Patent Publication Number: US-2012023969-A1

Title: Cooling system of an electromagnet assembly

Description:
BACKGROUND 
     Embodiments presented herein relate generally to electromagnet assemblies, and more specifically to electromagnet assemblies for a magnetocaloric refrigeration system. 
     Magnetocaloric refrigeration is based on alternate magnetization and demagnetization of a magnetocaloric material. While operating about Curie temperature, the magnetocaloric materials warm up when magnetized and cool down on demagnetization. A refrigerant may absorb heat from the magnetized magnetocaloric material and release the heat to the environment in one step of the refrigeration cycle. Similarly in another step of the refrigeration cycle, the refrigerant may absorb heat from a refrigerated enclosure and release the heat to the demagnetized magnetocaloric material. 
     In a magnetocaloric refrigeration unit, the alternate magnetization and demagnetization of the magnetocaloric material may be achieved by one or more electromagnets. Typically, the electromagnets generate heat by passing alternating current through the conducting coils. The generated heat may cause reduce the life of the current carrying coils of the electromagnets. Further, the heat from the current carrying coils may be transferred to the magnetocaloric material and may heat the magnetocaloric material beyond the Curie temperature. 
     However, for stable operation of the magnetocaloric refrigeration unit, the magnetocaloric material must be maintained at or around the Curie temperature of the magnetocaloric material. Active cooling methods such as liquid cooling and forced air cooling may be employed to cool the electromagnets of the magnetocaloric refrigeration unit. However, excessive cooling of the electromagnets may take away heat from the magnetocaloric material, thus causing the temperature of the magnetocaloric material to fall below the Curie temperature. 
     Therefore, there is a need for an improved system to regulate the cooling of an electromagnet assembly in a magnetocaloric refrigeration unit. 
     BRIEF DESCRIPTION 
     An assembly for magnetocaloric cooling includes a magnetocaloric core, one or more electromagnet coil wound around the magnetocaloric core, and one or more magnetic yokes having a top surface and a bottom surface disposed at longitudinal ends of the magnetocaloric core. At least one of the top surface and the bottom surface of the magnetic yokes has disposed thereon a micro-channel structure. The magnetic yokes are thermally coupled to the electromagnet coil and are thermally isolated from the magnetocaloric core. 
     An assembly for magnetocaloric cooling includes one or more magnetocaloric cores, and one or more electromagnet coil wound around the one or more magnetocaloric cores. The one or more electromagnet coils are thermally isolated from the one or more magnetocaloric cores. The assembly further includes a coil housing disposed around the one or more electromagnet coils. The coil housing includes a cooling structure disposed thereon, wherein the coil housing comprises a magnetic material. 
     A magnetocaloric cooling system includes a magnetocaloric heat pump which includes one or more magnetocaloric cores, one or more electromagnet coils wound around the one or more magnetocaloric cores, and an electromagnet cooling structure for extracting waste heat from the one or more electromagnet coils, thermally coupled to the one or more electromagnet coils and thermally isolated from the one or more magnetocaloric cores. The system further includes a source heat exchanger thermally coupled to the one or more magnetocaloric cores, and a sink heat exchanger thermally coupled to the one or more magnetocaloric cores. An energizing module supplies a time varying electrical current to the one or more electromagnet coils. A waste heat disposal module coupled to the electromagnet cooling structure rejects the waste heat generated by the electromagnet coils, to environment. 
    
    
     
       BREIF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example electromagnet assembly according to one embodiment; 
         FIG. 2  illustrates a yoke of an electromagnet assembly according to one embodiment; 
         FIG. 3  illustrates a section view of the yoke of the electromagnet assembly according to one embodiment; 
         FIG. 4  illustrates an example electromagnet assembly according to another embodiment; 
         FIG. 5-FIG .  8  illustrate various designs of cooling fins disposed on a yoke of the electromagnet assembly according to various embodiments; 
         FIG. 9-FIG .  15  illustrate various designs of cooling fins disposed on a cooling structure of the electromagnet assembly according to various embodiments; and 
         FIG. 16  illustrates an example magnetocaloric cooling system according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     An assembly for a magnetocaloric refrigeration unit is disclosed. Magnetocaloric refrigeration is based on cyclic adiabatic magnetization and demagnetization of magnetocaloric (MC) materials while keeping the MC materials at their Curie temperature. The MC materials warm up when they are magnetized and cool down when demagnetized. 
