Patent Application
20150136192
According to a first aspect, the invention relates to a thermoelectric module. According to a second aspect, the invention relates to an assembly of at least two thermoelectric modules. According to a third aspect, the invention relates to a process for making a thermoelectric module.
A major part of energy waste in industrial and domestic applications takes the form of thermal energy. High temperature (>400° C.) rejected gases can be efficiently used. However, there is a big challenge in using rejected gases that have a lower temperature. Thermoelectric converters or thermal conversion modules using the Seebeck effect could be used in order to generate electric energy from such a source of thermal energy. Thermoelectric converters can be used in other applications; for instance they can be used to convert into electricity geothermal energy, heat from sun or heat from the seas. Using the Peltier effect, thermoelectric converters can also be used to generate heat or coldness from electricity.
US2011/0226304 discloses a thermoelectric module that comprises p-type and n-type elements that are bonded to each others. In order to add the electric voltages induced in each of the n-type and p-type elements by the Seebeck effect, each p-type element is bonded to an n-type element with an electrically insulating material in a second region of a junction surface joining each p-type and n-type element. Hence, the p-type and n-type elements of this thermoelectric module are electrically connected in series whereas they are thermally connected in parallel. The fabrication of such a thermoelectric module is not simple as it requires the presence of such an electrically insulating material between each p-type and n-type element along a part of the junction surface joining each p-type and n-type element. Moreover, such a configuration is not adapted when one aims at having a thermoelectric module having the form of a layer or a plate with a small thickness because of the two following reasons. First, the electric voltage takes place over the thickness of each p-type or n-type element of the thermoelectric module in this case. If the thickness is small one cannot hope to have a large temperature difference between the upper and the lower surfaces of the thermoelectric module, and so to have a large induced electric voltage. Second, one needs to insert along a part of the junction surface joining each p-type and n-type element said electrically insulating material. Such an insertion renders the process of fabrication quite elaborate when the thickness of the thermoelectric module decreases.
FIG. 12 of WO2005/020340 proposes another geometry of a thermoelectric module. In this case, n-type and p-type elements are bonded to each other through shunts. As one can learn from line 23 of page 27, some shunts are cooled whereas others are heated. A thermal flux having a component parallel to the arrow 1209 of FIG. 12 takes place in the p and n-type elements. Contrary to the geometry of US2011/0226304, the electric current follows a straight line in this case and crosses the shunts separating each p-type and n-type elements. The shunts of this geometry are used to transport heat. Such shunts are not adapted if one aims at having a thermoelectric module that has the form of a plate or a layer. Indeed, the shunts of FIG. 12 extend at each side of the p and n-type elements in order to carry heat. The shunts shown in this FIG. 12 significantly increase the global thickness of the thermoelectric module. As known by the one skilled in the art, a thermoelectric module having the form of a layer has various advantages. One of them is to have a compact thermoelectric module that can be inserted around a chimney for instance. The presence of a shunt between each n-type and p-type element significantly increases the difficulty of processing a thermoelectric module having the form of a layer. Moreover, the shunts of the thermoelectric module of FIG. 12 of WO2005/020340 induce electric contact resistances along the way of the electric current induced by the electric voltage due to the Seebeck effect. Such electric contact resistances appear as there are different contacts between different elements (between the shunts and the p/n-type elements). Because of these electric contact resistances, there is a loss of energy in the thermoelectric module and finally a loss of efficiency of the thermoelectric module.
It is an object of the present invention to provide a thermoelectric module that can take the form of a layer and that can be processed more easily with respect to known thermoelectric conversion modules. To this end, the inventors propose a thermoelectric module comprising:
a thermoelectric layer of substantially constant thickness, t, comprising one p-type portion and one n-type portion, both portions presenting together an upper and a lower main surfaces separated by said thickness t and extending over the whole thickness t of the thermoelectric layer;
a first and a second thermal resistor elements in thermal contact with said lower main surface;
a first thermal bridge element in thermal contact with said lower main surface, between and adjacent to the first and second thermal resistor elements;
such that said first and second thermal resistor elements and said first thermal bridge element cover the whole lower main surface, and such that said first thermal bridge element is globally able to transfer heat between said thermoelectric layer and the surrounding environment at a substantially higher rate than the first and second thermal resistor elements.
