Experimental and modelling performances of a roof-integrated solar drying system for drying herbs and spices S. Janjai a, , N. Srisittipokakun a , B.K. Bala b a Solar Energy Research Laboratory, Department of Physics, Faculty of Science, Silpakorn Universtiy, Nakhon Pathom 73000, Thailand b Department of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh Received 12 June 2006 Abstract This paper presents experimental performance of solar drying of rosella flower and chili using roof-integrated solar dryer and also presents modelling of the roof-integrated solar dryer for drying of chili. Field-level tests for deep bed drying of rosella flower and chili demonstrated that drying in the roof-integrated solar dryer results in significant reduction in drying time compared to the traditional sun drying method and the dry product is a quality dry product compared to the quality products in the markets. The payback period of the roof-integrated solar dryer is about 5 years. To simulate the performance of the roof-integrated solar dryer for drying herbs and spices using hot air from roof-integrated solar collectors, two sets of equations were developed. The first set of equations was solved implicitly and the second set of equations was solved explicitly using finite difference technique. The simulated air temperatures at the collector outlet agreed well with the observed air temperatures. Good agreement was also found between experimental and simulated moisture contents. r 2007 Elsevier Ltd. All rights reserved. Keywords: Roof-integrated solar collector; Solar dryer; Rosella flower; Chili; Modelling; Simulation 1. Introduction Drying is the oldest preservation technique of herbs and spices. In developing countries, traditional sun drying method is commonly used for drying herbs and spices. Although it is the cheapest method, the dry products are of poor quality due to contamination by insects, birds and dusts. In the case of Thailand, most of herbs and spices are still dried using traditional sun drying method. Due to rewetting of the products during drying by rain and also because of too slow drying rate in the rainy season, toxic substances such as an alphatoxin produced by moulds is often found in the dry products. This is one of the main problems, which restricts the growth of exports of herbs and spices to international markets. In general, dry rosella flowers are used to make rosella juice by boiling these in water. It is a medicinal plant whose constituents are believed to help reduce high blood pressure. Rosella plants are grown mainly in the central region of Thailand and their harvesting season is in November to December. These months are in the dry season with clear days and low relative humidity of ambient air. This weather condition is favorable for solar drying. Chili is also a potential important cash crop in Thailand and there is an increasing demand for quality dry chili in the local and international markets. Thailand being situated near the equator receives abundant solar radiation [1]. Consequently, the utilization of a solar drying technology is considered to be an obvious option for drying agricultural products in this country. Although many types of solar dryers have been developed during the last two decades [2–8], the applica- tions of these dryers are still limited, mainly due to their unreliable performance and high investment cost relative to a production capacity. A reduction of losses, an improve- ment of quality of product and an investment cost are also important criteria dictating the adoption of the solar dryer. ARTICLE IN PRESS 0360-5442/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2007.08.009 Corresponding author. Tel.: +66 34 270761; fax: +66 34 271189. E-mail address: [email protected] (S. Janjai).
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Experimental and modelling performances of a roof-integrated solardrying system for drying herbs and spices
S. Janjaia,�, N. Srisittipokakuna, B.K. Balab
aSolar Energy Research Laboratory, Department of Physics, Faculty of Science, Silpakorn Universtiy, Nakhon Pathom 73000, ThailandbDepartment of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh
Received 12 June 2006
Abstract
This paper presents experimental performance of solar drying of rosella flower and chili using roof-integrated solar dryer and also
presents modelling of the roof-integrated solar dryer for drying of chili. Field-level tests for deep bed drying of rosella flower and chili
demonstrated that drying in the roof-integrated solar dryer results in significant reduction in drying time compared to the traditional sun
drying method and the dry product is a quality dry product compared to the quality products in the markets. The payback period of the
roof-integrated solar dryer is about 5 years. To simulate the performance of the roof-integrated solar dryer for drying herbs and spices
using hot air from roof-integrated solar collectors, two sets of equations were developed. The first set of equations was solved implicitly
and the second set of equations was solved explicitly using finite difference technique. The simulated air temperatures at the collector
outlet agreed well with the observed air temperatures. Good agreement was also found between experimental and simulated moisture
contents.
r 2007 Elsevier Ltd. All rights reserved.
Keywords: Roof-integrated solar collector; Solar dryer; Rosella flower; Chili; Modelling; Simulation
1. Introduction
Drying is the oldest preservation technique of herbs andspices. In developing countries, traditional sun dryingmethod is commonly used for drying herbs and spices.Although it is the cheapest method, the dry products are ofpoor quality due to contamination by insects, birds anddusts. In the case of Thailand, most of herbs and spices arestill dried using traditional sun drying method. Due torewetting of the products during drying by rain and alsobecause of too slow drying rate in the rainy season, toxicsubstances such as an alphatoxin produced by moulds isoften found in the dry products. This is one of the mainproblems, which restricts the growth of exports of herbsand spices to international markets.
