Abstract:
A multilayer piezoelectric generator is disclosed comprising a round, rectangular or other shaped box having a cover. In the box are top and bottom electrodes and a plurality of electricity generating layers. Each Layer comprises a plurality of piezoelectric rods held in place by a matrix layer that fits snugly in the box and configured to accept shear strains developed in the rods when pressure is applied to the cover. The layers are separated by central electrode layers. The structure is configures such that pressure is evenly spreads among all the rods and causes the rods to make contacts with the electrodes. Rods in adjacent layers are oppositely poled, and the electrodes are configured and wired such that all the rods are connected parallel such that their generated charge is summed. Adaptation of the generator to its application is done by changing the number and thickness of the layers.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
       [0001]    The present invention is a Continuation-in-Part Application of co-pending, commonly assigned U.S. patent application Ser. No. 12/195,670, filed Aug. 21, 2008; U.S. patent application Ser. No. 12/204,958, filed Sep. 5, 2008; and U.S. patent application Ser. No. 12/353,764, filed Jan. 14, 2009, all of which are incorporated herein in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a multilayer modular energy harvesting apparatus, system for using said apparatus and method for implementation said apparatus. 
       BACKGROUND OF THE INVENTION 
       [0003]    Piezoelectricity is the ability of certain crystalline 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. Today, piezoelectric materials for industrial applications are lead based ceramics available in a wide range of properties. Piezoelectric materials are the most well known active material typically used for transducers as well as in adaptive structures. 
         [0004]    Mechanical compression or tension on a poled piezoelectric ceramic 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 ceramic 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. 
         [0005]    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. 
         [0006]    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. 
         [0007]    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. 
         [0008]    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 discuses 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. 
         [0009]    Intl. Patent Application WO/07/038157A2 entitled “Energy Harvesting Using Frequency Rectification” to Carman Gregory P. and Lee Dong G.; filed: Sep. 21, 2006 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. 
         [0010]    U.S. Pat. No. 5,265,481 to Sonderegger, Hans C., et al. entitled “Force sensor systems especially for determining dynamically the axle load, speed, wheelbase and gross weight of vehicles” discloses sensor system incorporated in road surface—has modular configuration for matching different road widths. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention relates to a multilayer modular energy harvesting apparatus, system for using said apparatus and method for implementation said apparatus. 
         [0012]    According to an exemplary embodiment of the invention, a multilayer generator is provided comprising: a box having a top cover; a bottom electrode placed above the bottom of said box; a top electrode placed below said top cover; and a multilayer electricity generating structure positioned between said bottom electrode and top electrode, each layer of said multilayer electricity generating structure comprising a plurality of piezoelectric rods held in place by a matrix layers, wherein: thickness of said matrix layer is smaller than length of said plurality of piezoelectric rods in the corresponding layer, said piezoelectric rods are oppositely poled in alternating layers, adjacent layers are separated by a central electrode. 
         [0013]    In some embodiments, said box and said matrix layers are substantially cylindrical. 
         [0014]    In some embodiments, said box constructed of a round top cover, a round bottom cover and a cylindrical mid section. 
         [0015]    In some embodiments, said round top cover and said round bottom cover are substantially identical. 
         [0016]    In some embodiments, said matrix layers are shaped and sized to snugly fit inside said box. 
         [0017]    In some embodiments, said matrix layers are configured to withstand sheer stresses in said plurality of piezoelectric rods due to pressure applied on said cover. 
         [0018]    In some embodiments, said matrix layers are made of a non-conductive material selected from the group comprising: glass; Pertinex; Plexiglas; thermoplastics; reinforced resin and concrete. 
         [0019]    In some embodiments, said cover is further configured such when pressure is applied to said cover, said applied pressure causes mechanical and electrical contact between ends of said piezoelectric rods and the nearest electrodes. 
         [0020]    In some embodiments, said rods in each layer are poled in the same direction, and are electrically connected in parallel by two of said electrodes such that the charge generated by said layer is the sum of the charges generated by all the rods in said layer. 
