Solar Cooking AKS demonstrate knowledge of scientific processes and inquiry methods (GPS, ITBS) Third (3SC_A2006-2) apply computation and estimation skills necessary for analyzing data and following Grade scientific explanations (GPS, ITBS) (3SC_A2006-3) use tools and instruments for observing, measuring and manipulating objects in scientific activities utilizing safe laboratory procedures (GPS, ITBS) (3SC_A2006-4) communicate scientific ideas and activities clearly (GPS, ITBS) (3SC_A2006-6) explain how heat is produced and the effects of heating and cooling (GPS, ITBS) (3SC_C2006-10) demonstrate knowledge of scientific processes and inquiry methods (GPS, ITBS) Fourth (4SC_A2006-2) apply computation and estimation skills necessary for analyzing data and following Grade scientific explanations (GPS, ITBS) (4SC_A2006-3) use tools and instruments for observing, measuring and manipulating objects in scientific activities utilizing safe laboratory procedures (GPS, ITBS) (4SC_A2006-4) communicate scientific ideas and activities clearly (GPS, ITBS) (4SC_A2006-6) analyze weather charts/maps and collect weather data to predict weather events and infer patterns and seasonal changes (GPS, ITBS) (4SC_B2006-11) investigate the nature of light using tools (mirrors, lenses, prisms) (GPS, ITBS) (4SC_C2006-12) describe the roles of organisms and the flow of energy within an ecosystem (GPS, ITBS) (4SC_D2006-15) demonstrate knowledge of scientific processes and inquiry methods (GPS, ITBS) Fifth (5SC_A2006-2) apply computation and estimation skills necessary for analyzing data and following Grade scientific explanations (GPS, ITBS) (5SC_A2006-3) use tools and instruments for observing, measuring and manipulating objects in scientific activities utilizing safe laboratory procedures (GPS, ITBS) (5SC_A2006-4) communicate scientific ideas and activities clearly (GPS, ITBS) (5SC_A2006-6) distinguish between physical changes and chemical changes (GPS, ITBS) (5SC_C2006-10) Essential Question: How can alternative and sustainable energy resources advance technological systems? Solar Cooking Basics Most solar cookers work on basic principles: sunlight is converted to heat energy that is retained for cooking. Fuel: Sunlight is the "fuel." A solar cooker needs an outdoor spot that is sunny for several hours and protected from strong wind, and where food will be safe. Solar cookers don't work at night or on cloudy days, though during the best months for cooking, many foods can be cooked under intermittent clouds or a light haze, as long as food is put out early and there is definitely more sun than not overall. Extra covers or simple foiled boosters can help under marginal skies. Convert sunlight to heat energy Dark surfaces get very hot in sunlight, whereas light surfaces don't. Food cooks best in dark, shallow, thin metal pots with dark, tight-fitting lids to hold in heat and moisture. Retain heat A transparent heat trap around the dark pot lets in sunlight, but keeps in the heat. This is a clear, heat-resistant plastic bag or large inverted glass bowl (in panel cookers) or an insulated box with a glass or plastic window (in box cookers). Curved concentrator cookers typically don't require a heat trap. Capture extra sunlight energy One or more shiny surfaces reflect extra sunlight onto the pot, increasing its heat potential. The three most common types of solar cookers are heat-trap boxes, curved concentrators (parabolics) and panel cookers. Hundreds — if not thousands — of variations on these basic types exist. Additionally, several large-scale solar cooking systems have been developed to meet the needs of institutions worldwide. Box Cookers Curved Concentrator Panel Cookers Cookers Panel cookers incorporate Curved concentrator cookers, elements of box and curved Box cookers cook at moderate or "parabolics," cook fast at concentrator cookers. They to high temperatures and often are simple and relatively accommodate multiple pots. high temperatures, but require inexpensive to produce. Worldwide, they are the most frequent adjustment and widespread. supervision for safe operation. Great “Solar” Websites General Info http://www.solarcookers.org http://solarcooking.wikia.com/wiki/Introduction_to_solar_cooking http://solarcooking.wikia.com/wiki/Solar_Cooking:Frequently-asked_questions http://www.fsec.ucf.edu/en/education/k-12/curricula/index.htm http://www.fsec.ucf.edu/en/education/k-12/events/solar_cookoff/index.htm http://solarcookers.org/basics/health.html http://solarcooking.org/Solar-Ovens-for-Developing-Countries.htm http://solarcooking.wikia.com/wiki/Where_is_solar_cooking_possible%3F Cooker Plans http://solarcooking.org/plans/default.htm http://solarcooking.wikia.com/wiki/Category:Solar_cooker_designs http://media.fsec.ucf.edu/photos/2307-2009EnergyWhizOlympics/SolarCook- Off2009/index.html (examples of the 2009 solar cookers in Florida) http://www.solarnow.org/pizzabx.htm (pizza box design) http://www.energyquest.ca.gov/projects/solardogs.html (hot dog cooker) http://pbskids.org/zoom/activities/sci/solarcookers.html http://www.backwoodshome.com/articles/radabaugh30.html http://www.ethicurean.com/2007/08/14/solar-oven-1/ Recipes http://solarcooking.wikia.com/wiki/Recipes http://www.cookwiththesun.com/recipes.htm http://www.solarovens.org/recipes/ http://www.solarcooker-at-cantinawest.com/solarcookingrecipes.html http://www.solargadgetsinfo.com/solar-cooker-recipes.html Solar Curricula