The thermoelectric module of the invention is characterized in that:
said p-type and said n-type portions are adjacent and directly coupled by an interface; and in that
said first thermal bridge element spans at least over the orthogonal projection of said interface onto said lower main surface.
The thermoelectric module of the invention can take the form of a layer or a plate; this means that one dimension (the thickness) of the thermoelectric module can be much smaller than the other dimensions. Indeed, the thermoelectric layer, the first and second thermal resistor elements and the first thermal bridge element can each have a small thickness resulting in a thermoelectric module of small thickness. Even by choosing a small thickness t of the thermoelectric layer, one can obtain a non negligible electric voltage generated by the thermoelectric module. Indeed, from the specific position of the first thermal bridge element with respect to the first and second thermal resistor elements, a thermal flux that has a component perpendicular to the thickness t of the thermoelectric layer is induced in it if this thermoelectric layer is subjected to a temperature gradient between its upper and lower main surfaces. Hence, there will be an induced electric voltage in the thermoelectric layer having a component that is perpendicular to its thickness t. Because of the specific position of the first thermal bridge element with respect to the first and second thermal resistor elements, the component of the thermal flux that is perpendicular to the thickness t of the thermoelectric layer has opposite directions in the p-type and n-type portions when the thermoelectric layer is subjected to a temperature gradient between its lower and upper parts (the lower part is adjacent to the lower main surface of the thermoelectric layer and the upper part is adjacent to the upper main surface of the thermoelectric layer). Hence, the electric voltages that are generated in the p-type and n-type portions have a component perpendicular to the thickness t of the thermoelectric layer with a same direction. Finally, the electric voltages perpendicular to the thickness t generated in the p-type and n-type portions add and do not subtract to each other. Preferably, the first and second thermal resistor elements are continuous as well as the first thermal bridge element.
Contrary to the thermoelectric modules disclosed in US2011/0226304 and in WO2005/020340, neither any electrically insulating material nor shunt is needed between the p-type and n-type portions (elements) for the thermoelectric module of the invention. In particular, the p-type and n-type portions of the thermoelectric module of the invention are in contact all along the interface that crosses the substantially constant thickness t: in other words, there is no intermediate material that links the p-type and n-type portions in the thermoelectric module of the invention. Hence, its process of fabrication is easier or simplier. The global geometry of the thermoelectric module is simple and is adapted to take the form of a plate or a layer. This can reduce the cost of fabrication of the thermoelectric module of the invention with respect to known thermoelectric conversion modules. As mentioned before, a thermoelectric module having the form of a layer has other advantages such as to obtain a compact thermoelectric module that can be easily inserted around a chimney for instance.
Contrary to the thermoelectric modules proposed in US2011/0226304 and in WO2005/020340, one can imagine a continuous process of fabrication for making the thermoelectric module of the invention. By locally changing the doping of a thermoelectric layer, one can obtain the p-type and n-type portions of the thermoelectric layer. A continuous process of fabrication allows one to obtain thermoelectric modules of large dimensions. Another advantage of a continuous process of fabrication is to reduce the costs of fabrication. The thermoelectric module of the invention has other advantages. Neither wire nor conductor is needed for electrically connecting the p-type and n-type portions. An electric current is indeed able to flow through the interface between the p-type and n-type portions. As there is no shunt between p-type and n-type portions, there are no corresponding electric contact resistances between such shunts and the p-type/n-type portions (contrary to the thermoelectric module disclosed in WO2005/020340). The absence of electric contact resistances can lead to an increase of efficiency up to 20% of the thermoelectric module. Thanks to the geometry of the thermoelectric module of the invention, a large number of such thermoelectric modules can be easily assembled from a continuous fabrication process, resulting in a large thermoelectric converter that can take the form of a plate or of a cylinder for instance. Hence, the thermoelectric module of the invention allows obtaining large, low cost thermoelectric converters.
Preferably, the first thermal bridge element has a third thermal conductivity, κ3, that is higher than the thermal conductivities of said first and second thermal resistor elements (1r, 2r), κ1 and κ2.
As the first thermal bridge element has a higher thermal conductivity than the first and second thermal resistor elements, it is able to transfer heat between the thermoelectric layer and the surrounding environment at a substantially higher rate than the first and second thermal resistor elements do.
