X-Message-Number: 3157
Date: 18 Sep 94 12:59:17 EDT
From: "Andrew J. Clifford" <>
Subject: CRYONICS

SUPERCOLD REFRIGERATORS

	The 10th September issue of "New Scientist" carries an article
 (by Daniel Pendrick of Wisconsin) which details the imminent development 
of refrigerators capable of cooling their internal temperature to that of 
liquid HYDROGEN, (yes, they mean much colder than liquid nitrogen!).  And 
further that these supercold fridges will be available for domestic use in 
a hydrogen powered society.  These fridges are less than 10 years away 
from production.  There is no mention of cryonics but the implications are 
obvious.

"The principle behind the new refrigeration techniques is that some 
metals become hot when magnetised and cool down when demagnetised. The 
magnetocaloric effect.  So if the metals are moved during this cycle, they 
can transfer heat from one place to another.  The phenomenon was first 
discovered in 1918 and has been used by scientists since the 1920s to cool 
matter to temperatures close to absolute zero.  But what makes magnetic 
refrigeration a hot topic today is the discovery earlier this year of a 
magnetocaloric material, an alloy made of aluminium, erbium and 
dysprosium, that could make these machines cheap and efficient enough to 
compete with conventional refrigeration techniques at liquefying 
hydrogen."

	"Conventional fridges rely on a process known as the gas-compression 
cycle.  During this cycle, an easily liquefied gas is pumped round the 
fridge, absorbing heat from the inside and transferring it to outside. 
[..] the best gas-compression fridges are only 40 per cent efficient. But 
fridges using magnetic cooling do not use gases and can reach efficiencies 
of up to 60 per cent of the theoretical limit."

	The magnetic fridges also are environmentally safe as they do not 
use CFCs or similar substitutes.

	"Such systems could be available for certain niche markets within 
ten years says Lewis Lawton, director of the Astronautics laboratory. At 
present, magnetic refrigerators that work near room temperature use alloys 
of gadolinium, a rare-earth metal, but this costs about  $500 per 
kilogram."

	"Karl Gschneidner, a materials scientist at the US Department of 
Energy Ames Laboratory at Iowa State university, announced that an alloy 
made of aluminium, erbium and dysprosium can transfer heat 30 per cent 
faster." [and is 75% cheaper.]  

	Instead of improving the materials the fridges can be made to remove 
heat more efficiently, in stages.

	"The Astronautics fridge is a joint project with the US Department 
of Energy's Oak Ridge National Laboratory in Tennessee and together $2 
million on the programme.  The researchers believe that magnetic 
refrigeration could coot the next generation of superconducting devices to 
temperatures below -269 C.."

	The main problem with the fridges at present is heat leaking into 
the system until the efficiency of the cooling falls to 0% .  This means 
the temperature reduction can only be done for a limited range, whether 20 
C to -60 C or -200 C to -270 C.

"The cost of building superconducting magnets is one of the main 	
reasons why magnetic fridges are so expensive.  In 1989, a study sponsored 
by the Department of Energy estimated that a magnetic chiller for storing 
milk and processed foods in supermarkets would cost about $55 000 compared 
with about $10 000 for a conventional chiller.".

	"But John Barclay, a researcher who helped build the test devices at 
Astronautics and now at the University of Victoria in Canada, does not 
believe that customers will be willing to pay so much more for a 
relatively small increase in efficiency.  Magnetic cooling's best hope, he 
says, lies in liquefying gases at low temperatures.  Natural gas, for 
example, becomes liquid at -163 C and hydrogen at -253 C.  "The major 
advantages come when these machines span large temperature ranges," says 
Barclay.
	
	Further progress should see the fridges capable of cooling from room 
temperature to liquid hydrogen.  This has significant implications for 
cryonics.  Cooling would be simpler, cracking and damaged minimized, and 
the ultimate storage temperature set at an arbitrary level for optimum 
results.  The irony is that all those jokes about patients thawing when 
the electricity is cut off become plausible.  However, a diesel generator 
or solar panel backup is far easier to arrange than ones own source of 
liquid nitrogen in an emergency.


Regards,   AJC

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