CA1095596A

CA1095596A - High temperature magnetic refrigerator

Info

Publication number: CA1095596A
Application number: CA296,164A
Authority: CA
Inventors: William A. Steyert Jr.
Original assignee: US Department of Energy
Current assignee: US Department of Energy
Priority date: 1977-03-10
Filing date: 1978-02-01
Publication date: 1981-02-10
Also published as: FR2383410A1; DE2807093A1; US4107935A; IT7821027A0; JPS53113355A; IT1093658B

Abstract

ABSTRACT OF THE DISCLOSURE
A high temperature magnetic refrigerator which uses a Stir-ling-like cycle in which rotating magnetic working material is heated in zero field and adiabatically magnetized, cooled in high field, then adiabatically demagnetized. During this cycle said working material is in heat exchange with a pumped fluid which absorbs heat from a low temperature heat source and deposits heat in a high temperature reservoir. The magnetic refriger-tion cycle operates at an efficiency 70% of Carnot.

Description

~95~96 Background of the In~ention The utility Gf this invention is to provide a high capac-ity, efficient, inexD~nsive, and compact magnetic refrigerator in the temperature region of 2 K to room te~perature (293K).
Magnetic refrigeration involves the expulsion of heat into a high Lemperature sink from the magnetic working material thro~gh the application of a magnetic field. The subsequent removal of the magnetic field cools the working material and allows ab-sorption of heat from a iow temperature bath This process and l~ device operates at high Carnot efficiency, requires no massive gas compressors, and is compact because solids instead of gases are used as the working material.
Uslng the device and method of this inventiont the inven-tor has found that at high temperatures the lattice and elec-tronic contribution to the working material specific heat is very large. One of the problems solved is that although the wor~ing material ls able to remove a large quantity of heat frcm the fluid being refrigerated and to cool that fluid, it is unable to cool very far (not above a 30 K span) because of its own large specific heat. The inventor has solved this prob-lem by forcing a cold fluid to flow in intimate contact through a permeable wheel of magnetic working material and thus cool-ing the wheel in a high magnetic field. As the working ma-terial rotates out of the high field region, it gets even colder. This very cold wheel then is used to cool the fluid to this very low temperature. NOW the very cold fluid can ab-sorb heat from a thermal load~ In cooling this fluid, however, the whee]. has warmed up in preparation for being recooled by the fluid after it enters the high fiel~ region. During the 3~ cycle heat iQ expelled ~y Ihe fluid lnto the thermal reservoir.

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The magnetic working materials are operated near their ferro-magnetic Curle temperatures; thus, their own i~ternal spin-spin coupling enhances the externally applied magnetic field.
The principle of magnetic refrigeration is very old and, simply stated, the principle is that the application of magnetic field to a material warms the material and expels heat from the material into a high temperature thermal reservoir, The subse-quent removal of the magnetic field causes the material to cool and absorb heat from the substance to be refrigerated~ The fol-lowing is a list and abstracts of the most closely related artknown to applicant:
1. U. S. Patent 2,510,800, C, Chilowsky, is directed to the transformation of thermal energy into mechanical or elec-trical energy using paramagnetic bodies or ferromagnetic bodies and, using the Curie point of the materials, to effect the change from thermal to electrical energy, In addition, this patent suggests the use of liquid metals.
A distinction between the apparatus and method of the patent over the present invention is that the invention of the patent does not expose the stationary working material to a varying magnetic field. In this invention the rotat-in~ wor~ing material is exposed to both very large and zero magnetic fields during one cycle. The Chilowsky patent method is inefficient since the magnetization of a ferro-mag~let does not change sharply with temperature in an ap-plied field, even near its Curie point, 2. U. S. Patent 2,589,775, C. Chilowsky. The method of re-; fxigeration in an apparatus containing ferromagnetic sec~
tlons having Curie point temperatures approximating the desired temperature of refrigeration located in gaps in a ~955~6 closed ~err~magnet~c armature~ which co~prises causing a magn2tic flux to tra~er~e the armature, subjecting the ferromagnetic sections alternately to magnetization and de-magnetization, passing a fluid in heat-exchange relation with said sections alternately in opposite directions, such passage of fluid being so timed that the phase of magnetization of each section coincides with the passage of fluid in one direction and the phase o~ demagnetization coincides with passage in the opposite direction, removing heat from the fluid after passing a section in the ~ormer direction, and supplylng heat to the fluid after passing a section in the latter direction, whereby the space from which heat is supplied is refrigerated~ In particular, patent 2,589,775 differs in the following respects from the device and method of this invention in that the present invention uses rare earths instead of 3d elements (periodic table) because of their much 1 rger magnetic moment and a superconducting magnet with an intense and high field rather than an iron magnet. The primary distinction is - 20 that in this invention the material to be cooled is rota-ted in and out of the magnetic field, whereas in U. S.
Patent 2,589,775 the working material is stationary.

