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

Energy 35 (2010) 1754–1763

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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 b

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

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 June 2009 Received in revised form 6 November 2009 Accepted 21 December 2009 Available online 8 January 2010

This paper is concerned with the energy and exergy analyses of the thin layer drying process of mulberry via forced solar dryer. Using the first law of thermodynamics, energy analysis was carried out to estimate the 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 the second law of thermodynamics. The drying experiments were conducted at different five drying mass flow rate varied between 0.014 kg/s and 0.036 kg/s. The effects of inlet air velocity and drying time on both energy and exergy were studied. The main values of energy utilization ratio were found to be as 55.2%, 32.19%, 29.2%, 21.5% and 20.5% for the five different drying mass flow rate ranged between 0.014 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 was concluded that both energy utilization ratio and exergy loss decreased with increasing drying mass flow rate while the exergetic efficiency increased. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Energy analysis Exergy analysis Thin layer drying Forced solar dryer Mulberry

1. Introduction Mulberry trees are extensively grown for their leaves as food and 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, red mulberry in north and South America and black mulberry is from southern Russia. The fruits of white mulberries are often harvested by spreading a sheet on the ground and shaking the limbs. They have a high level of moisture content at harvest. Because of the short season and the sensitivity to storage drying is often used as a preservation method. In addition, mulberry is used in mulberry molasses, juices, paste, marmalade and wine production [10,14]. Drying is defined as reduction of moisture from the products and is a most important process for preserving agricultural products 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 food preservation. The major objective in drying agricultural products is the reduction of the moisture content to a level which allows safe storage over an extended period. Sun drying is the most common method used to preserve agricultural products in the World and also Turkey. However, it has some problems related to the contamination with dust, soil, sand particles and insects [10]. In * Corresponding author. Tel.: þ90 2742652031; fax: þ90 2742652003. E-mail addresses: [email protected] (A. Akbulut), [email protected] (A. Durmus¸). 0360-5442/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2009.12.028

spite of many disadvantage, sun drying is still practiced in many places throughout the world. Solar energy is an important alternative energy sources because it is abundant, inexhaustible, renewable, cheap and non-pollutant. Recently, several studies relating the mathematical modeling and drying kinetics of the vegetables, fruits and agrobased products have been conducted by several 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 design and analysis of the mass and heat transfer process during drying. Thermodynamic analysis, particularly exergy analysis, has appears to be an essential tool for system design, analysis and optimization of thermal system [8]. Exergy is defined as the maximum amount of work which can be produced by a stream of matter, heat or work as it comes to equilibrium with a reference environment [9]. In the drying process, the aim is to use a minimum amount of energy for maximum moisture removal for the desired final conditions of the product. Several studies have been conducted on exergy analyses of food drying. However, the detailed literature review for the present study has shown that there is no information on energy and exergy analyses of the thin layer drying process of mulberry via forced solar dryer. Therefore, this paper, as different other studies, concentrates on the energy and exergy analyses of the thin layer drying of mulberry via forced solar type dryer by using the first and second law of thermodynamics. It is believed that such a study will contribute to mulberry producers by removing their

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

Nomenclature A Cp E EUR Ex F g gc h I J _ m N P Q s t T U v V

area, m2 specific Heat, kJ kg1 K1 emissive power energy utilization ratio, % exergy rate, kW shape Factor gravitational acceleration, ms2 constant in Newton’s law enthalpy, kJ kg1 solar radiation, W m2 joule constant mass flow rate, kg s1 number of species pressure, kPa net heat rate, W specific entropy, kJ kg1 K1 time, minute temperature,  C specific internal energy specific volume, m3 kg1 velocity, ms1

