Abstract
A thermoelectric generator for converting heat of a hot gas flow into electric energy can include at least one thermoelectric module with a plurality of thermoelectric elements. A gas channel on a high-temperature side of the thermoelectric module can conduct the hot gas flow in a flow direction of the gas channel. A heat sink can cool the thermoelectric module and be in contact with the thermoelectric module on a low-temperature side thereof. At least one heat conducting body can extend into the gas channel in a direction running transversely to the flow direction on the high-temperature side of the thermoelectric module and can have a free end within the gas channel. The heat conducting body can be part of the thermoelectric module or connected thereto on the high-temperature side. The thermoelectric generator can have a seal which separates an area between the heat sink and the gas channel from the gas channel and seals the area from the hot gas flow.
Claims
1. A thermoelectric generator for converting heat of a hot gas flow into electric energy, wherein the thermoelectric generator comprises: at least one thermoelectric module with a plurality of thermoelectric elements, a gas channel on a high-temperature side of the thermoelectric module for conducting the hot gas flow in a flow direction of the gas channel, a heat sink for cooling the thermoelectric module, wherein the heat sink is in contact with the thermoelectric module on a low-temperature side of the thermoelectric module, and at least one heat conducting body which extends into the gas channel in a direction running transversely to the flow direction on the high-temperature side of the thermoelectric module and which has a free end within the gas channel, wherein the heat conducting body is part of the thermoelectric module or is connected to the thermoelectric module on the high-temperature side of the thermoelectric module, wherein the thermoelectric generator has a seal which separates an area arranged between the heat sink and the gas channel from the gas channel and seals the area from the hot gas flow flowing in the gas channel during the operation of the thermoelectric generator.
2. The thermoelectric generator according to claim 1, wherein the seal is made of a flexible material.
3. The thermoelectric generator according to claim 2, wherein the flexible material is designed to deform under the action of external forces.
4. The thermoelectric generator according to claim 1, wherein the material of the seal comprises a nonwoven or woven material formed of fibres.
5. The thermoelectric generator according to claim 1, wherein the seal is passed through at least at one point by the heat conducting body or one of the heat conducting bodies or by the thermoelectric module and/or parts of the seal are separated from one another at least at one point by the heat conducting body or one of the heat conducting bodies or by the thermoelectric module.
6. The thermoelectric generator according to claim 1, wherein protrusions formed by the heat conducting body or by a plurality of the heat conducting bodies and extending into the gas channel are formed as fins of a heat exchanger for transferring the heat of the hot gas flow to the thermoelectric module.
7. The thermoelectric generator according to claim 1, wherein the seal extends from the heat conducting body or from one of the heat conducting bodies to a gas channel wall or to a supporting part of the thermoelectric generator, but is not connected to the gas channel wall or the supporting part of the thermoelectric generator.
8. The thermoelectric generator according to claim 1, wherein the thermoelectric elements each comprise a pair of different materials, which are in contact with one another in a first contact region on the low-temperature side and are in contact with one another in a second contact region on the high-temperature side, so that an electrical voltage is produced between the first and the second contact region on account of a higher temperature in the second contact region than in the first contact region, and wherein the first contact region is disposed in the area separated by the seal from the gas channel.
9. The thermoelectric generator according to claim 1, wherein the area separated by the seal from the gas channel has, separately from the gas channel, an inlet and an outlet through which a flushing gas for flushing the area can be admitted and discharged before, during and/or after an operation of the thermoelectric generator.
10. The thermoelectric generator according to claim 1, wherein the heat sink is connected to a part supporting the weight of the thermoelectric generator and is supported by the supporting part, and wherein the heat sink supports the thermoelectric module.
11. A rail vehicle comprising a thermoelectric generator according to claim 1, wherein the rail vehicle comprises an engine and the thermoelectric generator is arranged in an exhaust gas tract of the engine or is thermally coupled to the exhaust gas tract.
12. A method for producing a thermoelectric generator for converting heat of a hot gas flow into electrical energy, comprising the following steps: providing at least one thermoelectric module with a plurality of thermoelectric elements, providing a gas channel for guiding the hot gas flow in a flow direction of the gas channel on a high-temperature side of the thermoelectric module, arranging a heat sink for cooling the thermoelectric module on a low-temperature side of the thermoelectric module, coupling the heat sink to the thermoelectric module on the low-temperature side, and arranging at least one heat conducting body on the high-temperature side of the thermoelectric module, so that the heat conducting body extends into the gas channel in a direction transverse to the flow direction and has a free end within the gas channel, wherein the heat conducting body is part of the thermoelectric module or is connected to the thermoelectric module on the high-temperature side of the thermoelectric module, wherein a seal is arranged so that it separates an area arranged between the heat sink and the gas channel from the gas channel and, during operation of the thermoelectric generator, seals the area with respect to the hot gas flow flowing in the gas channel.
