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re: parabolic cooker question
18 apr 2001
nospam)acer-access.com (m russon wrote:
> i work with a guy who is interested in making a parabolic cooker.
>the parabolic dish is the easy part. however, the design would include
>a hole in the center of the large dish to reflect the concentrated
>light back through the center of the dish using a smaller parabolic at
>the larger dishes focal point... i am not sure how we would figure
>the correct focal point with the smaller dish in order to get proper
>beam size without too much divergence...
sounds overly complex. you might look up "cassagrainian telescope."
>the whole idea behind this is to eliminate any need to be in front
>of the dish, and to eliminate drippage or spilling onto the large
>parabolic dish reflectors.
what's wrong with being in front of the dish? it might be large compared
to your shadow, especially if it is far away, with a long focal length.
you might put a secondary mirror under a rusty galvanized metal selective
surface under some thermal mass (a 2' steel cube, sand, rebar, concrete,
copper pipes...) under a grill, with a hinged cover containing 6" (r20)
fiberglass insulation and r20 sides. with the cover on, keeping the cube
1000 f on a 30 f day might require 24h(1000-30)20ft^2/r20 = 23k btu/day,
eg 46 ft^2 of reflector at 500 btu/ft^2-day.
steel and water have roughly the same heat capacity by volume. if rc
= r20/20ft^2x2^3(64btu/f) = 512 hours, the 1000 f cube would cool to
400 after -512ln((400-30)/(1000-30)) = 493 hours, or 21 days...
nick
the most serious mistake was making the outer container of the receiver
of plywood. we thought that the plywood would be sufficiently insulated
from the copper panel which was the receiver proper, that it would not
get too hot. the copper panel was separated from the plywood by 4" of
fiberglass insulation. nevertheless, the plywood caught fire and the unit
was completely destroyed. we suppose this is a success, of sorts...
from "a solar collector with no convection losses," (a downward-facing
receiver over a 4:1 concentrating parabolic mirror) written by
h. hinterberger and j. o'meara of fermilab, ca 1976
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