Apparatus for magnetocaloric refrigeration

An apparatus for magnetocaloric refrigeration, comprising a ferromagnetic terial arranged in a rotor and alternately entering and exiting from a stationary magnetic field when the rotor rotates, and also comprising a circuit for a working gas coming into successive heat exchange contact with a ferromagnetic material arranged outside the magnetic field and thereby cooled, a refrigeration load, a ferromagnetic material located within the magnetic field and thereby heated as well as an external negative heat source. In order to avoid problems with seals in such an apparatus at low temperatures, it is suggested that discrete members consisting of ferromagnetic material be arranged around the rotor circumference so as to be angularly offset, that two members be connected each time to form a pair by a flow path for the working gas, this flow path being arranged within the rotor and leading from an outer surface of the rotor via heat contact with one member of the pair into the center of the rotor and then via heat contact with the other member of the pair to an outer surface of the rotor again, that at least one stationary supply conduit ending in a sealed manner at the outer surface of the rotor and one stationary outlet conduit beginning in a sealed manner at the outer surface of the rotor be provided for the working gas, both conduits communicating with the flow path of one pair in a predetermined angular position of the rotor, that each outlet conduit be associated with a stationary magnetic field such that the member adjacent the outlet conduit is located within this stationary magnetic field and the other member of the pair outside the magnetic field in the predetermined angular position of the rotor, and that a cooling conduit be guided through a hollow bearing shaft of the rotor and have a refrigeration medium flowing through it, this refrigeration medium acting as refrigeration load and coming into heat contact with the working gas in the center of the rotor.

The invention relates to an apparatus for magnetocaloric refrigeration, 
comprising a ferromagnetic material arranged in a rotor and alternately 
entering and exiting from a stationary magnetic field when the rotor 
rotates, and also comprising a circuit for a working gas coming into 
successive heat exchange contact with a ferromagnetic material arranged 
outside the magnetic field and thereby cooled, a refrigeration load, a 
ferromagnetic material located within the magnetic field and thereby 
heated as well as an external negative heat source. 
It is known that ferromagnetic substances are cooled when they are 
withdrawn from a magnetic field. If this process is carried out in cycles, 
a continuous refrigeration may be obtained in this way when it is possible 
to supply the refrigeration capacity resulting during demagnetization to a 
refrigeration load and also to remove the heat resulting during subsequent 
magnetization of the ferromagnetic substance. In the practical embodiment 
of such arrangements an attempt is made to transfer the quantities of heat 
by means of working gases which flow in heat contact with the 
ferromagnetic substances. Quite considerable difficulties are encountered 
in this respect since the ferromagnetic substances have to be displaced so 
that they enter and are withdrawn again from the magnetic field. It is 
difficult or even impossible to manufacture reliable seals for low 
temperatures. 
It is known, for example, to rotate a ring of ferromagnetic material such 
that one region is always in a stationary magnetic field while another, 
oppositely located region is not in a magnetic field. The ring consists of 
a porous ferromagnetic material, into which a working gas may be 
introduced. This gas may flow in the circumferential direction of the ring 
and then leave the ring again. The working gas is hereby guided in a 
circuit such that it is first introduced from an external negative heat 
source into the annular region of the ring not located in the magnetic 
field and cooled in this region. The gas then comes into heat exchange 
contact with the refrigeration load, exits in the region of the ring 
located in the magnetic field, hereby absorbing the heat resulting in this 
region, and finally supplies this heat to an external negative heat source 
(J.Appl.Phys. 49 (3), pages 1216 et seq., March 1978). Although this 
method appears to be operable in theory, in practice considerable 
difficulties have resulted due to the sealing problems encountered, 
particularly in the cold region. 
The object of the invention is therefore to develop an apparatus of the 
type in question further such that difficulties with seals in the cold 
region can be avoided. 
This object is accomplished in accordance with the invention, for an 
apparatus of the type described at the outset, in that discrete members 
consisting of ferromagnetic material are arranged around the rotor 
circumference so as to be angularly offset, that two members are connected 
each time to form a pair by a flow path for the working gas, this flow 
path being arranged within the rotor and leading from an outer surface of 
the rotor via heat contact with one member of the pair into the center of 
the rotor and then via heat contact with the other member of the pair to 
an outer surface of the rotor again, that at least one stationary supply 
conduit ending in a sealed manner at the outer surface of the rotor and 
one stationary outlet conduit beginning in a sealed manner at the outer 
surface of the rotor are provided for the working gas, both conduits 
communicating with the flow path of one pair in a predetermined angular 
position of the rotor, that each outlet conduit is associated with a 
stationary magnetic field such that the member adjacent the outlet conduit 
is located within this stationary magnetic field and the other member of 
the pair outside the magnetic field in the predetermined angular position 
of the rotor, and that a cooling conduit is guided through a hollow 
bearing shaft of the rotor and has a refrigeration medium flowing through 
it, this refrigeration medium acting as refrigeration load and coming into 
heat contact with the working gas in the center of the rotor. 
With this arrangement, the entire circuit for the working gas on the cold 
side is conducted through the interior of the rotor and no parts which are 
movable against one another occur in this region. A seal is required only 
on the outer surface of the rotor, i.e. on the warm side of the working 
gas circuit. This means that no sealing problems caused by low 
temperatures will occur. 
In a preferred embodiment of the invention, with one pair the beginning and 
end of the flow path form a circumferential angle of 360/2n degrees, 
wherein n is a whole number, that n supply conduits and n outlet conduits 
are uniformly distributed around the circumference, supply conduits and 
outlet conduits hereby alternating, and that each outlet conduit is 
associated with a stationary magnetic field and the member disposed 
upstream of the respective outlet conduit is located within said magnetic 
field. 
Due to this construction, each magnetic member is subject to a plurality of 
magnetizations and demagnetizations during one revolution of the rotor and 
these are used by various working gas circuits, one after the other, for 
cooling the refrigeration medium flowing through the center of the rotor. 
This results in a quasi-continuous removal of heat from the refrigeration 
medium. The uniformity of heat removal may be increased by increasing n. 
It is particularly favourable in this respect for a plurality of supply 
conduits and outlet conduits to be connected in parallel with one another 
and communicate with an external negative heat source. This results in a 
quasi-continuous operation on the external side. 
This uniformity of operation may be improved in that the rotor has up to n 
pairs distributed around the circumference of the rotor such that the flow 
paths of all the pairs communicate simultaneously with supply and outlet 
conduits. For example, a total of four ferromagnetic members may be 
arranged in the rotor and offset relative to one another by an angle of 
90.degree. in the circumferential direction. Each magnet of each pair then 
passes into a magnetic field twice during one rotation of the rotor, i.e. 
each of the four magnets is magnetized twice and demagnetized twice during 
each rotation, and so cooled working gas comes into heat contact with the 
refrigeration medium a total of four times during each rotation. 
It is advantageous for the members to consist of porous ferromagnetic 
material and to fill the cross section of the flow path so that the 
working gas passes through the porous ferromagnetic material. 
In this respect it is possible for the members of ferromagnetic material to 
be arranged in rotor chambers, through which the flow path passes, and for 
a heat exchanger duct to be disposed between the chambers accommodating 
the two members, this heat exchanger duct connecting the chambers and 
leading through the hollow bearing shaft of the rotor. 
This heat exchanger duct preferably has the shape of a helix. 
In a preferred embodiment, the supply and outlet conduits are encircled by 
stationary seals abutting sealingly against the outer surface of the 
rotor. In this way, the supply and outlet conduits communicate each time 
with the flow paths of the individual pairs when the rotor is in 
predetermined angular positions. In the remaining angular positions of the 
rotor, the supply and outlet conduits are closed by the outer surface of 
the rotor.

A disc-shaped rotor 5 is rotatably mounted by a central hollow shaft 6 in a 
stator housing 1 comprising a bottom wall 2, a cover 3 and a 
circular-cylindrical side wall 4. The bottom wall 2 and cover 3 have 
central openings 7, in which suitable bearing rings 8 are held. The hollow 
shaft 6 communicates with a central interior space 9 in the disc-shaped 
rotor 5. In a manner not apparent from the drawings, the hollow shaft 6 
communicates outside the stator housing 1, and in a sealed manner, with a 
gas supply and a gas exhaust means and so the hollow shaft 6 and the 
interior space 9 have a gas flowing through them which is designated in 
the following as refrigeration medium. 
Located in the disc-shaped rotor 5, in the vicinity of its periphery, are 
four chambers 10 which are offset relative to one another through 
90.degree. and are filled with a member 11 consisting of a porous 
ferromagnetic material, for example a gadoliniumgallium garnet Gd.sub.3 
Ga.sub.5 O.sub.12. 
Each chamber 10 is separated from the hollow interior space 9 of the rotor 
by a partition 12. Every two adjacent chambers 10 are connected with one 
another by a helical heat exchanger duct 13 which is disposed in the 
interior space 9 such that the refrigeration medium flowing through the 
hollow shaft 6 and the interior space 9 is in intimate heat contact with 
the heat exchanger ducts 13. 
Each chamber 10 has an opening 14 which is directed radially outwards and 
leads to the periphery of the rotor 5. 
Two diametrally opposed supply conduits 15 and two diametrally opposed 
outlet conduits 16 for a working gas are provided in the side wall 4 of 
the stator housing 1. These conduits are arranged on a level with the 
central plane of the rotor. The points at which the supply conduits 15 and 
the outlet conduits 16 pass through the side wall are each surrounded by 
an annular seal 17 held on the side wall 4. The front faces of the annular 
seals abut sealingly on the circumferential surface 18 of the rotor. 
Supply conduits and outlet conduits are at right angles to one another so 
that, when seen in the circumferential direction, a supply conduit and an 
outlet conduit are alternately disposed every 90.degree.. 
