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PERFORMANCE EVALUATION OF FIVE SOLAR COOKERS
PERFORMANCE EVALUATION OF FIVE SOLAR COOKERS Dr. H.P. Garg, Physicist Dr. H.S. Mann, Director Mr. K.P. Thanvi, S.R.A. Central Arid Zone Research Institute, Jodhpur, India
HOT BOX TYPE SOLAR COOKER
ABSTRACT This paper reports the design details and perform ance studies carried out on five different types of solar cookers that were fabricated and tested at the Central Arid Zone Research Institute, Jodhpur. The study was undertaken to determine the cookers' potential usefulness. Two of them are of reflector type, two are of hot box type, and one uses the flat-plate collector. Of these five cookers, the hot box type solar cooker (solar oven) showed the greatest promise due particularly to its efficiency, ease of operation, ease of construction with local techniques, material, and simplicity. INTRODUCTION According to a report of the United Nations Envi ronmental Day, the real energy crisis for one-third of the world's population is a shortage of firewood. This has encouraged mass tree planting schemes and the development of alternative cooking devices such as solar and biogas cookers. This crisis is so severe that, in many places, the task of collecting wood, which once took an hour or two, now takes a whole day. In rural India cooking is done mainly with firewood, dungcake, and vegetable waste, which together account for 94.6 percent of the total fuel consumption; and firewood provides 58.6 percent of this. In India, about 150 million tons of firewood, 52 million tons of dried dung, and 36 million tons of agricultural waste are consumed annually for cooking purposes. This 253 million tons of non commercial fuel constitutes about 45 percent of the total energy supply in the country. Villagers will continue cutting trees and using dungcake for fuel unless some kind of alternative cooking (such as biogas or solar cookers) is made available. India was a pioneer in the development of a solar cooker (1), but poor performance and high cost made it less than popular. Solar cookers are generally of two types: concentration type (2,3) and hot box type (4,5). In the concentration type cooker, para boloid or spheroid reflectors are used, with the cooking pot placed at their sun-ray focal point. In the hot box type solar cooker, the direct and diffuse radiation goes directly through the double glass window; in addition, radiation is reflected from the side reflectors. The design details and performance studies carried out on five different types of solar cookers, both concentration type and hot box type, are described in this paper.
This solar cooker (figure 1) is a double-walled box (basal area, 45 x 45 cms). The outer walì is made of 1.25 cm thick teakwood and the inner wall of blackened galvanized iron sheet (20 gauge). There is a 3.75 cm fiberglass insulation between the two walls. The top of the box contains two clear window glasses (each 60 x 55 x 0.3 cm) placed 5.0 cm apart in an airtight wooden frame hinged on one side. The inclination of glasses is equal to the latitude of the location (26.3° for Jodhpur). For the protection of the glasses when the cooker is not in use, and also for the reflection of solar radiation, a wooden cover with a mirror lining (60 x 55 cm) on the inner face is used. This cover is also hinged on one side of the cooker and can be adjusted to any angle with the help of two slotted kamanis provided.on either side of the box. Four castor wheels are fitted in the legs of the box to follow the sun. The cooker was installed in the solar energy yard of the Institute and its performance with respect to inside plate temperature and the time taken for cooking various dishes was observed. It was noted (6) that the temperature in the cooker on a typical day in summer reaches 178°C, and 143°C in winter. To maintain focus on the sun, the orientation of the cooker was adjusted eyery 30 minutes or so. A variety of foods have been cooked satisfactorily, in winter as well as summer. The taste and quality of the solar cooked foods have been found to be good. A sample of the foods prepared during the winter and the approximate length of time taken are given in Table 1. Table 1. Cooking trials with simple hot box type solar cooker Type of food 1. 1.0 kgm potatoes kept in a vessel with water 2.
Cooking time (hr) 2.5
1.0 kgm rice kept in a vessel with water
1.5
3. 1.0 kgm arhar dal kept in a vessel with water
3.0
4. 1.0 kgm vegetables kept in a vessel with water
2.5
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It is evident that most of the dishes can be cooked in 1.5-3 hours. Moreover, more than one dish can be cooked simultaneously, though it may take slight ly longer.than a single dish. Thus, both lunch and dinner can be prepared when the sun is shining; the food will remain warm even till dinner time. The cost of fabrication of the cooker at the present market rate works out to about Rs.200/-. SOLAR OVEN This solar oven consists of a well insulated seraicylindrical form, made of sheet aluminum and wood. Two shells are made and the space between them, 7.5 cm, is filled with fiberglass insulation. The interior shell is made for keeping and taking out the food. The window (40 x 40 cm) of the oven con sists of two transparent glass sheets (3 mm thick), with a spacing of 2.0 cm. Eight reflectors of silvered glass mirrors were used. Four of these were square and four were triangular. The size and angle of the reflectors were so fixed that they made the concentration factor equal to 3.5. The stand and the orientation device were made of mild steel angle iron having castor wheels, so the re flectors could follow azimuthal movement of the sun. The altitude position of the sun was followed with the help of a slotted kamani fixed with the oven. A cradle-like cooking platform was made in the oven to help keep the vessel containing food horizontal, irrespective of the inclination of the oven. Two food-containing vessels at a time can be placed on this platform. The total cost of this oven is about Rs.350/-. Four solar ovens have already been fabricated at this Institute and a number of cooking trials have been conducted. Practically any food can be cooked with this solar oven. Some of the cooking tests conducted are listed in Table 2. Table 2.
