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Convective solar drying of Vitis vinifera & Momordica charantia using thermal storage materials
Accepted Manuscript Convective solar drying of Vitis vinifera and Momordica charantia using thermal storage materials
N. Karunaraja, Singh Thokchom Subhaschandra, Verma Tikendra Nath, Nashine Prerana PII:
S0960-1481(17)30600-6
DOI:
10.1016/j.renene.2017.06.096
Reference:
RENE 8963
To appear in:
Renewable Energy
Received Date:
23 February 2017
Revised Date:
30 May 2017
Accepted Date:
28 June 2017
Please cite this article as: N. Karunaraja, Singh Thokchom Subhaschandra, Verma Tikendra Nath, Nashine Prerana, Convective solar drying of Vitis vinifera and Momordica charantia using thermal storage materials, Renewable Energy (2017), doi: 10.1016/j.renene.2017.06.096
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Convective solar drying of Vitis vinifera & Momordica charantia using thermal storage materials
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Karunaraja N a, Thokchom Subhaschandra Singh b,*, Tikendra Nath Verma b, Prerana Nashinec
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a
Department of Mechanical Engineering, Dr. Mahalingam College of Engineering and Technology, Pollachi- 642003, India bDepartment of Mechanical Engineering, National Institute of Technology Manipur,Imphal-795004, India cDepartment of Mechanical Engineering, National Institute of Technology Rourkela, Rourkela-769008, India
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* Corresponding author, Email: [email protected]; Ph: +91-94029-32585
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Abstract
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Solar drying is one of the most feasible and cheap method in preservation of foods and commodities. An experimental analysis was performed on a solar tunnel dryer under the meteorological conditions of Negamam, India, for drying samples of Vitis vinifera and Momordica charantia. Sand bed, rock bed and aluminum filings were used as thermal storage materials for the study. A comparison was made between the dryer (with and without the application of thermal storage materials) and open sun drying to check the effectiveness in dehydrating the samples. There were effective reductions in drying time as well as moisture removal rate (85% to 10% in 27hrs and 88% to 6% in 6hrs) using the solar tunnel dryer (with or without thermal storage materials) as compared with open sun drying. Average thermal efficiency of the solar tunnel dryer with thermal storage materials was found to be greater by 2-3% as compared with the one without thermal storage material. Amongst the thermal storage materials, sand was found to have higher effectiveness with an average thermal efficiency of 19.6% and 15.46% while drying Vitis vinifera and Momordica charantia respectively.
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Keywords: Solar tunnel dryer; Vitis vinefera; Momordica charantia; Thermal storage materials
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1. Introduction
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A solar tunnel dryer is one among the many techniques available for convective thermal drying process. The principle of green house effect is used in a solar tunnel dryer, in which a thin plastic greenhouse of 200 micron thickness (UV Polyethene) covers the dryer. Low capital and operation cost along with high volume capacity of load provides an economical aspect of the dryer. Nabnean S et al. (2016) [1] have performed an experiment where they proposed a new design of solar dryer for drying cherry tomatoes, in the month of May to June 2014, with a volume of 100kg for each batch. They have found the efficiency of the dryer to be 21% to 69%, with a payback period of 1.37 years. Natural rubber sheet drying was performed by Racha Dejchanchaiwong et al. (2016) [2] and found that solar drying decrease the drying time by 2-3 days as compared to 7 days by conventional drying. They have investigated 30 natural rubber sheets on a mixed mode and indirect solar drying system. The mixed mode dryer have a higher efficiency of 15.4% as compared to indirect drying system (13.3%). The moisture contents on wet basis in mixed mode and indirect solar drying of the rubber sheets were reduced from 32.3 to 2.0% and 29.4-8% respectively in 4 days. The evaluation of performance of a solar tunnel greenhouse dryer was performed by Morad M.M et al. (2017) [3] for drying peppermint plants. They have found that reduction of drying time can be achieved if the peppermint were dried as leaves rather than the drying the whole plants. With a loading of 4kg/m2 and flow rate of 2.10m3/min with continuous fan operation increases the drying rate by 24.8% and 22.78% for leaves drying and 1
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whole plant respectively. A new direct and integrated type solar hybrid dryer was proposed for drying of pineapple [4]. Extra heat for the dryer is generated by hot water at 80°C passing through copper helical tube wherein the tube lies at the bottom of a black pan. A transparent vinyl film is used as cover for the dryer and its base and walls were isolated using fibreglass of 0.0254m thickness. They have found that evaporation efficiency of conventional dryer (22.7% to 24.0%) was higher than the proposed hybrid dryer (9.3% to 14.0%). The moisture removal rate (24.0% humid base or 0.32% dry base) on the other hand, was found to be faster in the case of hybrid dryer (3.0 to 6.8 hrs) than conventional method (8.0 to 8.8 hrs). Infrared drying systems were proposed by Mustafa A et al. (2016) [5] for drying melons. A computational 3D CFD simulation was also used for the investigation purpose. The initial temperature of melons ranged from 50-60 °C with air velocity of 0.5 m/s. The theoretical prediction and experimental analysis yields similar efficiency of the dryer (50.6%). Woods chips of Pinus pinaster were dried on a solar green house dryer by Alberto-Jesús et al.[6] in Autumn season conditions with average daily solar radiation of 13.74 MJ/(m2d) for a period of 15 days. The results were also mathematically modelled to depict the advantages of the solar greenhouse dryer for reaching the desired relative humidity of 10%.
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Bitter gourd drying on a forced convection solar dryer using evacuated tube air collector was performed by Venkatesan N et al. (2014) [7]. The collector temperature was found to vary between 47.4 °C to 91°C. The moisture content of bitter gourd was reduced from 90.93% to 7.61% in 6 hrs, as compared to 10 hrs in open sun drying. Umayal Sundari P et al. (2013) [8] have also performed an analysis on drying bitter gourd on a forced convective solar dryer and found that the moisture content was reduced from 91% to 6.25% in 6 hours. Mango slices were dried on a solar dryer using the standard static gravimetric method [9]. The moisture adsorption isotherms were determined at three different temperatures (20°C, 30 °C and 40°C) for water activity range of 0.111 to 0.813. Madhlopa et al. (2007) [10] have experimented on an indirect type natural convection solar dryer with integrated biomass backup heater using three modes of operation (solar, biomass, solar-biomass) on Ananas comosus and found that the moisture content was reduced from 669 to 11% (db) while yielding a nutritious dried product.
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Forson F K et al. (2007) [11] have modeled a mixed mode natural convection solar crop dryer using a single pass double duct solar air-heater (MNCSCD) for drying cassava and found that nocturnal drying accounts for 23.9% of moisture loss during solar drying. The mean moisture content of the product was reduced from 201.7% to 21.1% (db) in 21hrs. Benon B et al. (2000) [12] have experimented solar drying using a direct type natural convection solar dryer equipped with a simple biomass burner on pineapples, arranged in a single layer of 0.01m thick slices. The drying efficiency of solar component alone account for 22% while other trials suggest the efficiency of biomass burner in production of useful heat in drying to be 27%. The energy performance of a solar ambient hybrid source heat pump drier (SAHSHPD) in drying copra under hot-humid weather conditions was performed by Mohanraj et al. (2014) [13]. The COP of the dryer was found to range from 2.31 and 2.77 with average value of 2.54. The moisture content (w.b.) was reduced from 52% to 9.2%. The specific moisture extraction rate was calculated as 0.79 kg/kWh. A natural convective heat flow solar dryer has been investigated by Gbaha P et al. (2007) [14] in drying perishable foods such as cassava, banana and mango. It has been found that the moisture content has been reduced to 80% in 19hrs. Early uses of solar dryers for drying of cassava chips was found to be economical and promising as it reduces the moisture content to about 16% (w.b.) in 70hrs. It was found that the thickness of the layer influences the nature and pattern of drying [15].
