2009-solar drying of peeled longan using a side loading

8/2/2019 2009-Solar Drying of Peeled Longan Using a Side Loading

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Solar Drying of Peeled Longan Using a Side LoadingType Solar Tunnel Dryer: Experimental and

Simulated PerformanceS. Janjai,1 N. Lamlert,1 P. Intawee,1 B. Mahayothee,2 Y. Boonrod,1

M. Haewsungcharern,3 B. K. Bala,4 M. Nagle,5 and J. Muller5

1Solar Energy Research Laboratory, Department of Physics, Faculty of Science, Silpakorn

University, Nakhon Pathom, Thailand2Department of Food Technology, Faculty of Engineering and Industrial Technology, Silpakorn

University, Nakhon Pathom, Thailand3Department of Food Engineering, Chiang Mai University, Chiang Mai, Thailand4Department of Farm Power and Machinery, Bangladesh Agricultural University,

Mymensingh, Bangladesh5Institute of Agricultural Engineering, Hohenheim University, Stuttgart, Germany

This article presents experimental and simulated results ofdrying of peeled longan in a side-loading solar tunnel dryer. Thisnew type of solar tunnel dryer consists of a flat-plate solar air heaterand a drying unit with a provision for loading and unloading fromwindows at one side of the dryer. These are connected in seriesand covered with glass plates. A DC fan driven by a 15-W solar cellmodule supplies hot air in the drying system. To investigate theexperimental performance, five full-scale experimental runs wereconducted and 100 kg of peeled longan was dried in each experimen-tal run. The drying air temperature varied from 32 to 76C. Thedrying time in the solar tunnel dryer was 16 h to dry peeled longan

from an initial moisture content of 84% (w.b.) to a final moisturecontent of 12% (w.b.), whereas it required 16 h of natural sun dryingunder similar conditions to reach a moisture content of 40% (w.b.).The quality of solar-dried product was also good in comparison tothe high-quality product in markets in terms of color, taste, and fla-vor. A system of partial differential equations describing heat andmoisture transfer during drying of peeled longan in this solar tunneldryer was developed and this system of nonlinear partial differentialequations was solved numerically by the finite difference method.The numerical solution was programmed in Compaq VisualFORTRAN version 6.5. The simulated results agreed well withthe experimental data for solar drying. This model can be used toprovide the design data and it is essential for optimal design ofthe dryer.

Keywords Experimental performance; Mathematical modeling;Side-loading solar tunnel dryer; Simulation; Solardrying of peeled longan

INTRODUCTION

Longan (Dimocarpus longan Lour.) is an important fruitof Thailand with an export value of US$109 million in2004[1] and it shows an increasing trend in productionrate since 1990.[2] It is a seasonal fruit and its price is lowduring the harvest season. Several preservation techniquesare available to add value to this seasonal product such ascanning and drying.

Drying is one of the oldest methods for preservationof fruits and vegetables and it also an energy-intensive

operation. Improved drying technology is needed forimprovement of the quality of dried products for valueaddition, reduction of postharvest losses, and conservationof energy.

Mechanical hot air drying is a common method for dry-ing of whole longan fruit for large-scale drying for exportindustries and peeled longans derived from broken-shelllongans are also dried in small- to medium-scale mechanicaldryers. Most of the peeled longan is dried in a large, openflat bed using hot stoves burning fuel wood and charcoals.This procedure results in poor quality dried product dueto dirt and dust from the burning stoves. Some peeled long-ans are also dried under the sun on mats. During sun drying,

peeled longan fruits on mats are exposed to the openenvironment and are contaminated by dust and insects,resulting in poor quality dried fruits with less value addition.

Thailand, located in the tropical regions of SoutheastAsia, receives annual average daily solar radiation of18.2 MJm2day1.[3] Thus, utilization of solar energy to pro-duce quality solar-dried peeled longan has been consideredto be the most promising option. Furthermore, solar energyis a renewable and environmental friendly energy source.

