Abstract:
A stack of piezoelectric elements, in the form of an elongated rod divided in to segments, for generating electric energy in response to compressive stress is provided comprising: piezoelectric elements stacked one on top of the other such that electrodes of same polarity of adjacent disks are touching A holding structure, such as a screw holds the piezoelectric elements together between a top and a bottom end pieces which transfer mechanical compressive stress to the elements in the stack. The holding structure accepts shear stresses, provides preloading stress on the stack and prevents buckling of the stack under pressure. A recess in the end piece, deeper than the head of the screw, ensures that load placed on the stack will compress the piezoelectric elements and not on the screw.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an energy harvesting apparatus having stack piezoelectric elements, and system, method and applications for implementation of said apparatus. 
       BACKGROUND OF THE INVENTION 
       [0002]    Piezoelectricity is the ability of certain materials to develop an electrical charge proportional to an applied mechanical stress. The converse effect can also be seen in these materials where strain is developed proportional to an applied electrical field. The Curie&#39;s originally discovered it in the 1880&#39;s. Piezoelectric materials are the most well known active material typically used for transducers as well as in adaptive structures. Mechanical compression or tension on a poled piezoelectric element changes the dipole moment, creating a voltage. Compression along the direction of polarization, or tension perpendicular to the direction of polarization, generates voltage of the same polarity as the poling voltage. Tension along the direction of polarization, or compression perpendicular to the direction of polarization, generates a voltage with polarity opposite that of the poling voltage. These actions are generator actions—the piezoelectric element converts the mechanical energy of compression or tension into electrical energy. This behavior is used in fuel-igniting devices, solid state batteries, force-sensing devices, and other products. Values for compressive stress and the voltage (or field strength) generated by applying stress to a piezoelectric ceramic element are linearly proportional up to a material-specific stress. The same is true for applied voltage and generated strain. 
         [0003]    The review article “Advances In Energy Harvesting Using Low Profile Piezoelectric Transducers”; by Shashank Priya; published in J Electroceram (2007) 19:165-182; provides a comprehensive coverage of the recent developments in the area of piezoelectric energy harvesting using low profile transducers and provides the results for various energy harvesting prototype devices. A brief discussion is also presented on the selection of the piezoelectric materials for on and off resonance applications. 
         [0004]    The paper “On Low-Frequency Electric Power Generation With PZT Ceramics”; by Stephen R. Platt, Shane Farritor, and Hani Haider; published in IEEE/ASME Transactions On Mechatronics, VOL. 10, NO. 2, April 2005; discusses the potential application of PZT based generators for some remote applications such as in vivo sensors, embedded MEMS devices, and distributed networking. The paper points out that developing piezoelectric generators is challenging because of their poor source characteristics (high voltage, low current, high impedance) and relatively low power output. 
         [0005]    The article “Energy Scavenging for Mobile and Wireless Electronics”; by Joseph A. Paradiso and Thad Starner; Published by the IEEE CS and IEEE ComSoc, 1536-1268/05/; reviews the field of energy harvesting for powering ubiquitously deployed sensor networks and mobile electronics and describers systems that can scavenge power from human activity or derive limited energy from ambient heat, light, radio, or vibrations. 
         [0006]    In the review paper “A Review of Power Harvesting from Vibration using Piezoelectric Materials”; by Henry A. Sodano, Daniel J. Inman and Gyuhae Park; published in The Shock and Vibration Digest, Vol. 36, No. 3, May 2004 197-205, Sage Publications; discusses the process of acquiring the energy surrounding a system and converting it into usable electrical energy—termed power harvesting. The paper discuss the research that has been performed in the area of power harvesting and the future goals that must be achieved for power harvesting systems to find their way into everyday use. 
         [0007]    Patent application WO07038157A2; titled “Energy Harvesting Using Frequency Rectification”; to Carman Gregory P. and Lee Dong G.; filed: 2006 Sep. 21 discloses an energy harvesting apparatus for use in electrical system, having inverse frequency rectifier structured to receive mechanical energy at frequency, where force causes transducer to be subjected to another frequency. 
         [0008]    More general background and information concerning piezoelectric devices may be found in other patents and patent applications 
         [0009]    For example:
       U.S. Pat. No. 6,277,299 to Seyed   U.S. Pat. No. 5,825,386 to Ohashi   U.S. Pat. No. 4,412,148 to Kilcker   U.S. Pat. No. 5,340,510 to Bowen   U.S. Pat. No. 4,404,490 to Taylor   US 2006/118678 to Wells   US 2006/087201 to Spinelli   US 2005/0258717 to Mullen   US 2005/127677 to Luttrull,   JP2006-197704A to Mutou;   JP2005-353015A to Kokatsu;   JP10-073073A to Okawa   JP 08-098564 to Yamamoto   JP141478 to Kimura   JP2002-063685 to Tamura   EP 1,783,026 to Zoll   WO2006/053479 to Cao   CN1633008 to Cao et al.   CN 1,633,009   GB 2,389,249 to Mark Colin Porter       
 
       SUMMARY OF THE INVENTION 
       [0030]    The present invention relates to an energy harvesting apparatus having stack piezoelectric elements, and system, method and applications for implementation of said apparatus. 