Description
[0077] Exemplary embodiments of the invention will now be described with reference to the accompanying drawing. The individual figures of the drawing show:
[0078] FIG. 1 a longitudinal section through an arrangement having two heat sinks on opposite sides of a gas channel, wherein thermoelectric modules with heat conducting bodies secured thereto extend starting from each of the heat sinks in the direction of the interior of the gas channel,
[0079] FIG. 2 a cross-section through the arrangement illustrated in FIG. 1 along the line II-II,
[0080] FIG. 3 an isometric three-dimensional illustration of an arrangement with a heat sink, from which thermoelectric modules with heat conducting bodies secured thereto extend on mutually opposed sides,
[0081] FIG. 4 an isometric three-dimensional illustration of a plurality of the arrangements illustrated in FIG. 3, which are arranged adjacently and in levels one above the other,
[0082] FIG. 5 an illustration of three arrangements similar to the arrangement illustrated in FIG. 3, wherein the three arrangements are arranged in succession in the flow direction of the hot gas and have different lengths transversely to the direction of the gas flow, over which the heat conducting bodies extend into the gas channel,
[0083] FIG. 6 the arrangement illustrated in FIG. 4 with additional supporting frame, a gas inlet, and a gas outlet,
[0084] FIG. 7 a cross-section through a heat sink, which for example can be used as a heat sink in one of the arrangements in FIG. 1 to FIG. 6 and FIG. 8 and FIG. 9,
[0085] FIG. 8 a longitudinal section similar to the longitudinal section in FIG. 1, but through an arrangement with cuboid thermoelectric modules instead of with strip-like thermoelectric modules,
[0086] FIG. 9 a cross-section through the arrangement illustrated in FIG. 8 along the line IX-IX,
[0087] FIG. 10 a detail corresponding to the central region of the arrangement illustrated in FIG. 2 to illustrate a specific embodiment of the heat conducting body with wave shape, deviating from FIG. 2, and
[0088] FIG. 11 a specific exemplary embodiment of a supporting structure illustrated in FIG. 4.
[0089] The arrangement illustrated in FIG. 1 and FIG. 2 has two heat sinks 1a, 1b, which each have a plurality of channels 5a, 5b for a cooling liquid flow. The channels 5, in the illustration of FIG. 1, run with their flow direction perpendicularly to the plane of the drawing, whereas in FIG. 2 they run with their flow direction from top to bottom or vice versa in the plane of the drawing. Outer surfaces of the heat sinks 5a, 5b not facing the channels 5 face towards one another. Areas in which strip-like thermoelectric elements 3a, 3b are arranged and in the middle between the heat sinks 5a, 5b an exhaust gas channel 6 are disposed between the heat sinks 5a, 5b. The flow direction for the exhaust gas of a combustion engine flowing through the exhaust gas channel 6 runs in FIG. 1 from bottom to top in the plane of the drawing and runs in FIG. 2 perpendicularly to the plane of the drawing towards the viewer.
[0090] In the exemplary embodiment illustrated in FIG. 1 and FIG. 2, fins 4a, 4b are formed as an integral part of the heat sinks 1a, 1b. There is thus no joining of fins and heat sink. Ten fins 4a, 4b extend in the exemplary embodiment from each of the heat sinks 1a, 1b in the direction of the exhaust gas channel 6. The fins 4a, 4b serve as a connection to the strip-like thermoelectric elements 3a, 3b. Two thermoelectric elements 3a, 3b are connected to each of the fins 4a, 4b, for example adhesively bonded or soldered, so that during operation of the arrangement excess heat is transferred from the thermoelectric elements 3a, 3b via the fins 4a, 4b to the inner wall of the heat sink 1a, 1b and from there is transferred to the cooling liquid within the channel 5a, 5b. The cooling liquid transports the excess heat away. In this way, the low-temperature side of the thermoelectric elements 3a, 3b connected to the fins 4a, 4b is held at a low temperature.
