Patent Publication Number: US-2010115968-A1

Title: Heating apparatus comprising a thermoelectric device

Description:
The invention relates to a heating apparatus, with a wall, the inside of which has adjoining it a region for the routing of hot exhaust gases, a housing which at least partially surrounds the wall, a medium to be heated flowing through an interspace between the wall and the housing, and a thermoelectric device which is arranged in the interspace and has a plurality of thermoelement legs and which is suitable for generating an electrical voltage on account of a temperature difference between the exhaust gases and the medium to be heated. 
     A device of this type is known from DE 102 35 601 A1. The ability to convert heat energy into electrical energy by means of a device of this type makes it possible to generate the electrical energy required for operating the heating apparatus, for example for feeding a blower and a control, in the heating apparatus itself. The electrical energy thus generated may likewise be made available to other consumers in the motor vehicle, for example by feeding into the vehicle electrical power supply. The arrangement shown in DE 102 35 601 A1 has a thermoelectric device in the form of a thermoelectric module which is arranged between a first wall portion forming the “hot side” and a heat exchanger forming the “cold side”. When the heating apparatus is in operation, therefore, a voltage can be picked up on the thermoelectric module, although problems with regard to maintaining as high temperature differences as possible, to thermal expansions and to a deficient thermal conductivity between the thermoelectric module and the wall portion or the heat exchanger may arise. 
     The object on which the invention is based is to overcome at least partially these problems of the prior art and in this case, in particular, to improve the thermal and mechanical conditions of the arrangement and the configuration possibilities in the set-up. 
     This object is achieved by means of the features of the independent claim. 
     Advantageous embodiments of the invention are specified in the dependent claims. 
     The invention builds on the generic heating apparatus in that the wall is designed to be at least partially electrically insulating, in that the thermoelement legs are supported with a first end region, via electrically conductive layers, on a heat contact portion of the wall, in that an at least partially electrically insulating heat exchanger is provided, which is arranged opposite the heat contact portion of the wall, in that the thermoelement legs are supported with a second end region, via electrically conductive layers, on the electrically insulating heat exchanger, and in that the electrically conductive layers connect the respective end regions of the thermoelement legs at least partially to one another. In contrast to the prior art, an arrangement is selected consisting of individual thermoelement legs which are supported directly on the heat source or the heat sink via layers, in particular printed metallic conductor tracks. In order to make this possible, both the wall or at least that region of the wall on which the thermoelement legs are arranged and the heat exchanger are manufactured from electrically insulating, but nevertheless thermally conductive materials. The electrical conditions are determined by the arrangement of the electrically conductive layers which selectively connect the end region of the thermoelement legs to one another. This arrangement also affords a suitable starting point, as described in more detail below, for avoiding thermal shunts between the heat sink and heat source and for providing a sufficient mechanical compensation capacity with regard to thermal expansions. 
     It is preferable that the wall consists at least partially of an electrically insulating ceramic having high thermal conductivity. 
     It is likewise advantageous that the electrically insulating heat exchanger consists at least partially of a ceramic having high thermal conductivity. 
     For example, there may be provision for the ceramic for the wall and/or for the electrically insulating heat exchanger to have aluminum nitride. Ceramics of this type can fulfill the requirements of high thermal conductivity, at the same time with electrical insulation. Moreover, the printed conductor tracks can, for example, be manufactured from aluminum, with the result that, because of the high affinity between aluminum and aluminum nitride, the connection between the thermoelement legs and the fastening points is stabilized. 
     With regard to the electrical design of the arrangement, there may be provision for the electrically conductive layers to connect the thermoelement legs in series with one another. 
     It is likewise possible for the electrically conductive layers to connect the thermoelement legs in parallel with one another. 
     In a development of the embodiments mentioned, there may likewise be provision for the electrically conductive layers to connect groups of thermoelement legs in parallel with one another and to connect these groups in series with one another. Depending on the electrical requirements, the voltages desired for given temperature conditions can thus be generated by means of the series connection of the thermoelement legs, while sufficiently high currents are made available by virtue of the parallel connection of individual thermoelement legs or of the thermoelement legs combined into groups. 
