US20070186560A1

US20070186560A1 - Hybrid heat pump / refrigerator with magnetic cooling stage

Info

Publication number: US20070186560A1
Application number: US11/700,854
Authority: US
Inventors: Robert Schauwecker; Johannes Boesel
Original assignee: Bruker Biospin SAS
Current assignee: Bruker Biospin SAS
Priority date: 2006-02-11
Filing date: 2007-02-01
Publication date: 2007-08-16
Also published as: EP1818628A2; JP2007212128A; DE102006006326B4; EP1818628A3; DE102006006326A1

Abstract

A device for transporting heat from a cold reservoir to a warm reservoir, in which at least two cyclic processes are employed for transporting heat thereby absorbing work, of which at least one is a regenerative cyclic process, and at least one is a magnetocaloric cyclic process, wherein the regenerative cyclic process has a working fluid and a heat storage medium, is characterized in that the heat storage medium of the regenerative cyclic process comprises a magnetocaloric material for the magnetocaloric cyclic process, wherein the magnetocaloric material is in a regenerator area with a cold end and a warm end, the working fluid of the regenerative cyclic process additionally serving as a heat transfer fluid for the magnetocaloric cyclic process. This produces a compact device with low apparative expense, wherein the power density and also the efficiency of the device are increased. The device may advantageously be used for cooling a superconducting magnet configuration.

Description

This application claims Paris Convention priority of DE 10 2006 006 326.0 filed Feb. 11, 2006 the complete disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention concerns a device for transporting heat from a cold reservoir to a warm reservoir, in which at least two cyclic processes are employed for transporting heat, thereby absorbing work, of which at least one is a regenerative cyclic process, and at least one is a magnetocaloric cyclic process, wherein a working fluid and a heat storage medium are provided for the regenerative cyclic process.
A device of this type has been disclosed in the document “A multi-stage continuous-duty adiabatic demagnetization refrigerator” (P. J. Shirron et al., Adv. Cry. Eng., Vol. 45B, page 1629). It combines a regenerative, cyclic gas refrigeration process with magnetic cooling by using a Gifford-McMahon gas refrigerator for precooling several, series-connected magnetic cold stages in order to produce extremely low temperatures in the mK range.
A further device is disclosed in the document “Performance testing of a 4 K active magnetic regenerative refrigerator” (S. F. Kral et al., Adv. Cry. Eng., Vol. 45A, page 329). This arrangement suggests a combination of a Gifford-McMahon cooler with a separate magnetic cold stage in order to generate temperatures in the range of 4 K. The Gifford-McMahon cooler thereby precools the magnetic cold stage to approximately 10 K.
A further device of this type is described in the document “Prospects of magnetic liquefaction of hydrogen” (Barclay, J. A. Le froid sans frontières, vol. 1, page 297, 1991). It proposes a magnetic cold stage for liquefying hydrogen (at 20 K). The magnetic cold stage usually does not work between room temperature and 20 K but e.g. between approximately 80 K and 20 K. The magnetic stage may be precooled to approximately 80 K either via liquid nitrogen or a refrigerator, e.g. a regenerative gas refrigerator. The active regenerator bed of the magnetic cold stage thereby consists of several ferromagnetic materials which have different magnetic transition temperatures (Curie temperature) and are disposed in layers next to each other from the maximum to the minimum Curie temperature.
Common to all these examples is that precooling of the magnetic cold stage is effected either via a gas refrigerator, such as a Stirling, pulse tube or Gifford-McMahon cooler or liquid nitrogen. The gas refrigerators often have a multi-stage construction. In each stage thereby a gas refrigeration process is employed which is divided into several process phases such as compression, heat release, expansion and supply of heat. Heat is intermediately stored between the compression and expansion phases in a (passive) regenerator matrix to cool the gas. After expansion and supply of heat from the outside (=“release of cold”), the stored heat is once more absorbed by the gas.
In the following magnetic cold stage, the working medium which passes through a thermodynamic process is not gas, but magnetocaloric material, a solid. Heat is exchanged between the magnetocaloric material and a heat source or heat sink via a thermal connection (in the form of a thermal switch) that can be connected or disconnected, or an additional heat transfer fluid. The temperature in the process is increased and decreased through magnetization or demagnetization of the magnetocaloric material, e.g. by a permanent magnet. The temperature change is maximum when the average temperature of the material corresponds to its Curie temperature. In order to bridge large temperature differences between the heat source and sink, several materials having different Curie temperatures may be used, which are disposed in layers next to each other in bulk form. The magnetocaloric material provided at each location in the regenerator bed runs through its own cycle between different temperatures (active magnetic regeneration) using the heat transfer fluid. Coupling to the outside is provided at the ends of the regenerator bed, where the heat transfer fluid flows through a heat exchanger. Heat is thus finally transported from a cold heat source to a warm heat sink. The magnetocaloric material may be introduced into a magnetic field in cycles or a magnetic field may be periodically switched on and off. The heat transfer fluid (a liquid or a gas depending on the application) must be supplied at the right moment through the magnetocaloric material, e.g. using a pump.
Refrigeration at low (4-20 K) or very low temperatures (<4 K) is primarily suited as an application of such devices for transporting heat from a cold reservoir to a warm reservoir, wherein at least two cyclic processes are performed for transporting heat thereby absorbing work (a gas cycle and a magnetic cycle), in order to liquefy e.g. gases having a low boiling temperature, such as hydrogen or helium. It is, however, also possible to use such an arrangement as a heat pump, i.e. for heating. One common feature of a refrigerator and a heat pump is that heat is transported from a colder to a warmer reservoir by supplying work. For the refrigerator, the heat is absorbed at a temperature below the ambient temperature. For a heat pump, heat is released at a temperature above the ambient temperature. The physical principle, however, remains the same.
Since there are always two independent cooling mechanisms in the conventional devices of this type, the apparative expense is relatively large. Thus, a separate drive mechanism must be provided for the precooling stage(s) with a gas cooling cycle or with liquid nitrogen, and also for the magnetic cooling stage, e.g. in the form of a compressor or an apparatus which moves the magnetic material in and out in cycles in a permanent magnet. A heat transfer fluid must also be supplied through the active regenerator bed of the magnetic cooling stage. The working fluid of the regenerative cyclic process and the heat transfer fluid of the magnetic cooling stage are two different media or at least hydraulically separated from each other. The (passive) heat storage medium of the regenerative process and the active regenerator bed are also not identical.
Two-stage cryocoolers which are merely based on a gas refrigeration cycle have also recently been used for liquefying gases having a low boiling point, such as helium. A particularly interesting application is the use of a pulse tube cooler for reliquefying evaporated helium in an apparatus with a superconducting magnet, as is described e.g. in the patent document U.S. 2002/0002830A1. The magnet coil is still cooled through evaporating helium. However, in contrast to conventional systems, no helium or other cryogen is lost to the outside. Magnetic materials are also used in the heat storage medium (passive regenerator) of the second cold stage of these coolers, but for a completely different reason than in active regenerator beds of magnetic coolers. These materials have a large heat capacity in the area of their magnetic transition compared to the working gas, which is a prerequisite for running the cooling process. Frequently, the magnetic materials must even be shielded from the magnetic field of the superconducting magnet in order to maintain their efficiency. An external magnetic field is thereby undesired in this case. Moreover, a pulse tube cooler does not work very efficiently in contrast to magnetic coolers, with the consequence that the operating costs for cooling the magnet system are relatively high. A thermodynamically more efficient method for producing cold would therefore be advantageous.
It is therefore the object of the present invention to improve a device for transporting heat from a cold reservoir to a warm reservoir, wherein at least two cyclic processes for transporting heat, thereby absorbing work, are employed in the device in such a manner that the apparative expense of the device is considerably reduced, and the efficiency and the power density of the device increased.

