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Energy Procedía 109 (2017) 409 - 416
International Conference on Recent Advancement in Air Conditioning and Refrigeration, RAAR 2016, 10-12 November 2016, Bhubaneswar, India
Design, Development and Performance of Indirect Type Solar Dryer
for Banana Drying
Abhay Lingayat*, Chandramohan V.P., V.R.K. Raju
Department of Mechanical Engineering, National Institute of Technology Warangal, Warangal, Telangana - 506004, India
Abstract
Due to higher prices and shortages of fossil fuels and to reduce the fuel consumption used in the drying process, more importance is given to solar energy sources as it is freely available. For these purposes, an indirect type solar dryer was designed and developed to dry agricultural products. Solar dryer consists of solar flat plate air collector with V-corrugated absorption plates, insulated drying chamber, and chimney for exhaust air. The total area of the collectors is 2 m2. The size of the drying cabinet is 1 m x 0.4 m x 1 m (width, depth, and height). An experiment was conducted to study drying characteristics of banana. The qualitative analysis for drying of banana showed that moisture content of banana was reduced from initial value of 356% (db) to final moisture content of 16.3292%, 19.4736%, 21.1592%, 31.1582%, and 42.3748% (db) for Tray1, Tray2, Tray3, Tray4, and open sun drying respectively. The average thermal efficiency of the collector was found to be 31.50% and that of drying chamber was 22.38%. The temperature of drying air is the most important and effective factor during drying. The humidity of air as well as air velocity is also an important factor for improving the drying rate.
©2017 The Authors.Publishedby ElsevierLtd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of RAAR 2016. Keywords: Solar drying; Indirect type solar dryer; Moisture content; Banana.
* Corresponding author. Tel.: +919440105045; E-mail address: abhay.new12@gmail.com
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of RAAR 2016. doi:10.1016/j.egypro.2017.03.041
1. Introduction
The energy, for drying, supplied from various sources (fossil fuel, natural gas, solar etc.). Rapid depletion of natural fuel resources and rising fossil fuel cost, environmental damages caused due to fossil fuel, the use of solar energy for drying is expected to become popular source [1]. People, from rural India, are using open sun drying method to dry their agricultural products, but it has a number of disadvantages such as contamination of dust, pollution, damage by birds, animals, insect, etc. Indirect type solar dryer is one of the options to overcome the above issues [2].
The energy from the sunlight can be utilized for drying of food products. Solar drying of fruits and vegetable is an ancient food preservation technology. Drying is very important and essential process for preservation of agricultural products. Also, it is important for other industries, such as textile, cement, tea industry, tiles, wood processing, paper industry, etc. [3-5]. Although the solar radiation is used for drying of food materials, it has not yet been widely commercialized because of high investment cost, time-consuming operation etc.
Solar drying has a number of advantages as solar energy is non-polluting, free, abundant renewable energy source. But several practical difficulties arise and it should be overcome. The intensity of incident radiation is not constant throughout the day, therefore, heat storage is needed to store the solar energy at its peak value. The auxiliary energy source is required after sunset and at the time of bad weather. Also, solar radiation has a very low energy density, which requires the large surface area to collect solar radiation (collectors). Because of these things, investment costs are notably larger [6, 7].
An alternative solution for traditional drying method and to overcome the problem of open sun drying, indirect type solar dryer is used. The main reasons are as follows,
• Indirect type solar drying maintains good product quality compared to open sun drying.
• Time for drying process can be significantly reduced as compared to open sun drying.
• Dried foods can be preserved for a long time period and the product becomes extremely lightweight hence easy for
transportation.
Therefore the main objectives of this present experimental work are, (i) to design and develop an experimental setup for indirect type solar dryer, (ii) to conduct the drying experiments with the sample product of banana, (iii) to find the initial moisture content of banana using hot air oven, (iv) to estimate the transient moisture content distribution of banana placed at different trays of drying chamber (v) to estimate the collector efficiency and dryer efficiency and (vi) to develop drying correlations.
