Abstract:
Embodiments of the invention disclose a method and a system configured to transfer energy wirelessly, comprising a source configured to transfer the energy wirelessly to a sink via a coupling of evanescent waves, wherein the source generates an electromagnetic (EM) near-field in response to receiving the energy; and an energy relay arranged such that to increase the coupling between the source and the sink, wherein the source, the sink, and the energy relay are electromagnetic and non-radiative structures.

Description:
RELATED APPLICATIONS 
       [0001]    This application is related to U.S. patent application Ser. No. (MERL- 2218 )  12 / 630 , 498  filed Dec. 3, 2009, entitled “Wireless Energy Transfer with Negative Index Material” filed by Koon Hoo Teo, and U.S. patent application Ser. No. (MERL-2259) 12/xxx,xxx filed Dec. xx, 2009, entitled “Wireless Energy Transfer with Negative Index Material” co-filed herewith by Koon Hoo Teo and incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to transferring energy, and more particularly, to transferring energy wirelessly. 
       BACKGROUND OF THE INVENTION 
       [0003]    Wireless Energy Transfer 
         [0004]    Inductive coupling is used in a number of wireless energy transfer applications such as charging a cordless electronic toothbrush or hybrid vehicle batteries. In coupled inductors, such as transformers, a source, e.g., primary coil, generates energy as an electromagnetic field, and a sink, e.g., a secondary coil, subtends that field such that the energy passing through the sink is optimized, e.g., is as similar as possible to the energy of the source. To optimize the energy, a distance between the source and the sink should be as small as possible, because over greater distances the induction method is highly ineffective. 
         [0005]    Resonant Coupling System 
         [0006]    In resonant coupling, two resonant electromagnetic objects, i.e., the source and the sink, interact with each other under resonance conditions. The resonant coupling transfers energy from the source to the sink over a mid-range distance, e.g., a fraction of the resonant frequency wavelength. 
         [0007]      FIG. 1  shows a conventional resonant coupling system  100  for transferring energy from a resonant source  110  to a resonant sink  120 . The general principle of operation of the system  100  is similar to inductive coupling. A driver  140  inputs the energy into the resonant source to form an oscillating electromagnetic field  115 . The excited electromagnetic field attenuates at a rate with respect to the excitation signal frequency at driver or self resonant frequency of source and sink for a resonant system. However, if the resonant sink absorbs more energy than is lost during each cycle, then most of the energy is transferred to the sink. Operating the resonant source and the resonant sink at the same resonant frequency ensures that the resonant sink has low impedance at that frequency, and that the energy is optimally absorbed. An example of the resonant coupling system is disclosed in published U.S. Patent Applications 2008/0278264 and 2007/0222542, incorporated herein by reference. 
         [0008]    The energy is transferred, over a distance D, between resonant objects, e.g., the resonant source having a size L 1  and the resonant sink having a size L 2 . The driver connects a power provider to the source, and the resonant sink is connected to a power consuming device, e.g., a resistive load  150 . Energy is supplied by the driver to the resonant source, transferred wirelessly and non-radiatively from the resonant source to the resonant sink, and consumed by the load. The wireless non-radiative energy transfer is performed using the field  115 , e.g., the electromagnetic field or an acoustic field of the resonant system. For simplicity of this specification, the field  115  is an electromagnetic field. During the coupling of the resonant objects, evanescent waves  130  are propagated between the resonant source and the resonant sink. 
         [0009]    Coupling Enhancement 
         [0010]    According to coupled-mode theory, strength of the coupling is represented by a coupling coefficient k. The coupling enhancement is denoted by an increase of an absolute value of the coupling coefficient k. Based on the coupling mode theory, the resonant frequency of the resonant coupling system is partitioned into multiple frequencies. For example, in two objects resonance compiling systems, two resonant frequencies can be observed, named even and odd mode frequencies, due to the coupling effect. The coupling coefficient of two objects resonant system formed by two exactly same resonant structures is calculated by partitioning of the even and odd modes according to 
         [0000]      κ=π|f even −f odd |  (1)
 
         [0011]    It is a challenge to enhance the coupling. For example, to optimize the coupling, resonant objects with a high quality factor are selected 
         [0012]    Accordingly, it is desired to optimize wireless energy transfer between the source and the sink. 