         [0021]    In some embodiments, said alternating electrode layers are joined such that the voltage generated by said entire multilayer generator is substantially equal to the voltage generated by one rod, and the charge generated by said entire multilayer generator is substantially equal to the sum of charges generated by all said rods. 
         [0022]    In some embodiments, said generator further comprising fasteners, wherein said fasteners are configured to apply preloading force on said cover. 
         [0023]    In some embodiments, said cover is stiff and is configured to substantially evenly spread force applied to said cover among said piezoelectric rods. 
         [0024]    In some embodiments, said generator further comprising a load spreading layer situated between said cover and said top electrode, said load spreading layer configured to receive force applied to said cover and to substantially evenly spread force among said piezoelectric rods. 
         [0025]    In some embodiments, said at least one of said box and said cover is made of concrete. 
         [0026]    In some embodiments, said box and said matrix layers are shaped as polygons. 
         [0027]    In some embodiments, said generator further comprising a seal situated between said box and said cover. 
         [0028]    According to another exemplary embodiment of the invention, a multilayer super-module generator is provided comprising: a box having a plurality of chambers; and at least one top cover, wherein in each chamber: a bottom electrode placed above the bottom of said chamber; a top electrode placed below said top cover; and a multilayer electricity generating structure positioned between said bottom electrode and top electrode, each layer of said multilayer electricity generating structure comprising a plurality of piezoelectric rods held in place by a matrix layers, wherein: thickness of said matrix layer is smaller than length of said plurality of piezoelectric rods in the corresponding layer, said piezoelectric rods are oppositely poled in alternating layers, adjacent layers are separated by a central electrode. 
         [0029]    In some embodiments, each chamber has a separate top cover. 
         [0030]    In some embodiments, all the chambers are covered by same top cover. 
         [0031]    In some embodiments, said rods in each layer in each chamber are poled in the same direction, and are electrically connected in parallel by two of said electrodes, alternating electrode layers in each chamber are joined such that the voltage generated by said entire multilayer generator in each chamber is substantially equal to the voltage generated by one rod, and the charge generated by said entire multilayer generator is substantially equal to the sum of charges generated by all said rods, and said generators in said chambers are electrically joined in parallel. 
         [0032]    According to another exemplary embodiment of the invention, a method of installing an energy harvesting system in a road comprising the steps of: drilling an array of circular holes in said road; cutting slots in said road, joining said circular holes; placing a round piezoelectric energy generator in each of said drilled holes; placing connecting cables in said cut slots; and covering said connecting cables in said cut slots. 
         [0033]    In some embodiments, said method further comprising covering said round piezoelectric energy generators placed in said drilled holes. 
         [0034]    In some embodiments, said round piezoelectric energy generators are placed in said drilled holes such that upper surface of said round piezoelectric energy generators are flush with the surface of said road. 
         [0035]    In some embodiments, the step of drilling an array of circular holes in said road comprises using cup drill. 
         [0036]    In some embodiments, said method further comprising pouring a reinforcing layer at the bottom of said drilled hole before placing a round piezoelectric energy generator in each of said drilled holes. 
         [0037]    According to another exemplary embodiment of the invention, a method of installing an energy harvesting system in a railroad comprising the steps of: providing a sleeper having a cavity under the track mount; placing in said cavity a multilayer generator comprising: a bottom electrode placed above the bottom of cavity; a top electrode placed below said mount; and a multilayer electricity generating structure positioned between said bottom electrode and top electrode, each layer of said multilayer electricity generating structure comprising a plurality of piezoelectric rods held in place by a matrix layers, wherein: thickness of said matrix layer is smaller than length of said plurality of piezoelectric rods in the corresponding layer, said piezoelectric rods are oppositely poled in alternating layers, and adjacent layers are separated by a central electrode, alternating electrode layers are joined such that said rods are electrically connected in parallel; and attaching said mount and said track to said sleeper such that force applied by a train traveling over said track causes generation of electric energy in said piezoelectric rods. 
         [0038]    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 
         [0039]    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. 
           [0040]    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. 
