WO1996011372A1

WO1996011372A1 - Improved heat transfer system for thermoelectric modules

Info

Publication number: WO1996011372A1
Application number: PCT/US1995/012885
Authority: WO
Inventors: David F. Gallup; Christian T. Gregory; Rankiri T. Karunasiri; David R. Noles; Lon E. Bell
Original assignee: Amerigon, Inc.
Priority date: 1994-10-05
Filing date: 1995-10-04
Publication date: 1996-04-18
Also published as: AU3999295A

Abstract

A heat transfer system (48) comprises at least one thermoelectric module (50) that includes at least one thermoelectric element (52) interposed between a pair of thermally conductive surface portions (54). Thermally conductive heat transfer elements (60) are in contact with a respective surface portion (54) to provide conductive heat transfer. The heat transfer elements (60) may be integral or independent with one another and are stress relieved to accommodate the different thermal expansion rate of the surface portions (54) and, consequently increase thermoelectric module life. Contact between the heat transfer elements (60) and each respective surface portion (54) is maintained by a flexible thermally conductive bonding material or by solder connection or by an outer structure (68). Compressible sheets (64) are disposed over outer facing portions of the heat transfer elements (60) and the outer structure (68) is placed over the sheets to impose a compressive force onto the sheets. A thermally conductive material (62) is interposed between the heat transfer elements (60) and the surface portion (54) to promote thermal conductivity therebetween.

Description

IMPROVED HEAT TRANSFER SYSTEM FOR THERMOELECTRIC MODULES

Field of the Invention

The present invention relates generally to a thermoelectric module and, more specifically, to an improved heat transfer system used in conjunction with a thermoelectric module to enhance heat transfer response time, reduce size, reduce cost, and increase module service life.

Background of the Invention

In certain heating and cooling applications it is necessary that a relatively small device be used that can be mounted or concealed within the device being heated or cooled. A popular heating and cooling device is a thermoelectric module. The thermoelectric module is preferred because of its relatively small size and the fact that it does not require the use of moving parts. Rather, the thermoelectric module comprises a number of thermoelectric elements that generate heat or cooling via Peltier effect by the application of electricity across a junction of two non-similar metals making up each element. Heat is generated at one side of the thermoelectric elements by applying electricity in one direction across the junctions, at the same time cold is generated at the opposite side of the thermoelectric elements. By applying electricity in a reverse direction across the junctions the direction of heat transfer is reversed. The heat and cold generated by the thermoelectric elements is transferred to a pair of ceramic surfaces of the thermoelectric module that are attached to common ends of each thermoelectric element.

A second conductor plate is attached to each thermoelectric ceramic surface to facilitate conductive heat transfer of the heat and cold generated by the thermoelectric module. Typically, aluminum base plates are attached to the surface of each ceramic surface. The baseplates are either clamped into place, sandwiching the thermoelectric module therebetween, or bonded into place by using an epoxy adhesive between each adjoining ceramic surface and baseplate surface. The aluminum base plates each comprise a plurality of fins that extend outwardly away from the baseplate to facilitate conductive transfer of heating or cooling away from the thermoelectric module and convective transfer of heating or cooling to surrounding air. The fins are typically non-integral members of the conductive baseplate and are each brazed into place prior to attaching each baseplate to the ceramic surfaces.

Although the use of mounting aluminum base plates comprising a plurality of non- integral fins to the thermoelectric module does facilitate convection heating or cooling somewhat, i.e., via the plurality of fins, it also results in a heat pump that is costly to produce, is bulky in size, and has a slow response time. A thermoelectric module including the base plates is costly to produce because of the added cost of the base plates themselves, and the time and labor involved in attaching the base plates to the thermoelectric module and attaching the fins to the base plates. The use of the base plates also adds to the bulk or size of the thermoelectric module due to the added thickness of the base plates themselves. Because the relatively small size of the thermoelectric module is what makes the device a popular choice in applications where space is a primary concern, added bulk or size serves to defeat or limit its application. Also, the use of the base plates as an intermediary between the thermoelectric module and the fins acts to slow the response time associated with the conduction transfer of heating or cooling to the fins and, therefore, slows the convection transfer of heating and cooling to the surrounding air by the fins.

An additional problem with including the aluminum base plates in the construction of the thermoelectric module is the difference in the coefficient of thermal expansion between the thermoelectric module ceramic surfaces and the interfacing base plates. As the thermoelectric module is operated in the heating or cooling mode, the difference in thermal expansion between the interfacing members causes thermal stresses to develop and buildup in the thermoelectric module. After time, repeated use and continued thermal cycling causes thermal fatigue, ultimately resulting in the failure of the thermoelectric module due to the transmission of such stresses to the thermoelectric elements.

It is, therefore, desirable that a heat transfer system for use with a thermoelectric module be constructed in a manner that would enhance the response time in providing a desired degree of convection heating and/or cooling. It is desirable that the heat transfer system not add significantly to the size of the thermoelectric module so as to facilitate its use in applications of limited space. It is desirable that the heat transfer system eliminate or minimize the potential for thermoelectric module failure due to thermal stress and failure. It is also desirable that the heat transfer system be economically feasible to construct, in terms of the materials that are used and the time and labor involved in construction, and be constructed using conventional manufacturing techniques.

Summary of the Invention There is, therefore, provided in practice of this invention a heat transfer system for use with one or more thermoelectric modules. A heat transfer system comprises at least one thermoelectric module that includes at least one thermoelectric element interposed between a pair of thermally conductive ceramic surfaces. A plurality of heat transfer elements are in contact with adjacent and distinct portions of a respective ceramic surface to effect conductive heat transfer from the ceramic surface. The heat transfer elements may be either integral or non-integral with one another and are formed from a thermally conductive material. Contact between the heat transfer elements and a respective ceramic surface can be maintained through the use of a flexible thermally conductive bonding material or the use of solder connection. Use of the flexible thermally conductive bonding material promotes thermal conductivity between the heat transfer elements and respective ceramic surfaces and accommodates thermal expansion and contraction movement of the heat transfer elements vis¬ a-vis respective ceramic surface, thereby minimizing thermally induced stress in the ceramic surfaces.

