AU2013203947B2 - Wireless non-radiative energy transfer - Google Patents

Wireless non-radiative energy transfer Download PDF

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AU2013203947B2
AU2013203947B2 AU2013203947A AU2013203947A AU2013203947B2 AU 2013203947 B2 AU2013203947 B2 AU 2013203947B2 AU 2013203947 A AU2013203947 A AU 2013203947A AU 2013203947 A AU2013203947 A AU 2013203947A AU 2013203947 B2 AU2013203947 B2 AU 2013203947B2
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resonant
resonant structure
source
system
device
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John D. Joannopoulos
Aristeidis Karalis
Martin Soljacic
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Massachusetts Institute of Technology
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Massachusetts Institute of Technology
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Abstract

There is disclosed a system, comprising: a source resonant structure and a device resonant structure, the structures capable of performing wireless near-field energy transfer with a coupling rate K when separated a variable distance D from each other, said source resonant structure having a resonant frequency fi=oi1/27u, an intrinsic loss rate F 1, and a first Q-factor Qi=oi/(2Fi), where co is the angular frequency corresponding to the resonant frequency fi, said device resonant structure having a resonant frequency f2=o2/2n, an intrinsic loss rate F 2 , and a second Q-factor Q2=oi2/(2F 2), where 02 is the angular frequency corresponding to the resonant frequency f2, wherein the absolute value of the difference of said angular frequencies co and 02 is smaller than the magnitude of the coupling rate, K, and wherein at least one of the resonant structures comprises a high-Q capacitively-loaded conducting-wire loop. G) ~ C) C-) C-) CI) r

Description

H pail - r cNRP~ornhDCCPXA -N,0%44 1 dm21]11,201t. WIRELESS NON-RADIATIVE ENERGY TRANSFER The entire content of US provisional application Ser. No. 60/698,442 filed July 12, 2005, is incorporated herein by reference. 5 The invention relates to the field of oscillatory resonant electromagnetic modes, and in particular to oscillatory resonant electromagnetic modes, with localized slowly evanescent field patterns, for wireless non-radiative energy transfer. In the early days of electromagnetism, before the electrical-wire g id was deployed, serious interest and effort was devoted towards the development of schemes to transport 10 energy over long distances wirelessly, without any carrier medium. These efforts appear to have met with little, if any, success. Radiative modes of omni-directional antennas. which work very well for information transfer. are not suitable for such energy transfer. because a vast majority of energy is wasted into free space. Directed radiation modes, using lasers or highly-directional antennas, can be efficiently used for energy transfer, even for long distances 15 (transfer distance L TRy5.ANSD)L/)1 where L/)Av is the characteristic size of the device). but require existence of an uninterruptible line-of-sight and a complicated tracking system in the case of mobile objects. Rapid development of autonomous electronics of recent years (e.g. laptops, cellphones, house-hold robots, that all typically rely on chemical energy storage) justifies revisiting 20 investigation of this issue. Today, the existing electrical-wire grid carries energy almost everywhere; even a medium-range wireless non-radiative energy transfer would be quite useful. One scheme currently used for some important applications relies on induction, but it is restricted to very close-range (LTRANSc'LDEI) energy transfers. According to a first aspect of the present invention, there is provided a system, 25 comprising: a source resonant structure and a device resonant structure, the structures capable of performing wireless near-field energy transfer with a coupling rate K when separated a variable distance D from each other, said source resonant structure having a resonant frequency f 1 =0o/2n, an intrinsic loss rate F 1 , and a first Q-factor Q 1 =o 1 /(2F 1 ). where o ) is the angular frequency corresponding to the resonant frequency f, said device resonant structure 30 having a resonant frequency f2=o2/2n. an intrinsic loss rate F 2 , and a second Q-factor

Q

2 =o 2 /(2F 2 ), where a2 is the angular frequency corresponding to the resonant frequency f 2

.

H p r NRo orthh DCC\PX22AW %44_Ldos 21: 2.'6 -2 wherein the absolute value of the difference of said angular frequencies co and co2 is smaller than the magnitude of the coupling rate, x. and wherein at least one of the resonant structures comprises a high-Q capacitively-loaded conducting-wire loop. According to a second aspect of the present invention, there is provided a method. 5 comprising: providing a source resonant structure and a device resonant structure. the structures capable of performing wireless near-field energy transfer with a coupling rate K when separated a variable distance D from each other, said source resonant structure having a resonant frequency f,= co ,2t, an intrinsic loss rate F 1 , and a first Q-factor Qi= co 1 / (21 1 ), where co, is the angular frequency corresponding to the resonant frequency fl. said device resonant 10 structure having a resonant frequency f,=co2/2n. an intrinsic loss rate F, and a second Q-factor

