Solar Oven Mark II

This is my second attempt at a solar oven. My first was made from cardboard and worked okay. The first I built to make sure I understood the science. This one is an improved version. 

Step 1: Tools/Materials

You can get by with fewer tools. These are the ones I used.

Tools
Table Saw
Miter Saw
Jig Saw
Drill
Drill bits
Square
Ruler
Corner Clamps
Staple Gun
Clamps
Sandpaper

Materials
Plywood
Boards
Screws
Insulation (in my case I used an emergency blanket)
High Heat Grill spray paint
Aluminum foil
Duct tape
Mirrors
Mirror holders
Pane of glass
Zip Ties
Small clip
Staples

Comments

[bajablue]

Excellent build... and a definite improvement over cardboard.

How hot does it get... and where are the pics of your homemade granola bars, hmmm? ;-)

[ehudwill]

I managed to get the temp up to 225, and we ate all the granola bars. The kids eat them like candy. :-)

[bajablue]

YAY for you, ehudwill!!! 

Congratulations on a well-deserved win!!! ;-D

[ehudwill]

Thanks!

[Paulus44]

Great. I am in the Philippines and use almost the same model Solar cooker/oven. We cook, rice, meat, sweet potatoes, eggs and bananas without water and use it to heat some water for dishwashing and cleaning. I try to promote, but people are not always willing to give up old way to cook.
Thanks for posting this one. I go my oven give an update.

[ehudwill]

Thanks. I would like to see your oven.

[Paulus44]

Sorry for late replay. I made some pictures with meassures and other information and will post it within a week. I cooked this week rice, hotdog and eggs. The last 2 without water. You will see it in the pictures how to do that.
Paul

[Dale 1944]

How about using 2" insulation board for the box? Plenty of duct tape should give sufficient rigidity, and that would constitute the inner box, outer box and insulation all in one.

Could someone with some creativity devise a parabolic structure to which cheap (and thin) mirror tiles could be attached to concentrate the sun's energy? Moving this framework slightly to keep the focus on the stationary box oven should be easier than moving the entire box and reflector structure.

Thank you for your efforts. I keep waiting for you people with creativity to come up with ideas that I can steal!

[DIY-Guy]

Dale 1944 said- "I keep waiting for you people with creativity to come up with ideas that I can steal!"
Ahem. (Polite cough) Dale, we call this sharing ideas.

As to cheap parabolics with mirror tiles, you've exactly got it. Any cheap substrate parabola form that's weatherproof can have small mirror tiles glued on to it. Mirror tiles are made by cutting straight lines on standard 12" mirror tiles, then cutting the resulting strips apart into little squares.

[ehudwill]

Thank you everyone for all the comments. I have a ton of ideas for my next build of the oven. Also of note my wife reminded me that we did get temperatures of 250 so... her memory is better than mine.

[Manny B]

They usually are.

[zawy]

Looks excellent.

The corners of this design are not reflecting into the box. They need to "cut across", connecting the sides of the rectangular mirrors. So they will be upside down triangles. If the 4 square mirrors are the same size as the top of the box (which helps packing it up) then they should be tilted back almost exactly 30 degrees. The trigonometry to prove this is surprisingly hard, but a simple experiment will show it to be true. Tilted further back and it starts missing the target. Tilted further up and the area of collected sunlight is reduced. 4 mirrors like this make collected sunlight total to be 3x total regular sunlight (3 kW/m^2), minus 10% reflection and absorption loss from the glass, if you make sure to use low-iron glass (should not be too green when looking at the edge) that is NOT insulation (low E) glass or doubled-pane which causes even greater losses. It looks like if the corners are improved, it might add another 1x sun (4 kW/m^2 total). The small addition can mean an important increase in max temp. Yes, R=4 insulation board on the back and sides is important.

[ehudwill]

I did not have a material that would hold the sides together without ripping. I will have to explore more options.

[gsmith29]

For the insulation, try using perlite (or expanded clay). Perlite is an exceptional, light insulator - it is used as the insulating medium for some LNG marine tankers.
If it gets wet its insulating properties drop off sharply.

http://en.wikipedia.org/wiki/Perlite

[ehudwill]

Thanks. I will check this stuff out.

