Abstract:
The thermoelectric facility has a thermoelectric generator and a structure for limiting the temperature thereof. The structure has a flat compartment which is at least substantially filled with an evaporable working medium. The dimensions of the compartment are adapted to those of the thermoelectric generator and the compartment is thermally connected to a heat source or to the thermoelectric generator across a large surface of its opposite surfaces. The temperature-limiting structure also includes a conduit system, connected to the compartment, into which a recirculation cooler is integrated to which a gaseous portion of the working medium can freely rise from the compartment. The working medium should have a boiling point that is at least below a critical temperature above which the thermoelectric generator will be permanently damaged. The thermoelectric facility is especially useful for motor vehicles that are operated by an internal combustion engine.

Description:
[0001]    This application is based on and hereby claims priority to German Application No. 10 2006 040 855.1 filed on Aug. 31, 2006 and PCT Application No. PCT/EP2007/058717 filed on Aug. 22, 2007, the contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a thermoelectrical device having
   a) a thermoelectrical generator, a heat source and a heat sink, wherein the thermoelectrical generator is thermally connected on a first side to the heat source and on a second side to the heat sink,   b) a chamber,
       which is thermally connected over a large area to the heat source and to the thermoelectrical generator,   which is at least largely filled with a working medium which can be vaporized, and   can circulate in the liquid and gaseous working medium by virtue of a thermosiphon effect, and   
       c) a structure for temperature limiting on the thermoelectrical generator,
 
wherein
 
the working medium has a boiling temperature T s  which is below a critical temperature above which the thermoelectrical generator is permanently damaged. One such thermoelectrical device is disclosed in U.S. Pat. No. 3,881,962.
   
 
         [0009]    Heat can be converted directly to electrical energy using a so-called thermoelectrical generator. A thermoelectrical generator is a component composed of two different materials which are connected to one another, preferably two different or differently doped semiconductors, which produces an electrical voltage on the basis of the Seebeck effect when the junction points of the different materials are at different temperatures. 
         [0010]    The Seebeck effect describes the creation of an electrical voltage in an electrical conductor along a temperature gradient, caused by thermodiffusion flows. In order to allow technical use to be made of the Seebeck effect, it is necessary to bring two different electrical conductors with a different electronic heat capacity into contact with one another. As a result of the different electronic heat capacity, the electrons in the two conductors have different energies of motion at the same temperature. If these conductors are brought into contact with one another, then a diffusion flow of relatively high energy electrons will take place in the direction of the conductor with the low-energy electrons until this results in a dynamic equilibrium. If these two different conductors are denoted A and B and are brought into contact in the sequence A-B-A and, furthermore, if the junction A-B is at a temperature T 1  and the junction B-A is at a temperature T 2 , then the resultant voltage is dependent only on the difference between the temperatures T 1  and T 2  and the respective Seebeck coefficient of the two conductors A and B. In consequence, a voltage which can be tapped off on a thermoelectrical generator is dependent only on the temperature difference applied to the thermal generator and on the Seebeck coefficients of the materials used. 
         [0011]    In principle, a thermoelectrical generator can be constructed analogously to a Peltier element. Identical or similar materials as for the production of Peltier elements, for example bismuth-tellurite or silicon-germanium, can also be used for a thermoelectrical generator. 
         [0012]    The use of semiconductor materials allows the efficiency of a thermoelectrical generator for conversion of thermal energy to electrical energy to rise to several percent. Thermoelectrical generators have recently been increasingly used for exhaust gas waste heat, for example in the case of motor vehicles, cogeneration units or refuse incineration installations. 
         [0013]    DE 33 14 166 A1 discloses a high-efficiency thermoelectrical system. Starting with a hot fluid flow, for example an exhaust gas flow, thermally conductive tubes which are provided with ribs for better thermal linking are heated at one end. The thermally conductive tubes which are heated by the fluid flow conduct the heat to the thermoelectrical generators which are mounted at the opposite end of the thermally conductive tubes, and act as heat sinks. The thermally conductive tubes are filled with an operating fluid in order to improve their thermal conductivity, which operating fluid is vaporized on the hot part of the thermally conductive tubes and recondenses on the somewhat cooler part, on which the thermoelectrical generators are arranged. The thermoelectrical system disclosed in DE 33 14 166 A1 can be used to achieve particularly effective thermal coupling of thermoelectrical generators, for example to an exhaust gas flow. The disclosed system is particularly suitable for use in the high-temperature range at working temperatures of more than 400° C. 
