Abstract:
An energy harvester including first and second sheets; and a plurality of walls, each wall being sandwiched between the first and second sheets and surrounding a cavity, wherein each cavity houses at least one curved plate adapted to change from a first shape to a second shape when its temperature reaches a first threshold and to return to the first shape when its temperature falls to a second threshold lower than said first threshold.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of French patent application number 12/52996, filed on Apr. 2, 2012, which is hereby incorporated by reference to the maximum extent allowable by law. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to an energy harvesting device and to a method of forming such a device. In particular, the present disclosure relates to a device that converts thermal energy into electrical energy. 
         [0004]    2. Discussion of the Related Art 
         [0005]    It has been proposed to use a bimetal plate, which changes shape under varying temperature conditions, in combination with a layer of piezoelectric material, to convert thermal energy into electrical energy. 
         [0006]      FIG. 1  substantially reproduces  FIG. 2  of U.S. patent application 2011/083714. As illustrated, a curved bimetal plate  100  comprises a support layer  102 , which changes shape in response to temperature variations. Plate  100  is shown having a first shape in the form of an arch, and for example changes shape to the form of an inverted arch when its temperature changes. A layer  104  of piezoelectric material is superposed over the support layer  102 . A piezoelectric material is one that has the property of generating a voltage difference between its main surfaces that varies depending on the stress applied to it. During a shape change of the curved metal plate  100 , a stress S occurs in the piezoelectric layer  104 , represented by arrows in  FIG. 1 , resulting in variations in the voltage signals V −  and V +  present on the top and bottom surfaces of the piezoelectric layer  104 . The curved metal plate  100  is, for example, positioned in a cavity between hot and cold walls, such that its middle section contacts with the hot and cold walls when the curved plate  100  assumes its respective shapes. This results in a periodic shape change of the metal plate  100 , leading to the generation of a periodic voltage signal from which electrical energy can be extracted. 
         [0007]    There is a need in the art for a simple and low cost energy harvester that operates based on the above principles and that can provide an efficient conversion of thermal to electrical energy in a range of different environments. 
       SUMMARY 
       [0008]    It is an aim of embodiments to at least partially address one or more needs in the prior art. 
         [0009]    According to one aspect, there is provided an energy harvester comprising: first and second sheets; and a plurality of walls, each wall being sandwiched between the first and second sheets and surrounding a cavity, wherein each cavity houses at least one curved plate adapted to change from a first shape to a second shape when its temperature reaches a first threshold and to return to the first shape when its temperature falls to a second threshold lower than said first threshold. 
         [0010]    According to one embodiment, each of said cavities houses a single curved plate. According to another embodiment, each of said cavities houses a plurality of curved plates interconnected by fingers to form a matrix. 
         [0011]    According to another embodiment, between said first and second sheets, there is a space separating a first of said walls from a second of said walls. 
         [0012]    According to another embodiment, the energy harvester further comprises, within each of said cavities, a printed layer of piezoelectric material adapted to be deformed by said curved plate. 
         [0013]    According to another embodiment, said piezoelectric layer is printed onto an inner surface of each cavity on a surface of said first sheet. 
         [0014]    According to another embodiment, said piezoelectric layer is printed on a surface of each curved plate. 
         [0015]    According to another embodiment, said inner walls are arranged in at least one column and in at least one row. 
         [0016]    According to another embodiment, each of said curved plates comprises a layer of a first metal superposed by a layer of a second metal, the first and second metals having different coefficients of expansion. 
         [0017]    According to another embodiment, each of said curved plates is formed of a shape-memory material. 
         [0018]    According to a further aspect, there is provided a method of manufacturing an energy harvester comprising: forming a plurality of walls on a first sheet of material, each wall defining an opening which it surrounds; placing at least one curved plate into each of said openings, each curved plate being adapted to change from a first shape to a second shape when its temperature reaches a first threshold and to return to the first shape when its temperature falls to a second threshold lower than said first threshold; and sandwiching each of said walls between said first sheet and a second sheet of material. 
