Patent Application
20130019610
The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art.
The strong interaction of a ferromagnetic material, such as iron, with an applied magnetic field derives from the ability of the atomic spins in the material structure to coherently align themselves with the applied field. Above a certain temperature, which is characteristic of the magnetic material and called the Curie temperature, thermal agitation prevents this coherent spin alignment, and the interaction with the applied field becomes much weaker. Above the Curie temperature, the material is paramagnetic, rather than ferromagnetic. Near the Curie temperature, the coherent alignment of atomic spins in an applied field results in a decrease in the magnetic entropy of the material. If the material is thermally isolated, so that its total entropy is conserved, this decrease in its magnetic entropy is compensated by an increase in its thermal entropy, and its temperature rises. This rise in temperature upon exposure to a magnetic field is known as the magnetocaloric effect. When the applied field is removed, the magnetic entropy rises and the thermal entropy decreases, lowering the temperature of the material.
An illustrative method includes identifying at least partial degradation of a magnetocaloric material in a magnetic cooling system, wherein the magnetiocaloric material has a Curie temperature. The method also includes regenerating the magnetocaloric material by maintaining the magnetocaloric material at a regenerating temperature, wherein the regenerating temperature is different from the Curie temperature of the magnetocaloric material.
Another illustrative method includes forming at least one bed of a magnetic cooling system, wherein the at least one bed includes a magnetocaloric material, wherein the magnetocaloric material has a Curie temperature, and wherein a heat transfer fluid is configured to transfer heat to or from the magnetocaloric material in the at least one bed. The method also includes forming at least one valve of the magnetic cooling system to control a flow of the heat transfer fluid through the at least one bed and either a heater or a heat exchanger, wherein flow of the heat transfer fluid between the at least one bed and the heater regenerates the magnetocaloric material by maintaining the magnetocaloric material at a regenerating temperature, and wherein the regenerating temperature is different from the Curie temperature of the magnetocaloric material.
An illustrative apparatus includes a heat transfer fluid and a bed comprising a magnetocaloric material that has a Curie temperature. The bed is configured to allow the heat transfer fluid to transfer heat to or from the magnetocaloric material. The apparatus also includes a heater configured to maintain the magnetocaloric material at a regenerating temperature for an amount of time to regenerate the magnetocaloric material, wherein the regenerating temperature is different from the Curie temperature of the magnetocaloric material.
An illustrative system includes a first subsystem and a second subsystem. The first subsystem includes a first heat transfer fluid and a first bed having a first magnetocaloric material, wherein the first magnetocaloric material has a first Curie temperature. The first subsystem also includes a first valve configured to control whether the first subsystem operates in regeneration mode or cooling mode. The second subsystem includes a second heat transfer fluid and a second bed having a second magnetocaloric material, wherein the second magnetocaloric material has a second Curie temperature. The second subsystem also includes a second valve configured to control whether the second subsystem operates in regeneration mode or cooling mode.
The foregoing and other features of the present disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only several embodiments in accordance with the disclosure and are, therefore, not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings.
In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and make part of this disclosure.
A magnetic refrigerator (MR) uses the magnetocaloric effect to pump heat out of a colder system and exhaust that heat to a warmer environment. The magnetocaloric effect refers to the rise in temperature of a material upon exposure to a magnetic field. When the applied field is removed, the magnetic entropy rises and the thermal entropy decreases, lowering the temperature of the material. This temperature change is shown in
Modern room-temperature MR systems may employ an Active Magnetic Regenerator (AMR) cycle to perform cooling. An early implementation of the AMR cycle can be found in U.S. Pat. No. 4,332,135, the entire disclosure of which is incorporated herein by reference. In one embodiment, the AMR cycle has four stages, as shown schematically in
In the first stage of the cycle (“magnetization”), while the fluid in the bed is stagnant, a magnetic field is applied to the MCM, causing it to heat. In the second stage of the cycle (“cold-to-hot-flow”), the magnetic field over the bed is maintained, and fluid at a fixed temperature TCi (the cold inlet temperature) is pumped through the bed from the cold side to the hot side. This fluid pulls heat from each section of the bed, cooling the bed and warming the fluid as it passes to the next section of the bed, where the process continues at a higher temperature. The fluid eventually reaches the temperature THo (the hot outlet temperature), where it exits the bed. Typically, this fluid is circulated through a hot side heat exchanger, where it exhausts its heat to the ambient environment. In the third stage (“demagnetization”), the fluid flow is terminated and the magnetic field is removed. This causes the bed to cool further. In the final stage of the cycle (“hot-to-cold-flow”), fluid at a fixed temperature THi (the hot inlet temperature) is pumped through the bed from the hot side to the cold side in the continued absence of the magnetic field. The fluid is cooled as it passes through each section of the bed, reaching a temperature TCo (the cold outlet temperature) which is the coldest temperature reached by the fluid in the cycle. Typically, this colder fluid is circulated through a cold side heat exchanger, where it picks up heat from the refrigerated system, allowing this system to maintain its cold temperature.