Cooking trials in a solar oven conducted at the CAZRI, Jodhpur
Type of food
Cooking time (rain.)
Cooking (a) Rice (1.0 kg in water) 45 (b) Potatoes (1.0 kg in water) 50 (e) Arhar dal (1.0 kg in water) 75 (d) Other vegetables (1.0 kg in water)60
2. Roasting
(a) Potatoes (1.0 kg) (b) Chicken (1.5 kg) (c) Fish (1.0 kg)
3. Baking 4.
60 60 20
(a) Cake (1.0 kg)
50
Boiling (a) Tea (4 cups) (b) Milk (1.0 litres) (c) Water (1.0 litres)
25 45 45
The main advantage of the solar oven (7) is that its efficiency is high; this is because its perform ance is not affected by the wind, i.e. there are no chances of dust falling in the cooking pot - which is a problem in the arid zone of Rajasthan. More over, the food kept in the oven remains warm for hours, even after sunset. The plate and the inside air temperatures were measured using thermocouples and a potentiometric recorder. On a very few clear days the maximum temperature in the oven with glass reflectors reaches 350°C in the summer and 250°C in the winter. The maximum temperatures reached in winter in the oven with various reflectors versus the oven with a silvered glass reflector, are shown in Table 3. Tabi e 3. S.No.
Showing the maximum temperature in solar oven with different reflectors
Silvered glass reflector
Chromium plated reflector
1.
233
191
2.
240
3.
245
Anodized aluminum reflector
Aluminum reflector
183 165
From this table and from the weathering performance studies at Jodhpur, it is observed that silvered glass reflectors perform better. SOLAR STEAM COOKER This solar cooker, which is relatively simple in construction, absorbs direct and diffuse solar radiation and does not require adjustments toward the sun. It consists of two parts: an efficient tube-in-plate type of flat-plate collector and an insulated steam cooker. The collector plate (absorbtion area - 0.9 sq.m. [120 x 75 cm]) consists of a wire wound steel fin (29 gauge) with a steel pipe (19 mm diameter) spaced at 25 cm centers. First of all, a pipe gridwork with four steel pipes (each 19 mm in diameter, 120 cm long, and spaced 25 cm apart) are welded with two steel headers (each 25 mm in diameter, 75 cm long). Over this a steel plate (23 gauge thick), grooved to the equivalent of half the diameter of the pipe, is tied to the pipe with the help of binding wire at a 5 cm spacing. A steel pipe (19 mm in diameter and 15 cm long) is also welded at the center of one of the headers; this acts as the steam outlet. One side of this collector plate is painted with mat blackboard paint. This absorber plate is placed in a mild steel tray (130 x 85 x 18 cm); underneath is 5 cm thick fiberglass insulation and on top are two colorless glass sheets, each 3 mm thick and 5 cm apart. This absorber box is made airtight using tar plastic putty and aluminum angle iron (25 x 25 x 1.5 mm). The steam cooker is fixed at the top of the absorber with a threaded coupling.
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The steam cooker consists of a double walled in sulated cylindrical box made of a 24 gauge gal vanized iron sheet. Its stand is made of wooden batten. The inner box has an openable lid, a diameter of 15 cm and a height of 26 cm. The outer box also with an openable lid is 26 cm in diameter and 36 cm high. A steel pipe of 19 ram diameter and 15 cm long is welded at the bottom of the inner tank; this acts as a water inlet and steam outlet for the absorber. A metal ring 12 cm in diameter and 4 cm high is also placed in the inner box. The tiffin, containing food, is placed over this; it is 13 cm in diameter by 22 cm high and is made of aluminum. Food is put in any or all of three compartments in the tiffin. The entire cooker is fixed at an inclination of 41° from horizontal at Jodhpur; all morning it faces where the sun rises, and all afternoon where the sun sets. More frequent adjustment is not required. The solar steam cooker is shown in figure 3. The solar collector always contains water; about a cup of water should be added each morning to replace that which has boiled away. Steam is produced within an hour of sunrise and will continue to be produced for the rest of the day (as long as the sun shines on the collector). Because the cooking pot is located above the collector, the same con densate returns by gravity flow and boils up again. The entire cooker can be cleaned by opening the gate valve provided at the bottom of the collector and flushing the water through the cooker. This solar cooker can be used for cooking or boiling cereals, rice, potatoes, dal, vegetables, etc. Some of the cooking trials conducted and the time taken with this solar steam cooker (8) are listed in Table 4. Table 4. Cooking trials with solar steam cooker conducted at the CAZRI, Jodhpur Type of food
Cooking time (hr)
1. Rice (0.5 kg with some water)
1.5
2. Potatoes (0.5 kg with some water)
2.5
3. Vegetables (0.5 kg with some water)
2.5
From this table it is seen that it is possible to cook both the midday and evening meals. It can also be said that this cooker is a slow cooking device and is therefore suitable only for preparing those foods which require slow boiling, like rice, cereals, etc. Three copper contanton thermocouples, 32 SWG, were used with the automatic potentiometric recorder for recording the absorber plate temperature, water temperature in the steam cooker, and ambient air temperature. Measurements were taken for a number of summer and winter days. Hourly variation of the absorber plate temperature, hot water temperature, and ambient air temperature for a typical day in -1493-