problems related to energy and exergy throughout the drying process. The primary objective of this study is to present energy and exergy analyses of thin layer drying of mulberry at different conditions of drying mass flow rates in a forced solar dryer. As remarkable studies about energy and exergy analysis of drying process, the following may be presented. Midilli and Kucuk [16] performed the energy and exergy analyses of the drying process of shelled and unshelled pistachios using solar drying cabinet. Dincer and Sahin [8] developed a new model for thermodynamic analysis of drying process. Akpinar [5] performed energy and exergy analyses of drying of red pepper slices via convective type dryer. Colak and Hepbasli [6] performed an exergy analysis of thin layer drying of green olive in a tray dryer. Akpinar et al. [4] conducted the first and second law analyses of thermodynamic of pumpkin drying process. Corzo et al. [7] performed energy and exergy analyses of the thin layer drying of coroba slices at three different air temperatures. 2. Material and methods Turkey has great solar energy potential due to its location in the Mediterranean Region (36 and 42 North latitudes). The sunshine period of Turkey is 2624 h/year with a maximum of 365 h/month in July and a minimum of 103 h/month in December. The main solar radiation intensity is about 3.67 kWh/m2day. The solar cabinet dryer is installed in the Technical Education Faculty of Firat University, Elazıg˘, Turkey. The solar drying experiments were carried out during the period of June 2005. Each test started at 09:00 am and continued till 17:00 pm. The drying of mulberries was conducted in solar cabinet dryer. A schematic diagram and photograph view of the solar dryer system is illustrated in Fig. 1 a and b, respectively. The system consist mainly four subsystems, namely (a) drying cabinet, (b) solar air collector, (c) air fan and AC hertz converter (d) data logger. In the experiments, weather temperature, inlet and outlet temperature of solar collector and dryer, temperature of the mulberry center, relative humidity just above the mulberry bed surface and solar radiation were recorded at 15 min intervals. In the measurements of temperatures, T Type copper – constant thermocouples were connected with a ZA9000FST connector element to 5990 – 0 Almemo

w z

1755

specific humidity, g g1 altitude coordinate, m

Greek letters 4 relative Humudity, % hex exergetic efficiency, % m chemical potential, kJ kg1 Subscripts a air c chemical col collector cp connection pipe da drying air f fan i inlet dc drying chamber L loss mp moisture of product o outlet sat saturated N surrounding or ambient

digital data logger, with reading accuracy of 0.1  C. A thermo anemometer (FVA645TH3) was used to measure air speed, with reading 0.1–15 m/s range. Pressure drop in a collector was measured by a FDA612MR pressure module. Mass loss of the mulberry were recorded during drying for determination of drying curves by FKA0251 strain strengetch in the measurement range of 0.02–10 kN an accuracy of 0.01 kN. The solar radiation during the operation period of drying system was measured with a Kipp and Zonen solarimeter. Fresh mulberries were purchased at a local market in Elazıg˘, Turkey. All data were collected using an Almemo 5990-0 data logger interfaced to the personal computer and then recorded at a 15 min time intervals. Prior to placing the sample in a dryer, the drying system was run for at least 60 min to conduct calibration. The solar dryer system consisted of a centrifugal fan that was used to blow air into the solar collector through a 82 mm diameter flexible aluminum duct. By using AC hertz converter, the mass airflow was controlled. The dryer unit was made of an inner chamber 1.2  0.74  0.74 m made of a 0.8 mm thickness stainless steel sheet that was in turn enclosed in an outer chamber 1.5  0.75  0.75 m made of stainless steel sheet. The space between the two chambers was filled with polystyrene insulating materials. On leaving the heating chamber the air passed through a (0.3  0.2  0.2 m) chimney chamber to allow it to mix and achieve uniform temperature before entering the drying chamber. The experiments were carried out using fresh mulberry with an average initial moisture content of approximately 3 kg of water/kg of dry solids and fresh mulberry were placed in the dryer. No pretreatment was applied to the fresh product. 3. Analysis In first and second law analyses of thermodynamics, the drying process was considered as steady flow process. The main basis of these 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 energy analysis of the thin layer drying process of mulberry is performed to

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

determine more about the energy aspects and behaviour of drying air throughout the forced solar dryer. The air conditioning process throughout the mulberry drying includes the heating, cooling and humidification processes. Actually, the air conditioning process can be modeled as steady flow processes that are analyzed by employing the steady flow conservation of mass and conservation of energy principles. For the energy and exergy analyses of the thin layer drying process, the following equations are generally employed to compute mass conservation of the drying air and moisture, the energy conservation of the process and relative humidity and enthalpy of the drying air. General equation of mass conservation of drying air:

X

_ da;i ¼ m

X

_ da;o m

_ ¼ Q_  W

X

_ da;o m

V2 ho þ o 2

_ da;o ¼ m _ da;i ¼ m _ da m

! 