Q

2 =co 2 /(2F2), where co, is the angular frequency corresponding to the resonant frequency f2. wherein the absolute value of the difference of said angular frequencies co, and (o is smaller than the magnitude of the coupling rate, K. and wherein at least one of the resonant structures comprises a high-Q capacitively-loaded conducting-wire loop. 15 According to one example of the invention, there is provided an electromagnetic energy transfer device. The electromagnetic energy transfer device includes a first resonator structure receiving energy from an external power supply. The first resonator structure has a first Q-factor. A second resonator structure is positioned distal from the first resonator structure, and supplies useful working power to an external load. The second resonator 20 structure has a second Q-factor. The distance between the two resonators can be larger than the characteristic size of each resonator. Non-radiative energy transfer between the first resonator structure and the second resonator structure is mediated through coupling of their resonant-field evanescent tails. According to another example of the invention, there is provided a method of 25 transferring electromagnetic energy. The method includes providing a first resonator structure receiving energy from an external power supply. The first resonator structure has a first Q factor. Also, the method includes a second resonator structure being positioned distal from the first resonator structure, and supplying useful working power to an external load. The second resonator structure has a second Q-factor. The distance between the two resonators can be 30 larger than the characteristic size of each resonator. Furthermore, the method includes transferring non-radiative energy between the first resonator structure and the second resonator Hp eneoenNRobnDdPAW -441_ do r2 16( structure through coupling of their resonant-field evanescent tails. The present invention will now be described, by way of non-limiting example only. with reference to the accompanying drawings. in which: FIG. I is a schematic diagram illustrating an exemplary embodiment of the invention: 5 FIG. 2A is a numerical FDTD result for a high-index disk cavity of radius r along with the electric field: FIG. 2B a numerical FDTD result for a medium-distance coupling between two resonant disk cavities: initially, all the energy is in one cavity (left panel); after some time both cavities are equally excited (right panel). FIG. 3 is a schematic diagram demonstrating two capacitively-loaded conducting-wire 10 loops; FIGs. 4A-4B are numerical FDTD results for reduction in radiation-Q of the resonant disk cavity due to scattering from extraneous objects; FIG. 5 is a numerical FDTD result for medium-distance coupling between two resonant disk cavities in the presence of extraneous objects: and 15 FIGs. 6A-6B are graphs demonstrating efficiencies of converting the supplied power into useful work (q.), radiation and ohmic loss at the device (7A), and the source (r/), and dissipation inside a human (r/h), as a function of the coupling-to-K/Ta; in panel (a) Tis chosen so as to minimize the energy stored in the device, while in panel (b) T,. is chosen so as to maximise the efficiency q for each K/T) 20 In contrast to the currently existing schemes, embodiments of the present invention provide the feasibility of using long-lived oscillatory resonant electromagnetic modes. with localized slowly evanescent field patterns, for wireless non-radiative energy transfer. The basis of this technique is that two same-frequency resonant objects tend to couple. while interacting weakly with other off-resonant environmental objects. It is desirable that embodiments of the 25 invention quantify this mechanism using specific examples, namely quantitatively addressing the following questions: up to which distances can such a scheme be efficient and how sensitive is it to external perturbations. Detailed theoretical and numerical analysis show that a mid-range (LTRANS ,kiv*LDEO) wireless energy-exchange can actually be achieved, while suffering only modest transfer and dissipation of energy into other off-resonant objects. 30 The omnidirectional but stationary (non-lossy) nature of the near field makes this mechanism suitable for mobile wireless receivers. It could therefore have a variety of possible 11PvmcueeWRobbC PXAW9"0%644 1d- '21 111am -4 applications including for example, placing a source connected to the wired electricity network on the ceiling of a factory room. while devices, such as robots. vehicles, computers. or similar, are roaming freely within the room. Other possible applications include electric-engine buses, RFIDs, and perhaps even nano-robots. 5 The range and rate of the inventive wireless energy-transfer scheme are the first subjects of examination, without considering yet energy drainage from the system for use into work. An appropriate analytical framework for modeling the exchange of energy between resonant objects is a weak-coupling approach called "coupled-mode theory". FIG. I is a schematic diagram illustrating a general description of the invention. A preferred embodiment 10 of the invention uses a source and device to perform energy transferring. Both the source I and device 2 are resonator structures., and are separated a distance D from each other. In this arrangement, the electromagnetic field of the system of source I and device 2 is approximated by F(rl) - a,(t) Fl(r)+ a2(t) F(r), where F 1 _r)=[Ei 2 (r) Hllir)] are the eigenmodes of source 1 and device 2 alone, and then the field amplitudes al(l) and c(t) can be shown to satisfy the 15 "coupled-mode theory": cit (1) =a 2 -i (o, iF) a12 i Ch- as I a d t where co 1 ,are the individual eigen-frequencies, T, are the resonance widths due to the objects' 20 intrinsic (absorption, radiation etc.) losses, K e are the coupling coefficients. and K, modell the shift in the complex frequency of each object due to the presence of the other. The approach of Eq. 1 has been shown, on numerous occasions, to provide an excellent description of resonant phenomena for objects of similar complex eigen-frequencies (namely |coi-co<x, 2 ol and Tl [2), whose resonances are reasonably well defined (namely Tl, & 25 Im{K 1 2 w} 1KI| 1 1) and in the weak coupling limit (namely x1 u(i co). Coincidentally, these requirements also enable optimal operation for energy transfer. Also. Eq. (1) show that the energy exchange can be nearly perfect at exact resonance (coi= coi and T-= [), and that the losses are minimal when the "coupling-time" is much shorter than all "loss-times". Therefore, the embodiments of the present invention require resonant modes of high Q=co/2T) for low 30 intrinsic-loss rates T1, and with evanescent tails significantly longer than the characteristic H 1p) ne: o eN O DCC PXA' 0%44_i dm21 01 2M6 - 4a sizes L1 and L of the two objects for strong coupling rate IK.Il over large distances D, where D is the closest distance between the two objects. This is a regime of operation that has not been studied extensively, since one usually prefers short tails, to minimize interference with nearby devices. 5 Objects of nearly infinite extent, such as dielectric waveguides. can support guided modes whose evanescent tails are decaying exponentially in the direction away from the object, slowly if tuned close to cutoff, and can have nearly infinite Q. To implement the inventive energy-transfer scheme. such geometries might be suitable for certain applications, but usually finite objects, namely ones that are topologically surrounded everywhere by air. 10 are more appropriate. Unfortunately, objects of finite extent cannot support electromagnetic states that are exponentially decaying in all directions in air, since in free space: k or c Because of this, one can show that they cannot support states of infinite 0. However, very long-lived (so-called "high-Q") states can be found, whose tails display the needed exponential-like decay away 15 from the resonant object over long enough distances before they turn oscillatory (radiative). The limiting surface, where this change in the field behavior happens. is called the "radiation caustic", and, for the wireless energy-transfer scheme to be based on the near field rather than the far/radiation field, the distance between the coupled objects must be such that one lies within the radiation caustic of the other. 20 The embodiments of the invention are very general and any type of resonant structure satisfying the above requirements can be used for its implementation. As examples and for deiniteness, one can choose to work with two well-known, but quite different electromagnetic resonant systems: dielectric disks and capacitively-loaded conducting-wire loops. Even without optimization, and despite their simplicity, both will be shown to exhibit fairly good 25 performance. Their difference lies mostly in the frequency range of applicability due to practical considerations, for example, in the optical regime dielectrics prevail, since conductive materials are highly lossy.

