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The Incredible Solar Bread Machine

Published: 01-04-2011
Source: JILA Scientific Communications

Alan Gallagher’s prototype solar frying pan assembly. The assembly includes a safe and efficient flat-paneled mirror and a low heat-loss pan with a nonstick anodized-aluminum cooking surface. Credit: Alan Gallagher

After he retired, Fellow Alan Gallagher decided to take his interest in solar energy in a whole new direction: He decided to design, build, and test a unique large-area frying pan heated by the Sun’s energy. The new solar frying pan was specifically tailored to the cooking of injera bread in East Africa.For more than 100 million people in East Africa, the thin, flexible, and pancakelike bread is a mainstay of their diet. It is usually eaten with a variety of thick sauces spread around the top of a large (~0.4 m diameter) slice, which serves as a shared “dinner plate.” Once the sauces are consumed, the diners eat the sauce-soaked plate.In villages, a typical “slice” of injera bread can be more than half a meter in diameter. The bread is cooked for about 2 minutes in a covered pan preheated to ~180 °C. When wood is used to cook this bread, cooking contributes to severe forest and environmental degradation because of the high demand for wood. In contrast, solar cooking would benefit the local environment by (1) decreasing deforestation and desertification, (2) curtailing the generation of particulates and greenhouse gases, and (3) reducing the health hazards associated with indoor-fire cooking. Solar cooking would also lower the cost of gathering or purchasing firewood.To encourage East Africans to save on the cost of firewood, Gallagher designed his solar frying pan to be efficient, safe, easy to construct, and inexpensive. For example, the mirror consists of flat hexagonal panels of aluminized Mylar inside of a parabolic framework. The flat panels uniformly illuminate (evenly heat) the bottom of the frying pan, which sits above the mirror. The flat panels make this mirror safe to use for cooking because they don’t ever concentrate the sun in just one place, which can burn the cooks or start fires.The pan bottom is coated with a special black surface that only emits very low levels of radiation. The black surface keeps the fryer from losing as much as half the power it gets from the Sun.  The pan’s nonstick surface is made from anodized aluminum whose pores are filled with cooking oil. On a sunny day, the pan preheats to 180 °C in 15-20 minutes. It can cook slices of injera bread as large as 0.42 m in diameter.The solar fryer can be used eight hours a day throughout the year. An easy-to-use mirror adjustment allows users to track the Sun during the day, and a mounting adjustment makes it possible to correct for seasonal variations in the position of the Sun. The prototype solar fryer provides about 640 W of heating power and can cook about 32 kg of bread per day, or enough bread to feed ~160 people.The large cooking capacity allows multiple options for using the solar fryer: (1) Several families could chip in to buy one and share it, (2) The owner of the solar fryer could sell cooked bread or cooking time as a means of paying for the device, or (3) a restaurant might purchase one to save on (wood) fuel costs.The retail cost of the materials (in the United States) to construct the solar fryer is about $100 dollars, but the estimated cost for bulk manufacturing is only about $25. Most of the construction requires only hand tools, which is ideal for local production in East Africa. The design is scalable, making it relatively easy to construct smaller or larger fryers, if desired. - Julie PhillipsReferenceAlan Gallagher, Solar Energy, in press.

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