The time that it takes to complete execution of the four stages of the AMR cycle is called the cycle time, and its inverse is known as the cycle frequency. The “temperature span” of the MR system is defined as THi−TCi, which is the difference in the inlet fluid temperatures. The AMR cycle is analogous to a simple vapor compression cycle, where gas compression (which causes the gas to heat) plays the role of magnetization, and where free expansion of the gas (which drops the gas temperature) plays the role of demagnetization. Although
Typically, a magnetic field of 1-2 Tesla is utilized to effectively exploit the magnetocaloric effect for refrigeration. This field is usually provided by an assembly of powerful NdFeB magnets. The remanent magnetization of the highest grade of NdFeB magnets is about 1.5 Tesla. The use of a stronger field than this would improve MR performance, but to achieve fields in excess of the remanent magnetization, a large (and potentially prohibitive) increase in magnet size and weight is required. Thus, 1.5 Tesla is the field strength that provides a roughly optimum balance between MR system size and performance. As permanent magnet technology improves, magnets with remanent magnetizations greater than 1.5 Tesla may be obtained. In this case, the optimum field strength of an MR system will increase accordingly.
The permanent magnet assembly is generally the most expensive component in the MR. To make the best use of this expensive resource, the magnetocaloric material used in the MR should possess the strongest possible magnetocaloric effect. This material should also avoid the use of any toxic, reactive, or rare (and therefore expensive) constituents. The former consideration rules out the commercial use of Gd, for example, which is nontoxic, inert, and inexpensive but has a weak magnetocaloric effect. MR systems employing Gd, or other materials of comparable magnetocaloric strength, would be too large for commercial utility. Lanthanum iron silicon hydride (LaFeSiH) is one of the most promising magnetocaloric materials for use in commercial MR systems. A description of LaFeSiH can be found in an article by Fujita et al. titled “Itinerant-electron metamagnetic transition and large magnetocaloric effects in La(FexSi1-x)13 compounds and their hydrides,” Physical Review B 67 (2003), the entire disclosure of which is incorporated by reference herein. This material has a strong magnetocaloric effect.
In most cooling applications, the temperature span will be substantial, typically about 30° C. (54° F.) or larger. Although the overall span supported by an MR system may be large, the temperature within a given axial section of a bed in the system will remain within a relatively narrow range over the refrigeration cycle.
The advantages associated with the use of layered beds of LaFeSiH are demonstrated in
Although LaFeSiH appears to be an ideal material for use in a MR, its properties are not stable. This material has been shown to undergo a gradual deterioration of its magnetocaloric strength when it is stored at a temperature very close to its Curie point, as described in an article by A. Barcza et al. entitled “Stability and magnetocaloric properties of sintered La(Fe,Mn,Si)13Hz alloys”, presented at the IEEE International Magnetics Conference (Taipei, Taiwan) 2011, session ED-07 (hereinafter “A. Barcza et al.”), the entire disclosure of which is incorporated by reference herein. This deterioration is most readily observed in Differential Scanning calorimetry (DSC).