         [0031]    A stack of piezoelectric elements, in the form of an elongated rod divided in to segments, for generating electric energy in response to compressive stress is provided comprising: piezoelectric elements stacked one on top of the other such that electrodes of same polarity of adjacent disks are touching A holding structure, such as a screw holds the piezoelectric elements together between a top and a bottom end pieces which transfer mechanical compressive stress to the elements in the stack. The holding structure accepts shear stresses, provides preloading stress on the stack and prevents buckling of the stack under pressure. A recess in the end piece, deeper than the head of the screw, ensures that load, placed on the stack, will compress the piezoelectric elements and not on the screw. 
         [0032]    It is an object of the current invention to provide a piezoelectric stack, in the form of an elongated rod divided in to segments, for generating electric energy in response to compressive stress comprising: a plurality of piezoelectric disks or other shapes, such as rings each having a positive and a negative electrode on their opposing faces, stacked one on top of the other such that positive electrodes of adjacent disks are touching, and negative electrodes of adjacent disks are touching; positive and negative wires connected to positive electrodes and negative electrodes respectively; a holding structure, holding said plurality of disks together; and top and bottom end pieces, in mechanical contact with the first and last disks in the stack and adopted to transfer mechanical compressive stress to said disks in said stack. 
         [0033]    In some embodiments, inert disks or rings are added to the structure, for example to adjust for the structure&#39;s length or to be used as electrodes or to add flexibility to the structure. 
         [0034]    In some embodiments the holding structure is a pin inserted in holes in said disks and said end pieces. 
         [0035]    In some embodiments the piezoelectric stack further comprises at least one screw and nut combination, capable of applying preloading compressive force between said end pieces by tightening it to said pin. 
         [0036]    In some embodiments the disks further comprising at least two grooves capable of accepting said positive and negative wires. 
         [0037]    In some embodiments the piezoelectric stack further comprises a moisture proof cover, capable of protecting said disks. 
         [0038]    In some embodiments the shape of said stack is substantially cylindrical. 
         [0039]    In some embodiments the holding structure is a pipe holding said disks and said end pieces. 
         [0040]    In some embodiments the pipe further comprises at least two internal grooves capable of accepting said positive and negative wires. 
         [0041]    It is another object of the current invention to provide a piezoelectric generator comprising: a top and a bottom stiff load plate; a piezoelectric stack, in the form of an elongated rod divided in to segments, placed between said top and bottom load plate, said stack comprising: a plurality of piezoelectric disks, each having a positive and a negative electrode on their opposing faces, stacked one on top of the other such that positive electrodes of adjacent disks are touching, and negative electrodes of adjacent disks are touching; and positive and negative wires connected to positive electrodes and negative electrodes respectively; and at least three pins holding said top and a bottom load plate together. 
         [0042]    In some embodiments the at least three pins holding said top and a bottom load plate together mechanically supports said stack against out of plate displacements. 
         [0043]    In some embodiments the stack further comprises a holding structure, stabilizing said stack against out of plate displacements. 
         [0044]    It is another object of the current invention to provide a piezoelectric block generator comprising: a block having a plurality of holes; a plurality of piezoelectric stacks, each in the form of an elongated rod divided in to segments, placed in said holes, each of said stack comprising: a plurality of piezoelectric disks, each having a positive and a negative electrode on their opposing faces, stacked one on top of the other such that positive electrodes of adjacent disks are touching, and negative electrodes of adjacent disks are touching; and positive and negative wires connected to positive electrodes and negative electrodes respectively; positive and negative cables connecting said positive and negative wires respectively; and a cover, covering said piezoelectric stacks and capable of transferring mechanical compressive stress applied to said cover to said plurality of stacks. 
         [0045]    In some embodiments the block is made by casting. 
         [0046]    In some embodiments the block further comprises a recess capable of holding an energy conditioning electronics, electronically connected to the positive and negative cables. 
         [0047]    It is another object of the current invention to provide a method of constructing a block piezoelectric generator, the method comprises the steps of: mechanically connecting a plurality of piezoelectric stacks to the lower side of a cover capable of spreading compressive stress among said stacks; electrically connecting positive and negative cables to positive and negative wires of said stacks respectively; pouring curable liquid into a block-shaped mold; lowering said cover into said mold such that said stacks are embedded in said curable liquid; and letting said curable liquid harden. 
         [0048]    In some embodiments the method of constructing a block piezoelectric generator comprises the steps of: pouring curable liquid into an upside-down block-shaped mold having a plurality of sleeves, wherein each of said sleeve is capable of snugly holding a piezoelectric stack, such that said sleeve remain empty; letting said curable liquid harden; turning said block upright; inserting a plurality of stacks to at least some of said sleeve, such that top end of each stack is slightly outside said sleeve; electrically connecting positive and negative cables to positive and negative wires of said stacks respectively; covering said block with a cover capable of spreading compressive stress among said stacks. 
