Abstract:
A heat exchanger for installation in a motor vehicle includes a cooling channel, a heating channel, and heat pipes to thermally couple the cooling channel with the heating channel. A thermoelectrical generator is arranged on a cold side of at least one of the heat pipes and coupled with the heat pipe by a material joint. The material joint is realized by a liquid metal.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of German Patent Application, Serial No. 10 2010 054 640.2, filed Dec. 15, 2010, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference in its entirety as if fully set forth herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a heat exchanger for installation in a motor vehicle. 
         [0003]    The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention. 
         [0004]    Exhaust-gas heat recovering systems are known, using thermoelectrical generator units arranged in an exhaust tract of a combustion engine to recover energy from heat carried by exhaust gases. The thermoelectrical generator units have thermoelectric elements which are arranged in an exhaust passage. The exhaust flow and the connection of the exhaust pipe to the environment cause a temperature difference which the thermoelectric elements convert directly to electric energy by the Seebeck effect. 
         [0005]    In order to provide a temperature difference at the thermoelectric element, the latter has a hot side and a cold side. A certain thickness of individual thermoelectric elements is required to maintain a respective temperature gradient during operation of the thermoelectric element. The configuration involves therefore predominantly the application of thin film technology or sheeting technology. Application of these technologies in an exhaust tract is however not optimal because of the prevailing high temperatures. 
         [0006]    In particular as a result of the high exhaust temperatures of significantly above 600° C. up to more than 1000° C., assemblies of thermoelectric elements are exposed to extreme temperature fluctuations. The highly corrosive properties of exhaust gas also adversely affect the durability of thermoelectric elements. Proposals have been made to crimp the thermoelectric elements directly in an exhaust-carrying exhaust pipe so as to be coupled directly or indirectly with exhaust gas. For example, U.S. Pat. No. 7,100,369 discloses a direct coupling, with the thermoelectric generator being exposed directly to the exhaust gas. 
         [0007]    It would be desirable and advantageous to provide an improved heat exchanger which obviates prior art shortcomings and which utilizes the potential of a thermoelectric generator in an optimum manner while yet being easy to produce and exhibiting a long service life. 
       SUMMARY OF THE INVENTION 
       [0008]    According to one aspect of the present invention, a heat exchanger for installation in a motor vehicle includes a cooling channel, a heating channel, heat pipes thermally coupling the cooling channel with the heating channel, and a thermoelectrical generator coupled with at least one of the heat pipe by a material joint. 
         [0009]    In accordance with the present invention, a heat pipe constitutes by itself a heat exchanger, using evaporation heat of a material to establish a high heat flux density. As a result, large amounts of heat can be transported across a small cross sectional area. A fluid in a heat pipe is hereby evaporated in a heating zone and the created vapor flows to a cooling zone for condensation. The created vapor ensures a heat transport, with the return flow of the condensate to the heating zone realized by capillary effect. The thermoelectric generator is hereby coupled with the heat pipe by a material joint. In view of the material joint of the heat pipe with the thermoelectric generator, the heat transfer of heat transported through the heat pipe to the thermoelectric generator is effectuated in an optimum manner. As a result, the efficiency is at an optimum. 
         [0010]    By using the heat pipe in accordance with the present invention, the cooling channel is thermally coupled by the heating channel. In other words, the heat pipe has at least one part which projects into the heating channel while another part extends into the cooling channel. 
         [0011]    In view of the configuration of the heat pipe according to the present invention, thermoelectric materials can be used as thermoelectric generator which otherwise would have been conceived for use only in a low-temperature range. The thermoelectric generators can now be used via the heat pipes also for high-temperature function. The linkage of the heat pipe ensures that the thermoelectric generator for the low-temperature range has high efficiency also in the high-temperature range. 
         [0012]    In addition, the heat pipe assumes a safety function, as the heat transport via the heat pipe takes place only up to a maximum temperature. Thus, the operating performance is restricted, protecting the thermoelectric generator against overheating. 
         [0013]    The present invention has a further benefit in that a direct contact of thermoelectric generator and heating channel or exhaust gas flowing in the heating channel is thermally decoupled. The thermoelectric generator is thermally coupled solely via the heat pipe. As a result, stress cracks and/or leakage caused by different thermal expansions are substantially eliminated so that the service life of a heat exchanger according to the invention increases. 
         [0014]    According to another advantageous feature of the present invention, the material joint can be realized by a liquid metal. Using liquid metal as coupling permits a transfer of heat substantially free of forces from heat pipe to the thermoelectric elements and at a same time a high heat transfer factor. The presence of possibly encountered thermal expansions or stress within the components caused by different thermal expansion coefficients is prevented in view of the coupling by liquid metal, without having to forego the application of a material joint to achieve an optimal heat transfer rate. 
