Magnetic Field Sources - Magnetocaloric Energy Conversion: From Theory to Applications - page 69

Fig. 3.27 Comparison of different cryocoolers a comparison of the Carnot efciency for different
cryocoolers depending on the temperature of cooling; b carnot efciencies of different cryocoolers
at a cooling temperature of 80 K. Modied gure, originally published in [ 29 ]; published with kind
permission of © [IOP Publishing, J Phys: Condens Matter 2009]. All Rights Reserved

active magnetic regeneration), there will be a temperature gradient established over
the matrix of the passive regenerator.
Figure 3.27 shows a comparison of the Carnot ef

ciency (exergy ef ciency) of

different cryocooler technologies. These are also brie fl y described in the following

text.
The temperature range of Brayton (recuperative) cryocoolers is from 4.2 to
120 K. The maximum refrigerating capacity of a single refrigerator is 20 kW at a
temperature of 4.5 K [ 31 ]. These kinds of refrigerators are broadly applied in LTS
magnets. Their advantages are that they have a steady operation, long durability,
good ef ciency for large sizes; however, they are expensive, require a large heat

exchanger, and therefore they are not so compact, and have strict operating
requirements [ 29 31 ].

The Joule-Thomson (JT) (recuperative) refrigerator represents a steady
operation and applies heat recuperation. This kind of refrigerator applies gases that
have a strong dependence of enthalpy versus pressure. It mainly consists of four
parts: compressor, counterflow heat exchanger, JT valve and a reservoir. The
refrigerant gas is compressed at room temperature and the generated heat is rejected
to the ambient. Gas then enters the warm part of the counter heat exchanger, where
it is precooled to a certain temperature. Then it enters the JT valve and expands
during the adiabatic-isenthalpic process where it is lique

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ed at a cryogenic tem-
perature and at the initial pressure. The liquid refrigerant leaves the system and is
used for refrigeration, whereas the remaining gas

ows through the counter current
heat exchanger towards the compressor. Since there are no

fl

“ cold ” moving parts in

the Joule Thomson cryocoolers, these kinds of devices can be scaled to microsizes

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[ 29 31 ].

Another type of cryocooler relates to the Gilford

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McMahon (GM) (regener-
ative) refrigeration cycle. This kind of thermodynamic cycle comprises the fol-
lowing four processes: adiabatic charging (compression), isobaric cooling, adiabatic

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