         [0019]    According to one embodiment, the method comprises placing a matrix of curved plates into each of said openings. 
         [0020]    According to another embodiment, the method further comprises printing a layer of piezoelectric material on either: each of said curved plates; or each of a plurality of zones on the surface of said first sheet, each opening being aligned over one of said zones. 
         [0021]    According to another embodiment, the method further comprises printing, on said first sheet, interconnecting tracks comprising a plurality of electrodes adapted to make contact with each of said piezoelectric layers. 
         [0022]    According to another embodiment, the material forming each of said first and second sheets is a plastic or insulated metal having a thickness of between 0.5 mm and 5 mm. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The foregoing and other purposes, features, aspects and advantages will become apparent from the following detailed description of embodiments, given by way of illustration and not limitation with reference to the accompanying drawings, in which: 
           [0024]      FIG. 1  (described above) illustrates a curved metal plate in order to demonstrate a technique for thermal energy harvesting; 
           [0025]      FIG. 2  is a cross-section view illustrating part of a thermal energy harvester according to an example embodiment; 
           [0026]      FIG. 3  is a cross-section, taken in a horizontal plane, of the energy harvester of  FIG. 2  according to an example embodiment; 
           [0027]      FIGS. 4A to 4E  are perspective views of an energy harvester at various stages during its manufacture according to an example embodiment; 
           [0028]      FIG. 5  is a cross-section view illustrating part of a thermal energy harvester according to an alternative embodiment; and 
           [0029]      FIG. 6  is a perspective view illustrating a matrix of curved plates according to an example embodiment. 
       
    
    
       [0030]    It should be noted that the structures illustrated in the various figures are not drawn to scale, the thicknesses of certain layers in particular being shown to be disproportionately large to aid representation. 
         [0031]    Furthermore, throughout the following description, relative orientations such as “top surface”, “bottom surface”, “upper” and “lower” are assumed to apply when the corresponding structure is orientated as shown in the drawings. 
       DETAILED DESCRIPTION 
       [0032]      FIG. 2  is a cross-section view illustrating a portion of an energy harvester  200  according to an example embodiment. Two curved plates labelled  202  are positioned in corresponding cavities  206 ,  208  of the energy harvester  200 . For example, each of these plates  202  corresponds to the curved bimetal plate  100  of  FIG. 1  described above, except that it does not comprise the layer  104  of piezoelectric material superposed over the support layer  102 . Instead, a top wall of each cavity  206 ,  208  is formed by a corresponding layer of piezoelectric material  210 . 
         [0033]    The curved plates  202  are, for example, bimetal plates, formed of a layer of a first metal superposed by a layer of a second metal, the first and second metals having different coefficients of expansion. For example, the metal of each layer is one of TiN, aluminium, copper, tungsten, FeNi and an alloy of any of these metals. Alternatively, one or both layers could be formed of non-metals. 
         [0034]    For example, the width and length of the curved plates are in the range of 1 μm to 10 mm. A method of forming curved plates having relatively small dimensions is for example discussed in more detail in co-pending patent application entitled “Curved plate and method of forming the same” filed on the same day as the present patent application and having the same inventors (law firm reference S1022.71770US00) which is hereby incorporated by reference in its entirety. 
         [0035]    In some embodiments, the curved plates  202  are formed such that their change of shape in response to temperature variations is progressive, for example between the two shapes of the plates  202  illustrated in cavities  206  and  208  of  FIG. 2 . 
         [0036]    In alternative embodiments, the curved plates  202  are bi-stable, such that they flip rapidly from one shape to another when heated to a first temperature threshold, and back to their original shape when cooled to a second temperature threshold, lower than the first temperature threshold. For example, the curved plates  202  may comprise, as one of its layers, a shape-memory material, for example a nickel and titanium alloy. Such a material for example comprises two crystal phases, and is capable of having two stable shapes. Alternatively, the curved plate  202  may have an inward force applied to its ends by one or more springs, resulting in such a bi-stable effect. 
