KR20130108381A - Thermoelectric modules for exhaust system - Google Patents

Abstract

The present invention is an integral thermoelectric module composed of p- and n-conductive thermoelectric material parts alternately connected to each other via electrically conductive contacts, wherein the thermoelectric module 19 is 1 mm or less through which the fluid heat exchanger medium can flow. An integrated thermoelectric module is thermally conductively connected with a micro heat exchanger 13 comprising a plurality of continuous channels of diameter.

Description

Thermoelectric Module for Exhaust System {THERMOELECTRIC MODULES FOR EXHAUST SYSTEM}

The present invention relates to a thermoelectric module suitable for installation in an exhaust system of an internal combustion engine.

Thermoelectric generators and Peltier arrangements have long been known. The p- and n-doped semiconductors, which are heated on one side and cooled on the other, can transfer electrical charge through external circuitry to perform electrical work on loads in the circuit. The efficiency thus achieved for the conversion of heat to electrical energy is thermodynamically limited by Carnot efficiency. Therefore, when the temperature on the hot side is 1000 K and the temperature on the “cold” side is 400 K, an efficiency of (1000-400) ÷ 1000 = 60% is possible. However, to date, only up to 6% efficiency has been achieved.

On the other hand, when direct current is applied to this arrangement, heat is transported from one side to the other. This Peltier system acts as a heat pump and is therefore suitable for cooling parts of the apparatus, vehicles or buildings and the like. Heating using the Peltier principle is also preferred over conventional heating, since more heat is always transported relative to the equivalent energy supplied.

Currently, it is used in space searchers for direct current generation, for cathodic corrosion protection of pipelines, for energy supply of light and radio buoys, and for operation of radios and televisions. The advantage of thermoelectric generators lies in their excellent reliability. This operates independently of atmospheric conditions such as relative humidity, does not generate any material transport that is susceptible to interference, and is merely charge transport.

Thermoelectric modules are composed of p- and n-type components electrically connected in series and thermally connected in parallel. 2 shows such a module.

The conventional structure consists of two ceramic plates, in which individual parts are alternately installed between these ceramic plates. The two parts are in each case electrically-conductively contacting through the end faces.

In addition to the electrically conductive contacts, various additional layers serving as protective or soldering layers are provided on the actual materials.

However, finally, an electrical contact between the two parts is achieved via the metal bridge.

An essential element of thermoelectric module components is contacting. The contact physically connects the material in the "heart" of the component (which provides the desired thermoelectric effect of the component) and the "outer world". The structure of this contact is shown schematically in FIG. 1.

The thermoelectric material 1 inside the component provides the actual effect of the component. This is a thermoelectric component. Current and heat flux flow through the material 1 so that it fulfills its function in the whole structure.

The material 1 is connected at two or more sides, respectively, to the leads 6 and 7 via the contacts 4 and 5. In this case, layers 2 and 3 are intended to represent one or more optionally required interlayers (blocking material, solder, binder, etc.) between the material and contacts 4 and 5. Segments of 2/3, 4/5, 6/7 each associated with each other may be identical, but need not be. This will eventually depend similarly on the particular structure and application, as well as the direction of flow of current or heat flux through the structure.

The contacts 4 and 5 have an important role. They ensure a tight bond between the material and lead. If the contacts are bad, high losses can occur here and can greatly limit the component's performance. For this reason, the parts and contacts are often pressed onto the material for use. The contacts are thus exposed to strong mechanical loads. Whenever elevated (or decreased) temperatures and / or heat cycles are involved, these mechanical loads are further increased. Thermal expansion of the material in the component inevitably leads to mechanical stress, which in extreme cases causes damage to the component by cracking of the contacts.

To prevent this, the contacts used must have specific flexibility and elastic properties so that these thermal stresses can be canceled out.

A carrier plate is essential to give stability to the entire structure and to ensure the required maximum uniform thermal coupling per part. For this reason, ceramics, usually composed of oxides or nitrides such as Al ₂ O ₃ , SiO ₂ or AlN, are usually used.

Conventional structures are often limited in application, since in each case only flat surfaces can be in contact with the thermoelectric module. Tight coupling between the module surface and the heat source / heat sink is essential to ensure sufficient heat flux.

