Abstract:
A thermoelectric generator including a membrane maintained by lateral ends and capable of taking a first shape when its temperature reaches a first threshold and a second shape when its temperature reaches a second threshold greater than the first threshold; and mechanism capable of converting the motions and the deformations of the membrane into electricity.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of French patent application Ser. No. 09/57106, filed on Oct. 12, 2009, entitled “THERMOELECTRIC GENERATOR,” which is hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thermoelectric generator, that is, a device capable of generating electricity from a heat source. It especially aims at the exploitation of the thermal power generated by certain types of equipment, for example, electronic boxes or car mufflers. 
     2. Discussion of the Related Art 
     In certain mobile devices, for example, telephones, watches, or pacemakers, piezoelectric microgenerators have been used to generate electricity from the mechanical vibrations resulting from the user&#39;s movements. This enables to at least partially recharge the device batteries. A disadvantage of such a solution is that it cannot be used in the case of fixed equipment, for example, a television set. 
     Devices capable of directly converting heat into electricity by the Seebeck effect have also been provided. It has indeed been observed that a potential difference appears at the junction of two conductive materials submitted to a temperature difference. However, such devices have a very low efficiency. In practice, the use of the Seebeck effect is mainly limited to temperature measurement applications. 
     SUMMARY OF THE INVENTION 
     Thus, an object of an embodiment of the present invention is to provide a thermoelectric generator overcoming at least some of the disadvantages of prior art solutions. 
     An object of an embodiment of the present invention is to provide such a generator having a high efficiency. 
     An object of an embodiment of the present invention is to provide such a generator which is easy to manufacture. 
     An object of an embodiment of the present invention is to provide such a generator which is easy to integrate in conventional equipment. 
     Thus, an embodiment of the present invention provides a thermoelectric generator comprising: a membrane maintained by lateral ends and capable of taking a first shape when its temperature reaches a first threshold and a second shape when its temperature reaches a second threshold greater than the first threshold; and means capable of converting the motions and the deformations of the membrane into electricity. 
     According to an embodiment of the present invention, the conversion means comprise a region made of a piezoelectric material connected to output terminals of the generator. 
     According to an embodiment of the present invention, the membrane, in its first shape, is capable of being in contact with a hot wall and, in its second shape, is capable of being in contact with a cold wall. 
     According to an embodiment of the present invention, the generator further comprises a package comprising first and second superposed portions delimiting a cavity, the membrane being arranged in the cavity and attached to the package by its lateral ends, wherein the membrane, in its first shape, comes in contact with the first portion of the package and, in its second shape, comes in contact with the second portion of the package. 
     According to an embodiment of the present invention, the membrane, in its first shape, takes the shape of the inner surface of the first portion of the package and, in its second shape, takes the shape of the inner portion of the second portion of the package. 
     According to an embodiment of the present invention, the membrane comprises a bimetallic support strip bearing against lateral ends on springs. 
     According to an embodiment of the present invention, the membrane comprises a support layer made of a shape-memory material. 
     According to an embodiment of the present invention, the shape-memory material is a nickel and titanium alloy. 
     According to an embodiment of the present invention, said region of a piezoelectric material has the shape of a piezoelectric layer deposited on the membrane. 
     According to an embodiment of the present invention, the membrane comprises, superposed in the following order, a layer of a material having a first thermal expansion coefficient, and a layer of a material having a second thermal expansion coefficient much greater than the first coefficient, the piezoelectric layer being deposited on the layer having the second coefficient. 
     According to an embodiment of the present invention, the thickness of the layer of a material having a first thermal expansion coefficient is greater than the thickness of the piezoelectric layer. 
     According to an embodiment of the present invention, the first thermal expansion coefficient is smaller than that of the piezoelectric material. 
     According to an embodiment of the present invention, the generator is capable of being assembled on an integrated circuit chip. 
     According to an embodiment of the present invention, the generator is capable of being assembled on a hot surface of a car such as a car muffler or a braking device. 
     According to an embodiment of the present invention, the generator is capable of being assembled in a battery charger operating from a heat source such as a heater or a cooking plate. 
     The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified cross-section view of an embodiment of a thermoelectric generator; 
         FIG. 2  is a simplified perspective view of a mobile membrane comprised in the thermoelectric generator of  FIG. 1 ; 
         FIG. 3  is a diagram schematically illustrating the operating principle of the thermoelectric generator described in relation with  FIGS. 1 and 2 ; 
         FIG. 4  is a simplified cross-section view of an example of a mobile membrane comprised in a thermoelectric generator; 
         FIGS. 5   a  and  5   b  are simplified cross-section views illustrating alternative embodiments of a portion of the membrane of  FIG. 4 ; 
         FIG. 6  is a simplified cross-section view of an alternative embodiment of a thermoelectric generator, 
         FIG. 7  is a simplified cross-section view of an embodiment of a thermoelectric generator in which the generator is assembled on an integrated circuit; 
         FIG. 8  is a simplified cross-section view of an embodiment of a thermoelectric generator in which the generator is assembled on a car muffler or car braking device within a car; and 
         FIG. 9  is a simplified cross-section view of an embodiment of a thermoelectric generator in which the generator is assembled on a heater or cooking plate within a battery charger. 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, the various drawings are not to scale. 