       SUMMARY OF THE INVENTION 
       [0013]    This invention is based on a realization that a coupling of evanescent waves between an energy source and an energy sink can be optimized by arranging strategically at least one or more energy relays in a neighborhood of the source and the sink such that some evanescent waves generated by the source are redirected by the energy relay to the sink. 
         [0014]    One embodiment of the invention discloses a system configured to transfer energy wirelessly, comprising a source configured to transfer the energy wirelessly to a sink via a coupling of evanescent waves, wherein the source generates an electromagnetic (EM) near-field in response to receiving the energy; and an energy relay arranged such that to increase the coupling between the source and the sink, wherein the source, the sink, and the energy relay are electromagnetic and non-radiative structures. 
         [0015]    Another embodiment of the invention discloses a method for transferring energy wirelessly via a coupling of near-fields, comprising steps of providing a source configured to transfer an energy wirelessly to a sink via the coupling of near-fields of the source and the sink, wherein the source and the sink are electromagnetic (EM) and non-radiative structures configured to generate EM near-fields in response to receiving the energy; providing an energy relay configured to increase the coupling between the source and the sink when the sink is arranged in a predetermined location; and transferring the energy wirelessly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a block diagram of a conventional resonant coupling system; 
           [0017]      FIG. 2A  is an example of a system for transferring energy using an energy relay according to embodiments of the invention; 
           [0018]      FIG. 2B  is a diagram of an electromagnetic structure according an embodiment of the invention; 
           [0019]      FIGS. 3-5  are diagrams of different energy distribution pattern; 
           [0020]      FIG. 6  is an example of a system for supplying energy wirelessly using multiple energy relays; 
           [0021]      FIG. 7  example of an implementation of the energy relay; and 
           [0022]      FIGS. 8-13  are schematics illustrating effects of different embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Embodiments of the invention are based on a realization that a coupling of evanescent waves between an energy source and an energy sink can be optimized by arranging strategically at least one energy relay in a neighborhood of the source and the sink such that some evanescent waves generated by the source are redirected by the energy relay to the sink. 
         [0024]      FIG. 2A  shows an embodiment of our invention configured to optimized wireless energy transfer from the source  210  to the sink  220 . When the driver  240  supplies the energy  260  to the source  210 , the source generates an EM near-field  215 . Typically, the near-field  215  is generated according to a particular energy distribution pattern. The pattern, as described below, has different zones such as optimal zones, wherein near-field intensities are optimal, i.e., maximum. In blind zones, the near-field intensities are suboptimal. 
         [0025]    Some of the evanescent waves  230 , which are confined to the near field  215 , directly reach and couple to the sink. However, some other evanescent waves  235  reach the energy relay  222  and are redirected to the sink within a near-filed  216 . Without the energy relay, the waves  235  are substantially useless for the energy transmission. 
         [0026]    A distance and an orientation between the source and the sink are used to determine a particular arrangement of the energy relay. In some embodiments, the energy relay is passive, i.e., the energy not connected to any external source of energy and redirects the evanescent waves received from the source. In other embodiments, the energy relay is active, i.e., configured to absorb some of the energy transferred with the near-field  215 , amplify the energy and regenerate the near-filed  216 . Accordingly, the embodiments increase the coupling between the source and the sink and facilitate transferring the energy wirelessly between the source and the sink over a longer distance than without the relay. 
         [0027]      FIG. 2B  shows a structure  200  according an embodiment of the invention. The system is configured to exchange, e.g., transmit or receive, energy wirelessly and includes the structure  210  configured to generate an electromagnetic near-field  220  when the energy is received by the structure and exchange the energy wirelessly via a coupling of evanescent waves. 
         [0028]    In one embodiment, the energy  260  is supplied by the driver  240  as known in the art. In this embodiment, the structure  210  serves as a source of the wireless energy transfer system. In an alternative embodiment, the energy  260  is supplied wirelessly from the source (not shown). In that embodiment, the structure  210  serves as the sink of the wireless energy transfer system. 
