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Figure 1. Natural convection solar dryer |
| Livestock Research for Rural Development 20 (9) 2008 | Guide for preparation of papers | LRRD News | Citation of this paper |
The solar dryer described in this paper can be used for drying various products in rural area under hygienic conditions. This solar drying system was constructed, consisting of two parts (solar collector and solar drying cabinet). Solar collector with area of 1.2m2 (1.2mx1mx0.2m) has black painted rocks to absorb solar radiation and a cabinet that is divided into five divisions separated by four removable shelves. Each shelf is 0.3m width and 0.5m length and made of nylon wire net framed in wooden border. Three sides of the drying chamber walls are covered by fibreglass sheet and a door in the back. Grapes were dried during the present work. The moisture content of grapes was reduced from 81.7% to 36.7% within five days of drying. The drying air flows through the product by natural circulation.
In this work two modes of operation are discussed. The results were applied to the design of modified large scale solar agricultural dryer. This paper deals with a suitable design of a solar agricultural dryer that can be built in rural area with locally available construction materials and skills.
Keywords: collector, grape, local materials, moisture
Traditional method for fruits and vegetables drying in rural areas in Iran is to spread the products on the ground with exposure to the sun in the open air. Sun-drying method may be efficient and cheap process but has disadvantages such as contamination by dirt, insects and bacteria and loss due to wetting by rain squalls. These are usually accepted as an inherent part of the method of processing. In order to protect the products from above mentioned disadvantages and also to accelerate the time for drying the products, control the final moisture and reduce wastage through bacterial action, different types of solar dryer can be used (Exell 1980, Fohr and Figueredo 1987, Ghazanfari and Sokhansanj 2002, Janjaia et al 2008, Khalil et al 2007, Roa and Macedo 1976, Ting and Shore 1983, Yaldyz and Ertekyn 2001).
Total system cost is a most important consideration in designing a solar dryer for agricultural uses. No matter how well a solar system operates, it will not gain widespread use unless it presents an economically feasible alternative to other available energy sources. This paper will therefore concentrate on a low-cost solar agricultural dryer that can be built in rural area from almost any kind of available building materials and by locally available workmen.
The basic mechanism of material drying is one of heat and moisture transfer between the material and the air. The heat is transferred to the surface of material by conduction and convection from adjacent air at temperature above that of the material being dried. If the air is passed through the material at a relative humidity of less than moisture content in material, the air will absorb moisture from the material while increasing its absolute and relative humidity.
A prototype unit has been designed, built and evaluated and is shown in Figure 1. The collector used consists of a 25mm wooden frame (1.2m long x 1.0m wide x 0.16m high), insulated by polystern foam. Black painted rocks were placed on the insulator to absorb solar radiation and to store thermal energy. In order to reduce the heat loss the collector was covered with fibreglass sheet.
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Figure 1. Natural convection solar dryer |
Air enters through the open bottom end of the collector and is heated while it passes over the rocks. The warm air outlet of the collector is connected to the front side of the drying chamber. The dimension of the chamber is 1m high x 0.5m wide x 0.3m deep. Solar heated air is passed through four removable trays each of size 0.5m length x 0.5m wide and made of nylon wire mesh framed in wooden border on which the drying product is spread. The trays are inserted and removed through the door provided in the back of the cabinet. To increase air circulation rates wind-operated ventilator is placed on the top of the dryer chamber.
Two modes of operation as follows are considered:
i- Direct and indirect solar drying
ii- Indirect solar drying
In first mode of operation, a solar collector can be employed for providing a supply of hot air to the drying unit in which a material is also directly irradiated by solar energy through transparent sheets covering the east, south and west sides of the drying chamber. An important property of materials processed by direct radiation drying is their absoptivity for radiation. Fortunately, most solids have relatively high absorptivities, but they may change as drying proceeds, the surface of the materials becoming less or sometimes more “black” during the process.
In second mode of operation, the sides of drying chamber are insulated in order to prevent the solar radiation and to decrease the heat loss through the sides. In this mode, the quality of product is improved but increases the rate of drying.
The experiments were performed at Solar Energy Laboratory, IROST; Tehran (latitude 35.7oN; longitude 52.3oE altitude 1190m).The experimental investigation was conducted to determine the rate of drying of grape. The collector was mounted on the stand and was oriented N-S tilted 35.7oN towards the south. This has been done since most of the drying is required during August and October months. The global radiation was measured with Kipp and Zonnen CM11 pyranometer and continuously recorded along with the rest of data streams. The field seen by pyranometer contained no significant reflecting surfaces nor was covered by any shadows during the course of experiment. The temperatures were measured with Nickel Chromel/Nickel Aluminium (Cr/Al) thermocouples (type K). Thermocouples were used to measure the ambient temperature, outlet air temperature from collector, air temperature between two successive racks and one thermocouple was used to measure inside temperature of grape.
The drying of grapes in modes (i) and (ii) was studied. For purpose of comparing rate of drying and quality of product with sun drying, the grape was also dried, simultaneously under open sun.