     In an embodiment, a magnetocaloric refrigeration unit may include a plurality of magnetocaloric modules each including an MC material with a specific Curie temperature. The Curie temperature of MC materials for each magnetocaloric module may be suitably selected so that the successive modules provide a wide range of operating temperature. 
     In each of the magnetocaloric modules, the MC material may be magnetized by an electromagnet assembly. The electromagnet assembly may include one or more electromagnet coils that are wound around a core. In one embodiment, the core may be made of the MC material. In one example implementation, the MC material is periodically magnetized and demagnetized by energizing the electromagnet coils with a square wave electrical signal of suitable frequency and amplitude. The passage of electrical current through the electromagnet coils causes the electromagnet coils to heat up. Such heating may affect the MC material, and may destabilize the operation of the magnetocaloric module. Embodiments described herein disclose a system for removing the heat generated by the electromagnet assembly such that the MC materials maintained at or around respective Curie temperature. 
       FIG. 1  illustrates an example assembly  100  for use in a magnetocaloric refrigeration unit, according to one embodiment. The electromagnet assembly  100  includes a core  102 , one or more electromagnet coils  104 , and one or more magnetic yokes  106 . The assembly  100  may further include a coil housing  108 . 
     In the embodiment illustrated in  FIG. 1 , the core  102  may be made of a magnetocaloric material. Exemplary magnetocaloric materials include gadolinium, lanthanum, manganese, praseodymium, and their alloys. The core  102  may include at least one passage for the flow of a heat exchange fluid. The electromagnet coils  104  may be wound around the core  102 . The electromagnet coils  104  may be made of a suitable conductor such as, but not limited to, laminated copper wire. The electromagnet coils  104  may not be in direct physical contact with the core  102 . Such an arrangement reduces heat conduction from the electromagnet coils  104  to the core  102 . The electromagnet coils  104  may be thermally isolated from the core  102  by inserting a suitable heat insulating material between the core  102  and the electromagnet coils  104 . The magnetic yokes  106  may be disposed at longitudinal ends of the core  102 . The magnetic yokes  106  may be made of a suitable ferromagnetic material such as soft iron. The magnetic yokes  106  may have disposed thereon a micro-channel structure. The micro-channel structure provides coolant flow through the magnetic yokes  106 , thus removing heat generated by the electromagnet coils  104  due to passage of electric current. The yokes  106  may be thermally coupled to the electromagnet coils  104  for efficiently removing the heat generated thereon. Further, the yokes  106  may be thermally isolated from the core  102 . The thermal isolation may be achieved by inserting a suitable thermal insulator between the yokes  106  and the core  102 . Alternatively, the thermal isolation may be achieved by providing an air gap between the yokes  106  and the core  102 . Exemplary magnetic yokes are described in conjunction with  FIGS. 5-8 . 
     The assembly  100  may further include the coil housing  108 . The enclosure  108  encloses the electromagnet coils  104  wound around the core  102 . In one embodiment, the coil housing  108  may be made of a heat conducting material such as, but not limited to, aluminium, copper, and so forth. In another embodiment, the coil housing  108  is made of a magnetic material. Exemplary magnetic materials include soft iron, cobalt, nickel, and alloys thereof. In some embodiments, the coil housing  108  may be provided with a cooling structure. In one embodiment, the cooling structure may be an integral part of the outer surface of the coil housing  108 . In an alternate embodiment, a separately designed cooling structure may be disposed circumferentially around the coil housing  108 . Such a separate cooling structure may be thermally coupled to the coil housing  108  either by mechanical pressure, or by a suitable thermal compound. 
