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Marthony
07-24-2012, 01:58 AM
Hello,

I've finished & cooked hot dogs in my first try at a solar oven! Empty the unit reached 138C/280F in 20 minutes. With 1 and then 2 hot dogs inside the temp reached & stayed at 120C/250F within 15 minutes.

The design type is 'Heaven's Flame'; essentially a cube with deflecting panels attached to funnel more energy into the box: The design is one cardboard box inside another, each with aluminum foil on its inside reflecting towards the center with an inch of space between the boxes to be filled with crumpled newspaper for insulation. The gap is then sealed. The lid is two oven bags sealed to cardboard; it is tightened to the oven when in use. The deflectors are set at 67 degrees, and are collapseable when removed. I've added to the design by sealing oven bag material around the outside of the inner box to better trap heat. (the limit for oven bags is 200C/400F fyi)

My unit's window is 17"x12.5", and the oven is 9" deep. A cube design means the unit must be tipped 20-30 degrees to get a 90 degree angle to the sun; this means items inside roll to the side; this can be countered by a metal rack to suspend foods. I knew a trapezoid shape with the top already at 20 degrees was the ideal, but wanted to keep the design simple for the first build.

I am happy with this first run at the method and design, and am now looking to improvements to it to increase the heat:
-Replacing the double oven bag lid with a pane of glass or similar, after study of the insulation ratings of misc transparent materials
-Replacing the crumpled newspaper insulation with layers of cardboard cut to fit, plus 1 layer of glass or whatever is chosen above
-Find a better reflecting material than al. foil for the deflectors
-Determine if coloring the inside of the oven black would raise the temp level

Of course this type of unit isn't going to be in anyone's BOB or BOV, but could be made & stored at home or at the BOL. While the oven bag lid will get degraded over time from the sun a glass lid wouldn't, so this unit would be functional for the long term.

After tinkering with improvements, my next design experiment with this concept is a much more compact & portable design; I've found designs for a solar oven that uses a vehicle windshield reflector, oven bag, small metal rack, velcro squares, and any black metal vessel. (google 'Windshield Shade Solar Cooker') It has a great space-saver advantage, but could fail the long term test depending on how fast oven bags are degraded by the sun.

Random thought: would deflectors like these increase the energy output of a solar panel?

Thoughts?

(why can't I post attachments?)

bacpacker
07-24-2012, 02:04 AM
Good write up. A solar oven is on my list of stuff to do (dam that list is huge) and I am going to look into your design. It don't sound real difficult to make so hopefully I can make that work.

Marthony
07-24-2012, 02:16 AM
You're right, it isn't difficult at all. It is time consuming, but I've a childish love of building with cardboard & duct tape. =) My unit is defective in that I don't have the 1" around the sides of the inner box to the outer, due to failing to account for the thickness of the cardboard in my math.

I'd say the deflectors are marginally more difficult than the oven. You'll find a good design by googling 'Heaven's Flame', and it describes the method decently; 4 panels of cardboard are made, and 2 are taped together at their sides. The 'pairs' are then shoelaced together when in use, and are collapseable when not in use.

Gearhead
07-24-2012, 02:26 AM
Nice post, brother! Solar cooking has always interested me.

Sniper-T
07-24-2012, 09:53 AM
(why can't I post attachments?)

You need to post them to something like 'photobucket' and then put the link to the image here.

tx for the write up. Hopefully your attachments are pictures

The Stig
07-24-2012, 12:07 PM
If your attachments are pictures THIS (http://www.shtfready.com/threads/838-Tutorial-on-Image-Uploads) thread might be helpful.

I prefer photobucket myself but the concept is the same.

Please let the staff know if you still need help/have other questions.

Thanks for posting - great thread.

Marthony
07-24-2012, 02:38 PM
Thanks for the link. I did photo-document this as I was building it, minus the design addition of the oven bag around the outside of the inner box; that retrofit was done after the box-only test. Ok...let's see if I've got this right!

http://profile.imageshack.us/user/marthony/

Thanks!

(Heh...pictures are in reverse order. Well, I had a 50-50 shot at getting that right! =))

realist
07-25-2012, 04:45 AM
Nice post. Have you considered using Mylar for the reflector? Making one of these has been on my to do list for a long time, it just has not surfaced up to the top.

Marthony
07-25-2012, 01:13 PM
As of lunch yesterday, yes. :) A co-worker told me that an emergency blanket is more reflective than al. foil. I plan to tape one over the existing foil and see how that works. If it helps I'll put in the time to line the inside of the box as well. If anyone has a better idea please let me know.

Last night I removed the crumpled newspaper insulation between the boxes and inserted measured cardboard; I expect that will improve the insulation R value. Again, if anyone has a suggestion pass it on...I'm willing to try a few options here, though as I said to my wife, expanding foam must be the last try as there's no going back after that! :D

I won't have the chance to run a test until after the weekend though as I'll be away visiting family...

bacpacker
07-25-2012, 01:37 PM
For insulation, maybe try some styrofoam cut to size. R value will be in the 3-7 range. That will be better than paper or cardboard.

