Abstract:
A thermoelectric power generator for converting thermal energy into electrical energy. The thermoelectric power generator includes a heat exchanger configured to extract thermal energy from an exhaust gas stream. The heat exchanger includes fins in contact with a boundary of the heat exchanger, where the fins are directly connected to a first set of thermoelectric modules. A second set of thermoelectric modules are directly connected to the boundary of the heat exchanger. The first and second sets of thermoelectric modules are configured to convert the thermal energy to electrical energy. By eliminating the metal wall that previously existed between the thermoelectric modules and the fins, the thermoelectric power generator improves the heat transfer between the exhaust gas and the thermoelectric modules, eliminates the thermal fatigue failures at the bond between the metal wall and the thermoelectric modules as well as allows for a higher density of thermoelectric modules.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to the following commonly owned U.S. Patent Application: 
     Provisional Application Ser. No. 61/892,767, “A Heat Exchanger for Thermoelectric Power Generation with the Thermoelectric Modules in Direct Contact with the Heat Source,” filed Oct. 18, 2013, and claims the benefit of its earlier filing date under 35 U.S.C. §119(e). 
    
    
     GOVERNMENT INTERESTS 
     The U.S. Government has certain rights in this invention pursuant to the terms of the National Science Foundation Grant No. CBET-1048767. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to heat exchangers, and more particularly to a heat exchanger for thermoelectric power generation with the thermoelectric modules in direct contact with the heat source. 
     BACKGROUND 
     A heat exchanger is a piece of equipment for efficient heat transfer from one medium to another. One such heat exchanger is a heat exchanger for thermoelectric power generation as illustrated in  FIG. 1 . Referring to  FIG. 1 ,  FIG. 1  illustrates a conventional thermoelectric power generator  100  that includes an exhaust gas heat exchanger  101 . Exhaust gas heat exchanger  101  is defined by an exhaust housing/wall  102 , such as an aluminum exhaust housing. Exhaust gas heat exchanger  101  receives a pressurized exhaust gas stream  103  from an internal combustion engine  104  and extracts thermal energy from exhaust gas stream  103 . 
     Exhaust gas heat exchanger  101  may include a flange (not shown) to sealingly connect to an exhaust pipe (not shown) of internal combustion engine  104 . The flange and wall  102  may be formed from a single piece, by, for example, upsetting. Alternatively, the flange may be attached to wall  102  by welding, brazing, or crimping. It is to be understood that the exhaust gas stream  103  from internal combustion engine  104  is at a higher pressure than the ambient atmosphere when the engine  104  is running and the pressurized exhaust gas stream  103  is contained in an exhaust system. 
     Exhaust gas heat exchanger  101  may include fins  105 A- 105 R (e.g., aluminum fins) that are in contact with wall  102  to increase the rate of heat transfer from exhaust gas stream  103 . Fins  105 A- 105 R may collectively or individually be referred to as fins  105  or fin  105 , respectively. 
     Thermoelectric power generator  100  further includes thermoelectric device modules  106  with their hot-side connected to wall  102 . Thermoelectric device modules  106  are configured to convert the thermal energy extracted by heat exchanger  101  to electrical energy  107  for consumption or storage by an electrical load  108  (e.g., batteries, electric motors, fans). 
     Furthermore, thermoelectric power generator  100  includes liquid cooled heat exchangers  109 A,  109 B disposed on the cold-side of thermoelectric device modules  106  to transfer the thermal energy from thermoelectric device modules  106  to a liquid coolant (e.g., water) passed through liquid cooled heat exchangers  109 A,  109 B. 
     Currently, conventional thermoelectric power generators, such as disclosed in  FIG. 1 , utilize a metal wall, such as wall  102 . However, by utilizing such a metal wall, there exists thermal contact resistance between metal wall  102  and the hot-side of thermoelectric device modules  106 . As a result, conduction losses occur thereby lessening the effectiveness of the thermal energy transferred to thermoelectric device modules  106  from heat exchanger  101 . 