     The cooling structure may include fins. Depending on the cooling requirement of the assembly  100 , the fins disposed on the cooling structure may be configured for natural convection cooling, or forced convection cooling. For example, closely spaced fins may be used for forced convection cooling, while widely spaced fins may be used for natural convection cooling. 
     Alternatively, the cooling structure may include a micro-channel structure. The micro-channel structures may be helically disposed on the outer surface of the cooling structure. A coolant may be passed through the micro-channel structure to remove waste heat from the assembly  100 . Depending on the cooling requirements, the coolant may be a gas such as, but not limited to, compressed air, or compressed nitrogen; or may be a liquid such as, but not limited to, water, ethylene glycol, propylene glycol, methanol, and mixtures thereof. The liquid coolant may include other additives such as corrosion inhibitors. 
     In yet another implementation, the cooling structure may include a heat pipe structure. The heat pipe structure is a sealed, evacuated tube structure made of metals such as, but not limited to, copper, and aluminum, filled with a suitable working fluid. The working fluid may be selected depending on the cooling requirement of the assembly  100 . For example, working fluids may include liquid helium for extremely low temperatures, mercury for high temperatures, and ethanol, methanol, water and ammonia for moderate temperatures. 
       FIG. 2  illustrates a top view of the yoke  106 , according to one embodiment. The yoke  106  includes a micro-channel structure  202 . In an embodiment, the micro-channel structure  202  may include spiral micro-channels. In an alternate embodiment, the micro-channel structure  202  may include at least one of radial micro-channels and a plurality of closely spaced studs or pins. The pins or studs may be arranged to provide a directional path for coolant flow along the upper surface of the yoke  106 . The studs may be provided on the surface such that the transfer of heat along the upper surface of the yoke  106  may be in the desired direction. Said studs or pins may be arranged on the yoke  106  in definite shapes such as, but not limiting to, a circle, a hexagon, a square a rectangle, and the like. In an alternate embodiment, the studs or pins may act as fins for transferring heat to the surroundings. The cross sections of the pins or studs may be suitably chosen. In various embodiments, the cross sections of said studs or pins may be circular, hexagonal, rectangular, square, and the like. 
     In some embodiments, the micro-channel structure  202  may be disposed on either the top surface or the bottom surface of yoke  106 . In other embodiments, both the top surface and the bottom surface of the yoke  106  may be provided with the micro-channel structure  202 . In one embodiment, the micro-channel structure  202  may be made of a non-magnetic material. The micro-channel structure  202  may provide coolant flow to extract the heat generated by the electromagnet coils  104  due to passage of electric current. Depending on the cooling requirements, the coolant may be a gas such as, but not limited to, compressed air, or compressed nitrogen; or a liquid such as, but not limited to, water, ethylene glycol, propylene glycol, methanol, and mixtures thereof. The liquid coolant may include other additives such as corrosion inhibitors. 
     The micro-channel structure  202  may include a plurality of inlet ports  204  and outlet ports  206  for the coolant flow. In one embodiment, the micro-channel structure  202  may be configured to circulate a coolant from the center of the yoke  106  towards the circumference of the yoke  106 . In such an implementation, the inlet ports  204  are provided near the center of the yoke  106  and the outlet ports  206  may be disposed near the circumference of the yoke  106 . The direction of flow of the coolant through the micro-channel structure  202  may be selected to provide an optimum temperature gradient across the yoke  106 . For example, a flow from the center of the yoke  106  towards the circumference of the yoke  106  may provide a temperature gradient where the yoke  106  is coolest at the center, and warmest at the circumference. Such a temperature gradient may minimize or prevent heat transfer to the core  102 . The micro-channel structure  202  may further have a plurality of inlets and a plurality of outlets placed on suitable locations within the yoke  106 , to obtain a desired temperature gradient across the yoke  106 . 
