Journal Pre-proof
Performance assessment of passive indirect solar dryer comparing
without and with heat storage unit by investigating the drying kinetics
of carrot
Mulatu C. Gilago , Vishnuvardhan Reddy Mugi ,
V.P. Chandramohan
PII:
DOI:
Reference:
S2772-4271(23)00008-6
https://doi.org/10.1016/j.nexus.2023.100178
NEXUS 100178
To appear in:
Energy Nexus
Received date:
Revised date:
Accepted date:
19 November 2022
20 January 2023
8 February 2023
Please cite this article as: Mulatu C. Gilago , Vishnuvardhan Reddy Mugi , V.P. Chandramohan ,
Performance assessment of passive indirect solar dryer comparing without and with heat
storage unit by investigating the drying kinetics of carrot, Energy Nexus (2023), doi:
https://doi.org/10.1016/j.nexus.2023.100178
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Highlights

An indirect solar dryer without and with thermal storage system (setups-I and II) was
developed.

Drying parameters and thermal performance of the dryers were estimated.

The thermal efficiency and actual heat supply of the solar heater were 56.84 & 60.79% and
705.64 & 730.45 W, respectively.

Surface transfer coefficients of carrot with drying air were calculated for both dryers.

The specific energy consumptions for setups-I and II were 3.5 and 0.28 kg/kWh, respectively.
1
Performance assessment of passive indirect solar dryer comparing without
and with heat storage unit by investigating the drying kinetics of carrot
Mulatu C. Gilagoa,b, Vishnuvardhan Reddy Mugia, V.P. Chandramohana,*
a
Mechanical Engineering Department, National Institute of Technology Warangal,
Warangal, Telangana, India, 506004
b
Mechanical Engineering Department, Wachemo University, Post Box 667, Hosaena
City, Ethiopia
*
Corresponding author: E-mail: vpcm80@nitw.ac.in, Mob: +91 8332969329, ORCID: 0000-
0002-8680-8363
2
Abstract
The present study examined the drying kinetics of carrots and the effectiveness of passive
indirect solar dryer without (setup-I) and with (setup-II) thermal energy storage (TES). A
module containing paraffin wax as a TES unit was incorporated after the test was completed
in setup-I to create setup-II. The sample slices were dried in setup-I and setup-II at a drying
rate of 0.5 and 0.59 kg/h, respectively. In setup-I, the sample dried from 9.13 to 0.478 dry
basis (db) in 16 h, whereas, setup-II took 15 h. The thermal efficiency of the solar heater and
actual heat supply for the same were 56.84 & 60.79% and 705.64 & 730.45 W, respectively.
Similarly, the average heat and mass transfer coefficients (h and hm) were 6.32 & 7.08 W/m2
K and 0.0055 & 0.0062 m/s, and average effective diffusivity (De) for the same setups were
6.7 × 10-9 and 7.24 × 10-9 m2/s, respectively. Furthermore, the specific energy consumption
and specific moisture extraction rate for the same were 3.5 & 0.28 kWh/kg and 0.29 & 3.62
kg/kWh, respectively. Setups I and II had an average drying efficiency of 7.5 and 10.25%,
respectively. The activation energy was evaluated for both setups. The De, h and hm were
correlated with moisture content for both setups. In addition to helping the drying process to
be completed in a day, setup-II performed better in all the evaluated parameters than setup-I.
To ensure the reliability of the results, an experimental uncertainty analysis was conducted.