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An indirect type forced convective solar dryer was utilized for dehydration of red chilli in Mexico. The authors in the paper have conducted the drying process at controlled air temperatures of 45 °C, 55 °C and 65 °C using an oven. During the course of study, two different air velocities were used: high velocity (1.4 and 2.6 m/s) and low velocity (0.7 and 1.48 m/s). It was observed that drying kinetics of 55 °C and 65 °C were close to each other, with drying time of 2.75hrs and 3.0hrs respectively. On the contrary, the drying time at 45 °C was observed to be 6.25hrs. A total drying time of 16hrs was found in obtaining final moisture content of 0.057 kg of water/kg dry matter and 0.90 kg of water/kg dry matter with use high velocity air, while 21hrs drying time was observed in using low velocity air, with final moisture content of 0.0611 kg of water/kg dry matter and 0.109 kg of water/kg dry matter 2
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respectively [16]. Drying of banana was successful in an indirect type solar dryer with drying cabinet of 1m x 0.4 m x 1m (width, depth and height). Corrugated V-type absorption plates, insulated drying chamber with a chimney constituted the drying chamber. The authors in the paper observed that the moisture content of the samples were reduced from 356% (dry basis) to 16.3292%, 19.4736%, 21.1592%, 31.1582% and 42.3748% (dry basis) for trays 1,2,3,4 and open drying. Average thermal efficiency was found to be 31.50% (solar collector) and 22.38% (drying chamber) [17]. A forced convective solar dryer with double pass air heater (series connection), semi continuous tunnel dryer, heat exchanger (shell and tube) and a blower, was used to study the drying kinetics of ghost chilli pepper (Capsicum Chinense Jacq.). Six (6) trays are used for drying nine (9) kg of the samples. A simultaneous ope sun drying was conducted to study the effectiveness of the dryer. It was observed that the moisture content of the samples was reduced from 589.6% (db) to a final value of 12% (db) in 123hrs and 193hrs in solar dryer and open sun drying respectively [18].
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A solar assisted solid desiccant dryer was used to study drying characteristics of crushed oil palm fond samples. The moisture content of the samples were reduced from 69% to 29% in 30hrs and 40mins using open sun drying, while using the dryer, drying time was reduced significantly by 64%, 44% and 33% for products in 1st, 2nd and 3rd columns of the dryer respectively. Solar energy contributes 65% of the total energy required for drying. The efficiency of the dryer was observed to be 19% at full load conditions. The desiccant wheel have improved the quality of drying air with latent and sensible heat effectiveness of 67% and 74% respectively [19]. Mixed mode solar dryer with thermal energy storage is used to study the drying characteristics of fresh apricot slices. Phase change materials were used to store latent heat and it was observed that drying rate is equal in throughout the drying chamber. The thermal efficiency of the dryer is 11% [20]. Thermal modeling of a mixed mode greenhouse type solar dryer was performed and various analytical expressions such as temperatures of crop, greenhouse, outlet air and cell have been found through a set of governing equations using MATLAB 2013a numerical solver. The authors observed that variation in the no. of air collector (from 1 to 5) have changed the equivalent thermal energy (from 3.24 to 10.57 kWh/day), thermal efficiency (61.56% to 42.22%) and exergy efficiency (28.96% to 19.11% [21]. Various optimization models of solar thermal systems such as artificial neural network, wavelength neural network, least square support vector machine etc. were used in evaluating the performance of solar air heaters. It was observed that the models have good predictability and they can be used effectively in estimating performance parameters of solar thermal systems with accuracy [22-23]. Use of aluminum cans have been found to increase efficiency of solar energy collection with minimum investment. The authors have reported that aluminum cans placed as zigzag on absorber plate with air mass flow rate of 0.05kg/s have the highest efficiency [24]. This model can be effectively used in a solar dryer to study the drying characteristics of the samples.
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In this present work, the authors have presented experimental evaluation of a proposed solar tunnel dryer for studying the drying kinetics of Vitis vinifera and Momordica charantia at the proposed site of Negamam (10.7426° N latitude, 77.1032° E longitude, 293m altitude above sea level), India. The samples have been chosen for this particular study as there is abundance availability of raw materials in the region (Vitis vinifera & Momordica charanti). Various heat storage materials were taken for this study, such as sand bed, rock bed and aluminum filings, as an attempt to increase the thermal performance of the proposed solar tunnel dryer. An uncertainty analysis was also conducted for the measured parameters to increase the accuracy of the results.
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Nomenclature
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A CFD db hfg I
area of solar dryer (m2) computational fluid dynamics dry basis enthalpy (J/kg K) incident radiation (W/m2)
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k MRR m M RH STD t UV W WTSM Ƞth
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f i in m OSD out SD w wb
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2. Materials and method
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drying constant (h-1) moisture removal rate (gm) mass (kg) moisture content of product in wet basis (%) relative humidity (%) solar tunnel dryer drying time (hrs) ultraviolet weight (kg) without thermal storage materials thermal efficiency (effectiveness)
Subscripts
final initial inlet mass (kg) open sun drying outlet solar drying water wet basis
The experimental analysis of the solar tunnel dryer consists of the following Experimental set-up Instrumentation Performance analysis
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2.1 Experimental set-up and procedure
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The experiments were conducted in the month of March-April 2015, under the meteorological conditions of Negamam, India (10.7426° N latitude and 77.1032° E longitude). Five (5) kilograms of Vitis vinifera and Two (2) kilograms of Momordica charantia were procured from the local market. Fruits of commercial maturity having uniform size were chosen for the experiment in which average dimension of Momordica charantia were found to range from 100-150mm long and 25-30mm diameter. The fruits are picked and separated from the bunch before loading into the dryer. To achieve outmost quality, quality check was performed and fruits with cracks or injured skin were rejected. The Momordica charantia was sliced to 5mm diameter. The selected fruits were pre-treated with water to ensure no unwanted particles were present on the fruit. No chemical treatment was performed on the fruits. From the total five kilograms of Vitis vinifera, 990gm of sample is kept for open sun drying remaining and 3.96kg is equally distributed into four (4) samples for each of the analysis for the respective study period such that for each study, an equal distribution of sample can be made. An initial 50gm is removed for use in convective oven to determine moisture content on wet basis. 4
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Heat storage materials or thermal storage materials are those materials which serve as an important source for storage of excess energy and release of the stored energy during requirement. Sand, rock and aluminum filings are used as sensible heat storage materials in this experiment. The properties of some of the thermal storage materials are given in the following table.1 Table 1. Properties of some of the commonly available thermal storage materials Property
Material
Density (Kg/m3)
Water 958.4
Rocks 2245
Pebbles 1350
MgO 3575
Al2O3 4000
Al Filings 2712
Sand 2600
Specific Heat (kJ/Kg K)
4.22
0.81
0.9
1.06
1.02
0.900
1.26
Thermal Conductivity (W/m K)
0.683
0.13
0.85
10.5
6.3
235
2.3
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2.1.1 Design considerations
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Fig.1. Schematic diagram of the STD
Figure.1 shows the schematic diagram of the solar tunnel dryer. The dryer is of 2134mm length and 912mm in width, with a height of 609mm. Six (6) numbers of purlin and seven (7) numbers of hoops are mounted on the wooden floor of the solar dryer. The radius of the tunnel dryer was calculated to be 1689.4mm. The total drying area (floor area) of the tunnel dryer is 1,946,208 mm2. Polyethylene transparent film of 200 micron thickness is used as the UV sheet. The recorded maximum temperature is 60°C with RH of 30%. The design considerations of the dryer are given in table 2.
Table 2. Design considerations and assumptions for the design of the dryer Items
Items condition
Location Orientation Loading capacity Initial moisture content Final moisture content Sunshine hour Drying period required Global solar radiation for 10°7426’ N and 77°1032’E (I)
10°7426’ N and 77°1032’E E -W direction 10 Kg 85-90% (db) 15-10% (db) 10 hrs 18 hrs 490-505 W/m2
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Collector material (cover)
UV polythene 200 micron sheet 305 mm
Height of chimney (H)
The front and back door are fixed on the hoops such that the unfilled gaps of the UV sheet are covered. The air ventilation is done by an exhaust chimney on top of the dryer as shown in fig.2.
