X-Message-Number: 2091
Date: Sat, 10 Apr 93 03:50:26 CDT
From: Brian Wowk <>
Subject: CRYONICS Hybrid System

        I have been thinking about Steve Harris' idea for a hybrid 
thermoelectric/mechanical refrigeration system.  Basically the idea 
is that a mechanical system cools a heat sink to -79'C and a 
thermoelectric system then operates between this temperature and 
-130'C.  If the mechanical system ever needs maintenance then 
ordinary dry ice can be put in the -79'C heat sink with no 
disturbance in system operation.
        While the idea of a simple backup is excellent, I think the 
hybrid system will be too inefficient.  Thermoelectrics have 
characteristically lower efficiencies than mechanical systems.  As
Marvin Minsky has intimated, 5% is a good coefficient of performance
for thermolectric modules operating between even modest temperature 
differences.  (We would certainly not be able to do better operating
between -130'C and -79'C.)  This means that moving 200 watts out of
the Cold Room thermoelectrically would dump 4000 watts into the dry
ice, with most of this being waste electric heat.
        I think a better solution is to refrigerate mechanically all 
the way down to -130'C and use LN2 as backup as Steve originally 
suggested.  Only 200 liters of LN2 per day would be required to keep 
things going, compared to *several tons* of dry ice per day in the 
hybrid system.
        Temperature control and stability with an LN2 backup system 
can be just as good as the dry ice system.  In the refrigeration 
system I currently envision, all heat would be pumped out of the room 
through a single heat exchanger located near the room fans.  (This 
assumes a system operating below the thermal ballast melting point).  
The central portion of the heat exchanger could be an LN2 reservoir 
(normally empty) with insulation designed to pass 500 watts between 
-130'C outside and the LN2 inside.  Refill the reservoir once a day 
or so, and the heat exchanger will absorb heat exactly as if your 
refrigerator was still running.  Your room would hardly know the 
                                       --- Brian Wowk

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