         [0015]    According to another advantageous feature of the present invention, Galinstan® can be used as liquid metal. Advantageously, the heat conductor may be configured as alloy having gallium (Ga) and indium (In) as constituents. Other examples include compositions of gallium and indium with special steel or ceramics. A heat conductor containing the afore-described alloying components forms compositions in the boundary zones of the ceramic carrier material and the heat exchanger wall. 
         [0016]    The connection by material joint is characterized by a high heat transfer while being less susceptible to stress at the same time. 
         [0017]    Indium causes slight surface tension and thus tends to wet many materials, including metal, glass or ceramics, and invades the lattice of the alloy and thus enhances mechanical properties. Indium forms a yellow bonding oxide which promotes adherence of ceramic compounds to metals. The oxidation behavior contributes hereby substantially to a metal-ceramics composite. 
         [0018]    Gallium lowers the melting temperature of the alloy and improves in addition the flowability and mold filling capacity of the alloy. 
         [0019]    According to another advantageous feature of the present invention, the alloy may include tin (Sn) as alloying element. Tin reduces carbon absorption and prevents decarburization of the steel alloys being joined. Moreover, tin promotes bonding oxide formation of the constituents gallium and indium. This increases strength of the afore-mentioned alloying elements. 
         [0020]    According to another advantageous feature of the present invention, the alloy may include 60 to 80% of gallium, 10 to 30% of indium, and 1 to 20% of tin. Other constituents such as bismuth or antimony may be added. The alloying constituents are important for adjusting the melting temperature. The material properties of the heat conductor as realized by the alloying constituents can be selected by taking into account processing temperature and operating temperature range. Currently preferred is the use of Galinstan® and sodium as heat conductor. 
         [0021]    According to another advantageous feature of the present invention, the heat pipe has a wall which may be wetted with a metallic carrier substrate. Currently preferred is the use of an austenitic special steel alloy as carrier substrate. 
         [0022]    According to another advantageous feature of the present invention, the thermoelectrical generator can be arranged on a cold side of the heat pipe. As a result, the thermoelectric generator is thermally and physically separated from the heating channel including exhaust-gas carrying channel. The heat pipe can be configured through selection of the heat pipe material and fluid within the heat pipe in such a way as to establish an optimal heat transfer from exhaust gas to the thermoelectric generator module. 
         [0023]    The configuration of the heat pipe may be realized in such a way that a respective combustion engine for which the heat exchanger is conceived is operated with a predominant operating range in a driving cycle. The heat pipe can be configured in an optimum manner for this operating range. When the operating range is exceeded, for example at high nominal load range, the heat pipe assumes a protection function to prevent thermal overload of the thermoelectric generator. Moreover, the thermoelectric generator is optimally fed with heat energy by using the heat pipe so as to achieve high efficiency. 
         [0024]    According to another advantageous feature of the present invention, the heat pipes extend with more than half of their length in longitudinal direction into the heating channel and with less than half of their length into the cooling channel. Currently preferred is a ratio of about two third of the heat pipe extending into the heating channel and one third extending into the cooling channel. 
         [0025]    According to another advantageous feature of the present invention, the heat pipes can have a circular, oval, rectangular and/or polygonal cross section. The configuration may also include round-oval or flat-oval cross sections or also transversely star-shaped or similar cross sectional shapes. This allows realization of an optimum flow around the heat pipe to best suit the flow conditions and field of use at hand. For example, it may be required to attain a particularly small back pressure, in which case the cross section should assume a more rounded configuration. In the event of a desired turbulent flow behavior, the cross section should be more polygonal. 
         [0026]    According to another advantageous feature of the present invention, heat exchanger ribs can be arranged on a hot side of the heat pipes and oriented in a flow direction of the heating channel. The presence of heat exchanger ribs increases the available surface area useable for heat transfer. The heat exchanger ribs may be provided to create a turbulent flow depending on their orientation and configuration. For example, the heat exchanger ribs may have break-off edges or other flow-influencing elements to promote heat transfer. 
         [0027]    According to another advantageous feature of the present invention, the heat pipes can be oriented transversely to a flow direction of coolant in the cooling channel and/or the heat pipes can be oriented transversely to a flow direction of a heating medium in the heating channel. As a consequence of the substantially transverse orientation in relation to the flow direction of the coolant or heating medium, optimum heat transfer is realized at the heat pipes. The generated back pressure or pressure loss of the coolant and heating medium flowing through the heat exchanger can thus be kept to a minimum. 
