Solar Cookers Update Dr N K Srinivasan Bangalore , India Solar cooker NKS 1
Why use solar cookers? § § § § ü ü You save fire wood, charcoal or cooking gas You cook slowly at low heat…retaining many nutrients Food would taste better Very little capital investment-- inexpensive Easy -portable No smoke, no pollution—avoid lung problems due to smoke Safe to use Enough reasons to try this! Note that 2 billions of world population[6. 5 billion ] use firewood/charcoal /bio-mass only for cooking, especially in thickly populated countries… think harder and promote solar cookers!! Solar cookers are easy to make, easy to use. Solar cooker NKS 2
Basic Principles § Ø Ø Ø Ø Two types of design are used for solar cookers Box type: an insulated box with glass cover---a kind of mini-glass house Mirror type: with flat mirrors to reflect sunlight or parabolic or focusing (trough-like) mirrors You can combine these in ingenious ways, called ‘HYBRID DESIGNS’. : add flat mirrors to box or in mirror type , keep the cooking pot in a box. One elegant design is the “panel “ type. Both have been popular and cost/usage varies You can cook between 9 AM to 3 PM in most tropical and semi-tropical countries for nearly 10 months—leaving the rainy days For quick reference: See Beth Halacy & Dan Halacy “Cooking with the Sun”, also D S Halacy: “Fun with the sun” (1959) Ø Solar cooker NKS 3
Direct and diffuse sun light § § § § Sunlight comes to us in two forms: Direct light that casts sharp shadows Diffuse light , mainly scattered by clouds and the sky…or reflected by nearby walls Box type cookers absorb both Mirror type cookers use only direct light Box type would gather some energy even on cloudy days! Box cookers gather energy even with passing clouds and retain heat longer, after sun set. Solar cooker NKS 4
Box type cookers § Make a wooden/plastic/metal/cardboard box § Paint inside with black paint to absorb sun light energy—keep a metal tray painted black-use poster paint § Have a cover glass/plastic to insulate—the hot pot gives out infra-red rays which cannot pass through the glass …. thus trapping heat… § Keep insulator [coir, peanut shells, newspaper, insulating tiles ] below tray and box and at the sides of the box. § Simple –right! § You can keep a flat mirror hinged on the cover---to throw additional light into the box § It can be slow cooking – 1 to 3 hours § Barbara Kerr made an oven on the Kitchen wall, used from inside the kitchen!!! Solar cooker NKS 5
Mirror type cooker § You can use a curve sheet of polished aluminum or stainless steel or aluminum foil § The solar energy is focused on the cooking pot kept in a frame…blacken the outside of the pot § Simple---yes but one additional work …You have to keep turning the mirror and tilting to focus the light § You have to keep standing near the cooker… § You can also keep an automatic tracker, but that will cost more. § Cooking is done fast with mirror type cooker, typically in one hour § Be carerful. . dont look at the focal spot. . You can hurt your eyes…wear sun-glasses Solar cooker NKS 6
Various types of mirror-cookers § Now we consider different types of cookers § Sun-basket was made in the early 70’s---use papermache shell like a parabolic mirror like a basket and stick small flat mirror on the inside surface as reflector § Keep a metallic curved sheet like a cylindrical mirror and keep the cooking pot in the focusing area. § You can use a simple § Umbrella too. Solar cooker NKS 7
Umbrella Cookers § Many solar designers have built simple, cheap nearparabolic cookers using umbrellas. § Paste the inner cloth of umbrella with aluminum foil or mylar coated with aluminum sheets § Fix a grate like plate to keep the pot at the focus § Keep it in the sun—what could be simpler than this? § Umbrella cooker is just fun on the beach---while you tan, the umbrella gives cooked food! Solar cooker NKS 8
Energy balance § § § How much solar energy you can collect in these cookers? You collect more energy with more area covered by the glass cover or mirrors. What is the amount of energy per square meter? Solar energy is typically 200 watts/square meter, though it could vary from 100 W/sqm to 900 W/sqm [This will vary with your location, cloud cover, dust/smoke in the air…It would be zero at night! Or very cloudy day! In practice take the energy received as 150 watts per square meter. Suppose you use 1 sq. meter area…You get energy of 150 watt-hour in one hour. or 0. 15 KWH or 0. 15 unit You may need 1 to 3 hours to cook your meal!--be patient—depends on the food stuff, quantity and of course, the solar flux. Temperatures reached in solar cookers: 80 to 120 degrees C. It could be as high as 170 deg C. IF IT REACHES 120 deg C[250 F] it is a good design. Most cooking is done between 80 and 110 deg C Thermal efficiency of cookers---40 to 60% For baking and frying, you will need higher temperatures. Effective insulation & absorption by blackened surface inside, enables higher temp Solar cooker NKS 9