         [0014]    U.S. Pat. No. 4,125,122 A discloses a method and an apparatus for thermoelectrical conversion of heat to electrical energy. The disclosed apparatus is designed as a heat exchanger which operates on the opposing-flow principle. The known apparatus provides two mutually separate circuits in which media circulate for heat transmission. A first medium transports heat from a heat source to a heat sink. At least one first thermally conductive tube makes thermal contact with the hot flow of the first medium; at least one second thermally conductive tube makes thermal contact with the cooler flow of the first medium. In the case of the known apparatus, the thermoelectrical generators are in thermal contact both with one of the hot thermally conductive tubes and with one of the cooler thermally conductive tubes. A second medium circulates within the thermally conductive tubes, in a second circuit, driven by a thermosiphon effect. In that thermally conductive tube which is in thermal contact with the hot flow of the first medium, the second medium which is located within the thermally conductive tube circulates in gaseous form from a hot end, which is in thermal contact with the first medium, of the thermally conductive tube to a cooler end, which is in thermal contact with the thermoelectrical generator. At this end, which is in thermal contact with the thermoelectrical generator, the gaseous second medium condenses and in this way emits the heat condensation to the thermoelectrical generator. The second medium passes back in the liquid phase to the first end of the thermally conductive tube, in order to be vaporized again. 
         [0015]    In the case of the apparatus which is disclosed in said U.S. Pat. No. 4,125,122 A, the second medium therefore circulates in the thermally conductive tube which is in thermal contact with the cold side of the thermoelectrical generator, is vaporized at the end of the thermally conductive tube which is in thermal contact with the cold side of the thermoelectrical generator, and condenses on the (even) colder side of the thermally conductive tube which is in contact with the first medium. 
         [0016]    Both the thermoelectrical system which is disclosed in DE 33 14 166 A1 and that disclosed in U.S. Pat. No. 4,125,122 A have the aim of thermal coupling of the thermoelectrical generators to a hot operating fluid in a manner which is as effective and free of losses as possible. However, in these systems, there is a risk of their thermoelectrical generators being subjected to excessively high temperatures, and they can therefore be damaged. 
         [0017]    A thermoelectrical device having the features mentioned initially is disclosed in said U.S. Pat. No. 3,881,962. In this device, a chamber-like pipeline system is provided and is filled with a working medium which can be vaporized, and the pipeline system runs between a heating area, which can be regarded as a heat source, and a condenser, which can be regarded as a heat sink. In order to provide temperature limiting, in order to prevent damage, on a thermoelectrical module, this model is arranged physically separated from the condenser. Furthermore, a pipeline is additionally connected to the condenser area and leads to a geodetically higher pressure valve by which the pressure of the working medium and thus the thermal flow from the heating area to the condenser can be limited. Temperature limiting such as this on the thermoelectrical module is physically complex. 
         [0018]    A further thermoelectrical device having two thermoelectrical generators, a heat source and a heat sink is also disclosed in JP 2003-219 671 A. Two working media with different boiling temperatures are used. 
         [0019]    Two operating media are also used in an energy recovery system having a thermoelectrical generator for hybrid cars, as disclosed in WO 2004/092662 A1. One of the operating media is in this case used to cool a heat sink while the other working medium is connected to a heat source in the car. 
         [0020]    JP 5-343 751 A discloses a thermoelectrical generator of a solar installation in which water is used as a working medium which can be vaporized. Temperature limiting is achieved on the thermoelectrical generator by the vaporization of the water at its boiling temperature. 
