quality of selected products such as biltong, apples, bananas and chilies under various solar drying conditions. Black-grey solid stone-slabs were used as the ...
A NOVEL SOLAR DRYER SYSTEM FOR DRYING AGRICULTURAL AND MARINE PRODUCTS F. L. Inambao University of KwaZulu-Natal, Howard College Campus, Durban 4000, South Africa
ABSTRACT Solar drying systems have been constructed and used for the purpose of drying agricultural and marine products. Dried products are to be stored before use, otherwise insects and fungi, which thrive in moist conditions, render them unusable. Such solar drying systems will consist of regenerative solar air heater and regenerative solar dryers and controls though heat storage tanks and heat exchangers may be incorporated. This paper discusses design and application of a solar dryer as well as effects of sunlight on changes in the quality of selected products such as biltong, apples, bananas and chilies under various solar drying conditions. Black-grey solid stone-slabs were used as the heat storage materials, assisted by copper finned absorbers. Experimental data shows that the solar collector efficiency of the solar dryer exceeds 70% on a clear day. 1.
INTRODUCTION
Sunshine has been used since time immemorial for the purpose of drying agricultural and marine products. This has traditionally been accomplished by either sunlight or hot gas. By simply exposing these products to the sun’s ray and hoping that no rain or snow would fall until the drying process had been completed depends on weather conditions and a problem may arise with the length of time drying is possible. On the other hand, hot gas drying is expensive and may also cause a problem with taste. In recent years, a more sophisticated technology has evolved to make more effective use of the sun’s thermal power and minimize the usual contamination associated with natural dehydration. The most common sources of contaminants are [1,2]: • • • •
Fungi, which thrive in moist conditions Insect infestation and presence of larvae, etc.; Animal and human interference and Airborne dust and wind-blown debris.
To minimize contamination and maximize the effectiveness of the sun’s rays, it is essential that the drying area be covered by a transparent material. Normally, glass is preferred because of its resistance to deterioration, while plastic films such as Mylar have merit
because of their low cost and freedom from breakage due to stones, hail, etc. An excellent description of a solar drying unit was designed and can be made by anyone who has access to simple carpentry tools. This dryer is essentially a hot box, which can be used to dehydrate fruits, vegetables, fish, or any other product which needs to be dehydrated. A domed fruit drier was developed in the USA [3] and box-like fish drier developed in Angola [2]. In both these heliothermal devices designs, heating and drying are combined, i.e. both processes are accomplished in one chamber. In the novel heliothermal system developed, heating and drying are separated. The system incorporates features such as a double glazed glass, control system, and copper fins to enhance higher storage or heating temperatures, which makes it novel and unique. The report focuses on two aspects: an overview of the solar drier that collects solar energy efficiently, stores thermal energy and separately dries the products and evaluation of the properties of the products during the drying process. The system could be applied for drying ranging from marine products such as fish, squid, mushrooms, fruits, cereals, etc for preservation during storage. However, in this report, product selection was limited to biltong, chillies, bananas, and apples. 2.