Non-flat surfaces, such as, for example, round waste heat pipes, are not suitable for direct contact with conventional modules or require corresponding straight heat exchanger structures to convert the non-flat surfaces into flat modules.

At present, attempts have been made to provide thermoelectric modules for exhaust systems or exhaust gas recirculation in vehicles such as automobiles and trucks in order to obtain electrical energy from a portion of the exhaust gas heat. In this case, the hot side of the thermoelectric module element is connected to the exhaust gas or the exhaust pipe, while the cold side is connected to the cooler. The amount of electricity that can be generated depends on the heat flux from the exhaust gas to the thermoelectric material and the temperature of the exhaust gas. In order to maximize the heat flux, devices are often installed in the exhaust pipe. However, this is limited, for example, because the installation of a heat exchanger leads to a pressure loss in the exhaust gas which in turn leads to an unacceptable increased consumption of the internal combustion engine.

Typically, thermoelectric generators are installed for use under an exhaust gas catalytic converter in an exhaust system. Including pressure loss of the exhaust gas catalytic converter, this often leads to excessive pressure loss such that thermally-conductive devices cannot be provided in the exhaust system; Rather, the thermoelectric module is external to the exhaust pipe. For this reason, the exhaust pipe must be structured to have a polygonal cross section, so that the flat outer surface can be firmly connected with the thermoelectric material.

No heat transfer or pressure loss generated is satisfactory to date.

It is an object of the present invention to provide a thermoelectric module for installation in an exhaust system of an internal combustion engine, by avoiding the disadvantages of known modules and ensuring good heat transfer with low pressure losses.

The object is, in accordance with the present invention, a thermoelectric module composed of p- and n-conductive thermoelectric material parts alternately connected to each other via electrically conductive contacts, wherein the thermoelectric module is capable of flowing through a fluid heat exchanger medium. It is achieved by a thermoelectric module which is thermally conductively connected with a micro heat exchanger comprising a plurality of continuous channels of diameter up to 1 mm.

In particular, in vehicle exhaust gas catalysts, it is particularly advantageous for the channels of the micro heat exchanger to be coated with a washcoat of the internal combustion engine exhaust gas catalyst. In this way, individual exhaust gas catalytic converters can be eliminated and pressure losses in the exhaust system are minimized. The one-piece design simplifies the overall structure and facilitates installation in the exhaust system.

By using a micro heat exchanger, it is possible to ensure an improved heat flux from the exhaust gas to the thermoelectric module, while at the same time ensuring a sufficiently low pressure loss. According to the invention, the exhaust gas flows through the microchannels of the micro heat exchanger. In this case, the channels are preferably coated with an exhaust gas catalyst which catalyzes at least one of the conversion of nitrogen from NO _x , C0 ₂ and H ₂ 0 from hydrocarbons, and C0 ₂ from CO. Especially preferably, all these conversions are catalyzed.

Suitable catalytically active materials such as Pt, Ru, Ce, Pd are known and are described, for example, in Stone, R. et al., Automotive Engineering Fundamentals, Society of Automotive Engineers 2004. This catalytically active material is applied to the channels of the micro heat exchanger in a suitable manner. Preferably, application of the washcoat form may be envisaged. In this case, the catalyst is applied to the inner wall of the micro heat exchanger or to its channel as a thin film in the form of a suspension. The catalyst may then consist of a single layer or of various layers having the same or different compositions. The applied catalyst can then replace, in whole or in part, the exhaust gas catalytic converter commonly used in internal combustion engines in use in vehicles, depending on the size of the micro heat exchanger and its coating.

1 is a schematic view showing a contact of a thermoelectric module.
2 shows a typical structure of a thermoelectric module composed of p- and n- type components.
3 is a three-dimensional view showing the configuration of a thermoelectric generator.
4 is a three-dimensional view showing the layer configuration of the thermoelectric generator.

According to the invention, the term "micro heat exchanger" is intended to mean a heat exchanger having a plurality of continuous channels of diameter of 1 mm or less, particularly preferably 0.8 mm or less. The minimum diameter is set only by technical feasibility and is preferably approximately 50 μm, particularly preferably approximately 100 μm.