         [0021]    A system which is disclosed in EP 1 522 685 A1 for exhaust-gas control of a motor vehicle comprises a thermoelectrical generator having a structure for temperature limiting. In this case, various working media such as oil can be used to transport heat from an exhaust-gas system as a heat source to the thermoelectrical generator. A thermal contact area, which can be varied with the temperature conditions, to the thermoelectrical generator, in particular using a meltable solder material, leads to temperature limiting on the generator. 
       SUMMARY 
       [0022]    One potential object is to specify a thermoelectrical device having the features mentioned initially, which allows good matching to the respective temperature such that the risk of unacceptable overheating that has been mentioned then does not exist. 
         [0023]    The inventor studied the idea of using the latent heat of a phase change for protection of a thermoelectrical generator against overheating. The thermoelectrical device should have a thermoelectrical generator, a heat source and a heat sink, wherein the thermoelectrical generator is thermally connected on a first side to the heat source and on a second side to the heat sink. The thermoelectrical device should furthermore have a chamber which is thermally connected over a large area to the heat source and to the thermoelectrical generator, which is at least largely filled with a working medium which can be vaporized, and can circulate in the liquid and gaseous working medium by virtue of a thermosiphon effect. Furthermore, a structure is intended to be provided for temperature limiting on the thermoelectrical generator. In this case, the working medium should have a boiling temperature T s  which is below a critical temperature above which the thermoelectrical generator is permanently damaged. 
         [0024]    The structure for temperature limiting on the thermoelectrical generator should comprise the chamber and a pipeline system which is connected thereto and in which a recooler is integrated. In this case:
       the chamber should be flat, with mutually opposite surfaces,   the dimensions of the chamber should be matched to those of the thermoelectrical generator,   the chamber should be thermally connected by one of the mutually opposite surfaces over a large area to the heat source and by the other, over a large area to the thermoelectrical generator,   the recooler should be integrated in the pipeline system at a point which is geodetically higher than the chamber,   the pipeline system should be designed such that a gaseous component of the working medium can rise without impairment to the recooler from the chamber in order to be liquefied again,
 
and
   liquid and gaseous working medium should be able to circulate at least in parts of the chamber and of the pipeline system by virtue of a thermosiphon effect.       
 
         [0031]    The advantages associated with this refinement of the thermoelectrical device are, in particular, that, when the temperature of the heat source rises, the thermoelectrical generator, which is thermally coupled thereto by the liquid-filled chamber, is protected against thermal destruction. When the heat source reaches the boiling temperature of the working medium, then excess thermal energy which would otherwise contribute to loading of the thermoelectrical generator is converted by the phase transition of the working medium. If further heat is supplied, vaporized working medium is liquefied again in the recooler, and excess energy is dissipated in this way. It is particularly advantageous that it is possible to use thermoelectrical generators in the thermoelectrical device which have a working temperature which is below the temperature of the heat source. A further advantage is that any temperature peaks that occur can be coped with when the temperature of the heat source fluctuates. 
         [0032]    In many cases, a liquid has a lower thermal conductivity than a solid body. The heat flow originating from the heat source is opposed by the arrangement of a further resistance as described above. This can contribute to additional protection of the thermoelectrical generator. 