SYSTEM DESCRIPTION AND METHODS
The developed Solar dryer consists of a solar heat storage “hot box” , with two layers of transparent covering of a double strength glass and a drying chamber. The box is made of plywood and is made to stand on four support legs. The casing, which holds the collector and drying chamber together and in combination with the glazing, makes it water and dust-proof. In order to resist deterioration, was adequately painted black inside. The dryer was glazed , with two layers, fitted with adequate room for thermal expansion. Having double glazing will somewhat improve the direct-gain system performance by reducing or eliminating the heat lost through the collector. The insulation on all sides, other than on top of the solar collector, consists of glass wool which is locally available and inexpensive and is not subjected to infestation by ants and termites. The glass wool is used as an insulating material since it can survive any temperature and moisture
conditions which the hot box will attain and will not support insect life. The angle of slope of the dryer cover normally varies with the local altitude and an angle of 30o was found to be satisfactory for the experiments. The drying trays are made of galvanized wire mesh. Ventilation is essential so that the moisture which is ‘distilled’ out of the products can be removed from the cabinet. This means that screened air holes in the bottom, sides, and back of the cabinet are essential. The ventilation holes at the bottom and top of the dryer allow air to enter and carry away the moisture from the products by the heat of the sun. In our system, cold air enters the bottom, is warmed and heated while in contact with the solar heater interior, and rises, out at the top. The access door in the rear panel enables the products to be placed on the drying trays, and to be removed from these trays when they are dry. The interior of the cabinet is painted black, while the exteriors offside and rear panels are covered with the reflecting insulation. The number of ventilation holes is determined by the amount of moisture which must be removed from the products which are being dried. A small fan was used situated at the back of the drying cabinet/chamber, to draw air through the dryer. The control device used in the investigation was the differential thermostat which measures the difference of temperature between two sensors and activates the relay. When the temperature difference exceeds some specific preset value, there is usually an on/off differential, so that once the differential thermostat activates the relay, the relay is not deactivated until the temperature difference drops below a preset value, known as the shut off temperature differential. Proportional differential thermostats are available to vary fan speed as a function of the temperature difference between the solar collector and the temperature required to dry the product. If this temperature is small, the fan operates at a low speed and conserves electric power. As the temperature differential, rises, the speed of the fan automatically increases. The fan and the controller used were powered by a 12-24V DC power supply. Type T thermocouples were installed in several and different places inside the solar heater and the drying chamber and each temperature was measured once an hour to calculate the collector efficiency of the solar air heater. The internal Ti and external temperature Text difference of the wall was measured and the amount of heat loss calculated by the equation below to measure heat loss from each surface.
The heat storage of the stone slabs not mentioned in the description earlier was obtained from the heat capacity and its temperature rise of the stone slabs and the thermal efficiency of the collector was calculated from the solar radiation and heat collection determined by the temperature rise of the air flow: η = w Cp ( Tout – Tin ) / I A
(2 )
Most agricultural crops and marine products which are intended to be stored before use have to be dried first. The drying involves transfer of water from the product to the air around it, therefore it is important to determine how much water the air can accept as water vapour. The moisture content on a ‘wet basis’ was determined by using: W = (m – mo )/m (3) Where m is the total mass of the samples ‘as is’, mo is the mass of the dry matter in the sample. The determination of both m and mo requires care and therefore were measured in the laboratory according to the standard procedures for each crop or product The temperature and time for drying to determine ‘oven dry mass’ is limited so that other chemical changes do not occur. Some chemically bound water may remain after this process. It is important to realize that there are limiting temperatures for drying for storage, so that product does not crack and allow bacterial attack. The amount of energy for product to be dried was calculated using: Q = mcp∆T
(4)
Where m mass of product Cp heat capacity, and ∆T temperature difference 3.
RESULTS AND DISCUSSION
3.1
TEMPERATURE PROFILES
Continuous measurements and monitoring of the temperature profiles over a period of one month was carried out, to provide the opportunity to quantify its potential impact on the system. Acquisition of reliable and valuable data conformed to specifications for drying some selected agricultural products shown in Table 1.
(1)
The variation of temperature with time on a bright clear day from the solar air heater, i.e. slab (stone), after tray 1 and drying entry to the drying chamber are shown in figs. 2 to 6.
A heat flow sensor was installed on the wall to measure heat loss and the measured value coincided in heat loss calculated in the equation above within approximately 8%.