         [0037]    The structure of the energy harvester  200  for example comprises an upper sheet of material  214  and a lower sheet of material  216 . For example, the upper and lower sheets  214 ,  216  are each formed of a plastic sheet or of an insulated metal sheet. The sheets  214 ,  216  are for example flexible and each have a thickness of between 0.5 mm and 5 mm, depending on the size of the energy harvester  200  and the desired extent of flexibility. 
         [0038]    On the left-hand side of the structure shown in  FIG. 2 , a peripheral wall  218 , for example formed of gum, silicon, silicon dioxide, or porous-silicon, separates the sheets  214  and  216 . The peripheral wall  218  for example extends around the whole device close to the edges of the sheets  214  and  216 , as will be described in more detail below. For example, the separation between the inner surfaces of the upper and lower sheets  214 ,  216  is in the range of 0.5 mm to 20 mm. 
         [0039]    The piezoelectric layers  210  of each cavity  206 ,  208  are positioned at regular intervals on the inner surface of the upper sheet  214 . An inner wall  220 , also for example formed of gum, silicon, silicon dioxide, or porous-silicon for example surrounds each cavity  206 ,  208 , and contacts the respective piezoelectric layers  210  above, and contacts the top surface of the lower sheet  216  below. 
         [0040]    The peripheral wall  218 , and the inner wall  220  corresponding to the left-hand cavity  206  in  FIG. 2 , are separated by a distance d 1 , for example of between 1 and 20 mm. The inner walls  220  corresponding to neighbouring cavities  206 ,  208  in  FIG. 2  are separated by a distance d 2  also of, for example, between 1 and 20 mm. 
         [0041]    As represented by dashed lines extending from the right-hand edge of the structure of  FIG. 2 , the structure may continue beyond what is illustrated in  FIG. 2 , with one or more further cavities containing further curved plates  202 . 
         [0042]      FIG. 3  illustrates an example of a cross-section view of the energy harvester  200 , in a horizontal plane represented by a dashed line A-A in  FIG. 2 , passing through the peripheral wall  218  and inner walls  220 . 
         [0043]    In the example of  FIG. 3 , the energy harvester  200  comprises  21  curved plates  202 , each housed in a corresponding cavity, and arranged in 3 rows and 7 columns. Of course, in alternative embodiments, the energy harvester could comprise any number of curved plates. In some embodiments, hundreds, thousands or even millions of curved plates may be provided, each housed in a corresponding cavity or grouped into cavities. In particular, in some embodiments, each cavity houses a single curved plate. In alternative embodiments described in more detail with reference to  FIG. 6 , each cavity houses a plurality of curved plates formed in a matrix. 
         [0044]    An advantage of housing the curved plates in cavities, each cavity being surrounded by an inner wall  220 , is that the structure may be relatively flexible. Furthermore, an advantage of arranging the inner walls  220  in rows and columns is that this adds to the flexibility of the structure. In alternative embodiments, rather than being arranged in rows and columns, the inner walls  220  could be arranged in different patterns. 
         [0045]    In plan view, the energy harvester  200  is for example rectangular in shape, and the peripheral wall  218  thus extends in a rectangle around the edge of the device. Furthermore, each of the inner walls  220  also for example extends around the corresponding cavity in the form of a rectangle, the rectangle being square in the example of  FIG. 3 . 
         [0046]    Such a rectangular shape of the inner walls  220  is well adapted to rectangular plates  202 . In alternative embodiments, the curved plates  202  and inner walls  220  could have other shapes, for example circular or hexagonal. 
         [0047]    A method of forming an energy harvester similar to that of  FIGS. 2 and 3  will now be described with reference to  FIGS. 4A to 4E . 
         [0048]      FIGS. 4A to 4E  are perspective views of an energy harvester  400  at various stages of manufacture, in this example comprising  35  plates  202  arranged in seven columns and five rows. 