         [0033]    The thermoelectrical device can also have the following features:
       The structure for temperature limiting may thus have a flat second chamber, which has mutually opposite surfaces, whose dimensions can be matched to those of the thermoelectrical generator, which can be connected by one of the mutually opposite surfaces over a large area to the heat source and by the other over a large area to the first chamber, and which can be at least largely filled with a second, meltable working medium. In this case, the second working medium should have a melting temperature T L  which is below a critical temperature above which the thermoelectrical generator is permanently damaged.   One particularly advantageous feature of this refinement of the thermoelectrical device is that excess thermal energy which originates from the heat source can be stored as latent heat of the “solid-liquid” phase transition of the second working medium. When the temperature of the heat source changes, this allows the temperature peaks to be coped with and to be stored. The stored thermal energy is passed to the thermoelectrical generator again, in the form of solidification heat, when the temperature of the heat source falls. This allows the temperature difference across the thermoelectric generator to be kept at a desired value in such a way that a power which is as constant as possible can always be demanded from the thermoelectrical generator.   Alternatively, the structure for temperature limiting may have a flat second chamber which has mutually opposite surfaces, whose dimensions can be matched to those of the thermoelectrical generator, which can be connected by one of the mutually opposite surfaces over a large area to the first chamber and by the other over a large area to the thermoelectrical generator, and which can be at least largely filled with a second, meltable working medium. In this case, the second working medium should have a melting temperature T L  which is below a critical temperature above which the thermoelectrical generator is permanently damaged. An arrangement of the second chamber such as this means that the heat flow which originates from the heat source first of all passes through the second chamber before it passes through the first chamber, which is filled with a liquid which can be vaporized, in order finally to arrive at the thermoelectrical generator. If the temperature of the heat source rises, when the melting temperature of the second working medium is reached, thermal energy is stored by virtue of the “solid-liquid” phase transition of the second working medium which is located in the second chamber. If the temperature rises further, or if a high temperature remains constant, with an ongoing heat flow, thermal energy is converted by the “liquid-gaseous” phase transition of the first medium. Finally, excess heat is dissipated via the recooler by condensation of the gaseous first working medium on the recooler. The refinement described above is particularly advantageous since excess heat is dissipated via the recooler only in the situation in which the heat store is saturated. This allows the overall efficiency of the thermoelectrical device to be improved, while at the same time ensuring effective protection for the thermoelectrical generator against overheating.   The second working medium may have a melting temperature which corresponds essentially to a preferred working temperature of the thermoelectrical generator, in which case the working temperature may be below the critical temperature above which the thermoelectrical generator is permanently damaged. According to the described exemplary embodiment, the thermoelectrical generator can be kept at an optimum working temperature, in a particularly advantageous manner, by melting and solidification of the second working medium.   However, the second working medium may also have a melting temperature which corresponds essentially to a preferred working temperature of the thermoelectrical generator, in which case the working temperature may be below the boiling temperature of the first working medium. The described choice of the melting temperature of the second working medium and the boiling temperature of the first working medium allows the thermoelectrical generator to be kept at a desired working temperature. When the temperature of the heat source rises above the preferred working temperature of the thermoelectrical generator, the excess heat is first of all changed to latent heat by the phase transition of the second medium from solid to liquid. Only in the situation in which the temperature of the heat source rises further after exhaustion of the heat store is the boiling temperature of the first working medium reached, and excess heat is dissipated. When the temperature of the heat source falls, the solidification heat of the second medium can be emitted to the thermoelectrical generator.   The second working medium may have a lower thermal conductivity in the liquid state than in the solid state. Every physical component has a specific thermal resistance. If the thermal resistance of the liquid phase of a material is higher than the thermal resistance of the solid phase, then the thermal resistance of the corresponding material rises when the melting temperature is exceeded. If a material such as this is used as the second working medium in a thermoelectrical device, then the thermoelectrical generator can be protected better by a rise in the thermal resistance of the second working medium.   The recooler may have a further thermoelectrical generator which is thermally connected on a first side to a third chamber, which is connected to the pipeline system, and on a second side to a heat sink. A refinementof the recooler such as this also allows the heat dissipated via the recooler to additionally be used to generate electrical energy. This makes it possible to improve the efficiency of the thermoelectrical device.   The heat source can be thermally connected at least to parts of an exhaust-gas system of an internal combustion engine, or may be formed by at least parts of the exhaust-gas system. The exhaust-gas heat of an internal combustion engine such as this can be made use of by using a thermoelectrical generator which is thermally coupled to the exhaust-gas system of an internal combustion engine.   