The solar radiation flowing in from the glass in the morning is acquired as a hot gas by means of storing its thermal energy by slabs. Around noon, when the ambient temperature is maximum, temperatures of slabs were
Q = KA ( Ti – Text )
The outside of the dryer is insulated with fibre glass fan
casing Galvanized wire mesh trays Finned copper h Double glazed water proof glass flat plate Interior surfaces painted
Drying chamber
Solar collector
(Access door open) Blackgray solid stone
legs
Air inlet
30
Ventilation holes
FIG.1
o
SECTION THROUGH THE SOLAR DRYER
Table.1 Specifications for drying agricultural produc Products
Humidity ( % ) Initial
Humidity difference
Drying temperature
Final
Corn
25
13
12
68 - 80
Onions
80
4
76
55
Potatoes
75
13
62
70
Legumes
80
10
70
-
Peas
80
5
75
65
Bananas
80
15
65
70
Coffee
51
11
40
-
Cotton
-
9
unknown
-
Peanuts
40
9
31
-
Chillies
80
10
70
65
Apples
75
15
60
70
Biltong
80
10
70
75
Temperture
100 80
Ambient
60
Stone
40
Temperature
80oC, figs. 2 and 3; 70oC, fig. 4; 75oC, fig. 5 and 85oC, fig. 6. Heat storage decreased over time. This was due to the heat balance with heat radiation from the air to outside because even if the temperature of the slabs rises, heat storage did not increase much.
Ambient Stone After Tray1 Drying entry
9:00 10:00 11:00 12:00 13:00 14:00 Tim e Fig. 5.Tem perture Vs tim e for 20/10/2005
1st tray
20
80 70 60 50 40 30 20 10 0
0 12 Tim e
13
14
Fig. 2. Tem perature Vs Tim e
Temperature
When the amount of solar radiation and outside air temperature fall in the afternoon, heat storage became negative, i.e., the slabs, which stored thermal energy of the solar radiation in the morning, begins to radiate it slowly. 90 80 70 60 50 40 30 20 10 0
Ambient Stone After Tray1 Drying
9:00
11:00 13:00 Tim e
Heat loss from the apparatus greatly decreased in the daytime to restrain the air temperature rise in the apparatus and to restrain heat radiation from the glass because a hot gas was always collected. One of the most important aspects of the solar drying system is to minimize heat loss to ensure maximum heat generated in the collector casing which will be forced by convection to the drying chamber. In our design, after tray 1, is the top most tray which experienced the lowest temperature.
Temperature
0.375 0.438
15:00
The phenomenon that the amount of heat collection as a hot gas grew bigger than the inflow amount of solar radiation was appeared with the influence of the buildup of the amount of heat radiation after 14:30 hours in the afternoon.
Temperature
80
40 20
Ambient Stone After Tray 1 Drying entry
9:30
Fig. 3 Tem perature Vs Tim e for 18/10/2005
60
90 80 70 60 50 40 30 20 10 0
Ambient Stone
11:30 13:30 Tim e
Fig. 6.Tem perature Vs Tim e for 22/10/2005
3.2
CHILLIES
Fig. 7 shows the view of chillies in the drying chamber and fig. 8, the dependences of their weight on time, corresponding to various values of temperatures for three consecutive days. The original moisture content or any other moisture content at any given temperature was determined by using a programmable IR 200nMoisture Analyzer, a product of Denver Instrument Company.
After Tray1 Drying entry
0 9:30 10:30 11:30 12:30 13:30 Tim e FIG. 4. Tem perature Vs Tim e for 19/10/2005
It is important to note that the maximum temperature rise depends on weather conditions, and the time at which the peak temperature occurs.
15:30
Fig. 7 Chillies in the drying tray.
Weight (grams)
The initial weight of chillies on day 1, prior to commencement of experiments, was 76 grams while the final weight on day 3 was 30.2 grams. This corresponded to initial moisture content of 65% and final moisture content of 12% respectively. The total amount of energy for the product to be dried using equation (4) was found to be 116. 51 kJ. 80 70 60 50 40 30 20 10 0
contents of 76% and 34.6%, the total energy needed for the product to be dried was found to be 285.19kJ. 3.4
APPLES
The photographs of sliced apples are shown in fig. 11. while fig. 12 depicts the investigation results, i.e. weight versus time. At the initial and final weights of 205 grams and 55 grams, corresponding moisture contents were 85% and 44%, respectively while the total energy required was 383.52kJ.
day 1 day 2 day 3 0.375
11
13
15
Tim e Fig. 8. Weight Vs Tim e for chillies
BANANAS
Fig. 11 Washed and sliced apples during drying
Peeled and sliced in thin rounds bananas are shown in fig. 9. Fig. 10 shows a non-linear variation of weights of bananas with time over a period of two days.