     According to a preferred embodiment, there is provision for the medium to be heated to be liquid, in particular water, and for the heat contact portion to lie opposite a flame tube end. 
     The invention can therefore be employed within the framework of what is known as a water heating apparatus. In this case, it is useful to provide the heat contact portion opposite to a flame tube end, that is to say in the region of the burner bottom. The highest temperatures occur at this point, so that the thermoelectric device can operate with high efficiency. 
     Expediently, there is provision for an interspace, receiving the thermoelement legs, between the wall and the electrically insulating heat exchanger to be sealed off by means of a sealing device against the ingress of the liquid medium to be heated, and for the sealing device to be suitable for compensating thermally induced expansions. The seal thus at the same time ensures that no water comes into contact with the thermoelement legs and that sufficient mechanical flexibility is made available for the compensation of thermal expansions. The seal may have, in particular, two components. One component is optimized in terms of heat insulation, and the other component serves for providing the useful elasticity which compensates thermal expansions. With a suitable choice of material, these two components may be implemented in the form of one unitary component, that is to say in the form of a highly heat-insulating elastic seal. A compound seal of this type may be designed, for example, as a silicone/Al 2 O 3  seal. 
     According to a further preferred embodiment, there is provision for the medium to be heated to be gaseous, in particular air, and for the heat contact portion to lie in the region of an inlet for the gaseous medium to be heated. The invention can therefore also be implemented within the framework of what are known as air heating apparatuses. Since the air has the lowest temperature in the region of the air inlet, but heats up rapidly when it passes through the heating apparatus, it is expedient, in order to achieve as high a thermoelectric efficiency as possible, to arrange the thermoelectric device in the region of the air inlet. 
     With regard to this arrangement of the thermoelectric device in the air heating apparatus, there is expediently provision for an interspace, receiving the thermoelement legs, between the wall and the electrically insulating heat exchanger to be protected by means of a windbreak against the ingress of the gaseous medium to be heated. Since the functioning capacity of the device would be adversely influenced by the contact of the thermoelement legs with cold air, the simple measure of a windbreak constitutes a useful development of the invention. 
     According to a further embodiment of the invention, there must be provision for the thermoelectric device to be supported on the housing via an elastic connection. This mounting of the thermoelectric device makes available an improvement in the compensation of thermally induced expansions. The thermoelectric device is thus held in a stable way, but the elastic mounting makes sufficient flexibility available. 
     For example, this may be implemented such that the thermoelectric device is supported on the housing via a heat exchanger and elastic means. The heat exchanger, around which the liquid or gaseous heat transfer medium flows, can thus assume the task of providing an elastic connection. 
     In a development of this idea, there may be provision for the thermoelectric device to be supported on the housing via a heat exchanger and a plate supporting the heat exchanger and equipped with elastic means. 
     In this regard, it is possible for the heat exchanger and the plate equipped with elastic means to be produced in one piece. 
     In a simple and effective implementation of these arrangements compensating thermal expansions, the elastic connection comprises cup springs. 
     Furthermore, the invention is developed particularly advantageously in that a mechanical connection between the wall and the heat exchanger has a heat-insulating material. Undesirable thermal compensation between the wall and the heat exchanger is thereby avoided, so that the thermoelectric device can operate with high efficiency. 
     For example, there may be provision for the heat-insulating material to contain aluminum oxide and/or mullite. 
     Furthermore, it is expedient that the connection between the first housing and the heat exchanger has a sealing device which is suitable for sealing off the thermoelectric device against the medium to be heated and for compensating thermally induced expansions. 
     It is likewise possible for the wall and the heat-insulating material and/or the sealing device to be produced in one piece. 
     In a comparable way, there may be provision for the heat exchanger and the heat-insulating material and/or the sealing device to be produced in one piece. The possibility of implementing these one-piece versions depends, in turn, on the suitable choice of material for the structural parts used. 
     For further improvement in the thermoelectric properties, there is provision for a heat conduction means to be provided between the thermoelectric device and the wall and/or between the thermoelectric device and the heat exchanger. By virtue of such a heat conduction means, the thermoelement legs are tied thermally to the heat source and the heat sink in an improved way. 
     This may be implemented, for example, in that the heat conduction means has a heat conduction paste. 
     Another possibility is for the heat conduction means to have a heat conduction foil. 
    