Nomenclature
Ac area of collector (m2) Tc temperature of collector outlet air (°C)
Cpa specific heat of air (J kg -1 k-1) Td temperature above tray (°C)
hi latent heat of vaporization of water (kJ/kg) Tatm atmospheric air temperature (°C)
I solar radiation (W/m2) t time (h)
Lc characteristic length (m) t* Dimensionless time
Wwet initial mass Wdry final mass
ma air flow rate (kg/s) Greek
mw amount of water (kg) nc efficiency of collector (%)
M moisture content (%) nd efficiency of dryer (%)
Mi moisture content (%) Subscripts
Me equilibrium moisture content (%) db dry basis
Ti, Tin temperature of collector inlet air (°C) wb wet basis
2. Methodology
2.1. Climate data collection
A natural indirect type convection solar dryer was fabricated and installed at NIT Warangal, Telangana (India). Longitude 79.58o E; Latitude: 18.0o N. Warangal has a moderate climate with hot summer and cold winter. Solar radiation over the year on horizontal surfaces in Warangal is found to be 833.333 W/m2 and it is maximum (1250 W/m2) during summer and average solar radiation found was 960 W/m2 during Feb to June 2016. Total solar radiation on a 23.5° tilt surface toward the south is observed as 676.367 W/m2 [8].
2.2. Design and construction
Dryer consists of solar flat plate air heater, insulated drying chamber provided with chimney for exhaust air. Fig. 1(a) shows the schematic view of the experimental setup. Gross dimension of the solar collector is 2 m x 1 m x 0.10 m. The solar air heater consists of V-corrugated absorption plate painted with selective black color, glass cover, insulation at bottom and frame. V-shape corrugated (42 in number) absorber of 2 m2 area was made from 0.4 mm thick copper sheet. Rectangular box for collector was made from galvanized iron frame of 5 mm thick. The air was allowed to pass through the gap between absorber and glass in the direction perpendicular to the V shape corrugation. Copper sheet was supported by an aluminium sheet at bottom of the absorber. Rock wool was provided between aluminium sheet and bottom of the collector for insulation. A simple window glass of 4 mm thick [9] was fixed on rectangular box frame at a distance of 0.04 m above the absorber as shown in Fig. 1(b). The glass was fitted to the rectangular frame with sponge rubber with the help of screws. The collector was tilted by the angle of 23.50 with the horizontal. Drying chamber of 1 m x 0.4 m x 1 m (width, depth, and height) was developed from the aluminium sheet (0.5 mm thick). Four aluminium trays (on which the product was placed) were stacked evenly, at distances of 0.011 m apart. The frame of each tray (0.9 mx0.4 m) was made from an aluminium angle of 0.04 mm thick. The tray was made from an aluminium wire mesh and fixed to the frame inside the drying chamber. Outlet air from collector enters into the drying chamber at the bottom. Then it flows in the upward direction through the drying material. The chamber was insulated from all sides except the top. The chamber was proved with chimney for exhaust air. The height of the chimney was 0.25m.
3. Instrumentation and experimentation
3.1. Instrument used for experiment
Temperatures at different locations are measured using RTD Pt-100 sensor (+/-10C) and connected to 16 channel data logger system (PPI Make) with the accuracy of ± 0.25%. Temperature readings were recorded on an hourly basis starting from 8:00 AM - 6:00 PM. RTD were fixed at inlet and outlet of the collector (Tin, Tout), in open air for measuring ambient temperature (Tamb) and just after each tray in the drying chamber (Td1, Td2, Td3, Td4) for dry bulb temperature measurement as shown Fig. 1(a). Instantaneous global components of solar radiation were measured using a solar power meter (Tenmar TM 207) with an accuracy of ± 10W/m2. Relative humidity was measured by hot wire anemometer (2% accuracy) (Tenmar, Model: TM 4002) and humidity transmitter (Model: RH-33, Make: PPI). For estimating the initial moisture content of the banana sample, a hot air oven was used (Make: PPI, MEMMERT Type). Weight loss of sample during the experiment was measured using an electronic weighing balance (OHAUS PA 214, linearity ±0.2 mg) of 200 g capacity.
3.2. Experimentation
Experiments were conducted to study the drying characteristics of banana. In India, production of banana is high and also has a substantial loss after harvesting. Bananas are having good nutritive values. During the experiment, the weather was generally sunny. The experiments were conducted from March to June 2016 in NIT Warangal, India. The reduction in moisture content was determined by weighing the sample at every hour. Experiments were performed
with Banana. Fresh ripe banana slices were used in the drying. For effective drying, 4 - 5 mm thickness of banana slices was selected [10, 11, 12]. The cylindrical slices of Banana (4mm thick and 30mm diameter) were prepared first by removing the outer skin. The slices of banana were spread uniformly on four trays. 500 g of banana slices were used for drying in each tray. Then four trays were placed inside the drying chamber. The door of the dryer was closed properly. While performing the experiment, the solar dryer was tested by measuring temperature, solar radiation, and humidity for 1-hour interval of time.
