Parabolic Solar Oven

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Posted in WorkshopSolar

Introduction: Parabolic Solar Oven

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!

2 People Made This Project!

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Is there a blueprint for this? You reference a blueprint in your spreadsheet.

63 Comments

Hi!

Many thanks for your great instructions!

I would like to replicate the model, but step 3, that is bending the plywood, seems to me quite a demanding task, because it is not clear to me how to bend the plywood in the right shape. Did you use any mold or the like?

Thanks in advance for the reply!

Davide

2 replies

Hi davide,

I didn't use any mold, the shape of the petals was enough to make the parabola. I tightened the cable ties going from the center to the exterior of the parabola, one row at time. The parabola will gradually take its shape. I also used ropes on the exterior side of the petals . Each rope connected two opposite petal and was tightened slowly to help the positionning of the cable ties. It did help for the bending.

Hi Yasintoda,

many thanks for your clarifications. It's a good information to know that there is no need for a mold and that the parabola will gradually take its shape thanks to the cable ties and the supporting ropes. Your example is really a good and economic solution for a parabolic oven, thanks again for having shared it, I hope to be able to replicate the model in a similar way!

Bests
Davide

Your parabol solar cooker is amazing!!! Congratulations for the project, and many thanks for sharing!!!!! :)

I wonder if poster board and mylar emergency blanket would work. It would be lighter maybe easier to work with (bend) It would need a rigid frame but I am thinking 3 different size rings of wood or something like irrigation tube. I do like the frame for holding not only the parabola but the cooking pot as well. It allows it to be rotated to safe position for storing.

Ask how to calculate a petal, please explain in detail

Awesome design

My ply wood wont bend without snapping. It is 5mm plywood exactly like you used. How can I fix this problem without spending crazy amounts of money?

1 reply

drop little quantity of water in the plywood. It Could help you to bend your plywood sheets.

Can 1.25m dia. parabolic collector work??

excellent work ,i was going to do the same thing,

how much the reflective duct tape cost you ?

2 replies

thanks! it was 20€ for the 2 adhesive aluminium tapes

thanks for answering。

Good job, Nice idea
to recycle old satellite dishes. You still can use those dishes and watch
satellite TV. http://pengangkutan-ekspedisi-di-medan.blogspot.com/
You should try it to recharge or create a power source. Use aluminium offset
printing plates. You can get them from offset printing companies. Using them
will be lighter, quicker, cheaper, water proof, more precise and pretty much
just as reflective.

youtube.com

yasintoda - I made a python script (based on your spreadsheet) to auto generate the svg templates (petals.svg, discs.svg) according to different parameters. I can send it to you if you like (let me know who)..

Thanks for the great instructable!

Following my comment that oven bags might not be suitable, Lonejack wrote: "Not true, The oven bags are made to withstand a standard over up to 450 degrees."

I would like to point out that parabolic reflectors solar ovens can reach temperatures at their focal point of 290°C (554°F).

Here is an upscale solar cooker on the market.

file:///C:/Users/PAUL/Documents/Solar%20cookers%20and%20water%20purifiers/One_Earth_Designs_Press_Kit.pdf

https://www.oneearthdesigns.com/shop/

The cost is $499 USD, 2014
If You are serious about solar cooking, this is a good buy, if You consider that this unit would last several years if maintained correctly.

Nice.

A clear oven bag around the pot will decrease your cooking time a lot, tho you'll have to vent the steam somehow or it'll fog.

Maybe look at using aluminium offset printing plates instead of the plywood and alu tape. You can get them super cheap from offset printing companies (they can only use them once) and using them will be lighter, quicker, cheaper, water proof, more precise and pretty much just as reflective. To cut them you can score them metal with an exacto, give it a couple of flexes, and tear out the line.

Main consideration with them is that they're highly anisotropic, which means they reflect tightly in one direction but completely blown out in the other, so you'll need to make sure to curve them across the grain. Contact me if you have any questions about this, I've worked with them a bit (currently making a pure alu parabolic trough) so can fill you in on the details.