X-Message-Number: 2122
Date: Fri, 16 Apr 93 18:02:00 CDT
From: Brian Wowk <>
Subject: CRYONICS Cost of LN2

        I talked to a local LN2 supplier today (Canadian Liquid Air) 
and was quoted 34 cents per liquid liter for bulk purchased LN2.  This 
is the retail price.  The rep wouldn't give me an exact wholesale 
price (after all, these things are negotiated) but suggested he might 
give a 1000-liter-per-week user a 30% discount off retail.  That would 
be 26 cents per liter.  Winnipeg is a reasonable-sized city with 
several competing LN2 suppliers.  It also has some of the cheapest 
electricity in the world (a major cost of producing LN2).  I therefore 
doubt you could do much better than 26 cents per liter anywhere.
 
        What follows below is intended to convince Mike Darwin and any 
other remaining sceptics that cooling a -130'C Cold Room with liquid 
nitrogen (except as emergency backup) is a profligate waste of money.
 
         Our Cold Room as currently conceived (100+ patient capacity) 
will require the removal of 500 watts at -130'C.  This would consume 
185 liters of LN2 per day, with an annual expense of $17500 at 26 
cents per liter (*not* including transfer losses).  Alternatively, we 
can pump 500 watts from -130'C using two P-550 cryocoolers with an 
annual electric bill of $7000 at 8 cents per kilowatt hour.
 
        The cryocoolers save $10,000 per year.  This is a 60% saving, 
not including amortization of the cryocooler cost ($12500 each).  What 
time period should we amortize over?  Well, it almost doesn't matter.  
Even if the reliability was so bad that we had to buy a brand new 
cryocooler every other year, we would *still* save money.
 
        None of this should be surprising.  As Perry Metzger has 
pointed out, even liquid nitrogen ultimately comes from a 
refrigerator.  And refrigerating down to -196'C (even using the 
relatively simple Joule 
Thomson process) is far more energy inefficient and mechanically 
complicated than refrigerating down to -130'C.  LN2 suppliers have to 
pay electric bills and repair breakdowns too.   All this is going to 
end up in the cost of LN2 that the end user pays.
 
        Let me recap: We have two P-550s operating together to keep 
the Cold Room at -130'C.  The room has 30000 liters of water ice 
ballast.  If one of the units fails, we have one month to repair it or 
replace it before the room temperature rises to -123'C and *stays* 
there, held by just one unit.  If *both* units fail, the room 
temperature will rise at 1'C per day until we get some LN2 for backup.  
Backup is simple.  Once a day, you just pour 200 liters of LN2 into 
the heat exchanger tank.  The tank insulation is fine tuned during 
commissioning of the room (before thermal ballast is added) to keep 
the room at -130'C automatically.
 
        Finally, there is one very important reason to design this 
room with a mechanical refrigerator even if the mechanical system was 
less economical than LN2.  (In such a case, the refrigerator would be 
the backup instead of LN2).  During times of civil unrest, a few 
thousand liters of diesel fuel could run an emergency generator that 
would keep one of the cryocoolers going for months.  At some point we 
might even consider a solar electric energy plant
 
        I hope this posting isn't overkill.  But when respected 
critics use language like "I hate them" (mechanical systems), I feel 
the need to respond strongly and decisively.  Are there still any 
dissenters?
 
                                        --- Brian Wowk    
 

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