         [0028]    According to another advantageous feature of the present invention, the thermoelectrical generators can be arranged substantially about a circumference of the heat pipe. This means within the scope of the invention that the thermoelectrical generators are placed in surrounding relationship to the heat pipes, at least in surrounding relationship to sections of the heat pipes. It can be beneficial to arrange the thermoelectrical generators only on a section of the heat pipes that faces away from the flow direction or on a section that faces the flow direction. 
         [0029]    According to another advantageous feature of the present invention, the heat pipe can be operated in a way that best suits an operating performance of the thermoelectrical generator. Thus, the thermoelectrical generator exhibits optimal operating performance at certain temperature differences. In this operating performance or in this operating point, the thermoelectrical generator operates at optimal efficiency. This efficiency characteristic may for example be established on the side of the thermoelectrical generator by means of a resistance or preceding conceptional adjustment. The heat exchanger pipe has advantageously also a matching operating behavior to thereby realize or ensure an optimum operating performance of the thermoelectrical generator. 
         [0030]    According to another advantageous feature of the present invention, the operation of the heat pipe can be constructed for closed-loop control and/or open-loop control. The use of valves in the heat pipes can ensure a closed-loop control and/or open-loop control. Also conceivable is the use of a throttle to vary the heat transport within the heat pipe. For example, the presence of a pressure valve or a throttle allows control of a heat transport across the heat pipes at high temperature ranges of for example more than 500° C., preferably above 5600° C. Currently preferred is a control at a temperature range of more than 700° C. In this way, the thermoelectrical generator cannot be overloaded as a result of high heat impacts. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0031]    Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
           [0032]      FIG. 1  is a schematic cross sectional view of one embodiment of a heat exchanger with heat pipes in accordance with the present invention; 
           [0033]      FIG. 2  is a cross sectional view taken along the line II-II in  FIG. 1 ; and 
           [0034]      FIG. 3  is a sectional view of another embodiment of a heat exchanger according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0035]    Throughout all the figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
         [0036]    Turning now to the drawing, and in particular to  FIG. 1 , there is shown a schematic cross sectional view of one embodiment of a heat exchanger in accordance with the present invention, generally designated by reference numeral  1  and including a cooling channel  2  and a heating channel  3 . A separation layer  4  is arranged between the cooling channel  2  and the heating channel  3 . The cooling channel  2  and the heating channel  3  are coupled with one another by heat pipes  5 . A flow direction S K  of the cooling channel  2  is established in parallel relation to a flow direction S H  of the heating channel  3 . A heat transfer takes place at the partition layer  4  on one hand, and across the heat pipes  5  on the other hand. The heat pipes  5  have a cooling section  6  which extends in the cooling channel  2  and a heating section  7  which extends in the heating channel  3 . In other words, coolant  8  in the cooling channel  2  sweeps about the heat pipes  5  in the area of the cooling section  6 , and a heating medium  9  in the heating channel  3  sweeps about the heat pipes  5  in the area of the heating section  7 . The surface area in the heating section  7  is increased by the provision of heat exchanger ribs  10  which are arranged on the heating section  7  of the heat pipes  5  and oriented in the flow direction S H  of the heating channel  3 . Arranged on the cooling section  6  are thermoelectric generators  11  which have connections, not shown here, for discharge of electric energy generated by the thermoelectric generators  11 . 
         [0037]      FIG. 2  shows a cross sectional view taken along the line II-II in  FIG. 1  and depicts by way of example three different types of arrangements of the thermoelectric generators  11  in the area of the cooling sections  6  of the heat pipes  5 . Reference numeral  2   a  relates to an illustration of an internal heat pipe  5  which, in relation to the flow direction S K  of the coolant  8 , has on the outside thermoelectric generators  11 , some of which face the flow direction S K  and some of which face away from the flow direction S K . Reference numeral  2   b  relates to an illustration of an internal heat pipe  5  which has a thermoelectric generator  11  only on the side of the heat pipe  5  in facing relation to the flow direction S K . Reference numeral  2   c  relates to an illustration of an internal heat pipe  5  which is completely embraced by a thermoelectric generators  11 . 
         [0038]    Referring now to  FIG. 3 , there is shown a sectional view of another embodiment of a heat exchanger according to the present invention, generally designated by reference numeral  1   a . Parts corresponding with those in  FIG. 1  are denoted by identical reference numerals and not explained again. The description below will center on the differences between the embodiments. In this embodiment, the heat pipes  5  project into the heating channel  3  from opposite sides and thus are intertwined. This type of configuration saves space and can be arranged, for example when a heating channel  3  is involved in the form of an exhaust pipe, on top or on the bottom or also in star formation or at an angular offset to one another. 
         [0039]    While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit and scope of the present invention. The embodiments were chosen and described in order to explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. 
         [0040]    What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: