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Indirect solar cookers
Indirect cookers separate the solar energy collection element from the cooking area, using a heat transport medium to bring the collected energy into the cooking area. Hypothetically a solar collector could be placed several feet away from the cooking area by using this technique. This would allow the collection elements to be placed on the roof, for example, with the cooking area located within the home. Most of the indirect cookers summarized here comprise a single unit with only limited separation between the collection element and the cooking area. Longer separation distances bring design challenges such as minimizing heat loss between the collection element and the cooking area, and circulation of the heat transport medium. Close proximity units allow the heat transfer through natural convection.
Whiller Indirect Cooker
In 1965, Whiller described one of the earliest indirect cooker designs.
The cooker is simple. A central finned pipe, connected to an insulated cooking box inside the house, is heated by cylindro-parabolic mirror. This boils the water in the finned pipe and the heat is used for cooking. Reports indicate that the cooker did not perform well (Lof 1963).
Brace Research Institute Steam Cooker
The Brace Research Institute (1972) developed a cooker on these lines and it was called the steam cooker. Here, the heart of the cooker was a double glazed flat plate. It heated the enclosed water to boiling and heated the insulated box kept inside the house. The cooking vessels could be kept inside this insulated box. Though interesting the design did not perform well (Bowman, Blatt 1978), and Bowman tried to improve the design by altering the connection between the heater and the insulated box so that the heater could be tilted to face the sun.
Venugopal Indirect Cooker
In India, Venugopal (1978) proposed a very promising variation, wherein he attached four reflector boosters to the flatplate collector. The whole assembly was on a wheeled platform for easy mobility. The insulated box had a separate chamber for keeping the milk hot. This cooker was designed for the roadside tea vendors. Another variation was reported from Iran (GATE 1979), where the unit was larger and the absorber/heater was kept outside the house.
Chinese Heat Pipe
Scientists from China (Fang, Susan 1979) had described another design in 1979. The design incorporated a heat pipe, which is heated by a cylindro-parabolic mirror, with a transparent cover on top to reduce heat loss. This heatpipe delivered the heat to an insulated box kept inside the house. With tracking concentrator and suitable heat transfer fluid inside the heat pipe the design should work well, provided, the reflector has a 2 sq. m surface.
Nijaguna Indirect Cooker
Nijaguna (1983) from Karnataka Regional Engineering College, Suratkal, has evolved a cooker using a heat pipe. A conical reflector covered with a transparent cover heats a heat pipe which in turn delivers heat to an insulated box kept inside the house.
Stam Type Indirect Cooker
In 1961, Stam presented another interesting and ingenious design. The central pipe containing oil was heated by a cylindro – parabolic mirror. This hot oil would then transfer heat to a cooking vessel. The insulated box had a depression to accommodate the vessel. He had also evolved a very simple orientation mechanism for the mirror similar to one mentioned by Von Oppen (1977)
Swet Type Indirect Cooker
Swet (1972) proposed another design which was very much similar to the Stam design but the tracking was done with the help of a bi-metallic heliotrope. A bar was provided which cast a shadow on the heliotrope and this setup made the mechanism of tracking more efficient. Test data on this design was not available.
Farber Type Indirect Cooker
Another design was proposed by Beanson ( 1976 ). It was supposed to supply heat for 24 hours a day. Once again the centre of the cooker was a pipe heated by a cylindro-parabolic reflector. The heat transfer medium suggested was oil and the hot oil circulating in a close loop was to accumulate in an insulated container kept inside the house. which double as a cooking ‘stove’ . The circulation of the oil was effected by a small circulating pump attached to the insulated vessels. The pump would be very useful especially if the collector is kept at a height above the insulated box. Reporters of Mechanix Illustrated, Who tested this design, claim that they could fry potato finger chips. But calculations presented by Walton et al. (1977) indicte that at least 100 litres of oil would be required to store sufficient heat to be able to cook in the evening, and naturally, a very large quantity of oil would be essential to store the heat for 24 hours. A cooker which would require 100 litres of oil and material to heat it to 200oC would be very costly.
Bowman CPC Indirect Cooker
Winston’s compound parabolic concentrators (CPC) have been very popular with the solar water heating enthusiasts, but it was Bowman who designed a cooker on this principle (Type IDT 6, Figure 60). Such a cooker did not come up to the expectations of Bowman, Blatt (1978). But CPC is a highly acclaimed design and if the cooker is designed with more care it should perform well. However, the comments of Walton et al. (1977) are applicable in this case also so the cooker should not be very large.
Kundapur "Concept XII"
Concept XII proposed by the author in 1980, envisaged the use of a cylindro-parabolic concentrator on top of the house. The medium of heat transfer was air. The air thus heated was to be circulated through a small but suitable pump running on solar power, to the insulated cooking chamber kept in the kitchen. Suitable precaution has to be exercised to take care of the pressure that may build up or alternatively more suitable heat transfer medium could be selected. The proposed design has been acclaimed as viable by William Beale of VITA, but he felt that solar sterling engine may not be suitable for circulation of air (Beale 1981). Recently, Olwi et al. (1994) have tested a few designs of this type, but the details are not available.