[0091] The strip-like thermoelectric elements, in the cross-section illustrated in FIG. 2, have a very much smaller cross-sectional area compared to the very much larger surface illustrated in FIG. 1. In particular, the material pairs of many thermoelectric elements can be arranged one above the other from top to bottom in the surface of the thermoelectric elements 3a, 3b illustrated in FIG. 1, as is already known per se from the prior art. The thermoelectric elements themselves are not illustrated in FIG. 1.
[0092] The thermoelectric elements 3a, 3b are connected in pairs to a fin-like heat conducting body 7a, 7b, for example again by adhesive bonding or soldering, on the high-temperature sides pointing towards the exhaust gas channel 6. The inner end regions of the thermoelectric elements 3a, 3b pointing towards the exhaust gas channel 6 enclose therebetween, in pairs, an end region of the heat conducting body 7a, 7b. The heat conducting body therefore mechanically stabilises the pairs of thermoelectric elements 3a, 3b in the same manner as the fins 4a, 4b.
[0093] The heat sinks 1a, 1b support the thermoelectric elements 3a, 3b via the fins 4a, 4b formed integrally on the heat sinks in the shown exemplary embodiment, and the thermoelectric elements in turn support the heat exchangers 7a, 7b. The total weight of the thermoelectric elements 3a, 3b and of the heat exchangers 7a, 7b is therefore supported by the heat sinks 1a, 1b.
[0094] A seal 2a, 2b made of flexible material, in the exemplary embodiment a nonwoven material formed of mineral fibres, runs close to the high-temperature side ends of the thermoelectric elements 3a, 3b. The heat exchangers 7a, 7b pass through the seal 2a, 2b, which is held by clamping forces and optionally by additional adhesive on the heat conducting bodies 7a, 7b. The heat conducting bodies 7a supported by the first heat sink 1a thus hold the seal 2a, and the second heat conducting bodies 7b supported by the second heat sink 1b hold the second seal 2b. The seals 2a, 2b are mat-like, i.e. they have two large-area surfaces, which are arranged on mutually opposed sides of the seal 2a, 2b. The outer of these surfaces faces towards the thermoelectric elements 3a, 3b and thus also the heat sink 1a, 1b, whereas the other of the large-area surfaces forms the wall of the exhaust gas channel 6.
[0095] Within the exhaust gas channel 6, the heat conducting bodies 7a, 7b, as has already been described, are interlaced with one another in a comb-like manner, that is to say in the cross-section of FIG. 2, in the order in the plane of the drawing from top to bottom, a first heat conducting body 7a, which extends from the left in the plane of the drawing into the exhaust gas channel 6, is followed by a heat conducting body 7b which extends from the right into the exhaust gas channel, etc. In accordance with the number of ten fins 4a, 4b in the exemplary embodiment and twenty strip-like thermoelectric elements 3a, 3b, ten first heat conducting bodies 7a and ten second heat conducting bodies 7b are provided. Compared to the overall flow cross-section of the exhaust gas channel 6, narrow flow channels 16 are formed in each case by a first heat conducting body and a second heat conducting body 7b. In total, nineteen narrow flow channels 16 of this type are formed in the exemplary embodiment. A further narrow or slightly wider flow channel can be formed at the top and at the bottom in FIG. 2 if there is a screen disposed there (not illustrated in FIG. 2), the inner, large-area surface of which faces towards the strip-like thermoelectric elements 3a, 3b and within the exhaust gas channel 6 faces towards the heat conducting bodies 7a, 7b. The seals 2a, 2b can be supported at the respective screen, so that they form the peripheral walls of the exhaust gas channel 6, jointly with the screens to be arranged at the top and bottom in the plane of the drawing of FIG. 2. The screens can therefore also be referred to as walls.
[0096] As can be clearly seen from FIG. 2, the seals 2a, 2b separate the exhaust gas channel 6 from an area in which, respectively, the first thermoelectric elements 3a (area 10) and the second thermoelectric elements 3b (area 20) are disposed. On account of the strip-like arrangement of the thermoelectric elements 3, the areas 10, 20 are divided into narrow sub-areas 9a, 9b. For example, the lowermost second thermoelectric elements 3b illustrated at the bottom on the right in FIG. 2 and denoted by the reference sign 8 delimit one of these sub-areas 9b. Optionally, the sub-areas 9a, 9b can be passed through by a flushing gas before, during and/or after operation of the arrangement, wherein the flow direction is parallel to the flow direction of the hot gas in the exhaust gas channel 6 and therefore runs for example perpendicularly to the plane of the drawing in FIG. 2.