September 1976, along with the total solar radi ation on a horizontal surface (Cal/sq.cm.hr.) and the direct solar radiation at normal incidence (Cal/sq.cm.day) were noted. The collector plate temperature reaches 115°C and water starts boiling within 60 to 75 minutes of sunrise; it continues to boil till sunset. The main advantage of the solar steam cooker is that it does not require adjustment every hour toward the sun. This cooker can be installed on the lawn or right on the chajja of the house and then connected with a pipe to the steam cooker inside the kitchen. Even in multistory buildings this solar steam cooker can be used if the kitchen is in a suitable place. N.P.L.-TYPE PARABOLOID SOLAR COOKER The simplest concentrator of solar energy is a convex lens or a concave mirror. Since lenses of large size are difficult to make, it is easier to employ metal concave mirrors. Generally parabolic metal reflectors with an area of 1.0 sq. m. to 1.5 sq. m are used for cooking purposes. The parabolic reflector cookers are suitable for boiling water or for cooking food (with continu ous stirring to prevent scorching of the food). The cooker must be adjusted frequently to keep the focus image of the sun at the center of the pot. The National Physical Laboratory, New Delhi, India was the pioneer in developing the concen tration-type solar cooker. This cooker as shown in figure 4 employs a paraboloid reflector of equation Y2 = 130 X, having a focal length of 45 cms. This was found to be the most desirable from design considerations. In elevation, the para boloid has a diameter of 120 cms, but with 24 cms cut horizontally off the top for a vertical height of 35 cms. The face area normal to the incident solar rays is 0.75 sq.m. which is reduced to a net or an effective area of 0.65 sq.m. by the neces sary attachments. The reflector is spun from aluminum sheets to the desired shape and is then anodized to give it protection against atmospheric corrosion and to avoid loss of reflectivity. The reflectivity of this surface is about 75 percent. A small circular portion from its center is also cut out without any loss of efficiency; this also helps in the proper adjustment of the cooker for solar radiation to fall normally on the reflecting surface. The reflector is mounted on a stand that provides for manual adjustment to accommodate the daily and seasonable movement of the sun. The support for the cooking utensil is a wire netting fixed to a steel ring that can be adjusted manu ally to provide a horizontal position for the vessel. This solar cooker was extensively studied by the authors (9) at the Central Arid Zone Research Institute, Jodhpur, India, and by the Scientists of the National Physical Laboratory, New Delhi, India. These institutes represent arid and semiarid regions of the country respectively.
steel ring that can be adjusted manually to provide a horizontal position for the vessel.
It was experimentally observed that cooking time can be reduced by about one-third by simply using the cooking utensil with a blackened bottom; this may be because a black surface (compared to a polished surface) is a good absorber of heat. The performance of this cooker very much depends on the wind speed and the dusty atmosphere (as it does, of course, on the direct solar radiation in tensity). It was observed that 1.0 litre of water in a black bottom vessel comes to boiling point within 25 to 30 minutes under a clear sky and with wind speeds of 3 to 4 km/hr, while 1.0 litre of water never reaches boiling with a wind speed of 10 to 15 km/hr, and the maximum temperature re corded under such conditions was only 70 to 80°C. This cooker was used for cooking at Jodhpur and it was found that all types of cooking can be done under yery clear and calm skies. The performance is \/ery much reduced by a wind speed greater than 5 km/hr, which is a general feature in arid and coastal areas of India. Moreover, frequent adjust ment of the cooker toward the sun [eyery 10 to 15 minutes) and continuous stirring of the food to prevent scorching require the presence of a cook near the cooker. The stand of the cooker must be redesigned to withstand high wind pressure and for easy adjustment toward the sun. As such, this paraboloid reflector-type solar cooker is not suit able for use in arid areas where there are frequent dust raising winds. STEP REFLECTOR-TYPE SOLAR COOKER The parabolic reflectors are limited to small size because of their low resistance to strong winds and because their fabrication requires highly skilled technicians and a big workshop. Their performance, however, can be sufficiently improved by using a number of small size plain mirrors or reflecting metal strips arranged in such a way as to allow concentration of solar radiation in a small area. Based on this principle, a new step reflector for use as a cooker has been designed and tested by the authors (10). This reflector is made, with the help of a carpenter, of wooden battens and waste aluminum strips The step reflector in the cooker has a diameter of 120 cm with a focal length of 60 cm. Seven aluminum strips, each 7.5 cm wide, were used; these were finally anodized to retain a high reflectivity for solar radiation. The angle of the strip, a n to get concentration, can be calculated with the help of the following formula:
The actual cooking trials on this step reflectortype solar cooker were also conducted by the authors (10) at Central Arid Zone Research Insti tute, Jodhpur. In this cooker also, it was ob served that 1.0 litre of water comes to boiling point within 30 minutes under clear and calm sky conditions. But under high wind velocities and dust storms, water never reaches the boiling point. With this cooker also, all types of cooking can be done under clear skies and at low wind speeds. But this solar cooker can definitely withstand more wind pressure. This step reflector type of solar cooker has a number of advantages over the para boloid type of cooker: it is lower in cost, lower in weight, and can easily be fabricated with the help of a carpenter. CONCLUSIONS Table 5 gives the comparative data on the proper ties and performance of five solar cookers fabri cated and tested at CAZRI, Jodhpur. Of these five solar cookers, the solar oven showed the greatest promise, due particularly to its efficiency, ease of operation, ease of construction with local tech nique, material, and simplicity. REFERENCES 1.