Correspondence: S. Janjai, Solar Energy Research Laboratory,Department of Physics, Faculty of Science, Silpakorn University,Nakhon Pathom, Thailand 73000; E-mail: [email protected]

Drying Technology, 27: 595605, 2009

Copyright# 2009 Taylor & Francis Group, LLC

ISSN: 0737-3937 print=1532-2300 online

DOI: 10.1080/07373930802716383

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Solar drying can be considered as an elaboration of sun

drying and is an efficient system of utilizing solar

energy.[46] Many studies have been reported on natural

convection solar drying of agricultural products.[715]

Considerable studies on simulation of natural convection

solar drying of agricultural products and optimization have

also been reported.[1619] The success achieved by natural

convection solar dryers has been limited due to lowbuoyancy-induced air flow. This prompted researchers to

develop forced convection solar dryers. Many research

and performance studies have been reported on forced

convection solar dryers.[2027] Studies on simulation and

optimization of forced convection solar dryers have also

been reported.[28,29] Numerous tests of forced convection

solar tunnel dryers were carried out in the different regions

of the tropics and subtropics. The results of the tests have

shown that fruits, vegetables, cereals, grain, legumes, oil

seeds, spices, fish, and even meat can be dried properly in

the UV-resistant plastic-covered solar tunnel dryer.[3039]

The UV-resistant plastic-covered solar tunnel dryer suffers

from several drawbacks. The plastic cover lasts for only a

few months during the drying season. The loading and

unloading of the products in this type of dryer are also

not convenient because they require the opening of the

plastic cover over the drying section. This prompted the

development of a side-loading glass-covered solar tunnel

dryer at Silpakorn University in which the loading and

unloading are conducted through the windows at one side

of the dryer.[40,41]

Solar drying systems must be properly designed in order

to meet particular drying requirements of specific products

and to give satisfactory performance. Designers should

investigate the basic parameters such as dimensions, tem-perature, relative humidity, air flow rate, and the charac-

teristics of products to be dried. However, full-scale

experiments for different products, drying seasons, and sys-

tem configurations are sometimes costly and not possible.

The development of a simulation model is a valuable tool

for predicting the performance of solar drying systems.

Again, simulation of solar drying is essential to optimize

the dimensions of solar drying systems and optimization

techniques can be used for optimal design of solar drying

systems.[4]

Although many studies have been reported on solar

drying of fruits and vegetables,[21,22,36,3943] limited studies

have been reported on drying of longan fruit.[42,43]

Achariyaviriya et al.[43] reported quality tests of dried

peeled longan. Recently, Varith et al.[2] reported drying

of peeled longan using combined microwavehot air and

also reported drying kinetics using Sherwoods exponential

equation, quality indices, and specific energy consumption.

Limited studies have been reported on drying of longan

fruit and no systematic experimental and simulated study

on solar drying of peeled longan has been reported. This

article presents a systematic experimental and simulated

study of solar drying of peeled longan using a side-loading

glass-covered solar tunnel dryer.

EXPERIMENTAL STUDY

The new version of the solar tunnel dryer (side loading

and unloading through windows with glass cover type)

was constructed at Silpakorn University, Nakhon Pathom,Thailand. It consists of two parts: a solar air heating collec-

tor and a drying unit. Both parts have similar structures

and are connected in series. The top side of both parts is

covered by glass plates fixed with silicon glue to the alumi-

num frames. These glass plates with the frames can be

easily removed from the dryer when the dryer needs to be

cleaned. The bottom side of the dryer is made of high-

density foam sandwiched between two galvanized iron

sheets, which functions as a back insulator to reduce heat

losses. The upper surface of this back insulator was painted

black and functions as a solar radiation absorber. The side

walls of the dryer are made of a galvanized iron sheet. They

were designed in such a manner that they are supported by

the back insulator at the bottom side and the side walls also

support the glass plates that cover the dryer, as shown in

Fig. 1. The two side walls have different heights, so that

the glass plates make a tilted angle of 10 to facilitate

drainage of water in case of rain. The higher side wall

has windows, each of which has the dimension of

20cm 80 cm for loading and unloading products. Each

window has a galvanized sheet cover to prevent rain and

air leakage. The top glass cover, the bottom insulator,

and the side walls were made in modules. Each module

can be interlocked at the junction to prevent air leakage

and to facilitate construction and transportation.For a ventilation system, a DC fan driven by a 15-W

solar cell module was installed at the collector to suck

ambient air into the dryer (Fig. 1).

Solar radiation passing through the glass cover heats the

absorber. Ambient air is forced through the collector and,

while passing it, the collector gains heat from the absorber.