SUMMARY OF THE INVENTION

This object is achieved in accordance with the invention in that the heat storage medium of the regenerative cyclic process comprises a magnetocaloric material for the magnetocaloric cyclic process, wherein the magnetocaloric material is in a regenerator area with a cold end and a warm end, the working fluid of the regenerative cyclic process additionally serving as a heat transfer fluid for the magnetocaloric cyclic process. The inventive device may be part of a multi-stage arrangement for transporting heat from a cold reservoir to a warm reservoir, in particular, when the heat is transported through greatly differing temperatures.
The advantage of an inventive device compared to conventional devices consists in that the apparative expense is greatly reduced. The advantages of the inventive device compared to prior art may be explained by means of example by the arrangement of the document “Performance testing of a 4 K active magnetic regenerative refrigerator” (S. F. Kral et al., Adv. Cry. Eng., Vol. 45A, page 329). When the working fluid of a cooler of a regenerative cyclic process, e.g. of the Gifford-McMahon cooler that is used in the conventional device is used in accordance with the invention as a heat transfer fluid of the magnetocaloric cold stage, both may be advantageously transported via the same drive mechanism. In the same way, the inventive use of the active material of the magnetocaloric cold stage as a regenerator material in the Gifford-McMahon cooler represents a simplification of the apparatus. Moreover, the inventive combined process is thermo-dynamically more efficient than a conventional regenerative gas cycle alone (as used e.g. for a gas refrigerator in a pulse tube cooler or Gifford-McMahon cooler). The cooling or heating performance may be increased without considerably increasing the volume of the machine, thereby increasing the power density. Moreover, an existing external magnetic field (as described e.g. in the patent document U.S. 2002/0002830A1) is not disturbing, but even advantageous, since it can be utilized for the magnetocaloric cycle.
A particularly preferred embodiment of the inventive device for transporting heat from a cold reservoir to a warm reservoir is characterized in that the cold reservoir has a temperature below the ambient temperature, and the warm reservoir has a temperature which is equal to or larger than the ambient temperature. Thus, a refrigerator may generate a temperature below the ambient temperature, which provides a plurality of possible applications. It is, however, also feasible to use the principle with a heat pump for heating.
The inventive device is particularly advantageous when the regenerative cyclic process is based on a Stirling, a Vuilleumier, a Gifford-McMahon or a pulse tube gas cycle. All processes are mainly used for producing cold, and in the present case, at temperatures preferably below 100 K. The machines based on these processes (in particular for a Gifford-McMahon or pulse tube cooler) in combination with the magnetocaloric process are generally more efficient and have a larger power density.
In another advantageous fashion, the magnetocaloric material has different components with different Curie temperatures which are disposed next to each other in layers in the order of decreasing Curie temperature in the regenerator area, such that the component of the magnetocaloric material with the highest Curie temperature comes to rest at the warm end and the component of the magnetocaloric material with the lowest Curie temperature comes to rest at the cold end of the regenerator area. This joining of different components permits active magnetic regeneration, and relatively large temperature differences (e.g. 60 K) can be covered with only little change in magnetic field strength (e.g. 2T).
The inventive device is particularly advantageous when a means for providing a magnetic field and/or shielding a magnetic background field is provided which provides and/or shields a magnetic field at least at the location of the magnetocaloric material. In this fashion, the magnetocaloric material may be alternately magnetized and demagnetized to change the temperature in the material.
In a further advantageous embodiment of the inventive device, the means for providing a magnetic field and/or shielding a magnetic background field comprise a permanent magnet. The relative position of the permanent magnet compared to the magnetocaloric material is varied through a suitable device such that the magnetocaloric material can run through the intended process. This provides a simple and robust embodiment of the inventive device.
In an alternative embodiment, the means for providing a magnetic field and/or shielding a magnetic background field comprises a magnet coil winding with a normally conducting and/or a superconducting wire. The magnetocaloric material may then alternately be magnetized and demagnetized (and thereby heated and cooled) via a variable current, without moving parts. This is particularly advantageous when the device is to be used in environments which are sensitive to vibration.
In a special embodiment of the inventive device, a magnetic shielding of soft magnetic material is provided as a means for shielding the magnetic background field. Through suitable variation of the relative position of the magnetic shielding compared to the magnetocaloric material, the magnetocaloric material may be efficiently magnetized and demagnetized with little additional expense.
The invention also concerns a superconducting magnet configuration with an inventive device for transporting heat from a cold reservoir to a warm reservoir. In a superconducting magnet configuration, at least the area of superconducting windings must be cooled down to low temperatures, and the configuration provides a magnetic field which can be variably shielded with time in the magnetocaloric material of an inventive device, thereby facilitating the magnetocaloric process required for cooling.
With particular advantage, the superconducting magnet configuration is thereby part of an apparatus for magnetic resonance (MR), in particular, for nuclear magnetic resonance imaging (MRI) or nuclear magnetic resonance spectroscopy (NMR). These are analysis methods which usually use liquid cryogens for cooling the superconducting magnet configuration, such that direct cooling (without refilling the otherwise evaporating cryogens) is very attractive for the user.
It is also, however, feasible for the superconducting magnet configuration to be part of an apparatus for ion cyclotron resonance spectroscopy (ICR) or electron spin resonance (ESR, EPR). The user-friendliness is thereby also improved by autonomous cooling of the superconducting magnet configuration.
The invention also concerns a method for transporting heat from a cold reservoir to a warm reservoir, wherein at least two cyclic processes are employed for transporting heat, thereby absorbing work, of which at least one is a regenerative cyclic process in which heat is transported via a working fluid, and at least one is a magnetocaloric cyclic process in which heat is exchanged via a magnetocaloric material. The inventive method is characterized in that the magnetocaloric material is also used as a heat storage medium in the regenerative cyclic process and the working fluid is also used as a heat transfer fluid for the magnetocaloric cyclic process.
In one variant of the inventive method, the working fluid runs through a compression phase, a heat release phase, an expansion phase and a heat absorbing phase in the regenerative cyclic process. This provides a thermodynamic cycle, wherein heat is transported from a cold reservoir to a warm reservoir, thereby absorbing work.
In a further advantageous variant, the field strength of a magnetic field is periodically varied at least at the location of the magnetocaloric material. The magnetocaloric material is thus heated and cooled and may run through a thermodynamic cycle, in which heat is transported from a cold reservoir to a warm reservoir.
It is, however, also feasible to vary the field strength through cyclic change of the position of the magnetocaloric material relative to a permanent magnet. This is a simple, robust and inexpensive variant.
The field strength may moreover vary through changing the current flow in a normally conducting and/or superconducting magnet coil. In this case, no parts are moved. This provides a variant of the inventive method that has particularly little vibration.
In a particularly advantageous variant of the inventive method, the strength of magnetic field shielding in a magnetic background field is periodically varied at least at the location of the magnetocaloric material. The magnetocaloric material can thereby be magnetized and demagnetized with little effort.
The regenerative cyclic process moreover includes the heat supply phase and the compression phase with a high magnetic field in the magnetocaloric material, the phase of heat release and expansion phase with a low magnetic field in the magnetocaloric material. In this fashion, the method permits both a regenerative and a magnetocaloric working cycle in one single device, which provides an associated machine having high power and efficiency.
The inventive method is essentially advantageous when the heat is transported within a superconducting magnet configuration, wherein the superconducting magnet configuration is part of an apparatus for magnetic resonance (MR), in particular, for nuclear magnetic resonance imaging (MRI) or nuclear magnetic resonance spectroscopy (NMR) or is part of an apparatus for ion cyclotron resonance spectroscopy (ICR) or for electron spin resonance (ESR, EPR). In this fashion, such an apparatus can be cooled efficiently and in a user-friendly fashion. Compared to conventional cooling using liquid cryogens, the user friendliness of the apparatus and the cooling costs can be reduced, in particular, when the price for cryogens increases in the near future.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below may be used in accordance with the invention either individually or collectively in arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but have exemplary character for illustrating the invention.
The invention is shown in the drawing and is explained in more detail with reference to embodiments.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 a shows a schematic construction of a device for transporting heat from a cold reservoir to a warm reservoir in a regenerative, cyclic process in accordance with Stirling (prior art);