In general, dry rosella flowers are used to make rosellajuice by boiling these in water. It is a medicinal plant whose
e front matter r 2007 Elsevier Ltd. All rights reserved.
ergy.2007.08.009
ing author. Tel.: +6634 270761; fax: +66 34 271189.
constituents are believed to help reduce high bloodpressure. Rosella plants are grown mainly in the centralregion of Thailand and their harvesting season is inNovember to December. These months are in the dryseason with clear days and low relative humidity ofambient air. This weather condition is favorable for solardrying. Chili is also a potential important cash crop inThailand and there is an increasing demand for quality drychili in the local and international markets.Thailand being situated near the equator receives
abundant solar radiation [1]. Consequently, the utilizationof a solar drying technology is considered to be an obviousoption for drying agricultural products in this country.Although many types of solar dryers have been
developed during the last two decades [2–8], the applica-tions of these dryers are still limited, mainly due to theirunreliable performance and high investment cost relative toa production capacity. A reduction of losses, an improve-ment of quality of product and an investment cost are alsoimportant criteria dictating the adoption of the solar dryer.
Ca specific heat of air (J/kgK)Cb specific heat of absorber (J/kgK)Cc specific heat of cover (J/kgK)Cf specific heat of air in the stream (J/kgK)CP specific heat of product (J/kgK)Cv specific heat of water vapor (J/kgK)D spacing between cover and absorber (m)Dh hydraulic diameter (m)G mass flow rate of air (kg/m2 s)hc convective heat transfer coefficient (W/m2K)hr, b–c radiative heat transfer coefficient from absorber
to cover (W/m2K)hc, c–f convective heat transfer coefficient from cover
to air stream (W/m2K)hc, b–f convective heat transfer coefficient from absor-
ber to air stream (W/m2K)hr, c–s radiative heat transfer coefficient from cover to
sky (W/m2K)hv volumetric heat transfer coefficient (W/m3K)hw convective heat transfer coefficient from cover
to ambient air (W/m2K)H humidity ratio (kg/kg)It incident solar radiation (W/m2)k thermal conductivity of air (W/mK)kb thermal conductivity of insulation material (W/
mK)Lb thickness of insulation material (m)Lp latent heat of evaporation of product (J/kg)ma mass flow rate of air (kg/m2 s)M moisture content (decimal, db)Me equilibrium moisture content (decimal, db)M0 initial moisture content (decimal, db)MBE mean bias error
Nu nusselt numberN number of observationRh air humidity (decimal)RMSE root-mean-square errorS cross-sectional area (m2)t time (s)Ta ambient air temperature (K)Tb absorber temperature (K)Tc cover temperature (K)Tf drying air temperature (K)TP product temperature (K)Ts sky temperature (K)ub heat loss coefficient (W/K)V speed of the air (m/s)Va wind speed (m/s)W width of collector (m)x length of collector (m)yexp, i ith experimental valueypre, i ith predicted valueY depth of the product bed (m)n viscosity of air (m2/s)t transmittance of absorber (decimal)a absorbance of absorber (decimal)ac absorbance (decimal)db thickness of absorber (m)dc thickness of cover (m)r density of product (kg/m3)ra density of air (kg/m3)rb density of absorber material (kg/m3)rc density of cover (kg/m3)rp density of product (kg/m2)s Stefan–Boltzmann’s constante porosity (decimal)ec emittance of cover (decimal)eb emittance of absorber (decimal)
A number of solar dryers do not meet these criteria.Therefore, development of a well-performed solar dryer isof significant economic importance.
As farmers usually have farmhouses with galvanized-iron sheets as a roof for use in various agriculturalactivities, with a proper design of this roof to producehot air and incorporation of a fan to suck the heated air topass through a product in a drying bin would be apromising forced convection solar drying system for its useat the farm levels. In such a drying system the farmhousewill provide space for the solar collectors and reduce thetotal investment cost.
Solar drying systems must be properly designed in orderto meet particular drying requirements of specific productsand to give satisfactory performance. Designers shouldinvestigate the basic parameters such as dimensions,temperature, relative humidity, airflow rate and thecharacteristics of products to be dried. However, full-scaleexperiments for different products, drying seasons and
system configurations are sometimes costly and may notpossible. The development of a simulation model is avaluable tool for predicting the performance of solardrying systems. Again, simulation of solar drying isessential to optimize the dimensions of solar drying systemsand optimization technique can be used for optimal designof solar drying systems [9].Roof-integrated solar dryer can be used for production
of quality dry products of international standard with asignificant reduction of drying time and thus can be usedfor production of value added products of export quality.Production of export quality dry products and theirmarketing would generate not only income but alsoprovide some employment opportunity in rural-basedagro-industries for production of dry products. Thesimulation model of roof-integrated solar dryer will be ofacademic interest in drying technology and also willprovide design data. This simulation model is also neededfor optimal design of the dryer.