3. U. S. Patent 3,841rlO7, Arthur C. Clark. A magnetic re-frigeration system includes thermal transfer means com-prising a serial arrangement of magnetocaloric elements and a source of magnetic field, The serial arrangement comprises a material having a large, negative magneto-caloric effect which cools upon application of a magnetic field; a paramagnetic material in abutting relationship therewith which cools upon removal of a magnetic field;

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and end elements functioning as thermal switches~ The ma~netic field is caused to move along the serial arrange-ment, permitting heat to be transferred from a heat source to a heat sink. Cascading of the serial arrangements in-creases the refrigeration effect.
A distinction of the cited patent over the present in-vention is that U. S~ Patent 3,841~107 uses magnetic switch-es and therefore is useful only as a very low power re-frigerator since metals carry heat poorly compared to the forced flow of the present invention. In addition, the present invention involves rotating the magnetic material~
thus providing very rapid cycle rates.

4. ~. S. Patent 3,108,~44, D. Kahn. A magnetocaloric cryo-genic refrigerator comprising: a pair of spaced, ther-mally isolated heat reservoirs, a material having super-conducting properties thermally connecting said reservoirs with said material being the sole thermal connecting means between said reservoirs, means for subjecting sai~ mate-rial to a temperature sufficiently low to cause super-conductivity therein, means for subjecting only a portion of said material to a magnetic field of critical field in-tensity to cause said sub~ected portion, while thermally isolated, to revert to its normal state with a subse~uent decrease in temperatu-e and means for effecting progressive relati~e movement be~ween said material and said magnetic field to cause a net heat transfer from one reservoir to the other.
The cited patent uses superconductors ~hile the appa ratus of this invention uses paramagnets and ferroma~nets The Kahn patent separates the field from the fluid by a ;:

- 5 -~e~brane and thus has poor contact between the working material and the fiuid, The present invention forces fluid through permeable magnetic working material, 5. U. S, Patent 3,393,526, J. Pearl. Heat i3 pumped from one chamber, which is below the critical temperature of a superconductive ~aterial, into another chamber~ which is also below the said critical temperature, by placing the ends of a rod or rods of that material in heat transfer relation to the two chambers respectively and by applying a magnetic field, which is strong enough to cause a zone of said rod or rods to become normal, to the end of the rod or rods that is in heat transfer relation with the first chamber. When the zone on the rods becomes normal~ it withdraws heat from the first chamber, cooling it. Then the masnetic field, and therefore the normal zone, is moved along the rod to the second chamber, whereby the - second chamber absorbs the heat that is trapped in the normal æone and that moves with it. The process may be repeated to still further cool the first chamber. The Pearl paten~ differs from the high temperature refrigera-tor of this invention in the following ways: it uses metal into a forced mass transport to carry the heat and uses a superconductor instead of a.rotating para- or ferro-magnet.

6. IJ. S. Patent 3,413,814, J. R~ ~ran Ceuns. A method and apparatus for producing cold in which the entropy of a paramagnetic substance is alternately varied by varying an external parameter such as a magnetic field~ and a fluid mediur,l such as helium gas is flowed in alternate directions in heat-exchange relationship with the substance~ 3uring ':"

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the directional 1OWS heat and cold, respectively, are dissipated from the substance to the fluid, and corres-ponding to these flows there is heat-exchange relationship first by a portion of the fluid with an area absorbing heat from the fluid, and su~sequently, by a remote portion of the fluid with an area to be cooled, Fluid in the first area is at a generally higher temperature than fluid in the area to be cooled, and portions of fluid in the two areas are not intermixed~
The Van Geuns patent uses nonrotating nonferromagnets while this invention is directed to rotating para- or ferro-magnets since cooling is to be done about 20 ~ which re-quires the use of ferromagnets. U. S~ Patent 3,413,814 teaches that the fluld present at any moment in a cooled area never reaches the area to be cooled. The device of this invention teaches the opposite in that all the fluid in the cooled area will reach the area to be cooled, ro-tates the ~agnetic material in and out of the magnetic field, and the fluid is pumped through ~he material. Thus, rapid cycle rates are allowed in this device, Enormous utility exists for the device of this invention in that the cost of a high temperature magnetic refri~erator such as descrihed in this application is one~tenth the cost of an equivalent gas refrigerator and operates on one-fifth of the electrical power. Because high pressure compressors and gas-gas heat exchangers are not required, the magnetic re~rigerator . .
of this invention has a high degree of mechanical reliability.