Contact between the heat transfer elements and a respective ceramic surface can also be maintained by an outer structure. The outer structure is placed around the heat transfer elements and thermoelectric modules, and is configured to impose a compressive force onto the heat transfer elements that is sufficient to cause the heat transfer elements to compressively engage the surface of a respective ceramic surface, and yet permit thermal expansion and contraction movement of the heat transfer elements with respect to the ceramic surfaces. Blankets of a compressible material can be interposed between outer facing portions of the heat transfer elements and the outer structure to facilitate compression of the heat transfer elements against a respective ceramic surface. A thermally conductive material such as thermally conductive grease and the like is interposed between the heat transfer elements and the surface of a respective ceramic surface for promoting thermal conductivity between the heat transfer elements and ceramic surfaces. The heat transfer elements can be disposed onto the surface of each ceramic surface in a staggered arrangement and/or the heat transfer elements can also be configured having a complex geometric structure to promote turbulent flow of air therethrough and, thereby, maximize convective thermal transfer to the surrounding air.

A heat transfer system constructed according to principles of this invention has: (1) a quickened response time for effecting convection heating or cooling, due to enhanced thermal conduction from direct contact between the heat transfer elements and each ceramic surface, a lower thermal mass due to the elimination of the aluminum baseplate, and turbulent air flow through the heat transfer elements; (2) is less bulky, due to the elimination of a conductive baseplate interposed between the heat transfer elements and each ceramic surface; (3) is less expensive to manufacture, due to the elimination of both the conductive base plates and the need to braze the heat transfer elements to the base plates; and (4) an increased service life, due to the reduction of transferred thermal stress to each ceramic surface, when compared to the case where the baseplate is interposed between the heat transfer elements and the ceramic surface and is bonded to the ceramic surface of the thermoelectric module. Brief Description of the Drawings

These and other features and advantages of the present invention will become appreciated as the same becomes better understood with reference to the specification, claims and drawings wherein: FIG. 1 is a fragmentary semi-schematic side elevational view of a prior art heat transfer system incorporating multiple thermoelectric modules;

FIG. 2 is a fragmentary semi-schematic side elevational view of a first preferred embodiment of a heat transfer system constructed according to principles of this invention incorporating a thermoelectric module; FIG. 3 is a semi-schematic front elevational view of section 3-3 of the first preferred embodiment of the heat transfer system of FIG. 2;

FIG. 4 is a fragmentary semi-schematic side elevational view of a second preferred embodiment of a heat transfer system constructed according to principles of this invention;

FIG. 5 is semi-schematic perspective view of a third preferred embodiment of a heat transfer system constructed according to principles of this invention;

FIGS. 6a through 6h are semi-schematic perspective views of different heat transfer element configurations that can be used in the third preferred embodiment of FIG. 5;

FIG. 7a is a top plan view of an integral multi-fin embodiment;

FIG. 7b is a side perspective view of the integral multi-fin embodiment of FIG 7a; FIG. 7c is a side perspective view of a second integral multi-fin embodiment; and

FIGS. 8a through 8d are semi-schematic perspective views of different types of third integral multi-fin embodiments.

Detailed Description A heat transfer system known in the art is shown in FIG. 1. A known heat transfer system 10 includes one or more thermoelectric module or device 12. Each thermoelectric module comprises a plurality of thermoelectric elements 14 interposed between surface portions 16 of the thermoelectric module. The surface portions are made from a material that has good strength, good resistance to thermally induced stresses, is thermally conductive and electrically nonconductive, and is of relatively low cost to manufacture. Typically, the surface portions 16 are made from a ceramic material and, hereafter shall be referred to as ceramic surface 16. Each thermoelectric element is made up of two dissimilar metal portions that are joined together to form a junction between the two ceramic surfaces. The thermoelectric elements are arranged within the thermoelectric module with common metal portions attached to electrical conductors (not shown) at each side of the thermoelectric elements. Electricity is applied across the thermoelectric elements via wires 18 connected to the electrical conductors and extending from the thermoelectric modules. The application of electricity across the electrical conductors in one direction causes the current to flow through each thermoelectric element and each respective junction, generating heat at one ceramic surface 16 of each thermoelectric module and generating cold at each opposite ceramic surface via Peltier effect. Reversing the polarity of the electricity across the electrical conductors causes the current to reverse its flow across each thermoelectric element and junction and, thus generating cold at the ceramic surface that was once hot and generating heat at the ceramic surface that was once cold. By applying electricity to the thermoelectric modules and controlling the direction of current flow through each thermoelectric element, the thermoelectric modules are used as a source of heating and cooling in a variety of different applications. The known heat transfer system 10 includes a pair of base plates 20 of thermally conductive material that are each attached to an outside surface of a respective ceramic surface 16. The base plates are preferably made from a material having high thermal conductivity to facilitate conductive heat transfer away from the thermoelectric modules. Typically, the base plates are formed from aluminum and have a thickness of approximately 0.2 inch. Thermal grease or a thin foil of conductive material 22 can be interposed between each ceramic surface 16 and baseplate 20 couple to facilitate better thermal transfer between the interfacing ceramic surface and baseplate surfaces. The base plates 20 are attached to respective ceramic surfaces 16 by conventional attachment means such as by screws 24, wherein the screws 24 extends through holes 26 positioned around a peripheral portion of adjoining base plates. The thermoelectric modules are positioned within the hole pattern and the base plates are fastened against respective ceramic surfaces by placing the screws through the aligned holes in one baseplate, threadably engaging each screw with a threaded opening in an adjacent baseplate, and fastening each screw to thereby impose a predetermined compressive pressure on each interposed thermoelectric module. Typically, the base plates are tightened together to impose a compressive pressure on the thermoelectric modules of approximately 200 psi. The use of the base plates 20 adds to the bulk and size of the heat transfer system and consequently reduces the response time of convective heat transfer to surrounding air, due to the need to first effect conductive heat transfer through each baseplate and to heat or cool the entire baseplate. A heat transfer stock 28 attached about an outside surface of each baseplate 20. The heat transfer stock comprises a wave-shaped sheet of thermally conductive material such as copper, aluminum and the like, wherein each wave acts as a heat transfer element or fin 30 for transferring heat or cold from an adjacent baseplate 20 to the air to facilitate convective heating or cooling. Accordingly, the terms "heat transfer stock" and "fin stock" and "heat transfer element" and "fin" shall hereinafter be used interchangeably. The fin stock is attached to an adjacent baseplate by a plurality of bonds 29 formed by brazing each interfacing fin stock portion, i.e., each interfacing wave-shaped portion, to the baseplate surface. The fin stocks may be surrounded about an outer facing portion by an outer structure 31 that is attached to each conductive baseplate 20. In some applications, the thin foil 22 is replaced by a conductive thermal epoxy placed in the same location to eliminate the screws 24 and enable a less costly assembly operation.

