First run at a solar cooker a success

[Marthony]

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/I...lues-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]

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]

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]

Thanks for sharing your project with us, there is some good info here.

[The Stig]

This is kick ass. Can't wait to see pics of your first cooked meal.

[Marthony]

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. =)