000

OSD
SCD
FD
HAD

0.2500

0.2000

0.1500

0.1000

0.0500

0.0000
0

1

2

3

4

5

6

7

8

9

10

Average moisture content, kg water kg-1dm

Figure 1.40 Plot for drying rate vs average moisture content
for Jawala
0.0500

0.0400

-1


-1

OSD
SCD
FD
HAD

0.0300

0.0200

0.0100

0.0000
0

0.5

1
1.5
Average moisture content, kg water kg -1 dm

2

2.5

Figure 1.41 Plot for drying rate vs average moisture content for Mandeli

0.0100

0.0080

-1


-1

OSD
SCD
FD
HAD

0.0060

0.0040

0.0020

0.0000
0

0.5

1

1.5

2

2.5


OSD
SCD
FD
HAD

0.0150

-1


-1

Figure 1.42 Plot for drying rate vs average moisture content for Pomfret

0.0100

0.0050

0.0000
0

0.5

1
1.5
2

2.5

3

Figure 1.43 Plot for drying rate vs average moisture content for Ribbon fish


Table 1.4 Quality and other drying parameters of Bombay duck, Prawn and Jawala dehydrated using various drying methods (Visavale,
2009)
Density,
RR

Dryer

Esp,

Shrinkage, %

E

kg m-3
25 °C

100 °C

True

Deff

kJ kg-1

×109,

Hardness,
OA

DT,h

g
m2s-1

Bulk

Bombay duck: IMC ~85% (w.b)
OSD

1.75a

0.85a

612.18a

112.12a

33.23a

1744.3a

5.12a

0a*

0.31

17.33

SCD

2.55b

1.13b

56.24b

700.12b

122.82b

21.33b

971.68b

6.86b

2098a

1.01

10.00

HAD

2.45b

1.12b

55.15b

760.52c

128.18b

19.29b

1121.52c

6.95b

18950 b

1.51

7.50

FD

2.85c

1.15b

5.32c

400.23d

65.23c

5.34c

892.61d

8.87c

21200 c

0.70

9.83

410.25a

85.21a

38.58a

1443.33a

5.87a

0a*

0.08

68.24a

Prawn: IMC ~78% (w.b)
OSD

2.56a,c

2.25a

SCD

3.2b

3.01b

45.24b

422.23a

87.81a

22.18b

1218.22b

6.66a

959a

0.21

4.00

HAD

3.05a,b

2.85b

43.23b

425.41a

89.19a

20.58b

1040.67c

6.89a

8081a

0.31

5.00

FD

3.65c

3.35d

6.41d

315.35b

50.12b

6.55d

516.00d

8.72b

13507b

0.10

6.00

65.14a

8.50

Jawala (Acetes indicus): IMC ~87% (w.b)
OSD

1.65a

0.75a

SCD

2.65b

1.23b

58.14a
54.18b

380.28a

91.51a

39.15a

1245.31a

4.28a

0a*

0.60

6.00

412.15b

98.14b

24.43b

645.24b

6.90b

625.14a

1.01

3.00

HAD

2.55b

1.15b

54.11b

450.14c

104.25b

17.19b

845.25c

6.95b

5023.19b

2.02

2.00

FD

2.90c

1.18b

4.12c

275.31d

45.15c

6.15c

541.29d

8.85c

12935.24c

0.40

6.00


Table 1.5 Quality and other drying parameters of Silver Pomfret, Mandeli and Ribbon fish dehydrated using various drying methods
(Visavale, 2009)