In addition to its use as a low temperature magnetic refriger-ator, the device has further utility in that it can be used as : 3 n a magnetic engine, i~e~, a refrigerator run backwards. The ' .

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1~95~6 engine would convert low grade heat, for example, reactor waste heat, geothermal heat, solar heat, and ocean heat, into electricity in a most efficient and economical manner.
Summary of the Invention A high temperature magnetic refrigerator capable of a temperature range of refrigeration load from 2 K to 400 K and operating a-t a Carnot efficiency of greater than 70~
comprises in combination: motor and shaft means for rotating a wheel in a first direction, wherein the wheel is contained in a housing and wherein a superconducting magnet with a field strength of about 7 Tesla surrounds a portion of the housing. The wheel rotates in and out of this magnetic field. The wheel has a rim composed of a permeable rare earth with inlet and outlet means positioned on the outer periphery of the rim. By means of a pump, a fluid is circulated within the housing in a second direction (opposite to the direction of rotation of the wheel) and is in thermal contact with the rim. The fluid passes back and forth by means of multiple separators through the rim and then flows through a refrigeration load and heat exchanger by means nf the inlet and outlet means.
Brief Description of the Drawings Figure 1 is a schematic diagram of the process of this invention when it is directed to a refrigeration cycle.
Figure 2 is a graph showing the relationships between entropy and temperature for the working material consisting of Gd metal chips during a typical refrigeration cycle using , the device and method of this invention.
Figure 3 is a perspective view of the preferred embodiment of this invention with the wheel housiny area shown in cross sec-tion.

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Figure 4 is a top view in cross section of the housing and the permeable rare earth rim of the wheel of Fig. 3, with Fig. 2.
Figure 5 is a side view of another embodiment of the wheel of Fig. 3.
Figure 6 is still another embodiment of the wheel of Fig 3 ., , .

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-with exploded views 6a and 6b showing the specific features of the rim of this wheel, Fi~ure 7 ls a schematic drawing showing the process of this invention being adapted to a heat engine, Description of the PreLerred Em~od-im_nt As shown in Fig, l, fluid pumped at 10, at temperature TH
17 warms the rim 4 of the wheel in the low field region, leaving it at a cold temperature TC 5~ The fluid absorbs heat QC, then cools the rim from TH + G down to TC + ~ 23, leaving the rim at T~ + ~ 9. The said fluld is cooled to TH giving up heat QH, and reenters the wheel. Delta (~) is the temperature increase of the working material when subjected to the magnetic field~
Work is done to pump heat from TC to TH. This comes from the energy required to rotate the wheel, since the side of the rim 4 of the wheel entering the high field region is hot and com-paratively nonmagnetic and the region leaving the field is cold and very magnetic.
Figure 2 shows the entropy temperature curves for ferro-magnetic gadolinium metal. The gadolinium metal is heated by the fluid in zero field from TC (268 K) to TH (308 R). The rim of the wheel is then adiabatically magnetized to TH + ~ (316 K) as it enters the high fluid region. It is then cooled by the fluid in the high field (7T) to TC + ~ ~276 K), adiabatically demagnetized to T (268 R), and is ready to start the cycle over again, The work re~uired to produce this cycle is given by the area of the parallelogram-like graph, roughly 8 J/mole-K x 8 - 64 J/mole.
It may be advantageous to apply a small magnetic field (l T) to the center of the 3ero field region of Fig, l in order to make the actual Gd entropy curves correspond more nearly with - g _ ~9S~96 that shown in Fig.2bet~en TC and TH' As shown ln Fig. 3, a cold fluid ha~ing temperature TC
leaving the housing 3 at outlet 5 absorbs heat fro~ the load 13 in this case represented by a counterflow heat exchanger in which fluid from the load enters at 7 and is cooled by the fluid before it exits at 6, The fluid having temperature TC + ~ en-ters housing 3 at 23, absorbs heat in the field of magne, 8 from the rim ~ of the wheel, and leaves through tubing 9 at temperature TH + ~, It flows through the circulation pump 10 and deposits its heat in the thermal reservoir fluid of exchanger 11 thus cooling the fluid to a temperature TH. The fluid at temperature TH enters the housing 3 by means of tubing 17, de-posits heat as it flows through the permeable rim 4 of the wheel ^ reducing the temperature of the fluid to TC after it makes mul-tiple axial passes through the rim 4 through channel 16~ each separated by separator 15, and exits the wheel by means of out-let 5. The wheel is driven by shaft 2 connected to motor 1 and causes the wheel to rotate in a counter-clockwise manner~ The ; thermal reservoir fluid enters and leaves the exchanger 11 20 through inlet 12 and outlet 14. The device as shown in Fig, 3 consists of a 12-in. wheel having a rare earth rim rotating at 0.3 to 3 revolutions per second~ The wheel is 1/2-in. thick ~ with a rim dimension of 2 in. containing the permeable rare t earth such as gadolinium metal chips, As shown in Fig. 4, in order to minimize leakage between the high and low magnetic field regions, the inventor has de-signed a particular housing which promotes the flow of the fluid in a direction axial to the wheel. The fluid flows back and forth through the rim 4, as the arrows indicate, the 'luid 30 entering at inlet 17 as TH and flows back and forth through the ~ 10 --;5~

