Patent Publication Number: US-2012037337-A1

Title: Heat transfer system, apparatus, and method therefor

Description:
BACKGROUND 
     This disclosure relates to a system, device, and method for protecting components of a heat transfer system from thermal damage. Heat transfer structures, such as thermal shoes, transfer heat from a heat source to a heat sink. A conventional thermal shoe is formed from a thermally conductive body that includes a heat-receiving surface and a heat-emitting surface. The heat-receiving surface engages the heat source to accept heat, and the heat-emitting surface engages the heat sink to transfer the heat. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of the disclosed examples will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
         FIG. 1  illustrates an example heat transfer system having a thermotransfer structure. 
         FIG. 2  illustrates a heat-affected zone generally surrounding the thermotransfer structure of  FIG. 1 . 
         FIG. 3  illustrates a cross-sectional view of the thermotransfer structure of  FIG. 1 . 
         FIG. 4  illustrates a cross-section of another example thermotransfer structure. 
         FIG. 5  illustrates another example thermotransfer structure. 
         FIG. 6  illustrates another example thermotransfer structure. 
         FIG. 7  illustrates another example thermal storage element. 
         FIG. 8  illustrates another example heat transfer system. 
         FIG. 9  illustrates a cross-sectional view of the thermotransfer structure of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  illustrates selected portions of an example heat transfer system  20  that includes a heat transfer apparatus  22  (hereafter “thermotransfer structure  22 ”). The heat transfer apparatus  22  may also be considered to be a hot shoe. The disclosed examples may be described with regard to use of the heat transfer system  20  within a solar power application. It is to be understood that the heat transfer system  20  and thermotransfer structure  22  may be used in other types of applications and is not limited to the examples disclosed herein. For instance, the heat transfer system  20  and/or thermotransfer structure  22  may alternatively be used in an application that would benefit from waste heat recovery, such as steel factories, concrete production, paper mills, or industrial batch processes that utilize high temperature processing. 
     In the illustrated example, the heat transfer system  20  includes a heat source  24 . The type of heat source  24  may vary, depending on the type of system. In a solar power system, the heat source  24  is a vessel that is operable to contain a molten heat transfer fluid. The vessel may be a storage tank of the solar power system and may be adapted to handle high temperature, molten materials, such as molten salts, molten metallic materials, or other working fluids. In this regard, it is to be understood that the heat source  24  may include components that are not shown in the examples herein, such as but not limited to, piping, pumps, heat transfer structures, controls, or other structures/components that may be in contact with the working fluid. 
     The thermotransfer structure  22  is operable to transfer heat between the heat source  24  and a heat sink  26 . In the example of the solar power system, the heat sink  26  may be a power conversion device, such as a Stirling power conversion device (e.g., a heat engine that operates by cyclic compression and expansion of air or other gas), thermoelectric power conversion device (e.g., a device that converts heat into electricity), or the like. 
     The thermotransfer structure  22  includes a first surface  22   a  at one end and a second surface  22   b  at the opposed end. The first surface  22   a  is located adjacent the heat source  24  and therefore is a heat-receiving surface. The second surface  22   b  is located adjacent the heat sink  26  and is therefore a heat-emitting surface. In the illustrated example, the thermotransfer structure  22  tapers such that the first surface  22   a  has a first cross-sectional area (as represented by the horizontal dimension in the figure) and the second surface  22   b  has a second cross-sectional area  22   b  that is smaller than the first cross-sectional area, for efficient heat transfer. 
     The thermotransfer structure  22  includes a thermally conductive element  28  that extends continuously from the first surface  22   a  to the second surface  22   b , and a thermal storage element  30  that is adjacent to the thermally conductive element  28 . The thermally conductive element  28  may be a unitary, monolithic body for efficient heat transfer. In this example, the thermal storage element  30  is flush with the first surface  22   a  and extends partially between the first surface  22   a  and the second surface  22   b.    
     The thermally conductive element  28  has a first heat capacity and the thermal storage element  30  has a second heat capacity that is greater than the first heat capacity. That is, functionally, the thermally conductive element  28  operates to transfer heat between the heat source  24  and the heat sink  26 , and the thermal storage element  30  operates to retain, or store, heat as will be described in further detail below. 
     The materials of the thermally conductive element  28  and the thermal storage element  30  influence the thermal conductivity and thermal storage properties. For instance, the thermally conductive element  28  may be made of a metallic material that has generally high thermal conductivity. In some examples, the metallic material may be a cobalt material, a nickel material, a tungsten material, a zirconium material, a molybdenum material, a copper material such as substantially pure copper or copper alloy, an iron material such as steel, an aluminum material such as substantially pure aluminum or aluminum alloy, or other type of metal or metal alloy having approximately equal or better thermal conductivity. Given this description, one of ordinary skill in the art will recognize other metallic materials or thermally conductive non-metallic material to meet their particular needs. 