Density,
RR

Deff

Shrinkage,

E

kg m-3

Dryer
%
25 °C

100 °C

True

Hardness,

Esp,
OA

G

×109,

DT,h

kJ kg-1
m2s-1

Bulk

Silver pomfret (Pampus argenteus): IMC ~75% (w.b)
OSD

1.07a

0.65a

600.15a

102.31a

29.21a

15000.40a

5.05a

0a*

91.23

16.00

SCD

2.01b

1.03b

51.24b

695.21b

135.01b

18.11b

863.51b

6.88b

2367.03a

101.33

11.33

HAD

2.10b

1.11b

52.44b

724.55c

144.51b

18.55b

995.32c

6.91b

21037.13b

101.33

8.33

FD

2.51c

1.21b

4.85c

413.31d

58.32c

5.50c

780.24d

8.50c

30187.29 c

81.12

14.00

58.14a

Mandeli (Coilia dussumieri): IMC ~85% (w.b)
OSD

2.35a,c

2.05a

390.28a

75.12a

37.05a

1264.13a

5.80a

0a*

30.39

SCD

3.5b

3.15b

43.13b

412.58a

79.32a

25.19b

1108.05b

6.50a

1045.15a

61.08

5.00

HAD

3.05a,b

2.78b

42.14b

435.21a

80.72a

21.08b

988.06c

6.65a

7572.23a

4.05

3.00

FD

3.75c

3.45d

5.15d

305.12b

48.14b

5.75d

628.51d

8.88b

16168.97b

2.02

7.50

62.19a

8.00

Ribbon fish (Lepturacanthus savala): IMC ~85% (w.b)
OSD

1.78a,c

2.05a

650.11a

350.41a

41.22a

1503.81a

5.55a

0a*

20.27

SCD

2.25b

2.75b

44.05b

680.24a

412.44a

25.41b

1342.43b

6.65a

1668.40a

40.53

8.00

HAD

2.65a,b

2.78b

43.21b

690.12a

425.43a

23.32b

1303.53c

6.75a

15153.12a

50.60

6.00

FD

3.75c

3.55d

6.01d

495.44b

98.22b

5.78d

686.41d

8.85b

32314.35b

25.27

15.00

58.04a

18.00

*During OSD Esp was assumed 0.00 kJ/kg water removal as natural solar energy was freely available. Similar letters indicate non significant difference (p>0.05)


It was seen that for all fish, hot air drying exhibited highest moisture diffusivity followed
by solar cabinet and freeze drying, whereas, open sun drying showed the lowest diffusivity
values. Freeze-drying was the best method of drying for Bombay duck and Prawns resulting in
high quality dehydrated product, but it was high energy intensive process. Solar cabinet drying
was found to be low energy intensive process compared to freeze and hot air drying resulting in
acceptable quality dehydrated fish product.

1.12.1

ECONOMICS OF SOLAR DRYERS

Solar dryers are generally capital intensive and therefore the financial viability is the key to
successfully compete with any of the other dryers. The financial analysis generally includes the
cost of dryer (fixed cost), cost of drying (operating expenses) and payback. They can be viable
only if the annual cost of extra investment (on the solar dryer) could be balanced against fuel
savings, or if the equipment cost could be reduced (Jayaraman, 1995). The user or dryer
designer looks for a favourable combination of cost, energy efficiency, quality, and price of the
final product (Crapiste and Rotstein, 1997). Payback is the measure of time (number of
days/months/years) it takes to recoup the total investment made on a dryer, in the form of
operational cash inflow. Payback analysis does not measure the profitability of dryer as it does
not take the service life of dryer into consideration. This is discussed in detail by Brenndorfer et
al. (1987). Continued use of the dryer rather than seasonal use will decrease the drying cost and
payback (Arinze et al., 1996). Economic analysis on a solar dryer should also incorporate the
cost benefits due to improved quality, higher yields, less floor area and quicker drying
(Sreekumar, 2010).

1.12.2

DYNAMIC METHODS OF ECONOMIC EVALUATION

The economic evaluation of solar dryer ultimately aims at determining the payback time. For
the calculation of payback time the dynamic method of calculation takes into account the
influence of inflation. The following considerations summarize this method, according to Boer
(1978). Payback occurs when the accumulated savings S equals the sum of investment capital
(I) plus yearly interest and the accumulated costs (E):
(1.18)
The annual accumulated savings can be calculated from the net income D by reducing the mass
and quality losses and thus by increasing the marketing price of the product in case of natural
solar dryers.
For passive solar dryers, savings can be calculated from the price of the substituted
conventional energy D. Considering the annual interest rate r and the yearly inflation rate (e)
for the prices of energy, for n years the annual accumulated savings can be calculated as follows:
(1.19)
The sum of first investment cost (C) with interest will be, during n years,
Accumulated yearly costs, taking the annual fixed charge rate
equipment into consideration, will be

(1.20)
and inflation rate

for

(1.21)
Knowing and , diagrams can be made for the determination of payback time , referring to
values of
and . From these diagrams the requirements for the expected payback time can
be easily seen. Figure 1.44 shows the payback time as a function of
for various values of


, and , following Boer (1978). One can see from the diagram which
values can bring
about the desired payback time when the various other parameters are at given values. With
parameters differing from the above, the calculation must be made separately following
Equation 1.16 through Equation 1.19. A comparison of curves 1 and 2 indicates the influence of
interest rate r in cases with no inflation; curves 3 and 6, when compared, lead to the effect of
energy prices.