The known heat transfer system provides convective heat transfer to the surrounding air by thermal conduction from the thermoelectric elements 14, through the ceramic surfaces

16, through the base plates 20, and to the fins 30. The ceramic surfaces and base plates are formed from materials having different coefficients of thermal expansion (CTE). Generally, the material forming each baseplate 20 is made from a metal having a CTE much higher than that of an adjacent ceramic surface 16. Accordingly, as one ceramic surface is heated and an opposite ceramic surface is cooled, with the application of electricity in one direction across each thermoelectric element, an interfacing baseplate is heated and cooled, respectively. The baseplate that interfaces with the heated ceramic surface will expand to a greater extent, and the baseplate that interfaces with the cooled ceramic surface will contract to a greater extent, than the interfacing ceramic surface. In embodiments wherein the baseplates are bonded directly to the thermoelectric module surfaces, the differences in the expansion characteristics of the interfacing ceramic surface and baseplate couples will cause the heat ceramic surface and the baseplate to bow slightly, with a convex portion oriented at the heated side and a concave portion oriented at the cooled side of the heat transfer system. As the thermoelectric modules are switched from a heating to a cooling operation, i.e. , during thermal cycling, the heat transfer system is caused to bow back and forth and, thereby fatigue the interposed thermoelectric modules. After repeated thermal cycling, the fatigue will cause the thermoelectric elements to fracture and fail, limiting the service life of the heat transfer system. In an embodiment of the above configuration, for one application, the service life of such heat transfer system is approximately 500 to 5000 cycles.

A heat transfer system constructed according to principles of this invention eliminates both the problem of thermally induced thermoelectric element failure and reduced response time, inherent in the known heat transfer system, using a construction that is less bulky and less costly to manufacture. Referring to FIGS. 2 and 3, a first preferred embodiment of a heat transfer system 32 includes at least one thermoelectric module 34 of the same construction previously described for the known heat transfer system 10, comprising elements 36, ceramic surfaces 38. and wires 40. In an exemplary embodiment, each thermoelectric module has a size of approximately 40 millimeters by 40 millimeters and is powered by a DC voltage of approximately 12 volts. A fin stock 42 is attached directly to an outside surface of each ceramic surface 38.

Each fin stock can be configured in the same manner as the fin stock 28 previously described for the known heat transfer system 10, comprising a wave-shaped configuration forming a plurality of integral heat transfer elements or fins 44. The fin stock may be formed from a thin sheet of conductive material such as metals and metal alloys. In an exemplary embodiment, the fin stock is formed from copper or aluminum. It is desirable that the fin stock comprise fins having a fin height as tall as possible to provide an increased heat transfer area and, thus facilitate the convective heat transfer to surrounding air. In an exemplary embodiment, the fin stock 42 comprises 3/8 inch windowpane fin made from aluminum or copper having a thickness in the range of from 0.010 to 0.015 inches, and approximately 15 fins per inch that are each approximately 1/2 inch in height. As shown in FIG. 3, each fin stock can be configured to have a length that is longer than the length of an interfacing ceramic surface to provide both an enhanced level of conductive heat transfer from the ceramic surface and convective heat transfer to surrounding air.

As best shown in FIG. 3, each fin stock is attached or bonded at an interfacing fin portion 45 to a respective ceramic surface by use of a flexible thermally conductive bonding material 46 such as an epoxy, a silicone mixture or the like that can be loaded with a metallic substance to enhance thermal conductivity. The use of a flexible thermally conductive bonding material will allow the two interfacing materials to expand or contract at their own characteristic expansion or contraction rate, by stretching or compressing the bonding material instead of stressing the ceramic surfaces, while still providing good thermal transfer. In an exemplary embodiment, the thermally conductive bonding material is a silicone mixture with a predetermined amount of copper. It is desired that the bonding material have a CTE that is close to or matches the CTE of the ceramic surfaces 38 to reduce the extent of thermal stress that may be transferred from the fin stock to a respective ceramic surface.

Referring again to FIG. 3, the fin stock is shown to comprise a number of slots 47 that are cut into each interfacing fin portion 45. The slots serve to reduce the extent of thermally induced stress that can accumulate between each fin stock and a respective ceramic surface, in a direction parallel to each interfacing fin portion 45, and be transferred from each fin stock to a respective ceramic surface along the bonded interface. During thermal expansion or contraction, the slots 47 act to accommodate the higher rate of thermal expansion in the fin stock 42 by enlarging or contracting, respectively. In this manner, the amount of thermal stress generated between the fin stock and a respective ceramic surface in a direction parallel to each interfacing fin portion is greatly reduced. The expansion and contraction of the fin stock in a direction perpendicular to the interfacing fin portion 45 is accommodated by the wave-shaped configuration, also serving to reduce the amount of thermal stress transmitted to a respective ceramic surface 38.