Keywords
Passive indirect solar dryer; paraffin wax; drying performance; surface transfer coefficients;
thermal energy storage
3
Nomenclature
cp
Specific heat of air
A
Area
Dab
Moisture diffusivity of air
db
Dry base
De
Effective moisture diffusion coefficient
Do
Pre-exponential factor
Eac
Average activation energy
h
Heat transfer coefficient
hm
Mass transfer coefficient
Ia
Average solar radiation
Is
Solar intensity
PCM
Phase change material
k
Thermal conductivity
L
Thickness
Le
Lewis number
Lw
Latent heat of water
MC
Moisture content
m
Mass
ṁ
Mass flow rate
MR
Moisture ratio
Q
Heat supply
R
Universal gas constant
2
R
Correlation coefficient
SEC
Specific energy consumption
SAC
Solar air collector
SMER
Specific moisture extraction rate
T
Temperature
TES
Thermal energy storage
G
Uncertainty
V
Volume
wb
Wet based
xi
Uncertainty of a measured parameter
Y
Calculated parameter
Greek symbols
α
Thermal diffusivity
η
Efficiency
Subscripts
c
Collector
d
Dryer
e
Effective
f
Final
i
Initial, inlet
ic
Input of collector
o
Output
w
Water
4
(kJ/kg K)
(m2)
(m2/s)
(m2/s)
(m2/s)
(kJ/mol)
(W/m2 K)
(m/s)
(W/m2)
(W/m2)
(W/m K)
(m)
(kJ/kg)
(db)
(kg )
(kg/s)
(%)
(W)
(J/mol K)
(%)
(kW-h/kg)
(kg/kW-h)
(℃)
(m3)
(m2/s)
(%)
-
1. INTRODUCTION
The agenda of energy saving together with utilizing a cleaner source is supposed to be a
sensitive issue for the global community. It should be the front-line itinerary of all
policymakers, researchers, scientists, industries, and organizations because life on the earth is
totally dependent on energy. The energy demand is sloping up at an alarming rate as the
population and energy dependent industries are growing very fast. Striving to exploit the
reserved potential of solar energy by any means may put a step forward to a solution of the
energy related challenges. It is essential to mention that solar energy is one of the most
attractive methods of utilizing renewable sources. Due to its environmental friendly
characteristics, accessibility and free availability, it is getting special attention in the present
day.
Food drying is another major energy intensive process and the usage of solar energy for food
drying is a hot research area for the past few decades. Food drying can prevent microbial
decomposition by removing moisture from them and the energy needed for the same can be
utilized from solar energy [1] instead of wasting commercial energy. A careful choice of
drying techniques and controlling of drying parameters as well as making the process
continuous and short are to be achieved while drying agricultural food products in order to
ensure the required quality. Continuous drying using solar energy may not be possible
because of fluctuations in sunlight but this issue can be overcome by providing thermal
energy storage (TES) unit with the solar dryer. The TES unit can absorb the excess energy
during day time and deliver the same when the sunlight is off so the food gets more
continuous drying tenure and the possibility of degradation may be avoided. Evaluating the
drying kinetics has a major impact on the optimization of drying parameters while doing
drying experiments [2].
There are a good number of studies existing for confirming the above facts. Lakshmi et al. [3]
experimentally investigated the performance of an indirect type solar (ISD) by drying black
turmeric. They applied paraffin wax as phase change material (PCM) in the TES unit.
Accordingly, they reported the overall efficiency of the collector and drying were 25.6 and
12%, respectively. The condition of the final product was comparatively good in the setup
with TES compared to open sun drying (OSD). Drying of green onion has been done by
5
Hadalgo et al. [4] with both passive and active solar dryers. They concluded that passive
convection had a thermal efficiency (ηc) of 38.3 % and a specific energy consumption (SEC)
of 16.39 kW/kg, while active convection had the same parameters of 34.2 % and 18.3
kWh/kg, respectively.
Tajudin et al. [5] performed an experimental study of drying Roselle calyx in solar and
convective heat pump dryers to analyze the impact of temperature variation and mass of the
sample on the drying kinetics. They reported that higher drying temperatures and lower mass
of the sample fostered faster drying. Additionally, as the temperature varied from 40 to 60 ºC,
the effective diffusivity (De) enhanced from 0.787 to 2.04 × 10−9 m2/s. Gilago and
Chandramohan [6] investigated the performance of natural and forced convection ISD using
PCM that was kept inside the drying chamber while drying ivy gourd. They evaluated the
impact of TES unit on drying. Accordingly, they reported that both setups effectively worked
till mid-night so the drying process completed within a day. The drying kinetics of ivy gourd
produced a logarithmic nature of variation on De and surface transfer coefficients over
moisture content.
Mohammad et al. [7] made hybrid passive and active ISDs to experimentally investigate the
performance during drying pineapple in Uganda (East Africa). Additionally, they assessed
the economic importance of the system and compared it with the OSD method. They
presented that the average drying air temperature was 31.9 ºC for the proposed system and
27.6 ºC for the OSD. ISD dried the sample for 10 h while OSD took 30 h. Halaby and Bek
[8] conducted drying experiments to investigate the drying kinetics of herbal leaves as well as
to identify the impact of integrating the TES system on the performance of active ISD
(AISD). Moreover, they observed the effect of mass flow rate (0.067–0.218 kg/s) on different
study parameters. Accordingly, they found that in the dryer without TES (setup-I), the
samples dried in 12 h, and it took 18 h in the dryer with TES unit (setup-II) with drying rates
of 0.124 and 0.09 kg/s, respectively. The drying was completed continuously in one day in
setup-II.
