This solar oven is made out of materials that are very easily available and not very expensive either. Most materials used to build this one were scrap materials. We use it in Hungary during summertime when the days are sunny and dry. Temperatures are usually 30-35 degrees Celsius, but for some food it will also work with lower temperatures. As long as it is sunny outside!
This is not the first solar oven we built. The first one was made out of cardboard and we have used it for about 4 summers. That one worked just as good as this one, only this one is somewhat more durable and even easier to use. I do not have any pictures of the construction of the oven, but I don't think that really matters. I'll explain how I built it and what materials I used but I think there's many ways to do this and I'll tell you what I think is important to make this thing work.
On a hot sunny day temperatures in this oven may reach 120 degrees Celcius. We have used the oven to cook many things, such as pasta, rice, pasta sauce, soup, sausages, chicken, vegetables, mince, muffins and more. It is convenient because it doesn't take much of an effort to cook your food this way. More important, of course, is that it is super cool to cook your food using only direct solar energy!
Please note, some pictures in this Instructable are those of the cardboard version. I don't have many pictures of the improved version yet and the cardboard version shows the principles just as well as the new one.
Step 1: A Well-insulating Box
One of the most important parts required to build this oven (if not the most important) is a box with great heat insulation. It doesn't need a lid (top). For this one we happened to have a styrofoam box that was originally used to transport Dry Ice, so that was very convenient. You can of course use another styrofoam box if you can find one, but if you cannot you can just get some styrofoam or other material from your local DIY to built a box from. What I think is important is that the box is not too high, because than it will be difficult to reflect the solar rays into the box, as your pots or pans will be at the bottom of it.
Step 2: An Outside Box
A styrofoam box will not be very firm, hence it is wise to built a box around it. For this one I used OSB plates that were left from the building of a shack (no idea what those plates are called in English. Anyway, just some kind of cheap 'wood', this stuff is actually quite water resistant) to built a nice firm box around it. I even gave it some handles on the side that came off an old kitchen cupboard to make it easier to move the whole thing. This one is always on grass but if it's not you could also consider adding wheels.
Step 3: A Glass Plate to Close the Insulating Box
To close the insulating box you'll need something that will let the light through. So I guess any kind of transparent plate will do the job. In this case I happened to have a poster frame that fit my styrofoam box really well. It doesn't need to be attached or anything, just lay it on top.
Step 4: Flaps
To make sure the solar rays are reflected into the box, you need flaps. The back flap is the most important one. The side flaps don't do so much but I think they're good for preventing convective heat flows that would carry heat away from your solar oven. The size of the back flap is not very important. In my case it is such that it exactly fits on top of the box when closed. This is large enough. The side flaps both go halfway when closing them, as can be seen in the images in the step of the outside box.
You'll also need something to function as a hinge. I took a rubber tube and cut it open, then nailed it to the box and the flaps as can be seen in the picture. This works quite well but there are many ways to do this of course. Furthermore you'll need something to keep the flaps at the right angle. I happened to have some of those things you use to keep your windows open available which I mounted to the box and the back flap as can also be seen in the pictures. This works really great and I liked it because it didn't require me to buy anything other to suit the purpose. The side flaps just lean to the sides of the back flap.
Step 5: Reflective Material
A wooden surface won't really reflect solar rays that well of course, so you'll have to add reflective material to your flaps. I just covered them in aluminum foil, which works great. Make sure you have the shiny side facing towards you. Aluminum foil does tend to rip very easily when touched, which could be reduced by using thicker aluminum foil or something else entirely. I have some silver-colored tape that is used under laminate flooring (to tape the isolation material beneath it) which I apply everywhere the aluminum foil gets damaged and I also use it to tape the different sheets of aluminum foil together, because the rolls are usually not broad enough to cover the flaps in one go.
Furthermore I covered the inside of the aluminum box with the insulating reflective stuff that is normally used behind radiators. This is to prevent your styrofoam box from melting, which actually happens due to radiating heat from the black pots and pans you'll be using. This stuff by itself is not enough so I recommend you put some mass in between, which is also good if you don't have material like this. For example a piece of plasterboard covered in aluminum foil will do the trick.
Now make sure you aim the back flap such that the light is reflected such that it falls onto your pot or pan or whatever you're trying to heat.
Step 6: Black Pots and Pans
To make sure the pot or pan you're cooking in absorbs as much of the heat in the oven as possible, make sure it is black. We also have a large black pan that does great and what we also use is one of those metal things in which you can make 12 muffins.
Just really make sure it is black and put a coaster underneath to prevent the hot pan from sinking into the bottom of your oven....
Step 7: Cooking Your Food
Now it is time to cook your food. Like said earlier, you can cook many things in this thing, just go and experiment with it, it's real fun.
Just to give you some examples, we cook our pasta and rice in this thing. You can do this even later in the day when it is no longer at its warmest. It will just take a little longer, but hey, no need to stay beside it and it won't boil over. We found that brown rice is difficult to cook in this oven because the outside won't soften that well. Your meat will do fine in this thing as will many sauces you try to make. Vegetables also cook well in this thing.
We've also used it to bake muffins. For these sort of things it is important that you start early such that it is in the oven when it is warmest outside, which is usually between 13:00 and 15:00, depending on where you live. The muffins need the high temperature to bake.
Now, all you have to do to cook your food is turn the solar oven a bit about every half an hour so it remains aimed at the sun. Maybe you could even create a system that automatically rotates the oven to go with the sun.... Also don't try to stir or turn your food every now and then, but don't take of the glass too often because you'll lose a lot of heat. Last, give it some time. I doesn't even take very long for your food to cook, but it doesn't go as fast as it does on the stove. The upside is that you don't have to do much about it, just remember to put it out there in time.
Step 8: Some Final Remarks
So now you know what I've used to build this solar oven. It can of course be even more extensive or, much simpler. As I said earlier, my first version was just a cardboard box from which I cut away one of the flaps. I covered the others with aluminum foil. The cardboard box also contained a styrofoam box just like this one and had a poster frame with glass plate on top. The flaps were held upright by sticks. You can see this version of the solar oven in some of the images. Just see what you can make without using too many new materials because it kind of opposes the general thought of durability.