     An aspect of an embodiment of the present invention is to provide a device capable of:
         converting thermal power into mechanical power by means of a membrane mobile between two positions provided to change shape when its temperature varies; and   converting the mechanical power generated by the membrane into electric power.       

       FIG. 1  is a simplified cross-section view of an embodiment of a thermoelectric generator. This generator comprises a membrane  1  attached by its lateral ends to a support  3 . In this example, membrane  1  has, in top view, a rectangular shape with a length on the order of a few centimeters and a width of a few centimeters. However, the membrane may have any shape or size. As an example, the membrane may have a circular shape, support  3  then being ring-shaped. 
     Membrane  1  is provided to alternate between two shapes when its temperature varies. In its first shape, membrane  1  is bent downwards, as shown in  FIG. 1 . This first shape corresponds to an idle state, taken by the membrane when its temperature is lower than a first threshold T 1 . In its second shape, the membrane is bent upwards. As an example, the second shape may be substantially symmetrical to the first shape with respect to the horizontal plane of symmetry of support  3 . The second shape is taken by the membrane when its temperature is greater than a second threshold T 2  greater than or equal to T 1 . A membrane operating according to a hysteresis cycle is here considered, that is, second threshold T 2  is greater than first threshold T 1 . 
     Membrane  1  may comprise a support layer made of a shape-memory material, for example, a nickel and titanium alloy. Such a material comprises two crystal phases and is capable of having two stable positions according to its temperature. 
     According to another embodiment, membrane  1  may comprise a bimetallic strip formed of two superposed layers of different metals having different thermal expansion coefficients. Due to the difference between thermal expansion coefficient, such a strip bends in a first direction or in a second direction when its temperature varies. However, conversely to the case of a shape-memory material, the deformation of a bimetallic strip is progressive, and not abrupt. Thus, to impose two shapes to the membrane, as described hereabove, and a fast transition from one shape to the other, springs  9   a ,  9   b , schematized in  FIG. 1  by the torsion of the lateral ends of the membrane, are provided. 
     It is provided to arrange membrane  1  between two opposite walls  5  and  7 . Lower wall  5  is a hot wall, for example a metal wall of a car muffler or of car brakes, a package of an integrated circuit chip, or any other hot surface of an integrated circuit chip or another electronic circuit. Upper wall  7  is a cold wall, that is, its temperature is substantially lower than that of the hot wall. It may be a radiator, an external casing of an electronic device, or simply ambient air. The arrangement is selected so that the membrane comes, in its first shape, in contact with hot wall  5 , and in its second shape, in contact with cold wall  7 . 
       FIG. 2  is a simplified perspective view of a portion of membrane  1  of  FIG. 1 . As described hereabove, membrane  1  comprises a support layer  11  capable of alternating between two shapes according to its temperature. At the surface of support  11 , a layer  13  of a piezoelectric material is provided. 
     When the temperature varies, the shape changes of support  11  and the phenomena of thermal expansion/compression of support  11  create mechanical stress in piezoelectric layer  13 . Such stress, shown in the drawings by arrows S, are mainly exerted along directions tangent to the membrane. In this example, the piezoelectric material is pre-biased so that the electric field created in the material is orthogonal (perpendicular to the membrane surface) to stress S (tangential to the membrane surface) exerted on the material. This generates a potential difference V + −V −  between the upper surface and the lower surface of piezoelectric layer  13 . 
     It is provided to form contact terminals connected to the upper and lower surfaces of the piezoelectric layer. For this purpose, an upper metal electrode (not shown) arranged at the surface of piezoelectric layer  13  may for example be provided. Support layer  11  of the membrane being metallic, it may be used as a lower electrode. It may then be provided to form output terminals of the generator, connected to the lower and upper electrodes at the level of the non-mobile ends of the membrane, as shown in  FIG. 1 . 
       FIG. 3  is a diagram schematically illustrating the operating principle of the thermoelectric generator described in relation with  FIGS. 1 and 2 . The left-hand portion of the diagram illustrates the variation of mechanical stress S in the piezoelectric layer according to temperature T of the membrane. The right-hand portion of the diagram illustrates the variation of voltage V at the output terminals of the generator along time. 
     In an initial phase P 0 , the heat source is not established and the membrane temperature is lower than T 1 . The membrane then is in its first shape, in contact with the hot wall (in this case, cold) of the generator. When the system starts heating up, the temperature of the membrane, in contact with the hot wall, increases progressively. 