           [0041]    In discussion of the various figures described herein below, like numbers refer to like parts. 
           [0042]    The drawings are generally not to scale. For clarity, non-essential elements were omitted from some of the drawings. 
           [0043]    In the drawings: 
           [0044]      FIG. 1(   a ) depict piezoelectric rods as used in the art. 
           [0045]      FIG. 1(   b ) depict piezoelectric rods as used in the art. 
           [0046]      FIG. 1(   c ) depicts a multilayer piezoelectric stack according to an exemplary embodiment of the current invention. 
           [0047]      FIG. 2(   a ) schematically depicts an isometric view of a multilayer modular generator according to an exemplary embodiment of the current invention. 
           [0048]      FIG. 2(   b ) schematically depicts across section of a multilayer modular generator according to an exemplary embodiment of the current invention. 
           [0049]      FIG. 3  schematically depicts an exploded view of the internal elements of a multilayer modular generator according to an exemplary embodiment of the current invention. 
           [0050]      FIG. 4(   a ) schematically depicts an isometric view of a round multilayer modular generator according to another exemplary embodiment of the current invention. 
           [0051]      FIG. 4(   b ) schematically depicts across section of a round multilayer modular generator according to another exemplary embodiment of the current invention. 
           [0052]      FIG. 4(   c ) schematically depicts a top view of a matrix layer used in a round multilayer modular generator according to another exemplary embodiment of the current invention. 
           [0053]      FIG. 5(   a ) illustrates a cross section of a road with round multilayer modular generators embedded in it and vehicle over it, according to another exemplary embodiment of the current invention. 
           [0054]      FIG. 5(   b ) schematically depicts the stress distribution caused by a passing vehicle on a round multilayer modular generator according to another exemplary embodiment of the current invention. 
           [0055]      FIG. 5(   c ) schematically depicts a graph of the stress distribution caused by a passing vehicle according to another exemplary embodiment of the current invention. 
           [0056]      FIG. 6  schematically depicts a system for energy harvesting implemented in a road and using a plurality of round multilayer modular generators according to another exemplary embodiment of the current invention. 
           [0057]      FIGS. 7(   a - f ) schematically depict steps of constructing an energy harvesting system by embedding a plurality of round multilayer modular generators according to an exemplary embodiment of the current invention. 
           [0058]      FIG. 7(   a ) schematically depicts drilling, in a road, holes for embedding round multilayer modular generators, preferably using a cup drill. 
           [0059]      FIG. 7(   b ) schematically depicts drilling, in a road, holes for embedding round multilayer modular generators, preferably using a cup drill. 
           [0060]      FIG. 7(   c ) schematically depicts cutting, in a road, slits for embedding connecting cable, preferably using a disk saw. 
           [0061]      FIG. 7(   d ) schematically depicts optional stage of pouring reinforcing layer, preferably made of concrete at the bottom of the drilled hole. 
           [0062]      FIG. 7(   e ) schematically depicts the stage of laying the round multilayer modular generators and the connecting cables in the drilled holes and the cut slits respectively; and 
           [0063]      FIG. 7(   f ) schematically depicts the stage of refilling the drilled holes and the cut slits, preferably with asphalt. 
           [0064]      FIG. 8  schematically depicts a cross section of a railway sleeper with a modular multilayer generator according to yet another exemplary embodiment of the current invention. 
           [0065]      FIG. 9(   a ) schematically depicts an isometric view of a multilayer super-module generator according to yet another exemplary embodiment of the current invention. 
           [0066]      FIG. 9(   b ) schematically depicts a system for energy harvesting, implemented in a road and using a plurality of multilayer super-module generators according to yet another exemplary embodiment of the current invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0067]    The present invention relates to a modular energy harvesting apparatus, system for using said apparatus and method for implementation said apparatus. 
         [0068]    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. 
         [0069]      FIG. 1  schematically depict piezoelectric elements. 
         [0070]      FIG. 1(   a ) and  FIG. 1(   b ) depict piezoelectric rods used in the art, while  FIG. 1(   c ) depicts a multilayer piezoelectric stack according to an exemplary embodiment of the current invention. 
