Solar oven testing and bread baking

[Tim K]

I've been wanting to build a solar oven since I first discovered the concept. I finally got it done a few weeks ago. I also had an opportunity to borrow a genuine Sun Oven brand solar oven. After getting mine built, I set them up side by side and did a few days of comparative testing.

My design goal was to come up with a design that was inexpensive enough that I could make a half dozen or so for the cost of a single Sun Oven, and do the majority of the cutting on my CNC. Of course, it had to work, too. My preliminary design is shown. It uses 3/4" particle board for the outer box. Next is 1/2" rigid foam insulation with foil on both sides. Finally, I lined the inside with 1/4" MDF painted black. The reflectors are 1/4" melamine with aluminum foil applied with spray adhesive. The cover is 1/4" glass. The "swing" on the inside is also 3/4" particle board. The reflectors are attached to the oven box with hinges, and velcro is used to hold them together at the top.

In side by side testing, the Sun Oven heated up much faster. Amazingly fast, in fact. Leave that thing in the sun for 20 minutes and it gets right to 325°-350°F. Pretty impressive. By comparison, my oven took twice as long to heat up. On the other hand, mine would sometimes get even a little hotter and would always stay warm much longer. It's less sensitive to the sun going in and out behind clouds. Overall, I give the win to the Sun Oven.

It's when you get to issues like set-up and tear-down, portability, and weight that the Sun Oven really shines (pun definitely intended). It's light, easy to carry, and stupidly fast to set up. Overall, the Sun Oven design is superb.

While my design works well, it's also stinky. I assumed that the volatile compounds in the MDF liner and PB swing would cook off after a few hours. I was wrong. It's baked in the sun for days now, and there is still an odor when the lid is opened. It's gradually getting better, but it's still strong enough to impart the smell to food. Not good. My new plan it to remove the MDF liner and try another material. I've seen others use sheetrock, and I may try that if I can't come up with a better solution. I'll make a new swing out of sheet metal. With those two changes, I think I'll have a decent design. The other place my design falls short is in the reflectivity of aluminum foil. Compared to the Sun Oven, my reflectors look positively dull. I can't seem to come up with a solution to that one, so I guess I'll live with it.

I met my design goal since I can make these for about $60 in materials. For that price, I can build 5 for the price of a Sun Oven. I might be cooking for a crowd, so having a fleet of these is appealing.

The ovens running side by side


Front view of Sun Oven


Front view of Tim's Oven


So what did we make, you ask? We made my Mom's french bread recipe using whole wheat. I have hard white winter wheat, and we ground that both in the blender (coarse, but easy) and with a hand mill (fine, not so easy).

4 cups whole wheat flour
1 1/2 tsp salt
1T sugar
2 tsp yeast
1T oil or soft butter
1 1/2C warm water

Mix dry ingredients. Stir in water and oil. Sit for 10 minutes and knead. Repeat for a total stand time of 1 hour. Shape into 2 loaves and place in greased bread pans. Cover with a towell and let raise 1 - 1 1/2 hours. Bake in normal oven at 375° for 30 minutes. We tried anywhere from 45 to 60 minutes, and it didn't seem to make much difference. Solar ovens are almost burn proof, so it's hard to make a mistake.

Makes 2 loaves.

I wish I'd taken a pic of the bread, but we wolfed every loaf we baked. It was fantastic. It was surprisingly light and absolutely delicious. Especially with butter and honey.

We'd intended to try some other dishes like beans or rice or a stew, but never did it. I have no doubt it will work great.

So, I'm interested in ideas for the liner material. Plywood might be a decent choice since it has less glue than MDF, but I'd like something with no glue. I'll try drywall if I can't come up with something smarter, but all the dust and ragged edges make it unappealing.

[Sparky]

I am interested. cool build

[rbeau30]

If you get a chance, would you try this no-knead recipe that I use often with a cast iron dutch oven and home oven? I would assume the only difference is you use your ovens like you did above and just cook until an internal temp of 180 degrees? I am seriously going to buy one of these real soon, and would genuinely like to know.

No Knead Bread


3 C Flour
1 1/2 C Water
1/4 tsp Yeast
1 1/2 tsp Salt


Mix into a bowl and sit overnight


Take dough out of bowl and turn it into a puffy round.
(Roll it in under itself.)


Let rise (under a floured towel and on a piece of
parchment paper) for 2 hours.


Preheat Dutch Oven for 30 minutes at 450 degrees.


Flour top of dough. Score dough.


Set dough parchment side down, into preheated dutch oven.


Put lid on top.


Bake at 450 for 1/2 hour.
And another 15-25 minutes with lid off.
(180 degree internal temp)

[SA Friday]

The other place my design falls short is in the reflectivity of aluminum foil. Compared to the Sun Oven, my reflectors look positively dull. I can't seem to come up with a solution to that one, so I guess I'll live with it.

Polished aluminium sheeting? should reflect better than aluminum foil.

[Tim K]

White flour?

[rbeau30]

White flour?

Yes sorry I just use white flour currently to keep it as simple as I can.

[gnihcraes]

Similar to mine, that I've yet to get completely working.

I used the spray sealant from home depot you see advertised on tv. The smell is gone quickly.
I put my oven on a tilting base, so I can point to the sun easily. Has internal swing too.

I'm still having issues with the reflectors and getting them to easily attach without falling off. Guess I'll have to stick with some regular hinges.?? I tried straps like you're using the velcro for, as I tilt it the reflectors change enough to lose a good angle.

A mild gust of wind also messes with the angle.

So... Keep the thread going with updates as others build theirs and test.

[TFOGGER]

Maybe polish the reflectors with Blue Magic? I remember making a cigarette lighter (solar) by polishing the bottom of a soda can with that stuff. It's relatively nonabrasive as well.

[Aloha_Shooter]

So, I'm interested in ideas for the liner material. Plywood might be a decent choice since it has less glue than MDF, but I'd like something with no glue. I'll try drywall if I can't come up with something smarter, but all the dust and ragged edges make it unappealing.

I was kind of surprised that you used particle board and MDF for this as I would have assumed both would have outgassing problems at the high oven temperatures. Have you thought about building with sectioned heavy corrugated cardboard with standard styrofoam sandwiched between inner and outer cardboard?

Try mylar instead of aluminum foil for the reflectors. I believe gold-colored mylar is better than silver-colored for reflecting IR -- have to check some astronomy books but I seem to remember something about using gold-backed mirrors for IR telescopes because of the reflectance.