Figure 1. 44 Payback time in years as a function of D/C, with different parameters r, m, I, and e.
(Boer, 1978)

As can be seen in all the cases examined, the D/C ratio needed for a 10-year payback time falls
in the 0.12–0.23 range. Since payback time is a function of D/C, it is obvious that cheaper
(smaller C) and less efficient (smaller D) installations are justified insofar as realization of less
expense does not mean a significant decrease in the durability (lifetime) of the system. Payback
calculations refer to the whole solar energy drying system (Kwon et al., 1980 and Gordon and
Rabl., 1982). With appropriate division of the costs, there is of course nothing in the way of
making the calculation only for the collector system (Vaishya aet al., 1981 and Kovarik , 1978).
Construction of the collectors can be planned on the basis of the economic optimum
(Vijeysundera at al., 1982 and Chiou aet al., 1965). When the application of a solar dryer results
in improved quality of the dried product, the value of D is savings S and has to be increased in
relation to the value of the quality enhancement. Table 1.8 shows the economic analysis for
Bombay duck fish as a test case using the data given in table 1.7.
Table 1.6 Data used for economic analysis
Batch size approx.(kg)
40
Total drier cost (US$)

5,400

Depreciation (%)

10

Recovery period (yrs)

3

Product cost price (US$/kg)
Selling price (US$/kg)

0.1
3.4

Annual sale (US$)

4,000

Annual cost (US$)

1,600


Table 1.7 Economic Analysis for Solar Cabinet Drying at ICT Mumbai
Bombay duck
Product
(Harpadon nehereus)
I.M.C approx. (% w.b)
85
F.M.C approx. (% w.b)
8-10
Drying time (hours)
10
Drying cost (US$/kg)
0.16
Dried product obtained per annum (kg)
1200
Payback period (years)
1.8
Economic Analysis of Bombay duck:
Depreciation
= (5,400-540)/3 = 1,620 US$/y
Annual profit
= 4,000-1,600
= 2,400 US$
Annual earnings =2,400-(5,400-540) × (0.65)/3
= 1,347 US$/yr
Pay-back period = Fixed capital/(Annual earning +Depreciation)
= 5,400/ (1,347 + 1,620)
= 1.8 years

1.13

CONCLUSIONS










A comprehensive review of the fundamental principles governing the drying process
with classification of the practically realized various designs and selection of solar
dryers for drying of agricultural and marine products has been presented.
The classification clearly illustrates that the solar dryer designs can be grouped
systematically according to their operating temperature ranges, heating
sources/modes and operational modes.
It is difficult to have one single criteria for the selection of a solar dryer for a specific
region or a product as solar insolation and other parameters change frequently with
geographical location, however the general rules of thumb are mentioned to assist in
making the final selection.
The final selection is generally based on the available insolation rate, production
throughput, flexibility requirements, cost of fuel to run accessories as well as on the
experience and judgment of the fabricator.
As the solar dryer has a long life of about 20-40 years, the effect of a poor design can
have a long-term impact on the economic health of the dryer.


NOMENCLATURE
SCD
OSD
FD
HAD
Esp

solar cabinet dryer
open sun dryer
freeze dryer
hot air dryer
Specific energy consumption, kJ kg-1