M.L. Ghai, "Manufacture of Reflector-Type Solar Cooker," J. Sei. Ind. Res., 13 A, 212216, 1953.
2.
J.A. Duffie, G.O.G. Lof and B. Beck, "Labora tory and Field Studies of Plastic Reflector Solar Cookers," Proc. United Nations Conf. New Sources of Energy, Rome, Vol. 5, 339-345, 1961.
3.
G.O.G. Lof and D.A. Fester, "Design and Per formance of Folding Unbrella-Type Solar Cooker," Proc. United Nations Conf. New Sources of Energy, Rome, Vol. 5, 347-352, 1961.
4.
M. Telkes, "Solar Cooking Ovens," Solar Energy, Vol. 3, 1-11, 1959.
5.
M.K. Ghosh, "The Design of a Sun Cooker," Paper presented in the International Congress, The Sun in the Service of Mankind, Paris, 1973.
6.
H.P. Garg and K.P. Thanvi, "Design and Perform ance Studies on Simple Hot Box-Type Solar Cooker," Research & Industry, 21(3), 136-187, 1976.
Where Rn is the distance of the strip from the center of the reflector and F is the focal length.
7.
H.P. Garg, "A Solar Oven for Cooking," Indian Farming, 27(5), 7-9, 1976.
A step reflector constructed using anodized alumi num strips (24 gauge) and wooden battens (2.5 x 2.5 cm) and used as a cooker is shown in figure 5. This reflector is mounted on a stand made out of mild steel angle iron (25 x 25 x 3 mm) with four legs with castor wheels at its bottom for azimuthal adjustments. A slotted kamani has also been fitted for altitude adjustment purposes. The support for the cooking utensil is a wire netting fixed to a
8.
H.P. Garg and K.P. Thanvi, "Studies on Solar Steam Cooker," Indian Farming, 27(1), 29-30, 1977.
9.
H.P. Garg, "Testing of Five Solar Cookers at Jodhpur," Proc. National Solar Energy Conven tion, Calcutta
■n
= tan"1 {*"}
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I
Very good Good
Good Good Occasional
Fair Fair Occasional
Fair Very good Frequent
Fair Very good Frequent
Occasional
30 70
60
30
30
350
200 300
200
200
30.6 79x79x94
34.6 67x72x69
55.3 106x103x167
-
-
1.0
23.0 123x92x124
11.8 116x82x72
0.16
.. -
— -
0.33
-
-
—.
0.85 0.33
-
1.13 60.0
4 . R e f l e c t o r area(m ) 5. Reflector focal l e n g t h (cm) 6 . Window area of oven(m ) 7 . Absorber area(m ) 8 . Net weight (kgm) 9 · S i z e of packing (cm) 1 0 . Approximate c o s t ( R s . ) 11. Time for bringing 1.0 litre of water from 25°C to 100°0(minutes) under clear sky conditions 12. Performance for cooking of food 13· Durability 1 4 . Adjustment towards sun
*~
Anodised aluminium sheet
Anodised aluminium sheet 0.76 45.0
3 . Reflector material
Oven External flat plane Silvered glass sheet
Oven External flat plane Silvered glass sheet
Flat-•plate NO reflector
S o l a r oven
Focussing Spherical
Hot box t y p e s o l a r cooker
Focussing Paraboloid
S o l a r steam cooker
1 . Cooker type 2 . Reflector type
"Step r e f l e c t o r " N.P.L.type type s o l a r paraboloid s o l a r cooker cooker
Table 5. Solar cooker characteristics tested at OAZRI, Jodhpur.
10.
H.P. Garg and H.S. Mann, "Design and Perform ance Studies on Five Different Types of Solar Cookers under Indian Arid Zone Conditions," Proc. Conf. on Physics of Solar Energy, Bengahazi, Oct. 29-Nov. 4, 1976.