         [0029]    The system  200  optionally includes negative index material (NIM)  231 - 234  arranged within the near-field  215 - 216 . In one embodiment, the NIM  233  substantially encloses the EM structure  210 . The NIM is a material with negative permittivity and negative permeability properties. Several unusual phenomena are known for this material, e.g., evanescent wave amplification, surface plasmoni-like behavior and negative refraction. Embodiments of the invention appreciated and utilized the unusual ability of NIM to amplify evanescent waves, which optimizes wireless energy transfer. 
         [0030]    The shape and dimensions of the near-field, i.e., the energy distribution pattern, depends on a frequency of the external energy  260 , and on a resonant frequency of the EM structure  210 , determined in part by a shape of the EM structure, e.g., circular, helical, cylindrical shape, and parameters of a material of the EM structure such as conductivity, relative permittivity, and relative permeability. 
         [0031]    Usually, a range  270  of the near-field is in an order of a dominant wavelength of the system. In non resonant systems, the dominant wavelength is determined by a frequency of the external energy  260 , i.e., the wavelength λ  265 . In resonant systems, the dominant wavelength is determined by a resonant frequency of the EM structure. In general, the dominant wavelength is determined by the frequency of the wirelessly exchanged energy. 
         [0032]    The resonance is characterized by a quality factor (Q-factor), i.e., a dimensionless ratio of stored energy to dissipated energy. Because the objective of the system  200  is to transfer or to receive the energy wirelessly, the frequency of the driver or the resonant frequency is selected to increase the dimensions of the near-field region. In some embodiments, the frequency of the energy  260  and/or the resonant frequency is in diapason from MHz to GHz. In other embodiments, aforementioned frequencies are in the domain for visible light. 
         [0033]    Evanescent Wave 
         [0034]    An evanescent wave is a near-field standing wave with an intensity that exhibits exponential decay with distance from a boundary at which the wave is formed. The evanescent waves  235  are formed at the boundary between the structure  210  and other “media” with different properties in respect of wave motion, e.g., air. The evanescent waves are formed when the external energy is received by the EM structure and are most intense within one-third of a wavelength of the near field from the surface of the EM structure  210 . 
         [0035]    Whispering Gallery Mode (WGM) 
         [0036]    Whispering gallery mode is the energy distribution pattern in which the evanescent waves are internally reflected or focused by the surface of the EM structure. Due to minimal reflection and radiation losses, the WGM pattern reaches unusually high quality factors, and thus, WGM is useful for wireless energy transfer. 
         [0037]      FIG. 3  shows an example of the EM structure, i.e., a disk  310 . Depending on material, geometry and dimensions of the disk  310 , as well as the dominant frequency, the EM near-field intensities and energy density are maximized at the surface of the disk according to a WGM pattern  320 . 
         [0038]    The WGM pattern is not necessarily symmetric to the shape of the EM structure. The WGM pattern typically has blind zones  345 , in which the intensity of the EM near-field is minimized, and optimal zones  340 , in which the intensity of the EM near-field is maximized. Some embodiments of the invention place the NIM  230  in the optimal zones  340  to extend a range of the evanescent waves  350 . 
         [0039]    Even and Odd Modes 
         [0040]      FIG. 4  shows a butterfly energy distribution pattern. When two EM structures  411  and  412  are coupled to each other forming a coupled system, the dominant frequency of the coupled system is represented by even and odd frequencies. The near-field distribution at even and odd frequencies is defined as even mode coupled system  410  and an odd mode coupled system  420 . Typical characteristic of the even and the odd modes of the coupled system of two EM structures is that if the EM field is in phase in the even mode then the EM field is out of phase in the odd mode. 
         [0041]    Butterfly Pair 
         [0042]    The even and odd mode coupled systems generate an odd and even mode distribution patterns of the near-field intensities defined as a butterfly pair. The EM near-field intensity distribution of the butterfly pair reaches minimum in two lines  431  and  432  oriented at 0 degree and 90 degree to the center of each EM structure, i.e., blind zones of the butterfly pair. However, it is often desired to change the intensity distribution and eliminate and/or change the positions and/or orientations of the blind zones. 