An experimental study with a TES unit to determine the drying performance parameters of an
ISD was studied by Aboul-Enein et al. [9]. Additionally, they examined the domination of the
design details of the dryer on the drying performance. The researchers found that the TES
system and the dryer design parameters considerably influenced the drying characteristics of
6
agricultural products. During an experimental investigation on the importance of TES unit
with solar dryers, Shukla [10] demonstrated that the final products quality was improved.
Different TES systems such as latent, sensible, and chemical energy storage provisions
improved the quality of end products in dryers as the drying process was completed in one
day without any interruption in drying.
For their experimental investigation of drying black pepper, Lakshmi et al. [11] fabricated a
dryer with a TES unit. The moisture content (MC) decreased from 3.46 - 0.1 dry basis (db)
within 14 h in a mixed-type dryer and to 0.12 (db) in an AISD dryer after 23 hours. The
report also presented that the dried product quality was improved and that dryer was more
cost-effective compared to OSD drying. Safri et al. [12] surveyed the attainment of different
solar-assisted dryers and drying kinetics of various agricultural food products. They reported
that drying agriproducts in solar-assisted dryers were more hygienic, nutritional, and healthy
than other types of dryers.
A drying experiment of jackfruit leather was analyzed by Chowdhury et al. [13] to identify
the drying kinetics of solar tunnel dryers. As a result, they concluded that the samples dried
from a mean MC of 3.17 - 0.14% (db) with the solar air collector (SAC) and drying
efficiencies of 27.45 - 42.5% and 32.34 - 65.30%, respectively, within two days. The recent
advancements in drying agricultural food products were reviewed by Lamidi et al. [14]
indicating that ISDs coupled with TES systems are effective for drying agricultural products.
The above review indicates that some studies focus on solar dryers and their drying
performances [15–17], some on drying kinetics, and some address the advantage of solar
dryers while drying farm food items [14, 18]. But a comprehensive examination of the
parameters of setup-II during drying agriproducts is not reported yet. Various works
contributed to the study of the solar drying of green onion [4], ivy gourd [19], muskmelon
[20], banana and bitter gourd [21], green chili and okra [22], apple and watermelon slices
[23], citrus Aurantium [24]. No data was found on carrot slices dried in an ISD supported
with TES.
Few studies reported on the effect of adding TES unit to solar dryers and the merits of drying
agriproducts using solar dryers [14, 16]. The drying kinetics of carrot without and with the
TES unit was not found and such data is necessary to know the impact of the TES unit. Some
7
data on the drying kinetics of agricultural produce such as heat supplied to the drying section
[9, 23, 24], heat and mass transfer coefficients [8, 25], diffusivity coefficient [4, 26], and rate
of drying [4, 27] have been reported. No studies reported such drying mechanics for both
setups as it needs to be done so that one can study the variation of these parameters without
and with TES unit. In the literature, only a few studies addressed the investigation of
activation energy (Ea), drying efficiency [2, 19], specific moisture extraction rate (SMER)
[19, 30], specific energy consumption (SEC) [29] and efficiency of the collector of the setup
without TES (setup-I). However, there is little data available concerning how the drying
performance parameters are analyzed for the setup without TES (setup-II) during carrot
drying. Furthermore, a comparative investigation of the setup without TES (setups-I) and
with TES (setup-II) is not found. The MC vs De, heat transfer and mass transfer coefficients
(h and hm) correlations were not developed in any studies as they are necessary data after
such expensive experiments.
The following objectives were rooted and achieved in this study based on the gaps found in
the assessment of literature: i) to estimate and analyze the parameters of drying performance
such as thermal efficiency, heat supplied to the drying section, and efficiency of the dryer
during drying carrot in setups-I and II, ii) to evaluate the drying kinetics of carrot slices such
as De, rate of drying, h and hm for both dryers, iii) to tabulate a comparative chart of
performance and drying parameters of both dryers, iv) to evaluate Ea, SEC, and SMER for
both dryers and v) to correlate the relationship between MC and De, h and hm during drying in
setups-I and II.