FIG. 1 b shows a schematic construction of a device for transporting heat from a cold reservoir to a warm reservoir in a magnetocaloric cyclic process (prior art);

FIG. 1 c shows a schematic construction of an inventive device for transporting heat from a cold reservoir to a warm reservoir;

FIG. 2 a shows the different process phases in a device for transporting heat from a cold reservoir to a warm reservoir in a regenerative, cyclic process in accordance with Stirling (prior art);

FIG. 2 b shows the different process phases in a device for transporting heat from a cold reservoir to a warm reservoir in a magnetocaloric cyclic process (prior art);

FIG. 2 c shows the different process phases in an inventive device for transporting heat from a cold reservoir to a warm reservoir;

FIG. 3 shows an embodiment of an inventive device for cooling a superconducting magnet system;

FIG. 4 shows the process phases III->IV and VI->I in an inventive device for transporting heat from a cold reservoir to a warm reservoir with volume elements of the working fluid;

FIG. 5 a shows all process phases of an inventive device for a first volume element of the working fluid in a temperature-entropy diagram (T-S diagram);

FIG. 5 b shows all process phases of an inventive device for any volume element of the working fluid in a temperature-entropy diagram (T-S diagram);

FIG. 5 c shows all process phases of an inventive device for a last volume element of the working fluid in a temperature-entropy diagram (T-S diagram); and

