Patent Publication Number: US-2011067416-A1

Title: Thermal exchanging device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority from U.S. provisional application No. 61/245,441, filed on Sep. 24, 2009, the entirety of which is incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention is in the technical field of a thermal exchanging device, more particularly to a magnetically driven thermal exchanging device utilizing a magneto-caloric material (MCM) to generate a two-phase flow of liquid and vapor by means of magnetization and demagnetization mechanism. 
     2. Description of the Related Art 
     A conventional thermal exchanging device that utilizes the magneto-calorific properties of certain materials, such as Gadolinium or certain alloys, has the particularity of heating up when a magnetic field is applied (magnetization process) and of cooling to a temperature lower than the initial temperature following the diminishing effect of the magnetic field (demagnetization process). For instance, when the magneto-calorific material (MCM) is magnetized, the magnetic moment of the MCM becomes aligned causing a rearrangement of the atoms to thereby generate heat from the MCM. On the other hand, when the MCM is demagnetized, the magnetic moment of the MCM becomes randomized causing a disorder of the atoms to thereby absorb heat from outside of the MCM. 
     As shown in  FIG. 1   a , a conventional thermal exchanging device  1  includes a bed  11  containing a pool of fluid immersed with a porous magneto-calorific material (MCM), a magnet  12 , and cold and hot side chambers  13 ,  14 . During the magnetization process, the magnet  12  applies the magnetic field onto the bed  11 , thereby the temperature of the MCM increases so as to induce a thermal flux. As the magnetic field constantly applies to the MCM, the fluid is pushed through MCM so as to arrive at the hot side chamber  14 , shown in  FIG. 1   b . On the other hand, during the demagnetization process or when a zero magnetic field is applied to the MCM, the temperature of the MCM cools down, shown in  FIG. 1   c . Thereafter, as shown in  FIG. 1   d , the fluid is pushed back to the cold side chamber  13 . Evidently, this phenomenon utilizes the fluid force convection to force the fluid through the boundary of the MCM. 
     It is to be noted that the operating efficiency of the thermal exchanging device is primarily determined by the amount of heat transfer between the MCM and the fluid. Thus, in order to optimize the operating efficiency of the thermal exchanging device, two contributing factors are considered significant: MCM&#39;s surface area and fluid flow rate. As such, the optimization can be achieved either by dispensing a large amount of fluid through the surface of the MCM, or by increasing the surface area of the MCM. However, as the surface area of the MCM increases, the surface area of gaps or the pores of the MCM decreases, thereby limiting the amount of fluid to pass therethrough. As a result, a higher driving force is required to force the fluid through the MCM, which in turn requires a stronger pump to achieve this task. Consequently, the coefficient of performance (COP) of the thermal exchanging device is reduced. 
     SUMMARY 
     The present invention overcomes the aforementioned disadvantages by offering a magnetically driven thermal exchanging device that is simple in design while having an efficient and reliable performance. 