The heat sink may be thermally connected at least with parts of a cooling system of an internal combustion engine, or may be formed by at least parts of the cooling system. A heat source and a heat sink are required in order to maintain a temperature difference, across a thermoelectrical generator, for operation of that thermoelectrical generator. Typically, an internal combustion engine has a cooling system and therefore in this way allows a heat sink to be provided in a simple and effective manner for the thermoelectrical generator.   The heat sink can be thermally connected to a surface which is to be cooled by an air flow. Since a surface which is to be cooled by an air flow can be used as a heat sink for a thermoelectrical generator, a simple, robust and low-cost component can be specified as a heat sink for the thermoelectrical generator.   The recooler may be thermally connected to at least parts of a cooling system of an internal combustion engine, or may be formed by at least parts of the cooling system. The thermal coupling of the recooler to the cooling system of an internal combustion engine ensures similar, or in some cases the same, advantages as the thermal coupling of a heat sink to the cooling system of an internal combustion engine.   The internal combustion engine may be part of a motor vehicle. Now motor vehicles require ever greater amounts of electrical energy in order to operate various electronic devices. The use of the exhaust-gas heat from the internal combustion engine of the motor vehicle reduces the primary energy demand in the motor vehicle for carrying the required electrical energy.   The first working medium may be an oil, preferably an engine oil, with a boiling temperature of between 100° C. and 500° C., preferably with a boiling temperature of between 200° C. and 300° C., at a pressure of 2 to 5 bar. The stated temperature ranges are particularly suitable for operation of a thermoelectrical generator. The cooling water in a cooling system of an internal combustion engine typically has a maximum temperature of about 100° C. The cooling water can be used as a heat sink for operation of a thermoelectrical generator. In order to ensure an effective energy yield as a result of the temperature difference across the thermoelectrical generator, the hot side of the thermoelectrical generator should be at a temperature of more than about 200° C. The maximum load capacity of typical thermoelectrical generators which are commercially widely available is about 300° C. Thermoelectrical generators, which are designed specifically for high-temperature applications, have a maximum load capacity of about 500° C. Since the boiling temperature of the first working medium defines the maximum temperature allowed by the structure for temperature limiting, a boiling point of the working medium in the stated temperature ranges is particularly advantageous.   The second working medium may be a solder which, in particular, contains lead, tellurium or bismuth at least as an alloy partner. A solder which contains one or more of the abovementioned elements or is formed by them provides the physical characteristics desired for the second working medium, and has also been proven in technical use.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0048]    These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
           [0049]      FIG. 1  shows the schematic design of a thermoelectrical device with a structure for temperature limiting, 
           [0050]      FIGS. 2 and 3  show the schematic design of a thermoelectrical device in which the structure for temperature limiting additionally has a second chamber, which is filled with a second working medium, 
           [0051]      FIG. 4  shows a schematic illustration of the temperature of a thermoelectrical generator of a device, as a function of time, 
           [0052]      FIG. 5  shows the schematic design of a thermoelectrical device in which the heat source is connected to parts of the exhaust-gas system of an internal combustion engine, 
           [0053]      FIG. 6  shows the schematic design of a thermoelectrical device in which the heat source is connected to parts of the exhaust-gas system and the heat sink and the recooler are connected to parts of the cooling system of the internal combustion engine, and 
           [0054]      FIG. 7  shows the schematic design of a thermoelectrical device in which the recooler has a further thermoelectrical generator. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0055]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0056]      FIG. 1  shows the schematic design of a thermoelectrical device according to one preferred exemplary embodiment. The thermoelectrical device, in particular in the form of one of its special refinements, can be used particularly advantageously in a motor vehicle with an internal combustion engine, wherein there is a gaseous working medium in the first chamber when the internal combustion engine is on full load or is subject to a peak load. In a particularly advantageous manner, the use of the abovementioned thermoelectrical device makes it possible to protect the thermoelectrical generator against overheating when the internal combustion engine is on full load or is subject to a peak load, for example when the motor vehicle is going uphill. In the device, a thermoelectrical generator  112  is thermally connected on one side over a large area to a heat sink  111 . On the opposite side, the thermoelectrical generator  112  is connected to a chamber  114  at least the majority of which is filled with a liquid  118 , which can be vaporized, as a first working medium. The chamber  114  which is filled with the liquid  118  which can be vaporized is in turn thermally connected over a large area to a heat source  117 . The thermal connection between the abovementioned components can preferably be provided by a mechanical connection, with an interlock. In this case, the above-mentioned components can be connected to one another, for example, by a solder. The thermal connection between the components can additionally be improved by the use of a thermally conductive paste. When there is a temperature difference across the thermoelectrical generator  112 , it generates electrical energy. The thermoelectrical generator can be electrically connected at the contacts  113  to a load, store etc. 