     The thermal storage element  30  (i.e., thermal capacitor) may be made of a material that has generally high heat capacity (i.e., specific heat). For instance, the material may be a ceramic material or a phase change material that has a higher heat capacity than the material selected for the thermally conductive element  28 . In some examples, the ceramic material may be an oxide, nitride, carbide or other type of ceramic material having a high heat capacity with regard to the material of the thermally conductive element  28 . Alternatively, the material of the thermal storage element  30  may be a phase change material, such as a salt or metal that is liquid at the operating temperatures of the heat source  24 . 
     In the example of a solar power system, the phase change material may have a melting temperature that is near the melting temperature of the working fluid contained within the vessel. For instance, the phase change material may have a composition that is based on the composition of the working fluid. In one particular example, the vessel of the solar power system contains sodium potassium nitrate salt that has a eutectic composition, and the phase change material selected as the thermal storage element  30  is a sodium potassium nitrate salt that has a hypoeutectic or hypereutectic composition such that the melting temperature of the phase change material is higher than the melting temperature of the working fluid within the vessel. Alternatively, the working fluid may be a eutectic metallic alloy and the phase change material may be a hypoeutectic or hypereutectic composition. 
     In a state of operation of the heat transfer system  20  (e.g., a first state), the thermotransfer structure  22  transfers heat from the heat source  24  to the heat sink  26 . The state of operation depends on the type of application. In a solar power system application, the state of operation may be defined by the operation of the power conversion device and/or heat source  24 . The heat transfer system  20  may be considered to be in operation or active when the power conversion device functions to generate electricity and/or the heat source  24  functions to generate heat. The heat transfer system  20  may be considered to be inoperative or inactive (e.g., a second state) when the power conversion device does not generate electricity and/or the heat source  24  does not generate heat. Thus, during operation, the thermal storage element  30  debits heat transfer efficiency of the thermotransfer structure  22  because of the high heat capacity and low thermal conductivity of the thermal storage element  30  relative to the thermally conductive element  28 . 
     In an inoperative or inactive state, there is the potential that the working fluid or components in thermal communication with the thermotransfer structure  22  will cool. For instance, the working fluid may cool to a temperature below its melt temperature (i.e., freezing). The freezing of the working fluid may damage the vessel or other components in the vessel. Additionally, the power conversion device or components in the vessel may be sensitive to abrupt changes in temperature. In this regard, the thermal storage element  30  facilitates heating the working fluid and/or power conversion device and components to avoid thermal damage. 
     As shown in  FIG. 2  when the heat transfer system  20  is inoperative or inactive, the thermal storage element  30  releases stored thermal energy to the surrounding environment and into the heat transfer fluid within the vessel. The released thermal energy heats the working fluid, as represented by a heat affected zone  32  surrounding the thermotransfer structure  22 , and thereby prevents freezing or reduces the cooling rate of the working fluid. Similarly, the heat sink  26  may absorb some of the thermal energy and thereby reduce the cooling rate of the heat sink  26 . 
     Depending upon the application of the thermotransfer structure  22 , the ability of the thermal storage element  30  to store heat during use and later release the heat during inactivity can be used for different advantages and purposes. For instance, the thermal storage element  30  generally delays the time for the heat transfer fluid within the vessel to freeze in a solar power system. This allows for additional time before the material within the heat affected zone  32  will freeze and potentially damage nearby components. For a Stirling power converter, the thermal storage element  30  may also be used to “coast down” the temperature change between the vessel and the converter. Thus, the thermal storage element  30  provides a “thermal buffer” by storing and then later releasing thermal energy, some of which will be absorbed by the thermally conductive element  28  and transfer to the power conversion device to facilitate reduction in the temperature drop at the power conversion device. Thus, the thermal storage element  30  facilitates protection of the heat transfer system  20 , which may allow for longer periods of shut down for maintenance and reduction in wear on the components of the system. 
     Referring also to the example of  FIG. 3 , the thermotransfer structure  22  generally has a frustoconical shape. The thermal storage element  30  may have a corresponding frustoconical shape. In this example, the thermally conductive element  28  includes a recessed cavity  40  and the thermal storage element  30  is located at least partially within the recessed cavity  40 . As shown, the thermal storage element  30  is located completely within the recessed cavity  40 . However, in other examples, the thermal storage element  30  may include portions which extend from the recessed cavity  40  past the first surface  22   a.    