Fig. 1 (a) Schematic view of experimental setup; (b) Solar flat plate collector
4. Result and Discussion
4.1. Initial Moisture Content
The initial mass of fresh banana (Wwet) and final mass of dried banana (Wdry) was measured with the help of weighing balance. Initial moisture content was calculated by the following equation,
dry basis, Mwb = Wwe' Wdry (1)
wet basis, Mwh = Wwet Wdry (2)
wb wwet v '
Banana slices were dried in hot air oven after maintaining the oven temperature 105 °C. The slices were kept up to 24 hours in hot air oven as per ASTM (American society for testing and methods) standards [11, 13].
Table. 1 Initial and final moisture content of banana
Sr. no Initial mass in Size Temperature Total time Final mass Initial MC Initial MC
(gm) °C (hr) (gm) (wb) (db)
1 4.3113 0.9555 0.7784 3.512
2 4.9711 Dia.3cm, 1.1093 0.7768 3.491
3 3.6638 4mm 105 24 0.7877 0.7851 3.651
4 3.431 0.7520 0.7808 3.562
5 3.8448 0.8422 0.7809 3.565
Avg. 4.0444 0.88934 0.7799 3.556
From Table. 1 it is noticed that the ripe banana has 78% of water (approximately). The average initial moisture content was estimated as 3.556 kg/kg of db.
4.2. Solar radiation, Air Temperature and relative Humidity
During the experiment, variation in solar irradiance, ambient air temperature (Tamb), the temperature of air at the outlet of collector (Tout) and at the outlet of drying chamber (Tdout) were observed and plotted as shown in Fig.2. Maximum solar radiation of 1219 W/m2 was noticed at 12.40 pm. Average solar radiation was obtained as 897.04 W/m2. For no load conditions, the maximum temperature of air at the outlet of the collector and the chamber was recorded as 81°C and 78 °C respectively. During the experiment, the daily mean values of air temperature at the dryer inlet vary from 38°C to 81°C and global solar radiations vary from 192 to 1220 W/m2. Almost similar variations were noticed during the full set of experiments from March to June 2016 in NIT Warangal, India.
o -I-t-1-1-1-1-1-1- 0
6 8 10 12 14 16 18 20
Drying time
Fig. 2 Variation of the solar irradiance, collector outlet air temperature, ambient air temperature, dryer outlet air temperature for the natural
convection solar dryer for no load conditions.
The variation in temperature inside the drying chamber after loading is shown in Fig. 3. From the Fig. 3 the temperature at the bottom Tray1 is higher and reduces when air goes through different trays in an upward direction. Chamber temperature is minimum for the upper Tray4. The products in Tray1 absorb heat energy from heated air and then the heated air flows to Tray2 and subsequently Tray3 and 4. Therefore, the chamber temperature is reduced from Tray1 to 4. Average temperature for collector outlet (Tout), Tray1 (Td1), Tray2 (Td2), Tray3 (Td3), Tray4 (Td4) and atmospheric (Tatm) are observed as 61.2 °C, 55°C, 52°C, 47°C, 44°C, 35°C respectively.
20 -I-1-1-1-1-1-1-1
6 8 10 12 14 16 18 20
Drying time
Fig.3 Diurnal variation of the collector outlet air temperature, Temperature above each tray and atmospheric air temperature for the natural
convection solar dryer for sample load conditions.
From Fig. 4 it is observed that the moisture content of banana is varied with tray location. Drying rate is higher for the bottom tray as sample comes directly in contact with high-temperature air. In the case of open sun drying, initially, the rate of drying is higher because of the air flow over the product from all the direction. When the temperature inside the dryer increases, then the drying rate is higher compared to the open sun drying. The drying rate gradually reduces in the later period. There was 2 drying period found from the results called 1st falling rate period and 2nd falling rate period. For the 1st falling period, drying rate is very fast due to the large difference in the moisture content of banana and dry air. In the 2nd falling period, the rate of drying is slow as moisture gradient of banana and outside air is reduced. The moisture content of banana was reduced from the average value of 356% (db) to final moisture content of 16.3292%, 19.4736%, 21.1592%, 31.1582%, and 42.3748% for Trayl, Tray2, Tray3, Tray4, and Tray atmospheric i.e. open sun drying respectively.