wheel by mec;ns G- channels 16 in the housin~ 3 until it reaches outlet ~ The housing 3 is so designed that the tolerances be-tween the hous1ng and the wheel are of the ordQr of a few thou sands of an inch, thus preventing any significant flow along the boundary edge between the rim of the ~rheel 4 and the separators 15. The flow through the porous rim 4 of the ~heel is greater than ~he leakage flow through the ~heel-housing gaps~
Figure 5 is a side view of another embodiment of the rim 4 in which multiple radial holes 22 have been drilled axially through the said rim. The diameter of these holes is of the order of 0.005 in., and occupies 10 to 40~ of the rim volume.
Figure 6 is another side view of ~he rim 4 of the wheel with blowups of two alternate designs incorporating grooves ; across (axially~ the rim. In particular, Fig. 6a shows layers of gadolinium metal in the lorm of a ribbon, said ribbon being about 0.01-in. thick and being spaced by means of wires 19, having a diameter of about 0.001 in., said wires acting as spacers. In addition to the spacers 19, the ribbons 20 and 21 have grooves 18 which axe embossed in the ribbon, said grooves 20 having a depth of about 0 005 in. The purpose of the grooves 18 ; and spacers 19 between the ribbons 20 and 21 is to provide a ; channel means for the fluid to flow across the spaced gadolinium ribbon. The number of windings of ribbon are of the order of ` 200-300. The grooves promote the flow of fluid across the rib-bon and provide rapid heat contact between the fluid and the working material--gadoliniu~ ribbon. Figure 6b is similar to Fig~ 6a except ~le flow is accomplished by means of spzcers 19 ~wires 0.001-in. in diameter~ inserted between successl~Te layers of ribbon 20 and 21.
Figure 7 is directed to a ~chematic flow diagram whereln the device O,r t~is invertion could be convexted to a heat en-gine, In this application the working material is heated by the hot fluid while it is inside o~ the magnetic field. Be-cause the working material is hot, the fer~omagnet is not very magnetic and little energy is expended in the process of rotat-ing it out oî the high field, The cold fluid cools the working material while it is outside the magnetic field at which point the working material is much more magnetic and will deliver . rotating power to the shaft 2 as it enters the high field re-gion, Temperature range of refrigeration load TC for various fluids is shown in Table l and working material is shown in Table 2. These fluids, if in the gaseous form, are pressurized to about 10 atmospheres, or enough to provide high density for good heat transfer. Generally, the highest magnetic field would be utilized, typically 7 T. The greatest fields result in the greatest capacity (most rapid cooling) and greatest temperature span.
Tahle 1 Fluid Ran~e for TC, K
-Llquid Na-K alloy 261 + 400 Water-ethanol mi~ture (pressurized) 156 t 400 Propane (pressurized) 90 ~ l90 N2 (pressurized) 65 ~ 120 Ne (pressurized) 30 + 90 H2 (pressurized) 25 + ~0 He (pressurized) 2 + 25 .

