We’ve talked about solar chargers, solar water heaters, solar garden lights, and many other innovations that take advantage of the free energy provided by the sun. Today, I’m going to add to that list and talk about how to make a solar oven.
We’ve all felt the cooking potential of the sun on a hot day! It’s a powerful resource.
Solar ovens, which are little more than a cardboard box, aluminum foil and some black paint, can make a wonderful family project, using mostly recycled materials, that save energy in the kitchen.
Solar ovens can reach temperatures of around 250oF (121oC) and though this is lower than you would generally use in a conventional oven, it simply means that things might take a little longer.
However, if you have the time, it’s a wonderful way to make a summer meal.
When I first started looking into solar ovens or solar cookers, I was overwhelmed by the number of different styles. I quickly realized that there were a few things that were important to me when it came to choosing a solar oven style:
1 – Portable. Many of them are made out of plywood and look very heavy. Not that I’m adverse to weight lifting, but I would like to have the option of packing it with us for a family day out at the park. Also, I want to be able to move it easily on my own.
2 – Reflectors or collectors. Some styles had no or just one reflector. I’m generally an impatient person, so I know I want something that’s going to direct as much heat into the oven as possible – even if it means it looks like I’m recreating Sputnik in my backyard.
3 – Easy. Basically, I don’t want to give my husband an excuse to go out and buy some new fangled tool that he just has to have to complete the job.
4 – Functional. Yes, this is a fun project, but it will be wholly unsatisfying if it doesn’t work at the end of it.
With that being said, the style that I liked the most was one designed by Joe Radabaugh, a long time advocate of the solar cooker.
Get together the materials:
For this project, you will need two cardboard boxes, square if you can find them, one slightly smaller than the other so that it fits inside. For a medium sized cooker, the inner box should be about 12”x12” and 9-12” deep. The outer box should be two to three inches bigger than the inner box in all dimensions.
You will need some additional pieces of cardboard (flat and single strength), some white paper glue, one small roll of 18” wide heavy duty aluminum kitchen foil, and a piece of double strength glass that fits about ½” larger than the length and width of the inner cardboard box.
Finally, you will need some flat black paint, a box cutter or other blade, some string, some cotton cloth (recycled clothes or rags will do), and a baking tin that fits snugly inside the inside box.
Together, this shouldn’t run you any more than $10. What a deal!
- Create some insulation in the base of the oven. Cut up some of your additional cardboard pieces and lie them flat in the bottom of the outer box. You will need enough layers to make it so that the inner box sits about 1” below the top rim of the outer box when placed on top of them.
- Take care of your flaps. Keep two opposite facing flaps of the outer box folded out and then fold the other two flaps in so that they are between the inner and outer boxes. Fold all the flaps of the inner box out, so they lie between the inner and outer boxes.
- Insulate the sides of the oven. Cut up your additional cardboard and start wedging it between the inner and outer boxes until you can’t get any more in. These insulation pieces must come to the same height or slightly lower than the height of the inner box as the glass will be placed on top and you want a tight seal. Also, note that this cardboard insulation may shrink after some uses, so you may need to repack with some additional cardboard pieces.
- Paint the inside of your oven black.
- You can cut some handholds in the outer box to make your life easier.
- On four flat pieces of cardboard, trace out four reflectors – the pattern is here. The dimensions will need to be adjusted depending on the size of your box. Cut the reflectors out and then using a blunt point, make a crease along where all the dotted lines are in the pattern. Bend in on these crease lines.
- Bend the upper and lower flaps all the way over and glue them down. Put some weights on them to keep them down while the glue dries.
- Roll out your kitchen foil over one of the collectors and make a mark where the side flaps are as you will want to cut the foil just short of these side flaps as you don’t want to cover them.
- Cut out all your foil for all four collectors and then mix together two parts glue with one part water. Lie your foil down dull side up and spread the glue mixture over this dull side. Take one of your collectors and hold it so that the folded upper and lower flaps are facing you and then lower the other side down onto the glue side of the foil and try and centre it. If you mess it up, peel off the foil and try again. Turn the collector over and smooth out any wrinkles in the foil.
- Gluing the collectors together. For a rectangular box, lie the collectors out as they would fit onto the box (i.e. square, rectangle, square, rectangle). Glue together the side flaps of each of the square-rectangle sets. When the glue has dried, put three to four holes into the remaining side flaps. When the oven is assembled, two opposing corners will be laced together with sting and the other two corners will be glued. This will enable you to take the collectors down and store or transport them as two pieces. Assembling the collectors for the square box is somewhat more complex and I refer you to Radabaugh’s instructions.
- Make an additional piece to help attach the collector to the oven. Cut out a piece of cardboard that’s 16” by whatever length the glass is. Make creases along the 16” side at the 6” and 12” mark from one end. This should create three segments, two 6” wide and one 4” wide. Then fold, along these creases starting at the four inch end so that the four inch part is folded right in. Cut a piece of your scrap cotton that is the length of the glass by 6”. Place the folded cardboard down on a table so it opens like a book with the folded 4” piece on the top (i.e. like the cover of the book). Take the piece of material you’ve cut and glue it with 3” hanging out to what would be the inside of the back cover of the book.
- Once this additional piece has dried, glue the 3 inches of material that is hanging out to the doubled-over lower flap of the collector.
- Place a dark baking tin in the bottom of the oven as a rack.
- Buff the edges of the glass piece you have so that they aren’t sharp.
- To assemble, direct your oven to maximize sun exposure.
- Poke holes in the flaps that were left out of the outside box as well as in the top and bottom of the collectors so that the collectors can be tied to the flaps using some cord.
- When the oven is fully assembled, the additional piece attached to the collectors should be slipped in between the cardboard insulation of the upper sidewall to easily attach it to the oven.
- Happy cooking!
Four tips to make your solar oven more effective:
- Use black pots – anyone who has sat on a black vinyl seat in summer knows the heat absorption power of dark colours!
- Use reflectors – though there are a number of solar oven styles that don’t include reflectors, you will find that the oven will be far more effective with at least some form of reflector to help direct more of the sun’s rays into the oven.
- Insulate well – Luckily insulation doesn’t have to be expensive. Cardboard is perfect and be sure to have a good thick wall (~1.5-2 inches) on all sides in order to prevent heat escaping from your oven.
- Aim it at the sun – as cooking will generally be over a long period of time, the position of the sun will change while cooking. It will be worth it to check on the oven every half hour or so to readjust it so that it is aimed as directly at the sun as possible. This may mean tilting the bottom up to catch the afternoon sun. A good way to know you have the best position is to check whether the shadow cast behind the oven is equal on both sides.
If you really aren’t the DIY type, there are commercial brands available. The Global Sun Oven is one of the most widely-used brands. It’s lightweight and can be carried easily and set up is very quick. However, it is going to cost you about $250 USD. There is also a hybrid out there by Sun BD Corporation. On sunny days it’s a solar cooker, and for those not so sunny days it can be plugged into a 120V outlet. A perfect combination for outdoor living. However, this one will run you about $300 USD.
Suddenly more interested in becoming the DIY type?
There are lots of videos out there on how to make a solar oven. This was actually one of my favourites simply because it showed a couple of different models at the beginning and then some very easy step-by-step instructions along the way. Unfortunately, he never finishes the reflectors, and I can’t find part 2, so if you can find it, please post the link!