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Solar drying can be considered as an elaboration of sundrying and is an efficient system of utilizing solar energy[10–11]. Many studies have been reported on naturalconvection solar drying of agricultural products [8,12–15].Considerable studies on simulation of natural convectionsolar drying of agricultural products and optimization havealso been reported [16–18]. The success achieved by naturalconvection solar dryers has been limited due to lowbuoyancy-induced airflow. These prompted researchers todevelop forced convection solar dryers. Many research andperformance studies have been reported on forced convec-tion solar dryers [19–22]. Numerous tests in the differentregions of the tropics and subtropics have shown thatfruits, vegetables, cereals, grain, legumes, oil seeds, spices,fish and even meat can be dried properly in the solar tunneldryer [23–27]. Studies on simulation and optimization offorced convection solar tunnel dryers have also beenreported [28–30].
Little preliminary work on experimental performanceshas been reported [31] but no work on simulation of roof-integrated system has been reported. The purposes of thisstudy are to determine the experimental performance ofroof–integrated solar collectors for drying rosella flower andchili and also to develop a simulation model for roof-integrated solar dryer for drying chili. This simulation modelcan be used to simulate temperature and moisture changesduring drying. The results obtained from the simulation canbe used as a guideline for improvement of the dryer.
2. Experimental set-up and procedure
2.1. Experimental set-up
The roof-integrated solar dryer was installed at SuanPhoeng Educational Park, Ratchaburi (1313205000N,
Fig. 1. Roof-integrated
9914804400E), 250 km west of Bangkok, Thailand and it isessentially a batch-type deep bed solar dryer.The dryer consists of a roof-integrated solar collector
and a drying bin with an electric motor (220V, 1 phase,0.373 kW) operated axial flow fan to provide the requiredairflow (Fig. 1). The bin is connected to the middle of thecollector through a T-type air duct.The roof-integrated collector consists of two arrays of
collector: one facing the south and other facing the northwith a total area of 108m2. These arrays of the collectorsalso serve as the roof of the building. The roof-integratedcollector is essentially an insulated black-painted roofserving as an absorber, which is covered with a poly-carbonate plate.The drying bin is essentially a deep bed batch dryer. The
capacity of the dryer is 1.3� 2.4� 0.8m3 and it is locatedinside the building (Fig. 2). The building was partitionedinto one space for the drying bin and another twoadditional rooms. The first room was used for thepreparation of the product to be dried and the second forthe storage of dried products.Solar radiation passing through the polycarbonate cover
heats the absorber. Ambient air is sucked through thecollectors and while passing it through the collectors gainsheat from the absorber. This heated air is passed throughthe drying bin. Various measuring sensors were placed atthe solar collector and the dryer to monitor the perfor-mance of the system, as explained in the next section.
2.2. Experimental procedure
Rosella flower and chili were dried to demonstrate thepotentiality of the dryer for drying of rosella flower andchili. The tests were carried out during the period ofNovember 2004 to February 2005. A total of 8 experimentswere carried out. Of which 6 experiments were conducted
solar drying system.
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76.5 cm 60.5 cm
Inlet air
Fan
Outlet air
137 cm
Product container
60 cm
185 cm
Fan controller
Air distributor
125 cm
30.0 cm
244 cm
Air from roof – integrated collector
Fig. 2. The drying bin.
for drying of rosella flower and 2 were conducted fordrying of chili.
Solar radiation was measured by pyranometers (Kipp &Zonen model CM 11, accuracy 70.5%) and these wereplaced on both the roofs. Thermocouples of type K wereused to measure air temperatures in the collectors, air ductsand drying bin (accuracy 72%). Hot wire anemometers(Airflow, model TA5, accuracy 72%) were employed tomonitor the air speed in the collectors and air ducts. Thisanemometer was also used to monitor the wind speed.
Relative humidities of ambient air and drying air wereperiodically measured with hygrometers (Electronik, modelEE23, accuracy 72%). Voltage signals from the pyran-ometers, hygrometers and thermocouples were recordedevery 10min by a 40-channel data logger (Yokogawa,model DC100). The air speeds in the solar collectors andthe air ducts were also manually read and recorded 2–3times during the drying experiments. Samples of productsin the dryer were weighed at 3-h intervals using a digitalbalance (Satorius, model E2000 D, accuracy 70.0001 g).Before the installations, the pyranometers were calibratedagainst a new pyranometer recently calibrated by Kipp &Zonen, the manufacturer. For the hygrometer, it wascalibrated 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. Duringdrying of chili and rosella flowers the solar radiation variedfrom 55 to 1122W/m2. The ambient temperature varied
from 11 to 36 1C while the ambient relative humidity variedfrom 22% to 67%.For the drying tests, 200 kg of fresh rosella flowers or
chili was used for each experimental run. The experimentswere started at 8.00 am and continued till 5.00 pm. Duringnighttime, the products were kept in the dryer. The processwas repeated until the desired moisture content (about18% on a wet basis) was reached. This final moisturecontent corresponds to the moisture content of high-quality dry product in local markets. About 100 g of theproduct sample was taken from the dryer as well as fromthe control sample and weighed at 3-h intervals. Themoisture 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 wasdetermined by using the air oven method. The sampleswere placed in the oven at the temperature of 103 1C for24 h (accuracy 70.5%).Electrical energy was used only to run the electric motor
to operate fan to provide the desired airflow (air velocity of0.2m/s in the drying) in this roof-integrated solar dryer.Electrical energies used for drying tests are 10 kWh perbatch for rosella flower and 9 kWh per batch for chili.