955~6 Table 2 Working Material Range for T , K Curie Temperature, K
Gadolinium metal 255 ~ 318 293 Gd39D~61 140 ~ 255 193 Gdl2DY88 80 ~ 160 120 GdN 40 ~ 100 65 Y2 rO.8 0.2 8 ~ 60 25 In order to achieve optimum performance of this device, it is necessary th~t the wheel and fluid capacity flow rates be exactly equal, i.e., MfCf = r~wCw whexe Mf is the mass of fluid per second moving from inlet to outlet and Mw is the mass of working material per second moving from fluid outlet to fluid inlet, The C's are the corresponding ,; .
` specific heats per unit mass.
The criteria for ade~uate heat transfer is that MfCfTH/hA~
be less than about 1. (hA is the product of the heat fluid-wheel heat transfer coefficient and A is the area of that con-tact. The refrigeration capacity is given by QC = MwCw~ when the heat transfer is ideal.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A high temperature magnetic refrigerator capable of a temperature range of refrigeration load from 2 K to 400 K
and operating at a Carnot efficiency of greater than 70%
comprising in combination:
(a) motor and shaft means for rotating a wheel in a first direction, said wheel contained in a housing, (b) surrounding a portion of the housing is situate a superconducting magnet with a field strength of about 7 Tesla, and wherein the wheel rotates in and out of this magnetic field, (c) said wheel having a rim composed of a permeable rare earth with inlet and outlet means positioned on the said rim's outer periphery, and (d) by means of a pump a fluid is circulated within the said housing in a second direction opposite to said first direction and is in thermal contact with the rim by passing back and forth by means of multiple separators through the said rim, and then flowing through a refrigeration load and heat exchanger by means of the said inlet and outlet.

2. The apparatus of claim 1 wherein the rare earth rim has a dimension l/2-in. thick with 2-in. radial depth, and having holes with a diameter of the order of 0.005-in.
drilled axially through the said rim such that these holes occupy 10% to 40% of the rim volume.

3. The apparatus of claim 1 in which the said permeable rare earth rim is composed of layers of Gd metal ribbon, said ribbon being about 0.01-in. thick and being spaced from each layer by means of multiple wires inserted axially, said wires creating a spacing of about 0.001-in. between each layer, and said ribbon having grooves axially emboss-ed in the upper surface of the ribbon to a depth of about 0.005-in., and the number of layers of ribbon are of the order of 200 to 300,

4. The apparatus of claim 1 in which at least one of the said fluids is selected from the class consisting of liquid sodium-potassium alloy, pressurized water-ethanol mixture, pressurized propane, pressurized nitrogen, pressurized neon, pressurized hydrogen, and pressurized helium.

5. The apparatus of claim 4 in which the temperature range of refrigeration load (TC) is 261 K to 400 K and the said fluid is liquid sodium-potassium

6. The apparatus of claim 4 in which the temperature range of refrigeration load (TC) is 156 K to 400 K and the said fluid is a pressurized water-ethanol mixture.

7. The apparatus of claim 4 in which the temperature range of refrigeration load (TC) is 90 K to 190 K and the said fluid is pressurized propane.

8. The apparatus of claim 4 in which the temperature range of refrigeration load (TC) is 65 K to 120 K and the said fluid is pressurized nitrogen.

9. The apparatus of claim 4 in which the temperature range of refrigeration load (TC) is 30 K to 90 K and the fluid is pressurized neon.

10, The apparatus of claim 4 in which the temperature range of refrigeration load (TC) is 20 R to 80 K and the fluid is pressurized hydrogen,

11. The apparatus of claim 4 in which the temperature range of refrigeration load (TC) is 2 K to 25 K and the fluid is pressurized helium.

12, The apparatus of claim 1 in which the rare earth rim is composed of at least one of the following working mate-rials: Gd metal, Gd39Dy61,Gd12Dy88, GdN, and Dy2Er0.8 A10.2.

13. The apparatus of claim 12 wherein the working material is Gd metal and the temperature range of refrigeration load (TC) is 255 K to 318 K.

14. The apparatus of claim 12 wherein the working material is Gd39Dy61 and the temperature range of refrigeration load (TC) is 140 K to 255 K.

15. The apparatus of claim 12 wherein the working material is Gd12Dy88 and the temperature range of refrigeration load (TC) is 80 K to 160 K,

16. The apparatus of claim 12 wherein the working material is GdN and the temperature range of refrigeration load (TC) is 40 K to 100 K.

17, The apparatus of claim 12 wherein the working material is Dy2Er0.8A10.2 and the temperature range of refrigeration load (TC) is 8 K to 60 K.