Preferably, said p type portion and said n type portion have substantially the same volume, and each of said first and second thermal resistor elements covers at least 30% of said lower main surface.
As each of the first and second thermal resistor elements covers at least 30% of the lower main surface of the thermoelectric layer, the first thermal bridge element covers maximum 40% of the lower main surface of the thermoelectric layer. Hence, if the thermoelectric module is subjected to a temperature gradient between its upper and lower parts, a significant percentage of the heat flux traversing the thermoelectric layer will have to follow a trajectory having a component perpendicular to the thickness t of the thermoelectric layer. Such an embodiment increases the component of the generated electric voltage in a direction perpendicular to the thickness t of the thermoelectric layer.
Preferably, the thermoelectric layer has a breadth B; and the p-type and n-type portions extend over substantially the whole breadth B.
Preferably, the thickness t of the thermoelectric layer is comprised between 1 mm and 10 mm. Such a value of the thickness t of the thermoelectric layer is a good comprise between having a significant temperature gradient between the upper and lower main surfaces of the thermoelectric layer and the possibility of having a compact thermoelectric module of small thickness.
However, a larger or a smaller thickness t is possible. As an example, the thickness t can be comprised between 1 and 100 μm, and is preferably equal to 10 μm.
Preferably, the thermoelectric module further comprises:
wherein:
Preferably, said third thermal resistor element has a fourth thermal conductivity, κ4, that is lower than the thermal conductivities of said second and third thermal bridge elements, κ5 and κ6.
Preferably, said third thermal resistor element covers at least 50% of the orthogonal projection of said first thermal bridge element on said upper main surface.
Preferably, said third thermal resistor element covers at least 100% of the orthogonal projection of said first thermal bridge element on said upper main surface.
In this embodiment no thermal flux can cross directly the thermoelectric layer along a direction that is parallel to its thickness t. Indeed, the third thermal resistor element covers at least 100% of the orthogonal projection of the first thermal bridge element. This ensures having a large component of the thermal flux that is perpendicular to the thickness t of the thermoelectric layer, and finally obtaining a generated electric voltage that has a large component perpendicular to the thickness t of the thermoelectric layer.
Preferably, said first, second and third thermal resistor elements comprise a same thermally insulating material of thermal conductivity κr, and said first, second and third thermal bridge element elements comprise a same thermally conductive material of thermal conductivity κc, and such that κr<κc. The fabrication of this embodiment of the thermoelectric module is still easier as the first, second, and third thermal resistor elements comprise a same thermally insulating material, and as the first, second, and third thermal bridge elements comprise a same thermally conductive material.
Preferably, said thermoelectric layer comprises a Fe, V, Al-based material.
Choosing a thermoelectric layer comprising a Fe, V, Al-based material presents different advantages. First, iron (Fe) and aluminium (Al) are relatively cheap elements that are available in large quantity. Adding vanadium (V) to the Fe and Al elements allows obtaining an alloy with a relatively high Seebeck coefficient and that can be doped for obtaining p-type and n-type portions. There exist different doping processes for a Fe2VAl material.
According to a second aspect, the invention relates to an assembly of a first and a second thermoelectric modules where:
According to a third aspect, the invention relates to a method for making a thermoelectric module and comprising the following steps:
Preferably, the multilayer material is formed in step (a) by a roll bonding process.
Preferably, the multilayer material is formed in step (a) by performing the following steps:
These and further aspects of the invention will be explained in greater detail by way of examples and with reference to the accompanying drawings in which:
The figures are not drawn to scale. Generally, identical components are denoted by the same reference numerals in the figures.