http://www.fsec.ucf.edu/en/education/k-12/curricula/index.htm (click on Solar Matters I, II, or III) http://astro.uchicago.edu/yerkes/outreach/activities/yaays-sa08- see/ActivitySolarCookers/YAAYS_Solar_Cooker.pdf http://www.solarenergy.org/students-and-educators Principles of Solar Box Cooker Design By Mark Aalfs, Solar Cookers International e-mail: firstname.lastname@example.org The purpose of this paper is to summarize the basic principles that are used in the design of solar box cookers. People use solar cookers primarily to cook food and pasteurize water, although additional uses are continually being developed. Numerous factors including access to materials, availability of traditional cooking fuels, climate, food preferences, cultural factors, and technical capabilities, affect people's approach to solar cooking. With an understanding of basic principles of solar energy and access to simple materials such as cardboard, aluminum foil, and glass, one can build an effective solar cooking device. This paper outlines the basic principles of solar box cooker design and identifies a broad range of potentially useful construction materials. These principles are presented in general terms so that they are applicable to a wide variety of design problems. Whether the need is to cook food, pasteurize water, or dry fish or grain; the basic principles of solar, heat transfer, and materials apply. We look forward to the application of a wide variety of materials and techniques as people make direct use of the sun's energy. The following are the general concepts relevant to the design or modification of a solar box cooker: Heat Principles Materials Requirements Design and Proportion Solar Box Cooker Operation Cultural Factors HEAT PRINCIPLES The basic purpose of a solar box cooker is to heat things up - cook food, purify water, and sterilize instruments - to mention a few. A solar box cooks because the interior of the box is heated by the energy of the sun. Sunlight, both direct and reflected, enters the solar box through the glass or plastic top. It turns to heat energy when it is absorbed by the dark absorber plate and cooking pots. This heat input causes the temperature inside of the solar box cooker to rise until the heat loss of the cooker is equal to the solar heat gain. Temperatures sufficient for cooking food and pasteurizing water are easily achieved. Given two boxes that have the same heat retention capabilities, the one that has more gain, from stronger sunlight or additional sunlight via a reflector, will be hotter inside. Given two boxes that have equal heat gain, the one that has more heat retention capabilities - better insulated walls, bottom, and top - will reach a higher interior temperature. The following heating principles will be considered first: Heat gain Heat loss Heat storage Heat Principles: Heat gain, Heat loss, Heat storage | Materials Requirements Design and Proportion | Solar Box Cooker Operation | Cultural Factors | To top A. Heat gain Greenhouse effect: This effect results in the heating of enclosed spaces into which the sun shines through a transparent material such as glass or plastic. Visible light easily passes through the glass and is absorbed and reflected by materials within the enclosed space. The light energy that is absorbed by dark pots and the dark absorber plate underneath the pots is converted into longer wavelength heat energy and radiates from the interior materials. Most of this radiant energy, because it is of a longer wavelength, cannot pass back out through the glass and is therefore trapped within the enclosed space. The reflected light is either absorbed by other materials within the space or, because it doesn't change wavelength, passes back out through the glass. Critical to solar cooker performance, the heat that is collected by the dark metal absorber plate and pots is conducted through those materials to heat and cook the food. Glass orientation: The more directly the glass faces the sun, the greater the solar heat gain. Although the glass is the same size on box 1 and box 2, more sun shines through the glass on box 2 because it faces the sun more directly. Note that box 2 also has more wall area through which to lose heat. Reflectors, additional gain: Single or multiple reflectors bounce additional sunlight through the glass and into the solar box. This additional input of solar energy results in higher cooker temperatures. Heat Principles: Heat gain, Heat loss, Heat storage | Materials Requirements Design and Proportion | Solar Box Cooker Operation | Cultural Factors | To top B. Heat loss The Second Law of Thermodynamics states that heat always travels from hot to cold. Heat within a solar box cooker is lost in three fundamental ways: Conduction, Radiation, and Convection Conduction: The handle of a metal pan on a stove or fire becomes hot through the transfer of heat from the fire through the materials of the pan, to the materials of the handle. In the same way, heat within a solar box is lost when it travels through the molecules of tin foil, glass, cardboard, air, and insulation, to the air outside of the box. The solar heated absorber plate conducts heat to the bottoms of the pots. To prevent loss of this heat via conduction through the bottom of the cooker, the absorber plate is raised from the bottom using small insulating spacers as in figure 6. Radiation: Things that are warm or hot -- fires, stoves, or pots and food within a solar box cooker -- give