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Step 14The Numbers and Of Course Math
This is a graph of the progress.
The base temperature of the water was 77 degrees F. The time was 12:01 EST on 05/29/2008. The maximum elevation of the sun for that day was 71.8 degrees above the horizon. As can be noted there seemed to be a linear progression in regard to temperature increase. As I sat and observed I saw that the temperature went up one degree every two minutes. By the end of the hour the water was at 113 degrees. I would assume that it would have been higher if it were not for some clouds in front of the sun at the end of the hour.
To calculate the power of the oven I first needed to get the temperature change in Celsius.
77 F = 25 C
113 F = 45 C
Temp change = 20 C
163ml of water = 163 g
20deg * 163g = 3,260 Calories
Next we need to convert calories to joules. 1 Calorie = 4.1868 Joules.
3,260 Cal * 4.1868 = 13648.968 Joules
Now that we have the power in Joules we need to factor in the time to get wattage. Se we divide 13,648.968 Joules / 3,600 seconds (1 Hour)
3.971 Watts. Not very much. Hell even an Easy Bake oven uses a 100-Watt light bulb. But in the end one must remember that the heat is cumulative. The better the insulation the more heat will acquire and the better things will cook. It’s sad to report that out of 87 potential watts from the sun for the given area only a little fewer than 4 watts was produced. It would appear the overall the oven was only about 4.5% efficient
If nothing else this taught me a valuable lesson in the design considerations and power calculations necessary to build a more robust unit.
But will it cook food? Build one and see.
The base temperature of the water was 77 degrees F. The time was 12:01 EST on 05/29/2008. The maximum elevation of the sun for that day was 71.8 degrees above the horizon. As can be noted there seemed to be a linear progression in regard to temperature increase. As I sat and observed I saw that the temperature went up one degree every two minutes. By the end of the hour the water was at 113 degrees. I would assume that it would have been higher if it were not for some clouds in front of the sun at the end of the hour.
To calculate the power of the oven I first needed to get the temperature change in Celsius.
77 F = 25 C
113 F = 45 C
Temp change = 20 C
163ml of water = 163 g
20deg * 163g = 3,260 Calories
Next we need to convert calories to joules. 1 Calorie = 4.1868 Joules.
3,260 Cal * 4.1868 = 13648.968 Joules
Now that we have the power in Joules we need to factor in the time to get wattage. Se we divide 13,648.968 Joules / 3,600 seconds (1 Hour)
3.971 Watts. Not very much. Hell even an Easy Bake oven uses a 100-Watt light bulb. But in the end one must remember that the heat is cumulative. The better the insulation the more heat will acquire and the better things will cook. It’s sad to report that out of 87 potential watts from the sun for the given area only a little fewer than 4 watts was produced. It would appear the overall the oven was only about 4.5% efficient
If nothing else this taught me a valuable lesson in the design considerations and power calculations necessary to build a more robust unit.
But will it cook food? Build one and see.
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I would like to echo what Haw1horne said: Heating plastic is a bad idea unless it is designed for heating. You may not see or taste the outgassed products, but they are there. Other sites also say that if you can't use glass then the baking bags will work. I think a layer on the inside of the cover sealed with a cardboard edge, and another layer on the outside of the cover also sealed. Of course, pull tight to reduce diffraction.
An aside to the previous; careful what paint you use. Some are not safe to be heated. Look for those labeled as non-toxic stove paint.
A glass top would help keep the lid sealed because of its additional weight. If you go with the turkey bag window route, I'm thinking paperclip and rubber band anchoring system around the outside would keep things tight and allow for less heat loss. My guess is this is where your heat loss came from.
An angled lid lined with foil (aluminum, tin, lead, mylar, whatever) would catch more light and protect the works when all finished and folded down.
I'll start my oven tomorrow. Again, fantastic job with this and thank you so much for your time and effort.
have a nice day
Instead of cooking food, it might accelerate microbial growth! ICK! lol
Dude, I really gotta say (in somewhat paraphrased agreeance wth fishhead445 had to say), I thoroughly enjoyed this entry! Clear, concise, highly informative, well documented data, and witty as alll freakin' hell! THANX! ^_^
HAIL!!!! ^_~
The real trick here, I suspect, is maximising insulation on all sides and the area of the window, while minimising total volume. Afterall, the more air inside, the more air to heat up.
I wonder how much light gets lost from diffusion by the trash bag, too. Probably not much, but it would be interesting to compare it with glass. (There's gotta be a broken flatbed scanner around the office somewhere...)
But yeah, what with the way a Fresnel lens focuses light into a concentrated beam (much as lighthouses do), so does the heat concentrate, as well. Like with one of those Fresnel lens panels that can be gotten through Edmund Scientifics that can melt asphalt, for instance. As far as a focal point, with a Fresnel lens, it's a beam (again, like with lighthouse lenses, which are of the Fresnel type), so there's not much as far as spacial restrictions go, I would imagine. But I could be wrong. I'm no scientist lol
True that, on the insulation... but with a focused beam (as opposed to an apex/singular focal point) and spacially-throughout-heat-intensity, there *may* be a need for more heat resistant materials... hmmm... insulative fiberglass, perhaps? Like the kind one can find in one's attick? Maybe...
hey that rhymes!
There's 3 complications with your implementation of the solar box oven (variation of the Kerr/Cole Oven patented in the 1980s)
One is the plastic glazing. If the oven did get hot enough, that sheet would very likely melt, warp, and/or outgas. This could be bad for your lunch! In any event, its probably not a food-safe grade of plastic if you found it around the work place. One work around is to use food-grade turkey bag plastic. Stretch it tight across a frame, and mount the frame in the lid.
Second is the plastic glazing. Yes, I know that sounds redundant. The biggest source of heat leaking is going to be out through that plastic. You can improve this by either using glass or lexan for the glazing, or by creating a double layer of food-grade plastic with a 1/4" gap in between the layers.
Third, most box ovens made for climates outside the Tropics have an additional reflector to help boost the heat up to cooking temperatures. Cookers that don't have the additional reflector are (1) Made for the tropics (2) Use double glazing (3) Use high-R polyisocyanurate insulation in the walls.