In September I made a "compound" parabolic solar cooker. I took half a parabola,
twisted it round its focus by 22.5 degrees, made a form to the curve and then
used this to make a dish mold. I folded and cut corrugated plastic sheet over
the mold to make the dishes. It worked really well. The theory is that it
concentrates the light (even when not pointed right at the sun) to an area
behind the focal point for up to 3 hours. (Which is where I place the cooking
pot)  This is very useful for solar cooking. Parabolic dishes on the other
hand, produce a point of concentrated light which rapidly becomes a spread out
streak of light as the sun moves. (So the dish has to be constantly re
adjusted).
So, it worked really well, BUT. I only did the math in 2 dimensions. I do not
know what happens in 3 d in real life.  Especially supposing the sun's path  is
 a bit above or below where the dish is pointed. What happens?
This is important if someone in a poor country makes a stationary large cheap
solar cooker with mud backing the reflector.  Will the light all still focus on
the cooking pot for almost 3 hours a day? Or will a large amount of light miss
the pot completely?  A 30 degree twist to a half parabola will give a 4 hour
collector if rotated round the axis.  This might be more suitable in that
situation.  I will be modeling the situation soon with a little clay model and
a laser pointer to represent the sun. (I will have to move the sun a lot to
model it!) Anyone want to try with povray?
Another useful task is modeling a parabolic dish NOT pointing directly at the
sun.
 People make parabolic dishes all the time but then find that there is no
tracking system accurate enough to make them useful!  I have video of the
compound parabolic dome mold on utube and there is some info at the link below
too. Thank you for your time.
 Brian White


http://www.instructables.com/id/Compound_parabolic_dish_making_a_template_with_te/

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