         [0071]      FIG. 1(   a ) shows a single layer piezoelectric rod used in the art for energy generation. When rode  100 , made of an elongated piezoelectric material  101 , is subjected to longitudinal force, high voltage is generated between the positive end electrode  102  and negative end electrode  104  which are bonded to the elongated piezoelectric material  101 . It should be noted that while the generated voltage is high, the electric current that can be harvested is small. It was found that controlling generated electrical signal having high voltage and low current is difficult. Converting such signal to useful electrical power that can be transported, used and stored is might be expensive and inefficient due to the high voltage generated. 
         [0072]      FIG. 1(   b ) schematically depicts a multilayer integrated piezoelectric rod  110  as used in the art. Integrated rod  110  comprises a plurality of layers  111 , each made of piezoelectric material, where adjacent layers are oppositely poled (three such layers are seen, but number of layer may be larger). The layers  111  are separated by electrodes, for example positive central electrode  113  and negative central electrode  115 . Central electrodes as well as end electrodes are bonded to their adjacent layer, thus creating a single monolithic, multi-layer stack. All positive electrodes (in this case positive central electrode  113  and positive end electrode  119 ) are connected together to the positive terminal  112 . Similarly, all negative electrodes (in this case negative central electrode  115  and negative end electrode  118 ) are connected together to the negative terminal  114 . 
         [0073]    When rode  110 , is subjected to longitudinal force, a relatively voltage is generated between the positive terminal  112  and negative terminal  114 . Compared to the signal generated by single layer rod  100 , the signal generated by monolithic multilayer rod  110  is N times smaller while the generated charge (or ccurrent for the close-loop circuit) is N times larger (wherein N is the number of layers). 
         [0074]    It should be noted that the process of bonding the electrodes and creating a monolithic multilayer stack might be expensive. 
         [0075]      FIG. 1(   c ) schematically depicts a multilayer piezoelectric stack  120  according to an exemplary embodiment of the current invention. Multilayer piezoelectric stack  120  comprises a plurality of separate piezoelectric rods  121 , each made of piezoelectric material, where adjacent layers are oppositely poled (three such layers are seen, but number of layer may be larger). The rods  121  are separated by electrodes, for example positive central electrode  123  and negative central electrode  125 . Central electrodes as well as end electrodes are not bonded to their adjacent layers, but instead make mechanical and electrical contact with the layer when the stack is under pressure. All positive electrodes (in this case positive central electrode  123  and positive end electrode  129 ) are connected together to the positive terminal  122 . Similarly, all negative electrodes (in this case negative central electrode  125  and negative end electrode  128 ) are connected together to the negative terminal  124 . 
         [0076]    When rode  120 , is subjected to longitudinal force, voltage is generated between the positive terminal  122  and negative terminal  124 . Compared to the signal generated by single layer rod  100 , the voltage output generated by multilayer stack  120  is N times smaller while the generated charge (or current) is N times larger (wherein N is the number of layers). 
         [0077]    As multilayer stack  120  is not bonded, it is preferably supported by an external support structure which will be seen in the following figures. Assembling the multilayer stack  120  is easy and cheap as it requires only stacking the layers and the electrodes within the supporting structure. Additionally, the support structure may be configured to prevent the multilayer stack  120  from bucking under pressure by resisting shear forces that may developed as the stack tries to bend. Thus, it may be possible to apply larger forces on a supported multilayer stack  120  than to the monolithic rod  100 . 
         [0078]    It should be noted that piezoelectric rods  121  are appears as having cylindrical shape in  FIG. 1(   c ) as exemplary embodiment. Other shapes may be used. For example, rods may have square rectangular, hexagonal, oval or any other cross-section. Length of the rods may vary, wherein for the same stress, shorter rods yields lower voltage than longer rods. Shorter rods enable situating more rods along the same length of multilayer stack  120 , thus generating larger charge for the same applied longitudinal force. 
         [0079]      FIG. 2(   a ) schematically depicts an isometric view of a multilayer modular generator according to an exemplary embodiment of the current invention. 