The ageing process for LaFeSiH appears to not depend on the synthesis method, as long as the hydrogen content is less than 1.5 per formula unit. The age-splitting process was seen in material that was arc melted, then annealed for several weeks to form the 1-13 phase, then hydrided. The age-splitting process was also seen in material that was rapidly solidified by melt spinning or atomization, and then annealed for a few hours or less to form the 1-13 phase, and then hydrided. The ageing process was seen in different samples of LaFeSiH with slightly different compositions, such as La1.29(Fe0.88Si0.12)13Hy and La1.2(Fe0.888Si0.112)13Hy. The ageing process was also seen in a sample of Pr0.6La0.6(Fe0.888Si0.112)13Hy, where Pr was substituted for some of the La to increase the magnetocaloric strength. Thus, the age-splitting process will generally occur in magnetocaloric materials of the form RE(TMxSi1-x)13Hy material (where RE represents a rare earth element such as La, Ce, Pr, or Nd, and TM represents a transition metal such as Fe, Cr, Mn, or Ni, x<0.15, and y<1.5). In an illustrative embodiment, the value of y can be between approximately 0.8 and 1.5. Alternatively, a different range of y values may be used. As discussed herein, different values of y can be used to generate magnetocaloric materials having different Curie temperatures.
When used in an MR system, the magnetocaloric material will inevitably be exposed to temperatures close to its Curie temperature. Indeed, in a layered bed, the material in a layer is selected to have a Curie temperature equal to the average temperature seen by that layer during the MR cycle. Thus, if partially hydrogenated LaFeSiH, or more generally RE(TMxSi1-x)13Hy, is used in an MR system, its magnetocaloric properties will degrade over time. In spite of its significant advantages over other magnetocaloric materials, this degradation in the magnetocaloric properties of partially hydrogenated RE(TMxSi1-x)13Hy material could potentially preclude its use in a commercial MR system.
Applicants have discovered that when degraded RE(TMxSi1-x)13Hy material is subsequently held at a temperature away from (e.g., either a higher or a lower temperature) its Curie point, the degradation process reverses and eventually the properties of the material return to their initial condition. Moreover, Applicants have found that the recovery of the material proceeds more quickly at higher temperatures, as shown in
Because the age-splitting degradation can be completely reversed in a relatively simple manner, RE(TMxSi1-x)13Hy materials can be used in suitably modified MR systems, which forms the basis of the subject matter described herein. In the usual mode of operation of an MR system with layered beds of magnetocaloric material, the material layers will remain close to their respective Curie temperatures, which will cause deterioration of the magnetocaloric material. In addition, when the system is not operating, the portion of the magnetocaloric material with Curie point near ambient temperature may also deteriorate. As such, Applicants have developed a modified MR system that is configured to hold the layers of magnetocaloric material at a temperature that differs from the Curie temperature of the magnetocaloric material to reverse whatever age-splitting degradation may have occurred and to recover their full magnetocaloric effect. The temperature at which the magnetocaloric material is held, which can be higher or lower than the Curie temperature of the magnetocaloric material, can differ from the Curie temperature by 10° C., 25° C., 50° C., 100° C., etc. depending on the desired rate of recovery, the system capacity, etc. In an illustrative embodiment, temperature at which the magnetocaloric material is held can differ from the Curie temperature by approximately 10° C.
In one illustrative embodiment, an MR system employs RE(TMxSi1-x)13Hy as the magnetocaloric material and has a heating element plumbed into the flow system. When the MR system would otherwise be idle (e.g., a residential air conditioner at night), the heating element can be activated. The MR system would then circulate heated fluid through the magnetocaloric material, completely reversing any age-splitting that may have occurred since the last high-temperature treatment.
In the particular case of a MR system that normally absorbs heat at a cold heat exchanger (CHEX) and exhausts heat at a hot heat exchanger (HHEX), a heater can be plumbed in parallel with the cold heat exchanger. In normal cooling mode, flow is directed through the CHEX and the HHEX, as shown in
In an alternative embodiment, in addition to having a heating element, a cooling system can include two independent MR subsystems. The first MR subsystem can provide cooling as in
In another alternative embodiment, the cooling system can have two stages, with each stage containing layered AMR beds. The cold stage can have Curie temperatures ranging from Tc to Tm, while the hot stage can have Curie temperatures ranging from Tm to Th, where Th>Tm>Tc. In an air conditioner implementation, Tc may have a value of 10° C., Tm may have a value of 25° C., and Th may have a value of 40° C. In alternative embodiments and/or implementations, different temperature values may be used. When recovery of the hot stage magnetocaloric material is desired, the cold stage can operate in cooling mode, generating a cold outlet fluid stream with temperature near Tc. This cold fluid, instead of flowing through the cold side heat exchanger, can be directed through the hot stage to bring the hot stage temperature near Tc. Because Tc is well below all Curie temperatures in the hot stage, exposure to this temperature would reverse any age-splitting in the hot stage. Similarly, when recovery of the cold stage magnetocaloric material is desired, the hot stage can operate in cooling mode and can therefore generate a hot outlet fluid stream with a temperature near Th. This hot fluid, instead of flowing through the hot side heat exchanger, can be directed through the cold stage, bringing its temperature to approximately Th. Because this temperature is well above all Curie temperatures in the cold stage, exposure to this temperature would reverse any age-splitting of the cold stage material.
