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"Compound" parabolic solar cooker! And testing your reflector curves. Options
gaiatechnician
#1 Posted : Saturday, September 20, 2008 8:08:03 PM
Rank: Guest

Posts: 134,494
Parabolic dishes must be accurately pointed at the sun and they must be re positioned often.
In an effort to make something more user friendly, I came up with a compound parabolic dish.
This is good for up to 3 hours! of unattended cooking and does not need to be exactly pointed at the sun.
It is a 1 sq meter dish and it heats up water (or food) in a 7 liter aluminium pot. The pot is painted black and is covered in use with a turkey oven bag. The pot is about 10 inches wide.
Compound parabolic dishes have an "acceptance angle" that the designer chooses. (mine is 45 degrees) hence up to 3 hours of cooking. (I actually move it every 2 hrs).
I do not usually be home during the day so I use it to steam soil for seedlings.
(Steam soil or compost to 90C and you kill off the weed seeds and the bugs).
Please check http://www.youtube.com/user/gaiatechnician for the most recent 2 videos that give details of making the cooker and the optics.
I made mine on a mold.  to make things easy for you, I marked out an approximate segment.
 I chalk lined  the mold at 15 degrees, and marked and cut a piece of clear plastic which I then laid out on paper and drew round it. 
The photo very closely approximates what I had. I have straight lines for simplicity.
I have the photo with segment instructions on
http://groups.google.ca/group/Sustain-the-development/web/compound-parabolic?hl=en
You just cut a template out of wood and cut round that to make 12 dish segments.
It is up to you how you hold it open. I suggest a wire round the rim and perhaps 2 cris crossing to keep the umbrella shape.
there is a pdf file on some yahoo groups (solar cooking) that gives some performance results.
Brian White

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