By Steve Belanger — Would you like to make a solar oven to cook with the sun’s energy, but think it’s too expensive, or too complicated? Think again!
Countryside’s Steve Belanger made the ovens shown here at a cost of pennies, in 1-2 hours each. Note that this is a project anyone can use, even on an apartment balcony! Once you see how easy it is to build a solar oven, you’ll want to have one in your off the grid living supplies.
Box in a Box
This simple solar oven consists of two cardboard boxes, one inside the other. The tops of the two boxes are level, but there’s about an inch of space between them on the sides. This space is filled with crumpled newspaper for insulation. The inside of the smaller box is covered with aluminum kitchen foil attached with Elmer’s glue.
The cover was constructed from the cardboard of a third box. It was cut, folded and taped to fit snugly over the outer box. The reflector was made by cutting, on three sides (see illustration), an opening as large as the inner box. On the inside of this cover, we glued a sheet of glass from an old window, which we cut to fit. Again, Elmer’s glue did the job. Glue aluminum foil on the inside of the lid flap reflector, prop the reflector up with a stick… and you’re ready to cook with the sun!
- Three cardboard boxes (or two, if you can find one with a ready-made cover);
- 8 feet of aluminum foil;
- 1 piece of glass;
- a stick;
- rubber band;
- Elmer’s glue.
Construction time: About an hour and a half, not counting scrounging up materials.
Cost: Pennies, for the foil; everything else is easily scrounged.
A solar oven can be any size you choose to make it, but it will be most efficient if it’s just large enough to hold the container you’ll be cooking in. Aside from that, the deciding factors might be the size of the glass or boxes you have on hand.
Note also that heavy plastic could be substituted for the glass, although with some loss of efficiency, and other insulating materials could be used instead of newspaper. Avoid foam materials that might be susceptible to outgassing, however.
This oven, known as the SunStar, is a bit more complex and takes longer to construct, but it’s still simple. The basic “oven” itself is a box in a box (in a third box, as shown in the drawing, but we only used two). The main difference here is in the more efficient reflector.
1. Find or make three boxes that will nest together with about an inch of space between them. Fill the spaces with crumpled newspaper or other insulating material.
2. A glass cover goes over the inner boxes. It should fit snugly to minimize heat loss. One oven maker suggests using weather stripping to help create a seal. Since this glass is the oven door and will be handled frequently, it’s a good idea to tape the edges for safety.
The inside can be painted black, or covered with black paper or other material.
3. The reflector is the tricky part. The pieces should be cut at a 65°-70° angle. You can determine this by using a protractor. If you don’t have one handy, fold a piece of paper in half at one corner as shown below, and then in half again, giving you a 22.5° angle (90° ÷ 2 = 45° ÷ 2 = 22.5°). Place this on a 90° corner of a piece of cardboard and cut it on that angle. (90° – 22.5° = 67.5°.)
You’ll need four of these reflector panels. If your oven is square, they’ll all be the same size. If you used rectangular boxes, measure the length and the width to determine the length of the bottom of the reflector panel and cut two of each.
Assemble the pieces with tape. Glue aluminum foil to the inside.
4. We found suggestions on several ways to attach the reflector to the oven box. One involved poking holes through the outside oven box flaps and the reflector, and lacing them together, as shown here. (Note that extra flaps were left on the four reflector panels so they can be laced together too. This would be a good idea if storage or transportation is a concern because the reflector can then be dismantled when not in use.) We simply taped it all together.
5. The oven needs to be tilted toward the sun. This could be accomplished by propping it up on rocks or a log, by making a stand, or by setting it inside a larger box that will hold it at the correct angle.
With the box tilted as shown here, a level shelf inside is a necessity when cooking anything liquid. Taken into account when determining the size, this merely involves one more piece of cardboard.
Using the Solar Oven
We’re located above 45° latitude (closer to the North Pole than to the equator), and when these ovens were tested in April the noon sun was only about 45° off the horizon. On a cloudless day (also somewhat rare here in April) with an air temperature of 50°, these ovens easily reached 200°, and with a little time and proper positioning, 250°. It took about four hours to bake four small potatoes, but the wait was worth it: in a taste test, the slow-cooked sun-baked spuds won hands-down over those baked in the kitchen. Not bad for a small outside oven!
We baked some potatoes in a covered cast iron pan, and wrapped others in foil. The cast iron took longer to heat up, but resulted in a superior product. The pan also stayed hot when the sun went behind a cloud. Dry beans also take about 3-4 hours, but other vegetables, cut-up pieces of chicken and fish should be cooked in an hour or two.
Using A Solar Oven: Cooking with the Sun
Solar ovens are made-to-order for simple homesteading creativity … meaning there’s nothing “made-to-order” about them! The size might be determined by the size of the boxes you can find, or on the size of the piece of glass you have, just as easily as by what you intend to cook. Some people use thermopane insulated glass to increase efficiency; others use two sheets of ordinary window glass, providing an airspace between the two by cutting a “gasket” sort of spacer from cardboard or other material. Some people paint the cardboard to preserve its lifespan, while others build ovens from plywood or even metal.
In other words, there are no rules, no specific measurements, or even ironclad directions. You study the concept and let your creativity go.
After building one, you might want to try another design, or to experiment with improvements. Indeed, many solar oven enthusiasts have several, often cooking and baking at the same time. (Yes, with a little experience, you can bake cookies, pizza, and even bread, in a solar oven. Putting a heat sink—a dark-colored brick or rock or chunk of metal—in the oven will help “preheat” it while the bread dough is rising and will result in more uniform baking.)
When using a solar oven you’ll have to pay attention to a few things that microwave users never consider. Obviously, the sun must be shining, and the brighter the better, and the higher the angle the better.
For maximum efficiency, make your oven as small and as shallow as you can for the utensils you’ll be using. Then use the smallest pot possible, preferably black. Cast iron works, but a thinner pot will heat up faster.
Use as little water as practical, and cut the foods into small pieces for faster cooking.
Making a Solar Oven: The “Cookit” Folding Panel
This model has no glass cover, making it the simplest and cheapest oven we made, but was the least effective. (Many solar oven designs were developed for use in Third World countries where cooking fuel is scarce or unavailable, and where glass is probably hard to come by too. This one was reportedly widely used by refugees in Kenya, where they get more sun than we do in Wisconsin in April.) Cooking in boilable bags might work well with this unit, but we haven’t tried that.
Technically, this is considered a cross between a solar oven and a curved concentrator. Ours measures 12 inches deep and 38 inches wide, with the back reflector being 13 inches high and the front one 11 inches.
Originally published in the July/August 1998 issue of Countryside & Small Stock Journal and regularly vetted for accuracy.