FIG. 1. Schematic diagram of the side-loading solar tunnel dryer.

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This heatedair, while passing through the drying unit, absorbsmoisture from the peeled longan. The product receivesheat from solar passing through the glass cover of the dryingunit in addition to the heat received from the collector andthis enhances the drying rate of the product.

A total of five full-scale experimental runs were conductedand the dryer was loaded to a capacity of 100kg of peeled

longan for each experimental run. The drying was started at8 am and continued until 6 pm. The control sample was alsodried naturally under the same weather conditions.

To investigate the dryer performance, various sensors wereplaced at the dryer. Solar radiation was measured by a pyran-ometer (Kipp & Zonen model CM 11, precision 0.5%) andthis was horizontally placed on the top of the dryer. K-typethermocouples were used to measure the air temperatures inthe collector and drying unit (precision 2%). The positionsof the thermocouples for the measurements of temperaturesin the collector and drying unit are shown in Fig. 2. A hot-wire anemometer (Airflow, model TA5, precision 2%) wasemployed to monitor the air speed in the collector and dryingunit. This anemometer was also used to monitor the windspeed. Relative humidities of ambient air and drying air wereperiodically measured with hygrometers (Electronik, modelEE23, precision 2%).

Voltage signals from the pyranometer, hygrometers andthermocouples were recorded every 10 min by a 40-channeldata logger (Yokogawa, model DC100). The air speeds inthe solar collector and drying unit were also manually readand recorded every hour during the drying experiments. Sam-ples of products in the dryer were weighed at 2-h intervalsusing a digital balance (Satorius, model E2000 D, precision0.0001 g). The positions of the measurements of the sample

weights in the drying unit are also shown in Fig. 2. Before theinstallations, the pyranometer for measuring solar radiationwas calibrated against a new pyranometer recently calibratedby Kipp & Zonen, the manufacturer. The hygrometers werecalibrated using standard saturated salt solutions suppliedby the manufacturer. Before being used, the thermocoupleswere also tested by measuring the boiling and freezingtemperatures of water to ensure the accuracy.

For the drying tests, 100 kg of peeled longan was usedfor each experimental run. The experiments were startedat 8.00 am and continued until 6.00 pm. During the night,the products were kept in the dryer. The process wasrepeated until the desired moisture content (about 12%

wet basis) was reached. This final moisture contentcorresponds to the moisture content of high-quality dry

product in local markets. About 100 g of the product sam-ple was taken from the dryer and weighed at 2-h intervals.The moisture content during drying was estimated from theweight of the product samples and the estimated dried solidmass of the samples. At the end of the drying process, theexact dry solid mass of the product samples was deter-mined by using the air oven method. The samples were

placed in the oven at the temperature of 103C for 24h(precision 0.5%).

MATHEMATICAL MODELING

Energy Balances in the Collector (see Fig. 3)

Energy Balance for Glass Cover

The energy balance for the glass cover is as follows:Rate of thermal energy accumulation in the cover rate

of radiative heat transfer between the absorber and thecover rate of the convective heat transfer between thecover and air in the collector rate of the convective heatloss from the cover due to the wind rate of radiative heat

transfer between the cover and the sky rate of solarradiation absorption in the coverThis energy balance can be formulated as:

qcdcCc@Tc@t

hr;bcTb Tc hc;fcTf Tc

hwTa Tc hr;csTs Tc acIt 1

Energy Balances in the Air Stream

The energy balance in the air stream in the collector canbe written as:

Rate of enthalpy change in the air stream convectiveheat transfer between the air steam and the cover

convective heat transfer between the air stream and theabsorber.

FIG. 2. Positions of the thermocouples (T) and hygrometers (rh) andproduct sample for moisture determination (M).

FIG. 3. Heat transfer in the solar collector: (1) Radiative heat transferbetween absorber and cover. (2) Radiative heat transfer between

cover and sky. (3) Convective heat transfer between absorber and air.(4) Convective heat transfer between cover and air. (5) Convective heattransfer of cover due to wind. (6) Conduction loss through back insulator.

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This energy balance can be expressed mathematically as:

DcGCf@Tf@x

hc;cfTc Tf hc;bfTb Tf 2

Energy Balance for the Absorber

The energy balance for the absorber is considered as

follows:Rate of thermal energy accumulation in the absorber

rate of convective heat transfer between the absorber and

the air stream rate of radiative heat transfer between thecover and the absorber rate of the heat loss from theabsorber through the back insulator to ambient air rateof solar radiation absorbed by the absorber.

