Instructables

A mould for cardboard and plastic solar cookers

video A mould for cardboard and plastic solar cookers
This video shows a way you can make a mold for big cardboard or plastic or sheet metal solar cookers. I have made one solar cooker with this method. It is a compound parabolic reflective dish and it keeps the reflected sunlight in one area for up to 3 hours.
If you want to try a a similar dish, without making a mold, I may put a template (made from my mold) online soon. Comment if you want it.
thank you
Brian White
gaiatechnician (author) 5 years ago
Please note that the main video has changed. Originally it was suggesting something to try and hoping that one of you might go out and do it. I ended up making a COMPOUND parabolic dish. Compound parabolic dishes have AWESOME advantages from the DIY perspective. See my optics experiments video for more details on compound parabolic dishes. 1 They do not need to be exactly pointed at the sun. 2 They have an acceptance angle. With mine, the acceptance angle is 45 degrees which means that you can get a maximum of 3 hours cooking before you need to repoint the dish at or ahead of the sun. 3 You can make them to suit your constraints. Mine, for instance was made to heat a 7 liter metal pot. 4 If you do a tracking parabolic dish, you will find that it has to be aimed very accurately or it simply will not heat anything. But with compound parabolics, you can adjust the acceptance angle to allow for error in you tracking and timing systems. This means that you can use much cruder or cheaper tracking and aiming and still get almost as good performance. This is extremely useful! Brian
nagutron6 years ago
Great idea, by the way. You should definitely make an Instructable with higher-resolution images of your device and the diagram you drew. It would also be cool to see the cooker in action!
gaiatechnician (author)  nagutron6 years ago
http://www.instructables.com/id/ERKOK1WF5Y3YSRE/ has a very similar picture of the mechanical mathematician for making parabolic dishes directly. (And the mathematician does not need a spring so it is more reliable). Perhaps it is just as easy to form the cardboard on that dish? I have scavenged the dish I made. It is now part of the walls of my cob shed. Somebody could try with an old satelite dish. It should work nearly exactly as what i did with the globe in the video. Please! Somebody, give it a try with cardboard, tape it together add kitchen foil and see if it is any good!
This looks to me like a compass. A curve drawn with a compass isn't actually a parabola. While it will still cook, a true parabola will cook much faster.

Also, the globe isn't parabolic either (same reason as above)

As seen on this instructable's comments, it's a big deal.
I said that I didn't have a parabolic mould handy so the globe substituted to show how the cuts were done. It isn't a compass, it is a variation on the mechanical mathematician. It draws a parabolic dome. Why not make one and see?
Nagutron clarified how your device works for me.
I was pretty confused about this, too, but it made more sense to me once I spent a few minutes sketching it out on paper. The string is attached to the saddle and to the top of the central post, sliding freely through the eyelet at the top of the sliding tube. This means that the constant length of the string will cause the bottom of the sliding tube to describe a perfect parabolar, not a sphere as you suggest. I've attached an image below that explains why this works. See also the Wikipedia entry on Parabolas.

Your link to the other Instructable's comments was useful. Here are some other cool ways to get a parabola, without calculation:

  • Shine a flashlight at an angle to a wall; the shape that it makes is a "conic section" and is thus a perfect parabola.
  • Hang a string from two points. The shape that it hangs in is also a perfect parabola.
  • Spin a container of liquid. The shape that the liquid will pull out into is a perfect parabola.
  • Stretch a membrane over a container with a circular opening, and then lower the pressure inside the container. The suction will pull the membrane inwards into the shape of a parabola.
parabola.png
Note also that the device in your instructible produces a solar cooker that gives a line of concentrated light. This one gives a solar cooker that concentrates to a point (more or less).
cool video, but you should make it an instructable with good pictures, just like what nagurton said
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