Introduction: Flat Pack Solar Oven (200°+)
Addendum: The kind folks at Pop Science were kind enough to feature this project in their print version, as well as online, so please check out their link as well and support them for supporting many makers in this community! It is truly and honor and a blessing for them to reach out! (http://www.popsci.com/build-diy-solar-oven)
In this instructable I will show you how to create a DIY solar oven which can get up to around 200°F. I was able to use all materials I had laying around, so for me there was no additional cost, but you may need to buy a small piece of glass or acrylic. The entire base of this project was basically saved from the garbage, and I used all materials I had around, and even used many extra pieces from other projects. It's lightweight, compact, and ergonomic design makes it great for bringing camping or hiking, and it's sure to capture the attention of others.
- Flat pack design
- Heavily insulated
- Sturdy frame
- Adjustable angle reflective panels
- Fold out expandable reflective panels
- Gets up to around 200°F
- Endless possibilities for add-ons and customizing
For this instructable you will need:
- 5mm Black Craft Foam
- Duct Tape
- Construction Paper
- Aluminum Foil
- Glass, Acrylic, or any clear plastic sheet
Step 1: Adding Insulation
The first step is finding a sufficient box, and insulation that will go with it. I used a box I saved from fresh chocolate covered strawberries my family received, and the insulation that went with it. Any box will do, and for insulation you can use styrofoam, Eva foam, sheets of craft foam, fiberglass insulation, old shirts and sweatshirts, foam insulation sheets (come in large sheets from places like Lowe's and Home Depot), or anything else. Worse case you could even use newspaper or thick sheets of cardboard.
The insulation I used that came with the box was a simple closed cell foam which was about 1.5 to 2 inches thick. It came in 2 pieces (see pictures) which were cut so that they overlapped and made a sort of miter joint. I had to cut off the top flap to allow for a space to let the sun through, but depending on your insulation you probably won't have to do that. Lastly, use any adhesive (preferably not hot glue for obvious reasons) and secure the insulation to the inside of the box.
Step 2: Structural Additions
I figured that for a solar cooker you want as much surface area for reflecting the sun towards your oven, and I didn't think the flaps on my box were sufficient, so I measured them and duplicated 4 more flaps the same exact size. I used a strong duct tape to connect the flaps so that they could still fold out when the cooker is deployed. (See pictures)
To support the reflective panels I used bamboo skewers and poked a hole through the side and flap of the box to hold it up. This works phenomenally well and is very useful for this particular application because you can poke holes in multiple spots vertically on the box so that you have multiple different places to put the skewer. This allows you to adjust the angle of the reflective side panels, which is a great feature for a solar cooker. For a more durable approach you may also experiment with adding a separate piece of foam board, but for me the cardboard has worked fine and there have been no issues yet.
Just found this article on calibrating your reflective panels while writing this and figured it could really help:
Step 3: Black Insulation “Absorption” Liner
After the general structure and all the mechanics were finished, I started adding the details. The first important detail was adding black 5 mil Eva craft foam on top of the main foam. I cut sheets to fit in the bottom and on all the sides, then I cut skinny strips the cover up like trim around the top. Again, you can use whatever adhesive you might like, but try to avoid hot glue because the intense heat will weaken it, and there is a possibility the craft foam will come apart from the other insulation. It is important that this is black because it has to absorb the light, turning the energy of the light from the sun into heat for cooking. This style of cooker is formally known as a “solar box cooker”, but I use “solar oven” because it is more unique and it works similar to an oven in the way it cooks and heats the food. A solar box cooker lets UV light rays in through the transparent lid, then converts them to longer infrared light rays when it comes in contact with the black foam walls. The longer infrared light rays cannot escape the box because they do not have the energy to penetrate the glass, a surprisingly great insulator. (See the graph in Step 5)
Step 4: Adding the Foil
In this step you will see how I added foil to the sides and how I added foldout corners to capitalize on the addition surface area. The reflective foil is crucial to the design of this oven style cooker because it directs a lot more sunlight, and therefore, heat, into the oven. Remembering back to the last step, the side panels can move and change angles due to the design of the supporting mechanism, and that feature plus the corners helps tremendously at increasing the efficiency of your oven because you can direct maximum sunlight into the oven regardless of where the sun is located (basically mandatory features for evening cooking and early morning cooking when the sun isn't directly overhead).
Adding the reflective foil to the side flaps is quite simple, and again you can just use any adhesive you would like. In this case, it is fine to use hot glue because the area will not be experiencing intense heat, but it might just be easier to use the same adhesive throughout the project. As silly as it sounds, just make sure to use reflective side of the foil to optimise the efficiency.