This energy balance can be written as:

qbdbcb@Tb@t

hc;bfTf Tb hr;bcTc Tb

UbTa Tb saIt 3

The radiative heat transfer coefficient (hr,cs) from thecover with temperature of Tc to the sky with the equivalent

temperature Ts was computed as:[44]

hr;cs ecrT2c T

2s Tc Ts 4

where ec is the emissivity of the cover and r is the Stefan-

Boltzmann constant.

The sky temperature (Ts) was computed from

Ts 0:552T1:5a 5

where Ta is the ambient temperature in K.

The radiative heat transfer coefficient (hr,bc) between

the absorber with the temperature of Tb and the cover withtemperature Tc was computed as:

[44]

hr;bc rT2b T

2c Tb Tc

1

eb

1

ec 1

6

where eb is the emissivity of the absorber.

The convective heat transfer coefficient from the cover

to the ambient air due to wind (hw) was calculated from:[44]

hw 5:7 3:8Va 7

where Va is the wind speed in m=s.The convective heat transfer coefficient inside thecollector for either the cover hc,cf or absorber hc,bf was

computed from the following relationship:

hc;bf hc;cf Nu ka

Dh8

where Nu is the Nusselt number, ka is thermal conductivity

of the air, and Dh is the hydraulic diameter of the collector.

Nusselt number, Nu, was computed from the following

relationship:[45]

Nu 0:0158Re0:8 9

where Re is the Reynolds number, which is given by:

Re Dh Vqa

n

10

where V is the air speed in the collector, qa is air density,

and n is the viscosity of the air.

Overall heat loss coefficient from the bottom of the

absorber (Ub) was computed from the following relation:

Ub kb

Lb11

where kb and Lb are the thermal conductivity and the

thickness of the back insulator, respectively.

The system of Eqs. (1)(3) was solved numerically using the

finite difference technique. The length of the collector is divided

into a number of sections with the section length ofDx so thatthe properties of the materials are constant or nearly so within

each section. The time interval should be small enough for the

air conditions to be constant over the distance Dx. A compro-

mise between the computing time and precision must be

considered for the economy of computing.

Energy and Mass Balances in the Drying Unit (see Fig. 4)

For each component of the drying unit, the energy

balances and mass balances were written as follows.

Energy balance for the cover of the drying unit:

Rate of energy accumulation in the cover rate ofthe radiative heat transfer between the cover and the

product (peeled longan) rate of the convective heattransfer between the moist air steam in the drying unit

and the cover rate of heat loss from the cover due towind rate of radiative heat loss from the cover to thesky rate of solar energy absorption by the cover.

This energy balance can be expressed mathe-

matically as:

qcdcCc@Tc1@t

hr;pcTp Tc1 hc;fcTf Tc1

hwTa Tc1 hr;csTs Tc1 acIt 12

Energy balance for the moist air steam in the drying unit:

Rate of enthalpy change of the moist air steam

rate

of the convective heat transfer between the product and

the moist air steam rate of convective heat transferbetween the moist air steam and the cover.

This energy balance can be written as:

DGCf CvH@Tf1@x

hc;pfTp Tf1 hc;fcTc Tf1

13

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Energy balance for the product (peeled longan):Rate of enthalpy change of the product rate of

the sensible and latent heat for the evaporization ofthe moisture from the product rate of the convectiveheat transfer between the air steam and the productrate of the radiative heat transfer between the coverand the product rate of heat loss through the back

insulator from the product to ambient air rate ofsolar radiation absorbed by the product.

This energy balance can be represented mathemati-cally as follows:

qs;pCp CwM@Tp@t

hfg CvTp TfDG@H

@x

hc;pfTf Tp

hr;pcTc Tp

UbTa TpscapIt 14

Mass balance equation:The rate of moisture transfer between the product

and the air steam can be written as:

DG@H

@x qs;p

@M

@t15

Thin-layer drying equation of the product:To obtain the thin-layer drying equation, thin-layer

drying experiments of peeled longan were conductedunder controlled conditions of the drying air in a labora-tory dryer.[46] A number of experiments were carried outfor different values of the temperature (T) and relativehumidity (rh). For each experiment, the temperatureand relative humidity were fixed and the weight of thepeeled longan in this laboratory dryer was measured

during drying. Afterwards, this weight was used to deter-mine the moisture content (M) of peeled longan.