off heat waves, or radiate heat to their surroundings. These heat waves are radiated from warm objects through air or space. Most of the radiant heat given off by the warm pots within a solar box is reflected from the foil and glass back to the pots and bottom tray. Although the transparent glazings do trap most of the radiant heat, some does escape directly through the glazing. Glass traps radiant heat better than most plastics. Convection: Molecules of air move in and out of the box through cracks. They convect. Heated air molecules within a solar box escape, primarily through the cracks around the top lid, a side "oven door" opening, or construction imperfections. Cooler air from outside the box also enters through these openings. C. Heat storage: As the density and weight of the materials within the insulated shell of a solar box cooker increase, the capacity of the box to hold heat increases. The interior of a box including heavy materials such as rocks, bricks, heavy pans, water, or heavy foods will take longer to heat up because of this additional heat storage capacity. The incoming energy is stored as heat in these heavy materials, slowing down the heating of the air in the box. These dense materials, charged with heat, will radiate that heat within the box, keeping it warm for a longer period at the day's end. Food Safety and Solar Cooking Food safety for food cooked by any method requires meeting specific rigid conditions. Cooked food at temperatures between 125° F and 50° F (52° C - 10° C) can grow harmful bacteria. This temperature range is known as the danger zone. To protect against food poisoning, microbiologists and home economists strongly recommend that food be kept either above or below these temperatures. These precautions are the same whether food is cooked with gas, electricity, microwaves, wood fire, or solar heat as well as foods cooked by retained heat, crock pot, barbecue pit or any other method. In cooked food held at room temperature, there is a chance of Bacillus cereus food poisoning, a major intestinal illness. Worse, if the food is not thoroughly reheated before consumption, there is a chance of deadly botulism poisoning or salmonella. Even if it is reheated, when cooked food has been in the danger zone for three to four hours, there remains a risk of food poisoning in solar cooked food as in food cooked by any other method. It has been carefully documented with regard to solar box cookers that it is safe to place raw refrigerated or frozen food, even chicken or other meat, in a solar box cooker (SBC) in the morning several hours before the sun begins to cook it. Refrigerated food placed in an SBC remains sufficiently cold until the sun starts to heat the SBC. Once the full sun is on the oven, the heating of food proceeds quickly enough so that there is no danger of food poisoning. Uncooked grains, beans and other dried raw foods can also be placed in an SBC in advance. Both of these methods facilitate absentee cooking. There are three main points at which caution is required: it is dangerous to keep cooked food more than three or four hours in an unheated or cooling SBC unless both the SBC and food have been cooled rather quickly to below 50° F (10° C) in which case the SBC is serving as a cool box; it is dangerous to let cooked food remain overnight in an SBC unless it is likewise cooled; and it is dangerous for food to partially cook and then remain warm in the SBC when temperatures are not sustained as might occur on a poor solar cooking day, at the end of the day or when clouds move in. Cooked or partially cooked food should either be cooled to below 50° F (10° C) or cooking should be finished with an alternate fuel. If food has remained in the temperature danger zone for 3 to 4 hours it should be considered spoiled and should be discarded. Reheating the food does not correct the problem as heat does not inactivate all toxins. Food does not have to be visibly spoiled in order to be toxic and cause illness evidenced by nausea, vomiting and diarrhea. Even if food has not been at the incubating temperatures of the danger zone for the full 3 to 4 hours, absolutely discard food that is bubbling, foaming, has a bad smell, is becoming discolored, or gives any other indication of spoilage. Discard it out of reach of animals and children and thoroughly wash the pot. Discard it without tasting it as even small amounts can make an adult very sick. If temperatures below 50° F (10° C) cannot be obtained, it is still valuable to drop food temperatures as low as possible and as quickly as possible rather than allowing food to remain warm since bacteria grow more slowly at lower temperatures. An alternative method of holding cooked food is to reliably maintain the temperature of the entire food mass above 125° F (53° C). This can be achieved by first heating the food to boiling, simmering for a few minutes to allow heat to penetrate to the center of each particle and for a pocket of steam to collect under the lid. Then proceed as for retained heat cooking. This provides the level of temperature needed throughout the food, whereas leaving a pot of food on a very small flame may allow food at the edges to remain in the danger zone. Where neither of these methods can be used, it is best to cook amounts of food that will be consumed in one meal relatively soon after being cooked. This article was excerpted from The Expanding World of Solar Box Cooking, by Barbara Kerr.