     A thermal exchanging device is provided to exchange heat with a heat source. The heat source has hot and cold sources. The thermal exchanging device includes a vacuum chamber, a working material capable of being excited magnetically, and a working fluid capable of undergoing a transition between two phase flows of liquid and vapor. The working fluid is provided in the chamber to communicate with the working material. When the working material is magnetically excited, the working fluid is configured to exchange heat with the cold heat source. Otherwise, the working fluid is configured to absorb heat from the hot heat source. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1   a ,  FIG. 1   b ,  FIG. 1   c , and  FIG. 1   d  are side schematic views of a conventional thermal exchanging device; 
         FIG. 2   a  is a side schematic view of the first preferred embodiment of a thermal exchanging device according to the present invention, illustrating how heat is transferred inside a chamber thereof and exchanged with an external heat source when the working material is magnetically excited; 
         FIG. 2   b  is an side schematic view of the first preferred embodiment of the thermal exchanging device, illustrating how heat is transferred inside the chamber thereof and absorbed from the external heat source when the working material is not magnetically excited; 
         FIG. 3  is a side schematic view of the thermal exchanging device having a thermal controlling switch according to the first preferred embodiment of the present invention; 
         FIG. 4  is a side schematic view of the second preferred embodiment of the thermal exchanging device, illustrating an activation unit having one valve so as to be operable to selectively close one of the two portions of the chamber, according to the present invention; 
         FIG. 5  is a side schematic view of the second preferred embodiment of the thermal exchanging device, illustrating the two valves of the activation unit closing the top and bottom portions of the chamber, according to the present invention; and 
         FIG. 6  is a side schematic view of the third preferred embodiment of the thermal exchanging device according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 2   a ,  FIG. 2   b , and  FIG. 3 , the first preferred embodiment of a thermal exchanging device of the present invention can operate with an external heat source for performing a heat exchange therebetween. As shown in  FIG. 3 , the thermal exchanging device  2  includes a magnetic-generating source  26 , a vacuum chamber  21 , a magnetically excited working material  22 , a working fluid  25 , a heating unit  24 , a wick structure  23 , and a thermal controlling unit  27 . The external heat source Q has hot and cold sources Q 1 , Q 2 , in which the hot heat source Q 1 , for example, has a temperature that is higher than the Curie temperature of the working material  22  and the cold heat source Q 2 , for example, has a temperature that is lower than the Curie temperature of the working material  22 . The working material  22  is excited by the magnetic-generating source  26  to thereby create a thermal flux in the chamber  21 . The working fluid  25  is provided in the chamber  21  so as to communicate with the working material  22  for exchanging heat therebetween. In addition, the working fluid  25  is capable of undergoing a transition between single and/or two phase flows of liquid and vapor. 
     The magnetic-generating source  26  is selected from one of a permanent magnet, a Halbach magnet, and an electrical conductive coil magnetic set. In this preferred embodiment, the electrical conductive coil magnetic set is a superconductor coil, and the working material  22  is a porous magneto-caloric material made of gadolinium and the grain size of the porous magneto-caloric material which is between 50 to 150 micrometers. The porous magneto-caloric material is provided with more than one Curie temperature. When the working material  22  is near its Curie temperature, the magnetic state of the working material  22  will change between ferromagnetism and paramagnetism so as to cause a change of the magnetic entropy of the working material  22 . Further, the porous magneto-caloric material is selected from one of a powder shape and a wire mesh shape. In this preferred embodiment, the magneto-caloric material is a bulk material with thin slits provided therethrough and/or has a plurality of stacked plates with gaps spaced therebetween. The working fluid  25  is characterized by having more than one boiling point temperature. In this preferred embodiment, the working fluid is water. 
     The working material  22  can be magnetically excited by the magnetic-generating source  26 . When the working material  22  is magnetically excited, the working fluid  25  operates to exchange heat with the cold heat source Q 2 . Similarly, when the working material  22  is not magnetically excited, the working fluid  25  operates to absorb heat from the hot heat source Q 1 . The heating unit  24  is disposed on an outer surface of the chamber  21  and operable to exchange heat with the external heat source Q. The working material  22  is surrounded by the wick structure  23  in the chamber  21  so as to facilitate flowing of the working fluid  25  and the working material  22  in the chamber  21  by means of, for example, capillary action. The wick structure  23  also has a portion  231  in thermal contact with the heating unit  24 . Additionally, as shown in  FIG. 3 , the thermal controlling unit  27  has a switch  271  that is switchable between a first position P 1  and a second position P 2  for controlling the flow direction of heat exchange between the heating unit  24  and the external heat source Q. The thermal controlling unit  27  further includes a first port  272  that is in contact with the heating unit  24 , a second port  273  that is in contact with the hot heat source Q 1 , and a third port  274  that is in contact with the cold heat source Q 2 . One of the first position P 1  and second position P 2  is defined by selectively connecting the first port  272  to one of the second and third ports  273 ,  274 . For instance, during a heat generation stage where the working material  22  is magnetically excited by the magnetic-generating source  26 , the switch  271  bridges the first port  272  and the second port  273  so as to allow the working fluid  25  to expel heat to the cold heat source Q 2 . Similarly, during a heat absorption stage where the magnetic field is weakened or removed, the switch  271  bridges the first port  272  and the second port  273  so as to allow the working fluid  25  to absorb heat from the hot heat source Q 1 . Thus, by using the thermal controlling unit  27  to control the thermal cycle (heat generation and heat absorption) of the thermal exchanging device  2 , both heating and cooling process can be realized and operated continuously. 