         [0049]    It is another object of the current invention to provide a system for energy harvesting comprising: a plurality of piezoelectric stacks, wherein each stack comprises: a plurality of piezoelectric disks, each having a positive and a negative electrode on their opposing faces, stacked one on top of the other such that positive electrodes of adjacent disks are touching, and negative electrodes of adjacent disks are touching; positive and negative wires connected to positive electrodes and negative electrodes respectively; a holding structure, holding said plurality of disks together; and top and bottom end pieces, in mechanical contact with the first last disks in the stack and adopted to transfer mechanical compressive stress to said disks in said stack; and positive and negative cables electrically connecting positive and negative wires of said stacks respectively. 
         [0050]    In some embodiments the stacks are embedded in railway sleepers. 
         [0051]    In some embodiments the system stacks are embedded under railway sleepers. 
         [0052]    In some embodiments the stacks are embedded under railway tracks. 
         [0053]    In some embodiments the stacks are embedded in holes drilled in a road or a pavement. 
         [0054]    In some embodiments the stacks are under a vibrating machine. 
         [0055]    It is yet another object of the current invention to provide a method of constructing an energy harvesting system comprising the steps of: placing a plurality of block piezoelectric generators on a foundation; and connecting electrical cables from block piezoelectric generators to a user electrical load; and covering said block piezoelectric generators, wherein each block piezoelectric generator comprises: a block having a plurality of holes; a plurality of piezoelectric stacks placed in said holes; and a cover, covering said piezoelectric stacks and capable of transferring mechanical compressive stress applied to said cover to said plurality of stacks. 
         [0056]    In some embodiments the step of placing a plurality of block piezoelectric generators comprises placing said blocks side by side; and the step of covering said block piezoelectric generators comprises covering said generator with a cover such as asphalt, bitumen; concrete or tiles. 
         [0057]    In some embodiments the step of placing a plurality of block piezoelectric generators comprises: digging a trench in a road; and placing said blocks side by side in said trench; and the step of covering said block piezoelectric generators comprises refilling the trench with asphalt or bitumen. 
         [0058]    Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0059]    The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
           [0060]    The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
           [0061]    In discussion of the various figures described herein below, like numbers refer to like parts. The drawings are generally not to scale. For clarity, non-essential elements were omitted from some of the drawings. 
           [0062]    In the drawings: 
           [0063]      FIG. 1   a  schematically depicts an isometric view of a piezoelectric ring element according to an exemplary embodiment of the current invention. 
           [0064]      FIG. 1   b  schematically depicts a top view of the piezoelectric ring element seen in  FIG. 1   a  according to an exemplary embodiment of the current invention. 
           [0065]      FIG. 1   c  schematically depicts cross section of the piezoelectric ring element seen in  FIGS. 1   a  and  1   b  according to an exemplary embodiment of the current invention. 
           [0066]      FIGS. 1   d ( i ) and  1   d ( ii ) respectively depicts schematic top view cross section of the piezoelectric ring element according to another exemplary embodiment of the current invention. 
           [0067]      FIG. 2   a  schematically depicts cross section view of a piezoelectric stack device using several ring elements seen in  FIGS. 1   a - c  according to an exemplary embodiment of the current invention. 
           [0068]      FIG. 2   b  schematically depicts an isometric view of a piezoelectric stack device seen in  FIG. 2   a  according to an exemplary embodiment of the current invention. 
           [0069]      FIG. 2   c  schematically depicts a top or bottom view of a piezoelectric stack device seen in  FIGS. 2   a - b  according to an exemplary embodiment of the current invention. 
           [0070]      FIG. 2   d  schematically depicts a 3D isometric view of a piezoelectric disk element according to another exemplary embodiment of the invention. 
           [0071]      FIG. 2   e  schematically depicts a horizontal cross section through a stack constructed from a plurality of disks according to another exemplary embodiment of the invention. 
           [0072]      FIG. 2   f  schematically depicts a vertical cross section along the line B-B seen in  FIG. 2   d , of a stack according to the exemplary embodiment of the current invention. 
           [0073]      FIGS. 3(   a - e ) respectively depict 3D isometric view; side view; vertical cross section; top; and bottom views of a stack using ring piezoelectric elements according to yet another embodiment of the current invention. 
           [0074]      FIG. 4   a  schematically depicts an isometric view of an optional piezoelectric generator using a stack device seen in  FIGS. 2   a - c  according to an exemplary embodiment of the current invention. 
           [0075]      FIG. 4   b  schematically depicts a top view of a piezoelectric generator seen in  FIG. 3   a  according to an exemplary embodiment of the current invention. 
           [0076]      FIG. 4   c  schematically depicts a side view of a piezoelectric generator seen in  FIGS. 3   a - b  according to an exemplary embodiment of the current invention. 
           [0077]      FIG. 4   d  schematically depicts a cross section view of a piezoelectric generator seen in  FIGS. 3   a - c  according to an exemplary embodiment of the current invention. 
           [0078]      FIG. 4   e ( i ) to  3   e ( iii ) schematically depict: a side view, an isometric view and a top view of a piezoelectric generator according to another exemplary embodiment of the current invention. 