         [0057]    Like virtually all electronic components, a thermoelectrical generator  112  has a maximum thermal load capacity. This means that a predetermined critical temperature  141  exists (cf.  FIG. 4 ) above which the thermoelectrical generator  112  can be damaged if it is subjected to this predetermined critical temperature  141 , or to a higher temperature, for too long. A thermoelectrical generator  112  is preferably formed from a plurality of semiconductor elements which are soldered to one another. The thermoelectrical generator  112  can also be destroyed by a thermal load on the thermoelectrical generator  112  which is higher than the melting temperature of the solvent that is used for connection of the semiconductor elements. 
         [0058]    In order to protect the thermoelectrical generator  112  against thermal damage, the chamber  114  is connected to a pipeline system  115  in which a recooler  116  is integrated. As is indicated in  FIG. 1 , the pipeline system  115  may be connected at one end to the chamber  114 . In the same way, the pipeline system may comprise further parts which are connected to the chamber  114  at further points. In this way, the pipeline system may have parts which, for example, are connected to two opposite sides of the chamber  114 . A plurality of parts of the pipeline system  114  can likewise be connected to a common side of the chamber. The working medium  118  which is located in the chamber  114  can preferably have a boiling temperature T s  which corresponds to a preferred working temperature  143  (cf.  FIG. 4 ) of the thermoelectrical generator  112 . The boiling temperature T s  should preferably be below the critical temperature  141  above which the thermoelectrical generator  112  is permanently damaged. Further details will be explained in conjunction with  FIG. 4 . 
         [0059]    When the temperature of the heat source  117  rises above the boiling temperature of the working medium T s , then at least parts of the working medium  118  are vaporized in the chamber  114 . Gaseous working medium  118  can rise without any impediment from the chamber  114  via the pipeline system  115  to the recooler  116  which is integrated in the pipeline system  115 . For this purpose, the recooler  116  is located at a geodetically higher point than the chamber  114 . Gaseous working medium  116  can be liquefied again by the recooler  116 , and can then be passed back into the chamber  114  by the influence of the force of gravity. 
         [0060]    Liquid and gaseous working medium  118  can circulate in at least parts of the chamber  114  and of the pipeline system  115 , by virtue of a thermosiphon effect. 
         [0061]    Thermal energy originating from the heat source  117  can be carried away to the recooler  116  by the working medium  118  which can be vaporized in a specific manner. In this case, the thermoelectrical generator  112  can be protected against thermal overheating. 
         [0062]    Thermal peak loads can originate from the heat source  117  for a limited time or else continuously over time. If the heat source  117  is continuously at a temperature which is above the preferred working temperature  142  of the thermoelectrical generator  112  and is also above the building temperature T s  of the working medium  118 , excess heat is carried away continuously to the recooler  116 , by the boiling working medium  118 . If the temperature of the heat source  117  rises for a limited time, working medium  118  can be changed temporarily to the gaseous phase and can then liquefy again on relatively cool parts, for example those of the thermoelectrical generator  112 , or on parts of the pipeline system  115 , even without this being influenced by the recooler  116 . 
         [0063]    The preferred exemplary embodiment of a thermoelectrical device as illustrated in  FIG. 1  is not restricted to a flat arrangement, as illustrated in  FIG. 1 , of a heat source  117 , chamber  114 , thermoelectrical generator  112  and heat sink  111 . Just as advantageously, a multi-layer structure may be produced, which has a plurality of heat sources  117 , heat sinks  111  and a plurality of chambers  114 , which are filled with a working medium  118 , and thermoelectrical generators  112 . The thermoelectrical arrangement may likewise advantageously be in a curved form. 