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Fig.2. Fabricated model of the STD
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The minimum temperature for drying of foods is recorded to be 30 °C and maximum temperature to be 60 °C. Hence, a temperature range of 45 °C and above is considered to be average for drying of crops such as vegetables, fruits, roots, crop seeds etc. The design of the dryer was made for optimum temperature distribution, with outlet and inlet temperatures at 60 °C and 30 °C respectively. Relative humidity depends on the temperature and pressure of the system of interest, and hence it is one of the important design considerations. Temperature drop and rise in relative humidity depends on the volume of the airflow. If more volume of air moves across the lumber surface, there will be less temperature drop and increase in relative humidity. At low air velocity, there is a risk of wrap, staining and non-uniform drying. Therefore air velocity is a tool for controlling the rate of drying. Readings were taken on a daily basis from 0900 hrs to 2000 hrs. During the course of the experiment, temperatures were recorded from various locations in the dryer chamber on hourly using the temperature sensor. The relative humidity sensor is also kept inside the dryer to record RH on hourly and daily basis. The moisture reduction in each day was calculated on wet basis.
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2.2 Instrumentation & uncertainty analysis
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In the performance investigation of the proposed dryer, the following are the instruments which were employed during the course of the experiment. A data acquisition system (DAQ) from SIMEX (SRD-99, ±0.25% uncertainty) was employed for acquiring the data from the sensors. Table.3 gives the list of instruments utilized for data collection. The humidity/temperature transmitters have a stability of 1% RH per year and the temperature 6
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compensation is ±0.0008% RH/°C. The response time is less than 15 sec. The operating temperature of the pyranometer falls between -40 °C and 80 °C and the typical response time is less than 28 sec. The instruments were calibrated before the readings were taken to evaluate the uncertainty of the measurements. Standard methods were used for validating the results.
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Table 3. Uncertainty, accuracy and range of instruments Instrument
Range
Uncertainty
Accuracy
Humidity/ Temperature Transmitter SIMEX Italy (RIXEN TRH-303W) Pyranometer Delta Ohm, Italy
RH: 0-100% Temperature: 0-100°C 0-2000 W/m2
±1%
±2% RH (at 25 °C), ±0.3 °C
±5%
±0.01%
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Multiple-sample experimental analysis has less accuracy in determining the uncertainty of experiments. The plus or minus notation of determining the uncertainty is based on the researcher conducting the experiment. Hence, this is uncertain because the experimenter is uncertain about the accuracy of the measurements. So, a more precise way of determining the uncertainty is to use the single sample experiment uncertainty analysis [25]. The following equation (1) is used to determine the uncertainty arising in measurement & calculation of temperature, solar radiation, RH and consumption of energy. Total uncertainty arising from the parameters calculated such as, moisture content of samples, rate of drying, moisture removal rate and thermal efficiency of dryer are 3.1, 2.8, 2.6 and 3.9 respectively.
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R R R wr w1 w2 wn x1 x2 xn
2
2
2
(1)
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2.3 Performance analysis
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The performance of the solar tunnel dryer depends on the duration of drying and the quality of the end product, moreover factors such as collector performance and drying temperature. Elaborate testing of the solar tunnel dryer was carried out under full load capacity conditions on the month of March-April 2015, for drying of the samples. Three (3) phases of drying of the samples were carried out in each of the case and the most optimum reading was taken for the study. The following equations (2-6) were used to evaluate the performance parameter for drying the samples.
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2.3.1 Estimation of moisture content of samples.
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The moisture content of the samples are calculated using the following equation (2)
mi m f
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M wb
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They are represented on wet basis and expressed in percentage. 50gm sample was kept in a convective electric oven with a temperature of 105±5 °C. The initial (mi) and final (mf) mass of the samples were recorded using an electronic weighing machine.
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2.3.2 Estimation of mass of evaporated water vapor
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mi
(2)
100
The mass of water vapor evaporated can be calculated by the following equation (3).
Win Wout 100 Wout
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mw
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2.3.3 Drying rate
(3)
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The drying rate of the samples in the dryer can be calculated by using the following equation (4). It is use to signify the duration taken by the system to vaporize and remove water vapour from the samples.
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k
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2.3.4 Determination of dryer efficiency
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The thermal efficiency of the solar tunnel dryer can be estimated by using the following equation (5). The thermal efficiency of the solar dryer depends primarily on the mass of water vapor evaporated from the samples.
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2.3.5 Percentage of time saving
th
Mw t
mw h fg A I
(4)
100
(5)
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The percentage of time saving for the dryer can be calculated using equation (6). The calculated value provides the savings in time (usually expressed in hours) upon comparison of drying the samples between solar tunnel dryer and open sun drying.
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Percentage of Time saving
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3. Results and Discussion
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An initial load test was conducted on a solar dryer under full load condition without application of any heat storage materials and the following observations were made.
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tOSD tSD 100 tOSD
(6)
The moisture content of Vitis vinifera was reduced from 85% to 10% in 53hrs as compared to 58hrs in open sun drying. The average thermal efficiency was found to be 21.2%. Similarly, moisture content of Momordica charantia was reduced from 88% to 6% in 7hrs as compared to 10hrs in open sun drying. Average thermal efficiency was found to be 9.9%.
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The following is the discussion about the study of performance characteristics for drying the samples in the dryer (with and without thermal storage materials) with comparison to open sun drying.
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3.1 Drying time and relative humidity
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Relative humidity is the amount of water vapour present in the air. It is desired that the RH of the air which is to be used for drying be as low as possible. The variation of RH with respect to drying time of the samples of Vitis vinefera and Momordica charantia were plotted in fig. 3 (a) and (b).
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Fig.3. Variation of RH values with drying time of (a) Vitis vinefera and (b) Momordica charantia.
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There is significant reduction of RH in the solar tunnel dryer than the open sun drying. It was observed that the maximum and minimum RH inside the dryer are 62% and 25%, while that of ambient RH were found to be 70.8% and 28.6% while analyzing Vitis vinefera. Upon analysis of Momordica charantia, the maximum and minimum RH inside the dryer were found to be 50% and 20% while that of ambient were found to have RH of 75% and 60% owing to low solar intensity during the period of study (March-April, 2015). Usage of sand in the dryer reduces the RH of the solar thermal dryer by about 8-11% as compared to without/other thermal storage systems. Therefore the air in the dryer has significant drying potential than that of ambient air.
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3.2 Moisture content removal with respect to time.
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Fig.4. Reduction of moisture content of samples with increase in time (a) Vitis vinefera and (b) Momordica charantia.
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Moisture content removal is the ability of the drying air to evaporate water content from the samples to facilitate dehydration. From fig. 4, it can be clearly seen that, the moisture content (w.b.) of the samples reduces notably in the solar tunnel dryer, as compared to open sun drying. Initial moisture content of Vitis vinefera (a) was found to be 85% and it was reduced to desired moisture content of 10% within 28hrs (sand) as compared to 58hrs in open sun drying. Similarly, reduction of moisture content of Momordica charantia (b) from 88% to 7.82% in 5.3hrs was achieved while using sand, as compared to 10.2hrs in open sun drying. It can be noted that there was increase in moisture content of the samples by 1-2% during night which is due to re-absorption.
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In the proposed solar dryer by Venkatesan N et al. (2014) [7], the moisture from the samples were reduced from 90.93% (w.b.) to 7.61% in 6hrs, while the authors in this paper have achieved in 5.3hrs. Hence the proposed solar tunnel dryer is more efficient in removing moisture from the samples.