FIG. 6 shows the power increase and the exergetic efficiency in dependence on the pressure ratio for an exemplary inventive hybrid Stirling refrigerator.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 a schematically shows the structure of a (one-stage) regenerative Stirling gas refrigerator (or heat pump) according to prior art. The Stirling machine represents the basic form of all machines which are based on a regenerative cycle. The other gas refrigerators, such as Gifford-McMahon coolers or pulse tube coolers, are derived from this basic form.
The machine consists of a heat storage medium, a so-called (passive) regenerator 1 which is limited at its warm end 2 by a warm heat exchanger 3 (for a refrigerator approximately at ambient temperature) and is limited at its cold end 2′ by a cold heat exchanger 3′ (for a refrigerator below ambient temperature). The regenerator 1 consists of a finely distributed solid, e.g. in the form of woven metal screens or bulk particles, and has a high heat capacity compared to the working fluid, e.g. helium gas. The regenerator 1 absorbs heat from the working fluid during passage, and releases it after flow reversal to the working fluid without considerably changing the temperature distribution in the regenerator 1. The working fluid is compressed by a warm piston 4 in a compression space 5 and is expanded (after transferring it) in an expansion space 5′ by a cold piston 4′. In order to perform the various steps one after the other, the motion of the pistons 4, 4′ is controlled by suitable drive mechanisms 6, 6′ in such a manner that the warm piston 4 leads the cold piston 4′ by a quarter of a period (90° phase shift).
In a one-stage magnetic refrigerator or heat pump (FIG. 1 b), there is an (active) regenerator 1′ which consists of a magnetocaloric working medium, mostly in the form of bulk particles in a regenerator bed. The regenerator 1′ is, in turn, bordered by two heat exchangers 3, 3′: at its warm end 2 by the warm heat exchanger 3, and at its cold end 2′ by the cold heat exchanger 3′. In order to bridge large temperature differences between the two heat exchangers 3, 3′, several components of magnetically active materials having different Curie temperatures may be used in the regenerator 1′, which are disposed next to each other in layers, such that the component of the magnetocaloric material with the highest Curie temperature comes to rest on the warm heat exchanger 3 and the component of the magnetocaloric material with the lowest Curie temperature comes to rest on the cold heat exchanger 3′ (not shown). The two pistons 4, 4′ transport a heat transfer fluid (gas or liquid) through the (active) regenerator 1′; its motion via the drive mechanism 6, 6′ is performed in phase and the heat transfer fluid is neither compressed nor expanded. The working medium (magnetocaloric material) in the regenerator 1′ is magnetized via a magnet 7. The magnet 7 may be designed as a permanent magnet which is cyclically shifted over the regenerator bed, or as a coil of normally and/or superconducting wire. Then the magnetocaloric material in the regenerator 1′ is magnetized or demagnetized through charging and discharging the magnet coil. The motion of the two pistons 4, 4′ and the motion of the permanent magnet or charging and discharging of the magnet coil must be matched in time such that the magnetocaloric material runs through a magnetocaloric cycle.
FIG. 1 c schematically shows the construction of the inventive device. Components of both machines in accordance with FIGS. 1 a and 1 b are combined into a one-stage refrigerator or heat pump. The (passive and active) regenerator 1″ comprises the magnetocaloric working medium, e.g. in the form of bulk particles of preferably several magnetically active materials with different Curie temperatures, wherein the component of the magnetocaloric material with the highest Curie temperature comes to rest at the warm end 2 of the regenerator 1″ and the component of the magnetocaloric material with the lowest Curie temperature comes to rest at the cold end 2′ of the regenerator 1″. The regenerator 1″ then also provides intermediate storage of heat (as in the Stirling machine of FIG. 1). The working fluid is compressed in the compression space 5 by the compression piston 4, as in the regenerative process of the machine in accordance with FIG. 1 a, and is expanded in the expansion space 5′ by the expansion piston 4′, and also serves as a heat transfer fluid for the magnetocaloric process. The two pistons 4, 4′ transport the heat transfer fluid (working fluid of the regenerative process) through the regenerator 1″ where it releases or absorbs heat. The magnetocaloric material in the regenerator 1″ is magnetized via magnet 7. The magnet 7 may thereby also be a permanent magnet which is cyclically shifted over the regenerator bed, or a coil of normally and/or superconducting wire. Then the magnetocaloric material in the regenerator 1″ is magnetized and demagnetized through charging and discharging the magnet coil. Another possibility is to shield a magnetic field, that is already present in the environment, by a suitable device (e.g. of soft metal or a further magnet coil). Since a regenerative gas cycle and a magnetocaloric process are combined in one machine, the process phases (compression, expansion, transferring the working fluid through the regenerator 1″, magnetization, demagnetization) must be thoroughly matched to each other in time.
Prior to a more detailed description of the process phases that are employed in an inventive device, the process phases of a Stirling machine (according to FIG. 1 a) or a magnetocaloric machine (according to FIG. 1 b) are considered separately.
FIG. 2 a schematically shows the different process phases of a regenerative gas cycle in a one-stage Stirling machine for transporting heat in a simplified form. In detail, the following process steps are performed:

I->II Isothermal Compression

The working fluid in the compression space 5 is isothermally compressed by the piston 4, thereby releasing heat. The released heat 8 may be absorbed e.g. by a cooling medium at a constant temperature.

II->III Isochore Cooling (Heat Release)

The working fluid in the compression space 5 is transferred at a constant volume into the expansion space 5′ through the regenerator 1. In the regenerator 1, the working fluid releases heat to the heat storage medium of the regenerator 1 (matrix of the regenerator 1) for intermediate storage, and is cooled.

III->IV Isothermal Expansion

The working fluid in the expansion space 5′ is isothermally expanded by the piston 4′, thereby absorbing heat. The absorbed heat 9 is thereby supplied from the outside by the object/space to be cooled.

IV->I Isochore Heating (Absorption of Heat)

The working fluid in the expansion space 5′ is transferred at a constant volume into the compression space 5 through the regenerator 1. The working fluid absorbs the intermediately stored heat from the heat storage medium in the regenerator 1 and is thereby heated. The initial state is reached again.
The temperature profile 10 in the regenerator 1 remains unchanged during all process phases. In the machine net work is exerted and the released heat 8 exceeds the absorbed heat 9 by this energy amount.
Under ideal conditions, a Stirling machine has the maximum coefficient of performance COP of the Carnot process (COP_Car) and thereby an efficiency η (COP/COP_Car) of 1. Other coolers, such as e.g. pulse tube coolers do not work reversibly, not even in the ideal case, and are therefore not that efficient.
FIG. 2 b shows the different process phases of a one-stage magnetocaloric refrigerator or heat pump in a simplified and schematic fashion.