s Consider a 2D dielectric disk cavity of radius r and permittivity e surrounded by air that supports high-Q whispering-gallery modes, as shown in FIG. 2A. Such a cavity is studied using both analytical modeling, such as separation of variables in cylindrical coordinates and application of boundary conditions, and detailed numerical finite 5 difference-time-domain (FDTD) simulations with a resolution of 30pts/r. Note that the physics of the 3D case should not be significantly different, while the analytical complexity and numerical requirements would be immensely increased. The results of the two methods for the complex eigen-frequencies and the field patterns of the so-called "leaky" eigenmodes are in an excellent agreement with each other for a variety of 10 geometries and parameters of interest, The radial modal decay length, which determines the coupling strength K nIK 2 =IrKil, is on the order of the wavelength, therefore, for near-field coupling to take place between cavities whose distance is much larger than their size, one needs subwavelength-sized resonant objects (red). High-radiadon-Q and long-tailed 15 subwavelength resonances can be achieved, when the dielectric permittivity e is as large as practically possible and the azimuthal field variations (of principal number in) are slow (namely mn is small). One such TE-polarized dielectric-cavity mode, which has the favorable characteristics Q,ad=19 9 2 and Alr =20 using e =147.7 and n= 2, is shown in FIG. 2A, 20 and will be the "test" cavity 18 for all subsequent calculations for this class of resonant objects. Another example of a suitable cavity has Q, 0 =91 00 and A/r=10 using e - 65.61 and m =3. These values of c might at first seem unrealistically large. However, not only are there in the microwave regime (appropriate for meter-range coupling applications) many materials that have both reasonably high enough dielectric constants 25 and low losses, for example, Titania: e 96, Ims)/e = 10 3 ; Baium tetratitanate: e ~37, Im(E)/s 10 4 ; Lithium tantalite: 6 - 40, lI/e}/ - 10 4 ; etc.), but also s could instead signify the effective index of other known subwavelength (A/r>1) surface-wave systems, such as surface-plasmon modes on surfaces of metal-like (negative-S) materials or metallodielectric photonic crystals, 30 With regards to material absorption, typical loss tangents in the microwave (e.g. those listed for the materials above) suggest that Q,~&i(6)-J10000. Combining the effects of radiation and absorption, the above analysis implies that for a properly designed resonant device-object d a value of Qd-2 0 0 0 should be achievable. Note though, that the resonant source . will in practice often be immobile, and the restrictions on its allowed 35 geometry and size will typically be much less stringent than the restrictions on the design 6 of the device; therefore, it is reasonable to assume that the radiative losses can be designed to be negligible allowing for Q,-1 0000 , limited only by absorption. To calculate now the achievable rate of energy transfer, one can place two of the cavities 20, 22 at distance D between their centers, as shown in FIG. 2B. The normal 5 modes of the combined system are then an even and an odd superposition of the initial modes and their frequencies are split by the coupling coefficient K, which we want to calculate. Analytically, coupled-mode theory gives for dielectric objects

