Home Power's Indigenous Materials Cooker Contest Kathleen Jarschke-Schultze c.1992 Kathleen Jarschke-Schultze The sun shines on the rich and poor, hungry and well-fed alike. In the United States, a growing number use the sun's energy to cook food, with solar cookers built from scrap and low cost materials, such as cardboard, foil, and glass. What are some low cost or scrap materials in other countries that could be used to make solar cookers? In Home Power issue #28, we asked readers to design and build a solar cooker using materials readily available in a developing country of their choice. We received numerous phone calls; eight entries made their way to HP Central. Alan Nichols sent his design for a tracking solar cooker. Another reader sent a sample of fiber cement that could be formed into walls for a cooker. Philip Hodes' simple waterproof cooker required a plastic milk crate, plastic mirrors for reflectors, and foil-backed foam for insulation. We chose four cooker designs to build for the cookoff Saturday at the Solar Energy Expo and Rally 1992 in Willits, California. The four finalists were chosen based on their use of simple, "low tech" materials and included a bamboo-type box cooker, a hole-in-the-ground model, a parabolic-type design, and a foldable cooker. Judgment Day We built the four finalists' models from their instructions.The top four designs were judged on validity of materials, ease of assembly, clear instructions, ruggedness, beauty of design, and ability to cook food. Each cooker held a yam, and equal amounts of black beans and brown rice cooked in black painted jars. The cookers were placed in the sun at 10:30 am and adjusted throughout the day until 3:00 pm. Our four judges were Paul Mellersh, Board of Directors SBCI; Johnny Weiss and Felicia Trevor of Solar Technology Institute; and myself. C. Jay Campbell's hole-in-the-ground design took 1st place, winning a Solarex MSX-60 solar panel. Michael Diogo placed 2nd with his carrizo cooker, winning a PowerStar 200 inverter. Maria Gonzales' foldable design won 3rd place, and Lu Yoder's frustum-based model placed 4th; they chose either an Osram compact fluorescent light or a Kyocera Jetski PV module as their prize. The Top Four Jay Campbell's design, targeted for Guatemala, was beautifully simple. His cooker required a hole in the ground insulated with newspaper, and a conical reflector to concentrate the sun's rays onto the black plastic pan holding the cooking pot. Jay used a junked car's side window for glazing and fashioned reflectors from cardboard and aluminum foil. Jay's design scored high on all criteria; the lowest scores were for ruggedness, because of the cardboard elements. Judges' comments, "Good instructions, could be totally pictorial, maintained heat well." Overall score: 258.5 Michael Diogo, from Baja California, Mexico, scored high in material use, ruggedness and clear instructions with his box cooker built from carrizo (a native plant similar to bamboo). He wired lengths of carrizo to make the walls and floor of the interior and exterior box. Dried grass was stuffed between the boxes for insulation. The interior box was daubed with mud and black magnetic sand was poured on the bottom. Michael removed the bottoms of over 100 bottles before finally succumbing to sheet glass for glazing. For reflectors, he cut open rectangular tin cans and banged them flat. Building the cooker was labor intensive. Judges' comments, "Very imaginative and elegant in design." Overall score: 202.5 Maria Gonzales' triangular cooker uses velcro straps so it unfolds flat for travel or storage. She uses cardboard for the interior and exterior boxes, and adds foil and glass to the interior box which holds the cooking pot. The insulation between the boxes can be a blanket, newspapers, or whatever is on hand. Maria's cooker consistently scored high on ruggedness and beauty of design. Judge's comments, "Great idea, may need to be tilted back in countries close to the equator. Clean design." Overall score: 185.5 Lu Yoder wrote that since he'd never been to a developing country, his Liberation Technology: "no weld" solar cooker design was made from materials readily scavenged from an Albuquerque, New Mexico barrio. Tools and supplies were bartered and bought at the local flea market. He used three frustums, or cone reflectors to approximate a parabola. A metal conduit frame support the aluminum foil and cardboard reflectors. Lu wrote that there are rich and poor in all countries of the world. "The poor in both countries stand to benefit very much from technology which partly frees them from the toil of gathering fuel and destroying their own ecosystems...." He pointed out that the world's resources would be most affected if we changed our cooking habits in the United States. "Solar cookers made from secondary and low cost materials have the potential to help people in all parts of the world struggle for economic justice." While scoring high on most criteria, Lu's design scored low on ease of assembly and clear instructions. Judges' comments, "Attained highest temperature, instructions hard to understand." Overall score: 161 FOUR PHOTOS 1/4 PG FROM THE CONTESTANTS The Winning Design As promised, here are the