     Since the working fluid  25  is encapsulated in the vacuum chamber  21 , a portion of the working fluid  25  vaporizes to a higher vapor stream so as to fill up a portion of the empty space of the chamber  21 . The vaporization process will stop when the vapor pressure reaches to the working fluid&#39;s  25  saturation point. At this moment, the vapor and liquid phases are in equilibrium with each other until the temperature of the working fluid  25  changes again. Contrarily, when the temperature of the working material  22  decreases to result in cooling the working fluid  25 , the working fluid  25  condenses to a lower vapor pressure stream. 
     Referring to  FIG. 2   a , which illustrates the thermal exchanging device  2  undergoing the heat generation stage, where the working fluid  25  is expelling heat to the cold heat source Q 2  of the external heat source Q. It is noted that the magnetic-generating source  26  and the thermal controlling unit  27  are not shown in  FIG. 2   a  in this view. When the magnetic field is applied to the working material  22 , the thermal flux generated by the working material  22  will increase the temperature of and absorb by the working fluid  25  to result a higher vapor pressure stream of the working fluid  25 . The higher vapor pressure then moves to and comes in contact with the right hand side of the chamber  21 . The vapor then expels to the cold heat source Q 2  of the external heat source Q through the heat exchanging unit  24  and condenses into the liquid phase again. Thereafter, the liquid phase of the working fluid  25  is drawn to the wick structure  23  through the capillary action so that the working fluid  25  is absorbed by the working material  22  again. This cycle continues to process until the temperatures of both the working material  22  and the cold heat source Q 2  of the external heat source Q reach to equilibrium. 
     Referring to  FIG. 2   b , which illustrates the thermal exchanging device  2  undergoing the heat absorption stage, where the working fluid  25  is absorbing heat from the hot heat source Q 1  of the external heat source Q. It is noted that the magnetic-generating source  26  and the thermal controlling unit  27  are not shown in  FIG. 2   a  in this view. When the magnetic field is weakened or removed, the temperature of the working material  22  decreases such that the surrounding vapor of the working fluid  25  condenses. As such, the vapor pressure of the working fluid  25  that is proximate to the working material  22  is now lower than the vapor pressure at the right hand side of the chamber  21 . Then, the vapor pressure at the right hand side of the chamber  21  will circulate back to the working material  22  and continue to further condense. As a result, the liquid at the right hand side of the chamber  21  will continue to absorb heat from the cold heat source Q 2  through the heat exchanging unit  24 . This process continues to cycle until the heat absorbed by the working material  22  can sufficiently compensate the change in the magnetic entropy of the working material  22 . 
     Reference is now made to  FIG. 4 , there is shown the second preferred embodiment of the present invention. The second embodiment of the present invention discloses a thermal exchanging device  3  which differs from the thermal exchanging device  2  of the first embodiment in that this embodiment uses an activation unit instead of the thermal controlling unit for controlling the heat exchange flow direction. This embodiment also includes a support member  39  disposed in a vacuum chamber  31  for separating the chamber  31  into two portions  311 ,  312 . The activation unit  38  has a valve  381  provided on the support member  39  and operable to selectively close one of the two portions  311 ,  312  of the chamber  31 . Moreover, in this embodiment, the thermal exchanging device  3  has two heating units  341 ,  342  that are opposite to each other and respectively in contact with the hot and cold sources Q′ 1 , Q′ 2  of an external heat source. In this embodiment, the working material  32  is magnetically excited by the magnetic-generating source  36 , and the two portions  311  and  312  are top and bottom portions of the chamber  31  respectively. The working material  32  is surrounded by the wick structure  33  in the chamber  31 . The top and bottom portions  311 ,  312  are respectively adjacent to and in contact with the heating units  341 ,  342  respectively. During the heat generation stage, the valve  381  closes the bottom portion  312  of the chamber  31  so that heat can flow into the top portion  311  of the chamber  31 , as indicated by the right hand arrow shown in  FIG. 4 , to thereby exchange heat with the heating unit  341 . Similarly, during the heat absorption stage, the valve  381  flips from the bottom portion  312  to close the top portion  311  of the chamber  31  so that heat can be absorbed from the hot heat source Q′ 1  through the heating unit  341 , indicated by the left hand arrow shown in  FIG. 4 . In an alternative to this embodiment, as shown in  FIG. 5 , the thermal exchanging device  4  can also comprise a support member  49  disposed in a vacuum chamber  41  for separating the chamber  41  into two portions  411  and  412 . Two heating units  441 ,  442  are opposite to each other and respectively in contact with the hot and cold sources Q′ 1 , Q′ 2 . The thermal exchanging device  4  further comprises an activation unit  38  having two valves  481  and  482 , and each of the valves  481  and  482  closes a respective one of the top and bottom portions  411  and  412 . Likewise, in this embodiment, the working material  42  is surrounded by the wick structure  43  in the chamber  41 , and the working material  42  is magnetically excited by the magnetic-generating source  46 , wherein the valve  481  is open and the valve  482  is closed during the heat generation stage. During the heat absorption, the valve  482  is open and the valve  481  is closed. 