3. Uncertainty analysis
Uncertainty analysis refers to the uncertainty or error inexperimental data. Systematic error in the experimental
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data is a repeated error of constant value and the randomerror is due to imprecision. Systematic error can beremoved by calibration but random error cannot beremoved. The imprecision due to random error canbe defined numerically by calibration.
The measured data on solar radiation, temperature andrelative humidity were recorded during calibration. Themean value of the measurements and standard deviation ofthe random errors of the data on solar radiation,temperature and relative humidity were determined. Thevariable xi that has an uncertainty dxi is expressedas [32–34]
xi ¼ mmeanðmeasuredÞ � dxi, (1)
where xi is actual value, xmean is measured value (meanvalue of the measurements) and dxi is uncertainty in themeasurement. There is an uncertainty in xi that may be aslarge as dxi. The value of dxi is the precision index that istaken as 2 times the standard deviation and it enclosesapproximately 95% of the population for a single sampleanalysis.
Statistical analysis (analysis of variance) was carried outto assess whether there exists any significant difference indrying (moisture removed) in the solar dryer in comparisonto drying in the traditional sun drying system with level ofsignificance.
4. Modelling and analysis
The modelling was carried out only for chili because thethermal properties and necessary data of rosella flower arenot available.
4.1. Modelling and analysis of collector performance
Consider an element dx at a distance x from the inlet inthe direction of airflow in the roof-integrated collector(Fig. 3) and the energy balances on the collectorcomponents are:
Air flow
Polycarbonate cover
Black absorber Insulation
Radiative heat transfer
Conduction heat transfer
Convective heat transfer
It
hw
hr,e–s
hr,b–chc,c–f
hc,b–fUb�c
Ta
Tf
Tb
Tc
Δ x
W
Fig. 3. Energy balances in the roof-integrated solar collector.
Energy balance on the polycarbonate as the cover:
rcdcDxWCc Tc þdTc
dtDt
� �� rcdcDxWCcTc
¼ hr;b2cDxW ðTb � TcÞDtþ hc;c2f DxW ðTf � TcÞDt
þ hwDxW Ta � Tcð ÞDtþ hr;c�sDxW ðTs � TcÞDt
þ DxWacI tDt, ð2Þ
where T is temperature of collector component, h is heattransfer coefficient, r is density, d is thickness, W is thewidth of collector, c is specific heat, It is incident solarradiation, a is absorptance, Dx is increment of distance andDt is increment of time. Subscript c, b, f, a and s representcover, absorber, air in collector, ambient air and sky,respectively. For the heat transfer coefficients, subscript c
and r represent convection and radiation heat transfers,respectively, and hw is convection loss due to wind.It can be further simplified as
rcdcCcdTc
dt¼ hr;b2cðTb � TcÞ þ hc;c2f ðTf � TcÞ
þ hwðTa � TcÞ þ hr;c2sðTs � TcÞ þ acI t. ð3Þ
Energy balance on the absorber:
rbDxWdbCb Tb þdTb
dtDt
� �� rbDxWdbCbTb
¼ hc;b2f DxW ðTf � TbÞDtþ hr;b2cDxW ðTc � TbÞDt
þ ubDxW ðTa � TbÞDtþ DxW ðtaÞI tDt ð4Þ
can be further rearranged as
rbdbcb
dTb
dt¼ hc;b2f ðTf � TbÞ þ hr;b2cðTc � TbÞ
þUbðTa � TbÞ þ ðtaÞI t. ð5Þ
Energy balance in the air stream:
DGW ðCf þ CvHÞ Tf þdTf
dxDx
� �Dt
�DGW ðCf þ CvHÞTf Dt ¼ DxWhc;c2f ðTc � Tf ÞDt
þ DxWhc;b2f ðTb � Tf ÞDt, ð6Þ
where H is moisture ratio, D is spacing between cover andabsorber, G is flow rate of the air, Cf is specific heat of theair and Cv is specific heat of water vapor.It can be further simplified as
DGCf
dTf
dx¼ hc;c2f ðTc � Tf Þ þ hc;b2f ðTb � Tf Þ. (7)
Radiative heat transfer coefficient from the cover to thesky (hr, c–s) is computed as [17]
hr;c2s ¼ �csðT2c þ T2
s ÞðTc þ TsÞ, (8)
where Ts is blackbody-equivalent sky temperature, Tc isthe temperature of the cover, ec is the emissivity of thecover and s is Stefan–Boltzmann’s constant. The sky
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temperature is computed from
Ts ¼ 0:552T1:5a . (9)
Radiative heat transfer coefficient between the absorberand the cover (hr, b–c) is computed as [17]
hr;b2c ¼sðT2
b þ T2cÞðTb þ TcÞ
ð1=�bÞ þ ð1=�cÞ � 1, (10)
where Tb is the temperature of the absorber and eb is theemissivity of the absorber.