The thermoelectric module 10 of the invention further comprises a first and second thermal resistor elements, 1r and 2r, that are in thermal contact with the lower main surface 12 of the thermoelectric layer 15. A first thermal bridge element 3c is also in thermal contact with this lower main surface 12 of the thermoelectric layer 15. This first thermal bridge element 3c is adjacent to and positioned between the first and second thermal resistor elements, 1r and 2r. These three elements (1r, 3c, 2r) cover the whole lower main surface 12 of the thermoelectric layer 15 as shown in
S=ΔU/ΔT (Eq. 1),
where ΔU represents the voltage generated by a temperature difference ΔT in the material. As defined by the one skilled in the art, S is negative for n-type materials and positive for p-type materials. Hence, the induced voltages in the p-type 7p and n-type 7n portions of the thermoelectric layer of
Different reasons can cause that the first thermal bridge element 3c is able to transfer heat between the thermoelectric layer 15 and the surrounding environment at a substantially higher rate than the first and second thermal resistor elements (1r, 2r) do. These different reasons are linked to different preferred embodiments of the thermoelectric module 10 of the invention that present different thermal properties. In a preferred embodiment, this rate difference in heat transfer is due to different thermal conduction properties of the first and second thermal resistor elements (1r, 2r) and the first thermal bridge element 3c: in this preferred embodiment, the first thermal bridge element 3c comprises a material that has a third thermal conductivity, κ3, that is higher than the thermal conductivities of the first and second thermal resistor elements (1r, 2r), κ1 and κ2. Hence, in this preferred embodiment, κ3>κ1, and κ3>κ2. Preferably, the first and second thermal resistor elements (1r, 2r) comprise aluminium oxide, glass (silicate), or polymer. Preferably, first and second thermal resistor elements (1r, 2r) are electrical insulators which means that they have an electric resistivity higher than 1010 Ω*m, more preferably higher than 1012 Ω*m, and still more preferably higher than 1016 Ω*m. Preferably, the first thermal bridge element 3c comprises a material that is an electrical conductor such as copper. Using a first thermal bridge element 3c comprising a material that is an electrical conductor allows having a higher generated power. Internal resistance of the generator is reduced in this case.
In another preferred embodiment, the rate difference in heat transfer is due to different thermal radiation properties of the first and second thermal resistor elements (1r, 2r) and the first thermal bridge element 3c. So, in this preferred embodiment, the first thermal bridge element 3c is able to transfer heat by radiation between the thermoelectric layer 15 and the surrounding environment at a higher rate than the first and second thermal resistor elements (1r, 2r) do.
In a still preferred embodiment, the first thermal bridge element 3c is able to transfer heat between the thermoelectric layer 15 and the surrounding environment at a higher rate than the first and second thermal resistor elements (1r, 2r) do because of different convection properties. In this preferred embodiment, the first thermal bridge element 3c preferably comprises air and the first and second thermal resistor elements (1r, 2r) are typically solid elements that are also preferably good thermal insulators then also reducing heat transfer by conduction and radiation. Hence, one can combine different conduction, radiation and convection properties of the first thermal bridge element 3c and the first and second thermal resistor elements (1r, 2r) in order to obtain the desired property that the first thermal bridge element 3c is able to transfer heat between the thermoelectric layer 15 and the surrounding environment at a higher rate than the first and second thermal resistor elements (1r, 2r) do.
Different processes can be used for joining the first thermal bridge element 3c and the first and second thermal resistor elements (1r, 2r) to the thermoelectric layer 15. The elements (3c,1r,2r) can be deposited on the thermoelectric layer 15 by lithography or any other printing technique for instance. When oxides are used as thermal insulators for the first and second thermal resistor elements (1r, 2r), one can obtain such layers by local oxidation for instance. Alternatively the first thermal bridge element 3c and the first and second thermal resistor elements (1r, 2r) can be joined to the thermoelectric layer 15 by gluing. Such techniques are known by the one skilled in the art.
Preferably, the p-type 7p and n-type 7n portions extend over substantially the whole breadth B of the thermoelectric layer. The breadth B can be non constant. This preferred embodiment is shown in
Values of different parameters of the thermoelectric module 10 that result from optimization calculations are now presented. These preferred values can lead to a higher efficiency of the thermoelectric module 10 of the invention. Preferably, the thickness t of the thermoelectric layer 15 is comprised between 1 and 10 mm. More preferably, thickness t of the thermoelectric layer 15 is equal to 8 mm. However, a larger or a smaller thickness t is possible. As an example, the thickness t can be comprised between 1 and 100 μm, and is preferably equal to 10 μm. Preferably, the lengths Lp and Ln of the p-type 7p and of the n-type 7n portions (see
Preferably, the third thermal resistor element 4r has a fourth thermal conductivity κ4, that is lower than the thermal conductivities of the second and third thermal bridge elements (5c, 6c), κ5 and κ6. Preferably, the third thermal resistor element 4r covers at least 50% (and more preferably at least 100%) of the orthogonal projection of the first thermal bridge element 3c on the upper main surface 11 of the thermoelectric layer 15. Preferably, the first, second and third thermal resistor elements (1r, 2r, 4r) comprise a same thermally insulating material of thermal conductivity κr, and the first, second and third thermal bridge element elements (3c, 5c, 6c) comprise a same thermally conductive material of thermal conductivity κc, and such that κr<κc. Such a thermally insulating material is preferably aluminium oxide, glass (silicate), or polymer. Preferably, the first, second, and third thermal resistor elements (1r,2r,4r) comprise a material that is an electric insulator which means that it presents an electric resistivity higher than 1010 Ω*m, more preferably higher than 1012 Ω*m, and still more preferably higher than 1016 Ω*m.