What is the ‘Oven temperature’ ? § What do we measure as the oven temperature? § If the oven thermometer is kept in the oven space, it measures the temperature of the air in the oven § You can measure the temperature inside the cooking pot by drilling a hole on the lid and inserting themometer § Or else, measure the outside temp of the cooking pot or the metallic tray kept at the bottom by contact thermometer , say , using a thermo-couple § The cooking pot lid always shows higher temp , due to direct heating from sun’s rays. Solar cooker NKS 10
Box vs Concentrator Cookers § § § § § BOX Easy to make and costs less Slower cooking –takes 2 to 3 hours Requires no attention—leave the pots inside cooker and attend to other work Safe and easy to use § § Bulky and heavy to carry; use caster wheels Uses both direct and diffuse or scattered light from the sun; can operate on cloudy days or with passing clouds Can be modified as a food dryer , water pasteurizer, nut roaster and so on Easy to maintain, repair Box cookers can reach 120 to 150 deg C § § § § Solar cooker CONCENTRATOR Needs tools and higher skills , cost lot more Faster cooking— 30 mins to one hour Needs attention and often adjustments or tracking the sun Protect your eyes; bright spot can irritate eyes or cause blindness Can be light Uses only direct light; useless on cloudy days Cannot be modified for food drying and other uses Difficult to maintain; needs care of reflecting surface Can reach much higher temp, 140 deg C and above. NKS 11
Time to Cook in Box cookers § Easy to cook-Rice, egg, fruit, vegetables, chicken, fish— 1 to 2 hours § Moderate--Root vegetables, potatoes, beans, lentils, dhal, most meat, bread— 3 to 4 hours § Hard---Soup, dried beans, large meat— 5 to 8 hours § ‘minimum’ solar cooker § Cooking is mostly done in the range 80 to 120 deg C Solar cooker NKS 12
Simple cookers for developing countries § There is a great need for solar cookers in hot , underdeveloped or developing nations of the world, mainly to save fire wood and also to save time spent in gathering fire wood. . to reduce deforestation § Think about the rural folk in very poor nations § Also places like refugee camps § In Gaza strip, one inventor has built a solar oven with bricks, mud plaster and two glass windows. Solar cooker NKS 13
New designs at low cost § Several cookers have emerged ---tried in various countries and readily available § Use cardboard panels, with aluminum foil glued to them § Shape several panels –stick them together ---to form roughly a focusing basket. § Keep a black cooking pot , enclosed in a plastic bag! § That is it! You have a simple solar cooker for the masses---in underdeveloped countries. § This simple design –Panel type-is due to Roger Bernard of Univ of Lyons, France. § These panel type cookers have been widely used due to the efforts of SCI Solar cooker NKS 14
Coo. Kit § Here is a simple kit with cardboard panels made out of cardboard sheet [3 feetx 4 feet] § Aluminum foil [‘Reynolds’ Kitchen foil] is pasted on this to form reflectors § A black pot is wrapped in a plastic bag and kept on the cooker. § Simple and elegant ---the Coo. Kit is widely used in many countries due to the efforts of Solar Cooking International [SCI]—including in refugee camps in Africa. See their website: www. solarcooker. org § May be, you can build your own similar cooker to suit your needs. Solar cooker NKS 15
Low cost DIY- Solar Box Cooker § Two cardboard boxes, one inner and the other outer , can be used § Insulate the space between the boxes with dry insulators like dried peanut shell, newspaper , coir [coconut fibre] …. § Cover with glass or plastic sheet § Use cardboard +aluminum foil pasted panel as reflector. . some use lithogaph aluminum printing sheet… § Keep a matt-black steel plate as absorber at the bottom---this is important to absorb all the heat and transfer it to the pots § Use a black cooking pot--use poster paint to color black § You can use glass jars for cooking-paint outside with black paint…also for drying grains/fruits/veggies § Barbara Kerr is the pioneer in these cardboard box cookers § Box solar cookers are more widely used than mirror type --- easy to make and use ! Solar cooker NKS 16
