This is an instructable to show how to make a parabolic solar oven out of plywood.
It's perfect for my utilisation : it makes one liter of water boil in 15-20 minutes and cooks a meal for 3 people in something like 45-60 minutes on good days
The parabola is approximately 1.5 m wide and 50 cm deep. The larger the aperture size, the more powerful it becomes. It's quite stiff as it is with the bamboo ring, but if you want to make it bigger, you'll need to find some other ways to stiffen it.
Step 1: The Petals
The idea is that if the parabola is cut symmetrically along 8 radial directions and flattened out, then it would appear like an 16-petalled flower. So once you have the petals made of any flexible material you can bend them in the shape of a parabola.
I used 5 mm thick plywood. I think you can use a thinner plywood, but then you'll need to add a stronger structure to stiffen the parabola. Have a look here, they use a metallic structure to keep everything stiff.
I used this publication to calculate the dimensions of the petals. I had to tweak it a little bit to have something i could actually use.
Here is a google spreadsheet with everything you need (you can download it to modify it). I've also included an .ods file called parabola calculator.ods. You can :
modify the focal length of the parabola
modify the number of petals
choose the length of the petals
I've also included a petal in svg that I created using Inkscape. You can laser-cut the petals if you have access to a fablab !
Once you've cut the petals, drill holes every 20 cm along the sides of the petals so that you attach them together with the plastic cable ties.
Step 2: The Discs
The discs will force the bottom of the parabola to be plane. Without them, it's just impossible to bend the plywood !
Cut two discs of 20 cm out of plywood. Divide them like a pie in 16 equal parts and drill holes 2 cm from the outside of the discs.
Drill a hole of the same width 8 cm from the base of each the petal.
Sandwich the petals between the two discs using bolts.
Step 3: Bending the Plywood
I've used plastic cable tie to hold the plywood together. I've found it easier to go gradually from the center to the edges of the parabola as shown in the pictures. You can also circle the parabola with a rope, it will help.
Step 4: Adding the Aluminium
The aluminium will reflect the sunlight on the cooking pot. I used 2 adhesive aluminium tapes of 25mx5cm which cost me 10 € each.
I've found that is better to add the aluminium tape when the plywood is already bended (else the surface won't be flat once bended)
Step 5: Welding the Pot Holder
The pot holder is on the focal axis of the parabola so that it can also be used as a pivot for the parabola.
The position of the pot holder depends on the shape and the size of your parabola but in any case you can find it in the spreadsheets of the first step.
Drill a hole on two opposite petals. I used an electric wood saw to cut an ellipse (the intersection of a parabola and a cylinder can be approximated by an ellipse). The cylinder I used has a diameter of 4cm so I cut an ellipse of 4x6cm.
The welding should be very strong! I've added two extra "branches" on each side because of the weigth of the pot (I have a pot of 20L).
The place where you put the pot should be a bit lower than the rest so that the center of the pot will be on the focal point.
Step 6: The Bamboo Ring
I used a big green bamboo pole that I cut in 4 to stiffen the parabola.
The parabola could be stiffer, but it's alright like this !
Bamboo poles are very easy to cut, use a big knife and a hammer. Cut it while green and flexible and then let it dry on the parabola.
Step 7: The Support and Extras
The support should be strong enough for the parabola and the cooking pot. I made it stiffer by adding wood along the diagonal.
I've also added a little handle to the parabola so that it is easier to rotate and to transport.
That's all! If there's any problem, feel free to comment or to contact me. Have fun!