     Furthermore, a source of failure for conventional thermoelectric power generators occurs at the bond between the metal wall, such as wall  102 , and the hot-side of the thermoelectric device modules, such as modules  106 . Such a failure occurs because of the large differences in the coefficient of thermal expansion between the metal wall and the ceramic materials used on the hot-side of the thermoelectric device modules thereby resulting in thermal fatigue failure of the bond between the wall and the thermoelectric device modules. 
     As a result, such conventional thermoelectric power generators are subject to conduction losses and thermal fatigue failures. 
     BRIEF SUMMARY 
     In one embodiment of the present invention, a thermoelectric power generator comprises a heat exchanger configured to extract thermal energy from an exhaust gas stream, where the heat exchanger comprises a first plurality of fins in contact with a boundary of the heat exchanger, where the first plurality of fins are directly connected to a first plurality of thermoelectric device modules. The heat exchanger further comprises a second plurality of thermoelectric device modules directly connected to the boundary of the heat exchanger. The first and second plurality of thermoelectric device modules are configured to convert the thermal energy to electrical energy. 
     In another embodiment of the present invention, a thermoelectric power generator system comprises an internal combustion engine configured to generate an exhaust gas stream through an exhaust pipe. The system further comprises a thermoelectric power generator connected to the internal combustion engine via the exhaust pipe. Additionally, the system comprises an electrical load connected to the thermoelectric power generator. The thermoelectric power generator comprises a heat exchanger configured to extract thermal energy from an exhaust gas stream, where the heat exchanger comprises a first plurality of fins in contact with a boundary of the heat exchanger, where the first plurality of fins are directly connected to a first plurality of thermoelectric device modules. The heat exchanger further comprises a second plurality of thermoelectric device modules directly connected to the boundary of the heat exchanger. The first and second plurality of thermoelectric device modules are configured to convert the thermal energy to electrical energy. 
     The foregoing has outlined rather generally the features and technical advantages of one or more embodiments of the present invention in order that the detailed description of the present invention that follows may be better understood. Additional features and advantages of the present invention will be described hereinafter which may form the subject of the claims of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A better understanding of the present invention can be obtained when the following detailed description is considered in conjunction with the following drawings, in which: 
         FIG. 1  illustrates a conventional thermoelectric power generator; 
         FIG. 2  illustrates a thermoelectric power generator that eliminates the metal wall between the thermoelectric device modules and the fins in accordance with an embodiment of the present invention; 
         FIG. 3  illustrates thermoelectric device modules being connected to a fin in accordance with an embodiment of the present invention; and 
         FIG. 4  illustrates a second set of fins being integrated with the thermoelectric module housing on the hot-side of the thermoelectric device modules which are connected to a fin in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     While the following discusses the present invention in connection with thermoelectric power generators implementing a single housing, the principles of the present invention may be applied to thermoelectric power generators implementing multiple housings. A person of ordinary skill in the art would be capable of applying the principles of the present invention to such implementations. Further, embodiments applying the principles of the present invention to such implementations would fall within the scope of the present invention. 
     As stated in the Background section, currently, conventional thermoelectric power generators, such as shown in  FIG. 1 , utilize a metal wall, such as wall  102 . Referring to  FIG. 1 , by utilizing such a metal wall, there exists thermal contact resistance between metal wall  102  and the hot-side of thermoelectric device modules  106 . As a result, conduction losses occur thereby lessening the effectiveness of the thermal energy transferred to thermoelectric device modules  106  from heat exchanger  101 . Furthermore, a source of failure for conventional thermoelectric power generators occurs at the bond between the metal wall, such as wall  102 , and the hot-side of the thermoelectric device modules, such as modules  106 . Such a failure occurs because of the large differences in the coefficient of thermal expansion between the metal wall and the ceramic materials used on the hot-side of the thermoelectric device modules thereby resulting in thermal fatigue failure of the bond between the wall and the thermoelectric device modules. As a result, such conventional thermoelectric power generators are subject to conduction losses and thermal fatigue failures. 