The channel may have any suitable cross section, for example round, oval, polygonal, for example square, triangular or star shaped. Here, the shortest distance between the opposite edges or points of the channel is taken as the diameter. The channels can also be formed to be flat, in which case the diameter is defined as the distance between the bounding faces. This is especially the case for heat exchangers consisting of plates or layers. During operation, the heat exchanger medium flows through the continuous channel, where heat moves to the heat exchanger. The heat exchanger, on the other hand, is thermally connected to the thermoelectric module, for excellent heat transfer from the heat exchanger to the thermoelectric module.

The micro heat exchanger can be constructed in any suitable manner from any suitable material. For example, it can be made from a block of thermally conductive material, into which continuous channels are introduced.

Any suitable materials such as plastics such as polycarbonate, liquid crystalline polymers such as Zenith®, DuPont, polyether ether ketones (PEEK) and the like can be used as the material. Can be. Metals such as iron, copper, aluminum or suitable alloys such as aluminum-iron, Fecralloy can be used. Ceramic or inorganic oxide materials such as aluminum oxide or zirconium oxide or cordierite may further be used. It may also be a composite material composed of a plurality of the aforementioned materials. The micro heat exchanger is preferably composed of a hot-resistant alloy (1000-1200 ° C.), pecralloy, an Al-containing iron alloy, stainless steel, cordierite. The microchannels can be introduced into the block of thermally conductive material in any suitable manner, for example by laser method, etching, perforation or the like.

As an alternative, the micro heat exchanger can also be composed of different plates, layers or tubes, which are subsequently connected to one another, for example by adhesive bonding or welding. The plate, layer or tube can in this case be provided in advance of the microchannels and then assembled.

Particular preference is given to producing a micro heat exchanger from the powder by the sintering method. Both metal powders and ceramic powders can be used as the powder. Mixtures of metals and ceramics, or mixtures of different metals, or mixtures of different ceramics are also possible. Suitable metal powders include, for example, powders composed of ferritic steel, pecralloy or stainless steel. The production of the micro heat exchanger by the sintering method makes it possible to produce any desired structure.

The use of metal as the material for the micro heat exchanger provides the advantage of good thermal conductivity. In contrast, ceramics have good heat storage capacity and they can be used particularly to compensate for temperature fluctuations.

If plastic is used as the material for the micro heat exchanger, it is necessary to apply a coating to protect the plastic from the temperature of the exhaust gases flowing through the micro heat exchanger. Such coatings are also referred to as “heat barrier coatings”. Given the high temperatures of the exhaust gases, it is necessary to coat all surfaces of micro heat exchangers made of plastic material.

The external dimensions of the micro heat exchanger used according to the invention are preferably from 60 x 60 x 20 mm ³ to 40 x 40 x 8 mm ³ .

The specific heat transfer area of the micro heat exchanger with respect to the volume of the micro heat exchanger is preferably 0.1 to 5 m ² / l, particularly preferably 0.3 to 3 m ² / l, especially 0.5 to 2m ² / l.

Suitable micro heat exchangers are commercially available, for example, from Institut fur Mikrotechnik Mainz GmbH. IMM provides microstructured heat exchangers of various structures, in particular microstructured high temperature heat exchangers for maximum operating temperatures of 900 ° C. The dimensions of this high temperature heat exchanger are about 80 x 50 x 70 mm ³ (for other applications) and operate according to the counterflow principle. Its pressure loss is less than 50 mbar and the specific heat transfer area is about 1 m ² / l.

Another micro heat exchanger was published by VDI / VDE-Technologiezentrum Informationstechnik GmbH (www.nanowelten.de). The micro heat exchanger is additionally supplied by SWEP Market Service (Fuerte), a branch of Ehrfeld Mikrotechnik BTS GmbH, Wendelsheim and Dover Market Services GmbH. .

The micro heat exchanger is intended to be connected to the thermoelectric module in such a way as to have the highest thermal conductivity possible. Depending on the structure and material construction, it may be directly thermally conductively connected to the thermoelectric module. It is also possible for the thermoelectric module to be flat, and it is also possible for the thermoelectric module to have a carrier plate thermally conductively connected to the micro heat exchanger on the hot side. Suitable materials for the carrier plate are mentioned in the introduction.

