Natural Resources, 2011, 2, 92-97
doi:10.4236/nr.2 011.22012 Published Online June 2011 (http://www.scirp.org/journal/nr)
Exergetic Analysis of Solar Energy drying Systems
Bukola Olalekan Bolaji
Department of Mechanical Engineering, University of Agriculture, Abeokuta, Nigeria.
Received December 7th, 2010; revised June 3rd, 2011; accepted June 10th, 2011.
ABSTRACT
An exergetic analysis of three basic types of solar drying systems is presented. The analysis is used to find the available
useful energy and the quality of energy that is obtainable from the dryers. The dryers were installed side by side and
tested simultaneously to eliminate influence of solar radiation and environmental changes in comparing their perform-
ances. The results obtained show that mixed mode and indirect mode solar dryers are more effective in utilizing the
captured energy than direct mode dryer, and mixed mode has a slight edge in superiority over indirect mode system.
78.1% and 77% of energy collected in the mixed mode and indirect mode systems, respectively, were available as useful
energy, while direct mode system could only convert 49.3% of collected energy to useful energy. The overall exergetic
efficiencies of mixed mode, indirect mode and direct mode systems were found to be 55.2%, 54.5% and 33.4%, respec-
tively.
Keywords: Solar, Exergy, Direct, Indirect, Mixed Mode , D ryer
1. Introduction
Solar drying is one of the important means of utilizing
solar energy for low and moderate temperature applica-
tions. Solar drying of crops and some fruits and vegeta-
bles has been practiced in various parts of the world for
centuries the conditions in tropical countries make the
use of solar energy for drying foods particularly attrac-
tive. The introduction of solar dryers in developing
countries can reduce crop losses and improve the quality
of dried product significantly compared to traditional
drying methods [1-3].
Solar drying may be classified into direct, indirect and
mixed modes [4]. In direct solar dryers the air heater
contains the grains and solar energy passes through a
transparent cover and is absorbed by the grains. Essen-
tially, the heat required for drying is provided by radia-
tion to the upper layers and subsequent conduction into
the grain bed. In indirect dryers, solar energy is collected
in a separate solar collector (air heater) and the heated air
then passes through the grain bed. In the mixed mode
type of dryer, the heated air from a separate solar collec-
tor is passed through a grain bed, and at the same time,
the drying cabinet absorbs solar energy directly through
the transparent walls or roof. Grain is dried simultane-
ously by both direct radiation through the walls or roof
of the cabinet and the convection of heat from the solar
collector.
Several authors [5-8] have reported both theoretical
and experimental studies on the optimization of solar air
heaters. Most of them consider the pertinent performance
indicator to be the collector’s thermal efficiency. Also,
many other researchers [9-11] have developed design
principles for various classes of solar drying systems. In
their studies, thermal efficiency was used to evaluate the
performance of the systems.
The ideal thermodynamic efficiency of a system is the
ratio of useful work performed to the amount of energy
supplied to the system. Since the solar collector absorbs
energy at a higher temperature than the ambient, the en-
ergy will be partially converted to thermal energy in the
system and partially lost to the environment. Therefore,
for the evaluation of the thermal performance of such a
system, descriptive parameter that rates the quantity and
quality of energy is required. The objective of this study
is to use exergetic analysis that employed heat exergy
and anergy as descriptive parameters to rate the avail-
ability of energy in three different types of solar dryers
(direct, indirect and mixed modes).
2. Material and Methods
2.1. Basic Theory
The energy gained by the collector can be expressed by
the following relation [12]:

Exergetic Analysis of Solar Energy Drying Systems 93

a

Q= IAUA
ucLcc
TT   (1)
where, c = area of transparent cover (m2); = inci-
dent insolation (W/m2);
AI
U
L
= overall heat loss for the
collector (W/K);

= solar absorptance;

= trans-
mittance; c = collector temperature (K); a = ambi-
ent air temperature (K). The energy per unit area of the
collector is
T T

u
L
ca
c
Q
I
UTT
A
   (2)
If the heated air leaving the collector is at collector
temperature, the heat gained by the air
g
Q is:


g
apa ca
QmCTT
 (3)
where, = mass of air leaving the dryer per unit time
(kg/s−1);
a
m

p
a = specific heat capacity of air (kJ/kg·K).
The collector heat removal factor,
C
R
F
, is the quantity
that relates the actual useful energy gained of a collector,
Equation (1), to the useful gained by the air, Equation (3).
Therefore,



apa ca
R
cLc
mCT T
FAIUTT






a

(4)
or (5)

gcR Lcca
qAF IUATT 



2.2. Exergetic Analysis
In energy systems, not all the energy supplied is avail-
able to do work. The part of the supplied energy avail-
able to do the required work is known as Heat Exergy,
X
, while the unavailable energy is known as Heat An-
ergy, [13]. Total energy, . YEXY
Heat exergy, ra
r
TT
X
q
T
 