           [0079]      FIG. 5  schematically depict a top view of an implementation in a road of an energy harvesting system using piezoelectric generators seen in  FIG. 2  or  3 , according to an exemplary embodiment of the current invention. 
           [0080]      FIG. 6   a  schematically depict a cross section view of a piezoelectric generator implementation in a railway sleeper according to an exemplary embodiment of the current invention. 
           [0081]      FIG. 6   b  schematically depict a cross section view of a piezoelectric generator implementation in a railway according to other exemplary embodiments of the current invention. 
           [0082]      FIGS. 7(   a - f ) schematically depict steps of constructing an energy harvesting system in a road by embedding a plurality of separate stacks according to an exemplary embodiment of the current invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0083]    The present invention relates to an energy harvesting apparatus having stack piezoelectric elements, and system, method and applications for implementation of said apparatus. 
         [0084]    Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. It can be applied for any surface where mechanical load can be transferred efficiently into electrical load. 
         [0085]      FIG. 1   a  schematically depicts an isometric view of a piezoelectric disk  10  according to an exemplary embodiment of the current invention. 
         [0086]    Washer shaped bioelectric element  10  comprises a round disk, rings  11  made of piezoelectric material polled in the direction depicted by the arrow  12  such that compression axial load applied in the thickness direction, yield to compression stress, where parallel faces  13  and  14  would create a voltage due to the piezoelectric coefficient d 3,3 . Top electrode is formed by coating the top face  14  with a conductive coating for example a conductive metal such as silver, copper or Nickel or other conducting material. Top electrode is connected to a positive wire  16  at a contact  17  located within the first optional groove  18  on the side of the disk. 
         [0087]    Similarly, bottom electrode is formed by coating the bottom face  13  with a conductive coating for example a conductive metal such as silver, copper or Nickel. Bottom electrode is connected to a positive wire  26  at a contact  27  located within the second optional groove  28  on the side of the disk. 
         [0088]    Central hole  20  is preferably at the center of disk  10 . This optional hole may be used for stabilizing a stack as will be seen in  FIG. 2   a.    
         [0089]      FIG. 1   b  schematically depicts a top view of the piezoelectric element  10  seen in  FIG. 1   a  according to an exemplary embodiment of the current invention. 
         [0090]    The flush central hole  20  is clearly seen. First optional groove  18  and second optional groove  28  are seen opposite to each other, however, locations of grooves, their shapes, and sizes may vary. Grooves  18  and  28  are sized so wires  16  and  26  can fit inside them. Dimensions indicated in this and all other figures should be viewed as non limiting examples. 
         [0091]    The dashed line A-A marks the cross section plane seen in  FIG. 1   c.    
         [0092]    Dimensions on this and other figures are for demonstration of one of the preferred embodiment and should not be viewed as limiting. 
         [0093]      FIG. 1   c  schematically depicts cross section of the piezoelectric disk element  10  seen in  FIGS. 1   a  and  1   b  according to an exemplary embodiment of the current invention. Connection  17  between wire  16  and bottom electrode  13  may be clearly seen. Similarly, Connection  27  between wire  26  and top electrode  24  may be clearly seen. 
         [0094]      FIGS. 1   d ( i ) and  1   d ( ii ) respectively depicts schematic top view cross section of the piezoelectric ring element according to another exemplary embodiment of the current invention. 
         [0095]    In this exemplary embodiment, electrodes  14  and  24  do not extend to the outer edge  119  of ring element  10 ″. Similarly, electrodes  14  and  24  do not extend to the inner edge  129  of the hole of ring element  10 ′. 
         [0096]    However, in this exemplary embodiment, electrodes  14  and  24  extend onto the outer edge of ring element  10 ″ in grooves  118  and  128  respectively. These extensions may be used to connect or solder wires  16  and  26  respectively. 
         [0097]    Note that dimensions in these figures are to be viewed as non limiting examples only. 
         [0098]      FIG. 2   a  schematically depicts cross section view of a piezoelectric stack device  30  using elements seen in  FIGS. 1   a - c  according to an exemplary embodiment of the current invention. 
         [0099]    A plurality of piezoelectric elements  10  (in the depicted example, six such elements:  10   a  to  10   f  are seen, but number of elements may vary according to the application) are placed one on top of the other such that the poling direction as depicted by arrows  12  (for drawing clarity, only  12   a  and  12   b  are marked in the drawings) alternate. 
         [0100]    As a result, two adjacent elements are placed such that top (positive) electrode of one element touches the positive electrode of its neighbor, and a bottom (negative) electrode of one element touches the negative electrode of its neighbor. A central pin  31 , preferably having an insulator sleeve or coated by insulator  33  is inserted through the central holes  20  of the elements  10  and holds them in place. Central pin  31  prevents horizontal displacements or buckling of the piezoelectric stack thus formed from the plurality of elements  10  when compression stress is applied to it by accepting sheer stresses that may develop. 