plans of the winning design by Jay Campbell. Jay has travelled extensively to Guatemala. "On my recent visits, however, I have become very disturbed by the ever rising tree line around the cities. The hills are literally bald up to a certain altitude. As heating is only an issue in the highlands, much of the tree loss is due to cooking. "Guatemala has a pleasant, springlike climate year round....Even during the rainy season, the sun shines most of the day, with about 3 hours of cloud cover. This pattern is typical throughout the interior of Mexico and Central America. "Guatemala has a well developed plastics manufacturing sector. All types of plastic containers, bags, toys and household items are available at low prices in the many village markets. One of the most ubiquitous items is known as a PALANGANA (pronounced just like it looks, accent on the PA). It resembles a common oil change pan here, but is far more than that. Bathing, food preparation, laundry, storage, and coffee picking are typical uses. Two small ones suspended from either end of a stick forms the standard market scale. They come in a variety of sizes, cost from $0.50 to $2.50 U.S., and are used in every household. The palangana is truly an indigenous part of Guatemalan life. "Construction time for the prototype was 6 hours. Total cost as built was $2.75. Maximum temperature witnessed was 150øC (300øF), but the temperature was still climbing at this point. Time to bring 1 liter of 20øC water to a full boil was 61 minutes. As designed, there is a maximum 4:1 ratio concentration of incoming radiation. When pointed at the sun, this would provide over 1000 BTU's per hour. Based on the boil test, about a third of that actually gets into the food. In actual use, 1 1/2 liters of black beans (the staple food in Guatemala) cook nicely when left unattended for the workday. Rationale "The heart of this cooker is a black palangana. The oil drain pan I bought in the U.S. is a little thinner and shallower than standard, but worked well with a cardboard heat shield in the bottom. In country, I would use a larger version to increase the volume. The glass used is from the side window on a junked car, another common item in the country. Standard window glass would work fine, but would probably cost more. All other materialsÄ cardboard, foil, glue, string, and newspapersÄare readily available in any population center in the country, for a low total cost. There is no hardware required, as the glass slides in and out of the cardboard frame like a drawer. "The conical reflector captures just as much energy as the same sized parabolic reflector. The difference is that where the cone reflects all light into a relatively wide area, the parabola reflects it all into a single point. For food preparation, the wider area is preferable. An inclined base is used to correct for both latitude and seasonal changes. For anywhere out of the equatorial region (ñ10ø), the tilting spacer is worth the effort. It can double the amount of incident radiation, and allows for tracking the sun. The tilt angle in the photo (22ø) was built for my latitude (34ø N), and should work well in Willits in August. For use in Guatemala, the tilt angle should be needed only during October-March, and would be 26ø. "Geometrically, a circle is the most efficient shape for a container, having the maximum possible area inside for a given amount of perimeter. What this means for cooker designs is that a maximum of sunlight will enter the oven while a minimum of heat will be lost through the sides. Also, the circular reflector is a good concentratorÄby doubling the diameter, the energy input is quadrupled. The circular geometry maximizes the energy input for a given quantity of materials. "I must justify the use of 'high tech' foil. lt is widely available, and used in such small quantity that a single roll can make 9-10 reflectors. Split open aluminum cans (also widely available) worked about as well, but are very labor intensive to prepare. They are available for free, however. The stated goal of this contest is to use local materials. For Guatemala, foil is such an item. "Another feature is the outer boxÄjust a hole in the ground. Some siting considerations must be made (shading, local elevation, drainage), but no more than for other types of solar cookers. The main advantage is that almost anybody can afford a hole in the ground. A lining would be recommended for long term use, but is not essential. Tightly crumpled newspaper provides the insulation between the palangana and the ground. Newspaper may not be the best insulator, but by making the hole a little bigger and adding more paper, it can have a competitive R value with any insulated box. Materials and Tools Materials include: a palangana or shallow plastic pan, cardboard, foil, glue, string, glass, and newspapers. Tools: Sharp stout knife, sharp stick, straight edge, pencil, shovel, and instructions. Construction "Obtain palangana and a piece of glass which will completely cover it.... I recommend an 18-24 inch diameter pan for sufficient volume. Directions are given based on whatever sized parts you can acquire. Cone "Get a large piece of cardboard, or make one out of several smaller pieces. Lay out a [string] as long as 4 diameters of the tub. See Figure 1. Draw an arc from the center of the line, connecting the two ends, and cut out. Lay out a similar arc [1 diameter smaller], and cut it. "Cut (score) the surface layer of the arc as shown in Figure 2 so that it can be rolled into a faceted cone. On the same side, score an arc near the two edges, and push a string into this cut. By pulling the strings tight, the cone will cinch up like a barrel. Paste foil completely over the unscored side and edges, and trim off excess. "Now pull up the strings and tie them off. Cut a ring to fit around the small end of the cone, as shown in Figure 3. Glue this in place. Punch 8 small holes spaced evenly around the ring. The cone is now done. This simple geometric layout produces a perfect 60ø cone for any sized palangana, which will give a 4:1 concentration of incoming radiation. A 60ø cone is not the optimal angle, but is close. Due to its simple pattern, however, it cries out to be used for this application. INSERT FIGURES 1,2 & 3 HERE Tilt Angle "Lay out another line 4 diameters long. Cut and fold the pattern shown below, then glue into a square (a little tab helps). The tilt angle should optimally be the latitude of the site, for year round use. Two different ones could be used to improve the efficiency, one for March 21-September 20 (Latitude minus 12ø) and one for September 21-March 20 (Latitude plus 12ø). Punch 2 small holes at the bottom of each side, as shown below. Turn the cone upside down and set the angle on top. Thread a string through the holes in both the cone and the base to tie them firmly together. INSERT FIGURES 4 & 5 HERE Frame "[This is] a drawer slide. The glass will go in and out one edge, and seal on the top, bottom, and other edges. It must be made for a specific piece of glass in order to seal well. In a large piece of cardboard, cut a hole to the size of the palangana body. The [pan] should fit completely inside, with the lip seated well on the cardboard. See Figure 6. Stack up cardboard to be slightly above the lip. Set the glass on top of this buildup, centered over the palangana. Cut strips of cardboard to outline the glass. Cut a final piece to cover the whole stack. Cut a round hole the size of the palangana in the top piece. Once all pieces have been dry fit, glue them together as assembled. The glass should slide freely, but should not be loose. Use one of the cutout holes as a heat shield at the bottom of the palangana. This will help diffuse the concentrated energy which could damage the plastic. Also, the piece just below the glass can be made to any thickness, making the cooking volume larger. INSERT FIGURE 6 HERE PHOTO FROM JAY Assembly "Dig a round hole, about 10 inches larger in diameter than the palangana. Level out the ground around the hole. Place frame over hole, without glass or palangana. Pack newspaper around the inside of the hole; stepping on it and stuffing as much as possible without interfering with the palangana. Place in palangana, slide in glass and set cone assembly on top. The reflector can be weighted down with rocks around the base, or by tying 3 tethers to stakes in the ground. High winds are not a real problem in the interior of Guatemala, so only rocks were used during testing. INSERT THE SECOND FIGURE 6 (WHICH IS REALLY FIGURE 7) HERE Use "Tip the reflector onto its side. Slide the glass back and put in the food. Slide glass back snugly into frame, and replace the reflector. The reflector can be rotated to follow the sun without disturbing the food or cooker. It's important to tip the reflector for access, to avoid looking straight into the cone. To fully utilize the volume advantages of this design, round cookware should be used. Conclusion "This is a simple, inexpensive, rugged cooker, easily constructed of local materials. lt can meet the cooking needs of a typical family in Guatemala throughout much of the year. "I claim no financial interest in this design. Anyone is free to duplicate, distribute or modify it at will. Covering expenses is reasonable, but I only request that it not be produced for a profit." Contest Conclusions C. Jay Campbell's cooker was taken to an Earth Stewards/Peace Tree gathering and shared with people from fourteen different countries. The plans will be made available to all who wish to help spread the design to indigenous people everywhere. Congratulations to all of our entrants for your time and creativity! For you readers who had an idea for a solar cooker, but did not think you had enough time to develop one, there is always next year's competition. Look for the details in the next issue of Home Power. Go for it. Access Author: Kathleen Jarschke-Schultze, c/o Home Power, POB 130, Hornbrook, CA 96044 C. Jay Campbell, Applied Engineering, 218 Dartmouth SE, Albuquerque, NM 87106-2220 ù 505-848-7674 ù 505-256-1261 Michael Diogo, c/o Bill Keys, 8111 Stanford, #159, Garden Grove, CA 92641 Maria C. Gonzales, 48 Sycamore #3, San Francisco, CA 94110 Lu Yoder, 315 B Edith SE, Albuquerque, NM 87102 ù 505-242-8111 Milk crate oven: H. Philip Hodes, 3137 Capri Rd., Palm Beach Garden, FL 33410 Tracking solar oven (plans available for $2): Alan Nichols, 4220 N. Bear Canyon Rd., Tucson, AZ 85749