The micro heat exchanger is preferably formed integrally with the thermoelectric module. In this case, it is also possible to sinter the micro heat exchanger, for example, directly into the thermoelectric module. This has the advantage that a highly thermally conductive connection is obtained, regardless of the shape of the surface of the thermoelectric module.

The pressure loss occurring through the continuous channels of the heat exchanger for the gas flowing through it is preferably 100 mbar or less, in particular 50 mbar or less. This pressure loss does not lead to increased fuel consumption of the internal combustion engine. In particular, pressure loss may occur when the micro heat exchanger is arranged such that the channels through which the exhaust gas flows run in parallel and are connected to one inlet and the other outlet. The length of the channel through which the exhaust gas flows is in this case preferably 60 mm or less, in particular 40 mm or less. If more than one micro heat exchanger is used, the micro heat exchangers are similarly connected in parallel and connected to a common inlet and common outlet so that the channels of the individual heat exchangers run similarly in parallel.

The heat-exchange surface of the micro heat exchanger can be installed directly in the exhaust pipe or exhaust system of an internal combustion engine, in particular an internal combustion engine in a vehicle. In such cases it may be fixed or removable. The heat-exchange surface can also be tightly encapsulated in a thermoelectric module.

If the microheat exchanger is provided with a washcoat of catalytic material, it may be installed in the exhaust system at the location of the original exhaust catalytic converter. In this way, high exhaust gas temperatures can be supplied to the micro heat exchanger. Even chemical conversion in the exhaust gas catalyst of the micro heat exchanger may increase the temperature, resulting in more efficient heat transfer in known systems.

The improved efficiency of the thermoelectric module is also achieved by the improved heat flux.

A protective layer for protection against excessive temperatures may additionally be provided between the thermoelectric module and the micro heat exchanger. Such a layer, referred to as a phase change layer, is composed of an inorganic metal salt or metal alloy, preferably having a melting point in the range of 250 ° C to 1700 ° C. Suitable metal salts are, for example, fluorides, chlorides, bromide, iodides, sulfates, nitrates, carbonates, chromates, molybdates of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium and barium , Vanadate and tungstate. Mixtures of suitable salts of this type, which form binary or ternary eutectics, are preferably used. They may also form quaternary eutectics or five-way eutectics.

Alternatively, phase-change materials and their forming, for example, binary, tertiary, quaternary or 5-membered eutectics starting from metals which are zinc, magnesium, aluminum, copper, calcium, silicon, phosphorus and antimony It is also possible to use metal alloys as a combination. The melting point of the metal alloy is in this case in the range of 200 ° C to 1800 ° C.

Thermoelectric modules can be encapsulated in a protective layer, in particular when using metals such as nickel, zirconium, titanium, silver and iron, or when alloys based on nickel, chromium, iron, zirconium and / or titanium are used.

For example, one or more thermoelectric modules, connected in series, may be integrated with the exhaust system of the internal combustion engine. In such cases, thermoelectric modules comprising different thermoelectric materials may also be combined. In general, it is possible to use all suitable thermoelectric materials which are suitable for the temperature range of the exhaust gas of the internal combustion engine. Examples of suitable thermoelectric materials include skutterudite, for example CoSb ₃ , RuPdSb ₆ , TX ₆ (where T = Co, Rh, Ir and X = P, As, Sb); X ₂ Y ₈ Z ₂₄ , wherein X = lanthanide, actinium group, alkaline earth metal, alkali metal, Th, group IV element; Half-Heusler compounds such as TiNiSn, HfPdSn and intermetallic alloys; Clathrates such as Zn ₄ Sb ₃ , Sr ₈ Ga ₁₆ Ge ₃₀ , Cs ₈ Sn ₄₄ , Co ₄ TeSb ₄ ; Oxides such as Na _x CoO ₂ , CaCo ₄ O ₉ , Bi ₂ Sr ₂ Co ₂ O _y Sr ₂ TiO ₄ , Sr ₃ Ti ₂ O ₇ , Sr ₄ Ti ₃ O ₁₀ , R _1-x M _x CoO ₃ Where R = rare earth metal and M = alkaline earth metal; Sr _{n + 1} Ti _n O _{3n + 1} where n is an integer; YBa ₂ Cu ₃ O _7-x ; Silicides such as FeSi ₂ , Mg ₂ Si, Mn ₁₅ Si ₂₆ ; Borides such as B ₄ C, CaB ₆ ; Bi ₂ Ce ₃ and its derivatives, PbCe and its derivatives, antimonides such as zinc antimonide, Zintl phase, for example Yb ₁₄ MnSb ₄ .