The following thin-layer drying model based on Pagesequation was used to fit the drying data obtained fromthese experiments:

M MeM0 Me

expktA 16

where

k 0:00046 0:002553 T 0:39939 rh 17

A 0:0526026 0:013001797 T 7:31486676 rh 18

The equilibrium moisture content of peeled longan (Me)proposed by Janjai et al.[47] was used in this work.Radiative heat transfer coefficient (hr,pc) between

the cover with temperature Tc and the product withtemperature Tp was computed as:

[44]

hr;pc rT2p T

2c Tp Tc

1

ep

1

ec 1

19

where ep is the emissivity of the product.The system of Eqs. (12)(14) was solved numerically

using the finite difference technique.On the basis of the drying air temperature and relative

humidity for all the sections of the drying unit, the con-stants k and A were computed from Eqs. (17)(18) andequilibrium moisture content (Me) of the peeled longan

was calculated by using the model proposed by Janjaiet al.[47] Using the k, A, and Me values, the change in mois-ture content of peeled longan DM for all the sectionsfor a time interval D t were calculated using Eq. (15). Nextthe system of finite difference equation derived fromEqs. (12), (13), and (14) for the interval Dt for the entirelength of drying unit was formulated. This system of equa-tions is a set of implicit equations and was solved usingthe Gauss-Jordan elimination method. Using the recentvalue of drying air temperature and relative humidity forthe different sections of the entire length of drying unit,the change in moisture content of peeled longan, DM, fornext time interval for the different sections of the entirelength of drying unit was calculated and the process wasrepeated until the final time was reached. The numericalsolution was programmed in Compaq Visual FORTRANversion 6.5.

RESULTS AND DISCUSSION

Experimental Results

Full-scale tests of the side-loading solar tunnel dryer fordrying of peeled longan were carried out during Augustand September 2007. The typical results for drying ofpeeled longan are shown in Figs. 510. Figure 5 shows

FIG. 4. Energy balances in the drying unit: (1) Radiative heat transfer

between the product and the cover. (2) Radiative heat transfer betweencover and sky. (3) Convective heat transfer between the product and

air. (4) Convective heat transfer between cover and air. (5) Convectiveheat transfer of cover due to wind. (6) Heat loss through back insulator.(7) Mass transfer.



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FIG. 5. Variations of the solar radiation during a typical experimental run.

FIG. 6. The variations of the temperatures (T) at different positions inside the collector (see Fig. 2).

FIG. 7. The variations of the temperatures at different positions inside the drying unit (see Fig. 2).

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the variations of solar radiation during a typicalexperimental run of solar drying of peeled longan in a side-loading solar tunnel dryer. The solar radiation variedbetween 0 and 1108 W=m2 during the drying period. Therewere some fluctuations in solar radiation due to clouds andrainfall. In the early part of the first and second days of thedrying test, there were clouds, which resulted in fluctua-tions in solar radiation and during the later part of thesecond day, it rained, which also resulted in fluctuationsin solar radiation. However, the overall tendency of thepatterns of changes in solar radiation was sinusoidal.

Figure 6 shows the comparison of the temperaturechanges at different positions inside the collector for a typi-cal experimental run of solar drying of peeled longan in theside-loading tunnel dryer. The temperatures in the differentpositions increased until noon and then decreased in the

afternoon. The patterns of temperature changes are almostsimilar for all the days and the temperature increased withthe increase in distance from inlet inside the collector.There was a significant difference in temperature levels inthe different positions of the collector at a significance levelof 5%. The temperature at outlet of the collector increasedup to 72C.

Figure 7 shows the variations of the temperatures atdifferent positions inside the drying unit for a typicalexperimental run of solar drying of peeled longan insidethe dryer. The temperatures in the different positionsincreased until noon and then again decreased in theafternoon. Temperatures in the different positions insidethe drying unit changed within narrow band for thepositions T6 to T15 and are different at a significancelevel of 5%.