     Referring to  FIG. 6 , there is shown the third preferred embodiment of the present invention. The third embodiment of the present invention discloses a thermal exchanging device  5  which differs from the thermal exchanging device  4  of the second embodiment in that the valves  581 ,  582  are provided on opposite sides of the working material  52  such that each of which is disposed adjacent to a respective one of the heating units  541 ,  542 . In this embodiment, the working material  52  is surrounded by the wick structure  53  in the chamber  51 , and the valve  581  is open so as to allow the vapor of the working fluid  55  to flow through an vacuum chamber  51  to thereby generate heat towards the cold heat source Q″ 2  through the heating unit  541 . Similarly, the valve  582  is open so as to allow the working fluid  55  to absorb heat from the hot heat source Q″ 1 , indicated by the right hand arrows shown in  FIG. 6 . 
     The present invention operates in a fundamentally the same manner as the refrigeration. It should be noted that, in other embodiments of the present invention, the thermal exchanging device can be used in conjunction in other applications, such as in cooling or heating devices for operating as a power-conversion device. In addition, it is to be noted that the chamber  21 ,  31 ,  41 , or  51  is kept airtight in a vacuum manner so that the internal pressure of the chamber  21 ,  31 ,  41 , or  51  is lower than the atmosphere (atm) pressure, which results in decreasing the boiling point of the working fluid  25 ,  35 ,  45 , or  55  inside the chamber accordingly. As a consequence of this phenomenon, the working fluid  25 ,  35 ,  45 , or  55  can be more convenient to undergo the two-phase transition. 
     As described from the foregoing, the advanced design of the thermal exchanging device according to the present invention provides a high efficiency, high speed and low cost solution with operational advantages explained below: 
     1. The convection coefficient (h) for heat transfer using a two-phase flow of liquid and vapor is about 5˜50 times higher than that of a pure liquid. Also, the heat absorption for vaporizing one gram of water is 574 Calorie (2400 Joule), whereas the heat absorption for raising one centigrade degree of one gram of water is one Calorie (4.184 Joule). By comparison, the heat absorption for evaporating the two-phase flow fluid is 500 times higher than raising one-centigrade degree of the water. Therefore, the two-phase flow can absorb or expel a greater amount of heat than that of the traditional forced-liquid flow used on the thermal exchanging device. As a result, the two-phase flow phenomena can increase the speed at which the working fluid and the working material exchange heat. 
     2. The wick structure, based on the principle of capillary action, can transport the working fluid and the working material seamlessly through the chamber. In effect, by virtue of the capillary action, less energy is required to draw the working fluid across the chamber, compared to the conventional forced-liquid flow of the working fluid. Consequently, the operating efficiency of the thermal exchanging device is much higher than that that of the conventional thermal exchanging device. 
     Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments, will be apparent to persons skilled in the art. It is, therefore, contemplated that the appended claims will cover all modifications that fall within the true scope of the invention.