Fig.4 Drying time vs moisture content
Fig.5 Drying time vs relative humidity
From Fig. 5 it is observed that relative humidity is very low in the dryer which is important for higher drying rate as low humid air have more ability to absorb more moisture. The average relative humidity of the atmospheric air and of air inside the chamber is found to be 25.33% and 15.18%.
4.3. Collector Efficiency
The efficiency of flat plate collector is influenced by many factors such as the size of collector, geographical location, velocity, humidity, the temperature of the surrounding air etc. The thermal efficiency for solar collector can be determined by Eq. (3).
mqCpaiTc-Tj)
Thermal efficiency for solar collector was found to be 31.50% for the average values of solar radiation of 724 W/m2 and an average temperature of 42 °C and 62 °C for inlet and outlet air respectively.
4.4. Dryer efficiency
The thermal efficiency, of solar dryer system, was calculated by Eq. (4). mwhi
The initial mass of banana sample was 2 kg. After solar drying, the mass of dried banana was 0.5628 kg. So total
1.4372 kg of water was removed from the sample during the total drying time of 10 hours. The thermal efficiency of 22.38 % was found in the drying chamber.
4.5. Drying models or correlations from experiments
The experimental data of banana drying were used to develop the drying model or drying correlations. The moisture ratio (MR) can be determined by Eq. (5).
MR = (M- Me)/(Mj - Me) (5)
Where, Mi is initial and Me is equilibrium moisture content of the sample. During the drying process, continuous fluctuation in temperature, the relative humidity, and velocity of the drying air was observed (The humidity variation is explained in Fig. 5) therefore, a simplified form of moisture ratio, MR = M / Mi [14] is selected for the dimensionless parameter. Also non-dimensional parameter for time can be taken as,
t* = ats/Lc2 (6)
Where, a is thermal diffusivity of banana [15], ts time in second and Lc is characteristic length of cylindrical shape of banana slice (Lc = (6x Volume)/Surface area). An average value of moisture content data was taken for curve fitting. A regression analysis is carried out using the tool DATAFIT 9.0 to make the drying models. The experimental data was correlated and the drying correlations were developed in terms of moisture and time in both format of dimensional and non-dimensional terms respectively as,
MR = 1 + at + bt2 (7a)
and MR* = 1 + at* + bt*2 (7b)
Values of constant and correlationt coefficient are shown in Table 2. In the dimensional correlation (Eq. 7a), MR is moisture ratio and t is time in hour. And the developed drying correlations were compared with existing drying model in literature [16] and reasonable matching was noticed. Fig. 6 and 7 mentioned the average moisture content data with curve fitting equations in both dimensional and non-dimensional formats.
Table 2. Constants for drying models
Constant a b Correlation coefficient, R
Dimensional correlation - 0.246631 0.015541 0.9915
Non-dimensional correlation - 0.040981 0.000431 0.9915
Fig. 6 Time vs Moisture ratio (MR)
Fig. 7 Non-dimensional time vs Moisture ratio (MR)
5. Conclusion
An indirect type solar dryer was developed in NIT Warangal to dry the banana slices. Experiments were performed and found that indirect type of solar drying is more effective than open sun drying as it reduced the drying time. It is observed that the dried product is free from dust, environmental pollution. Mass of banana is reduced from 2 to 0.5628 kg. The moisture content of banana was reduced from average initial value of 356% (db) to final value of 16.3292%, 19.4736%, 21.1592%, 31.1582%, and 42.3748% (db) for Tray1, Tray2, Tray3, Tray4, and open sun drying respectively. The average thermal efficiency of the collector and drying efficiency were 31.50% and 22.38% respectively. Drying correlations were developed and the constants were tabulated. Solar dryer reduced the drying time with a quality product, so it is more efficient than open sun drying.
Acknowledgements
We would like to thank the Center of Excellence (CoE) under TEQIP-II, National Institute of Technology Warangal for funding this project.
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