FIGURE 1
Hot box type solar cooker
FIGURE 4
FIGURE 2
Solar oven
FIGURE 5
FIGURE 3
Paraboloid solar cooker
Solar steam cooker
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Step reflector type solar cooker
HOT BOX TYPE SOLAR COOKER
ABSTRACT This paper reports the design details and perform ance studies carried out on five different types of solar cookers that were fabricated and tested at the Central Arid Zone Research Institute, Jodhpur. The study was undertaken to determine the cookers' potential usefulness. Two of them are of reflector type, two are of hot box type, and one uses the flat-plate collector. Of these five cookers, the hot box type solar cooker (solar oven) showed the greatest promise due particularly to its efficiency, ease of operation, ease of construction with local techniques, material, and simplicity. INTRODUCTION According to a report of the United Nations Envi ronmental Day, the real energy crisis for one-third of the world's population is a shortage of firewood. This has encouraged mass tree planting schemes and the development of alternative cooking devices such as solar and biogas cookers. This crisis is so severe that, in many places, the task of collecting wood, which once took an hour or two, now takes a whole day. In rural India cooking is done mainly with firewood, dungcake, and vegetable waste, which together account for 94.6 percent of the total fuel consumption; and firewood provides 58.6 percent of this. In India, about 150 million tons of firewood, 52 million tons of dried dung, and 36 million tons of agricultural waste are consumed annually for cooking purposes. This 253 million tons of non commercial fuel constitutes about 45 percent of the total energy supply in the country. Villagers will continue cutting trees and using dungcake for fuel unless some kind of alternative cooking (such as biogas or solar cookers) is made available. India was a pioneer in the development of a solar cooker (1), but poor performance and high cost made it less than popular. Solar cookers are generally of two types: concentration type (2,3) and hot box type (4,5). In the concentration type cooker, para boloid or spheroid reflectors are used, with the cooking pot placed at their sun-ray focal point. In the hot box type solar cooker, the direct and diffuse radiation goes directly through the double glass window; in addition, radiation is reflected from the side reflectors. The design details and performance studies carried out on five different types of solar cookers, both concentration type and hot box type, are described in this paper.
This solar cooker (figure 1) is a double-walled box (basal area, 45 x 45 cms). The outer walì is made of 1.25 cm thick teakwood and the inner wall of blackened galvanized iron sheet (20 gauge). There is a 3.75 cm fiberglass insulation between the two walls. The top of the box contains two clear window glasses (each 60 x 55 x 0.3 cm) placed 5.0 cm apart in an airtight wooden frame hinged on one side. The inclination of glasses is equal to the latitude of the location (26.3° for Jodhpur). For the protection of the glasses when the cooker is not in use, and also for the reflection of solar radiation, a wooden cover with a mirror lining (60 x 55 cm) on the inner face is used. This cover is also hinged on one side of the cooker and can be adjusted to any angle with the help of two slotted kamanis provided.on either side of the box. Four castor wheels are fitted in the legs of the box to follow the sun. The cooker was installed in the solar energy yard of the Institute and its performance with respect to inside plate temperature and the time taken for cooking various dishes was observed. It was noted (6) that the temperature in the cooker on a typical day in summer reaches 178°C, and 143°C in winter. To maintain focus on the sun, the orientation of the cooker was adjusted eyery 30 minutes or so. A variety of foods have been cooked satisfactorily, in winter as well as summer. The taste and quality of the solar cooked foods have been found to be good. A sample of the foods prepared during the winter and the approximate length of time taken are given in Table 1. Table 1. Cooking trials with simple hot box type solar cooker Type of food 1. 1.0 kgm potatoes kept in a vessel with water 2.
Cooking time (hr) 2.5
1.0 kgm rice kept in a vessel with water
1.5
3. 1.0 kgm arhar dal kept in a vessel with water
3.0
4. 1.0 kgm vegetables kept in a vessel with water
2.5
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It is evident that most of the dishes can be cooked in 1.5-3 hours. Moreover, more than one dish can be cooked simultaneously, though it may take slight ly longer.than a single dish. Thus, both lunch and dinner can be prepared when the sun is shining; the food will remain warm even till dinner time. The cost of fabrication of the cooker at the present market rate works out to about Rs.200/-. SOLAR OVEN This solar oven consists of a well insulated seraicylindrical form, made of sheet aluminum and wood. Two shells are made and the space between them, 7.5 cm, is filled with fiberglass insulation. The interior shell is made for keeping and taking out the food. The window (40 x 40 cm) of the oven con sists of two transparent glass sheets (3 mm thick), with a spacing of 2.0 cm. Eight reflectors of silvered glass mirrors were used. Four of these were square and four were triangular. The size and angle of the reflectors were so fixed that they made the concentration factor equal to 3.5. The stand and the orientation device were made of mild steel angle iron having castor wheels, so the re flectors could follow azimuthal movement of the sun. The altitude position of the sun was followed with the help of a slotted kamani fixed with the oven. A cradle-like cooking platform was made in the oven to help keep the vessel containing food horizontal, irrespective of the inclination of the oven. Two food-containing vessels at a time can be placed on this platform. The total cost of this oven is about Rs.350/-. Four solar ovens have already been fabricated at this Institute and a number of cooking trials have been conducted. Practically any food can be cooked with this solar oven. Some of the cooking tests conducted are listed in Table 2. Table 2.