Evacuated Tube Indirect Solar Cooker
In 2008 John Grandinetti publicized his Evacuated Tube Indirect Solar Cooker, which has some similarities to the Whiller indirect solar cooker. Grandinetti's design replaces the finned pipe with an evacuated tube solar collector as the collector element, and uses vegetable oil instead of water as the heat transport medium. With these changes and some other refinements, the design provides a practical solar appliance. Tests have indicated the unit can reach cooking temperatures (300°F) with 1 hour of morning sun at tropical lattitudes. Cooking, water pasteuization and autoclaving can be readily achieved throughout the remainder of the solar day, with cooking times similar to an electric stove. Frying, baking, boiling and steaming have all been successfully accomplished.
Solar Steamer, an Evacuated Tube Indirect Solar Cooker without reflectors
The Solar Steamer invented by Heinz-Joachim Muller uses no reflectors but concentrates the solar energy by creating steam from a relatively large collector area and applying it to a smaller cooking area. The indirect cooking system allows the design of a split system where the thermal solar collector can be placed at some distance (e.g. on the roof) apart from the place of cooking (e.g. in the kitchen). The cook is not exposed to the sun shine and can use the steamer in a convenient position.
In its technical design the solar steamer makes use of the availability of standard evacuated tube solar collectors which are available from China at low cost.
Solar cookers using chemicals have existed since 1961. The need was felt much earlier as solar energy is very intermittent in nature. Many ideas have been proposed but none of them appear to be very promising.
The VITA group (1961) and Huxtable (1976) have proposed another system with H2SO4 and water (Type IDT 8b). When water is mixed with acid, heat would be liberated and later the acid could be concentrated by driving away the water using solar heat. Though very simple, it did not meet the safety standards.
Hall et al. (1977) propose the use of simple salts like MgC12 or Cacl2 (Type IDT 8a). Ammoniated MgCl2 and CaCl2 are kept in separate but interconnected boxes. Solar heat was used to drive away the ammonia from MgCl2 which would then combine with CaCl2 in the other box. Now, the system could be considered as charged. When heat is required the box containing CaC12 is slightly heated, this releases the ammonia which would then move to the other box, combine with MgCl2, and release heat at 3000C. This apparently simple system has not been studied in detail.
Japanese scientists had announced a chemical system (Anon. 1981 a) which would store solar energy and release it at very high temperature when silver salts are sprinkled over it. The cost of these chemicals and other details are not known.
Certain salt mixtures melt on heating and then they would release the heat at fairly high temperature when they solidify. The phenomenon is known as the latent heat of fusion. Salt mixtures of NaNO2 and NaOH melt at 2400C and 1 g of this mixture releases 58 cal at 2400C before it solidifies. If the same quantity of heat has to be liberated from oil then about 120 ml of oil will be required (Walton et al. 1977).
Dr. S.D. Sharma (now working at Japan) and his group, working at Indore, (India) along with Dr. Buddhi seem to have answers for people who were keen on storing solar heat for late evening, if not late night cooking. As mentioned earlier there were many attempts earlier but this one uses a commercial grade acetanilide (melting point 118.90 C and latent heat of fusion at 222 kJ/kg.). It was heartening to see that they have used a box type of cooker of 50 x 50cm and 19 cm deep, with three reflectors (in fact I, too, had suggested a three reflector design way back in 1980, but did not build and cook in it). The group used a ball & socket joint on the reflectors to turn them suitably to direct sun light into the box. The stagnant temperature they attain in their box was in the range of 204 C. They use a hollow cylindrical container to keep the phase change material PCM. The inner diameter of the cylinder was 20 cm , the outer 30 cm, and the height 125 cm. They load about 4 kg of PCM. Using suitable aluminium (?) vessels they were able to cook nearly 1.6 kg of food (0.40 kg rice and 1.2 kg water along with water). The food was loaded at about 19.30 hrs and was found well cooked at about 21.30 hrs. The data has been acquired using HP 3852 Data Acquisition system and based on what has been presented, I would say the trio have done a commendable work. Now it has to be seen as to the cost of the unit, its durability, especially the part which holds the PCM, and the things like it. If you are interested in knowing more about it you may visit their site http://www.sciencedirect.com/science/journal/01968904.
Though attractive, and apparently simple, extracting useful heat from this type of system has posed several difficulties. For example, the molten salt would release heat and solidify at the surface, act as insulation, and impair further transfer of heat. Further, the cost of salt to store sufficient heat to cook two square meals a day may also be prohibitive.
Under indirect type cookers, the author has included the biogas digester. Plant products of any type could be used here and, as plants use and store solar energy, the inclusion of this type of cooker should not be out of place. The Government of India has supported this type of cooker and it has solved fuel problems in many a household.
Another inclusion under this category is, solar hydrogen, the fuel of the future. When solar research work was launched here way back in 1979, the author had thought that solar hydrogen would be available at least in developed countries by 1990-95. But this has not been the case even by the end of 1997. There are many hurdles and some of the basic problems are: a) cost of production is still considered not competitive enough with conventional fossil fuels, and b) difficulties with storing the highly explosive gas have not been sorted out.
The popular solar water heater is considerded as another indirect type cooker. Those who find it inconvenient to cook directly in the sun can think of using the solar water heater. Solar heated water could reduce the cost of cooking by about 40%, and at present, it is probably the cheapest method by which to store solar heat. Conventional thermosiphon heaters are a bit costly but a modified box in the box-type solar water heaters could become popular. Several scientists have suggested very promising improvements in the passive box-type solar water heaters.
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