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"Compound" parabolic solar cooker. Suggest a way forward! Answered

I have a new instructable (Late December) about making your own templates with technical drawing tools. You can basically make the compound parabolic dish to catch 1,2,3 or 4 hours worth of sunlight. I have made a 3 hour dish and it worked great as a solar cooker. I have not however solved the math for exactly how the light reflects to the cooking pot. It is pretty good but not yet optimized. Would anyone like to join in? I can either make the instructable a collaboration or you can post your refinements on appropedia on their wiki.
I do not want this project to lie stillborn like the pulser pump did for 20 years but how to engage people? Thats the real problem. Thank you
Brian White 21st dec 2008

and it has worked really well. I would like others to try it so I made a template. I chalklined across the dome at 15 degrees intervals, Put clear plastic over it, marked the lines on the plastic and cut the plastic. Basically all you have to do is cut a piece of ply to be like the template, use it to cut out 12 mylar segments and then tape them together. (I do not know how you will hold this umbrella open, thats up to you!)
My dimension lines were vertical on the paper, it just looks angled from the picture. Figures (all in inches) are 0.5,15.5,17,31,7,12,2.5 and 3/4 just in case you cannot read them. Its a funny looking template but I think it is good enough of an approximation for this. I devided the 2 long curves into straight lines.
Should be fine. I also have a pdf file containing results, etc as the second image file. Just click on it and it should open. (solar for group.pdf) Brian


I am interested in making a dish like this, however, I can't make out the dimensions on the image showing the template. Do you have a cleaner image? I really don't want to make the cob mold, and prefer the template. thanks.

I am making an instructable just on how to make a template using ordinary technical drawing. But I will just put in template measurements here until that instructable is done. Check above or in this reply, I should have it done some time today 17th dec 2008 the template is for a dish with the focus 7.6 cm from the bottom. the total depth of the dish is 11.7 cm and the radius of the dish is 12.4 cm You have to multiply out the dimensions to suit your project.


There's a big empty space in your post. Did it hold a video?

I haven't got the time to make the actual reflector, but as to holding it in shape, I would suggest making two parabolas. One out of the mylar, the other out of transparent plastic. Stick them edge-to edge, insert a valve, inflate with air and you have a reflector that is light and rigid enough to mount with sticks and string.

Yeah, I think it is fixed now. Sorry. the video shows how I made it.
I have the template to save people the trouble of making a big cob dome in their back yards.
Just note that my shape is a "compound" parabolic dish. It isn't a parabolic dish.
I have made one with the dome mold and it works extremely well.
At my latitude 48 in September on a good day I brought a 7 liter pot of water to 90C. On the 13th it brought water in the pot to 99C. (The outside of the pot was painted black and the pot was covered with a turkey bag).
The really good thing is that this shape has to be turned only once every 2 hours. A parabolic dish needs to be moved every 15 or 20 minutes!
I thought it might be a good emergency design for hurricane victims.
So if you make it and you find it works you might be doing a good thing.
Perhaps if it was made from mylar, all you need to make it hold its shape would be a wire round the rim and perhaps 2 wires curving to the top of the dome. Like in those collapsable laundry bags they have everywhere for a couple of dollars.
You can pasturise water in it and cook stew no problem.
For instance you could leave water in it overnight with it facing the right way for the morning sun. At say, 10 or 11 am, you water is at 70C, you remove some for drinking water, and add food to the rest, readjust the aim and in an hour or 2 you will be eating a for a big pot of stew!
The optics are explained at http://www.youtube.com/watch?v=uX9Z-nsUHiA
Once again, it isn't parabolic, it is compound parabolic.
It is designed for up to 3 hours of unattended cooking at a time in a big pot.
A parabolic solar cooker cannot do this!

OK, I get it now.

Like I said, I can't help physically right now, but that looks like a really good scheme.

A little googling informs me that [http://en.wikipedia.org/wiki/Cob_(material) cob] can shrink, which may wreck your parabola, but adding sand or fine gravel prevents this.

Of course, places that use this could also build their dome out of concrete or clay.

Old plastic signs aren't always available, but simple papier mache (paper-and-paste style) should work well, and kitchen foil or silver paint should serve as acceptable replacements for the mylar. I have a feeling that, in the areas most likely to benefit from this, even white paint could provide a sufficiently reflective surface.

Good luck, and keep up the good work.

With the compound parabola, it does not need to be perfect as an ordinary parabola has to be so shrinkage is fine. I have a pdf file with results but I do not know how to post it here. Is there a way, anybody?

Just a note that I tried my template to make a "umbrella" from mylar and it did work. (but it had lots of wrinkles) It did cook potatoes on the first go. So the template should work for anyone who wants to do the plastic sign thing. (without doing a cob dome). My dish gave results that are comparable (not quite as good) to results I saw for a sk14 bringing water to near the boil in India. SK14 is a German factory made parabolic solar cooker. (It has to be moved more often to follow the sun) than my cooker. Brian

Treat it as an image - click "add images" at the top of a post, and go from there.