DT

drying time, h
heat transfer area, m2

D

diameter, m

X

moisture content, kg/kg dry solid
mass of water, kg

dw/dθ

C, K
cp
cs
n
t
Z
DL
DL0
S
I
E
D
C

mass of dry solid, kg
saturation humidity, kg/kg dry air
partial pressure of water vapor in the air
vapour pressure of water
absolute pressure
percent relative humidity
humid heat of moist air, kJ/(kg⋅K)
temperature, K
heat-transfer coefficient by convection, J/(m2⋅s⋅K)
drying rate, kg water/s
mass-transfer coefficient, kg/(s⋅m2⋅atm)
thickness of bed, m
constants
specific heat, J kg-1 K-1
humid heat, J kg-1 K-1
number of molecules
void fraction
drying time
height of cylinder, m
effective diffusivity, m2 s-1
effective diffusivity at reference temperature, m2 s-1
savings
capital
accumulated cost
net income
investment cost
annual interest rate
annual inflation rate
years/ pay back period


Greek letters

λ

s

water activity,
drying time, h
latent heat of evaporation, J/kg
density, kg/m3
relative humidity
normalized drying rate, latent heat of vaporization, J kg-1

Subscripts
sp
c
f
t
mon
eq
w

specific
constant rate period
falling rate period
total
monolayer
equilibrium
water


REFERENCES
1.

2.

3.
4.
5.
6.
7.

8.
9.
10.
11.

12.
13.
14.
15.
16.
17.
18.
19.
20.

A. Sreekumar, 2010, Techno-economic analysis of a roof-integrated solar air heating
system for drying fruit and vegetables, Energy Conversion and Management, 51, pp. 22302238.
Afriyie, J.K., Rajakaruna, H., Nazha, M. A. A., Forson, F.K., 2011, Simulation and
optimisation of the ventilation in a chimney-dependent solar crop dryer, Solar Energy, 85,
pp. 1560-1573.
Aguilera, J. M., Oppermann, K., Sanchez, F., 1987, Kinetics of browning of sultana grapes.
Journal of Food Science, 52, pp. 990-994.
Akachukwu A. E., 1986, Solar kiln dryers for timber and agricultural crops, Int J Ambient
Energy, 7(2), pp. 95-101.
Amer, B.M.A., Hossain, M.A. , Gottschalk, K., 2010, Design and performance evaluation of a
new hybrid solar dryer for banana, Energy Conversion and Management, 51, pp. 813-820.
Anderson, R.B., 1946, Modification of the BET equation, Journal of the American Chemical
Society, 68, pp. 656–691.
Arinze, E. A., Sokhansanj S., Schoenau G. J., Trauttmansdorff F. G., 1996, Experimental
evaluation, simulation and optimisation of a commercial heated-air batch hay drier,
Journal of Agricultural Engineering Research, 63, pp. 301–314.
Ayensu A., Asiedu-Bondzie V., 1986, Solar drying with convective self-flow and energy
storage, Solar and Wind Technology, 3(4), pp. 273-279.
Baird, C.D., Deng, J.C., Chau, K.V., Heinis, J. J., Perez, M., 1981, Solar drying of seafood
products, ASAE, 3, pp. 640-644.
Bala, B.K., Mondol, M. R. A., 2001, Experimental investigation on solar drying of fish using
solar tunnel dryer, Drying Technology, 19, pp. 1532-1537.
Banout, J., Ehl, P., Havlik, J., Lojka, B., Polesny, Z., Verner, V., 2011, Design and performance
evaluation of a Double-pass solar drier for drying of red chilli (Capsicum annum L.), Solar
Energy, 85, pp. 506-515.
Belessiotis, V., Delyannis, E., 2011, Solar drying, Solar Energy, 85, pp. 1665-1691.
Bena, B., Fuller, R.J., 2002, Natural convection solar dryer with biomass back-up heater,
Solar Energy, 72 (1), pp. 75–83.
Bennamoun, L., Belhamri, A., 2003, Design and simulation of a solar dryer for agriculture
products, Journal of Food Engineering, 59, pp. 259–266.
Beuchat, L.R., 1981, Microbial stability as affected by water activity, Cereal Foods World 26
(7), pp. 345–349.
Boer, K.W., 1978, Payback of solar systems, Solar Energy, 20, 225–232.
Bolin, H.R., Huxsoll, C.C., Salunkhe, D.K., 1980, Fruit drying by solar energy, Confructa, 25,
147-160.
Bolin, H.R., Salunkhe, D.K., 1982, Food dehydration by solar energy, Crit. Rev. Food Sci.
Nutri, 16, pp. 327-354.
Brace Research Institute, 1980, Types of solar agricultural dryers, Sunworld, 4(6), pp. 181999.
Brenndorfer, B., Kennedy, L., Bateman, O., Trim, D. S., Mrema, G. C., Brobby, C. W., 1987,
Solar dryers—their role in post-harvest processing, London, Commonwealth Science
Council.