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3.3 Drying time and MRR
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Fig.5. Variation of drying time and MRR (a) Vitis vinefera and (b) Momordica charantia
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The moisture removal rate (MRR) is the rate of evaporation of water vapour from the samples with respect to time. Fig. 5 (a) Vitis vinefera and (b) Momordica charantia, shows the variation of MRR with respect to drying time of the samples. The average moisture removal rate of Vitis vinefera were reduced from 50gm to38gm in about 10hrs (WTSM) and good MRR were found in case of aluminum bed, which reduces the MRR to about 26gm in the same time frame. In case of Momordica charantia, the reduction of 50gm to 8gm can be seen in about 8hrs, in which the sand bed coupled solar dryer gives better MRR compared to the rest systems. The constant phase in both the cases is due to absence of heat in the air after sunset.
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3.4 Days and dryer efficiency
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Fig.6. Average thermal efficiency of STD with and without thermal storage materials (a) Vitis vinefera and (b) Momordica charantia
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The average thermal efficiency of solar tunnel dryer while using Vitis vinefera [Fig. 6 (a)] and Momordica charantia [Fig. 6 (b)] as samples were shown in the above figure. The average thermal efficiency during drying of Vitis vinefera was found to be 21.2%, 19.6%, 15.4% and 16.80% in WTSM, sand bed, rock bed and aluminum 10
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conditions. Also, it was found that the thermal efficiency of solar tunnel dryer was 9.9%, 15.46%, 14.75% and 13.7% in WTSM, sand bed, rock bed and aluminum conditions. From the results, it is clear that sand have a higher advantage of using as a thermal storage media in solar tunnel dryer systems.
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3.5 Variation of temperature, time and solar intensity
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Fig.7. Variation of temperature with time (a) Vitis vinefera and (b) Momordica charantia
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The distribution of temperature with respect to time in various solar intensities while loading Vitis vinefera (a) Momordica charantia (b) is shown in the fig. 7. The ambient temperature ranged between 27 °C to 29 °C in open sun drying. In both cases of drying the samples, the temperature inside the dryer increases till 1400hrs and then there is a sudden drop in the temperature. This is because the highest solar intensity was recorded to be 612 W/m2 and 655 W/m2 during peak hours (1330 to 1430 hrs) in both cases of analyzing Vitis vinefera and Momordica charantia (March-April, 2015). As the sun approaches dusk, the solar intensity reduces gradually. The maximum temperature was recorded to be 67 °C and 57 °C in using sand for samples of Vitis vinefera & Momordica charantia respectively.
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4. Conclusions
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A solar tunnel dryer is proposed and constructed by the authors in Negamam, India with average global solar radiation of 490-505 W/m2. Thermal storage materials such as sand, rock bed and aluminum were selected as sensible heat storage media to facilitate accumulation of heat inside the dryer. Locally available variety of Vitis vinefera and Momordica charantia were chosen as raw materials for dehydration in the proposed solar dryer due to its abundance in the region. The samples were divided and a part of the sample is taken for open sun drying. Different test were conducted on various conditions such as- open sun drying, tunnel drying WTSM, tunnel drying with sand, rock bed and aluminum filings. An uncertainty analysis of the measured parameters from the instruments is conducted to determine the uncertainty factor. Drying time, relative humidity, moisture removal rate, dryer thermal efficiency is of main interest for determining the performance of the solar tunnel dryer. The removal of moisture from the samples of Vitis vinefera (85% to 10%) and Momordica charantia (88% to 6%) were observed in 80hrs and 11hrs [open sun drying], 53hrs and 7hrs [STD WTSM], 28hrs and 5.3hrs [STD with sand], 31hrs and 6hrs [STD with rock bed], 29hrs and 7hrs [STD with aluminum filings] respectively. Figure (8) and (9) shows the process of drying and end product of the samples.
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Fig.8. (a) Initial weight of sample; (b) loading of the sample ; (c) final product quality from open sun drying ; (d) final product quality from STD
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Fig.9. (a) Initial weight of Momordica charantia; (b) loading of Momordica charantia on the solar dryer; (c) final quality of product from open sun drying; (d) final quality of product from STD
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It can be summarized that the designed solar tunnel dryer served the purpose of drying quality products of the samples with higher superiority than open sun drying. Amongst the materials studied for the thermal storage material, sand bed was found to provide greater effectiveness in terms of moisture removal and thermal efficiency. Thus the authors concluded that the proposed STD coupled with sand can be used as an important food preservation technique and this can help in increasing the productivity and uplift the economy of the society without compromising the quality of the products.
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Acknowledgements
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The authors acknowledge the support provided by Dr. MCET Pollachi, Tamilnadu and JJCET Tiruchirapalli, Tamilnadu during the course of the project.
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References
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[2] Dejchanchaiwong R, Arkasuwan A, Kumar A, Tekasakul P. (2016) Mathematical modeling and performance investigation of mixed-mode and indirect solar dryers for natural rubber sheet drying. Energy for Sustainable Development, 34, 44-53. doi:10.1016/j.esd.2016.07.003.
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[3] Morad MM, El-Shazly MA, Wasfy KI, El-Maghawry HAM. (2017). Thermal analysis and performance evaluation of a solar tunnel greenhouse dryer for drying peppermint plants. Renewable Energy, 101, 992-1004. doi:10.1016/j.renene.2016.09.042.
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[4] Gudiño-Ayala D, Calderón-Topete Á. (2014). Pineapple drying using a new solar hybrid dryer. In: Energy Procedia, 57 (2), 1642-1650. doi:10.1016/j.egypro.2014.10.155.
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[5] Mustafa Aktas, Seyfi Sevik, Ali Amini, Ataollah Khanlari. (2016). Analysis of drying of melon in a solar-heat recovery assisted infrared dryer. Solar Energy, 137, 500-515. doi:10.1016/j.solener.2016.08.036
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[6] Perea-Moreno AJ, Juaidi A, Manzano-Agugliaro F. (2016). Solar greenhouse dryer system for wood chips improvement as biofuel. Journal of Cleaner Production, 135, 1233-1241. doi:10.1016/j.jclepro.2016.07.036.
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[7] Venkatesan N, Arjunan T.V. (2014). An Experimental Investigation and Performance Analysis of a Solar Drying of Bitter Gourd using an Evacuated-Tube Air Collector. International Journal of Chem Tech Research, 6(14), 55105518. doi:10.1016/j.solener.2016.08.036
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[8] AR. Umayal Sundari, P. Neelamegam, C.V. Subramanian. (2013). An experimental study and analysis on solar drying of Bitter gourd using an evacuated tube air collector in Thanjavur, Tamilnadu, India. International Conference on Solar Energy Photovoltaics, 19th to 21st December 2012, Bhubaneshwar, India, 2013.
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[9] Akoy E, Von Hörsten D, Ismail M. (2013). Moisture adsorption characteristics of solar-dried mango slices. International Food Research Journal, 20(2), 883-890.
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[10] Madhlopa A, Ngwalo G. (2007). Solar dryer with thermal storage and biomass-backup heater. Solar Energy, 81(4), 449-462. doi:10.1016/j.solener.2006.08.008.
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[11] Forson FK, Nazha MAA, Rajakaruna H. (2007). Modelling and experimental studies on a mixed-mode natural convection solar crop-dryer. Solar Energy, 81(3), 346-357. doi:10.1016/j.solener.2006.07.002.
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[12] Bena B, Fuller RJ. (2002). Natural convection solar dryer with biomass back-up heater. Solar Energy, 72(1), 75-83. doi:10.1016/S0038-092X(01)00095-0.
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[13] Mohanraj M. (2014). Performance of a solar-ambient hybrid source heat pump drier for copra drying under hothumid weather conditions. Energy for Sustainable Development, 23, 165-169. doi:10.1016/j.esd.2014.09.001.
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[15] Olufayo A.A, Ogunkunle O.J. (1996). Natural drying of cassava chips in the humid zone of Nigeria. Bioresource Technology, 58 (1), 89-91. doi: 10.1016/S0960-8524(96)00022-3.