[godbacon]

make your inner box out of cement board or slate (colored black) it will act as thermal-mass and store heat, combat your cloud problem.

[chesterdad]

We have a commercially purchased unit that works great. We used it last weekend and the oven temperature reached 350 degrees F. We found that after about 4:30 PM or so, we couldn't get the temperature above 250 degrees F. You are better off cooking with late morning to early afternoon sun. I live in Utah for reference.

The commercial one we have has a swivel pan in it and a leg on the back. The leg allows the oven to tilt into the sun. You can use the reflector pointed at the sun and ensure that it makes a shadow on the face of the box. This will help capture more sunlight. (If I were building one, I'd use more than one leg - possibly a frame- and work to ensure greater stability than the one post leg ours has.)

The swivel pan is merely a pinned, swinging shelf. The pins, two only, are opposite one another and on the sides rather than the front or back.

Be certain to heed the warning about knowing the pan size(s) before constructing your own. We had to shop for proper sized pans to use in ours. Walmart carries dark, thin walled pots, loaf pans, cookie sheets, and roasters. You want dark, thin walled cooking pots and pans so as not to waste time heating the container.

Do you think a solar blanket would make a good reflector? I wonder if the foil reflectors would survive the abuse of folding and reusing.

Excellent Constructable! Thanks for posting.

ChesterDad

[tig1]

Hell Utah ! you should be able to get 350 de without the oven :)

[ehudwill]

Thanks Chesterdad. I have thought about building a frame for mine, but have not had time. Also the plywood in kind of heavy. A frame would make it hard to move around.

Most emergency blankets I get are very thin and wrinkled. The wrinkles decrease the efficiency and the thinness makes it more susceptible to tearing.

Thanks for commenting. I like a good discussion.

[jhitesman]

Great job, I've been working on a similar project and noticed there weren't very many reproducible solar ovens on instructables. Mine was a quick afternoon build out of what was available but showed a LOT of potential. Unfortunately a surprise rain storm took it out. Working on a design for a more permanent one now:

https://plus.google.com/photos/118393350380853916534/albums/5762973954193916913

One thing I found on mine was that a plastic turkey bag with a tight fitting frame worked better than a piece of picture frame glass in a less than tight fitting frame. With the thin glass and somewhat loose frame I had a hard time getting it over 200 even here in the summer desert sun. With a turkey bag and a cardboard collar to seal it I was getting over 325 without having to worry about my focus nearly as much.

I can't wait to build my permanent oven. The solar oven was a blast to cook in while it lasted. Like a crock pot that uses no power and can't short out and burn your house down so I don't worry about leaving it alone while I go out. I made some great bread in it, cooked chick peas very nicely that I made into hummus, apples with some sugar and cinnamon in a jar made a great treat - and I did a number of small brownies, cakes, cookies and other treats as tests. Part of my plan for the new oven is to document building it for an instructable - but time is short right now so who knows when it will get done :D

[Beekeeper]

For most cooking you need 375 degrees Farenheit so your max at 225 F seems way to low to be of much use. At first I thought you were recording in Centigrade and thought this must be a good design. I have seen many similar ovens and the best have a sealed double glazed panel facing the sun and 4 inches of insulation between the outer and inner surfaces of the heat chamber with the inside painted matt black. The bigger the unit, the more efficient it is. The best one I have seen has reflectors about 4 feet x 6 feet, the whole thing being on a trailer so it can easily be turned every so often to face the sun. It is big enough to roast two turkeys in side-by-side. Incidentally that heat blanket material usually won't last more than one season. You can get mirrors at your local thrift store for very little money.

[bluerosei]

I made an oven years ago. A box in a box design, from cardboard with 2in. between them and as much cardboard scrap as I could fit between them for insulation. I used a large oven cooking bag with a cardboard "picture frame" that fit tightly inside the bag for the glass. covered the inside with foil. Had a single reflector covered with foil. Put a cooky sheet painted black in the bottom. I still have it and that thing gets HOT! You have to lift the whole top off to open it. But for the cost of the cooking bags and a role of tape, it was a whole lot of fun!. I plan to one day add three more reflectors.The card board was scrounged up from left over boxes.