K

12 = 2 /2 fd 3 rEI (r)E 2 (r) 1 (r)/Jd rI (r) e(r), where e, 2 (r) denote the dielectric functions of only object I alone or 2 alone excluding the background dielectric (free 10 space) and e(r) the dielectric function of the entire space with both objects present. Numerically, one can find x using FDTD simulations either by exciting one of the cavities and calculating the energy-transfer time to the other or by determining the split normal mode frequencies. For the "test" disk cavity the radius rc of the radiation caustic is rc ~ I Ir, and for non-radiative coupling D < rb, therefore here one can choose D/r=10, 15 7, 5, 3. Then, for the mode of FIG. 3, which is odd with respect to the line that connects the two cavities, the analytical predictions are t/2K=1602, 771, 298, 48, while the numerical predictions are w/2K=1717, 770, 298, 47 respectively, so the two methods agree well. The radiation fields of the two initial cavity modes interfere constructively or destructively depending on their relative phases and amplitudes, leading to increased or 20 decreased net radiation loss respectively, therefore for any cavity distance the even and odd normal modes have Qs that are one larger and one smaller than the initial single cavity Q=1992 (a phenomenon not captured by coupled-mode theory), but in a way that the average Fis always approximately 1hu/2Q. Therefore, the corresponding coupling-to loss ratios are /1=1.16, 2.59, 6.68, 42.49, and although they do not fall in the ideal 25 operating regime crl, the achieved values are still large enough to be useful for applications. Consider a loop 10 or 12 of N coils of radius r of conducting wire with circular cross-section of radius a surrounded by air, as shown in FIG. 3. This wire has inductance L=p 0

N

2 r[ln(8r/a)-2], where p t c is the magnetic permeability of free space, so 30 connecting it to a capacitance C will make the loop resonant at frequency o=1/ A0 . The nature of the resonance lies in the periodic exchange of energy from the electric field inside the capacitor due to the voltage across it to the magnetic field in free space due to the current in the wire. Losses in this resonant system consist of ohmic loss inside the wire and radiative loss into free space.