     The present invention provides a thermoelectric power generator that improves the effectiveness of the thermal energy transferred to the thermoelectric device modules from the exhaust gas heat exchanger by removing the metal wall thereby eliminating the conduction losses that were occurring. Furthermore, the thermoelectric power generator of the present invention provides a means for reducing the source of thermal fatigue failure by eliminating the metal wall thereby improving the durability of the thermoelectric power generator. Additionally, the thermoelectric power generator of the present invention allows for a higher density of thermoelectric device modules in a given thermoelectric power generator volume thereby allowing for greater electric power production for a given generator volume or size. In addition, the design of the thermoelectric power generator of the present invention is contained in a single housing as opposed to multiple housings, such as three housings as shown in  FIG. 1 , thereby reducing the cost in manufacturing the thermoelectric power generator. The thermoelectric power generator of the present invention that encompasses such benefits is discussed below in connection with  FIG. 2 . 
     Referring to  FIG. 2 ,  FIG. 2  illustrates a thermoelectric power generator  200  that is defined by an exhaust housing  201 , such as an extruded aluminum exhaust housing. In one embodiment, the cross-section of exhaust housing  201  is approximately 15 cm×30 cm. Thermoelectric power generator  200  includes a heat exchanger  202  that receives a pressurized exhaust gas stream  203  from an internal combustion engine  204  and extracts thermal energy from exhaust gas stream  203 . 
     In one embodiment, housing  201  may include a flange (not shown), such as a stainless steel flange, to sealingly connect to an exhaust pipe (not shown), such as a stainless steel exhaust pipe, of internal combustion engine  204 . In one embodiment, the flange and housing  201  may be formed from a’ single piece, by, for example, upsetting. Alternatively, the flange may be attached to housing  201  by welding, brazing, or crimping. It is to be understood that the exhaust gas stream  203  from internal combustion engine  204  is at a higher pressure than the ambient atmosphere when the engine  204  is running and the pressurized exhaust gas stream  203  is contained in an exhaust system. 
     Heat exchanger  202  may include fins  205 A- 205 F (e.g., aluminum fins) that are in contact with a boundary  206  within heat exchanger  202 . Fins  205 A- 205 F may collectively or individually be referred to as fins  205  or fin  205 , respectively. While  FIG. 2  illustrates fins  205 A- 205 F, heat exchanger  202  may include any number of fins  205  and the heat exchanger of the present invention is not to be limited in scope to the depicted number of fins  205 . In one embodiment, fins  205  are utilized in heat exchanger  202  to increase the rate of heat transfer from exhaust gas stream  203 . 
     Heat exchanger  202  further includes thermoelectric device modules  207 A- 207 S directly connected to either fins  205  or boundary  206 . Thermoelectric device modules  207 A- 207 S may collectively or individually be referred to as thermoelectric device modules  207  or thermoelectric device module  207 , respectively. While  FIG. 2  illustrates thermoelectric device modules  207 A- 207 S, heat exchanger  202  may include any number of thermoelectric device modules  207  and the heat exchanger of the present invention is not to be limited in scope to the depicted number of thermoelectric device modules  207 . A thermoelectric device module  207 , as used herein, is comprised of multiple thermoelectric device elements that are housed within module  207 . Each thermoelectric device element is comprised of thermoelectric materials that are bonded to electrical interconnects which are bonded to a non-electrically conductive housing of module  207 . 
     In one embodiment, thermoelectric device modules  207  are configured to convert the thermal energy extracted by heat exchanger  202  to electrical energy  208  for consumption or storage by an electrical load  209  (e.g., batteries, electric motors, fans). In one embodiment, the cold-side of thermoelectric device modules  207  are connected to fins  205  and boundary  206 . 