Marthony
07-25-2012, 03:35 PM
Excellent, thank you. If I don't have a better R-value suggestion by early next week I'll give that a shot. Cardboard is noted as R 3-4 at http://inspectapedia.com/interiors/Insulation-Values-Table.htm.

My co-worker also advised me that glass is a poor insulator and is unlikely to be better than doubled oven bag material. I found 1 site noting the doubled bags as have an R-value of 3-4, better than single-pane glass of 0.14 (if I'm reading it right).

Marthony
09-16-2012, 11:47 PM
Okay, I've since done two retrofitting atempts & tests. To summarize, while the latter run gave improvement, I think the lid material is the weak link that I'll work on next.

The first run had me replace the newspaper insulation with cardboard+bubble-mylar; it is essentially a layer of bubblewrap lined both sides with mylar. I also added this material to the deflectors, and used barbeque paint to blacken the inside of the oven (standard design on retail models). Result: 260F temp. Diagnosis: KISS principle broken; not clear what each change resulted in! The mylar-bubble material wasn't smooth and failed to improve the reflectors; all the wrinkles defracted light too much.

For the 2nd run I replaced the insulation with high density fibreglass wadding (same as retail versions). I removed the mylar-bubble material on the deflectors and replaced it with an emergency blanket, which was visually obvious as an improvement - it was almost mirror quality. At 21C outside, I reached 280F in the oven.

This is the same empty oven temp I reached with the original design BUT it was 26-30C outside back then, so I call this a minor improvement. E.blanket material is far better for the reflectors than Al.foil.

I'll post again after replacing the double-layer oven bag lid with a plexiglass version, followed by a glass version, and testing of each.

Marthony
09-20-2012, 05:05 AM
Victory! 360F!

Okay, victory in the sense that the lid change to plexiglass took the temp from a prior best of 280F to 360F; the temp would have gone higher but the tape melted and the seals for the insulation failed. The plexiglass (unknown specs) also sagged and wouldn't likely have taken much more heat either. Not a surprise really; I'm sure that's why the original instructions called for white glue. I never intended the unit to be a final product and used tape for easier building.

I will rebuild the unit from scratch minus the deflector panels. I will use cardboard again, mylar and no aluminum, more fibreglass wadding than before, and with a fibreglass lid of a better spec. A co-worker suggested a double pane with an almost sealed airspace could be even better; I'm not sure I'll go that far though. Finally, the unit will be made with an angled top so the oven doesn't have to be tilted to get a good angle to the sun. The ideal angle is TBA, but I'm thinking at least 20 degrees, maybe 30. More thought on the in-oven suspended plate is also needed, to secure food items or at least prevent them from sliding off the plate.

I don't think I'll bother with a glass lid.

And yes, there were noxious fumes when I took the lid off after the great melting. =) Who ever thought melting duct, scotch & packing tape would stink?

From a prepping perspective, I think the unit could be built from scavenged parts; abandoned ovens would supply the wadding. Any prepper would have survival blankets and hopefully find some white glue. I suppose folks would like a glass lid test because that would be easier to find than plexiglass...

Evolver
09-20-2012, 04:50 PM
Thanks for sharing your project with us, there is some good info here.

The Stig
09-22-2012, 12:15 PM
This is kick ass. Can't wait to see pics of your first cooked meal.

Marthony
09-23-2012, 06:18 PM
Thanks!

I've done a bit of Q&A with folks about materials; I won't try a generic type of glass - I'll seek out some tempered glass either new or salvaged from an old oven. The latter may pose interesting design challenges as a typical oven only has a small window...perhaps I'll try a pair? Generic glass would just be a safety hazard. On the plexiglass angle, my brother suggests trying Lexan. We'll see how these materials come available.

Looking at the original design specs, I've come to appreciate it more: The oven bag lid acted as a temperature control, and determined what materials were workable for the design. Using better insulation & deflectors would help it reach & hold that temp more quickly, but it was effectively an oven build to only ever be 250-280F while used. This meant white glue was functional for its design.

I seek to build a unit that will have a max 'occupied' temp of 400F. This is becuase I'm told the ignition point of cardboard is 451F. I'm told white glue will fail at higher temps, but I've found adhesive that will go to 450 & 600F (I'll use the former to save $). For tape I'll use aluminum tape. The reason I seek to go for 400 vs. be content with a longer cook time of 250 is that 370F is required for cooking pork. Thus at 400F one has a fully functional oven.

The final consideration is a simple means of controlling temperature once one has reached target temp. I suspect whatever is being cooked may do this for you without added devices, but it should be considered. The simplest means of limiting how hot the unit gets is by partly covering some of the deflector panels. More complicated, but offering a means of venting moisture, is some kind of lidded 1" pipe vent.

It started as a joke image, but it does beg the question: Couldn't a person drag a whole oven into the sun, block the vent, and arrange a deflector system to enter heat energy into the small window? The angling for the deflectors is an issue, but you get the idea. It is, you know, an oven. =)