Energy and Exergy Analyses of Thin Layer Drying of Mulberry in a Forced Solar Dryer

lable at ScienceDirect

Energy 35 (2010) 1754–1763

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Energy

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Energy and exergy analyses of thin layer drying of mulberry in a forced solar dryer

Abdullah Akbulut a,*, Aydin Durmus b

a Department of Mechanical Engineering, Dumlupinar University, 43000 Kutahya, Turkeyb Department of Mechanical Engineering, Ondukuz Mayis University, 55100 Samsun, Turkey

a r t i c l e i n f o

Article history:Received 26 June 2009Received in revised form6 November 2009Accepted 21 December 2009Available online 8 January 2010

Keywords:Energy analysisExergy analysisThin layer dryingForced solar dryerMulberry

* Corresponding author. Tel.: þ90 2742652031; faxE-mail addresses: [email protected] (A. Akb

(A. Durmus).

0360-5442/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.energy.2009.12.028

a b s t r a c t

This paper is concerned with the energy and exergy analyses of the thin layer drying process of mulberryvia forced solar dryer. Using the first law of thermodynamics, energy analysis was carried out to estimatethe ratios of energy utilization and the amounts of energy gain from the solar air collector. However,exergy analysis was accomplished to determine exergy losses during the drying process by applying thesecond law of thermodynamics. The drying experiments were conducted at different five drying massflow rate varied between 0.014 kg/s and 0.036 kg/s. The effects of inlet air velocity and drying time onboth energy and exergy were studied. The main values of energy utilization ratio were found to be as55.2%, 32.19%, 29.2%, 21.5% and 20.5% for the five different drying mass flow rate ranged between0.014 kg/s and 0.036 kg/s. The main values of exergy loss were found to be as 10.82 W, 6.41 W, 4.92 W,4.06 W and 2.65 W with the drying mass flow rate varied between 0.014 kg/s and 0.036 kg/s. It wasconcluded that both energy utilization ratio and exergy loss decreased with increasing drying mass flowrate while the exergetic efficiency increased.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Mulberry trees are extensively grown for their leaves as foodand fruits. There are three kinds of mulberry: white mulberry(Morus alba L.), black mulberry (Morus nigra L.) and red mulberry(Morus rubra L.). White mulberry originated in western Asia, redmulberry in north and South America and black mulberry is fromsouthern Russia. The fruits of white mulberries are often harvestedby spreading a sheet on the ground and shaking the limbs. Theyhave a high level of moisture content at harvest. Because of theshort season and the sensitivity to storage drying is often used asa preservation method. In addition, mulberry is used in mulberrymolasses, juices, paste, marmalade and wine production [10,14].

Drying is defined as reduction of moisture from the productsand is a most important process for preserving agricultural prod-ucts since it has a great effect on the quality of the dried products.Drying of fruit and vegetables is one of the oldest methods of foodpreservation. The major objective in drying agricultural products isthe reduction of the moisture content to a level which allows safestorage over an extended period. Sun drying is the most commonmethod used to preserve agricultural products in the World andalso Turkey. However, it has some problems related to thecontamination with dust, soil, sand particles and insects [10]. In

: þ90 2742652003.ulut), [email protected]

All rights reserved.

spite of many disadvantage, sun drying is still practiced in manyplaces throughout the world. Solar energy is an important alter-native energy sources because it is abundant, inexhaustible,renewable, cheap and non-pollutant. Recently, several studiesrelating the mathematical modeling and drying kinetics of thevegetables, fruits and agrobased products have been conducted byseveral researchers, such as those concerning pistachio [15], carrots[12], potatoes and apples [2], red peppers [3], figs [11], mint leaves[13], crop [1], mulberry [10,14], hazelnut [17], grapes [19], apricot[18]. Mathematical models have proved to be very useful designand analysis of the mass and heat transfer process during drying.