     In one embodiment, as illustrated in  FIG. 2 , a portion of thermoelectric device modules  207  are directly connected to fins  205  without a metal wall separating thermoelectric device modules  207  from fins  205 . For example, thermoelectric device modules  207 A- 207 L are directly connected to fins  205 A- 205 F as shown in  FIG. 2 . A close-up view of thermoelectric device module  207  being connected to fin  205  is shown in  FIG. 3 .  FIG. 3  illustrates thermoelectric device modules  207 , such as modules  207 A,  207 B of  FIG. 2 , being connected to a fin  205 , such as fin  205 A of  FIG. 2 , in accordance with an embodiment of the present invention. 
     Returning to  FIG. 2 , as a result of not including the metal wall, conduction losses that were occurring through the metal wall are eliminated thereby allowing more effective heat transfer between exhaust gas  203  and thermoelectric device modules  207 . Furthermore, by eliminating the metal wall, thermal fatigue failures at the bond between the metal wall and the hot-side of the thermoelectric device modules are eliminated. Additionally, as illustrated in  FIG. 2 , the design of thermoelectric power generator  200 , as opposed to conventional thermoelectric power generators, such as shown in  FIG. 1 , allow for a higher density of thermoelectric device modules  207  in a given thermoelectric power generator volume thereby allowing for greater electric power production for a given generator volume or size. In addition, the design of thermoelectric power generator  200 , as opposed to conventional thermoelectric power generators, such as shown in  FIG. 1 , only includes a single housing (element  201 ) as opposed to multiple housings, such as the three housings (elements  101 ,  109 A,  109 B) shown in the conventional thermoelectric power generator of  FIG. 1 , thereby reducing the cost in manufacturing thermoelectric power generators. 
     Furthermore, as shown in  FIG. 2 , a portion of thermoelectric device modules  207  are directly connected to boundary  206 . For example, thermoelectric device modules  207 M- 207 S are directly connected to boundary  206  as shown in  FIG. 2 . In one embodiment, the number of thermoelectric device modules  207  directly coupled to fins  205  corresponds to twice the number of fins  205  in heat exchanger  202  (two thermoelectric device modules  207  per fin  205 ). Thermoelectric power generator  200  of the present invention may include any number of thermoelectric device modules  207  and is not to be limited in scope to the depicted number of thermoelectric device modules  207  shown in  FIG. 2 . 
     To assist in the conversion of thermal energy to electrical energy, additional fins  209 A- 209 S are attached to or are integrated with the thermoelectric module housing on the hot-side of each thermoelectric device module  207 A- 207 S, respectively. Fins  209 A- 209 S may collectively or individually be referred to as fins  209  or fin  209 , respectively. A close-up view of fins  209  being integrated with the thermoelectric module housing on the hot-side of thermoelectric device modules  207  is shown in  FIG. 4 .  FIG. 4  illustrates fins  209 , such as fins  209 A,  209 B of  FIG. 2 , being integrated with the thermoelectric module housing on the hot-side of thermoelectric device modules  207 , such as modules  207 A,  207 B of  FIG. 2 , which are connected to a fin  205 , such as fin  205 A of  FIG. 2 , in accordance with an embodiment of the present invention. In one embodiment, the hot-side ceramic of thermoelectric device modules  207  may be contoured or textured in a shape that enhances heat transfer as opposed to being a simple flat ceramic. 
     Returning to  FIG. 2 , in one embodiment, fins  209  are comprised of ceramic material. In one embodiment, fins  209  aid in extracting more thermal energy from exhaust gas stream  203  thereby increasing the amount of electrical energy that is produced by thermoelectric device modules  207 . 
     Additionally, thermoelectric power generator  200  includes a series of holes  210 A- 210 T for cooling water which is used to transfer the thermal energy from thermoelectric device modules  207  to a liquid coolant (e.g., water). Holes  210 A- 210 T may collectively or individually be referred to as holes  210  or hole  210 , respectively. Each hole  210  may be designed in a circular shape or in other shapes, such as rectangular. The principles of the present invention are not to be limited in scope to the shape of holes  210  depicted in  FIG. 2 . 
     The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.