     A steady operating state is considered hereafter, that is, where the hot and cold walls of the thermoelectric generator are at substantially constant temperatures, respectively Th and Tc, with Tc&lt;T 1 &lt;T 2 &lt;Th. The generator then periodically follows a cycle P 1 -P 2 -P 3 -P 4 . 
     At point P 1 , the membrane temperature is equal to the first threshold, T 1 , and the membrane is in its first shape, in contact with the hot wall. The membrane temperature progressively increases, between points P 1  and P 2 . This results in a progressive variation of the stress in the piezoelectric material, due to the expansion of the metal support layer of the membrane. This causes a progressive variation of the output voltage of the generator. 
     At point P 2 , the membrane temperature reaches the second threshold, T 2 , at which its shape changes. The membrane then abruptly takes its second shape and comes into contact with the cold wall of the generator. This results in an abrupt variation of the mechanical stress in the piezoelectric layer, causing an abrupt variation of the voltage at the output terminals of the generator. 
     Then, from point P 3 , the temperature of the membrane, in contact with the cold wall, progressively decreases, thus causing a progressive variation of the stress in the piezoelectric material. This results in a progressive variation of the voltage at the output terminals of the generator. 
     At point P 4 , the membrane temperature reaches the first threshold, T 1 , at which its shape changes. The membrane then abruptly recovers its first shape and comes into contact with the hot wall of the generator. This results in an abrupt variation of the stress in the piezoelectric layer, causing an abrupt variation of the voltage at the output terminals of the generator. The cycle then resumes from phase P 1 . 
     Thus, the thermal power of the hot wall is transferred by the membrane to the cold wall by increments. This transfer causes displacements of the membrane and phenomena of expansion/compression of the membrane. The mechanical power associated with such phenomena is partially converted into electric power. 
     An advantage of the provided system is that it enables to use not only the mechanical stress linked to the shape changes of the membrane but also the mechanical stress linked to the expansion/compression of the membrane to actuate the piezoelectric material. 
     To optimize the efficiency of the generator, it is desirable for temperature difference Th−Tc between the hot and cold walls to be maximum. It is also preferable for the temperature difference between thresholds T 1  and T 2  at which the membrane changes shape to be large. As an example, if temperatures Th and Tc of the hot and cold walls respectively are 80 and 30° C., a membrane with respective shape change thresholds T 1  and T 2  at 40 and 70° C. may be provided. 
     To improve the thermal contact between the membrane and the hot and cold walls, an oil film or a thermally-conductive paste may if necessary be provided on each of the walls. Further, it is preferable for the membrane to have a low heat capacity to promote the frequency of heat transfers between the hot wall and the cold wall. 
     Further, at least one material having a large thermal expansion coefficient will preferably be selected to form the membrane support layer. 
     Further, the mass of the membrane (support layer+piezoelectric layer) should be sufficiently low to avoid slowing down or disturbing the displacements of the membrane. As an example, the thickness of the metal support layer may approximately range from 0.5 to 1 mm and that of the piezoelectric layer may range from 0.1 to 0.2 mm. 
       FIG. 4  is a simplified cross-section view of an embodiment of a membrane mobile between two positions of a thermoelectric generator of the type described in relation with  FIGS. 1 to 3 . In this example, the membrane comprises a bimetallic support comprising a lower layer  21  of a first metal having a very low expansion coefficient, for example, Invar, and an upper layer  23  of a second metal having a high thermal expansion coefficient. The second metal may be a shape-memory material with a high thermal expansion coefficient, for example a nickel and titanium alloy. If the second metal is not a shape-memory material, lateral springs capable of ensuring the hysteresis-type behavior of the thermal cycle may be provided. At the surface of the bimetallic support, on the side opposite to the layer of low expansion coefficient, a piezoelectric layer  25  is provided. In this example, the piezoelectric layer is covered with a metal electrode  27  capable of being connected to an output terminal V −  of the generator. Output terminal V +  may be directly connected to bimetallic support  21 ,  23 . 
     According to an advantage of this embodiment, the positioning of the piezoelectric layer on the side of the bimetallic support opposite to invariant layer  21 , enables to amplify, in the piezoelectric layer, the mechanical stress linked to phenomena of expansion/compression of the metal support. 
     In the case where layer  23  is made of a shape-memory material, when the temperature increases, layer  23  tends to expand and especially to lengthen. Due to its low thermal expansion coefficient, the piezoelectric layer tends to lengthen less than support layer  23 . Thus, in the absence of invariant layer  21 , the membrane tends to progressively bend during the progressive temperature increase phase. This bending promotes a decrease of the stress exerted in the piezoelectric material since it tends to restrict the elongation (forced by the support layer) of the piezoelectric layer. The presence of invariant layer  21  of a thickness greater than that of the piezoelectric layer and possibly having a thermal expansion coefficient substantially smaller than that of the piezoelectric material prevents this bending phenomenon and, conversely, tends to cause a bending of opposite direction, which significantly amplifies the stress exerted in the piezoelectric layer. 