2. EXPERIMENTAL TECHNIQUES
2.1. Procedure
Carrots purchased from Warangal local market were sliced into 5 mm thick cylindrical
shapes after making sure that the outer surface is cleaned by dry fabric. Initially, the tests
were performed in setup-I (without TES unit) which is constructed at NITW in Telangana,
India (17° 58′ 50.88′′ N, 79° 31′ 58.08′′ E). The setup-I was updated with a rectangular
framed TES unit that was placed just below the first tray and inside the drying section called
setup-II. It is made of 50 numbers of transparent polycarbonate tubes with concentric
aluminium fins filled by paraffin wax (34 kg, EC No. : 232-315-6, no caking, liquefaction
8
temperature 56-60 ºC, latent heat type, CAS No.: 8002-74-2, IMEDIA, India). Fig. 1 (a)
diagrammatically demonstrates its components and Fig. 1 (b) gives a single polycarbonate
pipe and its dimensions. The dimensions of the drying chamber and the SAC are 350 mm ×
850 mm × 700 mm and 2000 mm × 1050 mm × 700 mm, respectively. In setup-I, the
experiments were performed in the day time as it has no TES unit and hence drying took two
consecutive days. In setup-II, the drying process started at morning and continued even after
sunset (after 6:00 p.m.). Since, the setup-II has the TES unit, the drying continued up to
midnight but within the same day. Initial MC was estimated from the mass variation data
using a hot air oven (India, PPI-Unix96). OHAUS weighing balance (USA, 8-1415VAC,
PAG24, 50/60 Hz, readability-0.0019) was used to measure the mass of the samples. Indian
made RTD sensors and humidity transmitter (Testo635) were used to measure relative
humidity and temperature. More details are presented in the previous study of the authors
[32].
Figure 2 (a) illustrates the inside view of the experimental setup. 0.8 kg of carrot slices (0.2
kg × 4) were placed on four parallel trays (Fig. 2 b). Mass variation and drying parameters
data were found for both cases. Fig. 2 (c) shows the snapshot of the final dried sample. A hot
air oven was used for 24 h at 105 °C to determine the initial moisture content (MCi) of slices.
(a)
9
(b)
Paraffin wax
All dimensions are in mm
Fig. 1. Schematic representation of (a) overall setup-II and (b) paraffin wax holding unit
10
2.2. Moisture content
The MCi was estimated from five random out of 12 samples located and dried in the hot air
oven by:
(1)
Accordingly, the MCi of carrot was determined to be 9.13 (db) or 90.13 (wb), where, m
represents mass of the slices, i stands for initial, and f represents final.
Chimney
Dried slices
Slices
Trays
Trays
TES
Trays
Data logger
TES
Fig. 2. Photo snapshot of (a) inside view of setup-II, (b) sliced carrot slices and (c) dried
carrot slices
2.3. Estimation of energy involved in the experiment
To estimate the energy associated with drying, the principles of mass conservation have been
engaged, which states that the mass entered and the mass left in a specific control volume are
equal [25]. This can be determined by.
∑ ̇
∑ ̇
(2)
Energy that took part in the SAC can be evaluated as,
(3)
̇
(4)
Where, Qic is input heat (useful heat input), Qoc is collector heat output (actual heat supplied),
Is gives solar radiation (W/m2) and Ac represents area of SAC.
̇
(5)
11
Equation (5) is used to estimate the collector’s efficiency (ηc).
Evaluation of the energy inside the drying cabinet can be done using,
(6)
(7a)
(7b)
̇
Ia is the mean solar intensity for overall duration (W/m2), td represents the gross time used for
drying samples (h), mw gives the amount of eliminated water (kg), and Lw is latent heat
(kJ/kg), Ein is input energy (kWh), As represents area of sample exhibited to radiation (m2);
Ti,PCM and To,PCM are temperature of air at the inlet and outlet of the TES unit, respectively.
The SEC (kWh/kg) and SMER (kg/kWh) [32] can be evaluated by,
(8)
(9)
2.4. Determining the drying kinetics
Using experimental mass drop data, De [10] was determined by,
( )
(10)
Where, L (m) is the thickness of the slab and MR is the moisture ratio.
Energy of activation (Eac) can be estimated using the Arrhenius equation [22].
(
)
(11)
Where, Do (m2 /s) and R (J/mol K) are pre-exponential factor and universal gas constant,
respectively.
MR [35, 36] is evaluated using,
(12)
Where, the subscript tn stands for instantaneous.
(13)
Where, the subscript t stands for time and dt represents change in time.
The coefficient of mass transfer (hm) [37] can be calculated using,
12
(14)
Where, V (m3) and At (m2) represent the volume and the overall surface area, respectively.
(15)
(16)
Heat transfer coefficient (h) is evaluated using Eq. (15), where, Dab, α (m2/s), k (W/m K) and
Le stand for diffusivity of MC in air, drying air, thermal conductivity of air, and Lewis
number, respectively.