[0097] FIG. 3 shows a basic unit with a heat sink 1, which for example can be the heat sink 1a, 1b from FIG. 1 and FIG. 2, if thermoelectric elements, fins and heat conducting bodies are disposed on either side of the heat sink 1a, 1b in the arrangement of FIG. 1 and FIG. 2. In this case, the illustrations in FIG. 1 and FIG. 2 would be merely partial illustrations and would not show the fins, thermoelectric elements and heat conducting bodies arranged on the outside of the heat sinks 1a, 1b.
[0098] Starting from the heat sink 1 in FIG. 3, fins 4, to which strip-like thermoelectric elements 3 are secured, extend on either side to the right and left in the illustration of FIG. 3. Heat conducting bodies 7 are connected to the thermoelectric elements 3, so that, in each case starting from the heat sink 1, the sequence of fins 4, thermoelectric element 3 and heat conducting body 7 extends away from the heat sink 1. In the exemplary embodiment illustrated in FIG. 3, sixteen arrangements of this kind extend on each side of the heat sink 1 and on the whole form lamella-like stacks, wherein the narrow flow channels 16 are formed between the heat conducting bodies 7 of the lamellas. Each individual one of the lamellas with a fin 4, a strip-like thermoelectric element 3, and a fin-like heat conducting body 7 can be formed in particular as has been described with reference to FIG. 1 and FIG. 2.
[0099] The seals which are to be arranged one on the left and one on the right of the heat sink 1 illustrated in FIG. 3 are not illustrated in FIG. 3. Furthermore, neither a further basic unit nor a housing wall, which are to be arranged to the left and right of the heat sink 1 illustrated in FIG. 3, is illustrated.
[0100] The heat sink 1 in FIG. 3 comprises eight liquid channels 5 running from the front on the right to the rear on the left, wherein the ends of most of these channels 5 are preferably closed, with the exception of at least one end used as inflow opening or outflow opening in order to introduce cooling liquid into the heat sink 1 or discharge it therefrom. The heat sink 1 also comprises two channels 15 running perpendicularly to the channels 5, with one of said two channels being arranged in the foreground of the image and the other being arranged in the background. These perpendicularly running channels 15 connect all channels 5 to one another, so that a distributor is formed on the inflow side and a collector is formed on the outflow side. The ends of the perpendicularly running cooling channels 15 are preferably closed.
[0101] FIG. 4 shows an arrangement with nine of the basic units illustrated in FIG. 3, wherein in each case three adjacently arranged basic units 14 are arranged in three levels one above the other. In each level the heat conducting bodies 7 of the basic unit arranged in the middle of the level are interlaced in a comb-like manner with the inwardly pointing heat conducting bodies of the basic unit arranged on the outside in the level. As already discernible in FIG. 3 at the bottom and at the top in the image, the heat sinks 1 each have a fastening protrusion at the front and at the rear and at the bottom and at the top. At the transition between the levels, a supporting structure 12 is situated in each case, which runs around the stack at the height level of the transition between the levels and at which the heat sinks 1 are secured via their fastening protrusions 13. Here, merely either the lower fastening protrusion 13 or the upper fastening protrusion 13 of the heat sink 1 is preferably fixedly connected to the supporting structure 12. The other fastening protrusion is preferably guided merely movably on the supporting structure 12. In this way, the basic units can move relative to the other basic units in the event of thermal expansion and contraction. However, it is possible alternatively that all fastening protrusions 13 are fixedly connected to the supporting structures 12 at the transitions between the levels, and the supporting structures 12 are movable relative to a housing (not illustrated in FIG. 4).
[0102] In any case, it is preferred that the lower fastening protrusions 13 of the lowermost level of basic units 14 are fixedly connected to a housing (not illustrated in FIG. 4), but not the upper fastening protrusions 13 of the upper level. Alternatively, the upper fastening protrusions 13 of the upper level could be fixedly connected to the housing, whereas the lower fastening protrusions 13 of the lower level are not fixedly connected to the housing. The fastening protrusions 13 not fixedly connected to the housing are preferably guided movably relative to the housing so as to enable a thermally induced expansion and contraction. In FIG. 5 a stack of three is arranged in succession in the flow direction (illustrated by arrows pointing from bottom to top). Each of the basic units comprises a heat sink 1 and thermoelectric elements 3 connected on opposite sides to the heat sink 1 and heat conducting bodies 7 connected to the thermoelectric elements. The length of the heat conducting bodies 7 in the direction away from the heat sink 1 into the interior of the gas channel (the delimitations of the gas channel by the seals are not illustrated in FIG. 5) varies, however, in the basic units. The basic unit 14a arranged at the front in the flow direction has the heat conducting bodies 7 with the shortest length. The middle basic unit 14b has the heat conducting bodies 7 with a slightly longer length, and the basic unit 14c at the end of the arrangement in the flow direction has the heat conducting bodies 7 with the longest length, which is slightly longer than the length of the heat conducting bodies 7 of the middle basic unit 14b. The reduction of the temperature of the hot gas flow is compensated on account of the different length and therefore the different size of the surfaces of the heat conducting bodies 7.