The present invention preferably relates to the use of a thermoelectric module as described above in an exhaust system of an internal combustion engine in a vehicle such as an automobile or a truck. In this case, the thermoelectric module is used in particular for generating electricity from the heat of the exhaust gas.

However, if a washcoat is present on the micro heat exchanger, the thermoelectric module may be used in reverse to preheat the exhaust gas catalyst, preferably during the cold start of the internal combustion engine of the vehicle. In this case, the thermoelectric module is used as a Peltier element. When a voltage difference is applied to the module, the module transfers heat from the cold side to the hot side. This preheating of the exhaust gas catalyst reduces the startup temperature of the catalyst.

The invention further relates to an exhaust system of an internal combustion engine, preferably an internal combustion engine of a vehicle, comprising at least one thermoelectric module described above, which is integral with the exhaust system.

In this case, the exhaust system is intended to mean a system which is connected to the outlet of the internal combustion engine and in which the exhaust gas is processed.

The thermoelectric module according to the invention has many advantages. Especially when the micro heat exchanger is coated with the washcoat of the exhaust gas catalyst, the pressure loss in the exhaust system of the internal combustion engine is low. The structure of the exhaust system can be significantly simplified by one integrated component. Since the integrated component is integrated closer to the internal combustion engine in the exhaust system, higher exhaust gas temperatures can be supplied to the thermoelectric module. By the thermal use of the thermoelectric module as a Peltier element, the exhaust gas catalyst can be heated during the room temperature startup of the engine.

Exemplary embodiments of the invention are shown in the drawings and described in more detail in the following description.

3 shows a configuration of a thermoelectric generator in a state that can be inserted into, for example, an exhaust system of a vehicle.

The exhaust pipe 10 through which the exhaust gas is guided from the internal combustion engine is led to a manifold 11. The manifold 11 has a cross-sectional area that decreases in the flow direction of the exhaust gas. Manifold 11 is adjacent to micro heat exchanger 13. The latter is connected to the manifold 11 in such a way that the exhaust gas flows through the channel in the micro heat exchanger 13. The channel in the micro heat exchanger is directed to the collector 15, and after the exhaust gas flows through the channel in the micro heat exchanger, through the collector 15 the exhaust gas is directed to an additional exhaust pipe 17, further exhaust The pipe 17 generally ends in exhaust gas of the internal combustion engine.

The micro heat exchanger 13 is connected to the thermoelectric module 19 on one side, respectively. The thermoelectric module 19 is cooled on the opposite side of the micro heat exchanger. For this purpose, it is preferable to use a cooling liquid, for example cooling water, which flows over the thermoelectric module 19. In this case, it is first possible for the cooling water to flow through a channel in a heat exchanger, for example a micro heat exchanger. However, it is desirable to provide a cooling channel 21 through which the cooling liquid flows toward the thermoelectric module 19 to be cooled, wherein the wall of the cooling channel 21 is formed of a thermoelectric module 19.

In one preferred embodiment, the micro heat exchanger 13, the thermoelectric module 19 and the cooling channel 21 are stacked, wherein the micro heat exchanger 13 located in the inner part is in each case at opposite sides thereof. In connection with the thermoelectric module 19, the cooling channel 21 located therein is in each case connected with the thermoelectric module 19 on opposite sides thereof. The corresponding layer structure is illustrated with the example of FIG. 4. Here, the layer structure is in each case surrounded by cooling channels at the top and bottom. The cooling channel 21 is adjacent to the thermoelectric module 19, which is connected to the micro heat exchanger 13 on the opposite side. The micro heat exchanger 13 is followed by an additional thermoelectric module 19 and an additional cooling channel 21.