       FIG. 3  illustrates a cross-sectional view of the assembly  100 , according to one embodiment.  FIG. 3  illustrates a micro-channel structure  202  disposed on a surface of the yoke  106 . In an embodiment, the micro-channel structure  202  may be covered by a plate  302 . In the embodiment illustrated, the micro-channel structure  202  includes spiral micro-channels. The assembly  100  may further include one or more coolant tubes  304  disposed circumferentially around the electromagnet coils  104 . The coolant tubes  304  may provide coolant flow to extract the heat generated by the electromagnet coils  104  due to the passage of electric current. The coolant tubes  304  may be thermally coupled to the electromagnet coil  104  for effective transfer of heat. 
     Depending on the cooling requirements, the coolant may be a liquid such as, but not limited to, water, ethylene glycol, propylene glycol, methanol, and mixtures thereof. The coolant may include other additives such as corrosion inhibitors. 
     The micro-channel structure  202  and the coolant tubes  304  may be connected to a waste heat removal system. The waste heat removal system may include a coolant reservoir, a radiator, and one or more manifolds for providing flow of the coolant to and from the assembly  100 . The manifolds may include valves to regulate flow of the coolant, under manual or automatic control. 
     In various embodiments, the assembly  100  may include a flow regulating module to regulate the flow of the coolant through the micro-channel structure  202  and the coolant tubes  304 . The flow regulating module may include temperature sensors for sensing the temperature of the coolant, the electromagnet coils  104 , and the yoke  106 . The flow regulating module may also include a processor for computing power dissipated as heat from the electromagnet coils  104  due to the passage of electrical current, by measuring the current passing through the electromagnet coil  104 s, and using the formula: 
       H=I 2 R watts   Equation 1
 
     where H is the heat produced, I is the electrical current, and R is the electrical resistance of the electromagnet coils  104 . The value of the electrical resistance may be stored in a memory of the flow regulating module. The flow regulating module may then control the manifold valves, based on the computed heating or the sensed temperatures, to regulate the flow of the coolant. In various embodiments, the flow regulating module may be a microprocessor or microcontroller based system for controlling one or more of solenoid manifold valves, speed of a coolant pump, speed of radiator fan, and so forth. 
     According to various embodiments, a magnetocaloric refrigeration unit may have a plurality of assemblies  100  stacked, one on top of the other. Each assembly  100  may have a core  102  made of a different magnetocaloric material. In one embodiment, each assembly  100  of the magnetocaloric refrigeration unit may have dedicated flow regulating module independent of other such assemblies  100 . In such an implementation, each assembly  100  may include the flow regulating module. The flow of a coolant in the micro-channel structure  202 , and the coolant tubes  304  may be controlled by the regulating module associated with the particular assembly  100 . 
     In another embodiment, a central flow regulating module may regulate the flow of the coolant in each of the plurality of assemblies  100 . Such a central flow regulating module may determine the temperature and/or heating of each assembly  100  independently, and control the corresponding manifold valve to regulate the flow of the coolant. 
       FIG. 4  illustrates an example assembly  400  for use in a magnetocaloric refrigeration unit, according to one embodiment. The assembly  400  includes a core  402 , a spacer  404 , one or more electromagnet coils  406 , and one or more magnetic yokes  408 . The assembly  400  may also include a coil housing  410 . 
     In one embodiment, the spacer  404  may be a hollow cylinder. However, it will be appreciated that hollow tubes having for example, a square, rectangular, hexagonal cross section may also be used. The spacer  404  forms an enclosure within which a magnetocaloric regenerator may be disposed. The spacer  404  may be made of a suitable non-magnetic and thermally insulating material such as, but not limited to, Teflon, Delrin, ABS, PVC, Nylon and so forth. The electromagnet coils  406  may be wound around the spacer  404 . Such an arrangement facilitates modular construction of a magnetocaloric refrigeration unit. The electromagnet (i.e. assembly  400 ) may be connected to the magnetocaloric core such that the two may be engaged and disengaged with minimal disassembly. The electromagnet coils  406 , the yokes  408 , and the coil housing  410  are similar to those described above, in conjunction with  FIG. 1 ,  FIG. 2 , and  FIG. 3 . 