         [0101]    Two load transferring end pieces  34   a  and  34   b,  each having a hole  36   a  and  36   b,  and a recess  37   a  and  37   b  respectively are placed at opposite ends of the stack such that pin  31  goes through the holes  36  with its tapped end in the recesses  37 . Nuts  32   a  and  32   b  respectively are screwed to each end of pin  31  thus tightening the stack by applying compression between the two load transferring end pieces  34   a  and  34   b . The compression pressure is light and is used to ensure mechanical and electrical contact between the adjacent elements. The preloading pressure ensures that any compression stress applied between the load transferring end pieces  34   a  and  34   b  will result in electrical signal generation. 
         [0102]    It should be noted that load transferring end pieces  34   a  and  34   b  have a diameter of at least the diameter of the piezoelectric elements, and are made of rigid material such as metal. Holes  36   a  and  36   b  are sized at least slightly larger than the diameter of pin  31 , and recesses  37   a  and  37   b  are larger than nuts  32   a  and  32   b . Thus, when pressure is applied to load transferring end pieces  34   a  and  34   b  it is transferred to the stack of piezoelectric elements  10 . 
         [0103]    Positive wires  16  connected to contacts  17  of all elements  10  are connected together and run along the first groove  18  in all elements  10  to positive out wire  36 (+). Similarly, negative wires  26  from contacts  27  of from all elements  10  are connected together and run along the second groove  28  of elements  10  to negative out wire  36 (−). Optionally, only one wire  16  carries the signal from the two adjacent electrodes  14  which are in electrical contact with each other. Similarly, optionally, only one wire  26  carries the signal from the two adjacent electrodes  24  which are in electrical contact with each other. 
         [0104]    A protective cover  35  is used for covering the piezoelectric stack device  30 . Protective cover  35  is preferably made of insulating material such as plastic, shrink wrap plastic or may be formed by dipping the piezoelectric stack device  30  in thermoplastic material etc. Protective cover  35  protects the internal parts of piezoelectric stack device  30  against corrosion, moisture and prevents arcing. 
         [0105]    It should be noted that in some embodiments the stack may be sintered or glued to maintain its shape. This may be done for example using methods known in the art. Gluing or sintering may be used optionally, additionally or alternatively to applying preloading force. Gluing or sintering may be used optionally, additionally or alternatively to using a pin to maintain the shape. 
         [0106]    Dimensions on this and other figures are for demonstration of one of the preferred embodiment and should not be viewed as limiting. 
         [0107]      FIG. 2   b  schematically depicts an isometric view of a piezoelectric stack device  30  seen in  FIG. 2   a  according to an exemplary embodiment of the current invention. 
         [0108]    Preferably, the outer shape of piezoelectric stack device  30  is cylindrical, having an outer diameter defined by the protective cover  35 . Wires  36 (+) and  36 (−) exit the piezoelectric stack device  30  through opening in cover  35  which makes hermetic seal to the wires to prevent moisture leak. 
         [0109]      FIG. 2   c  schematically depicts a top view of a piezoelectric stack  30  device seen in  FIGS. 2   a - b  according to an exemplary embodiment of the current invention. 
         [0110]    The round, cylindrical shape of piezoelectric generator  30  enables easy insertion of the stack into round drilled holes. 
         [0111]    The dashed line A-A marks the cross section plane seen in  FIG. 2   a.    
         [0112]    In some embodiment, stack  30  is held together by gluing the disks together, by welding the electrodes together or by sintering the entire stack. 
         [0113]      FIG. 2   d  schematically depicts a piezoelectric disk  110  element according to another embodiment of the invention. 
         [0114]    In contrast to the disk  10  of  FIGS. 1   a - c , disk  110  has no central holes or grooves. Instead, wires  136 (+) and  136 (−), connected to top and bottom electrodes respectively are external to the perimeter of the disk. 
         [0115]      FIG. 2   e  schematically depicts a horizontal cross section through a stack  130  constructed from a plurality of disks  110 . 
         [0116]    A pipe  135  holds the plurality of disks  110  and prevents sideways motion of the disks. Wires  136 (+) and  136 (−) runs along grooves  137  and  138  in pipe  135 . The dashed line B-B marks the location of a vertical cross section plane seen in  FIG. 2   f.    
         [0117]      FIG. 2   f  schematically depicts a vertical cross section of a stack  130  according to an exemplary embodiment of the current invention. 
         [0118]    In this embodiment, pipe  135  holds the stack and prevents horizontal displacements or buckling of the piezoelectric stack thus formed from the plurality of elements  110  when compression stress is applied to it by accepting sheer stresses that may develop. 
         [0119]    Two load transferring end plugs  131  and  132  accept compression forces and apply it to the stack. Preferably, preloading compression force is applied between plugs  131  and  132 . Optionally, a protective coating is used for keeping moisture out. Alternatively, seals such as O ring  139  are used. 
         [0120]    It should be noted that in some embodiments the stack may be sintered or glued to maintain its shape. This may be done for example using methods known in the art. Gluing or sintering may be used optionally, additionally or alternatively to applying preloading force. Gluing or sintering may be used optionally, additionally or alternatively to using external case to maintain the shape. 
