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Abstract

We investigate whether, and to what extent, the physical phenomenon of long-lifetime resonant electromagnetic states with localized slowly-evanescent field patterns can be used to transfer energy efficiently over non-negligible distances, even in the presence of extraneous environmental objects. Via detailed theoretical and numerical analyses of typical real-world model-situations and realistic material parameters, we establish that such a non-radiative scheme can lead to ‘‘strong coupling’’ between two medium-range distant such states and thus could indeed be practical for efficient medium- range wireless energy transfer.

Introduction

In the early days of electromagnetism, before the electrical-wire grid was deployed, serious interest and effort was devoted (most notably by Nikola Tesla [1]) towards the development of schemes to transport energy over long distances without any carrier medium (e.g omni-directional antennas (which work very well for information transfer) are not suitablefor 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 (transfer distance LTRANS ≫ LDEV, where LDEV 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, cell-phones, house-hold robots, that all typically rely on chemical energy storage) justifies revisiting investigation of this issue. Today, we face a different challenge than Tesla: since the existing electrical-wire grid carries energy almost everywhere, even a medium-range (LTRANS _ few*LDEV) wireless energy transfer would be quite useful for many applications. There are several currently used schemes, which rely on non-radiative modes (magnetic induction), but they are restricted to very close-range (LTRANS ≪ LDEV) or very low-power (_mW) energy transfers. wirelessly). These efforts appear to have met with little success.

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Efficient wireless non-radiative mid-range energy transfer

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Abstract

We investigate whether, and to what extent, the physical phenomenon of long-lifetime resonant electromagnetic states with localized slowly-evanescent field patterns can be used to transfer energy efficiently over non-negligible distances, even in the presence of extraneous environmental objects. Via detailed theoretical and numerical analyses of typical real-world model-situations and realistic material parameters, we establish that such a non-radiative scheme can lead to “strong coupling” between two medium-range distant such states and thus could indeed be practical for efficient medium-range wireless energy transfer.

Introduction

These efforts appear to have met with little 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 (transfer distance LTRANS»LDEV, where LDEV 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, cell-phones, house-hold robots, that all typically rely on chemical energy storage) justifies revisiting investigation of this issue. Today, we face a different challenge than Tesla: since the existing electrical-wire grid carries energy almost everywhere, even a medium-range (LTRANS ≈ few∗LDEV) wireless energy transfer would be quite useful for many applications. There are several currently used schemes, which rely on non-radiative modes (magnetic induction), but they are restricted to very close-range (LTRANS«LDEV) or very low-power (~mW) energy transfers [2,3,4,5,6].

Range and rate of coupling

The range and rate of the proposed 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 this resonant energy-exchange is that of the well-known coupled-mode theory (CMT) [8]. In this picture, the field of the system of two resonant objects 1 and 2 is approximated by F(r,t)≈ a1(t)F1®+a2(t)F2®, where F1,2® are the eigenmodes of 1 and 2 alone, and then the field amplitudes a1(t) and a2(t) can be shown [8] to satisfy, to lowest order.

Capacitively-loaded conducting-wire loops

Consider a loop of radius r of conducting wire with circular cross-section of radius a connected to a pair of conducting parallel plates of area A spaced by distance d via a dielectric of relative permittivity ε and everything surrounded by air (Figure 3). The wire has inductance L, the plates have capacitance C and then the system has a resonant mode, where 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 absR inside the wire and radiative loss radR into free space. Mode-solving calculations for this type of RLC-circuit resonances were performed using again two independent methods: numerically, 3D finite-element frequency-domain (FEFD) simulations (which solve Maxwell’s Equations in frequency domain exactly apart for spatial discretization) were conducted [15], in which the boundaries of the conductor were modeled using a complex impedance /2ccημω= boundary condition.

Conclusion

In conclusion, we present a scheme based on “strongly-coupled” resonances for mid-range wireless non-radiative energy transfer. Although our consideration has been for a static geometry (namely κ and Γe were independent of time), all the results can be applied directly for the dynamic geometries of mobile objects, since the energy-transfer time (1κ−μ−∼1100s for microwave applications) is much shorter than any timescale associated with motions of macroscopic objects. Analyses of very simple implementation geometries provide encouraging performance characteristics and further improvement is expected with serious design optimization. Thus the proposed mechanism is promising for many modern applications. For example, in the macroscopic world, this scheme could potentially be used to deliver power to robots and/or computers in a factory room, or electric buses on a highway (source-cavity would in this case be a “pipe” running above the highway). In the microscopic world, where much smaller wavelengths would be used and smaller powers are needed, one could use it to implement optical inter-connects for CMOS electronics, or to transfer energy to autonomous nano-objects (e.g. MEMS or nano-robots) without worrying much about the relative alignment between the sources and the devices.