         [0064]      FIG. 2  shows a further preferred exemplary embodiment of a thermoelectrical device in which the arrangement, known in a general form from  FIG. 1 , has had added to it a second chamber  121  which is filled with a meltable second working medium  122 . The second working medium  122  can preferably have a melting temperature T L  which is below the boiling temperature T s  of the first working medium  118 . Further details will be given in conjunction with  FIG. 4 . If the temperature of the heat source  117  rises above the melting temperature T L  of the second working medium  122 , then the thermal energy originating from the heat source  117  is used to melt the second working medium  122 . Only when the second working medium  122  has been completely liquefied, and the heat store  121  has been effectively exhausted, does the temperature on the thermoelectrical generator  112  rise above the melting temperature T L  of the second working medium  122 . If the temperature of the heat source  117  rises further, the heat flow is carried away to the recooler  116  through the working medium  118  boiling in the chamber  114 . 
         [0065]      FIG. 3  shows a further preferred exemplary embodiment in which the second chamber  121 , which is filled with a second working medium  122 , is arranged between the chamber  114 , which is filled with the first working medium  118 , and the thermoelectrical generator  112 . The melting temperature T L  of the second medium can preferably be below the boiling temperature T s  of the first medium  118 . The thermal conductivity of a liquid is typically less than the thermal conductivity of a solid body. The heat flow originating from the heat source  117  is thus initially counteracted on its way to the thermoelectrical generator  112  by a thermal resistance in the form of the first chamber  114 . If a very hot heat source  117  is used to operate the thermoelectrical generator  112 , it may be advantageous to use a thermal resistance to reduce the high temperature of the heat source. 
         [0066]      FIG. 4  shows a schematic illustration of the temperature profile T TEG  on the hot side of the thermoelectrical generator  112 , as a function of time t. It is assumed that the heat source  117  is at a constantly high temperature, which should preferably be above the critical temperature  141  above which the thermoelectrical generator  112  is permanently damaged. The curve illustrated in  FIG. 4  is preferably based on an exemplary embodiment as shown in  FIG. 2 . 
         [0067]    If the temperature of the heat source  117  rises, the temperature of the thermoelectrical generator T TEG  initially follows that part of the graph annotated  144 . If the temperature of the thermoelectrical generator  112  reaches the melting temperature T L  of the second working medium, the temperature of the thermoelectrical generator T TEG  will also initially not rise any further, even if further heat is supplied. The position on the temperature axis of the resultant plateau is governed by the melting temperature T L  of the second medium  122 , and the mass or heat capacity of the second medium  122  governs the time over which the plateau extends. The melting temperature of the second working medium  122  preferably corresponds essentially to a preferred working temperature  142  of the thermoelectrical generator  112 . 
         [0068]    The temperature T TEG  of the thermoelectrical generator  112  will not rise any further until the second medium  122  has been melted completely. Because of the lower thermal conductivity of the liquid phase of the second working medium  122 , the temperature rises, as indicated by the part of the curve annotated  145  in  FIG. 4 , with a flatter gradient than before in the part of the graph annotated  144 . If further thermal energy is produced by the heat source  117 , the temperature of the thermoelectrical generator  112  rises to the boiling temperature T s  of the first working medium  118 , which preferably corresponds essentially to the maximum permissible working temperature  143  of the thermoelectrical generator  112 . Gaseous working medium  118  can rise to the recooler  116 , where it is liquefied again. This allows excess thermal energy to be carried away to the recooler  116  by the gaseous second medium  118 . 
         [0069]    Even if the temperature of the heat source  117  rises further and/or a heat flow continues at a temperature level above the critical temperature  141 , a further rise in the temperature T TEG  of the thermoelectrical generator  112  can be avoided by the vaporization and recooling of the first working medium  118 . This means that the thermal destruction threshold  141  of the thermoelectrical generator  112  will not be reached, and that it is protected against thermal overheating. 
         [0070]      FIG. 5  shows a further preferred exemplary embodiment of a thermoelectrical device. The design illustrated in  FIG. 5  is a design that is generally known from  FIG. 1  that has been added to such that the heat source  117  is connected to parts of the exhaust-gas system  152  of an internal combustion engine  151 . The chamber  114  can preferably be connected to the exhaust-gas system  152  of an internal combustion engine by the use of further measures, for example corrosion-protective measures. 