         [0080]    Multilayer modular generator  200  comprises a box  210  and a cover  212 . An electric cable  221 , preferably comprising a positive lead and a negative lead, transfers electrical signal generated by a plurality of supported multilayer stacks  120  within box  210  in response to pressure applied to cover  212 . 
         [0081]      FIG. 2(   b ) schematically depicts a across section of a multilayer modular generator according to an exemplary embodiment of the current invention. 
         [0082]    Multilayer modular generator  200  comprises a box  210  and a cover  212 . An electric cable  221 , preferably comprising a positive lead  221   a  and a negative lead  221   b,  transfers electrical signal generated by a plurality of supported multilayer stacks within box  210  in response to pressure applied to cover  212 . Preferably, a flexible seal  214  may used to prevent moisture and dirt from entering the box, while allowing small relative motion of the cover in response to force applied to it. 
         [0083]    Piezoelectric stacks within box  210  comprises of plurality of individual piezoelectric elements  201 , arranged in layers. Elements  201  in each layer are held in place by a matrix layer  231  (tree such matrix layers:  231   a ,  231   b  and  231   c  are seen, but number of layers may be two or larger than three). Piezoelectric elements in adjacent layers are oppositely poled and are separated by central electrode layers (two such electrode layers are seen, positive central electrode layer  224   a  and negative central electrode layer  224   c ). Top surfaces of piezoelectric elements in top layer make contact with top electrode layer (in the example depicted here—positive end electrode layer  224   d ). Similarly, bottom surfaces of piezoelectric elements in bottom layer make contact with bottom electrode layer (in the example depicted here—negative end electrode layer  224   a ). 
         [0084]    All positive electrodes (in this case positive central electrode  224   b  and positive end electrode  224   d ) are connected together to the positive terminal  221   a . Similarly, all negative electrodes (in this case negative central electrode  224   c  and negative end electrode  224   a ) are connected together to the negative terminal  221   a.    
         [0085]      FIG. 3  schematically depicts an exploded view of the internal elements of a multilayer modular generator  200  according to an exemplary embodiment of the current invention. 
         [0086]    In this figure, the arrangement of piezoelectric elements  201  in a layers and columns can be seen. In each layer the piezoelectric elements  201  are arranged in a substantially identical two-dimensional array and held in place in holes  232  in the corresponding matrix layer  231 . For clarity, only few of the piezoelectric elements  201  were presented. 
         [0087]    In the exemplary embodiment, a 3×3 array of piezoelectric elements  201  is depicted for clarity. However larger array are typically used. The array need not be square or symmetrical. Preferably, the shape of the matrix layer, the box and the cover is such that it can tile a surface (triangle, rectangular, square, or hexagonal). Similarly, any shape may be selected, such as oval, round, etc, within the general scope of the current invention. 
         [0088]    Number of Matrix layers  231  is preferably equal to the number of layers of piezoelectric elements  201 . In the exemplary embodiment, three layers of piezoelectric elements  201  are depicted for clarity (held in place by matrix layers  231   a ,  231   b  and  231   c ). However any number of layers may be used. 
         [0089]    Matrix layers  231  are preferably made of strong, nonconductive material so it can support piezoelectric elements  201  against shear stresses while electrically insulating them and the electrode layers. In a preferred embodiment of the invention, matrix layers  231  are made of glass sheets having holes  232  drilled in them. Optionally, the glass sheets are tempered or thermally treated to reduce internal stress or increase strength after holes  232  were drilled. Alternatively, glass sheet are casted. Using glass to manufacture the matrix layers is advantageous due to the mechanical and electrical properties of glass. Additionally, glass is cheap, nontoxic, easily processed, and easily disposable or recyclable. Alternatively, molded plastic is used. It should be noted that matrix layers may also be patterned with holes other than holes  232  for reducing cost and weight. 
         [0090]    Central electrodes  224   b  and  224   c  as well as end electrodes  224   a  and  224   d  are preferably patterned having contact pads  223  electrically connected by connecting lines  2621 . Electrode layers are preferably made of a thin conductive material, for example copper or copper alloy. This pattern reduces the amount of metal used for the electrode layer, thus reducing the cost. However, electrodes may be a full or perforated sheet or differently patterned. 