         [0121]      FIGS. 3(   a - e ) respectively depict 3D isometric view; side view; vertical cross section; top; and bottom views of a stack using ring piezoelectric elements according to yet another embodiment of the current invention. 
         [0122]    The stack  30  depicted in  FIGS. 3(   a - e ) is similar to the stack  30  depicted in  FIGS. 1   a - b  and  2   a - c . Thus only the main differences will be discussed herein. 
         [0123]    In contrast to piezoelectric ring elements  10  of stack  30 , piezoelectric ring elements  10 ′ of stack  30 ′ do not have groves ( 18  and  28  in  FIG. 1   a ). Positive wire  36 (+)′ is connected to the positive electrodes of all ring elements  10 ′ and continues to the energy utilization system. Similarly, negative wire  36 (−)′ is connected to the positive electrodes of all ring elements  10 ′ and continues to the energy utilization system. Positive and negative wires  36 (+)′ and  36 (−)′ may be connected for example near the top of the stack (as in  FIG. 3(   a ); near the center of the stack (as in  FIG. 3(   b ) or at any other location. 
         [0124]    Pin  30  is replaced with a screw  31 ′ having a head  301  and threaded end  302 . By screwing screw  31 ′ into the tapped hole in bottom plate  34   b ′, compressive pressure is created by top plate  34   a ′ and  34   b ′. This pressure holds the stack together, provide pre-loading and ensure mechanical and electrical contact between adjacent piezoelectric elements. The head  301  of screw  31 ′ does not protrude above the top face  340   a  of plate  34   a ′, and the threaded end  302  of screw  31  does not protrude below the bottom face  340   b  of plate  34   b . Thus, any compression applied to plates  34   a ′ and  34   b ′ is spread and transferred to the elements  10 ′ and not to screw  31 ′. 
         [0125]    Cover  35  is also missing. However, in some embodiments stack  30 ′ may be coated with a protective cover. 
         [0126]    In some embodiments plates such as  34   a ,  34   b,    34   a ′,  34   v ′,  131  or  132  are made of steel. 
         [0127]    In embodiments where number of piezoelectric elements  10 ,  10 ′,  10 ″ or  110  in the stack is even, top and bottom plates are in proximity to electrodes of same polarity (positive or negative, depending on the orientation of the elements). In these embodiments, the pin or pipe need not be electrically isolated from the plates, and the plates need not be electrically isolated from the electrodes, as both are at the same potential. However, the pin (or pipe) needs to be isolated from other electrodes in the stack having the opposite polarity. 
         [0128]    Dimensions on this and other figures are for demonstration of one of the preferred embodiment and should not be viewed as limiting. 
         [0129]      FIG. 4   a  schematically depicts an isometric view of a piezoelectric generator structure  40  using a stack device  30  seen in  FIGS. 2   a - c  according to an exemplary embodiment of the current invention. 
         [0130]    Generator  40  is made by placing a stack device  30  between two load plates  41   a  and  41   b . The entire structure is held in place by at least three screws  42   a ,  42   b  and  42   c  (screw  42   b  is hidden from view in this drawing), having screw heads  43   a  to  43   c  respectively. Screws or guide  42   a ,  42   b  and  42   c  traverses plate  41   a  through holes in plate  41   a , and heads  43   a - c  of screws  42   a - c  are within reassess  44   a - c  in plate  41   a . Plates  41  are preferably made of rigid material such as metal or hard plastic. Thus, pressure applied to plats  41   a  and  41   b  is transferred to piezoelectric stack  30 . The diameter of plates  41  is larger than the diameter of stack  30 , thus enabling load concentration to the smaller diameter stack. 
         [0131]    Screws or guide  42  are preferably placed to be in contact with the outer dimension of stack  30 , thus assist pin  31  in accepting any out of plate displacements that may develop, for example due to eccentric loading. Additionally, screws  42  may also be used to apply preloading compression on stack  30 . 
         [0132]    For clarity, wires  36  were omitted from the drawings. 
         [0133]    It should be noted that in some embodiments the stack may be sintered or glued to maintain its shape. This may be done for example using methods known in the art. Gluing or sintering may be used optionally, additionally or alternatively to applying preloading force. Gluing or sintering may be used optionally, additionally or alternatively to using external case to maintain the shape. Gluing or sintering may be used optionally, additionally or alternatively to using a pin to maintain the shape. 
         [0134]      FIG. 4   b  schematically depicts a top view of a piezoelectric generator  40  seen in  FIG. 3   a  according to an exemplary embodiment of the current invention. 
         [0135]      FIG. 4   c  schematically depicts a side view of a piezoelectric generator  40  seen in  FIGS. 3   a - b  according to an exemplary embodiment of the current invention. 
         [0136]    The dashed line A-A marks the cross section plane seen in  FIG. 3   d.    
         [0137]      FIG. 4   d  schematically depicts a cross section view along the A-A plane of  FIG. 3   c , of a piezoelectric generator  40  seen in  FIGS. 3   a - c  according to an exemplary embodiment of the current invention. 