7 For non-radiative coupling one should use the near-field region, whose extent is set roughly by the wavelength A, therefore the preferable operating regime is that where the loop is small (re). In this limit, the resistances associated with the two loss channels are respectively Rh,, =,4pc12. NrIa and Rrwd = c/6- 7N 2 (cr Ic) 4 , where p is the 5 resistivity of the wire material and 1, - 120n a is the impedance of free space. The quality factor of such a resonance is then Q = RoL/(l + Rrad ) and is highest for some frequency determined by the system parameters: at lower frequencies it is dominated by ohmic loss and at higher frequencies by radiation. To get a rough estimate in the microwave, one can use one coil (N=1) of copper 10 (p=J69-10O 8 m) wire and then for r=lcn and a=1mn , appropriate for example for a cell phone, the quality factor peaks to Q=1225 atf=38OMHz, for r=30cm and a=2mmi for a laptop or a household robot Q=1103 at f=17MHz, while for r=Jm and a=4mm (that could be a source loop on a room ceiling) Q=131 5 atf=5MHz. So in general, expected quality factors are Q41000-1500 at )/r'.50-80, namely suitable for near-field coupling. 15 The rate for energy transfer between two loops 10 and 12 at distance D between their centers, as shown in FIG. 3, is given by K12 = wcM /2 117, where M is the mutual inductance of the two loops 10 and 12. In the limit ra104 one can use the quasi-static result M =n/4-iuNN 2 (rr 2 )2/D 3 , which means that w/2K- (D/ 3.) For example, by choosing again D/r=10, 8, 6 one can get for two loops of r=Icm, same as 20 used before, that w/2K= 3 0 3 3 , 1553, 655 respectively, for the 'r=30cm that o/2K=7131, 3651, 1540, and for the r=lm that o/2K=6481, 3318, 1400. The corresponding coupling to-loss ratios peak at the frequency where peaks the single-loop Q and are K/f=0.4, 0.79, 1.97 and 0.15, 0.3, 0.72 and 0.2, 0.4, 0.94 for the three loop-kinds and distances. An example of dissimilar loops is that of a r=l1n (source on the ceiling) loop and a r=30cm 25 (household robot on the floor) loop at a distance D=3n (room height) apart, for which K/ Jf =0.88 peaks at f=6.4MHz, in between the peaks of the individual Q's. Again, these values are not in the optimal regime /r>'1, but will be shown to be sufficient. It is important to appreciate the difference between this inductive scheme and the already used close-range inductive schemes for energy transfer in that those schemes are 30 non-resonant. Using coupled-mode theory it is easy to show that, keeping the geometry and the energy stored at the source fixed, the presently proposed resonant-coupling inductive mechanism allows for Q approximately 1000 times more power delivered for work at the device than the traditional non-resonant mechanism, and this is why mid-range energy transfer is now possible. Capacitively-loaded conductive loops are actually being 8 widely used as resonant antennas (for example in cell phones), but those operate in the far field regime with r/.~1, and the radiation Q's are intentionally designed to be small to make the antenna efficient, so they are not appropriate for energy transfer. Clearly, the success of the inventive resonance-based wireless energy-transfer 5 scheme depends strongly on the robustness of the objects' resonances. Therefore, their sensitivity to the near presence of random non-resonant extraneous objects is another aspect of the proposed scheme that requires analysis. The interaction of an extraneous object with a resonant object can be obtained by a modification of the coupled-mode theory model in Eq. (1), since the extraneous object either does not have a well-defined 10 resonance or is far-off-resonance, the energy exchange between the resonant and extraneous objects is minimal, so the term K 12 in Eq. (1) can be dropped. The appropriate analytical model for the field amplitude in the resonant object aj(t) becomes: da=- irg-i)a,+ ixia (2) dt Namely, the effect of the extraneous object is just a perturbation on the resonance 15 of the resonant object and it is twofold: First, it shifts its resonant frequency through the real part of Kj; thus detuning it from other resonant objects. This is a problem that can be fixed rather easily by applying a feedback mechanism to every device that corrects its frequency, such as through small changes in geometry, and matches it to that of the source. Second, it forces the resonant object to lose modal energy due to scattering into 20 radiation from the extraneous object through the induced polarization or currents in it, and due to material absorption in the extraneous object through the imaginary part of Ku1. This reduction in Q can be a detrimental effect to the functionality of the energy-transfer scheme, because it cannot be remedied, so its magnitude must be quantified. In the first example of resonant objects that have been considered, the class of 25 dielectric disks, small, low-index, low-material-loss or far-away stray objects will induce small scattering and absorption. To examine realistic cases that are more dangerous for reduction in Q, one can therefore place the "test" dielectric disk cavity 40 close to: a) another off-resonance object 42, such as a human being, of large Relcl=49 and Ihe=)16 and of same size but different shape, as shown in FIG. 4A; and b) a roughened surface 46, 30 such as a wall, of large extent but of small Ref) =2.5 and Im(e}=0.05, as shown in FIG. 4B. Analytically, for objects that interact with a