[0103] FIG. 5 also shows, for each of the basic units 14, a connection point 25 for introducing or removing the cooling liquid into/from the heat sink 1, that is to say the corresponding end of the cooling channel is open at the connection point 25.
[0104] FIG. 6 shows a thermoelectric generator with a stack of basic units as in FIG. 4, wherein however additional housing parts and connection points are illustrated. Apart from the screens 11 already presented in FIG. 4, which close off the lamella stacks to the front and rear and form the walls of the gas channels to the front and rear, lateral walls 18 of the gas channels arranged to the left and right in the illustration are also shown. At the bottom, an exhaust gas inlet 10 with funnel-shaped widening to a lower supporting frame 41 and an exhaust gas outlet 30 with a funnel-shaped tapering starting from an upper supporting frame 31 are also illustrated. The lower supporting frame 41 comprises three openings for admitting flushing gas into the areas separated by the seals. The upper supporting frame 31 has three outlet openings 32 for discharging the flushing gas. What are not illustrated in FIG. 6 are outer housing walls, which can connect at the bottom to the lower supporting frame 41 and at the top to the upper supporting frame 31 or can be guided through the respective frames.
[0105] Merely the lower supporting frame 41 is preferably fixedly connected to the stack of the basic units 14, whereas the upper supporting frame 31 merely limits the freedom of movement of the upper level of basic units 14 and enables a movement on account of thermal expansion and contraction. The cross-section through a heat sink 1 illustrated in FIG. 7, for example the heat sink of one of the basic units from FIG. 3 to FIG. 6, shows the liquid channels 5 running parallel to one another in the exemplary embodiment 8 and the two liquid channels 15 running perpendicularly thereto. What are not illustrated are the connections to the thermoelectric elements (likewise not illustrated), which in the illustration of FIG. 7 can be, or are arranged on the heat sink 1 in the foreground and in the background.
[0106] The second-lowest of the eight cooling liquid channels 5 running in parallel comprises an opening on the left in FIG. 7, which opening forms the inlet 25 for the cooling liquid. The second-uppermost of the eight cooling liquid channels 5 running in parallel has an opening illustrated on top right in FIG. 7, which opening forms the cooling liquid outlet 26. All other ends of the channels 5, 15 are closed, for example by stoppers. The use of stoppers makes it possible to produce the cooling liquid channels 5, 15 by drilling in a solid block. The fastening protrusions 13 are illustrated to the right and left, in each case at the top and bottom.
[0107] The arrangement illustrated in FIG. 8 and FIG. 9 has, as a variant to the arrangement illustrated in FIG. 1 and FIG. 2, block-like thermoelectric modules 300a, 300b. Similarly to FIG. 1 and FIG. 2, a heat sink 101a, 101b with a plurality of cooling liquid channels 105 running parallel to one another is illustrated on the right and left in the arrangement. A plurality of the block-like thermoelectric modules 300a, 300b is connected over the entire area to the heat sink 101 on the inwardly pointing flat surface of the heat sink 101, optionally in each case via a compensating element (not illustrated). A heat conducting body 109a, 109b is arranged on the inner side of the thermoelectric modules 300a, 300b respectively, which heat conducting bodies in the exemplary embodiment have fins 107a, 107b tapering in cross-section in their extent to their free ends. The protrusions 107 are interlaced with one another in a comb-like manner. A meandering intermediate space 116, which is part of the exhaust gas channel 106, is thus formed in the cross-section illustrated in FIG. 9.