(6)
where, = rate of heat release (W); = temperature
at which heat is released (K).
qr T
Heat anergy, a
r
T
Y
T



q
(7)
Exergetic potential, ra
r
TT
T


 (8)
Therefore, Equations (6) and (7) can be expressed as
Equations (9) and (10) respectively:
X
q

 (9)

1Yq

 (10)
Exergetic efficiency,
x

is defined as:
out
xin
X
output exergy
input exergyX
 
therefore, out out
xin in
q
q








(11)
Thermal efficiency, th

is:
out
th in
q useful output energy
input energyq
  (12)
Therefore, Equation (11) becomes
out
x
th
in






(13)
These exergetic parameters were used in this study to
rate the effectiveness of three different types of solar
dryers.
2.3. The Experimental Set-up
The three types of solar dryers considered for compara-
tive investigations in this paper are direct, indirect and
mixed mode, and they are shown in Figure 1, 2 and 3
respectively.
2.3.1. Direct Mode Solar Dryer
This is essentially a rectangular cabinet with an inclined
transparent top cover of one sheet of 4 mm thick clear
Outlet vent
Inlet vent
Food item
Dryer stand
Access door
Transparent cover
Figure 1. Direct mode solar dryer.
Figure 2. Indirect mode solar dryer.
Solar collector
Inlet vent
Transparent cover
Chimney
Drying rack
Drying cabinet
Plenum chamber
Absorbers

94 Exergetic Analysis of Solar Energy Drying Systems
glass with a total surface area of 1220 mm by 900 mm.
The dryer cabinet is made of 25 mm plywood. The front
is higher than the rear giving the top cover an inclination
of about 17.5˚. This is approximately 10˚ more than
thelocal geographical latitude (Ado-Ekiti Nigeria, 7.5˚N),
which according to Adegoke and Bolaji [14], is the best
recommended orientation for stationary absorber. Air
inlet vent in the lower end of the front of the cabinet pro-
vide an entry for air which is then heated within the col-
lector metal base and rises to exhaust through outlet vent
in the upper parts of the back of the cabinet. The poten-
tial of the cabinet to absorb insolation is enhanced by
blackening all interior surfaces. The dryer is mounted on
legs to facilitate air entry through the inlet vent and to
reduce the risk of entry of pests into the cabinet.
2.3.2. Indirect Mode Solar Dryer
The dryer differs distinctly from direct mode dryer in
that the solar collector and the drying chamber are prin-
cipally separated as shown in Figure 2. Air is heated
during its flow through the collector and passes into the
drying chamber before exhausting through the chimney.
The function of the extended chimney is that the black
sides absorb insolation and so heat the air within, thereby
enhancing the natural convective flow of air through the
dryer. The drying chamber houses drying racks which
contained food item to be dried.
2.3.3. Mixed Mo de S olar Dryer:
In this dryer, the drying cabinet and solar collector are
separated like in the case of indirect dryer as shown in
Figure 3. But the dryer differs from the indirect mode in
that additional drying is achieved from direct solar radia-
tion incoming through the transparent walls and roof. An
Figure 3. Mixed mode solar dryer.
outlet vent is made at the upper end of back of the cabi-
net to facilitate and control the convection flow of air
through the dryer.
The solar collectors of both the indirect and mixed
modes are the same in their construction. The collectors
consist of an absorber back plate insulated with foam
material of about 30 mm thick at the bottom and covered
by transparent glass of 4 mm thick at the top. The solar
collectors are 420 mm × 980 mm × 150 mm in dimen-
sions; they are south-facing and titled 17.5˚ to the hori-
zontal so that they can take advantage of the maximum
insolation. Air enters through the inlet vent at the lower
end of the collectors. An absorber mesh screen midway
between the glass cover and the absorber back plate pro-
vides effective air heating because solar radiation that
passes through the transparent cover is then absorbed by
both the mesh and back-plate. The upper end of the col-
lectors is connected to the vertical drying cabinets, which
hold drying trays in layers.
2.4. Experimental Procedure
The solar dryers were tested and the parameters needed
for the evaluation of the systems were measured at in-
terval of one hour between the hours of 8.00 and 18.00.
Thermometers were placed through the walls in the col-
lectors and in the drying cabinets to measure their tem-
peratures. The relative humidity and ambient temperature
was measured using humidity-temperature meter. The
incident solar radiation intensity was measured using a
portable Kipps Solarimeter.
The dryers were installed side by side and tested si-
multaneously to eliminate influence of solar radiation
and environmental changes in comparing their thermal
performances. The dryers were used to dry shelled corn
(as a sample of food item). For each of the dryers, weight
of the food item was measured at the start and at
one-hour intervals thereafter. Knowing the initial weight
and the final weight at the point when no further weight
loss was attained, the weight loss was used to calculate
the moisture removed in kg water per kg dry matter at
intervals as the food item dried.
Outlet vent
Transparent roof
Drying rack
Drying cabinet
Transparent cover 3. Results and Discussion
Absorbers Plenum
chamber An exergetic analysis of the drying systems was carried
out to find out the effect of available energy (heat exergy)
on the performance of the systems. The total energy for
each of the dryers was calculated using Equation (5). The
exergetic potential, exergy and anergy were computed
using Equations (8), (9) and (10). The values of exergetic
efficiencies were obtained using Equation (13).
Inlet vent
Figure 4 shows a typical day hourly variation of an-
ergy (unavailable energy) for the solar dryers. It was
observed that the anergy in direct mode dryer increased
Solar collector
C opyright © 2011 SciRes. NR

