X-Message-Number: 2046
Date: 01 Apr 93 04:14:32 EST
From: "Steven B. Harris" <>
Subject: CRYONICS Peltier Heaven

Dear Cryonet:

   Look, I originally proposed the design with heat conductor at
liquid nitrogen temp, separated from the  -135 C space by
insulating foam to get the proper thermal gradient, in order to
simplify things, and allow liquid nitrogen to be used as primary
coolant.  If, however, we are complicate things and have a bunch
of Peltier effect coolers* keep our liquid nitrogen liquid, then
let me point out that we might as well use the coolers to
primarily keep our ethyl chloride vat slurried at the higher temp
of -135, and forget the liquid nitrogen completely!  In such a
design we'd have to have much thicker aluminum heat conductors
radiating from the central ethyl chloride heat dump out to our
hexagonal cryogrub cells ("cryogrub" = frozen TV dinner for
whatever creatures come to inhabit the future Earth...), and
essentially no insulation between conductor and the inside of our
cells at all.  Still, it would be a much simpler design than one
that had to deal with the 60 C temp gradient of LN2 to -135 all
the time.
  
   If the purpose of using liquid nitrogen as primary coolant is
for thermal balast, this can all be done with ethyl chloride.  If
the purpose of using liquid nitrogen is to have something as
backup if power fails, this is still very possible:  just immerse
a heat exchange coil in our ethyl chloride pool (or our vat of
vitrified silicone "silcool" containing sealed ethyl chloride
dinosaur eggs, if you like that safer design better), and when
the power fails you can still run liquid nitrogen through the
thing from a tank until all the ethyl chloride is frozen and the
temperature begins to drop.  Then stop until temp begins to rise
a bit again.  This is best a manual job, I think (designing a
reliable system that would do this automatically year in and year
out I'm sure would be a bigger peice of engineering than it first
appears), but in the special case of a powerout emergency such a
manual job performed every few hours (or days?) doesn't seem
excessive.  Even easier, of course, would be to have a backup
generator running on deisel, as has been suggested.

   Of course, all this may be academic.  Peltier-effect cooling
modules are available to cool down to -50 C (perhaps an 80 C
differential) but I've never seen one that will do to -135 C, let
alone -196 C.  Nor is it clear to me that you can simply stack
them-- although this should work in theory, in practice there may
well be absolute material limitations.  We'll need some expert
advice (let me see what I can do to dig it up).  If such things
as -196 Peltier modules were available, they'd perhaps be of
great benefit to cryonics NOW, in that we could simply stick a
few completely through the foam insulator caps on all our Dewars,
in order to pump some heat out of them electrically.  That would
keep LN2 boiloff down, which not only might be enough to pay for
the Peltier piles in a few years time (since as has been pointed
out electric cooling is a lot cheaper than liquid nitrogen
cooling), but also (maybe more importantly even if the economics
are bad) would be a great safety feature during long power
outages, since (again) it's a lot easier to stockpile diesel oil
to run generators than it is to stock liquid nitrogen.  And the
capital costs would be considerably less than for buying our own
small (room-sized) deisel generator driven liquid air plant,
which is our next step as is.  When it comes to LN2, best
conserve rather than make anew.  Waste not, want not.

                                                Steve


* Peltier effect: a sort of reverse thermocouple effect, where
application of voltage to a junction made of two materials of
dissimilar electromotive potential, results in heating of one
material and cooling of the other (i.e., heat is pumped "uphill"
against a temperature gradient by the power of the electricity).
This essential refrigeration function is done by a solid state
module working on low voltage, and without moving parts (neat,
eh?) although a fan is typically added to the design for better
heat sink efficiency (to keep the hot side closer to room temp). 
If you want to fool with them, bismuth based Peltier devices are
used in all the "auto cigarette-lighter socket powered" beer
coolers that can be found on the market now for around $125
(check your local Home Depo or large camping or hardware store). 

                                                S.

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