3.5. Analysis of uncertainty
In order to appraise the reliability of the results of the experiment, an experimental
uncertainty analysis was conducted. The root-sum square method can be used to evaluate the
uncertainty of dependent and independent variables [38]. The uncertainty evaluated for the
parameters is illustrated in Table 1, where G represents the degree of uncertainty, Y
represents a parameter's calculated value, and xi represents the degree of uncertainty for the
measured variable.
[(
)
(
)
(
)
(
) ]
Table 1. Uncertainties in the experiment
Variable
Uncertainty
T
± 1 oC
v
± 0.040 m/s
m
± 0.0002 g
I
± 10 W/m2
MC
±0.047 kg per kg db
RH
± 2%
De
±1.46%
MR
±0.031
ηc
±1.35%
Ea
±0.0674 kJ/mol
hm
±3.68×10-5 m/s
h
±0.029 W/m2K
13
(17)
Qoc
±24.02 W
ηd
±1.32%
3. ANALYSIS AND DISCUSSION
3.1. Data of solar radiation
Figure 3 presents the solar radiation data during carrot drying in setups-I and II. In setup-I,
the solar radiation was documented between 8:00 a.m. and 6:00 p.m., whereas, in setup-II, it
was recorded from 8:00 a.m. to midnight as it has a TES unit. Setup-I achieved a peak and
average solar radiation of 963 and 648.74 W/m2, and the same for setup-II were 975 and
616.27 W/m2, respectively. The values are almost the same because the tests were executed
on two successive days and also with the same weather. Therefore, it didn’t affect the drying
parameters significantly.
1200
Setup-I
Setup-II
Solar radiation (W/m2)
1000
800
600
400
200
0
8
10
12
14
16
18
20
Time (h)
22
24
26
28
Fig. 3. Solar flux noted for setup-I and setup-II
3.2. Heat supplied to the drying section (actual heat supply)
Equation (4) has been used to evaluate heat supplied to the drying section (Qoc). Fig. 4 shows
Qoc variation with time. The Qoc varies according to the solar radiation and hence it looks like
the same as Fig. 3. The average and maximum values of Qoc for setup-1 were 705.64 and
1056.86 W, while the same for setup-II were 730.45 and 1218.56 W, respectively. The
14
difference in average Qoc was 24.81 W (3.4%) between setup-I and setup-II, which may be
the result of a change in temperature and mass flow rate during testing which is
comparatively negligible.
1400
Setup-I
Actual heat supply (W)
1200
Setup-II
1000
800
600
400
200
0
8
10
12
14
16
18
20
Time (h)
22
24
26
28
Fig. 4. Supply of actual heat versus time for carrot drying experiment in setup-I and setup II
3.3. Thermal efficiency of the collector
The data obtained during carrot drying in setup-I and setup-II was used to evaluate the
thermal efficiency of the SAC (ηc). Fig. 5 describes the variation of ηc over time. The ηc
changes with solar radiation and hence it increases and reaches its maximum at midday.
Setup-I and setup-II had average and maximum ηc of 56.84 and 60.79% & 76.48 and 75.88%,
respectively. The difference in average ηc was 6.95%. Mugi and Chandramohan [31] reported
an average ηc of 63.3% during drying green chilli and Gilago and Chandramohan [6] reported
66.5% during drying ivy gourd and these values were formidable acceptance with the
outcomes of the current study.
15
90
Setup-I
80
Setup-II
70
ηc (%)
60
50
40
30
20
10
0
8
10
12
14
16
18
20
Time (h)
22
24
26
28
Fig. 5. Thermal efficiency of the collector during drying carrot in setups-I and II
3.4. Drying efficiency
The ηd was estimated while drying carrot slabs setup-I and setup-II and plotted in Fig. 6. The
ηd is the corollary of the amount of MC removed during drying and the average solar
radiation during the whole drying process [32] as mentioned in Eq. (7). As shown in Fig. 6
that the ηd values increase at a faster rate before midday for both setups, then decreased
similarly at afternoon. The variation of ηd was higher in setup-I compared to setup-II because,
in setup-II, the temperature of drying air is low due to heat absorption taking place in the TES
unit and hence MC removal is low. For setup-I, the average and maximum values of ηd were
7.5 and 23.66%, and the same for setup-II were 10.35 and 20.9%, respectively. There was an
enhancement of 38% in the average ηd of setup-II than setup-I. According to the study of
Caesar et al. [39], the ηd of 11% was noticed while drying pears which is in agreement with
the present study.
16
25
ηd (%)
20
Setup-I
Setup-II
15
10
5
0
8
10
12
14
16
Time (h)
18
20
22
24
Fig. 6. ηd variation during drying carrot in setup-I and setup-II
3.5. Temperature recordings
Figure 7 presents the recorded data of temperature during experiments in setups-I and II. Fig.