[0108] The flow direction of the exhaust gas channel 106 runs in the illustration of FIG. 8 from bottom to top and in the illustration of FIG. 9 from rear to front perpendicularly to the plane of the drawing. A first seal is combined with the heat conducting body 109a and has two parts 102a, 102c. A second seal with parts 102b, 102d is combined with the second heat conducting body 109b. It can be seen in the illustration of FIG. 9 that the parts 102a, 102b are disposed on a side of the arrangement with the interlaced heat conducting bodies 109, and the parts 102c, 102d are disposed on the opposite side of the arrangement of the heat conducting bodies 109. Accordingly, in the illustration of FIG. 8, the end faces merely of the seal parts 102c, 102d can be seen. The first seal with the parts 102a, 102c separates the gas channel 106 from an area 100 disposed between the seal and the heat sink and in which the first thermoelectric modules 300a are disposed. Accordingly, the second seal with the parts 102b, 102d separates the gas channel 106 from an area 200 in which the second thermoelectric modules 300b are disposed. In this way, it is prevented that hot gas flowing in the gas channel 106 comes into direct contact with the thermoelectric module 300. As also in the case of the strip-like thermoelectric modules of the arrangement in FIG. 1 and FIG. 2, a higher temperature difference across the thermoelectric modules can therefore be achieved. Here, heat from the gas channel is introduced with high efficiency into the high-temperature side of the thermoelectric modules.
[0109] FIG. 10 shows a detail of a preferred variant of the arrangement illustrated in FIG. 2. The detail relates to the shaping of the heat conducting bodies in the local region formed by the exhaust gas channel and delimited by the seals 2a, 2b on opposite sides. Whereas the heat conducting bodies 7a, 7b in the embodiment illustrated in FIG. 2 are planar with a straight extent along the longitudinal axis (running from right to left and from left to right in FIG. 2), the heat conducting bodies 207a, 207b in the embodiment illustrated in FIG. 10 have an undulating extent in the longitudinal direction with wave crests and wave troughs. The wave crests and wave troughs run perpendicularly to the plane of the drawing of FIG. 10. Merely two first heat exchangers 207a and one second heat exchanger 207b are illustrated. The wave troughs and wave crests of the most closely adjacent first and second heat conducting bodies 207a, 207b run parallel to one another, so that the distance of the surfaces of the most closely adjacent heat conducting bodies 207 is constant or approximately constant in the extent of the heat conducting bodies 207 from the thermoelectric elements 3a, 3b to the free ends of the heat conducting bodies 207. The term approximately constant is understood to mean that there is no change in the spacing in the aforementioned extent that can be attributed to an offset in the longitudinal direction or a different shape of the wave troughs and wave crests. However, a change in spacing in particular can be provided in practice because the undulating end portions of the heat conducting bodies 207 are unintentionally twisted.
[0110] Since FIG. 10 depicts a detail, merely the three aforementioned heat conducting bodies 207, a portion of each of the seals 200a, 200b, and merely the high-temperature side end regions of the thermoelectric elements 3a, 3b are illustrated, to which the three illustrated heat conducting bodies 207 are secured. In practice, further thermoelectric elements and heat conducting bodies are provided in the manner illustrated in FIG. 2, wherein, however, the heat conducting bodies likewise have undulating end portions.
[0111] The undulating interlaced arrangement of the end portions of the heat conducting bodies leads to an increased surface of the heat conducting bodies compared to the shaping in FIG. 2 and thus to an improved transfer of heat from the hot gas to the heat conducting bodies. The flow of the hot gas in the exhaust gas channel, however, preferably is not turbulent on account of the wave shape of the heat conducting bodies.
[0112] The surface of the end portions of the heat conducting bodies is increased based on the longitudinal portion, i.e. the quotient of the heat transfer coefficient and the length of the end portion in the longitudinal direction is greater. This makes it possible to choose the length of the end portion to be smaller and still attain a good transfer of heat from the hot gas to the heat conducting bodies. If the flow of the hot gas through the gas channel is not turbulent, the flow resistance is merely slightly higher than in the case of the straight embodiment of the end portions of the heat conducting bodies in FIG. 2.
[0113] FIG. 11 shows a supporting structure 212 similar to that in FIG. 4, which can be arranged in the flow direction of the hot gas between two levels of basic units arranged in succession so as to support at least the basic units of one of the two levels on the supporting structure 212. The supporting structure 212 is made in particular from a metal sheet and has three (with the exception of additional depressions) approximately rectangular cutouts 213, 214, 215 for one of the levels, the levels, or for a transition of the levels. In the perspective illustration of FIG. 11, the flow direction for the hot gas runs from bottom to top.
