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re: a 12' non-cube
13 mar 2001
georges wrote:
>...if it takes more than one bounce, then you are really not using
>the "focusing" properties of the parabolic reflector
but we don't care, for heat collection with a large target, except
that extra bounces are somewhat rare and only cost 10%. consider
y^2 = 12x, and a ray that hits the parabola directly above the focus
at (3,6) from some off-axis angle theta above the horizontal axis.
the reflected ray hits (0,0) when theta = atn(1/2) = 26.56 degrees.
where i live at 40 n lat, the sun only reaches 90-23.45-40 = 26.55
degrees at noon on 12/21, with no extra bounces.
duffie and beckman talk about this on pages 345-357 of their 1991 book
"solar engineering of thermal processes":
it is possible to construct concentrating collectors that can function
seasonally or annually with minimum requirements for tracking... these
non-imaging concentrators have the capability of reflecting to the
receiver all of the indident radiation on the aperture over ranges of
incidence angles within wide limits. the limits define the acceptance
angle of the concentrator. as all radiation incident within the
acceptance angle is reflected to the receiver, the diffuse radiation
within these angles is also useful input to the collector.
these concentrators had their origins in instruments for detection of
cherenkov radiation in high-energy physics experiments, a development
noted by hinterberger and winston (1966)... their potential as
concentrators for solar energy collectors was pointed out by winston
(1974) and they have been the basis of detailed study since then by
welford and winston (1978), rabl (1976a,b) and many others.
nick
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