"Compound" parabolic solar cooker, make a template with technical drawing

Step 4 A little about compound parabolic dishes and parabolic dishes

For now I am just leaving an animated gif here to show the differences between parabolas and compound parabolas for dishes. Please note that this is very much a shot in the dark for me. There are likely to be errors (but not substantial ones) in my explanation. Eventually others may confirm or improve on what I explain but I wouldn't hold my breath. Please go with this until something better comes along.
Note. Parapolas are like circles in that there are large and small ones but the fundimental shape is exactly the same. Unlike circles, nobody has ever seen a complete parabola. (the ends go on infinitely).
Compound parabolic dishes can have their curves come in and meet up. I show the MAXIMUM EXTENT for useful curve for a 30 degree (green), 45 degree(blue) and 60 degree(yellow) compound parabolic dish in the animated gif. Note of course that making the maximum extent would be costly and you can just use a cost effective portion nearer the focal point just as people do with parabolic dishes!
For my compound parabolic dishes all I did was twist one half of the parabola round its focal point.
This concentrates the light in the area between the back of the dish and the focal point as long as the incoming light is within the acceptance angle of the dish.

Comments

[robbtoberfest]

Is the energy less intense on the focus than a standard parabolic reflector because your sacrificing direct aim at the sun?

[gaiatechnician]

Yes, the energy ends up in an area behind the focus. So you put your cooking pot in that area to catch the heat. Ideally you make the cooking pot the same size as that area. The important points are that although it is less intense, it is (almost) the same amount of energy. And in my view, "sacrificing direct aim at the sun" is no sacrifice at all! I discovered last summer that keeping a parabolic dish automatically pointed exactly at the sun is really really hard! No matter what you do, the seasons can screw up your parabolic tracking device or at least add substantially to the cost. Having a device with virtually the same power ( without the need for exact pointing at the sun and constantly readjusting its aim is a super advantage). It means that cheap tracking (the clock based tracker or even the water clock tracker) might be good enough to keep food cooking or water pasturising all day. Brian