         [0043]    Crossing Pair 
         [0044]      FIG. 5  shows distribution patterns of the near-field intensities according embodiments of the invention define as a crossing pair  500 . The crossing pair distribution pattern has optimal zones  531  and  532  oriented at 0 degree and 90 degree to the center of each EM structure, i.e., the optimal zones of the crossing pair pattern corresponds to the blind zones of the butterfly pair pattern. Therefore, one important characteristic of the butterfly pair and the crossing pair patterns is that their respective blind zones are not overlapping, and thus allows for eliminating the blind zones when both kinds of patterns are utilized. Butterfly and crossing patterns have the system quality factor and the coupling coefficient of the same order of magnitude. 
         [0045]    Energy Relays Arrangement 
         [0046]    Some embodiments of the invention use the knowledge of butterfly and crossing pair energy distribution pattern to arrange the energy relays in the neighborhood of the source and the sink. In some embodiment the location of the sink is predetermined, and the energy relays are arranged such that to optimize the coupling between the source and the sink when the sink is arranged in the predetermined location. In some embodiments this objective is achieved experimentally. 
         [0047]    In another embodiment, the source is configured to transmit the energy to multiple sinks. Accordingly, the energy relays are arranged to increase the coupling of more than one sink. 
         [0048]      FIG. 6  shows an example of a system  600  configured to optimized transmission of the energy from the source  610  to the sink  620  using a first energy relay  630  and a second energy relay  640 . In this embodiment, the EM structures of the source, sink, and the energy relay are implemented as a loop  700  as shown in  FIG. 7 . The loop of a radius r is formed by a conductor wire  710  of a radius a, and by a capacitor  720  having a relative permittivity a A plate area of the capacitor is A, and the plates are separated over a distance d. The loop  700  has axis  705  and is a resonant structure. However, other embodiment uses different implementation of the structures, e.g., a disc. 
         [0049]    The source  610  and the sink  620  are arranged over a distance D from each other measured from their respective centers. The source and the sink are aligned such that axes of the source and the sink lie along the same line. The source is connected to the driver (not shown) and the sink is connected to the load (not shown). 
         [0050]    The first and the second energy relays are separated by a distance d S  and are arranged such as to increase the coupling of evanescent waves between the source and the sink. The distance d S  is selected such that the energy relays are not coupled strongly to each other. In one embodiment, the loops of the energy relays are rotated such that their axes points towards the sink. In another embodiment the axes of the loops of the energy relays are perpendicular to the axis of the source and sink. In yet another embodiment the orientation of the energy relays is arbitrary. 
         [0051]      FIGS. 8-11  show schematics illustrating dependencies of frequencies of the system on arrangements of the source  610  and the sink  620 , wherein the energy relays are inactive. For example, as the distance between the source and the sink increases the odd  805  and even  815  mode frequencies converge towards a dominant frequency  825 , as shown in  FIG. 8 . 
         [0052]      FIG. 9  shows a schematic illustrating effect of the rotation of either the source or the sink on the mode frequencies. In this embodiment, the two mode frequencies relatively stable despite of the rotation. 
         [0053]      FIG. 10  shows a schematic illustrating effect of displacement of the source or the sink from the coaxial alignment on the mode frequencies. In one embodiment, the displacement is within a range from 60 cm to 0. As shown, after the displacement reaches a threshold, e.g., 60 cm, the odd and even frequencies approach the individual resonator frequencies. 
         [0054]      FIG. 11  shows coupling coefficients for different arrangements of the source and the sink. As shown, the distance between the source and the sink affect the coupling coefficient the most, followed by the displacement and then the rotation. 
         [0055]      FIGS. 12 and 13  show graphs comparing embodiments of the invention with and without energy relays.  FIG. 12  shows that the coupling coefficient is larger for the system which includes the energy relays, i.e., the curves  1200  and  1220 , than for the system with the inactive energy relays, i.e., the curves  1210  and  1230 .  FIG. 13  shows the comparison between the coupling coefficients of the systems with and without energy relays. 
         [0056]    Some embodiments of the invention use a larger network of passive or active energy relays that allow the coupling to be optimized over a range of distances. Typically, the energy relays are arranged such that they do not strongly couple to the sink-source resonant link. 
         [0057]    Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications may be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.