       FIG. 4  illustrates a modified thermotransfer structure  122 , which may be used within the heat transfer system  20  in place of the thermotransfer structure  22  of  FIG. 1 . In this disclosure, like reference numerals designate like elements where appropriate and reference numerals with the addition of one-hundred or multiples thereof designate modified elements that are understood to incorporate the same features and benefits as the corresponding original elements. In this case, the geometry of the thermotransfer structure  122  differs from the prior example. The thermally conductive element  128  and the thermal storage element  130  have polygonal cross-sections, which in this case are square cross-sections that taper from end-to-end to form pyramidal shapes (not shown). Alternatively, the thermotransfer structure  22  may have a different geometric three-dimensional shape and need not necessarily taper from end-to-end (e.g., a cylinder). 
       FIG. 5  illustrates another example thermotransfer structure  222 . In this case, the thermotransfer structure  222  includes a cover  242  that retains the thermal storage element  230  within the recessed cavity  40  of the thermally conductive element  228 . The cover  242 , which may also be considered to be a stop, forms the first surface  222   a  of the thermotransfer structure  222 . The cover  242  may completely close the mouth of the recessed cavity  40  such that the recessed cavity  40  is hermetically sealed from the exterior environment of the thermotransfer structure  222 . Thus, the heat transfer fluid within the vessel cannot infiltrate the recessed cavity  40 . 
       FIG. 6  illustrates an example thermotransfer structure  322  that is somewhat similar to the example thermotransfer structure  222  of  FIG. 5 . In this case, the thermotransfer structure  322  includes an open gap  344  between the thermal storage element  330  and the walls that form the recessed cavity  40  of the thermally conductive element  328 . The open gap  344  allows relative movement between the thermal storage element  330  and the thermally conductive element  328 . For instance, the open gap  344  may function as an expansion gap or bellows between the thermally conductive element  328  and the thermal storage element  330 . If the material selected for each of the thermally conductive element  328  and the thermal storage element  330  are solid materials, the open gap  344  may be relatively small. Alternatively, if the thermal storage element  330  is a phase change material, the open gap  344  may be somewhat larger to accommodate the relatively larger difference in thermal expansion between the solid material of the thermally conductive element  328  and the phase change material of the thermal storage element  330 . 
       FIG. 7  illustrates another example thermal storage element  430  that may be used in combination with any of the prior examples. In this case, the thermal storage element  430  includes a core  450  that is made of a first material and a protective cladding  452  made of a second material that encases the core  450 . For instance, the protective cladding  452  completely encloses and seals the core  450  from the surrounding environment. In instances where the first material of the core  450  is incompatible with the heat transfer fluid, the protective cladding  452  may be used to limit or eliminate contact between the core  450  and the heat transfer fluid. 
     The first material of the core  450  has a first composition and the second material of the protective cladding  452  has a second composition that is different than the first composition. the first and second compositions may be metallic, ceramic, or combinations thereof. In one example, the first material is a metal or metal alloy and the second composition is different metal or metal alloy. In a further example, the protective cladding  452  may be a superalloy, such as a nickel-based, cobalt-based alloy, a steel alloy, or an aluminum alloy. In a further example, the core  450  is a ceramic material, such as an oxide, nitride, carbide, or the like. 
       FIG. 8  illustrates another example heat transfer system  520  that includes a thermotransfer structure  522 . In this case, the thermotransfer structure  522  includes an open gap  544  between the walls that form the recessed cavity  540  of the thermally conductive element  528  and the thermal storage element  530 . The open gap  544  substantially circumscribes the thermal storage element  530  such that there is reduced contact, or even no contact, between the thermal storage element  530  and the thermally conductive element  528 . 
     The open gap  544  is fluidly connected with the surrounding environment such that, in a solar power system, the heat transfer fluid within the vessel can flow through the open gap  544 . Thus, the open gap  544  provides access to additional surface area of the thermally conductive element  528  for contact with the heat transfer fluid while still allowing the thermal storage element  530  to absorb heat and, upon inactivity of the system as described above, release the thermal energy. 
     As illustrated in  FIG. 9 , the thermotransfer structure  522  includes multiple covers  542  that extend at least partially over the mouth of the recessed cavity  540  to retain the thermal storage element  530  within the recessed cavity  540 . In this case, the covers  542  (e.g., stops) extend partially over the recessed cavity  540  and thereby permit flow of the working fluid into and out of the open gap  544 . Optionally, the covers  542  may be bonded to the thermal storage element  530 . However, in other examples, the thermal storage element  530  is free of any attachments or bonds to the thermally conductive element  528 . That is, the thermal storage element  530  is suspended in the working fluid within the recessed cavity  540 . 
     Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments. 
     The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.