7 (a) gives the temperatures of outside air (Ta), the inlet of the collector (Tci), the outlet of the
collector (Tco), trays 1, 2, 3, and 4, respectively are denoted by T1, T2, T3, and T4 for setup-I.
Accordingly, the maximum Ta, Tci, Tco, T1, T2, T3, and T4 of setup-I were 41.6, 43, 75, 69, 65,
64, and 57 °C, respectively, and their corresponding averages were 35.8, 37.6, 61.2, 55.8,
51.4, 49.6, and 47.1 °C.
17
Similarly, the temperature records of setup-II are displayed in Fig. 7 (b). The average values
of Ta, Tci, Tco, T1, T2, T3, and T4 in setup-II were 33.6, 34.2, 45.5, 41.1, 40.5, 40, and 39.1℃,
respectively, while the maximum values of the same were 38.2, 41, 71, 52, 51, 51, and 51 ℃.
Overall, setup-I had higher average and maximum temperature values than setup-II. The
difference in temperature could have been due to the TES unit as it decreased the temperature
inside the drying section (on the trays) because of heat stored in TES in setup II.
(a)
80
Temperature (oC)
70
60
50
40
30
Ta
T1
T4
20
Tci
T2
Tco
T3
10
8
10
12
14
16
Time (h)
18
20
22
24
(b)
80
Ta
T1
T4
Tempeerature (oC)
70
Tci
T2
Tco
T3
60
50
40
30
20
10
8
10
12
14
16
Time (h)
18
20
22
24
Fig. 7. Data of temperature recorded during drying carrot slabs in (a) setups-I and (b) II
3.6. Kinetics of drying for carrot
3.6.1. Moisture content
18
To determine the MC, the mass loss of carrot during the drying experiment was recorded in
both setups. Fig. 8 shows how the MC varies with time in both setups. The MC started to
decrease slowly in the initial stage and a rapid drop was noticed after a certain period. This
indicates that the early phase of the removal of moisture is initiation and warming. As the
slopes of the graphs increased in the second phase, it was evident that there was a greater
amount of moisture being lost from the outer and interior surfaces. Towards the end of the
process, more moisture could be removed from the internal pores of the sample, but it
requires more energy, and thus the slope tends to be flattened for both setups. Throughout the
drying period, setup-I shows a steeper slope than setup-II. Setup-I and setup-II dried the slabs
from 9.13 to 0.478 (db) within 16 h and 15 h, respectively where setup-I required two
sunshine days but setup-II took only one day.
10
MC (db)
9
8
Setup-I
7
Setup-II
6
5
4
3
2
1
0
8
10
12
14
16
Time (h)
18
20
22
24
Fig. 8. The momentary MC (db) of carrot in setup-I and setup-II
3.6.2. Moisture ratio (MR)
It is the ratio between the MC available in the drying slab at a particular moment to the MCi
of the drying sample. Eq. (10) is used to estimate the same and is shown in Fig. 9. The
characteristic graph of MR with time is the same nature as MC reported in Fig. 8 because it is
a function of MC.
19
1.2
1
Setup-I
Setup-II
MR
0.8
0.6
0.4
0.2
0
8
10
12
14
16
Time (h)
18
20
22
24
Fig. 9. MR variation with time of carrot during drying in setups-I and II
3.6.3. Drying rate
In this study, the rate of drying (DR) of carrot slices in setup-I and setup-II has been plotted
(Fig. 10) and analyzed. It can be seen that DR increased at a steady rate until it reached its
maximum at noon. DR of setup-I increased at a higher rate than setup-II due to lower drying
temperature in setup-II (since TES unit is installed) than setup-I. The DR of setups-I and II
decreased at a faster rate after reaching their maximum value. For setup-I and setup-II, the
average and maximum DRs were 0.497 and 0.588 kg/h, and 1.38 and 1.24 kg/h, respectively.
The average DR in setup-II improved by 18.3% compared to setup-1. Vijayan et al. [2] dried
bitter gourds in a solar dryer at a maximum DR of 2.87 kg/h which is almost the same as the
present analysis.
