Magnetic Field Sources - Magnetocaloric Energy Conversion: From Theory to Applications - page 70

discharging and isobaric heating. The GM cryocoolers apply a passive regenerator
and a displacer. Their frequency of operation is rather low (1 Hz). The advantages
of these cryocoolers, despite their rather low exergy ef

ciency, are in the simplicity
of the device, compactness, small vibration and the reliability. There are two
general types of GM cryocoolers. The single-stage cryocoolers, which can operate
down to temperatures of liquid nitrogen (77 K), and two-stage cryocoolers, which
can operate down to temperatures of liquid hydrogen (20 K) or liquid helium
(4.2 K) [ 29 31 ].

The Pulse-Tube (regenerative) refrigerator is another type of the cryocooler,
which generally consists of a compressor, regenerator, gas piston, ori

-

ce, gas

reservoir and other elements. A Carnot ef ciency of about 20 % at 80 K has been

reported by Radebaugh [ 32 ]. Pulse-tube cryocoolers apply the pressure oscillating
fl

ow through the hot end ori ce (similar to a displacer in a GM or Stirling device).

In the rst type of pulse-tube cryocoolers, the compressor and the rotary valve are

applied. Pressure oscillators (valve-less compressor) are applied in another type of
pulsed tubes. Therefore, one can distinguish between the GM and Stirling type of
pulse-tube refrigerator, respectively [ 32 ]. The GM pulse-tube refrigerator operates
at very low frequency (1

2 Hz), and is ideal for small cooling powers and low
temperatures (e.g., down to 2 K). The Stirling type operates at high frequency
(50

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60 Hz) and with the lowest operating temperature of about 10 K. Compared to
the Stirling and Gifford-McMahon refrigerators, there is no need to apply a dis-
placer in a pulse-tube refrigerator. This brings the advantage of the latter because of
substantially reduced vibration, increased durability and reduced axial heat con-
duction [ 29

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31 ].
Stirling (regenerative) cryocoolers apply the Stirling cycle, and they can be
applied for the lowest temperature at about 12 K. The frequency of the operation
can be as high as 60 Hz. In these types of cryocoolers the heating of the gas occurs
during the compression, and cooling during the expansion of a gas refrigerant.
Stirling cryocoolers (similar to GM) apply a displacer in order to move the gas
refrigerant to the hot end during the compression (or to the cold end during the
expansion). These cryocoolers are suitable for applications of the HTS magnets
(e.g. in the range from 20 to 77 K and from the cooling power in the range from 50
to 500 W. Stirling cryocoolers are more ef

-

cient than Brayton cryocoolers, less
sophisticated, more compact and they do not require strict operation conditions,
which are necessary in Brayton cryocoolers [ 29 31 ].

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3.4 Permanent-Magnet Designs in Magnetic Refrigeration

The rst magnetic refrigerator based on permanent magnets was developed in 2001

and presented by Lee et al. [ 33 ], and later by Zimm et al. [ 34 , 35 ] (note there are
certain indicators of earlier prototypes see Chap. 7 , however, the information is

not supported by a photography). Since then, most of the magnetic refrigeration
prototypes that have been developed, whether for experimental purposes or as

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