Four reflectors--- slant faced Solar Oven Ø One of the efficient box cooker design is to add four flat reflectors on four sides of the window. . this becomes an oven! Ø The window is slanted to face the sun [towards south for northern hemisphere] Ø Or you tilt the oven Ø This design due to Maria Telkes [formerly a Prof at MIT] had been most efficient and popular Ø Several variations have been made---three reflectors, horizontal tray/cradle inside and others. Ø Keep a cradle inside to position the tray level. Ø If heavy, fit wheels or castors to move Ø Big ones have been made for village/community level cooking. Ø Solar cooker NKS 17
DIY-Home made solar cookers § Excellent designs are available for making your own cookers § Cardboard, aluminum foil, trays, transparent plastic glazing—these are required for your cooker § A detailed video presentation: www. williamgbecker. com/makesolaroven. html § A few technical pointers are given in the next slide Solar cooker NKS 18
Technical points for Box cookers § Use simple insulation material—crumpled newspaper is okay—but you need 25 to 50 mm thickness for effective heat trap § Much heat loss occurs thru glass cover and the sides § A shallow tray with less depth is better than a box with great depth in your box—this reduces side wall area. § Use aluminum or steel tray—Al tray may have less mass but has greater specific heat…so both are Okay § Good sealing is required to avoid leak of hot air. § Tilting the box or slant face is needed for latitude greater than 15 deg---that is subtropical… § For a family of three or four persons, you need a box cooker of at least 4 square feet collector area. § Box cookers collect both direct and diffuse solar rays and therefore better for cloudy skies. § Preheat box cookers by keeping in the sun from 9 AM before placing the pots. § You can use Al foil to keep in the air gap to improve the insulation. Test the cooker with actual cooking conditions with food stuff weighed, noting time and temperature. For very hot regions, reflectors may not be needed. Keep your cooker protected from the wind, adjacent to a wall or cover. Solar cooker NKS 19
Minimal insulation box cookers § § § § Standard cookers , including commercial ones have 1 to 2 inches of insuation--glass fibre, wool, foam material, crumpled newspaper and so on. Some pundits use only thin insualtion, about 1 in , with aluminum foil, cardboard barriers, dividers to reduce the convection in air space…after all air itself isa good insulator and small pockets of air spaces help…Al foil reflect the heat back into the inner box…[called ‘reflective barriers’] It is possible to have thin insulation with aluminum foil and cardboard pieces for cookers operating in very bright sun light, most probably in tropical regions. Thin insulation reduces the weight and size of the box cookers---an advantage for senior citizens and balcony users I would prefer thick insulation about 2 inches—for using even less bright days and for quick heating and slow cooling. Note that radiation is less important mode of heat loss , compared to convection or conduction losses in box cookers, because of low temperatures…. Amount of heat by radiation goes by T^4, where T is the temperature in Kelvin…For low temp, this factor would be less. [Stefan-Boltzmann law] Note , however, the radiation loss is almost double for a cooking pot at 100 deg. C, compared to the same pot at 50 deg C…therefore radiation losses affect the max temp attained. Solar cooker NKS 20
My “Box-pan” hybrid cooker § § § § Box cookers are simple to make, easy to us---, hence popular. Panel cookers throw more energy, faster to cook but needs orientation Box cookers are bulky and often heavy; panels are light weight, foldable. . I made hybrid called “Box-Pan “ cooker, easy to use, light weight and small Use a box, with slant face, keep back reflector. The box is not glazed as a whole… Keep a large metal tray, blackened at the bottom. The bottom insulation below the tray is a wad of newspaper. Cover this tray with oven bag or plastic cover Keep blackened pots inside the bag… Orient the box as you do for box cookers! You have a simple, light weight box cooker with quick heating of panel cookers…! Try this and you will love it…”Box-pan” will be your constant companion. !! Give me your feedback on “BOX_PAN” please!! Solar cooker NKS 21
Other uses for Solar Box Cookers § Pasteurize water---heat to 65 deg C and cool § Roast nuts and seeds—I use regularly to roast peanuts ! § Make pickles in glass jars kept in the box § Toast bread to the degree of browning § Dry fruits/veggies by keeping the lid partially open or providing vents at the sides § Making candies with nuts added Solar cooker NKS 22