Preferably, the thermal gradient imposed between the upper and lower main surfaces (11,12) of the thermoelectric layer 15 is equal to 400° C.
Preferably, the thermoelectric layer 15 of any of the preferred embodiments described above comprises a Fe, V, Al-based material. Such a material is less brittle than other thermoelectric materials such as ceramics. Fe, V, Al-based material also presents the advantage of having a relatively high electric conductivity while presenting a moderate thermal conductivity. By an appropriate doping of such a Fe, V, Al-based material, one can obtain p-type and n-type regions. The doping of the Fe, V, Al-based material also allows increasing the absolute value of the Seebeck coefficient, S. For obtaining a n-type (respectively p-type) portion, one can add silicon (respectively titanium) to a Fe2VAl material for instance. For obtaining a Fe, V, Al-based thermoelectric layer with n-type 7n and p-type 7p portions, one can also locally induce local stoichiometry variations in the alloy Fe2VAl.
According to a second aspect, the invention relates to an assembly 20 of a first 10a and a second 10b thermoelectric modules, as shown in
Even if the geometries shown in
The thermoelectric module 10 or the assembly 20 of the invention can be used for generating electric power from a temperature difference or for generating a temperature difference from electric power (Peltier effect).
According to a third aspect, the invention relates to a method for making a thermoelectric module 10 (
Different methods can be used in step (a) for forming the multilayer material 95. Two examples of such methods are detailed below.
Once the multilayer material 95 has been formed in step (a), a heat treatment is applied to it. Such a heat treatment allows obtaining a mixing of the different materials of the different layers of the multilayer material 95 by diffusion. Such heat treatments are commonly used by the one skilled in the art when one aims at obtaining a material having homogeneous properties. Preferably, the heat treatment of the multilayer material 95 is carried out under controlled atmosphere. More preferably, an atmosphere comprising 95% of argon and 5% of hydrogen is used. As known by the one skilled in the art, a heat treatment under controlled atmosphere allows preventing oxidation of the multilayer material 95.
When one aims at obtaining a thermoelectric layer 15 comprising a Fe, V, Al-based material, a multilayer material 95 comprising a layer of each of such material (Fe, V, Al) is made in a first time (step (a)). Then, a heat treatment is applied to the obtained multilayer material 95 with the following preferred parameters. Temperatures higher than 1400° C. are preferably used during the heat treatment of step (b). Such high temperatures allow obtaining a good diffusion of each material through the whole thickness of the multilayer material 95 formed in step (a). The duration of the heat treatment of step (b) when Fe, V, and Al materials are used for forming the multilayer material is preferably comprised between 1 and 17 hours, depending on the thickness of the multilayer material 95. More preferably, if the thickness of the multilayer material is larger than 1.5 mm, duration longer than 17 hours is used for still increasing the homogeneity of the resulting Fe2VAl phase.
A local doping of the multilayer material 95 is preferably carried out in order to obtain a thermoelectric layer 15 having different portions with different doping levels. When a thermoelectric layer 15 comprising Fe, V, and Al is used, such a doping process allows significantly increasing the thermoelectric properties of the thermoelectric layer 15, see for instance H. Matsuura, et al. in “Doping effects on thermoelectric properties of the pseudogap Fe2VAl system”, J. Japan Inst, Metals, Vol. 66, No. 7, pp. 767-771, (2002). When the thermoelectric layer 15 comprises Fe, V, and Al, adding silicon (Si) allows obtaining n-type portions whereas adding titanium (Ti) allows obtaining p-type portions. Other doping elements could be used. The doping of the thermoelectric layer 15 is preferably carried out during the heat treatment of step (b) of the method of the invention. However, the doping of the thermoelectric layer 15 can be carried out before or after the heat treatment of step (b) of the method of the invention. The one skilled in the art generally names such a doping process as doping by diffusion.