small perturbation the reduced value of radiation-Q due to scattering could be estimated using the polarization 9 S d3rrPKI(rf xf d'rIEi(r).Re{sX (r) induced by the resonant cavity 1 inside the extraneous object X=42 or roughened surface X=46. Since in the examined cases either the refractive index or the size of the extraneous objects is large, these first-order perturbation-theory results would not be accurate enough, thus one can only rely on 5 numerical FDTD simulations. The absorption-Q inside these objects can be estimated through Im{K 11 }=t 1 /2- d 3 rIE1 (r1 2 Im{e (r)}/Sd'rIEi(re E(r). Using these methods, for distances D/r=10, 7, 5, 3 between the cavity and extraneous-object centers one can find that Qr=1992 is respectively reduced to Qra=1988, 1258, 702, 226, and that the absorption rate inside the object is Qob=31 2 530, 10 86980, 21864, 1662, namely the resonance of the cavity is not detrimentally disturbed from high-index and/or high-loss extraneous objects, unless the (possibly mobile) object comes very close to the cavity, For distances D/r=10, 7, 5, 3, 0 of the cavity to the roughened surface we find respectively Qa= 2 1 0 1, 2257, 1760, 1110, 572, and Qabs> 4 0 00 , namely the influence on the initial resonant mode is acceptably low, even in 15 the extreme case when the cavity is embedded on the surface. Note that a close proximity of metallic objects could also significantly scatter the resonant field, but one can assume for simplicity that such objects are not present. Imagine now a combined system where a resonant source-object s is used to wirelessly transfer energy to a resonant device-object d but there is an off-resonance 20 extraneous-object e present. One can see that the strength of all extrinsic loss mechanisms from e is determined by E,(re)1 2 , by the square of the small amplitude of the tails of the resonant source, evaluated at the position r, of the extraneous object. In contrast, the coefficient of resonant coupling of energy from the source to the device is determined by the same-order tail amplitude |E,(rd)l, evaluated at the position rd of the device, but this 25 time it is not squared! Therefore, for equal distances of the source to the device and to the extraneous object, the coupling time for energy exchange with the device is much shorter than the time needed for the losses inside the extraneous object to accumulate, especially if the amplitude of the resonant field has an exponential-like decay away from the source. One could actually optimize the performance by designing the system so that the desired 30 coupling is achieved with smaller tails at the source and longer at the device, so that interference to the source from the other objects is minimal. The above concepts can be verified in the case of dielectric disk cavities by a simulation that combines FIGs. 2A-2B and 4A-4B, namely that of two (source-device) "test" cavities 50 placed 10r apart, in the presence of a same-size extraneous object 52 of 35 e=49 between them, and at a distance Sr from a large roughened surface 56 of c=2.5, as 10 shown in FIG. 5. Then, the original values of Q=1992, o>/2c=171 7 (and thus Ktl=1.16) deteriorate to Q=765, o/2c=965 (and thus W/V=0.79). This change is acceptably small, considering the extent of the considered external perturbation, and, since the system design has not been optimized, the final value of coupling-to-loss ratio is promising that 5 this scheme can be useful for energy transfer. In the second example of resonant objects being considered, the conducting-wire loops, the influence of extraneous objects on the resonances is nearly absent. The reason for this is that, in the quasi-static regime of operation (re4) that is being considered, the near field in the air region surrounding the loop is predominantly magnetic, since the 10 electric field is localized inside the capacitor. Therefore, extraneous objects that could interact with this field and act as a perturbation to the resonance are those having significant magnetic properties (magnetic permeability Refp)>1 or magnetic loss In(y)>0). Since almost all common materials are non-magnetic, they respond to magnetic fields in the same way as free space, and thus will not disturb the resonance of a 15 conducting-wire loop. The only perturbation that is expected to affect these resonances is a close proximity of large metallic structures. An extremely important implication of the above fact relates to safety considerations for human beings. Humans are also non-magnetic and can sustain strong magnetic fields without undergoing any risk. This is clearly an advantage of this class of 20 resonant systems for many real-world applications. On the other hand, dielectric systems of high (effective) index have the advantages that their efficiencies seem to be higher, judging from the larger achieved values of KIT, and that they are also applicable to much smaller length-scales, as mentioned before. Consider now again the combined system of resonant source s and device d in the 25 presence of a human h and a wall, and now let us study the efficiency of this resonance based energy-transfer scheme, when energy is being drained from the device for use into operational work. One can use the parameters found before: for dielectric disks, absorption-dominated loss at the source Q,~10 4 , radiation-dominated loss at the device Qr'~ (which includes scattering from the human and the wall), absorption of the source 30 and