         [0138]    In this figure, the holes  45   c  in plate  41   a , for screw  42   c  is clearly seen. Screw  42   c  is screwed to tapped hole  46   c  in pate  41   b  by rotating its head  43   c  within recess  44   c.    
         [0139]    Optional centering indentations  47   a  and  47   b  in plates  41   a  and  41   b  respectively are sized to fit stack  30 . 
         [0140]    When compression is applied between plates  41   a  and  41   b , the stress is transferred to stress load transferring end pieces  34   a  and  34   b    34   a  and from it to the piezoelectric  10  in stack  30 . 
         [0141]      FIGS. 4   e ( i )  4   e ( ii ) and  4   e ( iii ) respectively schematically depict: a side view, an isometric view and a top view of a piezoelectric generator  50  according to another exemplary embodiment of the current invention. 
         [0142]    Generator  50  is similar in construction to generator  40 , however, in contrast to piezoelectric generator  50 , screws  52   a  to  52   c  which connect plates  51   a  and  51   b , do not touch stack  30 . Just as in generator  40 , screw heads  53   a - c  are within recesses  54   a - c  in plate  51   a.    
         [0143]    In  FIG. 3   e ( i ), wires  36 (+) and  36 (−) may be seen. 
         [0144]    It should be noted that in some embodiments the stack may be sintered or glued to maintain its shape. This may be done for example using methods known in the art. Gluing or sintering may be used optionally, additionally or alternatively to applying preloading force. Gluing or sintering may be used optionally, additionally or alternatively to using external case to maintain the shape. Gluing or sintering may be used optionally, additionally or alternatively to using a pin to maintain the shape. 
         [0145]    In some embodiments, a plurality of stacks is used in generator  50 . For example three or four stacks  30  or  130  are placed between plates  41   a  and  41   b . In these embodiments, screws  41  apply a preloading force on all the stacks. 
         [0146]    It should be noted that the round shape of piezoelectric elements  10 ,  10 ′.  10 ″ and  110  is one of the preferred embodiments. Other shapes may be used, for example square or hexagonal or any other shape. Round shape of a stack may be achieved with non-round elements, for example by having round end plates  34 ,  35  or round pipe  135 . In some embodiments round cover  35  or round pipe  155  may have non-round internal hole matching the shape of the piezoelectric elements. 
         [0147]    However, stacks with shapes other than round may be used within the general scope of the current invention. 
         [0148]    In some embodiments round generators  40  or  50  may be constructed with a stack having shape different than cylindrical by using round plates  41  and  51 . 
         [0149]    However generators with shapes other than round may be used within the general scope of the current invention. 
         [0150]    In  FIGS. 4   a - e , one stack per generator is seen. However, generators with more than one stack may be used within the general scope of the current invention. Optionally, stacks are symmetrically placed between plates  41  or  51 . 
         [0151]      FIGS. 5 and 6  schematically depict some exemplary uses for the already disclosed stacks and generators. It should be clear that the stacks and generators according to the current invention may be used in other applications where conversion of compressive mechanical energy is to be converted to useful electrical energy. 
         [0152]      FIG. 5  schematically depict a top view of an implementation in a road, of an energy harvesting system  600  using energy harvesting device  800  which could be one of: stack  30 , generator  40  or generator  50  seen in  FIG. 2  or  3 , according to an exemplary embodiment of the current invention. 
         [0153]    In the depicted example, two lanes road  650  having curbs  651  is embedded with a plurality of round energy harvesting device  400 , each inserted in a round hole  710 , preferably located were wheels of traveling vehicles are most likely to pass. Connecting cables  614  and  612 , transfer generated electrical energy to a control unit  610  for storage or for delivery to energy user such as electrical main grid vial cable  690 . 
         [0154]    Preferably, Connecting cables  614  and  612  are also embedded beneath the surface of road  650 . 
         [0155]      FIG. 6   a  schematically depict a cross section view of an application of energy harvesting device  400  which could be one of: stack  30 ; stack  130 ; generator  40 ; generator  50 ; or block generator  60 , implemented in a railway sleeper  810 , according to an exemplary embodiment of the current invention. 
         [0156]    In this cross section, energy harvesting device  400  is seen placed in a sleeper  810 . The recess  800  in sleeper  810  preferably sized such that the internal recess acts as a box and mount  840  and elastomeric layer  850  acts as cover. 
         [0157]      FIG. 6   b  schematically depict a cross section view of an application of energy harvesting device  400  which could be one of: stack  30 ; stack  130 ; generator  40 ; generator  50 ; or block generator  60 , implemented in a railway, according to other exemplary embodiments of the current invention. 
         [0158]    In this cross section, energy harvesting device  400  is seen placed for example under sleeper  810  within the foundation layer  870 . Preferably a block  890 , placed under energy harvesting device  400  is used for supplying counter force to the compression force of sleeper  830  when a train is passing. 
         [0159]    In this cross section, energy harvesting device  400  is also seen placed for example under rail  830 . Preferably a block  880 , placed under energy harvesting device  400  is used for supplying counter force to the compression force of rail  830  when a train is passing. 