The layer structure makes it possible not only to use the heat of the exhaust gas but also to use the plurality of thermoelectric modules 19 in a narrow space.

In addition to the embodiments described in FIGS. 3 and 4, in which the individual layers have a layered structure running parallel to the main flow axis of the exhaust pipe 10, the individual layers are allowed to run perpendicular to the main flow axis of the exhaust pipe 10. It is also possible to devise a layer structure. Regardless of the orientation of the individual layers, the channel in the micro heat exchanger 13 preferably always runs transverse to the main flow direction of the exhaust pipe 10 from the manifold 11 to the collector 15.

In the case of the orientation of the layers as described in FIGS. 3 and 4, the individual layers may in each case comprise one micro heat exchanger 13 and one thermoelectric module 19, or alternatively a plurality of which are each positioned next to each other Micro heat exchangers 13 and / or a plurality of thermoelectric modules 19. If a plurality of micro heat exchangers 13 and a plurality of thermoelectric modules 19 are used, then they may have contact areas having the same size or different sizes. Preferably, the contact areas have the same size so that in each case one micro heat exchanger 13 is connected to one thermoelectric module 19. In this embodiment, a plurality of stacks are then formed that are connected in series with each other, with the individual stacks preferably oriented such that the individual cooling channels 21 adjoin one another with their inlet and outlet, thus contiguous stacks. It is desirable to form a continuous cooling channel. In this case, the orientation of the cooling channel is chosen such that the cooling liquid and the exhaust gas are guided in cross flow with respect to each other. Alternatively, it is also possible to orient the cooling channel in any other direction desired. Thus, for example, the cooling channel may run parallel to the channel in the micro heat exchanger.

Claims (14)

A thermoelectric module consisting of p- and n-conductive thermoelectric material pieces alternately connected to each other via electrically conductive contacts, wherein the thermoelectric module 19 has a diameter of 1 mm or less through which the fluid heat exchanger medium can flow. A thermoelectric module, thermally conductively connected with a micro heat exchanger (13) comprising a plurality of continuous channels. The method of claim 1,
The thermoelectric module (19) is flat and has a carrier plate on a hot side that is heat-conductively connected to the micro heat exchanger (13), on the interconnected thermoelectric material components. The method of claim 1,
The thermoelectric module, wherein the micro heat exchanger (13) is formed integrally with the thermoelectric module (19). 3. The method according to claim 1 or 2,
A thermoelectric module is provided between the thermoelectric module (19) and the micro heat exchanger (13) for protection against excessive temperatures. 5. The method of claim 4,
And the protective layer is composed of an inorganic metal salt or a metal alloy having a melting point in the range of 250 ° C to 1700 ° C. 6. The method according to any one of claims 1 to 5,
The channel of the micro heat exchanger is coated with a washcoat of the vehicle exhaust gas catalyst. The method according to claim 6,
Wherein the catalyst catalyzes one or more of conversion of NO _x to nitrogen, conversion of hydrocarbons to C0 ₂ and H ₂ 0, and conversion of CO to C0 ₂ . The method according to any one of claims 1 to 7,
Thermoelectric module with a pressure drop of up to 100 mbar occurring through the continuous channel of the heat exchanger for gas passage. The method according to any one of claims 1 to 8,
Wherein the micro heat exchanger is made of a block of thermally conductive material into which a continuous channel is introduced. 10. The method according to any one of claims 1 to 9,
A thermoelectric module, wherein the specific heat transfer area associated with the volume of the micro heat exchanger is 0.1 to 5 m ² / l. Use of a thermoelectric module according to any of the preceding claims in an exhaust system of an internal combustion engine, preferably an internal combustion engine in a vehicle. The method of claim 11,
For generating electricity from the heat of the exhaust gas. Use of a thermoelectric module according to claim 6 or 7 for preheating the exhaust gas catalyst during cold start of an internal combustion engine, preferably an internal combustion engine of a vehicle. An exhaust system of an internal combustion engine, preferably an internal combustion engine of a vehicle, comprising at least one thermoelectric module according to claim 1, which is integrated into the exhaust system.

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