It is desired that the portion of each fin stock extending beyond a respective ceramic surface not have slots to promote conductive heat transfer away from the ceramic surface. Slots located in the extending portion would impair conductive heat transfer to such portion. In an exemplary embodiment, the slots are positioned at equidistant locations along each interfacing fin at 6 millimeters (1/4 inch intervals). It is desirable that each slot have a slot width and slot length of sufficient size to provide a desired degree of thermal stress reduction without adversely affecting the heat transfer capability and structural rigidity of the fin stock. For a fin stock having a fin height of approximately 13 millimeters (1/2 inch) it is desired that each slot have a slot width of approximately 0.3 millimeters (0.010 inch) and a slot length of approximately 10 millimeters (0.4 inch).

The first preferred embodiment of a heat transfer system 32 provides a quickened response time, in terms of the amount of elapsed time between routing electricity to the thermoelectric modules and effecting a desired degree of convective heat transfer, due to the elimination of an interposed conductive baseplate and, the direct attachment of each fin stock to a respective ceramic surface. The direct attachment of each fin stock to a respective ceramic surface reduces the response time by the amount of time that was previously associated with the conductive heat transfer of the baseplate including its thermal mass. In an exemplary embodiment, the direct attachment of each fin stock to a respective ceramic surface has quickened the response time by a factor of approximately three. The direct attachment of each fin stock to a respective ceramic surface greatly reduces the cost and also acts to reduce the bulk or size of the heat transfer system, thereby making the heat transfer system a popular choice of heating and/or cooling source for applications where small size or light weight is important. Such applications may include occupant seats used in vehicles such as automobiles, trains, planes, buses, boats, and the like. The use of a thermally conductive bonding material 46 to attach each fin stock 42 to a respective ceramic surface 38, rather than the use of an intermediary base plate, acts to reduce the development thermal stresses at each ceramic surface due in one direction, e.g., in a direction parallel with the length of each ceramic surface, and the use of slots 47 through the fin stock act to reduce the development of thermal stresses at each ceramic surface in a different direction, e.g., in a direction parallel to the width of each ceramic surface. Reducing the development of thermal stresses at each ceramic surface in turn reduces thermal stresses that are transferred to each thermoelectric element, thereby reducing the potential for fracture or fatigue related failure of each thermoelectric element and extending the service life of each thermoelectric module. Additionally, the construction of such device allows for a faster responding, smaller and cheaper assembly that does not overly sacrifice thermoelectric module life. Tests have demonstrated that a first preferred embodiment of the heat transfer system have a service life in the range of from 5,000 to 12,000 thermal cycles without failure, up to three times longer than the known heat transfer system.

Referring to FIG. 4, a second preferred embodiment of a heat transfer system 48 includes at least one thermoelectric module 50 of the same type previously described, comprising thermoelectric elements 52 interposed between a pair of ceramic surfaces 54. A fin stock 56 of the type previously described, comprising a wave-shaped configuration forming a plurality integral heat transfer elements or fins 58, is disposed onto the outside surface of each ceramic surface 54. Each thermoelectric module 50 is interposed between the fin stocks 56. Unlike the first preferred embodiment, the interfacing fin portions 60 are not epoxied to a respective ceramic surface but, rather are placed in a predetermined degree of compressive engagement against a respective ceramic surface. In this manner each interfacing material is allowed to expand or contract at its own characteristic expansion and contraction rate to eliminate the transfer of thermal stress to the ceramic surface.

A conductive interfacing material 62, such as thermal grease, graphite sheet, or a thin foil layer of conductive material that will accommodate displacement of the interfacing fin portion 60 along a respective ceramic surface during expansion or contraction of the same, is interposed between each interfacing fin portion 60 and a respective ceramic surface 54. In an exemplary embodiment, thermal grease is used and includes a predetermined amount of copper to enhance thermal conductivity. Accordingly, each fin stock is placed in direct contact with a respective ceramic surface in a manner that accommodates the different thermal expansion characteristics of each interfacing material so as to minimize the transfer of thermal stress to the ceramic surfaces.

A compressible sheet 64 is disposed onto each fin stock 56 about outer facing fin portions 66. It is desired that the sheet be made from a compressible material such as high- density foam, hard rubber and the like for purposes of imposing a compressive force onto each fin stock and, thereby effecting compressive engagement between each interfacing fin portion 60 and respective ceramic surface 54. It is also desired that the material forming the compressible sheet have good oxidation resistance, good thermal resistance, have long term stability so that the compressible sheet does not creep over time, is capable of retaining its compressibility after exposure to repeated thermal cycling, and insulates the conditioned air from the outside environment. Each compressible sheet is placed onto the outer facing fin portions and serves to absorb a predetermined amount of compression force from a surrounding outer structure and transfer such compression force to each fin stock to both keep each fin stock in place against a respective sheet and ensure good interface between each fin stock and ceramic surface to provide good conductive heat transfer. The use of a compressible material also makes the design of the heat transfer system insensitive to variations in thermoelectric module design thickness, fin height, and other perturbations that are often encountered in production. In an exemplary embodiment, each compressible sheet is made from high-density foam, is configured to have a surface area of sufficient size to cover an underlying fin stock, and has a thickness of approximately 5 millimeters (0.2 inches). The compressible sheets 64 are compressively engaged against a respective fin stock

56 by an outer structure 68 that encases a heat exchanger assembly 70 that includes the compressible sheets 64, fin stocks 56 and thermoelectric modules 50. The outer structure can include one or more C-shaped structures or clamps that fit over the assembly 70 and are configured to impose a predetermined amount of compressive force onto the blanket to effect compressive engagement of the fin stocks against respective ceramic surfaces 54. The C- shaped structure can be made from a rigid material such as metal and the like that is capable of maintaining a compression force. Alternatively, the outer structure can include duct work that is configured to accommodate all or a portion of the heat exchanger assembly 70. The duct work can be made from a rigid material such as metal, plastic and the like that can be configured to accommodate the shape of the heat exchanger assembly. The duct work can include a one-piece structure configured to enclose the complete assembly therein, or may include a multi-piece structure that, when assembled together, is capable of enclosing the assembly therein.