device-energy at the human Q,.1 Qd.,, ~104_10 depending on his/her not-very-close distance from the objects, and negligible absorption loss in the wall; for conducting-wire loops, Q,~Q-10, and perturbations from the human and the wall are negligible. With corresponding loss-rates Thcw/2Q, distance-dependent coupling ic, and the rate at which working power is extracted 1,, the coupled-mode-theory equation for the device field 35 amplitude is 11 dad - -idjad+iKa--d-had wad. (3) di Different temporal schemes can be used to extract power from the device and.their efficiencies exhibit different dependence on the combined system parameters. Here, one can assume steady state, such that the field amplitude inside the source is maintained 5 constant, namely a,(t)=Ase'", so then the field amplitude inside the device is ad(t)=Ade' with A4=i/(Td+T.I+rW)A,. Therefore, the power lost at the source is P,=2iAs 12, at the device it is Pd=2rdlAdI 2 , the power absorbed at the human is Ph=2r.l,,A1+2a.hIAdl 2 , and the useful extracted power is P,=21flvAdI 2 . From energy conservation, the total power entering the system is P,,,= P,+P±+P,,+P,. Denote the total loss-rates r'0=f ,+r,.. 10 and r0 1 d = . Depending on the targeted application, the work-drainage rate should be chosen either r, = Id" to minimize the required energy stored in the resonant objects or r, = r'1+2/r rd > rl' such that the ratio of useful-to-lost powers namely the efficiency t/w=Pu/P, is maximized for some value of K. The efficiencies rI for the two different choices are shown in FIGs. 6A and 6B respectively, as a function of 15 the z/Fd figure-of-merit which in turn depends on the source-device distance. FIGs, 6A-6B show that for the system of dielectric disks and the choice of optimized efficiency, the efficiency can be large, e.g., at least 40%. The dissipation of energy inside the human is small enough, less than 5%, for values I/rd>1 and Qa>10 5 , namely for medium-range source-device distances (Dd/r<1O) and most human 20 source/device distances (D;/r>8). For example, for D/r=10 and Dw/r=8, if 1OW must be delivered to the load, then, from FIG. 6B, -0.4W will be dissipated inside the human, -4W will be absorbed inside the source, and -2.6W will be radiated to free space, For the system of conducting-wire loops, the achieved efficiency is smaller, -20% for i/ri, but the significant advantage is that there is no dissipation of energy inside the human, as 25 explained earlier. Even better performance should be achievable through optimization of the resonant object designs. Also, by exploiting the earlier mentioned interference effects between the radiation fields of the coupled objects, such as continuous-wave operation at the frequency of the normal mode that has the larger radiation-Q, one could further improve the overall 30 system functionality. Thus the inventive wireless energy-transfer scheme is promising for many modem applications. Although all considerations have been for a static geometry, all the results can be applied directly for the dynamic geometries of mobile objects, since -2 the energy-transfer time vc lps, which is much shorter than any timescale associated with mfotins of m~acroscoc.pic o es i ne embodinents o," tne nlvention provide a nance-basea seneme or nud-range wireless non-radiative enrgy.rnsfer. Analyses of very simnple implementaton geometries 5 provide cou-agin perfboan.co.. c erstics for th potential applicbily of Che proposed mechanism. For example, in th macroescopic worI, this s c e .sed t o deliver pOwer to robots and/or cters in a facry roam, or electric buses on ahig-w (sourCe cavity would in tis case be a -pipe" r above the high-way) i h mitems wrld where much smaller wave-enghswould bet ued andmlle powers arc nee one coul use 10 it to implenent optical nter-conlects for CMOS electronics or elseN to ransfe nie'rg to autonomIos nano-o-jects, without worying nauch about the retkative alignm-1en betwe the sources and the devices: eergt ranIsf er distance couple cmevenA onger compared tte objects' size. since hm~ o>) of dieletric materials can e muc lw ai the required-t. opticalU frequencies than it is a microufreqencies. 15 As a venue of future scientific research. different material systems shud he i.nvei-gted for enhanced perform-'ance or different range of applicability. For examl-e, it might be possible to significantly improve performance by exploring plasmi nic systems. These systems can -ften have spatial variations of IeS mn their surface that are nnuch shorter han the tree-space wavelengh, and it is precisely is feature niat n h required 20 dcouping o the scales: the resonant object c be sigmncantly smaller -ian tNe eotia like tails of its -field, Furthernore one should a-- invNiat usNig acoustic resonances ft r appi-cations in wich source and device are connected via a co-nion conee-ater ojt. Although the present invention has been shown and descri bed w rith respect to eral prefered embodimnents thereof, various changes, omissions and additions to the orma and 25 i treot may be made therein without departing front th Spirit aind scope of the inven-ion, Throughout this specification and the claims which folow unlcs's the cotext ries o01itrwi.se, the word comprise", and variations such as comprisess" and tcomprisng wl be understood to imply the inclusion of a stated integer or ste or goup of integers or steS but 30 not the exclusion of any other integer or step or group or integers or steps.