Convective heat transfer coefficient from the cover toambient due to wind (hw) is computed as [35]
hw ¼ 5:7þ 3:8Va, (11)
where Va is the wind speed in m/s.Convective heat transfer coefficient inside the roof-
integrated collector for either the cover or absorber (hc) iscomputed from the following relationship:
hc ¼Nuk
Dh
, (12)
where Nu is Nusselt number, k is the thermal conductivityof the air and Dh is hydraulic diameter.
Dh is given by
Dh ¼4WD
2ðW þDÞ, (13)
where W is the width of the collector and D is the spacingbetween the cover and the absorber.
Nusselt number, Nu is computed from the followingrelationship [36]:
Nu ¼ 0:0158 Re0:8, (14)
where Re is the Reynolds number which is given by
Re ¼DhVra
n, (15)
where V is the speed of the air in the collector, v is viscosityof the air and ra is density of the air.
Over all heat loss coefficient from the bottom of theabsorber, (Ub) is computed from the following relation:
Ub ¼kb
Lb
, (16)
where kb and Lb are the thermal conductivity and thicknessof the insulation materials, respectively.
4.1.1. Solution procedure
The collector model consists of three Eqs. (3), (5)and (7) and this set of equations is very difficult tosolve analytically. But this set of equations can besolved numerically using finite difference technique.The Eqs. (3), (5) and (7) in finite difference form is
The length of the collector is divided into a number ofsections Nsec so that the properties of the materials areconstant or nearly so within each section. The time intervalshould be small enough for the air conditions to beconstant over the distance Dx. But for the economy ofcomputing, a compromise between the computing time andaccuracy must be considered. Fig. 4 shows the schematicdiagram of roof-integrated collector for the numericalsolution.The system of equations consisting of Eqs. (20)–(22) is
expressed in the following form for the interval Dt for the
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Y+dY
Y
H, Ta, V, ρa, Ca
Tp, M, Cp, ρp, ε
S
Y
Fig. 5. Element of the bed.
Tc
Tf
Tb
flow
ΔX
N = 1 2 3 . . . . . N sec
Fig. 4. Schematic diagram of the roof-integrated collector for the
numerical solution using finite difference technique.
entire length of collector unit:
a11 a12 a13
a21 a22 a23
a31 a32 a33
264
375
Tc
Tf
Tb
�������������� ¼
b1
b2
b3
��������������. (23)
This system of equations is a set of implicit equations forthe time interval Dt for the entire length of the collector unitand was solved using the Gauss–Jordan elimination method.
4.2. Modelling and analysis of dryer performance
Consider an element of the bed of the dryer (Y, Y+dY)and S is the cross-sectional area with air speed, V from Y toY+dY (Fig. 5). There are four unknowns: moisturecontent M, humidity H, air temperature Ta, and producttemperature Tp. Thus, two conservation equations and tworate equations are made resulting in four equations.
Energy balance equation:
Change of enthalpy of air ¼ Sensible heat transfer from
the air to moisture prior to evaporation� change in
sensible heat of the moist product
þ enthalpy in evaporated moisture:
Mathematically this can be expressed as
ma Ca þ CvHð ÞqTf
qY¼ rpCv Tf � Tp
� � qM
qt
� rp Cp þ Cf M� � qTp
qtþ rpLp
qM
qt. ð24Þ
Heat transfer rate equation:
Change in enthalpy of the product
¼ Convective heat transfer between
the air and the product
þ enthalpy in evaporated moisture:
Mathematically this can be expressed as
rp Cp þ Cf M� � qTp
qt¼ hv Tf � Tp
� �þ rpLp
qM
qt, (25)
where hv is the volumetric heat transfer coefficient and it iscomputed from the following relationship [37]:
hv ¼ 8:69� 104m1:3a , (26)
where ma is mass flow rate of the air.
Mass balance equation:The mass balance equation gives that the moisture lost
by product is equal to moisture gained by the air.Mathematically it can be expressed as
rpS dYqM
qtdt ¼ maSH dt�maS H þ
qH
qYdY
� �dt
þ �S dYra
qH
qtdt. ð27Þ
Further simplification gives
ma
qH
qY¼ �rp
qM
qt. (28)
Drying rate equation:The rate of change of moisture content of a product in
the dryer can be expressed by an appropriate thin-layerdrying equation. Thin-layer drying equation in the form ofPage equation developed by Hossain [30], which fitted thebest to the experimental data of thin-layer drying of chiliwas used and this equation is
M �Me
M0 �Me
¼ exp ð�PtQÞ, (29)
where P and Q are function of drying air temperature, T
(K), relative humidity, Rh (decimal) and air speed, V (m/s).
P ¼ 0:00955þ 0:000372T � 3:20� 10�6T2
� 0:01127Rhþ 0:012408Rh2 þ 0:004737V
� 0:00381V 2 ðR2 ¼ 0:96Þ, ð30Þ
Q ¼ 4:89468� 0:137459T þ 0:001345T2
þ 0:386002Rh� 1:142445V
þ 0:920444V2 ðR2 ¼ 0:96Þ. ð31Þ
Equilibrium moisture content, Me (db, decimal) is com-puted from the following relationship [30]:
Fig. 8. Variation of the ambient temperature and temperature of the
outlet air from the north-facing collector for the drying test of rosella
flower.
0
20
40
60
80
100
9:00 12:00 15:00 9:00 12:00 15:00 9:00 12:0015:00
Time (hour)
Tem
pera
ture
(ºC
)
Outlet of south facing collectorAmbient
20/12/2004 21/12/2004 22/12/2004
Fig. 7. Variation of the ambient temperature and temperature of the
outlet air from the south-facing collector for the drying test of rosella
flower.
4.2.1. Solution procedure
The chili bed is divided into a number of layers(Y ¼ jDY) along the depth of the dryer. The drying timeis also divided into a number of intervals (t ¼ iDt). On thebasis of the air temperature, relative humidity and airflowat the entry of the dryer, the drying content (P) anddynamic equilibrium moisture content (Me) of the chiliwere computed. Using the P and Me values, the change inmoisture content of chili, DM, for a time interval, Dt, wascalculated using Eq. (29). Using the recent value of dryingair temperature and drying rate, the product temperatureof first layer of the dryer was computed using Eq. (25). Onthe basis of the recent value of the product temperature, theair temperature inside the first layer of drier was estimatedusing Eq. (24). The change in air humidity was computedusing Eq. (28). This process was repeated layer-by-layeruntil the top of the bed was reached. This process was thenrepeated for each time increment. When air relativehumidity exceeds 98%, the condensation routine depositsback the moisture from the saturated air. Air and chilitemperatures are adjusted for this condensation [38]. Thenumerical solution was programmed in Compaq VisualFORTRAN version 6.5 [39].
Goodness of fit of the model:The suitability of the model was evaluated using the
value of mean bias error (MBE) and the value of the root-mean-square error (RMSE):
Although the pyranometers was calibrated against a newpyranometer recently calibrated by Kipp & Zonen using apyranometer calibration facility of Kipp & Zonen, themeasurement of solar radiation by the pyranometer wasfound to possess an uncertainty of 72.3W/m2 with meanvalue of measurements of 1000.7W/m2. This indicates thatthere is almost no systematic error in the measurement ofsolar radiation but the imprecision due to random error is72.3W/m2 at 1000W/m2. The hygrometer had anuncertainty of 72% for the relative humidity range0–90% while it was found to be 70.1% when tested at arelative humidity of 52.9% maintained using standard saltsolution. The uncertainty of the measurement usingthermocouple was determined at the boiling point tem-perature and it was found to be 70.6 1C. Thus, themeasured data of temperature had an imprecision due to
random error of 70.6 1C. The uncertainty analysisindicates that the measured data are accurate enough toassess the performance of the roof-integrated solar dryingsystems.
Drying of rosella flower:Field tests of the dryer for drying of rosella flower were
carried out in the months of November 2004 andDecember 2004. The typical results for drying of rosellaflower are shown in Figs. 6–11. The variations of solarradiation on the south-facing collectors and the north-facing collectors are shown in Fig. 6. The south-facingcollectors received higher insolation than that of the north-facing collectors. This is due to the fact that the sun was inthe south of the celestial equator, creating a smallerincident angle of solar radiation on the south-facingcollectors compared to that of the north-facing collectors.With the orientation and tilted angle of these collectors, the
Fig. 11. Comparison of the moisture changes inside roof-integrated solar
dryer and open sun drying during drying of rosella flowers.
north-facing collectors and the south-facing collectorsreceive alternatively more and less radiation, compared toeach other during a year. However, the difference in theamount of energy received is small during most of the timeof a year. This is because of the fact that Thailand issituated near the equator with the noontime sun near thezenith year round and the tilted angle of the collectors issmall. The small difference in incident radiation resulted ina slight difference in outlet air temperature from thecollectors.
Figs. 7 and 8 show the ambient temperatureand the collector outlet temperatures at the middle of theroof for solar radiation incident on the south-facingcollectors and north-facing collectors, respectively. Sincethe more was the solar radiation incident on the south-facing roof as compared to the north-facing roof, thehigher was the temperature at the outlet of the south-facingroof.