Initial moisture in the grape was 81.7% and final moisture was measured from the same type of sultana available in the market. The moisture content in the dried materials was determined using the weight loss method with the aid of an air oven and sensitive balance . The specimen was weighed and then maintained for 72 h at constant temperature in the oven. The specimen was weighted again and the moisture content is determined from the relation:
r = 100 (Mw - Md ) / Mw (1)
Where:
r is the percentage moisture content,
Mw and Md are the mass of wet and dry matter in the sample.
The percentage moisture was 36.7%
The experimental results for one day (13th Oct.) of condition in solar dryer (for mode i) are represented graphically in (Figure 2).
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It shows the meteorological conditions (Ta and I ), outlet air temperature from collector (T1 ), temperatures in spaces II, III, IV and V and temperature inside the grape ( Tgrape). Ta, Tgrape, T1 , T2 , T3 , T4 and T5 are shown on the left scale of this figure, and solar insolation data I is shown on the right scale. The maximum collector outlet temperature on that day was 62 oC . Maximum dryer temperatures at point II, III, IV and V were 52, 47, 43.5, 39 oC respectively. The maximum inside temperature of grape was 55 oC. Thermal storage in the grapes themselves was noted, solar energy absorbed by the grapes being subsequently utilized for water vaporization during the shaded midday period as well as sundown (Lof 1962). The position of thermocouples (with Roman letter) is shown in (Figure 3).
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The outlet air velocity from solar collector can be calculated as follows (Exell 1980) :
v = velocity m/s
h = height of collector =
l = collector length m
ø = collector tilt angle
T1 = collector outlet temperature oC
Ta = ambient temperature oC
For T1 Ta
The drying curves for both solar and sun dried grape are shown in Figure 4.
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Figure 4. Moisture content changes in solar dryer (mode i) and sun-dried |
The percentage of moisture content is obtained from the following relations [5]:
Md = Mwi(1 - ri/100) (3)
Md = Mwf(1-rf/100) (4)
from relations (2-3) the final moisture content is obtained as:
where
Md mass of dry product, kg
Mwi initial mass of wet product, kg
Mwf final mass of wet product, kg
Mw weight of water to be evaporated, kg
ri initial moisture content
rf final moisture content
The moisture removal is more in the first rack than other racks and this is due to direct contact of hot dry air from the collector but in the upper rack (Figure 4), the moist hot air is passing through them and hence the moisture removal becomes slower. The drying time for grape using solar dryer was between 3 to 4 days, while for sun-dried it was 6 days.
The drying time can be predicted from knowledge of drying processes, the dryer characteristics and local meteorological data, the drying time in days, δ, is given by:
δ = Mw.λ / ( G.A.ηc.ηd ) (6)
where
δ duration time, day
λ latent heat of vaporization, 2.2 MJ/kg
G daily radiation, 15 MJ/m2-day
ηc collector efficiency assumed 10%
ηd dryer efficiency assumed 50%
Solar collector efficiency- defined as equal to the ratio (mass flow rate of air x specific heat of air x temperature rise across collector / (total incident radiation intensity)
The calculated duration time from equation (6) δ= 2.45 days is compared with experimental results which is 3 days for first tray. It can be seen that there is a good of similarity between measured and calculated duration time.
Drying characteristics of product for mode (ii) are shown in Figure 5.
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In this mode of operation, the air is heated only in the collector and it passes through the racks. In mode (ii) duration of drying time is longer than in mode (i) but the quality is better than mode (i) especially the colour is green while in mode (i) the colour is red.
The solar dryer was modified from results obtained in mode (i) and (ii). In mode (i) the faster drying rate and almost uniformity of moisture content in the product are the advantages but there is poor quality of product processed by direct radiation drying which causes the surfaces of grapes to become less or sometimes more “black” during the process. In mode (ii) the quality of product is better but drying rate is slower. In modified solar dryer, maximum drying rates are usually desired. Product quality must be considered, however, and excessive temperatures must be avoided in many materials. In addition, because drying occurs at the surface, those materials which have tendency to form hard, dry surfaces relatively impervious to liquid and vapour must be dried at a rate sufficiently low to avoid this crust formation. The modified model of solar dryer made of sun-dried mud bricks is shown (Figures 6-7).
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The south-facing window is tilted and behind the glass window the louver is painted matt black and tilted 35o for maximum absorbing solar radiation. The solar energy absorbed by black painted metal louver raises the inside air temperature and prevents direct entry of solar energy into the dryer. The size of the system depends on the amount of product since the side of east-west can be varied according to prerequisite. A number of chimneys can be mounted on the roof. Air velocity through the dryer can be increased by increasing the chimney height. The new design is required to be built and tested and the results compared with the present one.
These preliminary studies have shown the feasibility of using low cost solar agricultural dryer. This type of solar dryer offers the advantages of faster drying rate, ease of construction, no maintenance and, also, protection of the crop from weather, thus minimizing damage from inclement rains.
For required levels of temperature in the dryer, the present study also shows that even an inexpensive collector can be quite efficient.
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Received 23 April 2008; Accepted 7 June 2008; Published 4 September 2008