A key feature of the outer structure is that it impose a sufficient compression force onto the compressible sheets 64 to ensure good contact between each fin stock and respective ceramic surface to keep the fin stocks positioned well. However, at the same time, it is important that the compression force not be so great as to cause the fin stock to deform or to damage the thermoelectric modules. When enclosed in the outer structure 68, the compressible sheets 64 not only serve to effect compressive engagement between each fin stock and respective ceramic surface, but also act as thermal insulators to minimize unwanted conductive heat transfer from the outer structure to the environment.

Like the first preferred embodiment, the second preferred embodiment of a heat transfer system 48 also provides a quickened response time when compared with the known heat transfer system 10, due to the elimination of the interposed conductive base plates and, the direct attachment of each fin stock to a respective ceramic surface. In an exemplary second embodiment, the direct attachment of each fin stock to a respective ceramic surface has quickened the response time by a factor of approximately four. Also like the first preferred embodiment, the elimination of the conductive base plates also reduces the bulk and size of the heat transfer system.

The use of a thermally conductive grease between each interfacing fin portion and respective ceramic surface assists in allowing each interfacing material of different CTE to expand or contract at its own expansion or contraction rate, thereby minimizing the transfer of thermal stress to the thermoelectric modules and elements and extending the service life of each thermoelectric module. Tests have demonstrated that a second preferred embodiment of the heat transfer system may have a service life in the range of from 8,000 to 18,000 thermal cycles without failure, up to 36 times longer than known heat transfer system using bonded attachment methods. Referring to FIG. 5, a third preferred embodiment of a heat transfer system 72 includes at least one thermoelectric module 74 of the same type previously described, comprising thermoelectric elements 76 interposed between a pair of ceramic surfaces 78. A plurality of non-integral heat transfer elements 80 are attached to an outside surface of each ceramic surface 78 by use of a thermally conductive bonding material of the type previously described for the first embodiment. Alternatively, the heat transfer elements 80 may be attached to a respective ceramic surface 78 by use of solder pads. Solder pads 81 can be formed on each ceramic surface at locations that correspond to the desired placement of each heat transfer element. To accommodate thermal expansion movement of the heat transfer elements on respective ceramic surfaces, the solder pads 81 are arranged at predetermined locations with gaps of sufficient size between adjacent solder pads. Attachment of each heat transfer element to a respective ceramic surface is accomplished by placing the heat transfer element onto the solder pad 81 and applying heat to the heat transfer element to solder together the heat transfer element and the ceramic surface. Each heat transfer element may also have a corresponding solder pad (not shown) disposed on a base portion of the heat transfer element so that such heat transfer element is attached to the ceramic surface by placing the heat transfer element onto to solder pad 81 on the ceramic surface and applying heat, thereby causing each solder pad to melt together and form a bond therebetween. To facilitate the manufacture of the thermoelectric module, the solder pads 81 can be formed during the placement of solder pads (not shown) at the opposite side of the ceramic surface to attach each thermoelectric module thereto, and the heat transfer elements can be wave soldered to each respective ceramic surface 74.

Alternatively, the heat transfer elements 80 may be placed into communication with a respective ceramic surface of the thermoelectric module without solder pads or a thermally conductive boding material according to the practice of the second preferred embodiment of the heat transfer system discussed above, i.e., by arranging the heat transfer elements on each ceramic surface, placing a compressible sheet placed over tip portions of the heat transfer elements, and using a outer structure to engage the compressible sheet and effect compressive engagement of the heat transfer elements against each respective ceramic surface.

The use of non-integral heat transfer elements accommodates different rates of thermal expansion between the independent heat transfer elements and respective ceramic surfaces by allowing the heat transfer elements to expand in any direction along the ceramic surface, thereby minimizing the development of thermal stresses at each ceramic surface. The heat transfer elements 80 can be shaped in the form of a fin, post, wire mesh, tube, wire cloth or other type of configuration that could be used to effect conductive heat transfer from a heated or cooled ceramic surface and effect convective heat transfer to surrounding air. It is preferred that the heat transfer elements 80 be made from a material having high thermal conductivity such as metals, metal alloys and the like. In an exemplary embodiment, the heat transfer elements are configured in the form of a fin and are made from copper or aluminum. Accordingly, the term "fin" is hereinafter used to refer to the heat transfer elements 80. It is to be understood that the use of the term "fin" shall not operate in any way to limit the different types and configurations of heat transfer elements within the scope of this invention.

To maximize both conductive heat transfer from the ceramic surface to the fins and convective heat transfer from the fin to surrounding air, it is desired that the fins have a fin height as tall as possible. For purposes of maximizing conductive heat transfer from the ceramic surfaces 78 to the fins 80, it is desired that a base portion 82 of each fin have a planar surface with a large surface area. To maximize conductive heat transfer it is also desirable that the fins be nested together on the surface of the ceramic surfaces, i.e. , the base portions of each fin be in close proximity with one another. For purposes of minimizing the development of thermal stresses at the ceramic surface, it is desired that the fins be spaced apart from one another a small distance to accommodate thermal expansion movement of each fin. The base portions can be configured in a number of different geometric shapes that are capable of nesting together such as rectangular, square, hexagonal and the like. It is also desirable to have tall fins to allow more heat to be conducted away from the ceramic surfaces for convective heat transfer to the air.