X

_ da;i m

V2 hi þ i 2

! (3)

(4)

The changes in kinetic energy of the fan were take into consideration, while the potential and kinetic energy in other parts of the process were neglected:

f¼

wP ð0:622 þ wÞPsat@T

(5)

(1)

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:

(2)

h ¼ CPda Tda þ whsat@T

General equation of mass conservation of moisture:

X X  _ da;o $wo _ da;i $wi þ m _ mp ¼ m m

General equation of energy conservation:

(6)

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

_ da cpda Exergy ¼ m

In the scope of the second law analysis, the total exergy inflow, outflow and losses of the forced solar dryer were estimated. The basic procedure for exergy analysis of the drying chamber is to determine the exergy values at steady state points and the reason of exergy variation for the process. The exergy values are calculated by using the characteristics of the working medium from a first law energy balance For this purpose, the general form of exergy equation applicable for steady flow systems was employed [16].

Ekserji ¼



u  uN internal energy





 To s  sN entropy



  Po þ v  vN J work

 X g mc  mN Nc þ z  zN þ þ gravity gc J chemical c momentum   4 þ Eg Ag Fg 3T 4  TN  4TN T 3 þ / radiation V2 2gJ





ð12Þ

emission where the subscript Ndenotes the reference conditions. There are variations of this general exergy equation. In the analyses of many systems, some, but not all, of the terms shown in Eq. (12) are used. Since exergy is energy available from any source, the terms

X

Exo

(14)

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

  T _ da cpda ðTdci  TN Þ  TN ln dci Exdci ¼ m TN

(15)

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

  T _ da cpda ðTdco  TN Þ  TN ln dco Exdco ¼ m TN

(16)

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

hEx ¼

3.2. The second law analysis

Exi 

Exi  ExL Ex ¼ 1 L Exi Exi

(17)

4. Results and discussion The drying experiments were conducted during the period of July to August 2005 in Elazıg˘, Turkey. During the thin layer drying

4 m=0.014

3,5

m=0.02 3 m=0.026 2,5

m=0.033

2

m=0.036

1,5 1 0,5 0

Time (min) Fig. 2. Variation of moisture content with drying time.

780

(11)

X

720

  _ da hdc;i  hdc;o m   EUR ¼ _ da $Cpa Tcol;o  Tcol;i m

ExL ¼

660

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

(13)

600

(10)

X

Moisture content (kg-water/kg-dry matter)

Qdc

_ da $ðhdci@T  hdco@T Þ ¼ m



540

During the dehumidification process at the drying chamber, the heat used can be estimated by employing the following equation and using psychrometric chart.

Tg To

480

(9)

 T0 ln

420

  _ da $cpa $ Tcol;o  Tdc;i QL;cp ¼ m



360

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

120

(8)

60

  _ da $cPda $ Tcol;o  Tcol;i Qcol ¼ m

Tg  To

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

0

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



300

(7)

240

wcol;i ¼ wfo Tcol;i ¼ Tfo fcol;i ¼ ffo hcol;i ¼ hfo

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

180

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

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m Collector

Draying chamber

0.014kg / s

Energy utilation ratio(EUR)

450

100

400

90 80

350

70 60

250 50 200

EUR (%)

Q (Watt)