         [0091]    Although elements  201  and holes  232  are depicted cylindrical, other shapes may be used. For example, Elements  201  (or holes  232 ) may be shaped as rectangular boxes or bars. 
         [0092]      FIG. 4(   a ) schematically depicts an isometric view of a round multilayer modular generator  400  according to a preferred embodiment of the current invention. 
         [0093]    In contrast to the rectangular shape of multilayer modular generator  200 , round multilayer modular generator  400  is shaped as a wide cylinder. Positive lead  421   a  and negative lead  421   b  exit the casing of the round multilayer modular generator. 
         [0094]    In the non-limiting exemplary embodiment of  FIG. 4 , casing of round multilayer modular generator  400  comprises a top cover  405 , a bottom cover  407  and mid section  406 . Preferably, a flexible seal (not seen in these figures for clarity) may be used to prevent moisture and dirt from entering the box, while allowing small relative motion of the covers in response to force applied to them. 
         [0095]    Fasteners, such as screws  410  (three are seen, but different number of fasteners may be used), holds the casing together. Preferably, fasteners  410  apply some pressure on the layered structure within the casing, pre-loading the piezoelectric elements and ensuring mechanical and electrical contact between the piezoelectric elements and the electrode. Preloading the piezoelectric elements increases the energy yield of the generator and prevents energy loss for making contact in loose structure every time pressure is applied. Optionally, fasteners  410  comprise elastic elements that maintain the proper preloading force. Optionally, fasteners  410  are screws that are tightened with predetermined torque. It should be noted that mechanical and/or electrical contact between rods and electrodes may exist without preloading the screws, for example due to tightly fitting the internal elements of the generator into the box, or due to weight of the cover and/or the internal elements, or elastic properties of the internal elements or the box. 
         [0096]      FIG. 4(   b ) schematically depicts across section of a round multilayer modular generator according to another exemplary embodiment of the current invention. 
         [0097]    The layered structure of round multilayer modular generator is depicted, showing matrix layers  431   a  to  431   c , electrode layers  424   a  to  424   d  and piezoelectric elements  401 . 
         [0098]    In the depicted exemplary embodiment, top and bottom covers are similar or substantially identical to reduce design and production costs. In this design, number of layers can easily changed by changing the length of midsection casing part  406  and inserting different number of layers into the case. 
         [0099]      FIG. 4(   c ) schematically depicts a top view of round matrix layer  424  used in a round multilayer modular generator  400  according to another exemplary embodiment of the current invention. 
         [0100]    Round matrix layer  424  comprises holes  432  for piezoelectric elements  401  and holes  417  for fasteners  411 . 
         [0101]      FIG. 5  demonstrate an advantage of a round multilayer modular generator according to another exemplary embodiment of the current invention. 
         [0102]      FIG. 5(   a ) shows a cross section of a road with round multilayer modular generators embedded in it and vehicle over it, according to another exemplary embodiment of the current invention. When a vehicle (only axle  529  is seen for clarity), passes over a road  519 , the wheels  530  slightly are distorted  539  and make a substantially rectangular contact with the road. The asphalt layer  520  of the road, which is placed over the foundation  510  slightly, distorts  540  due to the pressure applied by wheels  530  and the strain is transferred to generators  400  embedded within asphalt layer  520 . For clarity, road distortion  540  was exaggerated. 
         [0103]      FIG. 5(   b ) schematically depicts the stress distribution caused by a passing vehicle on a round multilayer modular generator  400  according to another exemplary embodiment of the current invention. As stress spreads deeper and latterly within the asphalt layer  520 , zones of high stress  540  and zones of low stress  511  develop. By placing round multilayer modular generators  400  such that the majority of their volume is within the zones of high stress  540 , efficient use of the available energy is possible. 
         [0104]      FIG. 5(   c ) schematically depicts a graph of the stress distribution caused by a passing vehicle according to another exemplary embodiment of the current invention. 