Thermodynamic analysis, particularly exergy analysis, hasappears to be an essential tool for system design, analysis andoptimization of thermal system [8]. Exergy is defined as themaximum amount of work which can be produced by a stream ofmatter, heat or work as it comes to equilibrium with a referenceenvironment [9]. In the drying process, the aim is to use a minimumamount of energy for maximum moisture removal for the desiredfinal conditions of the product. Several studies have been conductedon exergy analyses of food drying. However, the detailed literaturereview for the present study has shown that there is no informationon energy and exergy analyses of the thin layer drying process ofmulberry via forced solar dryer. Therefore, this paper, as differentother studies, concentrates on the energy and exergy analyses of thethin layer drying of mulberry via forced solar type dryer by using thefirst and second law of thermodynamics. It is believed that sucha study will contribute to mulberry producers by removing their

Nomenclature

A area, m2

Cp specific Heat, kJ kg�1 K�1

E emissive powerEUR energy utilization ratio, %Ex exergy rate, kWF shape Factorg gravitational acceleration, ms�2

gc constant in Newton’s lawh enthalpy, kJ kg�1

I solar radiation, W m�2

J joule constant_m mass flow rate, kg s�1

N number of speciesP pressure, kPaQ net heat rate, Ws specific entropy, kJ kg�1 K�1

t time, minuteT temperature, �CU specific internal energyv specific volume, m3 kg�1

V velocity, ms�1

w specific humidity, g g�1

z altitude coordinate, m

Greek letters4 relative Humudity, %hex exergetic efficiency, %m chemical potential, kJ kg�1

Subscriptsa airc chemicalcol collectorcp connection pipeda drying airf fani inletdc drying chamberL lossmp moisture of producto outletsat saturatedN surrounding or ambient

A. Akbulut, A. Durmus / Energy 35 (2010) 1754–1763 1755

problems related to energy and exergy throughout the dryingprocess. The primary objective of this study is to present energy andexergy analyses of thin layer drying of mulberry at different condi-tions of drying mass flow rates in a forced solar dryer. As remarkablestudies about energy and exergy analysis of drying process, thefollowing may be presented. Midilli and Kucuk [16] performed theenergy and exergy analyses of the drying process of shelled andunshelled pistachios using solar drying cabinet. Dincer and Sahin [8]developed a new model for thermodynamic analysis of dryingprocess. Akpinar [5] performed energy and exergy analyses ofdrying of red pepper slices via convective type dryer. Colak andHepbasli [6] performed an exergy analysis of thin layer drying ofgreen olive in a tray dryer. Akpinar et al. [4] conducted the first andsecond law analyses of thermodynamic of pumpkin drying process.Corzo et al. [7] performed energy and exergy analyses of the thinlayer drying of coroba slices at three different air temperatures.

2. Material and methods

Turkey has great solar energy potential due to its location in theMediterranean Region (36� and 42� North latitudes). The sunshineperiod of Turkey is 2624 h/year with a maximum of 365 h/month inJuly and a minimum of 103 h/month in December. The main solarradiation intensity is about 3.67 kWh/m2day.

The solar cabinet dryer is installed in the Technical EducationFaculty of Firat University, Elazıg, Turkey. The solar drying experi-ments were carried out during the period of June 2005. Each teststarted at 09:00 am and continued till 17:00 pm. The drying ofmulberries was conducted in solar cabinet dryer. A schematicdiagram and photograph view of the solar dryer system is illus-trated in Fig. 1 a and b, respectively. The system consist mainly foursubsystems, namely (a) drying cabinet, (b) solar air collector, (c) airfan and AC hertz converter (d) data logger. In the experiments,weather temperature, inlet and outlet temperature of solarcollector and dryer, temperature of the mulberry center, relativehumidity just above the mulberry bed surface and solar radiationwere recorded at 15 min intervals. In the measurements oftemperatures, T Type copper – constant thermocouples were con-nected with a ZA9000FST connector element to 5990 – 0 Almemo

digital data logger, with reading accuracy of �0.1 �C. A thermoanemometer (FVA645TH3) was used to measure air speed, withreading 0.1–15 m/s range. Pressure drop in a collector wasmeasured by a FDA612MR pressure module. Mass loss of themulberry were recorded during drying for determination of dryingcurves by FKA0251 strain strengetch in the measurement range of0.02–10 kN an accuracy of 0.01 kN. The solar radiation during theoperation period of drying system was measured with a Kipp andZonen solarimeter. Fresh mulberries were purchased at a localmarket in Elazıg, Turkey. All data were collected using an Almemo5990-0 data logger interfaced to the personal computer and thenrecorded at a 15 min time intervals. Prior to placing the sample ina dryer, the drying system was run for at least 60 min to conductcalibration. The solar dryer system consisted of a centrifugal fanthat was used to blow air into the solar collector through a 82 mmdiameter flexible aluminum duct. By using AC hertz converter, themass airflow was controlled. The dryer unit was made of an innerchamber 1.2 � 0.74 � 0.74 m made of a 0.8 mm thickness stainlesssteel sheet that was in turn enclosed in an outer chamber1.5 � 0.75 � 0.75 m made of stainless steel sheet. The spacebetween the two chambers was filled with polystyrene insulatingmaterials. On leaving the heating chamber the air passed througha (0.3 � 0.2 � 0.2 m) chimney chamber to allow it to mix andachieve uniform temperature before entering the drying chamber.The experiments were carried out using fresh mulberry with anaverage initial moisture content of approximately 3 kg of water/kgof dry solids and fresh mulberry were placed in the dryer. No pre-treatment was applied to the fresh product.