In another alternative embodiment, the system can include two independent MR subsystems, with each subsystem having two stages, a hot stage and a cold stage as in the above-described embodiment. When maximum cooling power is desired, both subsystems can be run in parallel, with each providing cooling, as shown in
In another alternative embodiment, the possibly multiple beds of a magnetic refrigeration system can be designed to be easily removable and replaceable from the system. Beds that have been degraded from age-splitting can then be removed and replaced with pristine beds. In a separate device that can be physically remote from the magnetic refrigeration system, the degraded beds can be returned to pristine condition through exposure to temperatures sufficiently far from the Curie temperatures of all the layers they contain. This device, for example, could be a simple flow loop with a heater, capable of circulating fluid at an elevated temperature through the degraded beds, or an oven for holding the beds at an elevated temperature. Once restored to pristine condition, these beds can then be re-installed in the magnetic refrigeration system.
Any of the operations described herein can be performed by a computing system that includes a processor, a memory, a transmitter, a receiver, a display, a user interface, and/or any other computer components known to those of skill in the art. Any type of computing system known to those of skill in the art may be used. In one embodiment, any of the operations described herein can be coded into instructions that are stored on a computer-readable medium. A computing system can be utilized to execute the instructions such that the operations are performed.
To verify the effect on magnetic refrigerator performance of the age-splitting degradation, and to verify that elevated temperature treatment was effective at reversing this degradation, the beds of a magnetic refrigerator were packed with five layers of La(Fe0.885Si0.115)Hy material, with each layer having a different value of y and therefore a different Curie point. The Curie points of the layers were initially 8° C., 11° C., 15° C., 18° C. and 21° C. The machine was tested under a standard set of operating conditions, where the cycle frequency was 3.33 Hz, the flow rate was 6 lit/min, the hot inlet temperature was 25° C., and the cooling load, provided by an electrical heater, was 400 watts. Before operation as a MR, the LaFeSiH in the beds was suffused with 35° C. aqueous fluid for 80 hours to bring the material to its initial state. The temperature span of the machine with pristine material under the standard operating conditions was found to be 13.4° C. The machine was then left in a non-operating state at an ambient temperature of 22° C. for ten days. In this state, the materials with Curie temperatures of 18° C. and 21° C. would be expected to undergo age-splitting degradation, and indeed, the temperature span of the machine after this 10-day treatment under the standard operating conditions dropped to only 2.9° C. The LaFeSiH MCM was then suffused with 50° C. aqueous fluid for 19 hours to bring the material to its initial state, and then the temperature span of the machine in AMR mode at the standard condition of a cooling load of 400 watts and a hot inlet temperature of 25° C. was measured to be 13.2° C. Thus bringing the LeFeSiH MCM to a temperature more than 10° C. above the Curie point of the material for 19 hours was able to restore the performance of the MCM after a substantial reduction in performance that occurred when the MCM was kept close to its Curie point for ten days.
The herein described subject matter sometimes illustrates different components contained within, or connected with, different other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “operably connected”, or “operably coupled”, to each other to achieve the desired functionality, and any two components capable of being so associated can also be viewed as being “operably couplable”, to each other to achieve the desired functionality. Specific examples of operably couplable include but are not limited to physically mateable and/or physically interacting components and/or wirelessly interactable and/or wirelessly interacting components and/or logically interacting and/or logically interactable components.
With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.
It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”
The foregoing description of illustrative embodiments has been presented for purposes of illustration and of description. It is not intended to be exhaustive or limiting with respect to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosed embodiments. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.
This application claims priority to U.S. Provisional Patent Application No. 61/509,381 filed Jul. 19, 2011, the entire disclosure of which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61509381 | Jul 2011 | US |