    
     
       The invention, then, is explained by way of example by means of a preferred embodiment, with reference to the accompanying drawings in which: 
         FIG. 1  shows a partial sectional view of a heating apparatus according to the invention; 
         FIG. 2  shows a detail of a heating apparatus according to the invention; 
         FIG. 3  shows a partial sectional view of a further embodiment of a heating apparatus according to the invention; 
         FIG. 4  shows a partial sectional view of a further embodiment of a heating apparatus according to the invention. 
     
    
    
     In the following description of the drawings, the same reference symbols designate identical or comparable components. 
       FIG. 1  shows a partial sectional view of a heating apparatus according to the invention;  FIG. 2  shows a detail of a heating apparatus according to the invention. The heating apparatus  10 , that is to say, in particular, the heat exchanger arrangement illustrated in  FIG. 1 , is of essentially axially symmetrical construction, an inner wall  12  and an outer housing  16  being provided. A flame tube  28  is arranged within the wall  12 , exhaust gases which emerge from the flame tube  28  heating the inside of the wall  12 . Between the wall  12  and the housing  16 , an interspace  18  is provided, through which a heat transfer medium, for example water, flows, so that the wall  12  functions as a heat exchanger for the transmission of combustion heat to the heat transfer medium. A plurality of thermoelement legs  20  are arranged on a bottom of the wall  12 , said bottom being provided as a heat contact portion  24 . The thermoelement legs  20  are connected to the heat contact portion  24  of the wall  12 , in each case with an end region, via electrically conductive layers  22 . The thermoelement legs  20  are connected via their other end regions to a heat exchanger  26  which is arranged in the interspace  18  between the wall  12  and the housing  16 . The interspace  32  between the heat contact portion  24  and the heat exchanger  26  is surrounded by a seal  34 , so that the heat transfer medium cannot penetrate into the interspace  32 . The basic tie-up of the thermoelement legs  20  via the electrically conductive layers is illustrated in the detail A in  FIG. 2 . The electrically conductive layers  22  serve for electrically connecting the thermoelement legs  20  selectively to one another. This is possible, since both the wall  12  or at least the heat contact portion  24  and, at least partially, the heat exchanger  26  are manufactured from electrically insulating material which, however, easily has sufficient thermal conductivity for operating the thermoelectric device. Then, for example, two adjacent thermoelement legs  20  are connected to one another, on their side facing the wall  12 , via the electrically conductive layers  22 , while the next thermoelement leg is tied up electrically on the side facing the heat exchanger  26 . The next thermoelement leg  20 , in turn, is tied up electrically on the side facing the wall  12 , and so on and so forth. If this electrical tie-up is selected, a series connection of the thermoelement legs  20  is obtained. Consequently, a thermoelectric voltage, which represents the sum of thermoelectric voltages of the individual thermoelement legs  20 , can be picked up from the arrangement, using the electrical connection  44 . It is likewise possible to connect the thermoelement legs electrically in parallel in groups and then to connect these groups in series. Thus, with a low voltage, an increased current output is possible. The electrically insulating heat exchanger  26  is connected to the housing  16  in the region of the electrical connection  44 . The connection is made via a seal  46 . 
       FIG. 3  shows a partial sectional view of a further embodiment of a heating apparatus according to the invention. The embodiment of the heating apparatus  10  according to the invention, as shown in  FIG. 3 , differs from the embodiment described with regard to  FIG. 1  in that the heat exchanger  26  is supported on the housing  16  via a plate  40  equipped with cup springs  42 . A stable arrangement is thereby available, reliable thermal contact between the themoelement legs and the wall  12  and also the heat exchanger  26  being available due to the suitable rating of the spring forces. In terms of heat contact, there may also advantageously be provision for a heat conduction means, for example a heat conduction paste or a heat conduction foil, to be provided between the thermoelement legs  20  and the wall  12  or the heat exchanger  26 . 
     In the embodiments according to  FIG. 1  and  FIG. 3 , the thermoelectric device is arranged in the region in which the highest exhaust gas temperatures are available. In the embodiment according to  FIG. 4 , which relates to an air heating apparatus, this is different, as is described with reference to the following drawing. 
       FIG. 4  shows a partial sectional view of a further embodiment of a heating apparatus according to the invention. In addition to the components already described with regard to  FIGS. 1 to 3 , a fuel supply  46 , combustion air ports  48 ,  50 , an exhaust gas outlet  52  and a hot air outlet  54  can be seen. In contrast to the embodiments which were described with regard to  FIGS. 1 to 3  and which relate to a water heating apparatus,  FIG. 4  illustrates an air heating apparatus, the heat contact portion  24  of the wall  12  being arranged here in the region of the air inlet  36 . Thus, the heat exchanger  26  comes into contact with the cold air flowing in, so that the various sides of the thermoelement legs  20  have as high a temperature difference as possible. In the embodiment illustrated, the thermoelements  20  are connected in series via the electrically conductive layers  22 . As regards the other connection variants, reference is made to the statements relating to  FIGS. 1 to 3 . The connection variants should preferably be selected such that the voltages occurring in the case of customary temperature differences are such that, without additional direct voltage conversion, the voltage emitted by the thermoelement leg can be utilized either for components of the air heating apparatus or other vehicle components, for example by feeding into the vehicle power supply. 
     The features of the invention which are disclosed in the above description, in the drawings and in the claims may be essential both individually and in any desired combination for the implementation of the invention. 
     LIST OF REFERENCE SYMBOLS 
       10  Heating apparatus 
       12  Wall 
       14  Region 
       16  Housing 
       18  Interspace 
       20  Thermoelement leg 
       22  Electrically conductive layers 
       24  Heat contact portion 
       26  Heat exchanger 
       28  Flame tube 
       30  Flame tube end 
       32  Interspace 
       34  Sealing device 
       36  Air inlet 
       38  Windbreak 
       40  Plate 
       42  Cup spring 
       44  Electrical connection 
       46  Seal 
       48  Combustion air port 
       50  Combustion air port 
       52  Exhaust gas outlet 
       54  Hot air outlet