Development of A Simulation Model For The Hybrid Solar Dryers As Alternative Sustainable Drying System For Herbal and Medicinal Plants

A simulation model of hybrid solar drying as alternative sustainable drying system for herbal and medicinal plants was developed. Heat absorbed from the solar radiation. Heat gained by the collector and supplemented by the burner. Heat gained or lost by the product, heat gained or lost through the drying bin wall, and the latent heat of the moisture evaporation from the product were the main components of the equations describing the drying system. The model was able to predict the moisture loss from the product at wide ranges of temperatures (55, 60, 65 and 70 C and air circulation percentages (10, 20 and 30%). The model showed a dramatic effect of the drying air temperature on the moisture loss at the beginning of the drying process and became constant. Energy consumption at different drying temperatures was studied. Air recirculation has a profound effect on energy saving while drying herbs. In the next stages, high temperature without forcing air should be used. The model was validated by using experimental data of the drying temperatures and moisture loss under both direct sun and solar drying systems for herbal plants. The predicted values were in a reasonable agreement with the experimental data.
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  Development of A Simulation Model For The Hybrid Solar Dryers AsAlternative Sustainable Drying System For Herbal and Medicinal PlantsAli, S.A. and A.H. ahnasa!y  Agric. Eng. Dept., Moshtohor Agric. College Benha University, Moshtohor , Toukh, QaliobiaTel: ! #$%& $ 'o((ice)e*+ail: ael-bahnasay/   ritten for presentation at the #$%% &'() Section *' 'nternational Symposium on To!ards a Sustainable Food &hainFood Process, ioprocessing and Food +uality Managementantes, France - April %-#$, #$%% Abstract .    A si+ulation +oel o( hybri solar rying as alternative sustainable rying syste+ (or herbal an +eicinal plantsas evelope. 0eat absorbe (ro+ the solar raiation. 0eat gaine by the collector an supple+ente by the burner.0eat gaine or lost by the prouct, heat gaine or lost through the rying bin all, an the latent heat o( the +oistureevaporation (ro+ the prouct ere the +ain co+ponents o( the e1uations escribing the rying syste+. The +oel asable to preict the +oisture loss (ro+ the prouct at ie ranges o( te+peratures '22, %, %2 an & C an air circulationpercentages '!, # an 3). The +oel shoe a ra+atic e((ect o( the rying air te+perature on the +oisture loss atthe beginning o( the rying process an beca+e constant. Energy consu+ption at i((erent rying te+peratures asstuie.  Air recirculation has a pro(oun e((ect on energy saving hile rying herbs. 4n the ne5t stages, high te+peratureithout (orcing air shoul be use. The +oel as valiate by using e5peri+ental ata o( the rying te+peratures an+oisture loss uner both irect sun an solar rying syste+s (or herbal plants. The preicte values ere in a reasonableagree+ent ith the e5peri+ental ata. /ey!ords.  6i+ulation +oel, hybri solar ryer, herbal an +eicinal plants, air recirculation, energy consu+ption. Proceedings of the 0 th  &'() Section *' 'nternational Symposium1To!ards a Sustainable Food &hain2Food Process, ioprocessing and Food +uality Management antes, France - April %-#$, #$%%  'ntroduction The whole industry of exporting dried herbs and medicinal plants are at risk. The drying cost of dried herbsusing fossil energy with the governmental policy of liberalizing energy cost will become very crucial. At themoment, the drying cost of one Kg of mint ranges between L ! # relevant to type of energy used, diesel and $ or electric power. %ith the expected rising of energy cost, the total exporting cost will be critical compared tocompetitors from other producers such as &ndia and elsewhere. This is beside the environmental hazards of usingfuel as the source of energy.'rying of herbs and medicinal plants is one of the oldest forms of food preservation methods known toman and is the most important process for preserving food since it has a great effect on the (uality of the dried products. The ma)or ob)ective in drying agricultural products is the reduction of the moisture content to a levelwhich allows safe storage over an extended period. Also, it brings about substantial reduction in weight and volume,minimizing packaging, storage and transportation costs *+kos, arsimhan, -ingh,  %itnauer, /00!1. -olar energyis an important alternative source of energy and preferred to other energy sources because it is abundant,inexhaustible and non pollutant. Also, it is renewable, cheap and environmental friendly *2asunia  Abe, !33/1.Thin layer e(uations describe the drying phenomena in a united way, regardless of the controllingmechanism. They have been used to estimate drying times of several products and to generalize drying curves. &nthe development of thin layer drying models for agricultural products, generally the moisture content of the materialat any time after it has been sub)ected to a constant relative humidity and temperature conditions is measured andcorrelated to the drying parameters *4idilli, Kucuk,  5apar, !33!6 Togrul  7ehlivan, !3381.4any researches on the mathematical modelling and experimental studies have been conducted on the thinlayer drying processes of various vegetables, fruits and agro based products such as bay leaves *9u:nhan,'emir,;ancioglu,  ;epbasli, !33<1, hazelnut *+: zdemir  'evres, /0001, green pepper, green bean and s(uash *5aldiz  rtekin, !33/1, apricot *-arsilmaz, 5ildiz,  7ehlivan, !3336 Togrul  7ehlivan, !33#1, green chilli *;ossain  2ala, !33!1, pistachio *4idilli  Kucuk, !33#1, potato *Akpinar, 4idilli,  2icer, !33#a1, apple *Akpinar, 2icer,  4idilli, !33#1, pumpkin *Akpinar, 4idilli,  2icer, !33#b1, red pepper *Akpinar, 2icer,  5ildiz, !33#1, eggplant *rtekin  5aldiz, !3381, carrot *'oymaz, !3381, fig *'oymaz, !33<1, =itrus aurantium leaves *Ait 4ohamed et al., !33<1, rosehip *renturk, 9ulaboglu,  9ultekin, !3381, kiwi *-imal, >emenia, 9arau,  ?osella, !33<1.-olar drying systems must be properly designed in order to meet particular drying re(uirements of specific productsand to give satisfactory performance with respect to energy re(uirements. 'esigners should investigate the basic parameters namely dimensions, temperature, relative humidity, airflow rate and the characteristics of products to bedried. ;owever, full scale experiments for different products, drying seasons, and system configurations are sometimes costly and not possible. The development of a simulation model is a valuable tool for predicting of the performance of solar drying systems. Again, simulation of solar drying is essential to optimize the dimensions of solar drying systems and the optimization techni(ue can be used for optimal design of solar drying systems @/#B.;owever, there is no information about the mathematical models of the drying process using hybrid solar drying of herbs and medicinal plants in the literature. Therefore, the main ob)ectives of this study are to develop anappropriate model for the hybrid drying system to study the effect drying temperature and air recirculation percentages on the drying time, final moisture content and energy consumption for some herbal plants. Testing andverifying the model results with the experimental results. Applying the model results to design and construct anappropriate hybrid solar dryer for some herbal and medicinal plants. MODEL DEVELOPMENT ;eat and mass balances were carried out in order to describe the drying system. The temperature rise and moisture loss during the drying are described by means of the transient energy conservation e(uation, combined with an e(uation for the rate of moisture loss. ;owever, the following assumptions were made in developing the modelC The product is uniformity distributed in the drying space. #  7roduct leaves are characterized as homogenous ob)ects and have a uniform temperature. A steady state condition is achieved. The coefficient of evaporation remains constant. Heat alance3 The heat balance e(uation is based on the concept that the algebraic summation of the rate of sensible energy gain, the absorbed solar heat, heat gain or heat loss from the dryer room, and the heat loss due to the moisture evaporation. These could be explained as followsC 1- ;eat absorbed from the solar radiation .2- ;eat gained by the collector and supplemented by the burner.3- ;eat gained or lost by the product, 4- ;eat gained or lost through the drying bin wall, and 5- The latent heat of the moisture evaporation from the product.These components can be written as followsC Q s  +Q c ± Q p  ± Q w  – Q e  = 0(3) 7ith re(erence to 8ig. !, the values o( Q s , Q c , Q p , Q   an Q e  can be calculate (ro+ the (olloing e1uations:Q s 9 α s 0 s   α sky 0 sky  * Q conv  * E'$)Q p  9 + p  cp 'T in  ; T a+b )'2)Q  9 k   A   'T in  ; T a+b ) < = c '%)Q e  9 A o C t '> s  ; > a+b ) ? Q l '&) !lar rad at !n# The total solar radiation incident on a surface is the combination of the direct *subscript '1, diffuse*subscript d1 and ground reflected *subscript r1 irradiance of the surface, which givesC r d  Dt   E  E  E  E   ++= *D1The amount of solar irradiance was calculated according to A-;?, !330 !lar $!lle$t!r# The energy balance for the air flow through the collector given by e(uation * 1 can be written to give the temperature rise across the collector in according to the e(uations cited from 'ufee and 2echman, /00/enthalpy of air entering the bed is e(ual to the sum of the enthalpy of ambient air and the enthalpy gain in thecollector. The enthalpy gain in the collector can be expressed asCfor an actual collector and asC   for the electrically heated simulated collector where %= L is the area of collector. ow, the heat balance givesCh s  E h a  F Ghwhere ha is the enthalpy of ambient air and hs is the enthalpy of air leaving the bed which is assumed to be thesaturation enthalpy. The mode/ utilizes the following relations for enthalpy and saturated humidity ratio. Thosee(uations are approximations developed from the psychometric chart. h a  E */ H1xT a  F H x h s  h s  E !0.< F *!./ x*T /311 F *3.3I x *T /31 ! 1 J sat  E 3.33I F *3.3338< x *T /3.311 F*3.3333!I x *T /31 ! 1>irst, the saturation enthalpy of ambient air is calculated from e(uation *I<1 then e(uation *I81 is used to calculate the enthalpy of ambient air. The enthalpy of air leaving the grain bed is then computed using e(uation *I#1. -ince the air leaving the bed is saturation, its temperature, which is the saturation temperature, can be obtained from e(uation *I<1.%hen the ambient and final temperatures are known, e(uation *I1 is used to obtain the saturated humidity ratios of air entering the collector and that leaving the dryer. The humidity ratio of the air entering the collector can then becalculated using e(uation *81.The rate at which moisture is removed can then be computed by applying a moisture mass balance for the product  bed as followsCwhereC $
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