         [0071]    The preferred exemplary embodiment illustrated in  FIG. 5  is not restricted to the embodiment illustrated in the figure. The exhaust-gas flow can likewise be passed through an exhaust-gas guide system  152  which branches. In this way, the hot exhaust gas from the internal combustion engine  151  can be brought into thermal contact with a multiplicity of thermoelectrical generators  112 . Furthermore, the thermoelectrical generators may be arranged in a structure with a periodic design. For example, a first chamber  114  and the associated thermoelectrical generator  112  may in each case be arranged on the opposite sides of an exhaust-gas channel. A cooling channel or a cooling lug can be arranged on each of the cold sides of the thermoelectrical generators  114 , and these are used as heat sinks  111 . A further thermoelectrical generator  112  can also in each case be arranged with its cold side on this cooling channel. This makes it possible to design a periodic structure comprising exhaust-gas channels, thermoelectrical generators  112  with the structure for temperature limiting, and cooling channels. 
         [0072]      FIG. 6  shows a further preferred exemplary embodiment of a thermoelectrical device in which, in comparison to the exemplary embodiment illustrated in  FIG. 5 , the heat sink  111  is coupled to the cooling system  161  of an internal combustion engine  151 . The cooling system  161  may be a generally known cooling system, which is normally operated with cooling water, for an internal combustion engine  151 , or else, for example, the oil cooling system of an internal combustion engine  151 . By way of example, commercially available lubricating oil or cooling oil can be used as the first working medium  118 . It is likewise possible to use an oil which has been specifically modified for use in a thermoelectrical device with the structure for temperature limiting. 
         [0073]    The cooling water which is used to cool the internal combustion engine  151  can preferably be used to control the temperature of the heat sink  111 , that is to say it can be thermally connected to it. Furthermore, the recooler  116  can likewise be integrated in the cooling system  161  of the internal combustion engine  151 . This makes it possible also to ensure that the recooler  116  is cooled, and that this can be kept at a temperature as required for the gaseous first working medium  118  to be liquefied again. A surface  162  which is to be cooled by an airflow can likewise be thermally connected to the heat sink  111 . This refinement can be used in particular when the thermoelectrical device is used in a motor vehicle. In this case, for example, the surface  162  can be cooled by the wind of motion. 
         [0074]      FIG. 7  shows a further preferred exemplary embodiment of a thermoelectrical device. In comparison to the exemplary embodiment illustrated in  FIG. 1 , the recooler  116  is in the form of a further thermoelectrical device. For this purpose, the pipeline system  115  is connected to a further, third chamber  171 . This third chamber  171  may be at least partially filled with the first working medium  118 . The third chamber  171  is at least thermally, and preferably also mechanically, connected to the hot side of a further thermoelectrical generator  172 . The cold side of the thermoelectrical generator  172  is connected to a heat sink  173 . The integration of a further thermoelectrical generator  172  in the recooler  116  makes it possible to additionally use the thermal energy carried away via the recooler  116  to generate electrical energy. This makes it possible to improve the efficiency of the overall thermoelectrical device. Furthermore, the recooler  116  can also be designed such that, rather than using a single further thermoelectrical generator  172 , a cascade is used comprising a plurality of thermoelectrical generators  172  for recooling of the first working medium  118 . The cascade comprising a plurality of thermoelectrical generators  172  may in this context be created by a thermal parallel connection or else by a thermal series connection. In this context, thermal parallel connection means thermal coupling of a plurality of thermoelectrical generators  172  whose hot side is connected to a common heat source, for example the third chamber  171 . 
         [0075]    In the abovementioned context, thermal series connection means thermal coupling of a plurality of thermoelectrical generators  172 , in which the hot side of each thermoelectrical generator  172  is connected to the cold side of a further thermoelectrical generator  172 . 
         [0076]    The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in  Superguide v. DIRECTV,  69 USPQ2d 1865 (Fed. Cir. 2004).