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[16] Castillo-Téllez M, Pilatowsky-Figueroa I, López-Vidana, Sarracino-Martinez O, Hernandez-Galvez G. (2017) Dehydration of the red chilli (Capsicum annuum L., costesnoo) using an indirect-type forced convection solar dryer. Applied Thermal Engineering, 114, 1137–1144. https://doi.org/10.1016/j.applthermaleng.2016.08.114
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[17] Lingayat A, Chandramohan V.P, Raju V.R.K. (2017). Design, Development and Performance of Indirect Type Solar Dryer for Banana Drying. In Energy Procedia, 109, 409–416. https://doi.org/10.1016/j.egypro.2017.03.041
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[18] Rabha D.K, Muthukumar P, Somayaji C. (2017). Experimental investigation of thin layer drying kinetics of ghost chilli pepper (Capsicum Chinense Jacq.) dried in a forced convection solar tunnel dryer. Renewable Energy, 105, 583–589. https://doi.org/10.1016/j.renene.2016.12.091
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[19] Misha S, Mat S, Ruslan M.H, Salleh E, Sopian K. (2015). Performance of a solar assisted solid desiccant dryer for kenaf core fiber drying under low solar radiation. Solar Energy, 112, 194–204. https://doi.org/10.1016/j.solener.2014.11.029
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[21] Tiwari S, Tiwari G.N. (2017). Energy and exergy analysis of a mixed-mode greenhouse-type solar dryer, integrated with partially covered N-PVT air collector. Energy, 128, 183–195. https://doi.org/10.1016/j.energy.2017.04.022
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[22] Esen H, Ozgen F, Esen M, Sengur A. (2009). Artificial neural network and wavelet neural network approaches for modelling of a solar air heater. Expert Systems with Applications, 36(8), 11240–11248. https://doi.org/10.1016/j.eswa.2009.02.073
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[24] Ozgen F, Esen M, Esen E. (2009). Experimental investigation of thermal performance of a double-flow solar air heater aluminium cans. Renewable Energy, 34, 2391-2398
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HIGHLIGHTS OF THE MANUSCRIPT TITLED “Convective
1.
Solar Drying of Vitis vinifera & Momordica charantia using Thermal storage materials”
The manuscript was prepared based on original research of using a solar tunnel dryer for drying agricultural products. Locally available materials were used as samples for this purpose.
2.
New findings were proposed herein, in which thermal storage materials were utilized to check the performance characteristics of the solar dryer while drying the samples.
3.
Sand bed, Rock bed and Aluminum filings were used as thermal storage materials where the analysis was made in between the materials and also with solar tunnel dryer (without thermal storage materials) and open sun drying (conventional drying).
4.
There were effective reductions in drying time as well as moisture removal rate (85% to 10% in 27hrs and 88% to 6% in 6hrs) using the solar tunnel dryer (with or without thermal storage materials) as compared with open sun drying.
5.
Amongst the thermal storage material, sand was found to have higher effectiveness in reduction of moisture content of the samples with minimum time with an average thermal efficiency of 19.6% while drying Vitis vinifera.
N. Karunaraja, Singh Thokchom Subhaschandra, Verma Tikendra Nath, Nashine Prerana PII:
S0960-1481(17)30600-6
DOI:
10.1016/j.renene.2017.06.096
Reference:
RENE 8963
To appear in:
Renewable Energy
Received Date:
23 February 2017
Revised Date:
30 May 2017
Accepted Date:
28 June 2017
Please cite this article as: N. Karunaraja, Singh Thokchom Subhaschandra, Verma Tikendra Nath, Nashine Prerana, Convective solar drying of Vitis vinifera and Momordica charantia using thermal storage materials, Renewable Energy (2017), doi: 10.1016/j.renene.2017.06.096
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Convective solar drying of Vitis vinifera & Momordica charantia using thermal storage materials
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Karunaraja N a, Thokchom Subhaschandra Singh b,*, Tikendra Nath Verma b, Prerana Nashinec
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a
Department of Mechanical Engineering, Dr. Mahalingam College of Engineering and Technology, Pollachi- 642003, India bDepartment of Mechanical Engineering, National Institute of Technology Manipur,Imphal-795004, India cDepartment of Mechanical Engineering, National Institute of Technology Rourkela, Rourkela-769008, India
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* Corresponding author, Email: [email protected]; Ph: +91-94029-32585
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Abstract
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Solar drying is one of the most feasible and cheap method in preservation of foods and commodities. An experimental analysis was performed on a solar tunnel dryer under the meteorological conditions of Negamam, India, for drying samples of Vitis vinifera and Momordica charantia. Sand bed, rock bed and aluminum filings were used as thermal storage materials for the study. A comparison was made between the dryer (with and without the application of thermal storage materials) and open sun drying to check the effectiveness in dehydrating the samples. There were effective reductions in drying time as well as moisture removal rate (85% to 10% in 27hrs and 88% to 6% in 6hrs) using the solar tunnel dryer (with or without thermal storage materials) as compared with open sun drying. Average thermal efficiency of the solar tunnel dryer with thermal storage materials was found to be greater by 2-3% as compared with the one without thermal storage material. Amongst the thermal storage materials, sand was found to have higher effectiveness with an average thermal efficiency of 19.6% and 15.46% while drying Vitis vinifera and Momordica charantia respectively.
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Keywords: Solar tunnel dryer; Vitis vinefera; Momordica charantia; Thermal storage materials
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1. Introduction
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A solar tunnel dryer is one among the many techniques available for convective thermal drying process. The principle of green house effect is used in a solar tunnel dryer, in which a thin plastic greenhouse of 200 micron thickness (UV Polyethene) covers the dryer. Low capital and operation cost along with high volume capacity of load provides an economical aspect of the dryer. Nabnean S et al. (2016) [1] have performed an experiment where they proposed a new design of solar dryer for drying cherry tomatoes, in the month of May to June 2014, with a volume of 100kg for each batch. They have found the efficiency of the dryer to be 21% to 69%, with a payback period of 1.37 years. Natural rubber sheet drying was performed by Racha Dejchanchaiwong et al. (2016) [2] and found that solar drying decrease the drying time by 2-3 days as compared to 7 days by conventional drying. They have investigated 30 natural rubber sheets on a mixed mode and indirect solar drying system. The mixed mode dryer have a higher efficiency of 15.4% as compared to indirect drying system (13.3%). The moisture contents on wet basis in mixed mode and indirect solar drying of the rubber sheets were reduced from 32.3 to 2.0% and 29.4-8% respectively in 4 days. The evaluation of performance of a solar tunnel greenhouse dryer was performed by Morad M.M et al. (2017) [3] for drying peppermint plants. They have found that reduction of drying time can be achieved if the peppermint were dried as leaves rather than the drying the whole plants. With a loading of 4kg/m2 and flow rate of 2.10m3/min with continuous fan operation increases the drying rate by 24.8% and 22.78% for leaves drying and 1
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whole plant respectively. A new direct and integrated type solar hybrid dryer was proposed for drying of pineapple [4]. Extra heat for the dryer is generated by hot water at 80°C passing through copper helical tube wherein the tube lies at the bottom of a black pan. A transparent vinyl film is used as cover for the dryer and its base and walls were isolated using fibreglass of 0.0254m thickness. They have found that evaporation efficiency of conventional dryer (22.7% to 24.0%) was higher than the proposed hybrid dryer (9.3% to 14.0%). The moisture removal rate (24.0% humid base or 0.32% dry base) on the other hand, was found to be faster in the case of hybrid dryer (3.0 to 6.8 hrs) than conventional method (8.0 to 8.8 hrs). Infrared drying systems were proposed by Mustafa A et al. (2016) [5] for drying melons. A computational 3D CFD simulation was also used for the investigation purpose. The initial temperature of melons ranged from 50-60 °C with air velocity of 0.5 m/s. The theoretical prediction and experimental analysis yields similar efficiency of the dryer (50.6%). Woods chips of Pinus pinaster were dried on a solar green house dryer by Alberto-Jesús et al.[6] in Autumn season conditions with average daily solar radiation of 13.74 MJ/(m2d) for a period of 15 days. The results were also mathematically modelled to depict the advantages of the solar greenhouse dryer for reaching the desired relative humidity of 10%.