20
DR (kg/h)
1.6
1.4
Setup-I
1.2
Setup-II
1
0.8
0.6
0.4
0.2
0
8
10
12
14
16
Time (h)
18
20
22
24
Fig. 10. DR during drying carrot in setup-I and setup-II
3.6.4. Coefficient of moisture diffusion (De)
The De for carrot slices dried in setup-I and setup-II have been evaluated from the
observations and plotted in Fig. 11. The De increased at a faster rate for the first phase as a lot
of MC is eliminated in the first phase. The interior MC needs more energy to be removed as
it is trapped in the complex pores of carrot slabs and therefore, the De is increased at a slower
rate in the second phase compared to first phase of drying. The differences in the rate of
variation for De between the two setups match with the variations in MC. The average De for
setup-I and setup-II was determined to be 6.7 and 7.24 × 10-09 m2/s and their values were
0.229 - 1.07 × 10-08 m2/s and 2.29 × 10-09 - 1.00 × 10-09 m2/s, respectively. Existing literature
(Tagnamas et al. [1]) reported the De values those are almost in a similar range (1.2 × 10-9 4.2 ×10-9 m2/s).
21
1.2E-08
Setup-I
1E-08
Setup-II
De (m2/s)
8E-09
6E-09
4E-09
2E-09
0
8
10
12
14
16
18
Time (h)
20
22
24
Fig. 11. De variation during drying carrot in setup-I and setup-II
3.6.5. Coefficient of heat transfer (h)
Using Eq. (15), the h of carrot slices dried in hot air in setup-I and setup-II is calculated and
its characteristic graph is depicted in Fig. 12. The curve of h follows the same trend as De as
mentioned in Fig. 12. The mean values of h were 6.32 and 7.08 W/m2K for setup-I and setupII, respectively. In setup-II, there was an improvement of 12.03% in h in comparison with
setup-I. The current outcomes are in favorable consensus with those of the existing data [40]
while drying watermelon in a passive solar dryer as the reported range was 0.52 - 5.039 and
0.158 - 3.19 W/m2 K for apple and watermelon, respectively.
3.6.6. Coefficient of mass transfer (hm)
Equation (14) has been used to determine the hm of carrot slices dried in setup-I and setup-II.
It is mentioned in Fig. 13. The style of variation of hm is similar to those of h and De. Setup-I
had an average hm of 0.0055 m/s and setup-II had an average hm of 0.0062 m/s. There was a
12.73% improvement in average hm noticed in setup-II. The evaluated hm range was between
0-0.011 m/s. The published hm data of watermelon and apple was 0.00052 – 0.005 m/s and
0.00016 – 0.0032 m/s [40], respectively, which is almost similar to the present study.
22
14
Setup-I
12
Setup-II
h (W/m2 K)
10
8
6
4
2
0
8
10
12
14
16
Time (h)
18
20
22
24
Fig. 12. Coefficient of heat transfer during drying carrot in setup-I and setup-II
0.012
Setup-I
0.01
Setup-II
hm (m/s)
0.008
0.006
0.004
0.002
0
8
10
12
14
16
Time (h)
18
20
22
24
Fig. 13. Coefficient of mass transfer during carrot drying in setup-I and setup-II
3.7 Describing the relationship of MC with h, hm and h
The relationship of De, h, and hm with MC during the carrot drying experiment is represented
in Figs. 14, 15, and 16, respectively. A trending function of correlation was established by
tracing their variations. De, h, and hm showed significant correlations with the MC in the
logarithmic function F(MC) = b1 ln (MC) + b2, where, F represents the variables, b1 and b2
23
are the correlation constants. Table 2 summarizes the values for all variables, constants, and
coefficient of correlation (R2).
Figure 14 illustrates the variation in De with MC (db) of carrot slabs dried in setup-I and
setup-II. As the MC was decreased, De gradually increased for both setups. When the free
bound MC is removed from carrot slabs, more amount of unbound moisture is diffused from
the complex pores per unit area and hence such inverse variation is noticed between MC and
De. For both setups, hm (Fig. 15) and h (Fig. 16) change in the same manner as De (Fig. 14).
Findings from this study are in accordance with those from existing literature [4, 31].
1.2E-08
1E-08
Setup-I
De (m2/s)
8E-09
Setup-II
6E-09
4E-09
2E-09
0
0
1
2
3
4
5
6
7
Moisture content (db))
8
9
10
Fig. 14. De variation over MC (db) of carrot slices dried in setup-I and setup-II
0.012
Setup-I
0.01
Setup-II
hm (m/s)
0.008
0.006
0.004
0.002
0
0
1
2
3
4
5
6
MC (db)
7
8
9
10
Fig. 15. The hm variation over MC (db) of carrot slices in setup-I and setup-II
24
14
Setup-I
Setup-II
12
h (W/m2K)
10
8
6
4
2
0
0
1
2
3
4
5
6
MC (db)
7
8
9
10
Fig. 16. The h variation over MC (db) of carrot slices in setup-I and setup-II
3.8. SEC, SMER and Ea
The SEC refers to the specific energy consumed while removing a kg of MC from the drying
sample. Eq. (8) was used to determine the SEC of carrot slices dried in setup-I and II.