The base portion 82 can be an integral member of each fin or can be a separate component that is welded or otherwise fixedly attached to a fin element 84. In an exemplary embodiment, the base portion is integral with the fin to ensure a maximum degree of conductive heat transfer. In fact, the base portion 82 may be formed from an end portion of the fin element 84 that has been bent in a perpendicular orientation with respect to the fin element to accommodate attachment with the ceramic surface, as shown in FIG. 6a. Although it is preferred that the fin element be perpendicular to the base portion, it is to be understood that the fin element may depend away from the base portion at an angle other than 90 degrees, to facilitate convective heat transfer. The fin element 84 of each fin can be positioned at the center of the base portion or can be offset from the center a predetermined amount.

It is known that turbulent air flow increases convective heat transfer. Therefore, to further maximize convective heat transfer from the fins 80 it is desired to attach the fins to the ceramic surface in a manner that induces turbulent flow across the fin elements 84. Accordingly, as shown in FIG. 5, the fins may be nested on each ceramic surface so that each base portion 82 is staggered with respect to a base portion of an adjacent fin, thereby staggering each fin element 84. For example, the fins can be attached to each ceramic surface in a series of repeating rows, wherein the base portions of each row are staggered with respect to an adjacent row. The staggering of the base portions 82 in adjacent rows form columns of staggered fin elements 84 that causes turbulent flow of the air passed over the fin elements.

In an exemplary embodiment, the fin elements 84 are each configured in the shape of a rectangle and are positioned at the center of a square base portion 82. The base portions of the fins in adjacent rows are staggered to maximize convective heat transfer by inducing convective heat transfer. The fin elements have a fin height in the range of from 1/2 to 1- 1/2 inch, have a thickness in the range of from 0.006 to 0.20 inches, and have a width in the range of from 1/8 to 1/4 inches. Alternatively, each fin can be constructed so that the fin element is staggered differently with respect to each base portion 82. In this embodiment, the fins used in one row comprise a fin element 84 that is disposed on the base portion 82 at a different position than the fins used in an adjacent row so that the base portions of each fin are aligned and only the fin elements are staggered. This also causes turbulent flow of the air passed over the fin elements.

Turbulent air flow can also be induced by the shape of the heat transfer element or fin element 84 itself. Referring to FIGS. 6b-6h, the fins 80 may comprise fin elements 84 that are configured in a number of different geometries including a tapered fin element 86, a twisted fin element 88, a wavy fin element 90, a perforated fin element 92, split fin elements 94, 96 and 98, and the like. Although the fin elements illustrated in FIGS. 6b-6h are shown to include a square bottom base portion 82, it is to be understood within the scope of this invention that the fin elements may be integral with or attached to other configurations of base portions. Additionally, rather than being centered on each base portion, the fin elements may be staggered on each base portions. Fins comprising such fin element configurations can be attached to each ceramic surface 78 in the manner previously described to provide a staggered arrangement to further increase the turbulence of air passed over the fins.

Referring to FIG. 7a, the heat transfer elements or fin elements 102 may be integral members of a multi-element or multi-fin embodiment 100. The multi-fin embodiment comprises a number of fin elements 102 that are integral with adjacent fin elements via common connecting members 104 near a portion of the multi-fin embodiment that forms base portions 106 for the multi-fin embodiment. Each base portion 106 and respective fin element 102 is staggered from an adjacent base portion and fin element by a predetermined amount so that each fin element can induce turbulent flow. The base portions 106 are readied for attachment onto a surface of a ceramic surface by bending the base portion into a perpendicular orientation vis-a-vis a respective fin element 102 as shown in FIG. 7b. Alternatively, as disclosed above, the base portion may be bent so that it is at an angle other than 90 degrees from the fin element.

A plurality of the multi-fin embodiments 100 can be attached to the surface of each thermoelectric module ceramic surface in a staggered manner to both facilitate conductive heat transfer from the ceramic surface to the fins and maximize convective heat transfer from the fin elements 102 to the surrounding air by inducing turbulent air flow. A second integral multi-fin embodiment 108 comprising a number of heat transfer or fin elements 110 is shown in FIG. 7c. The second multi-fin embodiment 108 comprises a common tip portion 112 that extends between a tip portion of each fin element 110, and includes independent base portions 114 that are integral with each respective fin element 110 and are bent perpendicularly to each fin element to accommodate attachment to a respective ceramic surface. Alternatively, the base portion may be bent at an angle other than 90 degrees from a respective fin element as described above. The fin elements are configured so that a gap exists between each adjacent fin element to accommodate thermal expansion/contraction movement on the ceramic surface. The second multi-fin embodiment 108 may comprise any number of integral fin elements to accommodate a particular application. For example, the second multi-fin embodiment 108 may be configured having a predetermined number of fin elements that extend from one end portion of each ceramic surface to an opposite end, so that each ceramic surface is covered by repeating rows of second multi-fin embodiment, each row comprising integral fin elements. To enhance convective heat transfer, each row of second multi-fin embodiments may be staggered vis-a¬ vis second multi-fin embodiments in adjacent rows.

FIGS. 8a-8d illustrates different types of third integral multi-element or multi-fin embodiment that are each formed from a spirally wound or continuous winding of conductive material of the same type previously described for the first, second, and third preferred embodiments of a heat transfer system. Referring to FIG. 8a, the third multi-fin embodiment 116 is configured in the shape of repeating triangles each having heat transfer elements 118 that extend from a base portion 120, thereby forming a triangle. Each triangle forming the integral multi-fin embodiment shares a heat transfer element 118 and base portion 120 that is common with adjacent triangles. The base portion 120 of the triangular third multi-fin embodiment can either be attached to a respective ceramic surface by use of thermally conductive bonding material or by use of solder pads as previously described for the third preferred embodiment of the heat transfer system. Alternatively, the triangular third multi- fin embodiment may be compressively engaged to a respective ceramic surface by use of a compressible sheet and outer structure as previously described for the second preferred embodiment. Additionally, if the third multi-fin embodiment is flexible enough, the compressible sheet may be eliminated and the fin embodiment may be held in compressive contact with the ceramic surface of the thermoelectric module by the outer structure.