I->II Demagnetization

The magnetocaloric material in the (active) regenerator 1′ is demagnetized, i.e. a magnetic field of a strength B, that is present in state I, is reduced (e.g. by removing the permanent magnet, discharging of the magnet coil or activating the shielding). The magnetocaloric material of the regenerator 1′ (matrix of the regenerator 1′) thereby cools at each location over its length by a certain temperature difference, the temperature profile 11 in the regenerator 1′ before demagnetization differs therefore from the temperature profile 11′ in the regenerator 1′ after demagnetization.

II->III Cooling (Release of Heat)

The warm heat transfer fluid is transferred together with the pistons 4, 4′ through the magnetocaloric material of the regenerator 1′. It is thereby cooled (i.e. the magnetocaloric material absorbs heat). When it leaves the regenerator 1′, the heat transfer fluid is initially colder than the environment. The heat transfer fluid may thereby absorb heat 9′ from the (cold) environment. During transfer, the temperatures in the regenerator 1′ change and, in state III, the temperature profile 11 has been restored (as in state I).

III->IV Magnetization

The magnetocaloric material of the regenerator 1′ is magnetized to a field strength B (with permanent magnets, through charging a coil or removing a shielding). The magnetocaloric material of the regenerator 1′ is heated at any location over its length by a certain temperature difference such that the temperature profile 11″ is achieved.

IV->I Heating (Absorption of Heat)

The cold heat transfer fluid is shifted back by the pistons 4, 4′ through the magnetocaloric material of the regenerator 1′. It is thereby heated (i.e. the magnetocaloric material releases heat). When exiting the regenerator 1′, the heat transfer fluid is initially warmer than the environment. The heat transfer fluid releases heat 8′ to the (warm) environment. During transfer, the temperatures in the regenerator 1′ change and the original temperature profile 11 is restored in state I.
Work (by moving a permanent magnet or charging of a magnet coil) must also be exerted in the magnetic refrigerator or heat pump. The released heat 8′ exceeds the absorbed heat 9′ by this energy amount. High efficiencies can be realized as in a Stirling machine.
The inventive device is based on a combination of processes shown e.g. in FIGS. 2 a and 2 b. FIG. 2 c schematically shows the different process phases of such a refrigerator or heat pump combination. In the simplest case of only one cooling stage, a device of this type (e.g. for a hybrid Stirling refrigerator) comprises the compression space 5 and expansion space 5′ with an intermediate regenerator 1″. As in a magnetic machine, it is thereby also possible to switch on or off an external magnetic field B. At the start, the temperature profile 11 is prevailing in the regenerator 1″. This results in the following cycle:

I->II Isothermal Compression

When an external magnetic field B is present, the working fluid in the compression space 5 is isothermally compressed by the piston 4, thereby releasing heat. The released heat 8 may e.g. be absorbed by a cooling medium at a constant temperature.

II->III Demagnetization

The magnetocaloric material in the regenerator 1″ is demagnetized, i.e. a magnetic field of a strength B which is present in states I and II is reduced (by removing the permanent magnet, discharging the magnet coil or activating the shielding). The magnetocaloric material of the regenerator 1″ is thereby cooled at any location over its length by a certain temperature difference. The temperature profile 11 in the regenerator 1″ prior to demagnetization is therefore different from the temperature profile 11′ in the regenerator 1″ after demagnetization.

III->IV Cooling (Release of Heat)

The warm heat transfer fluid (working fluid in process step I->II) is transferred with the piston 4, 4′ through the magnetocaloric material of the regenerator 1″. It is thereby cooled (i.e. the magnetocaloric material absorbs heat). When it leaves the regenerator 1″, the heat transfer fluid is initially colder than the environment at that location. The heat transfer fluid may thereby absorb heat 9′ from the (cold) environment. During transfer, the temperatures in the regenerator 1″ change and, in state IV, the temperature profile 11 is restored (as in states I and II).

IV->V Isothermal Expansion

The heat transfer fluid in the expansion space 5′ is isothermally expanded by the piston 4′, thereby absorbing heat. The absorbed heat 9 is thereby supplied from the outside by the object/space to be cooled.

V->VI Magnetization

The magnetocaloric material of the regenerator 1″ is magnetized by applying a magnetic field with a field strength B (with permanent magnets, by charging a coil or removing a shielding). The magnetocaloric material of the regenerator 1″ is heated at any location over its length by a certain temperature difference to yield the temperature profile 11″.

VI->I Heating (Absorption of Heat)