Figure 8 shows relative humidities at three differentpositions inside the dryer for a typical experimental runduring solar drying of peeled longan in the side-loadingsolar tunnel dryer. The relative humidities in the differentlocations decreased in the early part of the day becauseof the decreased level of the ambient relative humidityand increased level of the drying air temperature with time,

whereas the relative humidities in the different locationsincreased in the later part of the day because of theincreased level of the ambient relative humidity anddecreased level of the drying air temperature with time inthe later part of the day. The relative humidity at differentpositions inside the drying unit depends on the moisturereleased from the product and the air temperature. For

FIG. 8. Variations of relative humidities (rh) at three different positions inside the drying unit during a typical experimental run (see Fig. 2).

FIG. 9. Variations of mass flow rate inside the dryer during a typical experimental run.



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most cases, the relative humidity at the outlet of the dryerwas higher than that at the middle of the dryer.

Figure 9 shows mass flow rate inside the dryer of a

typical experimental run. The mass flow rate inside dryerincreased until noon and then decreased in the afternoon.The pattern of changes in mass flow rate followed thepattern of the changes in solar radiation (Fig. 5). This isdue to the fact that the mass flow rate was provided by aDC fan operated by a solar cell module and the outputpower of the module depended strongly on solar radiation.However, there was a threshold value of solar radiationfor the operation of the DC motor used to run the fanto provide the mass flow rate in the dryer.

Figure 10 shows the variations of the moisture contentsof peeled longan in five different positions inside dryer andof the control sample for a typical experimental run of the

experiments. The moisture content of peeled longan in thesolar dryer was reduced from an initial value of 84% (w.b.)to the final value of 12% (w.b.) within 16 h, whereas themoisture content of the sun-dried samples was reducedto 40% (w.b.) within the same period under similarconditions.

Statistical analysis shows that there is no significantdifference in solar drying of peeled longan in the differentlocations of the side-loading solar tunnel dryer at a signifi-

cance level of 10%, but there is a significant difference indrying after hours of initial drying between drying insidethe solar dryer and sun drying of peeled longan.

Thus, the drying in the side-loading solar tunnel dryerresulted in reduction in drying time and production ofbetter quality dried product. The color of the dried peeledlongan was comparable to that of high-quality driedpeeled longan in markets when the color was tested.

For the economic analysis, it was assumed that the dryeris used to dry peeled longan during the harvest season andto dry off-season longan. Based on production of driedlongan and the capital and operating cost of the dryingsystem, the payback period of this system was estimated

to be 2.9 years.

Simulated Results

To validate the model, the predicted air temperatures atthe collector outlet and moisture contents of peeled longanduring drying were compared to the experimental values.

FIG. 10. Variations of the moisture contents (M) in the different locations of the drying unit for a typical experimental run during drying of peeledlongan. The positions of the measurement of moisture contents are shown in Fig. 2.

FIG. 11. Comparison of the simulated and observed collector outlet temperature during drying of peeled longan for a typical full-scaleexperimental run.

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Figure 11 shows a typical comparison between thepredicted and experimental values of the temperatures atthe outlet of the collector for 2 consecutive days startingfrom September 9, 2007, to September 10, 2007. Predictedtemperature shows plausible behavior and the agreement isgood. Figure 12 shows a typical comparison of the pre-dicted and observed moisture contents of peeled longaninside the dryer and the model predicts well the moisturecontent changes during drying.

The model predictions were evaluated on the basis ofroot mean square difference (RMSD). RMSD of theprediction of the collector outlet temperatures was 4%.This indicates that the model can predict the temperaturewith a reasonable accuracy. RMSD of the prediction ofthe moisture contents was 4.1%. Thus, the model predic-tions are reasonably good. Furthermore, predictions arewithin the acceptable limit (10%).[48]

CONCLUSION

From the drying experiments of peeled longan inthe side-loading tunnel dryer, the temperature inside thecollector varied with the positions but it varied within anarrow band for the positions starting from the middleof the dryer to exit of the dryer in the middle of the day.The pattern of changes of air velocity inside the solartunnel dryer followed the pattern of changes in the solarradiation. Field-level tests demonstrate the potentiality ofsolar drying of peeled longan in the side-loading solar tun-nel dryer. Solar drying of peeled longan in the side-loadingsolar tunnel dryer resulted in considerable reduction in

drying time compared to the natural sun drying and theproducts dried in the solar tunnel were quality dried pro-ducts. The payback period of the side-loading solar tunneldryer is 2.9 years.