3. Analysis

In first and second law analyses of thermodynamics, the dryingprocess was considered as steady flow process. The main basis ofthese analyses is the phenomena of thermodynamics of humid air.

3.1. The first law analysis

In the scope of the first law of thermodynamics, an energyanalysis of the thin layer drying process of mulberry is performed to

Fig. 1. a Schematic view of the solar dryer system. b Photograph view of the solar dryer system.

A. Akbulut, A. Durmus / Energy 35 (2010) 1754–17631756

determine more about the energy aspects and behaviour of dryingair throughout the forced solar dryer. The air conditioning processthroughout the mulberry drying includes the heating, cooling andhumidification processes. Actually, the air conditioning process canbe modeled as steady flow processes that are analyzed byemploying the steady flow conservation of mass and conservationof energy principles.

For the energy and exergy analyses of the thin layer dryingprocess, the following equations are generally employed tocompute mass conservation of the drying air and moisture, theenergy conservation of the process and relative humidity andenthalpy of the drying air.

General equation of mass conservation of drying air:X_mda;i ¼

X_mda;o (1)

General equation of mass conservation of moisture:

X�_mda;i$wi þ _mmp

�¼X

_mda;o$wo (2)

General equation of energy conservation:

_Q � _W ¼X

_mda;o

ho þ

V2o

2

!�X

_mda;i

hi þ

V2i

2

!(3)

_mda;o ¼ _mda;i ¼ _mda (4)

The changes in kinetic energy of the fan were take intoconsideration, while the potential and kinetic energy in other partsof the process were neglected:

f ¼ wPð0:622þwÞPsat@T

(5)

Where wdenotes the specific humidity, P atmospheric pressure,Psat@Tthe saturated vapor pressurem of the drying air.

The enthalpy of the drying air can be determined as follows:

h ¼ CPdaTda þwhsat@T (6)

0

0,5

1

1,5

2

2,5

3

3,5

4

0 60 120

180

240

300

360

420

480

540

600

660

720

780

Time (min)

Moi

stur

e co

nten

t (k

g-w

ater

/kg-

dry

mat

ter)

m=0.014

m=0.02

m=0.026

m=0.033

m=0.036

Fig. 2. Variation of moisture content with drying time.


In order to determine the outlet conditions of the solar collector,it is assumed that there is no heat loss throughout the connectionpipe between the fan and the solar collector, and thus, the inletconditions of the solar collector are approximately equal to theoutlet conditions of the fan as given in equation (7).

wcol;i ¼ wfoTcol;i ¼ Tfofcol;i ¼ ffohcol;i ¼ hfo

(7)

Using the values of the outlet and inlet temperatures of the solarcollector, the energy transmitted to the drying air from the solarcollector can be calculated by the following equation.

Qcol ¼ _mda$cPda$�Tcol;o � Tcol;i

�(8)

Temperature measurements show that some small heat losseswere taking place between the solar collector outlet and dryer inlet.Because of the heat losses in this part of the system, it should bedefinitely emphasized that the outlet conditions of the solarcollector would not be equal to the inlet conditions of the dryer.Hence, the quantity of heat losses throughout the connection pipebetween the solar collector and dryer can be estimated by thefollowing equation.

QL;cp ¼ _mda$cpa$�Tcol;o � Tdc;i

�(9)

During the dehumidification process at the drying chamber, theheat used can be estimated by employing the following equationand using psychrometric chart.