Cooking trials in a solar oven conducted at the CAZRI, Jodhpur
Type of food
Cooking time (rain.)
Cooking (a) Rice (1.0 kg in water) 45 (b) Potatoes (1.0 kg in water) 50 (e) Arhar dal (1.0 kg in water) 75 (d) Other vegetables (1.0 kg in water)60
2. Roasting
(a) Potatoes (1.0 kg) (b) Chicken (1.5 kg) (c) Fish (1.0 kg)
3. Baking 4.
60 60 20
(a) Cake (1.0 kg)
50
Boiling (a) Tea (4 cups) (b) Milk (1.0 litres) (c) Water (1.0 litres)
25 45 45
The main advantage of the solar oven (7) is that its efficiency is high; this is because its perform ance is not affected by the wind, i.e. there are no chances of dust falling in the cooking pot - which is a problem in the arid zone of Rajasthan. More over, the food kept in the oven remains warm for hours, even after sunset. The plate and the inside air temperatures were measured using thermocouples and a potentiometric recorder. On a very few clear days the maximum temperature in the oven with glass reflectors reaches 350°C in the summer and 250°C in the winter. The maximum temperatures reached in winter in the oven with various reflectors versus the oven with a silvered glass reflector, are shown in Table 3. Tabi e 3. S.No.
Showing the maximum temperature in solar oven with different reflectors
Silvered glass reflector
Chromium plated reflector
1.
233
191
2.
240
3.
245
Anodized aluminum reflector
Aluminum reflector
183 165
From this table and from the weathering performance studies at Jodhpur, it is observed that silvered glass reflectors perform better. SOLAR STEAM COOKER This solar cooker, which is relatively simple in construction, absorbs direct and diffuse solar radiation and does not require adjustments toward the sun. It consists of two parts: an efficient tube-in-plate type of flat-plate collector and an insulated steam cooker. The collector plate (absorbtion area - 0.9 sq.m. [120 x 75 cm]) consists of a wire wound steel fin (29 gauge) with a steel pipe (19 mm diameter) spaced at 25 cm centers. First of all, a pipe gridwork with four steel pipes (each 19 mm in diameter, 120 cm long, and spaced 25 cm apart) are welded with two steel headers (each 25 mm in diameter, 75 cm long). Over this a steel plate (23 gauge thick), grooved to the equivalent of half the diameter of the pipe, is tied to the pipe with the help of binding wire at a 5 cm spacing. A steel pipe (19 mm in diameter and 15 cm long) is also welded at the center of one of the headers; this acts as the steam outlet. One side of this collector plate is painted with mat blackboard paint. This absorber plate is placed in a mild steel tray (130 x 85 x 18 cm); underneath is 5 cm thick fiberglass insulation and on top are two colorless glass sheets, each 3 mm thick and 5 cm apart. This absorber box is made airtight using tar plastic putty and aluminum angle iron (25 x 25 x 1.5 mm). The steam cooker is fixed at the top of the absorber with a threaded coupling.
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The steam cooker consists of a double walled in sulated cylindrical box made of a 24 gauge gal vanized iron sheet. Its stand is made of wooden batten. The inner box has an openable lid, a diameter of 15 cm and a height of 26 cm. The outer box also with an openable lid is 26 cm in diameter and 36 cm high. A steel pipe of 19 ram diameter and 15 cm long is welded at the bottom of the inner tank; this acts as a water inlet and steam outlet for the absorber. A metal ring 12 cm in diameter and 4 cm high is also placed in the inner box. The tiffin, containing food, is placed over this; it is 13 cm in diameter by 22 cm high and is made of aluminum. Food is put in any or all of three compartments in the tiffin. The entire cooker is fixed at an inclination of 41° from horizontal at Jodhpur; all morning it faces where the sun rises, and all afternoon where the sun sets. More frequent adjustment is not required. The solar steam cooker is shown in figure 3. The solar collector always contains water; about a cup of water should be added each morning to replace that which has boiled away. Steam is produced within an hour of sunrise and will continue to be produced for the rest of the day (as long as the sun shines on the collector). Because the cooking pot is located above the collector, the same con densate returns by gravity flow and boils up again. The entire cooker can be cleaned by opening the gate valve provided at the bottom of the collector and flushing the water through the cooker. This solar cooker can be used for cooking or boiling cereals, rice, potatoes, dal, vegetables, etc. Some of the cooking trials conducted and the time taken with this solar steam cooker (8) are listed in Table 4. Table 4. Cooking trials with solar steam cooker conducted at the CAZRI, Jodhpur Type of food
Cooking time (hr)
1. Rice (0.5 kg with some water)
1.5
2. Potatoes (0.5 kg with some water)
2.5
3. Vegetables (0.5 kg with some water)
2.5
From this table it is seen that it is possible to cook both the midday and evening meals. It can also be said that this cooker is a slow cooking device and is therefore suitable only for preparing those foods which require slow boiling, like rice, cereals, etc. Three copper contanton thermocouples, 32 SWG, were used with the automatic potentiometric recorder for recording the absorber plate temperature, water temperature in the steam cooker, and ambient air temperature. Measurements were taken for a number of summer and winter days. Hourly variation of the absorber plate temperature, hot water temperature, and ambient air temperature for a typical day in -1493-