The supply conduits 15 communicate with the exit side of a heat exchanger 
20 by means of a common branching conduit 19. The outlet conduits 16 
communicate with the entry side to the heat exchanger 20 in the same 
manner, by means of two conduits 21 which are joined together. A 
circulating pump 22 or compressor is provided for conveying the working 
gas. In the heat exchanger 20, heat contact is provided with a negative 
heat source which is not illustrated in more detail in the drawings. This 
may be, for example, a magnetocaloric refrigeration stage operating at a 
higher temperature or any other refrigeration machine. 
Two diametrally opposed coils 23 and 24 of a superconductive magnet are 
arranged above and below the rotor 5. In this respect, coils arranged 
above and below the rotor 5 together form a magnetic field penetrating the 
rotor in its circumferential region. The arrangement of the coils 23 and 
24 is such that the chambers 10 with the ferromagnetic members 11 arranged 
therein enter the magnetic field when these chambers 10 are in flow 
connection with the outlet conduits 16 (FIG. 2). 
During operation, the hollow shaft 6 has refrigeration medium continually 
flowing through it. When the rotor 5 is located in an angular position in 
which the openings 14 of the chambers 10 are aligned with the supply 
conduits 15 or the outlet conduits 16, respectively, working gas can flow 
into the non-magnetized chambers 10 via the supply conduits 15 and through 
the ferromagnetic material contained therein. As this material has been 
demagnetized due to withdrawal from a magnetic field, it has a low 
temperature and cools the working gas flowing through it. In the heat 
exchanger duct 13, the gas subsequently transfers refrigeration capacity 
to the refrigeration medium flowing through the hollow shaft 6 and the 
interior space 9 and reaches the adjacent chamber 10 after it has been 
correspondingly heated. This chamber is located in a magnetic field. The 
temperature of the ferromagnetic material is thus increased due to 
magnetization, the working gas flowing through this ferromagnetic material 
is heated as it flows therethrough and, in this way, conveys heat to the 
external heat exchanger 20 where this heat is removed from the working gas 
again. This circulation of the working gas takes place only when the rotor 
is in an angular position in which the openings 14 of the chambers are 
aligned with the supply conduits 15 and the outlet conduits 16. If the 
rotor rotates out of this position, the outlet conduits and supply 
conduits are first closed until the openings 14 of the chambers are again 
aligned with supply conduits and outlet conduits following a rotation 
through 90.degree.. Due to this rotation, the chambers which were 
previously located in the magnetic field are now in the region without any 
magnetic field and vice versa. At the same time, the chambers which 
previously commuicated with supply conduits now communicate with outlet 
conduits and vice versa. When the working gas begins to circulate again, 
it is cooled in the same manner, refrigeration capacity is transferred to 
the refrigeration medium and heat is removed from the magnetized 
ferromagnetic material and transferred in the external heat exchanger 20. 
In this respect, the ferromagnetic materials in the two chambers 10 
connected via the heat exchanger duct 13 have exchanged roles in 
comparison with the previous position. Following a further rotation 
through 90.degree. the roles are again exchanged. The two chambers 10 
which are filled with ferromagnetic material and connected via a heat 
exchanger duct 13 therefore form a pair, the partners of which are 
alternately magnetized and demagnetized, whereby one partner is always 
magnetized and the other partner demagnetized. 
It is an advantage that the seals 17 are always located in the 
high-temperature region of the working gas, i.e., on the one hand, prior 
to cooling of the gas in the demagnetized ferromagnetic material and, on 
the other hand, following heating of the working gas by the magnetized 
ferromagnetic material. No seals are required in the central cold region 
of the rotor. 
The embodiment illustrated in FIG. 3 is essentially constructed in the same 
way as that of FIGS. 1 and 2 and the same parts therefore have the same 
reference numerals. It differs from the arrangement of FIG. 2 in that the 
rotor has a total of eight chambers 10 distributed uniformly around its 
circumference, two adjacent chambers being connected each time with one 
another via a heat exchanger duct 13. Consequently, four supply conduits 
15 offset relative to one another through 90.degree. and four outlet 
conduits 16, also offset relative to one another through 90.degree., are 
provided as well as four coils 23 and 24, again offset relative to one 
another through 90.degree.. By multiplying the number of ferromagnetic 
members 11 and also the number of magnetic fields around the 
circumference, each pair is subjected to magnetic cooling eight times 
during one rotation of the rotor. This means that, during one rotation, 
refrigeration capacity can be passed to the refrigeration medium from each 
heat exchanger duct 13 a total of eight times. This means that the 
operation is very uniform, i.e. a quasi-continuous cooling of the 
refrigeration medium is achieved. 
This multiplication of the number of magnetic members 11 around the 
circumference could be continued correspondingly. In each case, the 
advantage remains that the seals on the outer side of the rotor are 
located in the warm region and no seals are required in the cold region.