         [0160]    When a train traverses along rail  830 , stress caused by the train&#39;s weight it transferred via rail  830 , mount  840  and elastomeric layer  850  and compresses on energy harvesting device  400 , causing charge to be generated. Depth of recess  800  in sleeper  810  is limited by metal reinforcement cables or bars  820  in sleeper and design conclusions  810 . This depth limits the length of the energy harvesting device  400 . However, Sleeper  810  may be redesigned to allow deeper recesses. Similarly, width of for energy harvesting device  400  in sleeper  810  is limited by the distance between screws  825  and design conclusions which hold mount  840  to sleeper  810 ; however, sleeper  810  may be redesigned to allow wider or narrower recesses. 
         [0161]    Preferably recesses  800  are preferably round and may be easily drilled in a preexisting sleeper. Alternatively, recesses  800  may be created when a concrete sleeper is molded. Round recess  800  is preferably sized so that inserted stack  30 , generator  40  or  60  is snugly fits in it such that side walls of recess  800  are used to carry and accept some of the shear stress that may be developed due to train vibrations. 
         [0162]    It should be noted that cables (not seen in  FIG. 6  for clarity) collects the generated electric power and relay it to be used. 
         [0163]      FIG. 7  schematically depict steps of constructing an energy harvesting system  600  by embedding a plurality of round energy harvesting devices  800  according to an exemplary embodiment of the current invention. It is noted that figures are generally not to scale. 
         [0164]      FIG. 7(   a ) schematically depicts drilling, in a road, holes  710  for embedding round stacks or generators (For example  30 ;  130 ;  40 ; or  50 ), preferably using a cup drill  701 . Drilling circular holes in a road is easier than cutting rectangular holes. Drilling hole  710  in asphalt layer  520  having an upper surface  519  which is deposited over a foundation layer  510 , may be done using standard roadwork equipment, for example cup drill  701  may be used to remove a cylindrical core from the road leaving a cylindrical hole  710 . 
         [0165]      FIG. 7(   b ) schematically depicts cutting slits  720  in the road&#39;s asphalt layer  520 , for embedding connecting cable  614 , preferably using a disk saw  711 . 
         [0166]    Optionally, slits  720  and holes  710  are made only part way into asphalt layer  520 . However any of slits  720  and holes  710  may be made all the way to or into foundation layer  510 . 
         [0167]      FIG. 7(   c ) schematically depicts the prepared holes and slits. In some embodiments, slits  720  is missing. In this embodiment, connecting cables are laid on the surface of the road and a layer of asphalt is poured over it. This method is useful when resurfacing a road. The method comprises the step of: removing the top layer of the road by abrasion; drilling the holes; installing generators in the holes; laying the cables; and resurfacing the road with a top layer of asphalt. The method may be preferred as it does not require cutting the slots, thus saving time and cost and prevents weakening the road by the slots. 
         [0168]      FIG. 7(   d ) schematically depicts optional stage of pouring a reinforcing layer  730 , preferably made of concrete at the bottom of the drilled hole  710 . The optional reinforcement layer  730  acts as sturdy foundation for the round multilayer modular generator  400  to be placed in hole  710  and may be used to ensure desired depth of hole  710  which may not be easily drilled to the required accuracy. 
         [0169]      FIG. 7(   e ) schematically depicts the stage of laying the round energy harvesting device  400  which could be one of: stack  30 , generator  40  or generator  50  in drilled hole  710  over optional reinforcement  730 , and placing the connecting cables  614  in the cut slit  720 . 
         [0170]      FIG. 7(   f ) schematically depicts the stage of refilling the drilled holes and the cut slits, preferably with asphalt or bitumen  750 , thus embedding round multilayer modular generator  400  and cables  614  of system  600  below the surface  519  of the road. 
         [0171]    It should be noted that in some embodiments the stack may be sintered or glued to maintain its shape. This may be done for example using methods known in the art. Gluing or sintering may be used optionally, additionally or alternatively to applying preloading force. Gluing or sintering may be used optionally, additionally or alternatively to using external case to maintain the shape. Gluing or sintering may be used optionally, additionally or alternatively to using a pin to maintain the shape. 
         [0172]    It should be noted that round generators used in the embodiments depicted in  FIGS. 7   a - f  may be stacks such as stacks  30 ,  130  or other stacks known in the art; generators such as generators  40 ,  50 , or generators using a plurality of stack or other generators known in the art. 
         [0173]    It also should be noted that in some applications, standard piezoelectric stacks and generator as known in the art may replace the stacks and generators depicted herein. 
         [0174]    It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the various embodiments of the invention without departing from their scope. While the dimensions and types of materials described herein are intended to define the parameters of the various embodiments of the invention, the embodiments are by no means limiting and are exemplary embodiments. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the various embodiments of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
         [0175]    Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. §112, sixth paragraph, unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure. 
         [0176]    This written description uses examples to disclose the various embodiments of the invention, including the best mode, and also to enable any person skilled in the art to practice the various embodiments of the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the various embodiments of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if the examples have structural elements that do not differ from the literal language of the claims, or if the examples include equivalent structural elements with insubstantial differences from the literal languages of the claims. 
         [0177]    Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.