September 1976, along with the total solar radi ation on a horizontal surface (Cal/sq.cm.hr.) and the direct solar radiation at normal incidence (Cal/sq.cm.day) were noted. The collector plate temperature reaches 115°C and water starts boiling within 60 to 75 minutes of sunrise; it continues to boil till sunset. The main advantage of the solar steam cooker is that it does not require adjustment every hour toward the sun. This cooker can be installed on the lawn or right on the chajja of the house and then connected with a pipe to the steam cooker inside the kitchen. Even in multistory buildings this solar steam cooker can be used if the kitchen is in a suitable place. N.P.L.-TYPE PARABOLOID SOLAR COOKER The simplest concentrator of solar energy is a convex lens or a concave mirror. Since lenses of large size are difficult to make, it is easier to employ metal concave mirrors. Generally parabolic metal reflectors with an area of 1.0 sq. m. to 1.5 sq. m are used for cooking purposes. The parabolic reflector cookers are suitable for boiling water or for cooking food (with continu ous stirring to prevent scorching of the food). The cooker must be adjusted frequently to keep the focus image of the sun at the center of the pot. The National Physical Laboratory, New Delhi, India was the pioneer in developing the concen tration-type solar cooker. This cooker as shown in figure 4 employs a paraboloid reflector of equation Y2 = 130 X, having a focal length of 45 cms. This was found to be the most desirable from design considerations. In elevation, the para boloid has a diameter of 120 cms, but with 24 cms cut horizontally off the top for a vertical height of 35 cms. The face area normal to the incident solar rays is 0.75 sq.m. which is reduced to a net or an effective area of 0.65 sq.m. by the neces sary attachments. The reflector is spun from aluminum sheets to the desired shape and is then anodized to give it protection against atmospheric corrosion and to avoid loss of reflectivity. The reflectivity of this surface is about 75 percent. A small circular portion from its center is also cut out without any loss of efficiency; this also helps in the proper adjustment of the cooker for solar radiation to fall normally on the reflecting surface. The reflector is mounted on a stand that provides for manual adjustment to accommodate the daily and seasonable movement of the sun. The support for the cooking utensil is a wire netting fixed to a steel ring that can be adjusted manu ally to provide a horizontal position for the vessel. This solar cooker was extensively studied by the authors (9) at the Central Arid Zone Research Institute, Jodhpur, India, and by the Scientists of the National Physical Laboratory, New Delhi, India. These institutes represent arid and semiarid regions of the country respectively.
steel ring that can be adjusted manually to provide a horizontal position for the vessel.
It was experimentally observed that cooking time can be reduced by about one-third by simply using the cooking utensil with a blackened bottom; this may be because a black surface (compared to a polished surface) is a good absorber of heat. The performance of this cooker very much depends on the wind speed and the dusty atmosphere (as it does, of course, on the direct solar radiation in tensity). It was observed that 1.0 litre of water in a black bottom vessel comes to boiling point within 25 to 30 minutes under a clear sky and with wind speeds of 3 to 4 km/hr, while 1.0 litre of water never reaches boiling with a wind speed of 10 to 15 km/hr, and the maximum temperature re corded under such conditions was only 70 to 80°C. This cooker was used for cooking at Jodhpur and it was found that all types of cooking can be done under yery clear and calm skies. The performance is \/ery much reduced by a wind speed greater than 5 km/hr, which is a general feature in arid and coastal areas of India. Moreover, frequent adjust ment of the cooker toward the sun [eyery 10 to 15 minutes) and continuous stirring of the food to prevent scorching require the presence of a cook near the cooker. The stand of the cooker must be redesigned to withstand high wind pressure and for easy adjustment toward the sun. As such, this paraboloid reflector-type solar cooker is not suit able for use in arid areas where there are frequent dust raising winds. STEP REFLECTOR-TYPE SOLAR COOKER The parabolic reflectors are limited to small size because of their low resistance to strong winds and because their fabrication requires highly skilled technicians and a big workshop. Their performance, however, can be sufficiently improved by using a number of small size plain mirrors or reflecting metal strips arranged in such a way as to allow concentration of solar radiation in a small area. Based on this principle, a new step reflector for use as a cooker has been designed and tested by the authors (10). This reflector is made, with the help of a carpenter, of wooden battens and waste aluminum strips The step reflector in the cooker has a diameter of 120 cm with a focal length of 60 cm. Seven aluminum strips, each 7.5 cm wide, were used; these were finally anodized to retain a high reflectivity for solar radiation. The angle of the strip, a n to get concentration, can be calculated with the help of the following formula:
The actual cooking trials on this step reflectortype solar cooker were also conducted by the authors (10) at Central Arid Zone Research Insti tute, Jodhpur. In this cooker also, it was ob served that 1.0 litre of water comes to boiling point within 30 minutes under clear and calm sky conditions. But under high wind velocities and dust storms, water never reaches the boiling point. With this cooker also, all types of cooking can be done under clear skies and at low wind speeds. But this solar cooker can definitely withstand more wind pressure. This step reflector type of solar cooker has a number of advantages over the para boloid type of cooker: it is lower in cost, lower in weight, and can easily be fabricated with the help of a carpenter. CONCLUSIONS Table 5 gives the comparative data on the proper ties and performance of five solar cookers fabri cated and tested at CAZRI, Jodhpur. Of these five solar cookers, the solar oven showed the greatest promise, due particularly to its efficiency, ease of operation, ease of construction with local tech nique, material, and simplicity. REFERENCES 1.