21.
22.
23.
24.
25.

26.
27.
28.
29.
30.
31.

32.
33.
34.
35.
36.

37.
38.

39.
40.
41.
42.
43.

Brunauer, S., Emmett, P.H., Teller, E., 1938, Adsorption of gases in multi-component
layers, American Chemical Society Journal, 60, pp. 309–319.
Cheema, L.S., Robeiro, C. M., 1978, Solar dryers of cashew, banana, and pineapple, Sun, 3,
pp. 2075-2079.
Chiou, J. P., EI-Wakil, M. M., Duffie, J. A., 1965, Solar Energy, 9, pp.73-76.
Condori, M., Echazu, R., Saravia, L., 2001, Solar drying of sweet pepper and garlic using the
tunnel greenhouse dryer, Renewable Energy, 22, pp. 447–460.
Crapiste, G. H., Rotstein, E., 1997, Design and performance evaluation of dryers, In:
Valentas Kenneth J, Rotstein Enrique, Singh Paul, editors. Handbook of food engineering
practice, New York: CRC Press.
de Boer. J.H., 1953, The Dynamic Character of Adsorption, Clarendon Press, Oxford.
Doe P. E., Ahmed, M., Muslemuddin, M., Sachithananthan, K. A., 1977, A polythene tent
dryer for improved sun drying of fish, Food Tech Aust; 29, pp. 437-441.
Eisen, W., Muhlbaner, W., Kutzback, H. D., 1985, Solar drying of grapes, Drying
Technology, 3, pp. 63-64.
Eissen, W., Muhlbauer, W., 1985, Solar drying of grapes, Drying Technology, 3 (1), pp. 63–
74.
Ekechukwu, O.V., 1999, Review of solar-energy drying systems I: an overview of drying
principles and theory, Energy Conversion and Management, 40, pp. 593–613.
El-Sebaii, A. A., Aboul-Enein, S., Ramadan, M. R., El-Gohary, H. G., 2002, Experimental
investigation of an indirect type natural convection solar dryer, Energy Conservation and
Management, 43, pp. 2251-2266.
Esper, A., Muhlbauer, W., 1998, Solar drying–an effective means of food preservation,
Renewable Energy, 15 (1–4), pp. 95–100.
Exell, R. H. B., 1980, A basic design theory for a simple solar rice dryer, Renew Energy Rev
J, 1(2), pp. 1-14.
Exell, R. H. B., 1980, A simple solar rice dryer- basic design theory, Sunworld, 4(6), pp.
186-190.
Ezekwe, C.I., 1981, Crop drying with solar air heaters in tropical Nigeria, ISES, Solar World
Forum, Brighton, UK, Pergamon Press, Oxford, pp. 997-1005.
Fleming, P. D., Norton. B., Ekechukwu, O. V., Onyegegbu, S. O., Probert, S. D., 1986, A largescale facility for experimental studies of natural-ciruclation solar-energy tropical crop
dryers. In: Proc Int Drying Symp (Drying '86). Cambridge, Mass., U.S.A., Hemisphere Pub
Coy Washington, pp. 685-693.
Gordon, J.M., Rabl, C. J., 1982, Solar Energy, 28, pp. 519-523.
Grainger, W., Othieno, H., Twidel, J.W., 1981, Small scale solar crop dryers for tropical
village use-theory and practical experiences, ISES, Solar World Forum, Brighton, UK,
Pergamon Press, Oxford, pp. 989-96.
Guggenheim, E.A., 1966, Application of Statistical Mechanics, Clanderon Press, Oxford.
Henriksson, R., Gustafsson, G., 1986, Use of solar collectors for drying agricultural crops
and for heating farm buildings, Energy in Agriculture, 5, pp. 139-150.
Hollick, J. C., 1999, Commercial scale solar drying, Renewable Energy,16, pp. 714–719.
Huang, B. K., Toksoy, M., 1981, Greenhouse solar system for selective year-round solarenergy utilization in agricultural production, Agric Energy, pp. 1-152.
Hughes, B. R., Oates, M., 2011, Performance investigation of a passive solar-assisted kiln in
the United Kingdom, Solar Energy, 85, pp. 1488-1498.