Solar Cooking History § § § § Early pioneers: Horace de Saussure cooked fruits in 1767 John Herschel and Langley (USA) cooked food with box type cooker Mouchot did cooking with concentrating mirrors & wrote a book Charles Abbot of Smithsonian Inst made a concentrating cooker with mirror heating oil and cooking later in the evenings. M K Ghosh, an Indian, made a box cooker in 1945 Maria Telkes made cookers of different designs using four mirrors attached to a box type oven at MIT[1950’s] Renewed interests after 1980 Barbara Kerr developed a simple low cost box-type cooker Robert Metcalf developed water pasteurization using a solar cooker 1987 marked the founding of Solar Cooker International [SCI] by Kerr. Cole-Metcalf team in Sacramento, California Univ of Chile popularized solar cookers in a village in Chile In 1990, SCI developed the Coo. Kit from the earlier design of Roger Barnard of France. Coo. Kit has been used by more than 30000 refugees in Africa & elsewhere. Solar cooker NKS Since 2005, commercial solar cooker sales has gone up 5 times…. 23
Countries using Solar cooking § § § § African nations China India Sri Lanka Nepal Chile/Peru/Mexico…. . Israel/Palestine & others Solar cooker NKS 24
Home-made vs commercial cookers § Home –made § § Easy to make Very low cost Lot of fun Learn basic principles of energy devices and teach to others…you learn energy science § Use local materials and skills § Can change design and adopt to local requirements § Can start your own small business! § Commercial § Expensive [$100 to $300] § Built to last [some say 20 years] § More efficient/Easy to use § Standard designs/Performance specified § No learning experience § Mass produced in thousands § Can be locally manufactured in a small unit/generates employment Solar cooker NKS 25
Some commercial Box cookers § Global Sun Oven---with four large reflectors, largest selling ones— 14 inx 14 in area… § Sport-SOS---light weight, with or without reflectors, large cooking area, developed by Solar Oven Society as non-profit venture, acrylic glazing § Hot Pot-with several reflectors--single , cooking pot kept in a glass bowl—developed and tested by Florida Solar energy center…, made in Mexico, less expensive § Sun Stove --used in south Africa, India and elsewhere—low cost, uses litho printing plates , no reflectors § See the websites for details , comparison and pricing § Tulsi solar oven—original Indian design—hybrid , that is , has electrical heating too, comes like a small suitcase. Solar cooker NKS 26
My home-made box cooker § This is small---12 in x 10 in glass cover from picture frame, two cardboard boxes… § Outside insulation with styrofoam[thermocole], inner insulation with crumpled paper &al foil § Reflectors—cardboard pasted with Al foil § No tilting § Inside box—lined with Al foil. The bottom with a black enamelled steel tray § Important—check for air leaks and seal them. § Reaches 110 deg C in 30 mins in Bangalore, India in a cloudless day § Can cook with one or two pots, filled with 250 gms of food stuff… § Weight—about 3 Kg only § Used for roasting peanuts and toasting bread almost daily. § Can be placed in a balcony for urban people § Also made a larger one 16 inx 13 in –similar design Solar cooker NKS 27
General Remarks § Solar cookers have not been very popular among the rural people of even developing countries—they are too expensive for them, often inconvenient to use. § In India very small percentage of rural people use these cookers— total about 0. 6 million cookers. § Most people do not see the environmental connection of firewood burning, losing trees and carbon pollution. § Low cost card-board made cookers with aluminum foil and panel cookers like Coo. Kit may change the scenario § With card-board made cookers, solar cookers could become very popular among the rural people of several countries. —that is the hope! § There is tremendous interest in solar cookers now---to reduce carbon emissions and many African nations are serious about this. Solar cooker NKS 28
Contact & some references K § Dr Srinivasan Nenmeli § nksrinivasan@hotmail. com § www. solardevicesforthirdworld. webs. com § Kerr-Cole Solar Box Cookers Po Box 576, Taylor, Az 85939 , USA Ø WWW. solarcookers. org Ø Ref: The Expanding world of Solar Box cooker—Barbara Kerr, 1991 Ø www. backwoodshome. com [see Joe Radabaugh’s solar cooker design] Ø www. solarcookeratcatinawest. com Ø Read “Solar Cooking review ‘ from SCI---available online. Solar cooker NKS 29
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