M.L. Ghai, "Manufacture of Reflector-Type Solar Cooker," J. Sei. Ind. Res., 13 A, 212216, 1953.
2.
J.A. Duffie, G.O.G. Lof and B. Beck, "Labora tory and Field Studies of Plastic Reflector Solar Cookers," Proc. United Nations Conf. New Sources of Energy, Rome, Vol. 5, 339-345, 1961.
3.
G.O.G. Lof and D.A. Fester, "Design and Per formance of Folding Unbrella-Type Solar Cooker," Proc. United Nations Conf. New Sources of Energy, Rome, Vol. 5, 347-352, 1961.
4.
M. Telkes, "Solar Cooking Ovens," Solar Energy, Vol. 3, 1-11, 1959.
5.
M.K. Ghosh, "The Design of a Sun Cooker," Paper presented in the International Congress, The Sun in the Service of Mankind, Paris, 1973.
6.
H.P. Garg and K.P. Thanvi, "Design and Perform ance Studies on Simple Hot Box-Type Solar Cooker," Research & Industry, 21(3), 136-187, 1976.
Where Rn is the distance of the strip from the center of the reflector and F is the focal length.
7.
H.P. Garg, "A Solar Oven for Cooking," Indian Farming, 27(5), 7-9, 1976.
A step reflector constructed using anodized alumi num strips (24 gauge) and wooden battens (2.5 x 2.5 cm) and used as a cooker is shown in figure 5. This reflector is mounted on a stand made out of mild steel angle iron (25 x 25 x 3 mm) with four legs with castor wheels at its bottom for azimuthal adjustments. A slotted kamani has also been fitted for altitude adjustment purposes. The support for the cooking utensil is a wire netting fixed to a
8.
H.P. Garg and K.P. Thanvi, "Studies on Solar Steam Cooker," Indian Farming, 27(1), 29-30, 1977.
9.
H.P. Garg, "Testing of Five Solar Cookers at Jodhpur," Proc. National Solar Energy Conven tion, Calcutta
■n
= tan"1 {*"}
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I
Very good Good
Good Good Occasional
Fair Fair Occasional
Fair Very good Frequent
Fair Very good Frequent
Occasional
30 70
60
30
30
350
200 300
200
200
30.6 79x79x94
34.6 67x72x69
55.3 106x103x167
-
-
1.0
23.0 123x92x124
11.8 116x82x72
0.16
.. -
— -
0.33
-
-
—.
0.85 0.33
-
1.13 60.0
4 . R e f l e c t o r area(m ) 5. Reflector focal l e n g t h (cm) 6 . Window area of oven(m ) 7 . Absorber area(m ) 8 . Net weight (kgm) 9 · S i z e of packing (cm) 1 0 . Approximate c o s t ( R s . ) 11. Time for bringing 1.0 litre of water from 25°C to 100°0(minutes) under clear sky conditions 12. Performance for cooking of food 13· Durability 1 4 . Adjustment towards sun
*~
Anodised aluminium sheet
Anodised aluminium sheet 0.76 45.0
3 . Reflector material
Oven External flat plane Silvered glass sheet
Oven External flat plane Silvered glass sheet
Flat-•plate NO reflector
S o l a r oven
Focussing Spherical
Hot box t y p e s o l a r cooker
Focussing Paraboloid
S o l a r steam cooker
1 . Cooker type 2 . Reflector type
"Step r e f l e c t o r " N.P.L.type type s o l a r paraboloid s o l a r cooker cooker
Table 5. Solar cooker characteristics tested at OAZRI, Jodhpur.
10.
H.P. Garg and H.S. Mann, "Design and Perform ance Studies on Five Different Types of Solar Cookers under Indian Arid Zone Conditions," Proc. Conf. on Physics of Solar Energy, Bengahazi, Oct. 29-Nov. 4, 1976.
FIGURE 1
Hot box type solar cooker
FIGURE 4
FIGURE 2
Solar oven
FIGURE 5
FIGURE 3
Paraboloid solar cooker
Solar steam cooker
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Step reflector type solar cooker