I wanted to maximize my flat pack design, so I went through the extra effort of adding fold out corners. These corners went through multiple concept and design changes, which included everything from a flexible reflective sheet to an accordion design (see images). I ended up with a fold out design, which was the most logical for the flat pack design because it only added the thickness of 2 sheets of paper, and doesn't impede the folding at all. The process used to create these sheets is easy enough to replicate, and the majority of the time it takes to add corner panels comes during the sizing. I took 4, 12×12 inch sheets of scrapbooking paper, and rolled out a 50+ inch long strip of the 12 inch wide foil I was using to accommodate the 48 inches of paper. I used a glue stick and covered the sheets completely with glue, then pressed them down onto the foil and added weights to the top to hold it down. When the glue dried, I roughly cut between the sheets, then took them to a slicing machine and squared up the edges. I then set up the side panels and traced the rough shape between each side, and cut the panels to fit. I only attached the panels to one side to allow for flexibility when changing the angles of the sides. Once installed, fold the sheets along the edge of the side panels, and then once in between the original panel and the extention so that it can easily fold back up into the box, and close securely (see images- number 9 and 10 show it perfectly).
Although it took multiple concepts and a bit of engineering because I wanted it to accomadate the flat pack design, adding corners ended up being worth it because it added and extra 72+ sq. in. (15+%) to the already 403 sq. in. surface area.
Quantity Equation Combined Area
(4x) 12×12×0.5= 72 sq.in.
(2x) 13×13= 169 sq.in.
(2x) 13×18= 234 sq.in.
(Sub)Total Surface Area= 475* sq.in.
*Plus the 234 sq.in. of area inside the box
Total Surface Area= 709 sq.in.
Although it seems nominal, this 15% change by adding corners could be a change of almost 15°, which could be the difference between reaching boiling temperatures and not. Depending on your box size, I would seriously consider going through the extra effort of adding corners.
Step 5: Adding the Transparent Lid
This step is actually one of the simplest, and depends mostly on the size of your personal oven and the type of glass/acrylic/plastic sheet you are using. I built a simple frame from foam board, because it was the perfect thickness for my glass. I used a foam cutter to melt out a groove in the foam, and I slipped my glass inside of that. After I knew the glass fit inside, I slid it back inside and glued all of the corner together so that it was impossible for the glass to come out unless the device broke (which it hasn't yet and has proved to be super strong). Also note that the foam core I used was black so that it matched the rest of the interior of the oven. Again, this step relies heavily on what materials you are using, but if you roughly use my outline you should easily be able to make your own.
I chose glass because it has very high insulation properties, even better than the acrylic in the pictures. If you look above, I included a graph which I found while doing research on the best material to use.
Picture citation: http://www.wintergardenz.co.nz/glass-vs-poly-vs-film.html
Step 6: Painting
For painting you can obviously use any design you would like but I will be briefly walking you through on the particular style I used.
I personally came up with this painting style when I was working on a cardboard post-apocalyptic storage unit. I think it looks so cool and it is really quite simple to achieve. First you have to paint the entire exterior f your oven all black, then mix up some light gray paint, and dry brush it onto the face of the box with a large (1+ inch) brush. The natural ridges on the corrugated cardboard make this painting style look awesome, and I would highly recommend you try it!
Step 7: Creating a Custom Logo With a Cricut
For this build I also added a custom logo I created using the Cricut and a modeling software that goes with it. I know very few people own machines like the Cricut, so alternatives would include: cutting letters out by hand, using precut letters or stickers, printing and cutting out a custom logo, drawing or painting a logo, or anything else you might want to try. The process of creating custom letters and designs is actually quite long and difficult to explain, so I won't add it in this instructable, but look out for one on this topic in the future! (Plus I'm sure if you own a Cricut you most likely know how to use it) I will still include the pictures I took to show you, and if anyone would like, I will post screenshots in the description of me actually creating the logo. Again, just remember there are loads of alternatives, and I guarantee a few minutes in photoshop could also yield some great quality custom logos.
Step 8: Conclusion
I know there are probably thousands and thousands of different ideas for solar cookers and ovens, but I wanted to share my high quality design made from all previously owned materials and a salvaged insulating box for shipping food. I obviously knew what solar cookers were, but I had never really looked into them in great detail. I saw the box and insulation those strawberries came shipped in, and I figured that would be absolutely perfect to use as a base for a solar cooker. I hadn't done any research on the topic previously to me building this, and only did reasearch when writing this instructable (and the two websites I looked at I included links in the instructables). However, I am confident in my design and it has worked exceptionally well for me whenever I have used it.
The lightweight, ergonomic, flat pack design makes it easy to carry and great to bring on trips. All the engineering I put into it with the fold out, collapsible corners, and the adjustable side panels to allow you to track the sun really increase the potential surface area, making this solar oven incredibly efficient for its size in my opinion. You can store all of your cooking pots, pans, and utensils, and any other cooking tools neatly inside the box, and close up the lid for easy storage and transportation. I absolutely love the design I came up with, and is overall incredibly versatile.
I would also like to encourage you to expand on this “concept”. I know i will personally try to make one out of wood with hinges, handles, and other hardware in the future, but make sure to keep it light.
I would love to see people's different spins and takes on this concept to see how I may be able to improve mine. Mine is different from the rest because it uses insulation from shipped chocolate covered strawberries, and it seemed quite unique to me. Please reach out to me if you made this or if you have any ideas for improvements on my model.
- Brandon (16)