Qdc ¼ _mda$ðhdci@T � hdco@TÞ (10)

During the drying process, the energy utilization ratio of thedrying chamber (EUR) was calculated using the following equation:

EUR ¼_mda�hdc;i � hdc;o

�_mda$Cpa

�Tcol;o � Tcol;i

� (11)

3.2. The second law analysis

In the scope of the second law analysis, the total exergy inflow,outflow and losses of the forced solar dryer were estimated. Thebasic procedure for exergy analysis of the drying chamber is todetermine the exergy values at steady state points and the reason ofexergy variation for the process. The exergy values are calculated byusing the characteristics of the working medium from a first lawenergy balance For this purpose, the general form of exergy equa-tion applicable for steady flow systems was employed [16].

Ekserji ¼�

u� uNinternal

energy

�� To

�s� sNentropy

�þ Po

J

�v� vN

work

�

þ V2

2gJmomentum

þ�

z� zNgravity

�g

gcJþX

c

�mc � mNchemical

�Nc

þ EgAgFg

�3T4 � T4

N � 4TNT3

radiation

emission

�þ/ ð12Þ

where the subscript Ndenotes the reference conditions. There arevariations of this general exergy equation. In the analyses of manysystems, some, but not all, of the terms shown in Eq. (12) areused. Since exergy is energy available from any source, the terms

can be developed using electrical current flow, magnetic fields,and diffusional flow of materials. One common simplification is tosubstitute enthalpy fort the internal energy and PV terms that areapplicable for steady flow systems. Eq. (12) is often used underconditions where the gravitational and momentum terms areneglected. In addition to these, the pressure changes in thesystem are also neglected because of V¼VN. In this case, Eq. (12)is derived as:

Exergy ¼ _mdacpda

��Tg � To

�� T0ln

Tg

To

�(13)

Applying Eq. (13), the inflow, and outflow of exergy can bedetermined depending on the inlet and outlet temperatures of thedrying chamber. Then the exergy losses throughout the dryingprocess are determined by using Eq. (14).X

ExL ¼X

Exi �X

Exo (14)

The equation of exergy inflow can be written for the dryingchamber as below:

Exdci ¼ _mdacpda

�ðTdci � TNÞ � TNln

Tdci

TN

�(15)

where cpda is the average specific heat of the drying air. However,the equaition of exergy outflow can also be written as

Exdco ¼ _mdacpda

�ðTdco � TNÞ � TNln

Tdco

TN

�(16)

Finally, the quantity of exergy loss is calculated by applying Eq.(14). The exergetic efficiency can be defined as the ratio of theexergy use in the drying of the product exergy to exergy inflow forthe drying chamber. However, it is explained as the ratio of theexergy outflow to the exergy inflow for the drying chamber.Considering this definition, the exergetic efficiency of dryingchamber can be estimated. Thus, the general form of the exergeticefficiency is written as [16],

hEx ¼Exi � ExL

Exi¼ 1� ExL

Exi(17)

4. Results and discussion

The drying experiments were conducted during the period ofJuly to August 2005 in Elazıg, Turkey. During the thin layer drying

skgm /014.0

0

50

100

150

200

250

300

350

400

450

0 45 80 135 180 225 270 315 360 405 450 495 540 585 630 675 720 765

time (minute)

Q (W

att)

0

10

20

30

40

50

60

70

80

90

100

EU

R (%

)

Collector Draying chamber Energy utilation ratio(EUR)

first day second day

Fig. 3. The results of the energy analysis for 0.014 kg/s mass flow rate of the drying air.


via forced solar dryer experiments, the solar radiation changedbetween 123.3 W/m2 and 939 W/m2, the temperature of ambientair ranged from 28 �C to 45 �C. The air temperature reached atmaximum value between 11.00 h and 15.00 h. The initial moisturecontent of fresh mulberries was close to 80% (wet basis). Dryingwas continued until the sample reached the desired moisture level(8%, wet basis). The solar radiation energy is maximum at middayand minimum at evening in day of experiment. The energy andexergy analyses of the thin layer drying process of mulberry viaforced solar dryer were performed by using data from the

0

100

200

300

400

500

600

0 45 135 180 225 270 315 360 4

time (mi

Collector Draying chambe

first day s

Q (W

att)

80

Fig. 4. The results of the energy analysis for 0

experiments and the results obtained from these calculations arepresented in Figs. 3–12 and discussed in detail.

