Solar Oven Mark II

Intro: 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

STEP 2: Make the Inner Box

The plywood I used was from an old stage. It was 1/2 an inch thick. Make sure you know what pans/dishes you will be using in the oven so you can make it to the correct size. I wanted my inner box to fit a  9 inch pan with plenty of room to reach in and take it out. I made the inner box 13"x13"x6". Accounting for the 1/2" thickness of the wood, made the inside of the box 12"x12x6". Plenty of room for my pans. I used a table saw to cut the plywood, and sanded the wood after cutting it.

Here are the measurements for the panels that made up the box.
2 panels 12"x6" 
2 panels 13"x6"
1 panel   13"x13"

After that screwed the panels together using corner clamps. If you do not have any corner clamps do your best to make sure the box is square. You don't want any leaks that might let heat out. Then I sprayed the interior of the box with high heat black spray paint.

STEP 3: Make the Outer Box

The outer box needs to have some space to allow for your insulation. I made the outer box 16"x16"x10". This allowed for about an inch of insulation. I made this one in the same way I did the other.

Here are the measurements for the panels that made up the box.
2 panels 16"x10"
2 panels 15"x10"
1 panel   16"x16"

STEP 4: Insulation

I learned in my research that most of the heat lost from a solar oven is through the glass. In hindsight I might have been able to make the oven without the outer box. I did not perform any experiments with just the inner box. That would be a project for another time. For the insulation I decided to use an emergency blanket. It said it reflected 90% of body heat. I figured it would work well for what I needed. Also do not use Styrofoam. It can melt and release chemicals into your food. 

STEP 5: Combine the Two Boxes

Now put the two boxes together. I made a spacer so that the smaller box would be flush with the top of the larger box. At first I thought I might fill the box with stones since they hold heat well, but decided it would be too heavy. Besides most of the heat is lost out the top through the glass. 

The spacer I made was 3.5 inches high. Once you have the boxes flush use some wood to seal the two pieces together. 

Then I made a slot for the glass to slide in. I took the plywood I was using and cut a slot in it wide enough for the glass to easily slide into. The slot was an inch deep and about a quarter of an inch wide. I used the glass from a cheap picture frame to cover the opening. I lined the outside of the glass with duct tape to keep it from cutting me when I use it.

STEP 6: Make the Reflector

I got the measurements for the reflector from this instructable on solar ovens. The widest size of the trapezoid is 2.25 x cook chamber width and the height is 1.25 x cook chamber width. I used my jigsaw to cut the reflector pieces. I made one and used it as a guide for making the others. 

I drilled holes at the top and bottom corners of each reflector. Then I wrapped the reflectors in aluminum foil shiny side out. I used duct tape to secure the foil. Then I attached mirror holders to the reflectors. I used twist ties to attach the reflectors to each others. I also added a small piece of wood to the back of each reflector to help stabilize it.

STEP 7: Test

Test your oven. See how it is working and make see if there are any improvements you can make. You can see my testing data here. In the third test I added the glass from a scanner to the top of the oven. It did not completely cover it, but it did increase the temperature. Clouds were an issue that could not be helped, but I found if I checked it every half an hour to make sure it was angled toward the sun it did better.

From my testing I concluded that making the glass as thick as possible , and keeping the oven pointed toward the sun will help keep the temperature high in the oven.


We like to cook granola bars in ours. Post what you have cooked in your own solar oven in the comments below.

Comments

[bajablue]

YAY for you, ehudwill!!! 

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

[ehudwill]

Thanks!

[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. :-)

[Kwolf12]

Great job and great explanations! Thank you, I'll be looking at this as one of the parts of the one I'm planning to make. By the way, the solution to your in-between-box insulation is PERLITE. Doesn't hold heat, but it's awesome insulation that takes far greater heat than you'll generate--thousand degrees F or higher. It's 90 degrees outside and I am NOT going to turn our oven on if I can avoid it, so this is how I'm hoping to get our pizzas cooked!

[Paulus44]

Thanks for posting. I make some solar cookers too. I did not use a inner box. I only made 4 side panels with the aluminum folio. I painted my pans black and put them with food in a oven bag, close that and put it in the solar cooker. I cooked, rice, hotdog, potato, chicken and other meat.

here you can find more:

http://solarcooking.wikia.com/wiki/Paul_van_den_Hu...

.

[marieyoung]

how long does it take to cook stuff? Was in the winter, summer, spring or fall? I take it that it was outside in the direct sun light. With everything that is going on, I need how to cook solar wise. Thank.

[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