The cold heat transfer fluid (working fluid in the process step IV->V) is passed back by the pistons 4, 4′ through the magnetocaloric material of the regenerator 1″. It is thereby heated (i.e. the magnetocaloric material releases heat). When it leaves the regenerator 1″, the heat transfer fluid is initially warmer than the environment at that location. The heat transfer fluid releases heat 8′ to the (warm) environment. During passage, the temperatures in the regenerator 1″ change and in state I, the original temperature profile 11 has been restored.
The heat (heat 9′, 9) is thereby supplied from the space to be cooled (useful cold) during two process steps (III->IV and IV->V). The heat (heat 8′, 8) is released to the warm environment during the process steps VI->I and I->II. The required work increases correspondingly. The coefficient of performance (COP) and the efficiency of the device are, however, high. The volume of the device corresponds approximately to the individual volume of one of the non-combined machines from FIGS. 2 a and 2 b, which increases the power density.
Other machines may also be used which are based on a regenerative gas process, e.g. a Gifford-McMahon cooler or a pulse tube cooler. In order to obtain very low temperatures (<20 K), such a device will moreover be composed of several stages. It is then feasible to use combined cooling in accordance with the invention only at the coldest stage.
FIG. 3 shows a multi-stage embodiment of the inventive device based on a magnetic pulse tube cooler for cooling a superconducting magnet configuration. A first magnet coil 12 is thereby located in a helium container 14 filled with liquid helium 13. The helium container 14 is connected to an outer shell 16 via at least one suspension tube 15. A two-stage cold head 20 of a magnetic pulse tube cooler is installed into a neck tube 17 whose upper warm end 18 is connected to the outer shell 16 and whose lower cold end 19 is connected to the helium container 14. The helium container 14 is moreover surrounded by a radiation shield 21, which is connected both to the suspension tubes 15 and the neck tube 17 in a thermally conducting fashion. A heat-conducting solid connection 23 is provided between the first cold stage 22 of the cold head 20 and the neck tube 17, via which heat is conducted from the radiation shield 21 to the first cold stage 22 of the cold head 20. Evaporated helium from the helium container 14 is reliquefied at the second cold stage 24 of the cold head 20. In contrast to a conventional two-stage pulse tube cooler, magnetocaloric material is provided in the regenerator tube 25 of the second cold stage 24. The regenerator tube 25 is within the stray field of the first magnet coil 12 without magnetic shielding. The stray field may be shielded or increased by the second magnet coil 26 in such a manner that the magnetocaloric material may be used for producing cold in a magnetocaloric cyclic process in accordance with the invention, and also as a heat storage medium in the regenerative gas cycle. The working fluid of the regenerative gas cycle (helium gas) is moreover the heat transfer fluid for the magnetocaloric cyclic process. The power and efficiency of the cooler may thereby be increased, such that the cold head 20 may be decreased in size and the energy consumption during operation may be reduced.
The use of an inventive cooling device is therefore advantageous when the cold head 20 is located in the already existing stray field of a superconducting magnet, wherein the magnetic field can then be suitably shielded. Apparatus comprising a superconducting magnet configuration, such as e.g. for nuclear magnetic resonance spectroscopy, nuclear magnetic resonance imaging (MRI), ion cyclotron resonance spectroscopy (ICR) or electron spin resonance (ESR, EPR) can thereby be efficiently cooled in a user-friendly way.
The invention is explained below with reference to an exemplary calculation and further drawings.
An ideal inventive hybrid Stirling refrigerator is initially shown as an example during the process phases III->IV and VI->1 (FIG. 4). The (ideal) working fluid (e.g. helium gas) in the compression space 5 is divided into n small volume elements. Each of these volume elements a, i, n runs through its own thermodynamic cycle which is represented for the first volume element a in FIG. 5 a in a temperature-entropy diagram (T-S diagram). During displacement into the expansion space 5′, the volume element a cools from the temperature T_h(state III) to a temperature T_c. Due to change in temperature of the magnetocaloric material, the volume element a is further cooled and finally exits the regenerator 1″ with T_c-ΔT_ca, (state IV′ in FIG. 5 a). External heat is then supplied to the volume element a (at a constant volume) such that it is heated to a temperature T_c(state IV). Another arbitrary inner volume element i (see T-S diagram in FIG. 5 b) also enters the regenerator 1″ with a temperature T_h, and exits it with temperature T_cΔT_ci, which is higher than the exiting temperature of the first volume element a, since the regenerator 1″ has previously absorbed heat from the volume elements. After leaving the regenerator 1″, the volume element i is also heated to the temperature T_cby absorbing heat from the outside. The heat supplied to the volume element i, however, is then smaller than for the first volume element a. The last volume element n (see T-S diagram in FIG. 5 c) also enters the regenerator 1″ with the temperature T_h, but leaves it with the temperature T_c, since the original temperature profile 11′ has been shifted to the temperature profile 11. The volume element n can therefore no longer absorb any external heat.
The heat supplied to the volume elements a, i, n, during change of states IV′->IV can be represented in the T-S diagram as areas below the curve.
The heat Q_i,IV′-IVabsorbed by the volume element i (having the mass Δm_i) during exiting the regenerator 1″ can be calculated as follows:
Q _i,IV′-IV =Δm _i c _v,i └T _c−(T _c −ΔT _c,i)┘=Δm _i c _v,i ΔT _c,i,
wherein c_v,iis the specific heat capacity of the working fluid at a constant volume. The overall heat Q_IV′-IV(corresponds to heat 9′ in FIG. 4) absorbed by the working fluid (having a total mass M) is then
$\begin{matrix} Q_{{IV}^{'} - IV} = \sum_{i} Δ m_{i} c_{v, i} Δ T_{c, i} \\ = Δ m c_{v} \sum_{i} Δ T_{c, i} \\ = Δ m c_{v} n Δ T_{c}^{'} \\ = M c_{v} Δ T_{c}^{'}, \end{matrix}$
wherein ΔT_c′ is the average value of temperature changes ΔT_c,iand Δm_i=Δm for all i.
The subsequent process step IV→V (isothermal expansion) is the same for all volume elements a, i, n. The overall heat supplied to the working fluid at a temperature T_cis:
Q _IV-V =MT _c(s _V −s _IV),
wherein s_IVand s_Vare the specific entropies of the working fluid in states IV and V.
