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Start with a solar oven
The solar oven must be large because a larger solar oven takes in more of the sun's energy. A Heat Retention Solar Oven must take in enough energy to cook the food, to store some energy for later use, and to compensate for heat lost to the exterior. Therefore, the oven should also have an effective reflector, so that additional energy in the form of sunlight is reflected into the oven. A Heat Retention Solar Oven must be particularly well-insulated on the sides and bottom, so that it does not lose that energy too quickly (R50 is recommended). And it must have a window (solar collector) on the top that is insulated to R3 or better.
Add Heat Retention Material
Now let's add some material to the solar oven, material that retains heat well. Given a sufficient quantity of heat retention material, the same oven will take longer to heat up, but it will also hold that temperature for a longer period of time. With enough energy input each day, and enough heat retention material, we can cook in the solar oven even after the sun has set, because the heat retention material radiates sufficient heat to continue cooking.
Any one of a number of different Heat Retention Materials can be used for this purpose. Bricks have a high specific heat capacity (the ability of a material to store heat); they are relatively inexpensive and widely available. They should NOT be joined together with mortar or cement when used in this type of oven. Allow some air space between the bricks, so that the heat can be transferred to and from the bricks.
Salt (NaCl or table salt) has a fairly high specific heat capacity. It is also widely-available, inexpensive, and non-toxic. Salt also have sufficient density to store that retained heat in a reasonable amount of space. And it handles high temperatures well.
Ceramic tiles, or cinder blocks, or even sandstone would also work fine. Ceramic is also relatively inexpensive and widely-available. It has a good specific heat capaticy. Ceramic handles high temperatures, gives up heat slowly, and is available even in developing nations. Cinder blocks are easy to build with and have a high enough specific heat capacity for this task.
Cover the Oven at Night
No matter how well insulated the sides and bottom of the oven are, heat rises and so the top of the oven will tend to lose heat rapidly. Most of the heat loss in the typical solar oven is through the top of the oven, where the transparent material is located. It is difficult to both insulate well and allow plenty of light into the oven, so, when it is night we must cover the oven with a particularly well insulated top.
When the sun is too low in the sky to add much heat to the oven, or when it is too cloudy to provide much sunlight at any hour, or when it is night, the oven must be covered to retain the stored heat. With enough heat retention material, and a well-insulated oven with a well-insulated night cover, the oven will stay hot through the night and into the next day. The cover is removed during the day (for anywhere from 7 to 10 hours) and is replaced again in the evening.
When the Heat Retention Solar Oven is first built, it is at ambient temperature. On day 1, the sun begins to heat the interior of the oven. Some of that heat is stored in the heat retention material. On day 2, the oven begins at a warmer temperature than on day 1 and consequently reaches a higher cooking temperature. On day 3, the oven begins at an even warmer temperature, etc. Eventually, the oven reaches a stable maximum daytime and minimum nighttime temperature range.
The amount of time this takes depends on how much energy from the sun is available during the warm-up and whether or not the oven is used for cooking during that time. If the oven is used for cooking from day one (and it can be) it will take longer to reach a point where cooking can occur day or night. However, even during the warm-up period, the oven will maintain its temperature for longer and reach a higher temperature than a typical solar oven.
How long can this oven last without a good sunny day to add more energy and heat to the interior? With enough heat retention material, this solar oven design can cook for over 2 days, even while the cover remains on the oven day and night. And, if the oven is not being used, this solar oven design can maintain its cooking temperature for even longer, while the cover remains on the oven day and night. Eventually, a number of good sunny days will be needed to replace the lost energy and heat.
Cook Anytime, Day or Night, in a Solar Oven
So, now we have a large enough oven, enough energy entering through the collector (the window on top of the oven), and good insulation. Essential to that good insulation is a well-insulated cover to use whenever there is not enough sunlight. We also have a good material to store excess heat and to release that heat at night and on cloudy and rainy days. After the initial warm-up period, the oven can be used anytime, day or night, since it maintains its cooking temperature range 24 hours a day, 7 days a week.
The amount of cooking this oven can provide varies depending on how much sun a particular location receives each month, and on what the design specifications are like. However, it should provide more than the typical amount of cooking heat used by a family of four (about 1500 watt-hours/day). In very sunny locations, and with a larger oven design, this oven could serve several families.
What are the Design Specifics? Read On....
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.