First run at a solar cooker a success

[Marthony]

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]

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]

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]

Nice post, brother! Solar cooking has always interested me.

[Sniper-T]

(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]

If your attachments are pictures THIS 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]

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]

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]

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!

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

[bacpacker]

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.