According to the calculations, setups-I and II had an average SEC of 3.5 and 0.28 kWh/kg,
respectively. In comparison, setup-II exhibited 92% less SEC than setup-I.
Additionally, the amount of MC removed while applying 1 kWh of energy during the drying
of a sample is known as SMER. Accordingly, the average SMER of carrot slices dried in
setup-I was calculated to be 0.29 kg/kWh, and in setup-II, it was 3.6 kg/kWh. In comparison
to setup-I, setup-II showed an improvement of 3.31 kg/kWh of SMER. The results of Mugi
and Chandramohan [31] also showed a similar SEC and SMER and they were approximately
1.783 kWh/kg and 0.6526 kg/kWh while drying green chilli in a passive solar dryer,
confirming that the values of the present work are matching.
Moreover, the Eac for the carrot drying experiments was appraised for both dryers.
Accordingly, the average Eac of carrot slices was 41.25 kJ/mol in setup-I, and 39.5 kJ/mol in
setup-II. In comparison with setup-I, Eac improved by 4.73% in setup-II. During drying carob
kernels (Ceratonia Siliqua L.) in a solar dryer, Tagnamas et al. [1] observed an Eac of 41.46
kJ/mol which agrees well with the current work.
25
Table 2. Correlation details between De, h, and hm with MC
F(MC)= b1 ln (MC) + b2
Variable (Y)
De
h
hm
Constants
N Setup
R2
b1
b2
I
-3×10-9
8×10-9
0.9942
II
-3×10-9
8×10-9
0.9849
I
-4.033
8.5518
0.9891
II
-3.857
8.3415
0.9937
I
-0.004
0.0077
0.981
II
-0.003
0.0073
0.9934
4. Conclusions
Experimental research has been conducted to distinguish the drying kinetics of carrot in a
passive indirect solar dryer without (setup-I) and with thermal energy storage (TES) (setupII). The following main conclusions are retrieved for both setups.
o Having recorded an average solar radiation of 648.75 and 616.27 W/m2 for the drying
days of setup-I and II, the average thermal efficiency of the collector was 56.84 and
60.79%, respectively. The mean actual heat supply for the same was evaluated to be
705.64 and 730.45 W, respectively.
o Setup-II showed a 38% improvement in drying efficiency compared to setup-I, while
setup-I and setup-II had respective averages of 7.5 and 10.35%.
o The average drying rate was 0.498 and 0.588 kg/h for setup-I and setup-II,
respectively, indicating setup-II achieved an 18.3% improvement over setup-I.
o In setup-I and setup-II, the effective moisture diffusion coefficients (De) were 6.7 and
7.24× 10-9 m2/s, respectively. And the coefficient of heat transfer (h) was 6.32 and
7.08 W/m2K, respectively. Additionally, setup-I and setup-II showed an average mass
transfer coefficient (hm) of 0.00549 and 0.00615 m/s, respectively. Moreover, the
activation energy was 41.25 kJ/mol for setup-I and 39.5 kJ/mol for setup-II,
respectively.
26
o Based on the analysis of the specific energy consumption for setup-I and setup-II, 3.5
and 0.28 kWh/kg were estimated, respectively, while setup-II showed a reduction of
92% in energy consumption. Moreover, for Setup-II, the specific moisture extraction
rate was 3.31 kg/kWh higher compared to Setup-I, which had a specific moisture
extraction rate of 0.29 kg/kWh and 3.6 kg/kWh, respectively.
o As the moisture content decreased, the De, h, and hm were observed to be increasing in
a logarithmic trend, while they were also increasing over time.
o
The sample dried from 9.13 to 0.478 (db) in 16 h (setup-I) and 15 h (setup-II). SetupII performed well, as the drying lasted for one day and was more efficient than setupI.
The overall results of the evaluation suggest that setup-II would be reasonably
recommended for subsequent large applications if it is further studied with an economic
and computational analysis. Also, numerical optimization studies are necessary for the
dryer dimensions and TES unit to achieve the maximum performance of the dryers.
27
Acknowledgements
The authors acknowledge Department of Mechanical Engineering, NIT Warangal, India for
financing the research work. The reference number is, NITW/MED/Head/2015/408 dated 3rd
Dec. 2015.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
28
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