H \ NRi- Db DCC PXAi930%44_1 d-21/ fl - 13 The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in 5 the field of endeavour to which this specification relates. The disclosure of the complete specification of Australian Patent Application No. 20102000044 as originally filed is incorporated herein by reference. The disclosure of the complete specification of Australian Patent Application No. 2011203137 as originally filed is incorporated herein by reference. 10 DocuniIt33-I A)42013 - 13a The content of the complete specification of Australian patent application no. 2006269374 as originally filed is incorporated herein by reference.

Claims (31)

1. A system, comprising: a source resonant structure and a device resonant structure, the structures capable 5 of performing wireless near-field energy transfer with a coupling rate K when separated a variable distance D from each other, said source resonant structure having a resonant frequency f=oy/2x, an intrinsic loss rate FI,. and a first Q-factor Qi=o /(2F 1 ), where oi is the angular frequency corresponding to the resonant frequency f. 10 said device resonant structure having a resonant frequency f,=o2/27t, an intrinsic loss rate F2, and a second Q-factor Q2=o2/(2F), where (2 is the angular frequency corresponding to the resonant frequency f 2 , wherein the absolute value of the difference of said angular frequencies oi and o is smaller than the magnitude of the coupling rate, K, and 15 wherein at least one of the resonant structures comprises a high-Q capacitively-loaded conducting-wire loop.
2. The system of claim 1, wherein Q-factor of at least one of the resonant structures is greater than 100. 20
3. The system of claim 1, wherein QQ 2 >100.
4. The system of claim 1, wherein Qi>100 and Q2>100. 25 5. The system of any one of the preceding claims, further comprising a power supply coupled to the source resonant structure and an energy drain coupled to the device resonant structure.
6. The system of claim 5. wherein the energy drain comprises a robot., vehicle. computer. 30 cell phone, or a portable electronic device.
7. The system of any one of claims 1 to 4, further comparing a tird resoanttr optionally coupled to an energy drain. oated a able distance om the source resonant structure, and wherein he s ource resonn struu and the third resonant structure are coupled trzan4 s electromagnetc er fo the source resonant structure to the N th'rd r'sonant structure
8. The system of any one of claims 1 to 4, tr comprisin a thir resonant structure, optinlly coupled to a power supply, located at a variable distance from the deice resonant structure, ani wherein the third resonant strucueand the device resonamt structure. are 10 coupled to wirelessiy transfer electromagneticgnergy 1rm the third resonant structure to the device resonant structure.
9. The system of any one of the preceding claims, wherein at least one of the resonant structures is tunaNe. 15 l1, The system of any one of the preceding clans, fiker comprismng a feedback mechanism coupled to at least one of the resonant structures to correct for detuni. 11 T system of anyne of the r'ceding c s wrein the esat structurs 20 movable relative to one another and wherein the wireless energy transfer occurs over a range of distances.
12. The system of claim Iwherein the range of distances includes 5cm. 25 13, The system of claim 11. wherein ie range of distances includes 10cm,
14. The system of claim 11, wdherein the range of distances includes 30cm. I5. The system of any one ofclaims I1 to 14., wherein the enicieney of the wireless energy 30 transfer is at least 2M% over the range of distances. H:p'a ~cnr-o iNRPornDCC PXA 9 09644_ docv2 1U2116 - 16
16. The system of any one of claims I 1 to 15. wherein K/ F f;F 2 >0.2 over the range if distances, wherein K is the wireless energy transfer rate.
17. The system of any one of claims II to 15, wherein K/ jFfF 2 >0.5 over the range of 5 distances.
18. The system of any one of claims 11 to 15. wherein K/ fF2B>1 over the range of distances. 10 19. The system of any one of claims 11 to 18, wherein the source resonant structure and the device resonant structure are configured to be adjustably tuned to increase the ratio of useful-to-lost power for varying wireless energy transfer rates K between the source resonant structure and the device resonant structure over the range of distances. 15 20. The system of claim 1, further comprising a power supply coupled to the source resonant structure and an energy drain coupled to the device resonant structure, and wherein the power supply and energy drain are configured to be driven to increase the ratio of useful to-lost power for varying wireless energy transfer rates K between the source resonant structure and the device resonant structure. 20
21. A method, comprising: providing a source resonant structure and a device resonant structure, the structures capable of performing wireless near-field energy transfer with a coupling rate K when separated a variable distance D from each other, said source resonant structure having a 25 resonant frequency f= co 1 /2T, an intrinsic loss rate F , and a first Q-factor Qi= col/ (2F 1 ), where coi is the angular frequency corresponding to the resonant frequency f,. said device resonant structure having a resonant frequency f 2 =to 2 /2z, an intrinsic loss rate F), and a second Q-factor Q 2 =co 2 /(2F 2 ), where co2 is the angular frequency corresponding to the resonant frequency f,, wherein the absolute value of the difference of said angular frequencies (o and co2 is smaller 30 than the magnitude of the coupling rate, K, and wherein at least one of the resonant structures comprises a high-Q capacitively-loaded conducting-wire loop. H p nter NRPobDCCPX A 964 d-21 O2 - 17
22. The method of claim 21. wherein the Q-factor of at least one of the resonant structures is greater than 100.
23. The method of claim 21, wherein QQ, Q>l 00. 5
24. The method of claim 21, wherein QI>l00 and Q2>100.
25. The method of any one of claims 21 to 24, wherein a power supply is coupled to the source resonant structure and an energy drain is coupled to the device resonant structure. 10
26. The method of claim 25. wherein the energy drain comprises a robot, vehicle. computer, cell phone, or a portable electronic device.
27. The method of any one of claims 21 to 24, wherein a third resonant structure is 15 coupled to an energy drain and located at a variable distance from the source resonant structure., and further comprising wirelessly transferring electromagnetic energy from the source resonant structure to the third resonant structure.
28. The method of any one of claims 21 to 24, wherein a third resonant structure is 20 coupled to a power supply and located at a variable distance from the device resonant structure, and further comprising tirelessly transferring electromagnetic energy from the third resonant structure to the device resonant structure.
29. The method of any one of claims 21 to 28, wherein at least one of the resonant 25 structures is tunable.
30. The method of any one of claims 21 to 29, wherein the resonant structures are movable relative to one another and wherein the wireless energy transfer occurs over a range of distances. 30
31. The method of claim 30, wherein the range of distances includes 5cm. H!:p unnv ent NR44nb (DC '\PXAW30%44 .dc2 :Ol2016 - 18 32. The method of claim 30, wherein the range of distances includes 10cm.
33. The method of claim 30, wherein the range of distances includes 30cm. 5 34. The method of any one of claims 30 to 33, wherein the efficiency of the wireless energy transfer is at least 20% over the range of distances.
35. The method of any one of claims 30 to 34, wherein K! FiF 2 >0.2 over the range of distances. wherein K is the wireless energy transfer rate. 10
36. The method of any one of claims 30 to 34, wherein/ FI F 2 >0.5 over the range of distances.
37. The method of any one of claims 30 to 34, wherein K! F 2>1 over the range of 15 distances.
38. The method of any one of claims 30 to 37, further comprising correcting at least one of the resonant structures to correct for detuning of the resonant structures over the range of distances. 20
39. The method of any one of claims 30 to 38, further comprising adjustably tuning at [east one of the source and device resonant structures to increase the ratio of useful-to-lost power for varying wireless energy transfer rates K between the source resonant structure and the device resonant structure over the range of distances. 25
40. The method of claim 21, wherein a power supply is coupled to the source resonant structure and an energy drain is coupled to the device resonant structure, and wherein the power supply and energy drain are driven to increase the ratio of useful-to-lost power for varying wireless energy transfer rates K between the source resonant structure and the device 30 resonant structure.
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