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Bitter gourd drying on a forced convection solar dryer using evacuated tube air collector was performed by Venkatesan N et al. (2014) [7]. The collector temperature was found to vary between 47.4 °C to 91°C. The moisture content of bitter gourd was reduced from 90.93% to 7.61% in 6 hrs, as compared to 10 hrs in open sun drying. Umayal Sundari P et al. (2013) [8] have also performed an analysis on drying bitter gourd on a forced convective solar dryer and found that the moisture content was reduced from 91% to 6.25% in 6 hours. Mango slices were dried on a solar dryer using the standard static gravimetric method [9]. The moisture adsorption isotherms were determined at three different temperatures (20°C, 30 °C and 40°C) for water activity range of 0.111 to 0.813. Madhlopa et al. (2007) [10] have experimented on an indirect type natural convection solar dryer with integrated biomass backup heater using three modes of operation (solar, biomass, solar-biomass) on Ananas comosus and found that the moisture content was reduced from 669 to 11% (db) while yielding a nutritious dried product.
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Forson F K et al. (2007) [11] have modeled a mixed mode natural convection solar crop dryer using a single pass double duct solar air-heater (MNCSCD) for drying cassava and found that nocturnal drying accounts for 23.9% of moisture loss during solar drying. The mean moisture content of the product was reduced from 201.7% to 21.1% (db) in 21hrs. Benon B et al. (2000) [12] have experimented solar drying using a direct type natural convection solar dryer equipped with a simple biomass burner on pineapples, arranged in a single layer of 0.01m thick slices. The drying efficiency of solar component alone account for 22% while other trials suggest the efficiency of biomass burner in production of useful heat in drying to be 27%. The energy performance of a solar ambient hybrid source heat pump drier (SAHSHPD) in drying copra under hot-humid weather conditions was performed by Mohanraj et al. (2014) [13]. The COP of the dryer was found to range from 2.31 and 2.77 with average value of 2.54. The moisture content (w.b.) was reduced from 52% to 9.2%. The specific moisture extraction rate was calculated as 0.79 kg/kWh. A natural convective heat flow solar dryer has been investigated by Gbaha P et al. (2007) [14] in drying perishable foods such as cassava, banana and mango. It has been found that the moisture content has been reduced to 80% in 19hrs. Early uses of solar dryers for drying of cassava chips was found to be economical and promising as it reduces the moisture content to about 16% (w.b.) in 70hrs. It was found that the thickness of the layer influences the nature and pattern of drying [15].
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An indirect type forced convective solar dryer was utilized for dehydration of red chilli in Mexico. The authors in the paper have conducted the drying process at controlled air temperatures of 45 °C, 55 °C and 65 °C using an oven. During the course of study, two different air velocities were used: high velocity (1.4 and 2.6 m/s) and low velocity (0.7 and 1.48 m/s). It was observed that drying kinetics of 55 °C and 65 °C were close to each other, with drying time of 2.75hrs and 3.0hrs respectively. On the contrary, the drying time at 45 °C was observed to be 6.25hrs. A total drying time of 16hrs was found in obtaining final moisture content of 0.057 kg of water/kg dry matter and 0.90 kg of water/kg dry matter with use high velocity air, while 21hrs drying time was observed in using low velocity air, with final moisture content of 0.0611 kg of water/kg dry matter and 0.109 kg of water/kg dry matter 2
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respectively [16]. Drying of banana was successful in an indirect type solar dryer with drying cabinet of 1m x 0.4 m x 1m (width, depth and height). Corrugated V-type absorption plates, insulated drying chamber with a chimney constituted the drying chamber. The authors in the paper observed that the moisture content of the samples were reduced from 356% (dry basis) to 16.3292%, 19.4736%, 21.1592%, 31.1582% and 42.3748% (dry basis) for trays 1,2,3,4 and open drying. Average thermal efficiency was found to be 31.50% (solar collector) and 22.38% (drying chamber) [17]. A forced convective solar dryer with double pass air heater (series connection), semi continuous tunnel dryer, heat exchanger (shell and tube) and a blower, was used to study the drying kinetics of ghost chilli pepper (Capsicum Chinense Jacq.). Six (6) trays are used for drying nine (9) kg of the samples. A simultaneous ope sun drying was conducted to study the effectiveness of the dryer. It was observed that the moisture content of the samples was reduced from 589.6% (db) to a final value of 12% (db) in 123hrs and 193hrs in solar dryer and open sun drying respectively [18].
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A solar assisted solid desiccant dryer was used to study drying characteristics of crushed oil palm fond samples. The moisture content of the samples were reduced from 69% to 29% in 30hrs and 40mins using open sun drying, while using the dryer, drying time was reduced significantly by 64%, 44% and 33% for products in 1st, 2nd and 3rd columns of the dryer respectively. Solar energy contributes 65% of the total energy required for drying. The efficiency of the dryer was observed to be 19% at full load conditions. The desiccant wheel have improved the quality of drying air with latent and sensible heat effectiveness of 67% and 74% respectively [19]. Mixed mode solar dryer with thermal energy storage is used to study the drying characteristics of fresh apricot slices. Phase change materials were used to store latent heat and it was observed that drying rate is equal in throughout the drying chamber. The thermal efficiency of the dryer is 11% [20]. Thermal modeling of a mixed mode greenhouse type solar dryer was performed and various analytical expressions such as temperatures of crop, greenhouse, outlet air and cell have been found through a set of governing equations using MATLAB 2013a numerical solver. The authors observed that variation in the no. of air collector (from 1 to 5) have changed the equivalent thermal energy (from 3.24 to 10.57 kWh/day), thermal efficiency (61.56% to 42.22%) and exergy efficiency (28.96% to 19.11% [21]. Various optimization models of solar thermal systems such as artificial neural network, wavelength neural network, least square support vector machine etc. were used in evaluating the performance of solar air heaters. It was observed that the models have good predictability and they can be used effectively in estimating performance parameters of solar thermal systems with accuracy [22-23]. Use of aluminum cans have been found to increase efficiency of solar energy collection with minimum investment. The authors have reported that aluminum cans placed as zigzag on absorber plate with air mass flow rate of 0.05kg/s have the highest efficiency [24]. This model can be effectively used in a solar dryer to study the drying characteristics of the samples.
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In this present work, the authors have presented experimental evaluation of a proposed solar tunnel dryer for studying the drying kinetics of Vitis vinifera and Momordica charantia at the proposed site of Negamam (10.7426° N latitude, 77.1032° E longitude, 293m altitude above sea level), India. The samples have been chosen for this particular study as there is abundance availability of raw materials in the region (Vitis vinifera & Momordica charanti). Various heat storage materials were taken for this study, such as sand bed, rock bed and aluminum filings, as an attempt to increase the thermal performance of the proposed solar tunnel dryer. An uncertainty analysis was also conducted for the measured parameters to increase the accuracy of the results.
