Disclaimer: The drawings, procedures, and words on this site are for information purposes only. No claims are expressed or implied as to the safety, usefulness, or accuracy of this information. This site does not contain recommendations or actual plans for building a Heat Retention Solar Oven. This particular solar oven design is theoretical and experimental.
- Disclaimer: The drawings, procedures, and words on this site are for information purposes only. No claims are expressed or implied as to the safety, usefulness, or accuracy of this information. This site does not contain recommendations or actual plans for building a Heat Retention Solar Oven. This particular solar oven design is theoretical and experimental.
- The base of the oven should be set above ground level, so that rain water does not enter the oven.
- The bottom and sides of the oven must be very well-insulated. An R-value of R50 or better is desireable. A large oven will work much better than a small oven. Spun clay makes a good non-flammable insulation. Rock wool can be a problem if it has chemicals or plastics added to it. Fiberglass is probably not an option, because it is made with chemicals that are released when exposed to high temperature. Brick, concrete, and sand are each poor insulators.
- Many small solar ovens are made from cardboard or other flammable material. A Heat Retention Solar Oven will get much hotter than the typical solar oven and so it CANNOT be made from any flammable materials.
- Inside the solar oven, above the floor, is a thick layer of heat retention material, such as bricks, or cinder blocks, or salt (NaCl), or ceramic tiles. Its purpose is to store heat energy from the hottest part of one day, through the cooler portion of the rest of the day and night, and into the next day. The top layer over the bricks (or salt or other material) must be black ceramic or black metal, in order to turn the sunlight into heat.
- Further into the oven is an inner container, made of cast iron or steel, which would be used to contain the food being cooked. The oven will retain heat better when the oven door is opened, if that door goes only to the inner container, not to the space containing the heated ceramic tiles.
- The transparent window (solar collector) at the top of the solar oven should have minimum R-value of R3 and allow at least 70% light transmittance. The transparent material cannot be flammable. At least three panes of high-temperature glass will be needed to reach R3. Consider allowing two inches between each of the three panes. The window should be large enough to include both the inner container and the area with the heat retention material.
- The sides of the space between the panes should be black, to absorb light and produce heat. The exterior of the solar oven should be black to raise its temperature, thus improving the effectiveness of the insulation. (Insulation value depends in part on the difference between interior and exterior temperature. Raising the temperature of the exterior wall improves the insulation value.) The interior walls of the solar oven should also be black. The interior of the metal container (which holds the food) does not have to be black.
- Reflectors should be used around the top of the oven (front, back, and both sides) to increase light energy entering the oven. The angle of the reflectors should be adjustable. The reflectors should be removeable or should be able to fold over the window when the cover is placed over the oven.
- The shape of the oven should be a long rectangle, with the lengthwise ends facing east and west. The reflectors and the oven top should be angled towards the angle of declination of the sun. In this way, the largest area of the reflectors will not need to be re-adusted during the course of the day. The optimum length of the oven is 3x to 4x (or greater) of the width of the oven.
- The optimum angle for reflectors is 20 to 25 degrees on either side of the solar declination. So, if the sun is at 30 degrees, then the reflectors will be at 10 and 50 degrees or 5 and 55 degrees.
- The length of the reflectors should be between 1.2x (for 25 degrees angle) and 2x (for 20 degrees angle), where 'x' is the width of the oven window. The reflectors on the longest side only need to be set in angle once per day. They should reflect light directly into the oven window, or onto the end reflector, but not onto the reflector on the opposite side.
- The end reflectors (along the shortest sides) will need to be set twice per day. First, when the cover is removed in the morning, the west end reflector should be 90 degrees from horizontal, and the east end reflector should not be set up. This set up should remain until about solar noon, when the west reflector should be taken down and the east reflector should be set up to 90 degrees from horizontal.
- If the oven is located within 25 degrees of latitude on either side of the equator, the top of the oven should be horizontal. The oven will then have a rectangular shape, when viewed from the side or top.
- If the oven is located above 25 degrees latitude, AND if it will be used in the winter, then the top of the oven should be angled at latitude plus 10 degrees. Example: at 27 degrees latitude, the angle of the oven window should be 37 degrees facing South (in the Northern hemisphere).
- If the oven is located above 25 degrees latitude, AND if it will NOT be used in the winter (November thru February, in the Northern hemisphere), then the window of the oven should be tilted at latitude. Example: at 43 degrees latitude, the angle of the oven window should be 43 degrees facing South (in the Northern hemisphere).
- If the oven is located so far North or South that it can only be used in the summer, then the angle of the oven window should be latitude plus 10 degrees. Example: at 63 degrees North latitude (in Alaska), the angle of the window would be 73 degrees (at which point the triangle shape may need to be modified to have a short flat top, rather than a point at the top (in order to make room for the metal food container within the oven). Fairbanks, Alaska, may be able to make use of this type of solar oven as early as mid March and as late as early October.
- If the oven is located in the Northern Hemisphere, then the angle of the window should face South, because the sun will always be to the South. If the oven is located in the Southern Hemisphere, then the angle of the window should face North, because the sun will always be to the North.
- The most effective time of day for a solar oven is from about one or two hours after sunrise to about one or two hours before sunset. For those hours, the oven should be open to the sun.
- For the other hours of the day and night, the heat retention solar oven must be covered with a well-insulated cover. This cover is the key to the effectiveness of this solar oven design. Without the cover, the solar oven will not retain heat from one day to the next. Even if it is somewhat cloudy outside, the oven should be able to take in more energy than it loses with the cover off.
- On very cloudy or rainy days, the cover should be left on the oven in order to retain the stored heat.
- The oven goes through a warm up period. During this time the heat retention material is storing some energy from each day. The oven can be used during the warm up period, but it will not yet reach maximum temperature range, especially at night and in the early morning.
- It may take days or weeks for the oven to reach its maximum temperature range. If the maximum temperature is too high, decrease the area of the window or remove the reflectors. If the max temp is too low, increase the insulation on the window and/or the door and sides. If the minimum temp is too low, increase the amount of heat retention material or increase the insulation.
- Below is an example of a slightly larger Heat Retention Solar Oven that accomodates two interior separate cooking ovens, within the overall design.
- The Heat Retention Material in the drawings above could be bricks, or cinder blocks, or salt, or ceramic tiles. If I had to choose among those materials, probably bricks would be the first choice as they are easy to work with and have a high specific heat capacity.
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