4.1. Moisture content

Fig. 2 presents the variations of moisture content as a function ofdrying time for the mass flow rates of 0.014, 0.02, 0.026,0.033 and0.036 kg/s. As the moisture content of mulberry in the solar typedryer decrease, the moisture diffusion from the mulberry to the airdecreases as well. It can be seen that the relative humidity in the

m =0.02kg/s

05 450 495 540 585 630 675 720

nute)

0

5

10

15

20

25

30

35

40

45

50

EU

R (%

)

r Energy utilation ratio(EUR)

econd day

.02 kg/s mass flow rate of the drying air.

m = 0.026kg/s

0

100

200

300

400

500

600

700

0 30 58 80 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540

time (minute)

Q (W

att)

0

5

10

15

20

25

30

35

40

45

EU

R (%

)





drying air declines in accordance with the moisture content inmulberry.

4.2. Energy analysis

The energy analysis of thin layer drying process of mulberrywas performed by using data obtained from the forced solar dryer

m =0.0

0

100

200

300

400

500

600

700

0 30 58 80 120 150 180 210 240time (mi

Q (W

att)

Collector Draying chambe

Fig. 6. The results of the energy analysis for 0.

experiments. Figs. 3–7 present the results of the energy analysisof thin layer drying process of mulberry via forced solar dryer. Thevalues of energy utilization in the drying chamber were calculatedusing Eq. (10). EUR, calculated with Eq. (11), was defined as theratio of the energy utilization to the energy given from solarcollector. Maximum values of Qcol and Qdc were obtained 392.8 Wand 349 W with mass flow rates of 0.014 kg/s during 405 min for

33kg/s

270 300 330 360 390 420 450nute)

0

5

10

15

20

25

30

35

40

45

EU

R (%

)

r Energy utilation ratio(EUR)


033 kg/s mass flow rate of the drying air.

m =0.036kg/s

0

100

200

300

400

500

600

700

0 30 58 80 120 150 180 210 240 270 300 330 360

time (minute)

Q (W

att)

0

5

10

15

20

25

30

35

40

45

EU

R (%

)


a day



the first day experiments, respectively. The value of Qcol wasobtained in the ranges of 99.987–345.66 W during 375 min forthe second day experiment, respectively. Fig. 3 also shows thevariations of energy utilization ratio (EUR) of drying process for0.014 kg/s mass flow rate. It was obtained that EUR variedbetween 15.99% and 89.25% during the first day experiments. Forthe second day experiments, the EUR varied between 27.11% and45.37%. Fig. 4 shows the values of Qcol, Qdc and EUR for 0.02 kg/smass flow rate of the drying air. The value of Qcol varied between276 and 515 W in the first day experiment, 180–507.55 W in the

m

0

2

4

6

8

10

12

14

16

0 30 60 90 120 150 180 210 240 270 300 330 360 390 4

time (min

ExL

Exergy Loss

first day

Fig. 8. The results of the exergy loss and exergetic effici

second day experiment. For the first and second day dryingexperiments of 0.02 kg/s mass flow rate of the drying air, themaximum values of Qdc were obtained 225 W and 196 W,respectively. On the other hand, Fig. 4 displays the variations ofthe EUR as a function of the drying time. It was observed that EURranged between 16.3% and 43.79% in the drying chamber duringthe first day experiments, and between 18.33% and 38.61% duringthe second day experiments. Consequently, EUR of the first daywas found to be higher that of the second. This is due to thestructure and the moisture content of the dried mulberry sample.

= 0.014 kg/s

20 450 480 510 540 570 600 630 660 690 720 750 780

ute)

0

10

20

30

40

50

60

70

80

90

ex

exergetic efficiency

second day

ency for 0.014 kg/s mass flow rate of the drying air.

m = 0.02 kg/s

0

2

4

6

8

10

12

ExL

0

10

20

30

40

50

60

70

80

90

ex

Exergy Loss exergetic efficiency


0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540 570 600 630 660 690 720 750 780

time (minute)

Fig. 9. The results of the exergy loss and exergetic efficiency for 0.02 kg/s mass flow rate of the drying air.