         [0105]    The zone of high stress  540  and zones of low stress  511  can be seen in this figure. 
         [0106]      FIG. 6  schematically depicts a system  600  for energy harvesting implemented in a road and using a plurality of round multilayer modular generators  400  according to another exemplary embodiment of the current invention. 
         [0107]    In the depicted example, two lanes road  650  having curbs  651  is embedded with a plurality of round multilayer modular generators  400 , preferably placed at locations 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 . 
         [0108]    Preferably, Connecting cables  614  and  612  are also embedded beneath the surface of road  650 . 
         [0109]      FIG. 7  schematically depict steps of constructing an energy harvesting system  600  by embedding a plurality of round multilayer modular generators according to another exemplary embodiment of the current invention. 
         [0110]      FIG. 7(   a ) and  FIG. 7(   b ) schematically depicts drilling, in a road, holes for embedding round multilayer modular generators, preferably using a cup drill. 
         [0111]    Drilling circular holes in a road is easier than cutting rectangular holes. Drilling hole  710  in asphalt layer  520  having an upper surface  519  and 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 . 
         [0112]      FIG. 7(   c ) schematically depicts cutting in a road&#39;s asphalt layer  520 , slits  720  for embedding connecting cable  614 , preferably using a disk saw  711  (seen in  FIG. 7(   b )). 
         [0113]    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 . 
         [0114]      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 ad 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. 
         [0115]      FIG. 7(   e ) schematically depicts the stage of laying the round multilayer modular generator  400  in drilled hole  710  over optional reinforcement  730 , and placing the connecting cables  614  in the cut slit  720 . 
         [0116]      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. 
         [0117]      FIG. 8  schematically depicts a cross section of a part of railway sleeper  810  with a modular multilayer generator  200  according to yet another exemplary embodiment of the current invention. 
         [0118]    In this cross section, multilayer modular generator  800  is seen placed in a recess in sleeper  810 . Preferably multilayer modular generator  800  is multilayer modular generator  200  as depicted in  FIGS. 2 , but other types of multilayer modular generators may be used. For example round multilayer modular generator  400 . Optionally, internal elements depicted in  FIG. 3  or  4 ( b ) are placed in a recess in sleeper  810  such that the internal recess acts as a box and mount  840  and elastomeric layer  850  acts as cover. 
         [0119]    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 presses on multilayer modular generator  200 , causing charge to be generated in said generator. Depth of recess for multilayer modular generator  800  in sleeper  810  is limited by metal reinforcement cables or bars  820  in sleeper  810 . This depth limits the number of layers in multilayer modular generator  200 . However, Sleeper  810  may be redesigned to allow deeper recesses. Similarly, width of for multilayer modular generator  800  in sleeper  810  is limited by the distance between screws  825  which hold mount  840  to sleeper  810 ; however, sleeper  810  may be redesigned to allow wider or narrower recesses. 
         [0120]    It should be noted that rods  201  multilayer modular generator  200  ( 401  for round multilayer modular generator  400 ) are at least slightly longer than the corresponding matrix layer  231  ( 431 ), thus, when pressure is applied between the top and the bottom of the multilayer modular generator  200  ( 400 ), a physical and electrical contact is formed between the edges of rods  201  ( 401 ) and electrodes  224  ( 424 ). Electrode layers  224  ( 424 ) electrically connect rods  201 ( 401 ) in parallel. Pressure may be applied using fasteners  410  (not seen in  FIGS. 2(   a ) and  2 ( b ), but shown in  FIGS. 4(   a ) and  4 ( b )). Alternatively or additionally, pressure may be applied by the weight of cover  212  ( 405 ), which may be made of heavy material for example concrete or metal. Optionally, the entire box is made of concrete. Alternatively or additionally, pressure may be applied by encapsulation of the entire multilayer modular generator in elastic encapsulation. Encapsulation may also provide added protection against moisture, in addition to seal  214  (seen in  FIG. 2(   b ), but may be implemented in round modular generator  400  as well). Alternatively or additionally, pressure may be applied by the weight of embedding material used for covering the multilayer modular generator, for example asphalt or bitumen layer  750  seen in  FIG. 7(   f ) for the case of round multilayer modular generator  400 , but similarly applies when embedding multilayer modular generator  200 . When a multilayer modular generator is embedded in railroad sleeper (FIG.  8 ), the weight of the track  830 , and pressure applied to hold track  830  in place causes physical and electrical contact between the edges of rods and the electrodes. 