300

40 150

30

100

20

50

second day

first day 0

10 0

0

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

time (minute) 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 changed between 123.3 W/m2 and 939 W/m2, the temperature of ambient air ranged from 28  C to 45  C. The air temperature reached at maximum value between 11.00 h and 15.00 h. The initial moisture content of fresh mulberries was close to 80% (wet basis). Drying was continued until the sample reached the desired moisture level (8%, wet basis). The solar radiation energy is maximum at midday and minimum at evening in day of experiment. The energy and exergy analyses of the thin layer drying process of mulberry via forced solar dryer were performed by using data from the

experiments and the results obtained from these calculations are presented in Figs. 3–12 and discussed in detail. 4.1. Moisture content Fig. 2 presents the variations of moisture content as a function of drying time for the mass flow rates of 0.014, 0.02, 0.026,0.033 and 0.036 kg/s. As the moisture content of mulberry in the solar type dryer decrease, the moisture diffusion from the mulberry to the air decreases as well. It can be seen that the relative humidity in the

m =0.02kg/s Collector

Draying chamber

Energy utilation ratio(EUR)

600

50 first day

second day

45

500

40 35 30 300

25 20

200

15 10

100

5 0 0

45

80

135 180 225 270 315 360 405 450 495 540 585 630 675 720

time (minute) Fig. 4. The results of the energy analysis for 0.02 kg/s mass flow rate of the drying air.

0

EUR (%)

Q (Watt)

400

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

1759

m = 0.026kg/s Collector

Draying chamber

Energy utilation ratio(EUR) 45

700

first day

600

second day

40 35

500

400

25 20

300

EUR (%)

Q (Watt)

30

15 200

10 100

5 0

0 0

30

58

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

time (minute) Fig. 5. The results of the energy analysis for 0.026 kg/s mass flow rate of the drying air.

drying air declines in accordance with the moisture content in mulberry. 4.2. Energy analysis The energy analysis of thin layer drying process of mulberry was performed by using data obtained from the forced solar dryer

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

m =0.033kg/s Collector

Draying chamber

Energy utilation ratio(EUR)

45

700

40

600 first day

second day

35

500

400

25 20

300

15 200 10 100

0 0

5 0 30 58 80 120 150 180 210 240 270 300 330 360 390 420 450

time (minute) Fig. 6. The results of the energy analysis for 0.033 kg/s mass flow rate of the drying air.

EUR (%)

Q (Watt)

30

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

m =0.036kg/s Collector

Draying chamber

Energy utilation ratio(EUR)

700

45 a day

600

40 35 30

400

25 20

300

EUR (%)

Q (Watt)

500

15

200 10

100

5

0

0 0

30

58

80

120

150

180

210

240

270

300

330

360

time (minute) Fig. 7. The results of the energy analysis for 0.036 kg/s mass flow rate of the drying air.

the first day experiments, respectively. The value of Qcol was obtained in the ranges of 99.987–345.66 W during 375 min for the second day experiment, respectively. Fig. 3 also shows the variations of energy utilization ratio (EUR) of drying process for 0.014 kg/s mass flow rate. It was obtained that EUR varied between 15.99% and 89.25% during the first day experiments. For the second day experiments, the EUR varied between 27.11% and 45.37%. Fig. 4 shows the values of Qcol, Qdc and EUR for 0.02 kg/s mass flow rate of the drying air. The value of Qcol varied between 276 and 515 W in the first day experiment, 180–507.55 W in the

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

m = 0.014 kg/s Exergy Loss

exergetic efficiency

90

16

first day

second day 80

14

70

12

60

10 L

Ex

50 8 40 6

30

4

20

2 0

10 0 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. 8. The results of the exergy loss and exergetic efficiency for 0.014 kg/s mass flow rate of the drying air.

ex

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

1761

m = 0.02 kg/s Exergy Loss

exergetic efficiency

12

90

first day

second day

80

10 70 8

60 50

L Ex

6 40 4

ex

30 20

2 10 0

0

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 process for the mass flow rates of 0.026 kg/s. It was obtained that Qcol and EUR ranged between 394 W and 627 W, 7.86% and 29.18%, respectively. Maximum value of Qdc was obtained 275 W with mass flow rates of 0.026. Fig. 6 shows the variations of Qcol and Qdc on the left ordinate and the EUR on the right, as a function of drying 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 the results 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%,