Fig. 5 presents the results of the energy analysis of drying processfor the mass flow rates of 0.026 kg/s. It was obtained that Qcol andEUR ranged between 394 W and 627 W, 7.86% and 29.18%,respectively. Maximum value of Qdc was obtained 275 W withmass flow rates of 0.026. Fig. 6 shows the variations of Qcol andQdc on the left ordinate and the EUR on the right, as a function ofdrying time for the 0.033 kg/s mass flow rate of the drying air.Qcol, Qdc and EUR ranged between 340 W and 479 W, 43 W–106.22 W and 13.64 %–21.5%, respectively. Fig. 7 presents theresults of the energy analysis for the mass flow rates of 0.036 kg/s. It was obtained that Qcol and EUR ranged between 394.26 W –525.62 W, 16.22% and 20.95%, respectively. The mean value of Qdc

was obtained 111.18 W with mass flow rates of 0.036 kg/s.The mean values of EUR for 0.014, 0.02, 0.026, 0.033 and

0.036 kg/s mass flow rates of drying air were obtained as 55.2%,

m =

0

1

2

3

4

5

6

7

0 30 60 90 120 150 180 210 240 270 300

time (minut

ExL

Exergy Loss

first day secon

Fig. 10. The results of the exergy loss and exergetic effici

32.19%, 29.2%, 21.5% and 20.5%, respectively. These values show thatEUR of drying chamber decreased with the increase of mass flowrate of drying air.

4.3. Exergy analysis

The exergy analysis of thin layer drying process of mulberry viaforced solar dryer was performed by using data obtained from thedrying experiments. The values of ExLand hEx for each mass flowrate of the drying air can be observed in Figs. 8–12. In the dryingexperiments with five different mass flow rates performed exergyloss in the drying chamber increased during the first 6 h, and afterthat showed a decaying behaviour. Obviously, such time variationof the exergy loss appeared as a consequence of the changes in thesolar radiation.

0.026 kg/s

330 360 390 420 450 480 510 540

e)

0

10

20

30

40

50

60

70

80

90

ex

exergetic efficiency

d day

ency for 0.026 kg/s mass flow rate of the drying air.

m = 0.033 kg/s

0

1

2

3

4

5

6

0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450

time (minute)

ExL

0

10

20

30

40

50

60

70

80

90

ex



Fig. 11. The results of the exergy loss and exergetic efficiency for 0.033 kg/s mass flow rate of the drying air.


The mean values of ExL for 0.014, 0.02, 0.026, 0.033 and 0.036 kg/s mass flow rates of drying air were obtained 10.82 W, 6.41 W,4.92 W, 4.06 W and 2.65 W, respectively. Maximum value of exergyloss was obtained with a mass flow rate of 0.014 kg/s. Minimumvalue of exergy loss was obtained with a mass flow rate of 0.036 kg/s. These values show that the exergy loss decreased with increase ofthe mass flow rate of the drying air. Furthermore, it can be said thatthe value of radiation affected the exergy loss. The exergetic effi-ciencies of the drying chamber are also shown in Figs. 8–12. Theexergetic efficiency for each mass flow rate of drying air wascalculated by using Eq. (15) based on the inflow, outflow and loss ofexergy. The exergetic efficiency of the drying chamber increasedwith decrease of the temperature difference between inlet andoutlet of the dryer chamber.

The exergetic efficiency values with a mass flow rate of 0.014 kg/s were obtained as 21.3–78.7% for the first day experiments and

m = 0.036 kg/s

0

0,5

1

1,5

2

2,5

3

3,5

4

0 30 60 90 120 150 180 210 240 270 300 330 360time (minute)

ExL

0

10

20

30

40

50

60

70

80

90

100

ex


a day

Fig. 12. The results of the exergy loss and exergetic efficiency for 0.036 kg/s mass flowrate of the drying air.

35.2–75.6% for the second day experiments. For 0.02 kg/s mass flowrate of the drying air, the exergetic efficiencies values were recor-ded in the interval 30 %–61.7% dyring the first day experiments, andin the interval 38.7–83.3% during the second day experiments.However, for 0.026 kg/s mass flow rate of the drying air, the exer-getic efficiency altered between 40.8% and 82.1%. The exergeticefficiencies changed between 44.4% and 82.2% for the 0.033 kg/smass flow rate of the drying air. The exergetic efficiency value in theinterval 44.4 %–93.3% was recorded for the 0.036 kg/s mass flowrate of the drying air. These values show that the exergetic effi-ciency of the drying chamber decreased while the energy takenfrom the solar collector was productively utilized.

5. Conclusions

The effect of the convective solar dryer on drying of mulberriesunder five different mass flow rates was studied successively. Thedrying time considerably decreased when the mass flow rateincreased.

The drying process occurred in the falling rate period. Energyand exergy analyses of thin layer drying process of mulberry viaforced solar type dryer were performed in the scope of this study.Taking into consideration the results from these analyses, thefollowing remarks may be concluded.