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Nomenclature
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A CFD db hfg I
area of solar dryer (m2) computational fluid dynamics dry basis enthalpy (J/kg K) incident radiation (W/m2)
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k MRR m M RH STD t UV W WTSM Ƞth
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f i in m OSD out SD w wb
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2. Materials and method
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drying constant (h-1) moisture removal rate (gm) mass (kg) moisture content of product in wet basis (%) relative humidity (%) solar tunnel dryer drying time (hrs) ultraviolet weight (kg) without thermal storage materials thermal efficiency (effectiveness)
Subscripts
final initial inlet mass (kg) open sun drying outlet solar drying water wet basis
The experimental analysis of the solar tunnel dryer consists of the following Experimental set-up Instrumentation Performance analysis
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2.1 Experimental set-up and procedure
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The experiments were conducted in the month of March-April 2015, under the meteorological conditions of Negamam, India (10.7426° N latitude and 77.1032° E longitude). Five (5) kilograms of Vitis vinifera and Two (2) kilograms of Momordica charantia were procured from the local market. Fruits of commercial maturity having uniform size were chosen for the experiment in which average dimension of Momordica charantia were found to range from 100-150mm long and 25-30mm diameter. The fruits are picked and separated from the bunch before loading into the dryer. To achieve outmost quality, quality check was performed and fruits with cracks or injured skin were rejected. The Momordica charantia was sliced to 5mm diameter. The selected fruits were pre-treated with water to ensure no unwanted particles were present on the fruit. No chemical treatment was performed on the fruits. From the total five kilograms of Vitis vinifera, 990gm of sample is kept for open sun drying remaining and 3.96kg is equally distributed into four (4) samples for each of the analysis for the respective study period such that for each study, an equal distribution of sample can be made. An initial 50gm is removed for use in convective oven to determine moisture content on wet basis. 4
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Heat storage materials or thermal storage materials are those materials which serve as an important source for storage of excess energy and release of the stored energy during requirement. Sand, rock and aluminum filings are used as sensible heat storage materials in this experiment. The properties of some of the thermal storage materials are given in the following table.1 Table 1. Properties of some of the commonly available thermal storage materials Property
Material
Density (Kg/m3)
Water 958.4
Rocks 2245
Pebbles 1350
MgO 3575
Al2O3 4000
Al Filings 2712
Sand 2600
Specific Heat (kJ/Kg K)
4.22
0.81
0.9
1.06
1.02
0.900
1.26
Thermal Conductivity (W/m K)
0.683
0.13
0.85
10.5
6.3
235
2.3
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2.1.1 Design considerations
.
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Fig.1. Schematic diagram of the STD
Figure.1 shows the schematic diagram of the solar tunnel dryer. The dryer is of 2134mm length and 912mm in width, with a height of 609mm. Six (6) numbers of purlin and seven (7) numbers of hoops are mounted on the wooden floor of the solar dryer. The radius of the tunnel dryer was calculated to be 1689.4mm. The total drying area (floor area) of the tunnel dryer is 1,946,208 mm2. Polyethylene transparent film of 200 micron thickness is used as the UV sheet. The recorded maximum temperature is 60°C with RH of 30%. The design considerations of the dryer are given in table 2.
Table 2. Design considerations and assumptions for the design of the dryer Items
Items condition
Location Orientation Loading capacity Initial moisture content Final moisture content Sunshine hour Drying period required Global solar radiation for 10°7426’ N and 77°1032’E (I)
10°7426’ N and 77°1032’E E -W direction 10 Kg 85-90% (db) 15-10% (db) 10 hrs 18 hrs 490-505 W/m2
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Collector material (cover)
UV polythene 200 micron sheet 305 mm
Height of chimney (H)
The front and back door are fixed on the hoops such that the unfilled gaps of the UV sheet are covered. The air ventilation is done by an exhaust chimney on top of the dryer as shown in fig.2.
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Fig.2. Fabricated model of the STD
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The minimum temperature for drying of foods is recorded to be 30 °C and maximum temperature to be 60 °C. Hence, a temperature range of 45 °C and above is considered to be average for drying of crops such as vegetables, fruits, roots, crop seeds etc. The design of the dryer was made for optimum temperature distribution, with outlet and inlet temperatures at 60 °C and 30 °C respectively. Relative humidity depends on the temperature and pressure of the system of interest, and hence it is one of the important design considerations. Temperature drop and rise in relative humidity depends on the volume of the airflow. If more volume of air moves across the lumber surface, there will be less temperature drop and increase in relative humidity. At low air velocity, there is a risk of wrap, staining and non-uniform drying. Therefore air velocity is a tool for controlling the rate of drying. Readings were taken on a daily basis from 0900 hrs to 2000 hrs. During the course of the experiment, temperatures were recorded from various locations in the dryer chamber on hourly using the temperature sensor. The relative humidity sensor is also kept inside the dryer to record RH on hourly and daily basis. The moisture reduction in each day was calculated on wet basis.
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2.2 Instrumentation & uncertainty analysis
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In the performance investigation of the proposed dryer, the following are the instruments which were employed during the course of the experiment. A data acquisition system (DAQ) from SIMEX (SRD-99, ±0.25% uncertainty) was employed for acquiring the data from the sensors. Table.3 gives the list of instruments utilized for data collection. The humidity/temperature transmitters have a stability of 1% RH per year and the temperature 6
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compensation is ±0.0008% RH/°C. The response time is less than 15 sec. The operating temperature of the pyranometer falls between -40 °C and 80 °C and the typical response time is less than 28 sec. The instruments were calibrated before the readings were taken to evaluate the uncertainty of the measurements. Standard methods were used for validating the results.
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Table 3. Uncertainty, accuracy and range of instruments Instrument
Range
Uncertainty
Accuracy
Humidity/ Temperature Transmitter SIMEX Italy (RIXEN TRH-303W) Pyranometer Delta Ohm, Italy
RH: 0-100% Temperature: 0-100°C 0-2000 W/m2
±1%
±2% RH (at 25 °C), ±0.3 °C
±5%
±0.01%
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Multiple-sample experimental analysis has less accuracy in determining the uncertainty of experiments. The plus or minus notation of determining the uncertainty is based on the researcher conducting the experiment. Hence, this is uncertain because the experimenter is uncertain about the accuracy of the measurements. So, a more precise way of determining the uncertainty is to use the single sample experiment uncertainty analysis [25]. The following equation (1) is used to determine the uncertainty arising in measurement & calculation of temperature, solar radiation, RH and consumption of energy. Total uncertainty arising from the parameters calculated such as, moisture content of samples, rate of drying, moisture removal rate and thermal efficiency of dryer are 3.1, 2.8, 2.6 and 3.9 respectively.
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R R R wr w1 w2 wn x1 x2 xn
2
2
2
(1)
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2.3 Performance analysis
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The performance of the solar tunnel dryer depends on the duration of drying and the quality of the end product, moreover factors such as collector performance and drying temperature. Elaborate testing of the solar tunnel dryer was carried out under full load capacity conditions on the month of March-April 2015, for drying of the samples. Three (3) phases of drying of the samples were carried out in each of the case and the most optimum reading was taken for the study. The following equations (2-6) were used to evaluate the performance parameter for drying the samples.
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2.3.1 Estimation of moisture content of samples.
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The moisture content of the samples are calculated using the following equation (2)
mi m f
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M wb
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They are represented on wet basis and expressed in percentage. 50gm sample was kept in a convective electric oven with a temperature of 105±5 °C. The initial (mi) and final (mf) mass of the samples were recorded using an electronic weighing machine.
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2.3.2 Estimation of mass of evaporated water vapor
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mi
(2)
100
The mass of water vapor evaporated can be calculated by the following equation (3).
Win Wout 100 Wout
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mw
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2.3.3 Drying rate
(3)
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The drying rate of the samples in the dryer can be calculated by using the following equation (4). It is use to signify the duration taken by the system to vaporize and remove water vapour from the samples.
3
k
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2.3.4 Determination of dryer efficiency
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The thermal efficiency of the solar tunnel dryer can be estimated by using the following equation (5). The thermal efficiency of the solar dryer depends primarily on the mass of water vapor evaporated from the samples.
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2.3.5 Percentage of time saving
th
Mw t
mw h fg A I
(4)
100
(5)
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The percentage of time saving for the dryer can be calculated using equation (6). The calculated value provides the savings in time (usually expressed in hours) upon comparison of drying the samples between solar tunnel dryer and open sun drying.
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Percentage of Time saving
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3. Results and Discussion
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An initial load test was conducted on a solar dryer under full load condition without application of any heat storage materials and the following observations were made.
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tOSD tSD 100 tOSD
(6)
The moisture content of Vitis vinifera was reduced from 85% to 10% in 53hrs as compared to 58hrs in open sun drying. The average thermal efficiency was found to be 21.2%. Similarly, moisture content of Momordica charantia was reduced from 88% to 6% in 7hrs as compared to 10hrs in open sun drying. Average thermal efficiency was found to be 9.9%.