During subsequent shifting to the compression space 5, the volume element n is heated from a temperature T_cto a temperature T_h(VI→I in FIG. 5 c). Due to the temperature change of the magnetocaloric material, the volume element n is further heated and finally exits the regenerator 1″ with T_h+ΔT_h,n(state I′). The volume element n subsequently releases heat to the outside (at a constant volume) such that it is cooled again to a temperature T_h(state 1). Another arbitrary inner volume element i also enters the regenerator 1″ at a temperature T_c, and leaves it at a temperature T_h+ΔT_h,i, which is less than the exiting temperature of the last volume element n, since the regenerator 1″ has previously released heat to the volume element a and the volume elements i (FIG. 5 b). After leaving the regenerator 1′, the volume element i is also cooled to a temperature T_hby releasing heat to the outside. The heat released by the volume element i is now, however, smaller than for the last volume element n. The first volume element a finally also enters the regenerator 1″ at a temperature T_cand leaves it at a temperature T_h(FIG. 5 a), since the original temperature profile 11″ has shifted to temperature profile 11. The first volume element a can therefore no longer release heat to the outside. The heat released by the volume elements during the change of state I′→I can also be represented in the T-S diagram as areas below the curve.
The heat Q_i,I′-Ireleased by the volume element i (having the mass Δm_i) during exiting the regenerator 1″ (which is therefore negative in accordance with the normal conventions in thermodynamics) can be calculated as follows:
Q _i,I′-I =Δm _i c _v,i └T _h−(T _ch +ΔT _h,i)┘=−Δm _i c _v,i ΔT _h,i,
The heat Q_I′-I(which corresponds to the heat 8′ in FIG. 4) released in total by the working fluid (having the overall mass M) is:
$\begin{matrix} Q_{I^{'} - I} = - \sum_{i} Δ m_{i} c_{v, i} Δ T_{h, i} \\ = - Δ m c_{v} \sum_{i} Δ T_{h, i} \\ = - Δ m c_{v} n Δ T_{h}^{'} \\ = - M c_{v} Δ T_{h}^{'}, \end{matrix}$
wherein ΔT_h′ is the average value of temperature changes ΔT_h,iand Δm_i=Δm for all i.
The subsequent step I→II (isothermal compression) is the same for all volume elements a, i, n. The overall heat released by the working fluid at a temperature T_his thereby:
Q _I-II =−MT _h(s _I −s _II),
wherein s_Iand s_IIare the specific entropies of the working fluid in states I and II.
Since the isochores (V=const) in the T-S diagram are equidistant lines in case of an ideal working fluid (as assumed herein), the entropy changes during the state changes I→II and IV→V are equal:
s_I −s _II =s _V −s _IV =Δs.
During passage through the regenerator 1″, no heat is transferred to the outside, such that the following total amount of heat is supplied to the working fluid in the overall process:
Q _zu =M[T _c Δs+c _v ΔT _c′]
and the following total amount of heat is released from the working fluid:
Q _ab =−M[T _h Δs+c _v ΔT _h′].
In accordance with the first law of thermodynamics, the following applies for a working cycle with work W:
W+(Q_zu+Q_ab)=0, such that the work (with ΔT=T_h−T_c) is calculated as shown below:
W=−(M[T _c Δs+c _v ΔT _c ′]−M[T _h Δs+c _v ΔT _h′])=M(ΔTΔs+c _v(ΔT _h ′−ΔT _c′))
The heat supplied in a “pure” Stirling cooling process at a temperature T_cis:
Q _zu,Stir. =MT _c Δs,
such that the increase in cooling power can be calculated as follows:
$\frac{Q_{zu}}{Q_{zu, Stir .}} = 1 + \frac{c_{v} Δ T_{c}^{'}}{T_{c} Δ s} .$
In contrast to a “pure” ideal Stirling cooling process, even the ideal hybrid process contains irreversibilities which can be determined through exergy loss, as is common in thermodynamics. Below, it is assumed that the temperature T_hcorresponds to the ambient temperature. Then, the exergy of the working fluid would change as follows, through supply of heat during process phase IV′→IV:
$\begin{matrix} E_{{IV}^{'} - IV} = Q_{{IV}^{'} - IV} - {MT}_{h} (s_{IV} - s_{{IV}^{'}}) \\ = Q_{{IV}^{'} - IV} - {MT}_{h} c_{v} \ln (\frac{T_{c}}{T_{c} - Δ T_{c}^{'}}) \\ = M c_{v} (Δ T_{c}^{'} - T_{h} \ln (\frac{T_{c}}{T_{c} - Δ T_{c}^{'}})) . \end{matrix}$
The following exergy changes occur during the process phase IV→V (VI):
E _IV-V =Q _IV-V −MT _h Δs=−MΔsΔT.
The exergy of the working fluid changes through heat release during the process phase I′→I:
$\begin{matrix} E_{I^{'} - I} = Q_{I^{'} - I} - {MT}_{h} (s_{I} - s_{I^{'}}) \\ = Q_{I^{'} - I} - {MT}_{h} c_{v} \ln (\frac{T_{h}}{T_{h} + Δ T_{h}^{'}}) \\ = M c_{v} (- Δ T_{h}^{'} - T_{h} \ln (\frac{T_{h}}{T_{h} + Δ T_{h}^{'}})) . \end{matrix}$
Since heat is released during phase I→II (III) at ambient temperature (T_h) the exergy does not change.
An overall exergy balance may now be calculated:
E _VI′-VI +E _IV-V +E _VI-I +E _I′-I +E _I-II+E _III-IV +E _loss +W=0
Since the exergy changes of the working fluid cancel during passage through the regenerator 1″ (in both directions) (i.e. E_VI-I+E_III-IV=0), the exergy loss E_lossrrelated to work can be calculated after substitution and in a simplified fashion:
$\frac{E_{loss}}{W} = \frac{c_{v} T_{h} (\ln (\frac{T_{c}}{T_{c} - Δ T_{c}^{'}}) + \ln (\frac{T_{h}}{T_{h} + Δ T_{h}^{'}}))}{Δ T Δ s + c_{v} (Δ T_{h}^{'} - Δ T_{c}^{'})} .$
We obtain the following exergetic efficiency or efficiency factor η (which permits a statement about the “quality” of the process):
$η = 1 - \frac{E_{ver}}{W}$
As derived from the document “Prospects of magnetic liquefaction of hydrogen” (Barclay, J. A, Le froid sans frontières, vol. 1, page 297, 1991), the following applies for a magnetic refrigerator and also for the inventive hybrid machine:
$\frac{T_{h}}{T_{c}} = \frac{Δ T_{h}}{Δ T_{c}} \approx \frac{Δ T_{h}^{'}}{Δ T_{c}^{'}},$
such that an example can be calculated. With the given boundary values FIG. 6 shows the (cooling) power increase and the exergetic efficiency of a hybrid Stirling refrigerator compared to a “pure” Stirling refrigerator. The actually occurring irreversibilities are not taken into consideration in these two machines (“ideal” machines). It is obvious that the power can be considerably increased mainly with small pressure ratios without considerably reducing the process quality. For small pressure ratios, the two isochores (see e.g. FIG. 5 b) move closer together, such that the area Q_i,IV-Vis reduced compared to the area Q_{i,IV′ IV}. The exergy loss is still tolerable, since the temperature changes ΔT_h, ΔT_care still small compared to the absolute temperatures T_h, T_c. A small pressure ratio in a machine is always advantageous, since the alternating loads are reduced and the operating life of the machine is increased. This is another reason why the inventive hybrid machine is superior to a “pure” gas refrigerator.
In summary, we obtain a simple device with little apparative expense for transporting heat from a cold reservoir to a warm reservoir, in which at least two cyclic processes are employed for transporting heat thereby absorbing work, of which at least one is a regenerative cyclic process, and at least one is a magnetocaloric cyclic process. The device has high power density (mainly with small pressure ratio) and efficiency and can advantageously be used for cooling a superconducting magnet configuration, since a magnetic stray field, which is already present at that location, can be used for the magnetocaloric cycle.