44.
45.
46.

47.

48.
49.

50.
51.
52.
53.
54.
55.
56.
57.
58.
59.

60.
61.
62.
63.
64.

Imre, L., 2004, Solar drying, In: Mujumdar, A.S., (Ed.), Handbook of Industrial Drying, vol. 1.
second ed, pp. 373–451.
Islam, M.N., Flink, J. M., 1982, Dehydration of potato-II, Osmotic concentration and its
effect on air drying behaviour, Journal of Food Technology, 17, pp. 387-403.
Jadhav, D .B., Visavale, G. L., Sutar, N., Annapure, U.S., Thorat B.N., 2010, Studies on Solar
Cabinet Drying of Green Peas (Pisum sativum), Drying Technology an International
Journal, 28, pp. 600-607.
Jadhav, D.B., Visavale, G. L., Sutar, P. P., Annapure, U.S., Thorat, B. N., 2010, Solar Cabinet
Drying of Bitter Gourd: Optimization of Pretreatments and Quality Evaluation,
International Journal of Food Engineering, 6, Iss. 4.
Jain D., 2006, Determination of convective heat and mass transfer coefficients for solar
drying of fish, Biosystems Engineering, 94, pp. 429–435.
Jain, D., 2005a, Modelling the system performance of multi-tray crop drying using an
inclined multi-pass solar air heater with in-built thermal storage, Journal of Food
Engineering, 71 (1), pp. 44–54.
Jain, D., 2005b, Modeling the performance of greenhouse with packed bed thermal storage
on crop drying applications, Journal of Food Engineering, 71 (2), pp. 170–178.
Jain, D., Pathare, P. B., 2007, Study the drying kinetics of open sun drying of fish, Journal of
Food Engineering, 78 (4), pp. 1315-1319.
Jain, D., Tiwari, G. N., 2003, Thermal aspects of open sun drying of various crops, Energy
28, pp. 37–54.
Jangam, S.V., Visavale, G.L., Mujumdar, A. S., 2011, Use of renewable source of energy for
drying of FVF, In Drying of Foods, Vegetables and Fruits – Vol 3, Chapter 6, pp. 103-126.
Janjai, S., Intawee, P., Kaewkiew, J., Sritus, C., Khamvongsa, V., 2011, A large-scale solar
greenhouse dryer using polycarbonate cover, Renewable Energy, 36, pp. 1053-1062.
Jayaraman., Gupta Das., 1995. In: Mujumdar AS, Handbook of Industrial Drying. New York:
Marcel Decker Inc.
Kalra, S.K., Bhardwaj, K. C., 1981, Use of simple solar dehydrator for drying fruit and
vegetable, Journal of Food Science Technology, 18, pp. 23-26.
Keey, R.B., 1978, Introduction to Industrial Drying Operations, 1st ed., Pergamon Press,
New York, chap. 2.
Keey, R.B., 1980, Theoretical foundations in drying technology, in Advances in Drying, Vol.
1 (A.S. Mujumdar, Ed.), Hemisphere, New York, pp. 1–22.
Kituu, G.M., Shitanda, D., Kanali, C.L., Mailutha, J.T., Njoroge, C.K., Wainaina, J.K., Silayo,
V.K., 2010, Thin layer drying model for simulating the drying of Tilapia fish (Oreochromis
niloticus) in a solar tunnel dryer, Journal of Food Engineering, pp. 325-331.
Kostaropoulos, A. E., Saravacos, G. D., 1995, Microwave pretreatment for sun dried raisins,
Journal of Food Science, 60, pp. 344-347.
Kovarik, M., 1978, Solar Energy, 21, pp. 477-511.
Kwon, O. Y., Catania. P. J., 1980, in Drying ‘80 (A.S. Mujumdar, Ed.), Hemisphere, New York,
pp. 445-448.
Langmuir, I., 1918, The adsorption of gases on plane surfaces of glass, mica and platinum,
Journal of the American Chemical Society, 40, pp. 1361–1402.
Leon, M. A., Kumar, S., Bhattacharya, S.C., 2002, A comprehensive procedure for
performance evaluation of solar food dryers, Renewable and Sustainable Energy Reviews,
6, pp. 367-393.