Exergetic Analysis of Solar Energy Drying Systems 95
drastically reaching peak value of 63 kJ between 12.00
hr and 13.00 hr. On the other hand, the anergy was rela-
tively smaller in both indirect and mixed mode dryers.
This is an indication that direct mode system wasted
more energy than the other two systems. Also between
10.00 hr and 14.00 hr the anergy in indirect mode system
was found greater than that of mixed mode system, but
reverse was the case in the last 3 hours of effective sun-
shine.
Figure 5 shows the hourly variation of heat exergy for
the three systems. The figure indicates that more exergy
was available in both indirect and mixed mode systems
Figure 4. Variation of anergy with time in the solar dryers.
Figure 5. Variation of exergy with time in the solar dryers.
than direct mode system. The better results observed in
the mixed mode system justified the additional heat re-
ceived from direct solar radiation incoming through the
transparent walls and roof of the system. Also the ability
of the indirect mode system to retain energy and its bet-
ter performance especially during the hours of low solar
radiation intensity justified the insulation of its drying
chamber. Further analysis shows that 78.1% and 77% of
energy collected in the mixed mode and indirect mode
systems, respectively, were available as useful energy,
while direct mode system could only convert 49.3% of
collected energy to useful energy.
Figure 6. Drying curves of shelled corn in the solar dryers.
Figure 7. Variation of exergetic efficiency with time in the
solar dryers.

Exergetic Analysis of Solar Energy Drying Systems 96
The moisture content versus time plot (solar drying
curves) for shelled corn is shown in Figure 6. For the
three dryers, it was observed that the food item in the
mixed mode and indirect mode dryers dried faster than in
the direct mode dryer. Figure 7 shows the exergetic effi-
ciency curves of the three drying systems. The overall
analysis gave average exergetic efficiencies of 55.2%
and 54.5% for mixed mode and indirect mode systems,
respectively, while for direct mode, an average exergetic
efficiency of 33.4% was obtained.
4. Conclusions
Exergetic analysis was carried out on three types of
solar drying systems (direct, indirect, and mixed modes)
to find the useful and the quality of energy that are ob-
tainable from the systems. The results obtained show that
mixed mode and indirect mode solar dryers are more
effective in utilizing the captured energy than direct
mode dryer. For mixed mode and indirect mode systems,
about 78.1% and 77% of the collected energy respec-
tively is available or useful energy (exergy), while 21.9%
and 23% of the collected energy respectively is unavail-
able or wasted energy (anergy). But the direct mode sys-
tem could only convert 49.3% of the collected energy to
useful energy, while the rest 50.7% is wasted. In the
overall analysis of the systems, mixed mode and indirect
mode solar dryers were found more efficient than direct
mode dryer with mixed mode dryer having a slight edge
in superiority over indirect mode dryer. Average exer-
getic efficiencies of 55.2%, 54.5% and 33.4% were ob-
tained from mixed mode, indirect mode and direct mode
systems respectively.
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Exergetic Analysis of Solar Energy Drying Systems 97
NOMENCLATURE
Ac – total collector area (m2)
C
p
– specific heat at constant pressure (kJ·kg−1·K−1)
E – total energy (kJ)
R
F
– collector heat removal factor
I – incident solar radiation (W·m−2)
m
 – mass flow rate (kg·s−1)
q – rate of heat released (W)
Q – rate of heat collection (W)
T – temperature (K)
r
T – temperature at which heat is released (K)
L
U – heat loss coefficient (W·m−2·K−1)
X
– exergy (kJ)
Y – anergy (kJ)
Greek:

– solar absorptance

– efficiency (%)

– transmittance

– exergetic potential
Subscripts:
a – ambient air
g – energy gained by air
in – input
out – output
th – thermal
u – energy gained by collector
x – exergetic