LIST OF REFERENCE NUMERALS

1 passive regenerator (heat storing medium)
1′ active regenerator (magnetocaloric material)
1″ passive and active regenerator of the inventive device (magnetocaloric material)
2 warm end of the regenerator
2′ cold end of the regenerator
3 warm heat exchanger
3′ cold heat exchanger
4 warm piston
4′ cold piston
5 compression space
5′ expansion space
6 warm drive mechanism
6′ cold drive mechanism
7 magnet (permanent magnet, magnet coil)
8 released heat in the regenerative gas cycle
8′ released heat in the magnetocaloric cycle
9 absorbed heat in the regenerative gas cycle
9′ absorbed heat in the magnetocaloric cycle
10 temperature profile in the passive regenerator
11 temperature profile in the active regenerator prior to demagnetization
11′ temperature profile in the active regenerator after demagnetization
11″ temperature profile in the active regenerator after magnetization
12 first magnet coil
13 liquid helium
14 helium container
15 suspension tube(s)
16 outer shell
17 neck tube
18 warm end of the neck tube
19 cold end of the neck tube
20 two-stage cold head
21 radiation shield
22 first cold stage of the cold head
23 heat conducting solid connection
24 second cold stage of the cold head
25 regenerator tube of the second cold stage
26 second magnet coil
a first volume element
i any inner volume element
n last volume element

Claims

We claim:

1. A device for transporting heat from a cold reservoir to a warm reservoir using at least two cyclic processes for transporting the heat, thereby absorbing work, the device comprising:

means for transporting heat via a working fluid in a regenerative cyclic process;

means for exchanging heat via a magnetocaloric material in a magnetocaloric cyclic process;

means for storing heat with the magnetocaloric material during said regenerative cyclic process; and

means for transferring heat with the working fluid during said magnetocaloric cyclic process.

2. The device of claim 1, wherein said regenerative cyclic process has a heat storage medium comprising said magnetocaloric material, said magnetocaloric material being disposed in a regenerator area having a cold end and a warm end, wherein said working fluid of said regenerative cyclic process additionally serves as a heat transfer fluid for said magnetocaloric cyclic process.

3. The device of claim 2, wherein a cold reservoir has a temperature which is below ambient temperature, and a warm reservoir has a temperature which is at or above ambient temperature.

4. The device of claim 2, wherein said regenerative cyclic process is based on a Stirling, a Vuilleumier, a Gifford-McMahon, or a pulse tube gas cycle.

5. The device of claim 2, wherein said magnetocaloric material comprises different components with different Curie temperatures which are disposed next to each other in layers at said regenerator area in order of decreasing Curie temperature, wherein a component of said magnetocaloric material with a highest Curie temperature is disposed at said warm end and a component of said magnetocaloric material with a lowest Curie temperature is disposed at said cold end of said regenerator area.

6. The device of claim 2, further comprising means for providing a magnetic field and/or for shielding a magnetic background field to provide and/or shield a magnetic field at least at a location of said magnetocaloric material.

7. The device of claim 6, wherein said means for providing a magnetic field and/or shielding a magnetic background field comprise a permanent magnet.

8. The device of claim 6, wherein said means for providing a magnetic field and/or shielding a magnetic background field comprise a magnet coil winding with a normally conducting and/or a superconducting wire.

9. The device of claim 6, wherein a magnetic shielding of soft magnetic material is provided for shielding said magnetic background field.

10. A superconducting magnet configuration comprising the device for transporting heat from a cold reservoir to a warm reservoir of claim 1.

11. The superconducting magnet configuration of claim 10, wherein the superconducting magnet configuration is part of an apparatus for magnetic resonance (MR), nuclear magnetic resonance imaging (MRI), or nuclear magnetic resonance spectroscopy (NMR).

12. The superconducting magnet configuration of claim 10, wherein the superconducting magnet configuration is part of an apparatus for ion cyclotron resonance spectroscopy (ICR) or electron spin resonance (ESR, EPR).

13. A method for transporting heat from a cold reservoir to a warm reservoir using at least two cyclic processes for transporting the heat, thereby absorbing work, the method comprising the steps of:

a) transporting heat via a working fluid in a regenerative cyclic process;

b) exchanging heat via a magnetocaloric material in a magnetocoleric cyclic process;

c) storing heat with the magnetocaloric material during step a); and

d) transferring heat with the working fluid during step b).

14. The method of claim 13, wherein the working fluid passes through a compression phase, a phase of heat release, an expansion phase and a heat absorbing phase in the regenerative cyclic process.

15. The method of claim 13, wherein a field strength of a magnetic field is periodically varied at least at a location of the magnetocaloric material.

16. The method of claim 15, wherein the field strength is varied through cyclic change of a relative position of the magnetocaloric material relative to a permanent magnet.

17. The method of claim 15, wherein the field strength is varied by changing a current flow in a normally conducting and/or superconducting magnet coil.

18. The method of claim 13, wherein a strength of a magnetic field shielding in a magnetic background field is varied periodically at least at a location of the magnetocaloric material.

19. The method of claim 13, wherein, in the regenerative cyclic process, a heat absorbing phase and a compression phase are executed with a high magnetic field in the magnetocaloric material, and a heat release phase and expansion phase are executed with a low magnetic field in the magnetocaloric material.