65.
66.

67.
68.
69.

70.
71.
72.
73.

74.
75.
76.
77.
78.
79.
80.
81.

82.
83.

84.

85.

Lutz, K., Mulhbaner, W., 1986, Solar tunnel dryer with integrated collector, Drying
Technology, 4, 583-603.
Mathew, S., Visavale, G., Thorat, B.N., 2010, Making order in the Cabinet- Integrating CFD
in the Green energy design process for food industry helps identify and fix causes for
uneven drying in a Solar Cabinet Dryer, 2010-Ansys India Users Conference-Bangalore.
Mekhilefa, S., Saidurb, R., Safari, A., 2011, A review on solar energy use in industries
Renewable and Sustainable Energy Reviews, 15, pp. 1777–1790.
Miller, W.M., 1985, Solar drying coupled with solid desiccant energy storage, Trans. ASAE,
1985, 28, pp. 649-656.
Montero, I., Blanco, J., Miranda, T., Rojas, S., Celma, A. R., 2010, Design, construction and
performance testing of a solar dryer for agro-industrial by-products, Energy Conversion
and Management, 51, pp. 1510-1521.
Moyls, A.L., 1986, Evaluation of a solar fruit dryer, Canadian Agricultural Engineering, 28,
pp. 137-144.
Mujumdar, A.S., 2007, Handbook of Industrial drying, Taylor and Francis group, U.K.
Nandwani, S. S., 2007, Design construction and study of a hybrid solar food processor in
the climate of Costa Rica, Renewable Energy, 32, pp. 427–441.
Pangavhane, D. R., Sawhney, R. L., Sarsavadia, P. N., 1999, Effect of various dipping
pretreatment on drying kinetics of thompson seedless grapes, Journal of Food Engineering,
39, pp. 211-216.
Pangavhane, D. R., Sawhney, R. L., Sarsavadia, P. N., 2002, Design, development and
performance testing of a new natural convection solar dryer, Energy, 27, pp. 579-590.
Perry, J.H., Chemical Engineering Handbook, 2007, 8th ed., McGraw-Hill, New York, pp. 28.
Posomboon, W., 1998, Processing effect on quality of dried shrimp [Master of
Engineering], Asian Institute of Technology, Bangkok, Thailand.
Rahman, S., 1995, Food Properties Handbook. Contemporary Food Science Series. CRC
Press.
Raouzeous G.S., Saravacos, G.D., 1986, Solar drying of raisins, Drying Technology, 4, pp.
633-649.
Rathore, N.S. , Panwar, N.L., 2010, Experimental studies on hemi cylindrical walk-in type
solar tunnel dryer for grape drying, Applied Energy, 87, pp. 2764-2767.
Reza, M. S., Bapary, M. A., Islam, M. N., Kamal, M., 2009, Optimization of marine fish drying
using solar tunnel dryer, Journal of Food Processing and Preservation, 33, pp. 47 – 59.
Roca, E., Guillard, V., Guilbert, S., Gontard, N., 2006, Moisture migration in a cereal
composite food at high water activity: Effects of initial porosity and fat content, Journal of
Cereal Science, Vol. 43, pp. 144-151.
Sachithananthan, K., Trim D., Speirs C. I., 1983, A solar dome dryer for drying of fish, FAO
Fisheries Paper, Rome, Italy. RAB/81/002/INT/L8.
Sankat C. K., Mujaffar, S., 2004, Sun and solar cabinet drying of salted shark fillets, Drying’
2004 – Proceedings of the 14th International Drying Symposium, São Paulo, Brazil, C, pp.
1584-1591.
Shakya, B.R., Flink, J.M., 1986, Dehydration of potato III, Influence of process parameters
on drying behavior for natural convection solar drying conditions, Journal of Food
Processing and Engineering, 10, pp. 127-143.
Sharma, A., Chen, C.R., Lan, N. V., 2009, Solar-energy drying systems: A review, Renewable
and Sustainable Energy Reviews, 13, pp. 1185-1210.


86.
87.
88.
89.

90.
91.
92.
93.
94.
95.
96.