When air from the collector outlet was forced throughthe air ducts to the perforated floor of the productcontainer, the air temperature dropped by a few degreesdue to heat losses in the air ducts. The variations of drying
air temperature at the inlet and outlet of the dryer and thecorresponding relative humidity at the inlet and outlet ofthe dryer was well as the relative humidity of the ambientair are shown in Figs. 9 and 10 respectively. The drying airtemperature was maximum at noon and was about 60 1Cwhile the corresponding relative humidity was the mini-mum and it was about 10%. In sunny days as the drying airtemperature increases starting from morning till noon therelative humidity of the moist air decreases due to sensibleheating of the moist air by solar radiation while the relativehumidity of the moist air increases due to sensible heatingof the moist air by solar radiation in the afternoon. Similarpatterns are observed for ambient temperature and relativehumidity.In Fig. 9, it is observed that at the end of the third day of
the experiment the inlet and the outlet air temperatureshow the same trend. This is likely due to the fact thatenergy required for evaporation is relatively small at theend of the drying process and the temperature difference ismainly from heat losses. For the relative humidity (Fig. 10)it is also observed that after 15:00 h, the difference betweenthe inlet and outlet relative humidity is small because themoisture released from the product is relatively low due tolow radiation.Considering the temperature and the relative humidity of
the air entering the drying bin, this drying air has higherdrying potentials for drying the rosella flower, compared toambient air. The drying air was forced through the rosellaflower with an air speed of 0.2m/s. This low air speed isused because it is a deep bed drying which usually requiresrelatively low speed of the air to save energy for the fan,with acceptable ventilation to remove moist air from thedryer.For clear sky weather conditions, the moisture content
of the rosella flower in the drying bin was reduced from aninitial value of 90% (wb) to the final value of 18% (wb)within 3 days or with an effective drying time ofapproximately 27 solar hours whereas the moisture contentof the sun dried samples was reduced to 28% (wb) in thesame period as shown in Fig. 11. From Fig. 11, it wasobserved that the moisture content slowly decreased on thefirst and it rapidly decreased on the second and slowlyagain on the third day while the moisture content ofcontrol sample decreased in a similar fashion in the secondand third and the final moisture content was about 28%(wb). This can be explained as follows. The rosella flowershave a natural wax coated on their surfaces. This waxprevents most of the migration of moisture from theinside of the flowers into the drying air. After thesurface was dried the wax was broken, the moisture frominside can be easily released, thus increasing the drying rateon the second day. For the last day, the drying rate wasslow again because most of water to be evaporated was inthe mono-layer or multi-layer water with a high bindingenergy [39]. The color of the dried rosella flower wascomparable to that of a high quality dried rosella flower inmarkets.
Fig. 16. Variations of the relative humidity of the ambient air and the air
at the inlet and outlet of the dryer for the drying test of chili.
Drying of chili:Field tests of the dryer for drying of chili also were
carried out in the months of January and February 2005and the typical results are shown in Figs. 12–17. Thevariations of solar radiation on the south-facing collectorsand the north-facing collectors are shown in Fig. 12. As thesun was near to the celestial equator, the south-facingcollectors received higher insolation than that of the north-facing collectors. With the orientation and tilted angle ofthese collectors, the north-facing collectors and the south-facing collectors receive alternatively more and lessradiation, compared to each other during a year. However,the difference in the amount of energy received is smallduring most time of a year.
Figs. 13 and 14 show the ambient and the collector outlettemperatures at the middle of the roof for solar radiationincident on the south-facing collectors and north-facingcollectors, respectively. Since the more was the solarradiation incident on the south-facing roof as comparedto the north-facing roof, the higher was the temperature atthe outlet of the south-facing roof.
Fig. 17. Comparison of the moisture changes inside roof-integrated solar
dryer and open sun drying during drying of chili.
The variations of drying air temperature at the inlet andoutlet of the dryer and the corresponding relative humidityat the inlet and outlet of the dryer was well as the relativehumidity of the ambient air are shown in Figs. 15 and 16,respectively. The drying air temperature was maximum atnoon while the relative humidity was the minimum.However, the ambient temperature and relative humiditywere the lowest and the highest, respectively.For clear sky weather conditions, the moisture content
of the chili in the drying bin was reduced from an initialvalue of 80% (wb) to the final value of 18% (wb) within 3days or with an effective drying time of approximately 24solar hours whereas the moisture content of the sun driedsamples was reduced to 53% (wb) during the same dryingperiod as shown in Fig. 17. From Fig. 17, it was observedthat the moisture content slowly decreased on the first dayand it rapidly decreased on the second day and slowlyagain on the third day while the moisture content ofcontrol sample decreased very slowly in a similar fashion inthe second and third and the final moisture content wasabout 53% (wb).Statistical analysis shows that there is no significant
difference in solar drying in the roof-integrated solar dryerand sun drying during the first day of drying for both
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rosella flower and chili, but there is a significant differencein drying in the second and third days for rosella flowerand chili at a significance level of 5% and 1%, respectively.