Weight (grams)
3.3
250 200
Weight
150 100 50 0 0.375
10
11 12 Tim e
13
14
Fig 12 .Weight Vs Tim e for 20/10/2005
At relatively higher temperature differences of the ambient and the slabs, i.e. 10:00 and 13:00 hours, the dehydration was relatively higher, thus indicating a sharp drop in the weight of the product
Fig. 9 View of peeled and sliced bananas 200 Weight (grams)
day 1 150
day 2
3.5
BILTONG
Fig. 13 shows the view of marinated biltong during the drying process and fig. 12 presents some of the results from the experiments i.e., the variation in the products weight as a function of time. Prior to testing, total weight
100 50 0 9:30
10:30 11:30 12:30 13:30 14:30 Tim e
Fig.10 Weight Vs Tim e for bananas
The initial weight was 174.4 grams on day one and final weight of 64 grams on the second day. It was observed that a high level of dehydration was achieved on the sunniest day which corresponds to maximum temperature differences of the ambient and slabs or to be more specific, around noon. With the initial and final moisture
Fig. 14 Biltong in the drying chamber
of biltong was 360 grams and after drying operation -110 grams. Moisture contents were 76% and 21.5%, respectively with calculated total energy required of 637.6 kJ.
Weight (grams)
400
REFERENCES [1] Twidell, J.W. and Weir, A.D.: Renewable Energy Resources, E & F N Spon Publishers, , London, 2000. [2]
Garcia, A.A.,Boizan, M.A. and Alexandre, P.M.: Solar drying of fish in peoples republic of Angola. Drying ’92, 1992, 1967-1708.
[3]
Goswami, D.Y., Lavania, A., Shahbazi, A. and Masood, M.: Experimental study of a geodesic dome solar fruit dryer, Intersoc Energy Convers Eng Conf 1990, 25,156-161
Weight
300 200 100 0
9:30 10:30 11:30 12:30 13:30 14:30 15:30 Tim e Fig. 14 Weight Vs Tim e for 22/10/2005
From the presented results, sharp drops in weight were observed to take place after mid-day, when higher temperature differences were reached. In our experiments, the heat storage of the slabs sensitively varies with the fluctuation of solar radiation. When the solar radiation decreases and air temperature decreases in the system, slabs change the heat radiation from the heat storage, and when the amount of solar radiation increases some time around noon, heat storage begins again. When much solar radiation flows into the system, the slabs store this. When this inflow decreases, the slabs radiate stored thermal energy. A combination of slabs and the copper fins contribute higher temperatures in the drying chamber. Copper fins also assist in both direction and speed regulation of hot air movements. CONCLUSION A full scale experimental prototype of a solar energy collection system has been built and initial investigations have been performed which have shown proven the feasibility of such a system. To Develop A Solar Dryer, variations of temperature and weight for solar drying agricultural and marine products were investigated. Solar dryer unit and collector system are integrated to provide a compact unit of high efficiency. Solar collector efficiency of the collector exceeds 60% on a clear hot day due to the heat storage buffer action of both the black slabs and the copper fins and the radiation inhibition from glass by collecting hot gas. The unit generates hot air for drying operation with a maximum temperature rise of 55oC when ambient and slabs are 30o and 85oC, respectively. .We conclude that double glazing, air tightness, inclusion of copper fins, and excellent heat retention properties of slabs as well as good use of fibre glass insulation contribute both to high efficiency and high temperature applications for drying purposes. If this solar drying system is used, good-quality dried products appear to be a likely result.