32.19%, 29.2%, 21.5% and 20.5%, respectively. These values show that EUR of drying chamber decreased with the increase of mass flow rate of drying air. 4.3. Exergy analysis The exergy analysis of thin layer drying process of mulberry via forced solar dryer was performed by using data obtained from the drying experiments. The values of ExLand hEx for each mass flow rate of the drying air can be observed in Figs. 8–12. In the drying experiments with five different mass flow rates performed exergy loss in the drying chamber increased during the first 6 h, and after that showed a decaying behaviour. Obviously, such time variation of the exergy loss appeared as a consequence of the changes in the solar radiation.

m = 0.026 kg/s Exergy Loss

exergetic efficiency

7

90

first day

second day 80

6

70

5

60

4

50

3

40

L Ex

30

2

20

1

10

0

0 0

30

60

90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540

time (minute) Fig. 10. The results of the exergy loss and exergetic efficiency for 0.026 kg/s mass flow rate of the drying air.

ex

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m = 0.033 kg/s exergetic efficiency

Exergy Loss 6

90 80

5 70 4

60 50

L Ex 3

40 2

ex

30

first day

1

20

second day

10 0

0

0

30

60

90

120

150

180

210

240

270

300

330

360

390

420

450

time (minute) 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 exergy loss was obtained with a mass flow rate of 0.014 kg/s. Minimum value 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 of the mass flow rate of the drying air. Furthermore, it can be said that the value of radiation affected the exergy loss. The exergetic efficiencies of the drying chamber are also shown in Figs. 8–12. The exergetic efficiency for each mass flow rate of drying air was calculated by using Eq. (15) based on the inflow, outflow and loss of exergy. The exergetic efficiency of the drying chamber increased with decrease of the temperature difference between inlet and outlet 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

exergetic efficiency

4

100 90

3,5

80

3

70 2,5 L

Ex

a day

2

60 50 ex

40

1,5

30

1

20

0,5 0

10 0

30

60

90 120 150 180 210 240 270 300 330 360

5. Conclusions The effect of the convective solar dryer on drying of mulberries under five different mass flow rates was studied successively. The drying time considerably decreased when the mass flow rate increased. The drying process occurred in the falling rate period. Energy and exergy analyses of thin layer drying process of mulberry via forced solar type dryer were performed in the scope of this study. Taking into consideration the results from these analyses, the following remarks may be concluded.

m = 0.036 kg/s Exergy Loss

35.2–75.6% for the second day experiments. For 0.02 kg/s mass flow rate of the drying air, the exergetic efficiencies values were recorded in the interval 30 %–61.7% dyring the first day experiments, and in 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 exergetic efficiency altered between 40.8% and 82.1%. The exergetic efficiencies changed between 44.4% and 82.2% for the 0.033 kg/s mass flow rate of the drying air. The exergetic efficiency value in the interval 44.4 %–93.3% was recorded for the 0.036 kg/s mass flow rate of the drying air. These values show that the exergetic efficiency of the drying chamber decreased while the energy taken from the solar collector was productively utilized.

0

time (minute) Fig. 12. The results of the exergy loss and exergetic efficiency for 0.036 kg/s mass flow rate of the drying air.

 The mulberry samples were sufficiently dried until a final moisture content of approximately 0.1 kgwater/kgdry matter, at the ranges between 0.014 kg/s and 0.036 kg/s drying air mass flow 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 energy utilization ratio would be assumed as an important parameter to 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

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

radiation affected the exergy loss. The most exergy losses took place 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 improve collector efficiency by using different obstacles in the air flow duct for increasing the heat transfer area.  Consequently, it is suggested that the order, structure, and moisture content of the products on the drying chamber should be taken into consideration to decrease the energy utilization and exergy losses.  It is necessary to show the variations of exergy with drying time in order to determine when and where the maximum and minimum values of the exergy losses took place during the drying process. Acknowledgement The authors thank to Firat University Research Foundation (FUNAF) for financial support.

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