The different p-type and n-type portions of the thermoelectric layer 15 can also be induced by local stoichiometry variations of the material of the thermoelectric layer 15. If one slightly and locally changes the stoichiometry of the alloy Fe2VAl by locally modifying the concentrations of Fe, V, and Al, p-type or n-type portions of this alloy can be obtained. Local modifications of the elements concentration of this alloy can be achieved by providing before step (a) layers of different materials having a predetermined shape.
In step (c) of the method of the invention, the first and second thermal resistor elements (1r, 2r), and the first thermal bridge element 3c are coupled to the thermoelectric layer 15. When the thermoelectric module 10 further comprises a third thermal resistor element 4r, and second and third thermal bridge elements (5c,6c), step (b) of the method of the invention also includes the coupling of these last elements to one main surface of the thermoelectric layer 15. As explained above, different techniques known by the one skilled in the art can be used for coupling the elements (1r, 2r, 3c) and possibly also the elements (4r,5c,6c) to the thermoelectric layer 15. Examples of such techniques are: printing (such as lithography), local oxidation techniques, and gluing.
Preferably, a roll bonding process that is known by the one skilled in the art is used for step (a) of the method of the invention. Roll bonding is a process that allows joining different layers by stacking them and after by passing and pressing the obtained layup (or stack) between rolls 100.
Preferably, a method derived from Friction Stir Processing (FSP) is used for step (a). This method is preferably used when the multilayer material 95 of step (a) has three layers and comprises the following steps.
Preferably, the rotating tool 70 has a cylindrical shape. In this case, its axis of revolution is preferably tilted of an angle α with respect to a vertical axis VA that is perpendicular to the upper surface of the first layer 50. Preferably, in order to increase the heat that is generated, the rotating tool 70 penetrates the first layer 50 by about 100 μm when it is translated over the friction portion of the upper surface of the first layer 50. Preferably, the speed of rotation of the rotating tool 70 is comprised between 500 and 3000 revolutions per minute. More preferably, this speed of rotation is equal to 2000 revolutions per minute. Preferably, the rotating tool 70 is made of a material comprising cemented carbide (K20), or tungsten carbide. Preferably, the rotating tool 70 is translated over the friction portion of the upper surface of the first layer 50 with a speed comprised between 50 and 400 mm/min.
The present invention has been described in terms of specific embodiments, which are illustrative of the invention and not to be construed as limiting. More generally, it will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and/or described hereinabove. Reference numerals in the claims do not limit their protective scope. Use of the verbs “to comprise”, “to include”, “to be composed of”, or any other variant, as well as their respective conjugations, does not exclude the presence of elements other than those stated. Use of the article “a”, “an” or “the” preceding an element does not exclude the presence of a plurality of such elements.
Summarized, the invention may also be described as follows. According to a first aspect, the invention relates to a thermoelectric module 10 that comprises a thermoelectric layer 15 comprising one p-type 7p and one n-type 7n portions presenting together an upper and a lower main surfaces (11,12). The thermoelectric module 10 further comprises a first and a second thermal resistor elements (1r, 2r), and a first thermal bridge element 3c between and adjacent to the first and second thermal resistor elements (1r, 2r). The first and second thermal resistor elements (1r, 2r) and the first thermal bridge element 3c cover the whole lower main surface 12. The p-type 7p and the n-type 7n portions are adjacent and directly coupled by an interface 7i. The first thermal bridge element 3c spans at least over the orthogonal projection of the interface 7i on the lower main surface 12. According to a second aspect, the invention relates to an assembly 20 of such thermoelectric modules 10. According to a third aspect, the invention relates to a method for making such thermoelectric modules 10.
| Number | Date | Country | Kind |
|---|---|---|---|
| 12166123.5 | Apr 2012 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/058897 | 4/29/2013 | WO | 00 |