     In the case where invariant layer  21  and layer  23  form a conventional bimetallic support (with no shape memory), forced in high or low position by springs, when the temperature increases, the bimetallic support tends to bend. The piezoelectric layer having a low thermal expansion coefficient, if it were placed on the side of invariant layer  21 , would tend to enhance the bending of the support. This would result in restricting the elongation (forced by the expansion of the support) of the piezoelectric layer, and thus to decrease the stress exerted in the piezoelectric material. The positioning of the piezoelectric layer on the side opposite to the invariant layer prevents this bending phenomenon and, conversely, tends to cause a bending of opposite direction. This causes a significantly amplification of the stress exerted in the piezoelectric layer. 
     Simulations carried out by the present inventors show that the embodiment described in relation with  FIG. 4  provides an efficiency of conversion of thermal power into electric power on the order of 15%. 
       FIG. 5   a  is a simplified cross-section view of a piezoelectric layer  31  of a thermoelectric generator of the above-described type, and of its connection to output terminals V +  and V −  by metal electrodes. In this drawing, for clarity, the vertical dimensions are especially enlarged with respect to the horizontal dimensions. 
       FIG. 5   b  is a simplified cross-section view of an alternative embodiment of a piezoelectric layer of a thermoelectric generator of the type described hereabove, and of its connection to output terminals V −  and V + . In this drawing, for clarity, the vertical dimensions are especially enlarged with respect to the horizontal dimensions. In this variation, instead of a thick piezoelectric layer, a plurality of thin superposed piezoelectric layers  33  are provided. Metal electrodes, preferably very thin, alternately connect the front and rear surfaces of piezoelectric layers  33  to output terminals V +  and V − . 
     It should be noted that the voltage generated between the front and rear surfaces of a piezoelectric layer is related to the thickness of this layer. Thus, in the single-layer variation of  FIG. 5   a , the thick piezoelectric layer generates a specifically high voltage. An advantage of the multiple-layer variation of  FIG. 5   b  is that it enables providing the same electric power as with a single-layer structure, but at a voltage which is n times smaller, n being the number of thin layers  33 , the sum of the thicknesses of layers  33  ( FIG. 5   b ) being equal to the thickness of layer  31  ( FIG. 5   a ). 
       FIG. 6  is a simplified cross-section view of an alternative embodiment of a thermoelectric generator. Generator  41  comprises a package for example having, in top view, a rectangular shape. The package comprises a lower portion  43  capable of being put in contact with a heat source, and an upper portion  45  superposed to the lower portion and capable of being put in contact with a cold source. Portions  43  and  45  are preferably thermally conductive, and are separated from each other by a thermally-insulating material  47 . Portions  43  and  45  of the package delimit a cavity  49 . In this example, the lower surfaces of portions  43  and  45  of the package are basin-shaped. In cavity  45 , a membrane  51  mobile between two positions, for example, of the type described in relation with  FIG. 4 , is provided. Membrane  51  is attached to the package by its lateral ends. According to an aspect of the present embodiment, in its first shape, the membrane takes the shape of the inner surface of hot portion  43  of the package, and in its second shape, that of the inner surface of cold portion  45  of the package. 
     An advantage of this embodiment is that it provide a good thermal contact between the membrane and the hot and cold sources. 
     Further, such a package is easy to integrate in any device comprising a heat source. 
     Specific embodiments of the present invention have been described. Different variations and modifications will occur to those skilled in the art. 
     In particular, a thermoelectric generator in which a mobile membrane actuates a piezoelectric element having the shape of a piezoelectric layer directly deposited on the membrane has been described. Other arrangements of the piezoelectric element may be provided, for example, blocks of piezoelectric material on which are bearing the lateral ends of the membrane so that at each shape change, the membrane exerts a pressure on these blocks. 
     More generally, it will be within the abilities of those skilled in the art to provide other means capable of converting the mechanical power of the mobile membrane into electric power. 
     Further, the present invention is not limited to the use of the above-mentioned heat sources to operate the thermoelectric generator. According to an alternative embodiment, not shown, the cold wall directly is the rear surface of the substrate of an integrated circuit chip. In this variation, in its first shape, the membrane takes the shape of the rear surface of the substrate. 
     Similarly, a thermoelectric generator capable of recharging batteries, for example, lithium-ion batteries, from a heat source such as a radiator or a cooking plate, may be provided. 
     Further, the present invention is not limited to the above-described means of connection of the piezoelectric element to the output terminals of the generator. It will be within the abilities of those skilled in the art to use any adapted connection means. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.