97.
98.
99.
100.

101.

102.
103.
104.

105.
106.

Sharma, G. P.; & Prasad, S. Effective moisture diffusivity of garlic cloves undergoing
microwave convective drying. Journal of Food Engineering 2004, 65, 609–617.
Sharma, S., Ray, R. A., Sharma, K., 1987, Comparative study of solar dryers for crop drying.
Invention Intelligence, 22, pp. 105-113.
Sharma, S., Ray, R. A., Sharma, V. K., 1987, Comparative study of solar dryers for crop
drying, Invent Intell, pp. 105-113.
Sharma, V. K., Sharma, S., Ray, R. A., Garg, H. P., 1986, Design and performance studies of a
solar dryer suitable for rural application, Energy Conversion and Management, 26, pp.
111-119.
Singh, S., Subodh, K., 2011, Testing method for thermal performance based rating of
various solar dryer designs, Solar Energy, In Press.
Slama, R. B., Combarnous, M., 2011, Study of orange peels dryings kinetics and
development of a solar dryer by forced convection, Solar Energy, 85, pp. 570-578.
Sodha, M. S., Bansal, N. K., Kumar, K., Bansal, P. K., Malik, M. A. S., 1987, Solar Crop Drying
1, West Palm Beach, CRC Press.
Toei, R., 1996, Theoretical fundamentals of drying operation, Drying Technology, 14 (1),
pp. 1–194.
Treybal, R.E., Mass Transfer Operations, 3rd ed., McGraw-Hill, New York, 1980, chapter 7
and 12.
Trim, D. S., Ko, H. Y., 1982, Development of a forced-convection solar dryer for red
peppers, Tropical Agriculture (Trinidad), 59(4), pp. 319-323.
Usub, T., Lertsatitthankorn, C., Poomsa-ad, N., Wiset, L., Siriamornpun, S., Soponronnarit,
S., 2010, Thin layer solar drying characteristics of silkworm pupae, Food and Bioproducts
Processing, 88, pp. 149-160.
Vaghani, S.N., Chundawat, B. S., 1986, Sun drying of Sapota fruits (Achras sapota L.). Indian
Food Packer, 40, pp. 23-28.
Vaishya, J. S., Subrahmaniyam, S., Bhide, V.G., 1981, Solar Energy, 26, pp. 367-371.
Van Arsdel, W.B., 1965, Food dehydration, Food Technology, 19, pp. 484-487.
Van der Berg, C., 1981, Water activity and its estimation in food systems, theoretical
aspects, In: Rockland, B., Stuwart, G.F. (Eds.), Water Activity: Influence on Food Quality,
Academic Press, New York.
Van der Berg, C., 1984, Description of water activity of foods for engineering purposes by
means of a Gab model sorption, In: McKenna, B.M. (Ed.), Engineering and Food, vol. 1.
Engineering Sciences in the Food Industry, Elsevier, New York.
Vijeysundera, N. E., Ah, L.L., Tijoe, L.E., 1982, Solar Energy, 28:pp. 363-367.
Visavale, G.L., 2009, Design and Characteristics of Industrial Drying Systems. Ph. D. thesis,
Institute of Chemical Technology, Mumbai, India.
Visavale G. L., Sutar P. P. and B. N. Thorat., 2011, Comparative Study on Drying of Selected
Marine Products: Bombay Duck (Herpodon nehereus) and Prawn (Penaeus indicus),
International Journal of Food Engineering, Vol. 7: Iss. 4, Article 20.
Wagner, C. J., Coleman, R.L., Berry, R. E., 1979, A low cost small scale solar dryer for
Florida fruits and vegetables, Proc. Florida State Horticultural Society, 92, pp. 180-183.
Xingxing, Z., Xudong, Z., Stefan, S., Jihuan, X., Xiaotong, Y., 2012, Review of R&D progress
and practical application of the solar photovoltaic/thermal (PV/T) technologies,
Renewable and Sustainable Energy Reviews, 16, pp. 599-617.

Principles, Classification and Selection of Solar Dryers

More Related Content

What's hot (20)

Similar to Principles, Classification and Selection of Solar Dryers (20)

More from iMentor Education (20)

Recently uploaded (20)

Principles, Classification and Selection of Solar Dryers