       FIGS. 5-8  illustrate exemplary yokes  500 ,  600 ,  700 , and  800 , according to various embodiments. Yokes  500 ,  600 ,  700 , and  800  include a plurality of fins thereon. The fins may have suitable cross-sections such as, but not limited to, circular cross-section, a rectangular cross-section, a square cross-section, a hexagonal cross-section, and the like. The fins may be disposed on the top surface, on the periphery of the yokes, or on both. The fins provide a large surface for heat transfer to the surroundings by natural convection or forced air cooling. The yoke  500  includes fins disposed in the radial direction. The yoke  600  includes fins disposed in longitudinal direction across the surface. The yokes  700  and  800  include circular fins disposed on the surface. In some embodiments, the yokes may include a micro-fin structure. 
       FIGS. 9-15  illustrate exemplary cooling structures according to various embodiments. The exemplary cooling structures include at least one of fins and micro-channel structures disposed on the outer surface. The micro-channel structures may, without limitation include helically disposed on the outer surface of the cooling structure. The fins include, without limitation, automotive radiator type fins, vertical fins, peripheral fins, cross hatch fins, thread pattern fins, helix based fins and pin cluster fins, and so forth. The fins may have suitable cross-sections such as, but not limited to, circular cross-section, a rectangular cross-section, a square cross-section, a hexagonal cross-section, and the like. The fins facilitate the transfer of heat to the surroundings by natural convection or by forced air cooling. The cooling structures described herein, and the fins may be made of a suitable heat radiating material such as, but not limited to, aluminium, and copper. The fins provide a large surface for effective heat transfer to the surroundings. 
     In one embodiment, a thermal interface material may be applied to the inner surface of the cooling structures to facilitate the conduction of heat from the electromagnet coils  104  and  406  to the cooling structures. The thermal interface material includes, without limitation, metallic foam, thermal paste, thermal adhesive tape, and so forth. 
       FIG. 16  illustrates an example magnetocaloric cooling system  1600  according to one embodiment. The magnetocaloric cooling system  1600  includes a magnetocaloric heat pump  1602 , which further includes one or more magnetocaloric cores  1604 , an electromagnet coil  1606 , and one or more electromagnet cooling structures  1608 . The magnetocaloric cooling system  1600  further includes a source heat exchanger  1610 , a sink heat exchanger  1612 , an energizing module  1614 , and a waste heat disposal system  1616 . 
     The magentocaloric heat pump  1602  is similar to the assemblies  100 , and  400  described above in conjunction with  FIG. 1 , and  FIG. 4  respectively. The electromagnet cooling structures  1608  may be a microchannel structure, a fin structure, a coolant tube structure, or a heat pipe structure. Example cooling structures are described in conjunction with  FIG. 3 , and  FIGS. 5-15 . Although  FIG. 16  illustrates a single magnetocaloric heat pump  1602 , it will be appreciated that multiple magnetocaloric heat pumps may also be employed in the cooling system  1600 , as per the requirements of the cooling system  1600 . 
     The cooling system  1600  may extract heat from a source through the source heat exchanger  1610 , and transfers it to the environment through the sink heat exchanger  1612 . The energizing module  1614  supplies cyclic electrical current to cyclically energise and deenergise the electromagnet coil  1606  to magnetise and demagnetize magnetocaloric cores  1604  at a predetermined operating frequency. 
     The waste heat disposal system  1616  rejects the waste heat generated by the electromagnet coils  1606 , and extracted by the electromagnet cooling structures  1608 . The waste heat disposal system  1606  may include suitable heat transfer systems such as liquid-to-air radiators, heat pipes, evaporative coolers, and so forth. 
     Various embodiments presented herein describe a compact cooling system for an electromagnet assembly for use in a magnetocaloric refrigeration unit. It will be appreciated that such embodiments may be applied to other applications, like magnetocaloric refrigeration or cooling or heat-pump systems, which require cooling of an electromagnet structure. Embodiments presented herein are described solely for the purpose of illustration. Persons skilled in the art will recognize from this description that the embodiments presented herein may be practiced with modifications and alterations limited only by the spirit and scope of the appended claims.