A system of partial differential equations for heatand moisture transfer was developed for solar drying ofpeeled longan in a side-loading solar tunnel dryer and themodel was programmed in Compaq Visual FORTRANversion 6.5. The simulated air temperatures at the collector

outlet agreed well with the observed data on temperatures.Good agreement was found between the experimentaland simulated moisture content of peeled longan duringdrying and the precision was reasonable. This model canbe used for providing design data for a side-loading solartunnel dryer and for optimization of a side-loading solartunnel dryer.

NOMENCLATURE

Ac Collector area (m2)

Cb Specific heat of absorber material (J=kg K)Cc Specific heat of cover material (J=kg K)Cf Specific heat of air (J=kg K)Cp Specific heat of peeled longan (J=kg K)Cv Specific heat of water vapor (J=kg K)Cw Specific heat of water (J=kg K)D Average distance between the absorber and the

cover (m)Dh Hydraulic diameter of the collector (m)G Mass flow rate of air (kg=s m2)H Humidity ratio (kg=kg)hc,b-f Convective heat transfer coefficient between the

absorber and the air (W=m2 K)hc,c-f Convective heat transfer coefficient between the

cover and the air (W=m2 K)hc,p-f Convective heat transfer coefficient between the

product and the air (W=m2 K)hfg Latent heat of vaporization of moisture from

peeled longan (J=kg)hr,b-c Radiative heat transfer coefficient between the

cover and the absorber (W=m2

K)hr,c-s Radiative heat transfer coefficient between the

cover and the sky (W=m2 K)hr,p-c Radiative heat transfer coefficient between the

cover and the product (W=m2 K)hw Convective heat transfer coefficient between the

cover and the ambient due to wind (W=m2 K)It Incident solar radiation (W=m

2)k Drying constant

FIG. 12. Comparison of the simulated and observed moisture content during drying of peeled longan for a typical full-scale experimental run.



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ka Thermal conductivity of air (W=m K)kb Thermal conductivity of back insulator

(W=m K)Lb Thickness of back insulator (m)M Moisture content of peeled longan (d.b., decimal)Me Equilibrium moisture content of peeled longan

(d.b., decimal)Nu Nusselt numberRe Reynolds numberRMSD Root mean square differencerh Relative humidity (decimal)Ta Ambient temperature (K)Tb Temperature of the absorber (K)Tc Temperature of the collector cover (K)Tc1 Temperature of the cover of the drying unit (K)Tf Temperature of the air steam in the collector (K)Tf1 Temperature of the moist air steam in the drying

unit (K)Tp Product temperature (K)

Ts Sky temperature (K)t Time (s)Dt Time interval (s)Ub Heat loss coefficient of the absorber through the

back insulator (W=m2 K)V Velocity of air (m=s)Va Wind velocity (m=s)W Width of the solar collector (m)x Position (m)Dx Space interval (m)

Greek Letters

ac Absorptance of the cover material (decimal)

ap Absorptance of peeled longan (decimal)db Thickness of the absorber (m)dc Thickness of the cover (m)eb Emissivity of the absorber material (decimal)ec Emissivity of the cover material (decimal)n Viscosity of air (m2=s)q Density of air (kg=m3)qb Density of the absorber material (kg=m

3)qc Density of the cover material (kg=m

3)qs, p Density of the peeled longan (kg=m

2)r Stefan Boltzmanns constant (W=m2 K4)sc Transmittance of the cover material (decimal)(sa) Transmittance-absorptance product of the system

composing of the cover and absorber (decimal)

ACKNOWLEDGEMENTS

This research work is a part of the SFB-564 projectResearch for Sustainable Land Use and Rural Develop-ment in Mountainous Regions of Southeast Asia, fundedby Deutsche Forchungsgemeinschaft (DFG), Germany,

and cofunded by the National Research Council ofThailand and the Ministry of Science, Technology andEnvironment, Vietnam. We thank these organizations forthe financial support to this work. We also thank theDepartment of Alternative Energy Development andEfficiency of Thailand and Department of Physics,Silpakorn University, for the support in the development

of the side-loading solar tunnel dryer. We are grateful tothe German Academic Exchange Service (DAAD) forsupporting the modeling work.

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2009-solar drying of peeled longan using a side loading

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