� The mulberry samples were sufficiently dried until a finalmoisture content of approximately 0.1 kgwater/kgdry matter, atthe ranges between 0.014 kg/s and 0.036 kg/s drying air massflow rates during 360–780 min, and 123.3 W/m2 – 939 W/m2

solar radiation.� It is said that the energy taken from the solar collector

increased with the increase of the mass flow rate of drying air.� The energy taken from the solar collector was productively

utilized for drying chamber when the energy utilization ratio(EUR) increased. As an important note, it is said that the energyutilization ratio would be assumed as an important parameterto analyze the utilization of energy in thin layer drying process.� The exergy loss decreased with increase of the mass flow rate

of the drying air. Furthermore, it can be said that the value of


radiation affected the exergy loss. The most exergy losses tookplace for the 0.014 kg/s mass flow rate.� In order to increase the energy utilization for drying chamber,

an optimization study must be carry out leading to improvecollector efficiency by using different obstacles in the air flowduct for increasing the heat transfer area.� Consequently, it is suggested that the order, structure, and

moisture content of the products on the drying chambershould be taken into consideration to decrease the energyutilization and exergy losses.� It is necessary to show the variations of exergy with drying

time in order to determine when and where the maximum andminimum values of the exergy losses took place during thedrying process.

Acknowledgement

The authors thank to Firat University Research Foundation(FUNAF) for financial support.

References

[1] Afriyie JK, Nazha MAA, Rajakaruna H, Forson FK. Experimental investigationsof a chimney-dependent solar crop dryer. Renewable Energy 2008;34:217–22.

[2] Akpinar EK. Determination of suitable thin layer drying curve model for somevegetables and fruits. Journal of Food Engineering 2005;73:75–84.

[3] Akpinar EK, Biçer Y, Yildiz C. Thin layer drying of red pepper. Journal of FoodEngineering 2003;55:99–104.

[4] Akpinar EK, Midilli A, Biçer Y. The first and second law analyses of thermo-dynamic of pumpkin drying process. Journal of Food Engineering2006;72:320–31.

[5] Akpinar EK. Energy and exergy analyses of drying of red pepper slices ina convective type dryer. International Communications in Heat and MassTransfer 2004;31:1165–76.

[6] Colak N, Hepbasli A. Performance analysis of drying of green olive in a traydryer. Journal of Food Engineering 2007;80:1188–93.

[7] Corzo O, Bracho N, Vasquez A, Pereira A. Energy and exergy analyses ofthin layer drying of coroba slices. Journal of Food Engineering 2008;86:151–61.

[8] Dincer _I, Sahin AZ. A new model for thermodynamics analysis of a dryingprocess. International Journal of Heat and Mass Transfer 2004;47:645–52.

[9] Dincer _I. On energetic, exergetic and environmental aspects of drying systems._International Journal of Energy Research 2002;26:717–27.

[10] Doymaz _I. Drying kinetics of white mulberry. Journal of Food Engineering2004;61:341–6.

[11] Doymaz _I. Sun drying of figs: an experimental study. Journal of Food Engi-neering 2005;71:403–7.

[12] Doymaz _I. Convective air drying characteristics of thin layer carrots. Journal ofFood Engineering 2004;61:359–64.

[13] Doymaz _I. Thin layer drying behaviour of mint leaves. Journal of Food Engi-neering 2006;74:370–5.

[14] Maskan M, Gogus F. Sorption isotherms and drying characteristics of mulberry(morus alba). Journal of Food Engineering 1998;37:437–49.

[15] Midilli A. Determination of pistachio drying behavior and conditions in a solardrying system. International Journal of Energy Research 2001;25:715–25.

[16] Midilli A, Kuçuk H. Energy and exergy analyses of solar drying process ofpistachio. Energy 2003;28:539–56.

[17] Ozdemir M, Devres YO. The thin layer drying characteristic of hazelnuts duringroasting. Journal of Food Engineering 1999;42:225–33.

[18] Sarsılmaz C, Yıldız C, Pehlivan D. Drying of apricots in a rotary column cylin-drical dryer (RCCD) supported with a solar energy. Renewable Energy2000;21:117–27.

[19] Yaldız O, Ertekin C, Uzun H_I. Mathematical modeling of thin layer solar dryingof sultana grapes. Energy 2001;26:457–65.

Energy and Exergy Analyses of Thin Layer Drying of Mulberry in a Forced Solar Dryer

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