         [0121]    Optionally, a pressure spreading layer (not shown for clarity) may be inserted between cover  212  ( 405 ) and top electrode layer  224   d  ( 424   d ). In this case, the cover  212  ( 405 ) may be relatively thin and mechanical forces applied to the cover is transferred to said pressure spreading layer and spread among the plurality of rods  201  ( 401 ). Similarly, optionally, a pressure spreading layer (not shown for clarity) is inserted between the bottom of box  210  ( 407 ) and bottom electrode layer  224   a  ( 424   a ) to spread the forces. Alternatively, optionally or additionally covers  212  ( 405 ) are stiff and acts as force spreading member. 
         [0122]      FIG. 9(   a ) schematically depicts an isometric view of a multilayer super-module generator  900  according to yet another exemplary embodiment of the current invention. 
         [0123]    Super-module generator  900  preferably comprises a container  910  within which a plurality of multilayer modular generator  200  is placed. Optionally container  910  is divided to chambers by dividers  930 . 
         [0124]    Alternatively, internal parts of multilayer modular generator  200 , as depicted in  FIG. 3  are placed in each chamber of container  910  and covered with individual or a common cover (not seen in this figure). Leads  921  from each multilayer modular generator are united into electrical cable  912 . 
         [0125]    It should be noted that the array of 2×3 multilayer modular generators  200  in a super-module generator  900  is exemplary, and other shapes, number and orientations of multilayer modular generators is possible. 
         [0126]    Multilayer modular generator  200 , round generator  400 , and super-module generator  900  may be embedded in a road, airport runway, indoor floor or street pavement to harvest energy from vehicles or pedestrians. 
         [0127]      FIG. 9(   b ) schematically depicts a top view of a system for energy harvesting  960 , implemented in a road  650  and using a plurality of multilayer super-module generators  900 , according to yet another exemplary embodiment of the current invention. 
         [0128]    In this figure, system for energy harvesting  960  comprises a plurality of multilayer super-module generators  900  and connecting cables  912 , embedded below the surface of a road or a highway  650 . 
         [0129]    In the depicted example, two lanes road  650  having curbs  651  is embedded with a plurality super-module generators  900 , preferably placed at locations were wheels of traveling vehicles are most likely to pass. Connecting cables  912 , transfer generated electrical energy to a control unit  918  for storage or for delivery to energy user such as electrical main grid vial cable  990 . 
         [0130]    For simplicity, only one lane of road  650  is seen fitted with super-module generators  900 , however, few or all lanes may be fitted with super-module generators  900 . 
         [0131]    In the depicted example, super-module generators  900  are seen placed to form a single row. However, super-module generators  900  may be placed in two rows per lane, each where a wheel of a traveling care is likely to pass. Optionally, super-module generators  900  may be placed in a two dimensional array to tile a large area. 
         [0132]    It should be noted the novel modular multilayer construction of multilayer modular generator  200 , round multilayer modular generator  400  and super-module generators  900  enables flexible fitting of a piezoelectric generator to its specific application, preferably with minimal redesign of few mechanical parts such as box  210 , mid section  406  or container  910 . 
         [0133]    It should be noted that modular generator  200 , round modular generator  400  and super-module generator  900  may be embedded under the surface  519  of the road, or alternatively, modular generator  200 , round modular generator  400  and super-module generator  900  may be placed such that their upper surface is flush with the surface  519  of the road so that mechanical pressure is directly transferred to their covers. In this later case, asphalt or bitumen layer  750  seen in  FIG. 7(   f ) is missing. 
         [0134]    It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub combination. 
         [0135]    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.