Re: Here it comes
- From: Bill Ward <bward@xxxxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 16 Nov 2007 21:24:22 -0800
On Fri, 16 Nov 2007 12:54:27 -0600, jgraber wrote:
Jim Black <fmlast3@xxxxxxxxxxxxxxxx> writes:
Would the light on the solar panel still
be 1,200x as concentrated as the light striking the ground?
Across 1/1,200th the area - which is the point. Think of light on
the ocean floor at the beach. There are spots of light and spots of
darkness. The total amount of light doesn't change, merely its
distribution.
Which doesn't violate Liouville's theorem because, unlike the
scenario I'm asking you about, the ocean is not illuminated uniformly
from all directions.
But the ocean or a pool focusing light into spots of light and dark on
the bottom of the water describe precisely what's going on here.
Not at all. I asked you what would happen if your device (or a modified
version with a higher melting point) was illuminated with uniform and
isotropic 6000K blackbody radiation. If you did this in the case of the
water waves, you would not get bright and dark spots.
Is the Sun a source of non-uniform isotropic radiation at the collector?
How can isotropic radiation be uniform?
For example, you mean for the collector to be surrounded by a sphere
that was heated to 6000K? As the size of the surrounding sphere becomes
larger, the uniformity of the radiation as seen by the collector becomes
more uniform.
But you seem to be saying that under those conditions, your concentrator
would still work, and would focus all or most of the energy that hits
the fish-eye lens onto a much smaller region (casting a shadow on the
surrounding area, of course). This claim of yours is what I am
objecting to; it violates the second law of thermodynamics because the
increased power per unit area would enable you to heat something to a
temperature above 6000K. No calculation needs to be done to see that.
For a different concentrator:
Suppose a standard satellite dish parabolic reflector concentrates
sunlight from a 3m diameter to a 1cm diameter, and a synthetic ruby
downward reflector (some other name for it) concentrates the 1cm diameter
to a 1mm diameter; the 1mm focus is reported to be hotter than the sun. Is
this believable? (reported in Pop Science or some such)
If so, then it could theoretically use the sun as the cool end
of a Stirling engine. Does this violate the 2nd law? If not, why?
If you've transferred heat from a cool body (the sun) to a hot body (the
receiver), you've violated the second law. We're still here, so I kind of
doubt that.
.
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