In arranging the triangular third multi-fin embodiment on each respective ceramic surface it is desired that each triangular heat transfer element be separated from an adjacent element by a predetermined distance to provide room for thermal expansion/contraction movement. To provide maximum conductive and convective heat transfer, multiple numbers of triangular third multi-fin embodiments are arranged on respective ceramic surfaces in a nested configuration. As shown in FIG. 8b, the triangular third multi-fin embodiments 116 are arranged together to form an intertwined nest of triangular heat transfer elements 118.

The triangular third multi-fin embodiments are nested together such that an adequate space or gap is maintained between adjacent heat transfer elements to allow for thermal expansion/contraction movement, thereby minimizing the development of thermal stress on the ceramic surfaces. Nesting the triangular third multi-fin embodiments in this manner accommodates air flow in any direction along the ceramic surface, minimizing the development of a boundary layer, and creates a turbulent air flow along the ceramic surface, maximizing convective heat transfer.

Conductive heat transfer from the thermoelectric module is also maximized using the triangular third multi-fin embodiment due to the large surface area of each base portion 120 in contact with the ceramic surface. The technique of nesting the triangular third multi-fin embodiments also contributes to maximum conductive heat transfer from each respective ceramic surface as a surface coverage, i.e., the amount of ceramic surface in contact with a base portion, of up to ninety percent can be achieved. FIG. 8c illustrates a third multi-fin embodiment 122 configured in the shape of repeating rectangles each having heat transfer elements 124 that extend from a base portion 126. Like the triangular embodiment, each rectangle forming the integral multi-fin embodiment shares a heat transfer element 124 and base portion 126 that is common with adjacent rectangles. The base portion 126 of the rectangular third multi-fin embodiment can be attached to a respective ceramic surface in the same manner previously described for the triangular embodiment. Additionally, the rectangular embodiment can be arranged on a respective ceramic surface in a nested configuration with other rectangular third multi-fin embodiments to accommodate thermal expansion/contraction movement, provide maximum convective heat transfer, and to provide maximum surface coverage, therefore, maximum conductive heat transfer.

FIG. 8d illustrates a third multi-fin embodiment 128 in the shape of repeating circles each having heat transfer elements 130 that extend from a base portion 132. Like the triangular and rectangular embodiments, each circle forming the integral multi-fin embodiment 128 shares a heat transfer element 130 and base portion 132 that is common with adjacent circles. The base portion 132 of the circular third multi-fin embodiment can be attached to a respective ceramic surface in the same manner previously described for the triangular and rectangular embodiments. Additionally, the circular embodiment can be arranged on a respective ceramic surface in a nested configuration with other circular third multi-fin embodiments to accommodate thermal expansion/contraction movement and provide maximum convective heat transfer.

It is to be understood that although specific third multi-fin embodiments have been described and illustrated, other configurations of third multi-fin embodiments can be used and are within the principles of this invention. For example, a third multi-fin embodiment may comprise repeating integral square, hexagonal, oval shaped and the like heat transfer elements. Additionally, the third-multi-fin embodiments may be configured in the form of a wound pad or the like of a thermally conductive material, such as an aluminum wool or a wire cloth and the like. Like the first and second preferred embodiments, the third preferred embodiment of a heat transfer system 72, incorporating use of the fins disclosed in FIGS. 5-8, also provides a quickened response time when compared with the known heat transfer system 10, due to the elimination of an interposed conductive baseplate and, the direct attachment of each individual fin 80 to a respective ceramic surface. In an exemplary third embodiment, the direct attachment of each independent fin 80 to a respective ceramic surface 78 has quickened the response time by a factor of approximately three when compared to known heat transfer systems described earlier. Also like the first preferred embodiment, the elimination of the conductive base plates also reduces the bulk or size of the heat transfer system.

The attachment of individual fins 80 to a respective ceramic surface, rather than using a fin stock comprising integral fin elements, serves to minimize the development of thermal stresses at the ceramic surfaces because the fins are allowed to thermally expand or contract relatively unrestricted in any direction on the ceramic surface. Accordingly, the use of the individual fins serve to extend the service life of each thermoelectric module. Tests have demonstrated that a third preferred embodiment of the heat transfer system may have a service life in excess of 24,000 thermal cycles without failure.

Although limited embodiments of a heat transfer system have been described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that, within the scope of the appended claims, the heat transfer system according to principles of this invention may be embodied other than as specifically described herein.

Claims

What is Claimed is:

1. A heat transfer system comprising: at least one thermoelectric module, wherein each thermoelectric module comprises: at least one thermoelectric element interposed between surface portions of the thermoelectric module; a plurality of heat transfer elements disposed onto at least one surface portion, wherein the heat transfer elements are in contact with adjacent and distinct portions of a respective surface portion; and means for maintaining contact between the heat transfer elements and adjacent and distinct portions of each respective surface portion.

2. The heat transfer system recited in claim 1 wherein the heat transfer elements are formed from at least one sheet of thermally conductive material, wherein the heat transfer elements of each sheet are integral with other heat transfer elements of the same sheet, and wherein each heat transfer element includes an interfacing portion that is in contact with an adjacent and distinct portion of a respective thermoelectric module surface portion.

3. The heat transfer system recited in claim 2 wherein the heat transfer elements are formed from a sheet of thermally conductive material.

4. The heat transfer system recited in claim 3 comprising a plurality of slots through each heat transfer element, wherein the slots are through the sheet and extend a distance from each interfacing portion for accommodating thermal expansion and contraction of the sheet.