         [0049]    With reference to  FIG. 4A , in a first step, a grid of conductive tracks is printed or otherwise deposited on the surface of the upper sheet  214  of the structure of  FIG. 2 . The top surface of the sheet  214  shown in  FIG. 4A  corresponds to the bottom surface of the sheet  214  orientation as shown in  FIG. 2 . 
         [0050]    In the example of  FIG. 4A , the grid of conducting tracks comprises seven tracks  402  to  414  formed in columns. Each of the tracks  402  to  414  comprises five regularly spaced electrodes  416 , in this example formed as “U” shaped tracks. Thus there are a total of  35  electrodes. The respective ends of the tracks  402  to  414  are coupled together by respective tracks  418  and  420  running perpendicular to the column tracks  402  to  414 . The track  420  is for example coupled to a connection terminal  422  close to an edge of the sheet  214 . 
         [0051]    For example, the conductive tracks could be formed of copper or another suitable conducting material, and printed using PCB (printed circuit board) techniques, which are well known in the art. 
         [0052]      FIG. 4B  illustrates the upper sheet  214  after a subsequent step in which a piezoelectric layer  210  has been formed over each electrode  416 . For example, the piezoelectric material is formed of PZT (lead zirconate titanate), ZnO or a compound based on lead and zirconium. The piezoelectric layers  210  could be coated, deposited or printed. For example techniques for printing such a material are discussed in more detail in the publication entitled “Processing of Functional Fine Scale Ceramic Structures by Ink-Jet Printing”, M. Mougenot et al., the contents of which is hereby incorporated by reference to the extent permitted by the law. 
         [0053]    In some cases, the printing or depositing of the piezoelectric layers  210  may be followed by a baking step, for example at a temperature of 200° C. or less. 
         [0054]      FIG. 4C  illustrates the structure after a subsequent step in which a further grid of conducting tracks is formed over the surface of upper sheet  214 , this further grid being very similar to the grid discussed above with reference to  FIG. 4A . In particular, the further grid of conducting tracks comprises electrodes  424 , one of which is formed over each piezoelectric layer  210 . To prevent electrical contact between the conductive tracks of each of the superposed grids, an insulating layer is for example deposited in some areas prior to forming the further grid. The further grid of conducting tracks is coupled to a further terminal  426  near an edge of the upper sheet  214 . 
         [0055]    The further grid of conducting tracks comprising the electrodes  424  is for example printed or coated, for example using well known techniques, such as those used to print RFID (Radio Frequency Identification) antennas. 
         [0056]      FIG. 4D  illustrates yet a further step in which the peripheral wall  218  and inner walls  220  are formed over the surface of the upper sheet  214 , and a curved plate  202  is positioned within each inner wall  220 . In particular, the step of placing each of the inner walls on the surface of the upper sheet  214  for example defines a corresponding opening  428  surrounded by the inner wall, and into which the plates  202  are placed. 
         [0057]    In some embodiments, the curved plates  202  are individual elements. Alternatively, they could form a matrix, being interconnected by one or more fingers. Such fingers could be embedded in the inner walls  220 . 
         [0058]      FIG. 4E  illustrates a final step of the method in which the lower sheet  216  is glued to the structure opposite the upper sheet  214  to form the finished energy harvester  400 . In some embodiments, this final gluing step may be performed in a partial vacuum such that the cavities defined by each inner wall  220  are at a partial vacuum, and likewise the spacing between the inner walls  220  in the area between the sheets  214 ,  216  is also for example at a partial vacuum. Such a feature improves the insulation between the upper and lower sheets  214 ,  216 . 
         [0059]    The terminals  422  and  426  (not illustrated in  FIG. 4E ) are for example coupled to energy recuperation circuitry  430 , which recuperates the electrical energy resulting from the voltage changes across the surfaces of the piezoelectric layers  210 . This electrical energy is for example used to charge a battery and/or supply a load (not illustrated in the figures). 