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The following is the discussion about the study of performance characteristics for drying the samples in the dryer (with and without thermal storage materials) with comparison to open sun drying.
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3.1 Drying time and relative humidity
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Relative humidity is the amount of water vapour present in the air. It is desired that the RH of the air which is to be used for drying be as low as possible. The variation of RH with respect to drying time of the samples of Vitis vinefera and Momordica charantia were plotted in fig. 3 (a) and (b).
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Fig.3. Variation of RH values with drying time of (a) Vitis vinefera and (b) Momordica charantia.
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There is significant reduction of RH in the solar tunnel dryer than the open sun drying. It was observed that the maximum and minimum RH inside the dryer are 62% and 25%, while that of ambient RH were found to be 70.8% and 28.6% while analyzing Vitis vinefera. Upon analysis of Momordica charantia, the maximum and minimum RH inside the dryer were found to be 50% and 20% while that of ambient were found to have RH of 75% and 60% owing to low solar intensity during the period of study (March-April, 2015). Usage of sand in the dryer reduces the RH of the solar thermal dryer by about 8-11% as compared to without/other thermal storage systems. Therefore the air in the dryer has significant drying potential than that of ambient air.
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3.2 Moisture content removal with respect to time.
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Fig.4. Reduction of moisture content of samples with increase in time (a) Vitis vinefera and (b) Momordica charantia.
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Moisture content removal is the ability of the drying air to evaporate water content from the samples to facilitate dehydration. From fig. 4, it can be clearly seen that, the moisture content (w.b.) of the samples reduces notably in the solar tunnel dryer, as compared to open sun drying. Initial moisture content of Vitis vinefera (a) was found to be 85% and it was reduced to desired moisture content of 10% within 28hrs (sand) as compared to 58hrs in open sun drying. Similarly, reduction of moisture content of Momordica charantia (b) from 88% to 7.82% in 5.3hrs was achieved while using sand, as compared to 10.2hrs in open sun drying. It can be noted that there was increase in moisture content of the samples by 1-2% during night which is due to re-absorption.
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In the proposed solar dryer by Venkatesan N et al. (2014) [7], the moisture from the samples were reduced from 90.93% (w.b.) to 7.61% in 6hrs, while the authors in this paper have achieved in 5.3hrs. Hence the proposed solar tunnel dryer is more efficient in removing moisture from the samples.
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3.3 Drying time and MRR
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Fig.5. Variation of drying time and MRR (a) Vitis vinefera and (b) Momordica charantia
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The moisture removal rate (MRR) is the rate of evaporation of water vapour from the samples with respect to time. Fig. 5 (a) Vitis vinefera and (b) Momordica charantia, shows the variation of MRR with respect to drying time of the samples. The average moisture removal rate of Vitis vinefera were reduced from 50gm to38gm in about 10hrs (WTSM) and good MRR were found in case of aluminum bed, which reduces the MRR to about 26gm in the same time frame. In case of Momordica charantia, the reduction of 50gm to 8gm can be seen in about 8hrs, in which the sand bed coupled solar dryer gives better MRR compared to the rest systems. The constant phase in both the cases is due to absence of heat in the air after sunset.
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3.4 Days and dryer efficiency
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Fig.6. Average thermal efficiency of STD with and without thermal storage materials (a) Vitis vinefera and (b) Momordica charantia
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The average thermal efficiency of solar tunnel dryer while using Vitis vinefera [Fig. 6 (a)] and Momordica charantia [Fig. 6 (b)] as samples were shown in the above figure. The average thermal efficiency during drying of Vitis vinefera was found to be 21.2%, 19.6%, 15.4% and 16.80% in WTSM, sand bed, rock bed and aluminum 10
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conditions. Also, it was found that the thermal efficiency of solar tunnel dryer was 9.9%, 15.46%, 14.75% and 13.7% in WTSM, sand bed, rock bed and aluminum conditions. From the results, it is clear that sand have a higher advantage of using as a thermal storage media in solar tunnel dryer systems.
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3.5 Variation of temperature, time and solar intensity
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Fig.7. Variation of temperature with time (a) Vitis vinefera and (b) Momordica charantia
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The distribution of temperature with respect to time in various solar intensities while loading Vitis vinefera (a) Momordica charantia (b) is shown in the fig. 7. The ambient temperature ranged between 27 °C to 29 °C in open sun drying. In both cases of drying the samples, the temperature inside the dryer increases till 1400hrs and then there is a sudden drop in the temperature. This is because the highest solar intensity was recorded to be 612 W/m2 and 655 W/m2 during peak hours (1330 to 1430 hrs) in both cases of analyzing Vitis vinefera and Momordica charantia (March-April, 2015). As the sun approaches dusk, the solar intensity reduces gradually. The maximum temperature was recorded to be 67 °C and 57 °C in using sand for samples of Vitis vinefera & Momordica charantia respectively.
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4. Conclusions
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A solar tunnel dryer is proposed and constructed by the authors in Negamam, India with average global solar radiation of 490-505 W/m2. Thermal storage materials such as sand, rock bed and aluminum were selected as sensible heat storage media to facilitate accumulation of heat inside the dryer. Locally available variety of Vitis vinefera and Momordica charantia were chosen as raw materials for dehydration in the proposed solar dryer due to its abundance in the region. The samples were divided and a part of the sample is taken for open sun drying. Different test were conducted on various conditions such as- open sun drying, tunnel drying WTSM, tunnel drying with sand, rock bed and aluminum filings. An uncertainty analysis of the measured parameters from the instruments is conducted to determine the uncertainty factor. Drying time, relative humidity, moisture removal rate, dryer thermal efficiency is of main interest for determining the performance of the solar tunnel dryer. The removal of moisture from the samples of Vitis vinefera (85% to 10%) and Momordica charantia (88% to 6%) were observed in 80hrs and 11hrs [open sun drying], 53hrs and 7hrs [STD WTSM], 28hrs and 5.3hrs [STD with sand], 31hrs and 6hrs [STD with rock bed], 29hrs and 7hrs [STD with aluminum filings] respectively. Figure (8) and (9) shows the process of drying and end product of the samples.
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Fig.8. (a) Initial weight of sample; (b) loading of the sample ; (c) final product quality from open sun drying ; (d) final product quality from STD
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Fig.9. (a) Initial weight of Momordica charantia; (b) loading of Momordica charantia on the solar dryer; (c) final quality of product from open sun drying; (d) final quality of product from STD
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It can be summarized that the designed solar tunnel dryer served the purpose of drying quality products of the samples with higher superiority than open sun drying. Amongst the materials studied for the thermal storage material, sand bed was found to provide greater effectiveness in terms of moisture removal and thermal efficiency. Thus the authors concluded that the proposed STD coupled with sand can be used as an important food preservation technique and this can help in increasing the productivity and uplift the economy of the society without compromising the quality of the products.
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Acknowledgements
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The authors acknowledge the support provided by Dr. MCET Pollachi, Tamilnadu and JJCET Tiruchirapalli, Tamilnadu during the course of the project.
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HIGHLIGHTS OF THE MANUSCRIPT TITLED “Convective
1.
Solar Drying of Vitis vinifera & Momordica charantia using Thermal storage materials”
The manuscript was prepared based on original research of using a solar tunnel dryer for drying agricultural products. Locally available materials were used as samples for this purpose.
2.
New findings were proposed herein, in which thermal storage materials were utilized to check the performance characteristics of the solar dryer while drying the samples.
3.
Sand bed, Rock bed and Aluminum filings were used as thermal storage materials where the analysis was made in between the materials and also with solar tunnel dryer (without thermal storage materials) and open sun drying (conventional drying).
4.
There were effective reductions in drying time as well as moisture removal rate (85% to 10% in 27hrs and 88% to 6% in 6hrs) using the solar tunnel dryer (with or without thermal storage materials) as compared with open sun drying.
5.
Amongst the thermal storage material, sand was found to have higher effectiveness in reduction of moisture content of the samples with minimum time with an average thermal efficiency of 19.6% while drying Vitis vinifera.