Fig. 18 shows the comparison of the drying of rosellaflower and chili under almost similar drying conditions inroof-integrated solar dryer. At the initial stage of drying,the rosella flower dried at a slower rate because of its wax-coated surface. But once it was broken after first day ofdrying, the drying rate increased to such an extent that itquickly achieved the drying rate of chili and then followedalmost similar pattern of drying.
This dryer has an air distributor which is box locatedunder the product container. This air distributor helps toobtain almost uniform air entering the product container.Consequently, it is observed that most products in thedryer are uniformly dried. A slight problem of theuniformity is found at the corners of the product container.This problem will also be considered for the next version ofthis dryer.
Thus, the drying in the roof-integrated solar results inreduction in drying time and production of quality driedproduct. The color of the dried chili was comparable tothat of a high-quality dried chili in markets when the colorwas tested.
For the economic analysis, it is assumed that each yearthe dryer is used to dry rosella flower in November–De-cember and to dry chili for the rest of the year.Approximately, 450 kg of dry rosella flower and 3040 kgof dry chili are annually produced. Based on thisproduction and the capital and operating cost of the
Fig. 18. Comparison of the drying of rosella flower and chili inside the
roof-integrated solar dryer.
0
10
20
30
40
50
60
70
80
8:00 10:00 12:00 14:00 16:00 8:00 10:00 12:
Time
Tem
pera
ture
, ˚C
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Fig. 19. Predicted and experimental values of the ou
drying system, the payback period of this system isestimated to be 5 years.
5.2. Simulated performance
To validate the model the predicted air temperatures atcollector outlet and moisture contents of chili duringdrying were compared with the experimental values.Fig. 19 shows a typical comparison between the predictedand experimental values of the temperatures at the outlet ofthe collector. Predicted temperature shows plausiblebehavior and the agreement is good. Fig. 20 shows atypical comparison of the predicted and observed moisturecontents of chill inside the dryer and the model predictswell the moisture content changes during drying.The model predictions were evaluated on the basis of
RMSE and mean bias error (MBE). RMSE and MBE ofthe prediction of the collector outlet temperatures were10.0% and 1.0% with reference to the mean value,respectively. This indicates that the model can predict thetemperature with small fluctuation RMSE 8.3%) and alsooverpredicts the temperature with a very small mean bias(1%). RMSE and MBE of the prediction of the moisturecontents were 8.3% and �5.0%, respectively. Thisindicates that the model underpredicts the moisture contentwith a small mean bias (�5.0%) and also with a relativelysmall mean root square error of 8.3% compared with theprediction of the temperature. Thus, the model predictions
8:00 10:00 12:00 14:00 16:0000 14:00 16:00
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ExperimentModel
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tlet temperature of the roof-integrated collector.
Fig. 20. Predicted and observed values of the moisture content of chili.
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are reasonably good. Furthermore, predictions are withinthe acceptable limit (10%) [40].
5.3. Application
Roof-integrated solar dryer is costly in terms of capitalcost (construction and installation cost). But the operatingcost is extremely low and it is also environment friendly.The roof-integrated solar dryer is suitable for dryingapplications of value added products where the quality isreflected in price. These products are such as spices andherbs. The field-level tests demonstrate the potentiality ofthis type of dryer for small-scale industrial production ofquality dried spices and herbs. Although, this dryer wasinstalled and tested for demonstration of the dryingpotential of herbs and spices, it is still being used forproduction of quality dried products for sale to the visitorsof the Suan Phoeng Educational Park. The drying systemworks well without any problem of algae or fungal attackon polycarbonate cover. Due to the successful tests withthe roof-integrated solar dryer under this investigation, thistype of dryer with a slight modification has beenconstructed at Pakxe province (151070N, 1051470E) inLao’s People Democratic Republic. It is now being usedfor small-scale production of quality dried spices andherbs.
6. Conclusions
Solar drying of rosella flower and chili in roof-integratedsolar dryer resulted in considerable reduction in dryingtime as compared to the traditional sun drying and theproducts dried in the roof-integrated solar dryer werequality dried products. The solar collector functioned wellas the roof of the building as well as the solar energycollectors and the payback period of the roof-integratedsolar dryer is about 5 years.
The simulated air temperature at the collector agreedwell with the observed data. Good agreement was foundbetween the experimental and simulated moisture contentof chili during drying. Both the cases the accuracy wasreasonable. This model can be used for providing designdata for roof-integrated solar dryer and also for optimiza-tion of roof-integrated solar dryer.
Acknowledgments
The authors would like to thank the Department ofAlternative Energy Development and Efficiency, Ministryof Energy, Kingdom of Thailand for the financial supportto carry out this research work.
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