         [0060]    As represented in  FIG. 4E , due in part to the form of the inner walls  220 , the resulting energy harvester  200  is for example relatively flexible, for example being able to be bent around pipes or placed in contact with other uneven surfaces. Such flexibility improves the thermal contact between the energy harvester  400  and a heat source, and thus leads to a higher thermal gradient across the energy harvester. This in turn leads to greater energy recuperation. Indeed, the warmer the lower sheet  216 , the faster the curved plates  202  will be heated and change shape, thereby increasing the mechanical power generated by the curved plates and thus the electrical power generated by the piezoelectric layers  210 . 
         [0061]    The surface area of the device  200  could be anything from a few square millimetres to several square metres. For example, in some embodiments the device  200  has a surface area of at least 0.1 square metres. 
         [0062]    In an alternative embodiment, the upper sheet  214  and/or lower sheet  216  could comprise features contributing to the final structure. For example, the inner walls  220  and/or peripheral wall  218  could at least partially be formed of a protrusion from the surface of the lower sheet  216 . 
         [0063]      FIG. 5  is a cross-section view illustrating a portion of an energy harvester  500  according to an alternative embodiment. The energy harvester  500  is very similar to the energy harvester  200  of  FIG. 2 , and like features have been labeled with like reference numerals and will not be described again in detail. 
         [0064]    In energy harvester  500 , the piezoelectric layers  210  are removed, the inner walls  220  extending to the underside of the upper sheet  214 . Instead, each of the curved metal plates  202  comprises a piezoelectric layer  502 , which is for example similar to the layer  104  of  FIG. 1 . Furthermore, an electrode  504  is for example deposited or coated over the piezoelectric layer. In one example, the electrical signals generated by such a piezoelectric layer  502  are recuperated by electrodes (not illustrated in  FIG. 5 ), similar to electrodes  416 ,  424  described above, printed on the inner surfaces of the upper and lower sheets  214 ,  216 . As illustrated, a connecting wire  506  for example couples the electrode  504  to such an electrode formed on the underside of the upper sheet  214 , and a connecting wire  508  for example couples the metal layers of curved plate  202  to an electrode formed on the top side of the lower sheet  216 . 
         [0065]      FIG. 6  is a perspective view illustrating a portion of the structure of  FIG. 3  in more detail according to an example in which each of the cavities defined by the inner walls  220  houses a matrix  600  of curved plates  202 . In the example of  FIG. 6 , the matrix  600  comprises eight plates arranged in two columns and four rows, although in alternative embodiments the matrix could comprise any number of curved plates, such as hundreds or thousands of plates arranged in an appropriate number of columns and rows. 
         [0066]    As illustrated in  FIG. 6 , each of the curved plates  202  is for example attached by a single finger  602  to a common interconnecting rail  604 . In this way, despite being interconnected, each of the plates  202  may flip from one bi-stable state to another independently of the other plates. 
         [0067]    The interconnecting fingers  602  and rail  604  are, for example, all formed of the same layered structure as the curved plates  202 . The matrix  600  is, for example, formed by the method described in relation to  FIGS. 5 and 6  of the co-pending application indicated above entitled “Curved plate and method of forming the same”. 
         [0068]    While a number of specific embodiments of a method and device have been described herein, it will be apparent to those skilled in the art that there are various modifications and alterations that could be provided. 
         [0069]    For example, it will be apparent to those skilled in the art that while a few examples of arrangements of curved plates within an energy harvester have been described, other arrangements of the plates would be possible. 
         [0070]    Furthermore, while rectangular curved plates have been described, in alternative embodiments, the plates could have other forms, such as circular or hexagonal. Furthermore, the “U” shaped form of the electrodes  416 ,  424  is merely one example, many other forms being possible. 
         [0071]    The various features described in relation with the embodiments described herein could be combined, in alternative embodiments, in any combination. 
         [0072]    Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.