5. . The heat transfer system recited in claim 3 wherein the sheet of thermally conductive material is bent to produce a base portion, that is in contact with an adjacent and distinct portion of a respective thermoelectric module surface portion, and an element portion extending away from the base portion, wherein the heat transfer elements share a common base or element portion.

6. The heat transfer system recited in claim 2 wherein the heat transfer elements are formed from a wound configuration of thermally conductive material.

7. The heat transfer system recited in claim 6 wherein the wound configuration of thermally conductive material includes heat transfer elements having a number of similar or different repeating geometric shapes.

8. The heat transfer system recited in claim 1 wherein the means for maintaining contact between each heat transfer element and an adjacent and distinct portion of a respective surface portion comprises a flexible thermally conductive bonding material interposed therebetween.

9. The heat transfer system recited in claim 1 wherein the means for maintaining contact between each heat transfer element and an adjacent and distinct portion of a respective surface portion comprises a solder connection therebetween.

10. The heat transfer system recited in claim 1 wherein the means for maintaining contact between each heat transfer element and a respective surface portion comprises an outer structure configured to accommodate the thermoelectric modules and heat transfer elements therein and impose a compressive force onto the heat transfer elements to cause compressive engagement of the heat transfer elements against adjacent and distinct portions of each respective surface portion.

11. The heat transfer system recited in claim 10 comprising one or more sheets interposed between outer facing portions of the heat transfer elements and the outer structure, wherein each sheet is made from a compressible material.

12. The heat transfer system recited in claim 10 comprising a thermally conductive material interposed between the heat transfer elements and adjacent and distinct portions of each respective surface portion.

13. The heat transfer system recited in claim 12 wherein the thermally conductive material is selected from the group consisting of thermal grease, graphite sheet, and a thin foil of conductive material.

14. The heat transfer system recited in claim 1 wherein each heat transfer element is independent from each other heat transfer element and comprises a base portion that is in contact with an adjacent and distinct portion of each respective surface portion of the thermoelectric module, and an element portion extending away from the base portion and surface portion to effect convective heat transfer to a suπounding environment.

15. The heat transfer system recited in claim 14 wherein the base portion of each heat transfer element is attached to an adjacent and distinct portion of a respective surface portion by a flexible thermally conductive bonding material interposed therebetween.

16. The heat transfer system recited in claim 14 wherein the base portion of each heat transfer element is attached to an adjacent and distinct portion of a respective surface portion by solder connection.

17. The heat transfer system recited in claim 14 wherein the base portion of each heat transfer element is arranged on a respective surface portion a distance away from a base portion of an adjacent heat transfer element to accommodate thermal expansion and conduction movement.

18. A heat transfer system comprising: a thermoelectric module having a number of thermoelectric elements interposed between a pair of thermally conductive surface portions; a plurality of thermally conductive heat transfer elements in contact with at least one surface portion, wherein each heat transfer element is independent from each other heat transfer element and comprises: a base portion configured to accommodate contact with a respective surface portion; and an element portion extending away from the base portion and respective surface portion; and means for maintaining contact between each base portion and a respective surface portion.

19. The heat transfer system recited in claim 18 wherein the means for maintaining contact comprises a flexible thermally conductive bonding material interposed between each base portion and a respective surface portion.

20. The heat transfer system recited in claim 18 wherein the means for maintaining contact comprises a solder connection between each base portion and a respective surface portion.

21. The heat transfer system recited in claim 18 wherein the base portion of each heat transfer element is arranged on a respective surface portion spaced a distance away from the base portion of an adjacent heat transfer element to accommodate thermal expansion and contraction movement.

22. A heat transfer system comprising: at least one thermoelectric module comprising: a plurality of thermoelectric elements; a pair of thermally conductive surface portions, wherein the thermoelectric elements are interposed between the surface portions, and wherein each thermoelectric element is connected at opposite ends to respective surface portions; a plurality of thermally conductive heat transfer elements in contact with adjacent and distinct portions of at least one surface portion of the thermoelectric module on a side opposite from the thermoelectric elements; and an outer structure for accommodating the thermoelectric module and conductive heat transfer elements therein and effecting compressive engagement of the heat transfer elements against a respective surface portion.

23. The heat transfer system as recited in claim 22 comprising at least one sheet of compressible material disposed onto an outer facing portion of each heat transfer element, wherein the sheet is interposed between each heat transfer element and the outer structure.

24. The heat transfer system recited in claim 22 comprising a thermally conductive material interposed between each heat transfer element and respective surface portion.

25. The heat transfer system recited in claim 22 wherein the heat transfer elements are integral with one another and are formed from a sheet of thermally conductive material.

26. The heat transfer system recited in claim 25 comprising a plurality of slots through each heat transfer element, wherein the slots are through the sheet and extend a distance from each interfacing portion for accommodating thermal expansion and contraction of the sheet.

27. The heat transfer system recited in claim 22 wherein the heat transfer elements are integral with one another and are formed from a wound configuration of thermally conductive material.

28. The heat transfer system recited in claim 27 wherein the wound configuration of thermally conductive material includes heat transfer elements having a number of similar or different repeating geometric shapes.

29. The heat transfer system recited in claim 22 wherein the heat transfer elements are integral with one another and are formed from a sheet of thermally conductive material, wherein the sheet is bent to form a base portion, to accommodate contact with an adjacent and distinct portion of a respective thermoelectric module surface portion, and an element portion extending away from the base portion, and wherein each heat transfer element shares a common base or element portion.

30. The heat transfer system recited in claim 22 wherein the heat transfer elements are independent from one another, wherein each heat transfer element comprises a base portion in contact with a respective surface portion and an element portion extending away from the base portion to effect convective heat transfer.

31. The heat transfer system recited in claim 30 wherein the heat transfer elements are arranged on a respective surface so that the base portions of adjacent heat transfer elements are spaced a distance apart from each other to accommodate thermal expansion and contraction movement.