Power Transfer Through Strongly Coupled Resonances by André Kurs Master of Science in Physics at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY

Power Transfer Through Strongly Coupled Resonances by André Kurs

Submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Master of Science in Physics at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY September 2007.

 Chapter 1
 Introduction
At the turn of the 20th century, Nikola Tesla [1, 2, 3] devoted much effort to developing a system for transferring large amounts of power over continental distances. His main goal was to bypass the electrical-wire grid, but for a number of technical and financial difficulties, this project was never completed. Moreover, typical embodiments of Tesla's power transfer scheme (e.g., Tesla coils) involve extremely large electric fields and are potential safety hazards. The past decade has witnessed a dramatic surge in the use of autonomous electronic devices (laptops, cell-phones, robots, PDAs, etc) whose batteries need to be constantly recharged. As a consequence, interest in wirelessly recharging or powering such devices has reemerged [4, 5, 6]. Our attempts to help to fulfill this need led us to look for physical phenomena that would enable a source and a device to exchange energy efficiently over mid-range distances, while dissipating relatively little energy in extraneous objects. By mid-range, we mean that the separation between the two objects effecting the transfer should be of the order of a few times the characteristic sizes of the objects. Thus, for example, one source could be used to power or recharge all portable devices within an average-sized room. A natural candidate for wirelessly transferring powering over mid-range or longer distances would be to use electromagnetic radiation. But radiative transfer [7], while perfectly suitable for transferring information, poses a number of difficulties for power transfer applications: the efficiency of power transfer is very low if the radiation is omnidirectional (since the power captured is proportional to the cross-section of the receiving antenna, and most of the power is radiated in other directions), and requires an uninterrupted line of sight and sophisticated tracking mechanisms if radiation is unidirectional (which might also damage anything that interrupts the line of sight). An alternative approach, which we pursue here, is to exploit some near-field interaction between the source and the device, and somehow tune this system so that efficient power transfer is possible. A recent theoretical paper [8] presented a detailed analysis of the feasibility of using resonant objects coupled through their near-fields to achieve mid-range energy transfer. The basic idea is that in systems of coupled resonances (e.g. acoustic, electro-magnetic, magnetic, nuclear), there may be a general strongly coupled regime of operation [9]. It is a general physical property that if one can operate in this regime in a given system, the energy transfer is expected to be very efficient. Mid-range power transfer implemented this way can be nearly omnidirectional and efficient, irrespective of the geometry of the surrounding space, and with low losses into most off-resonant environmental objects [8]. The above considerations apply irrespective of the physical nature of the resonances. In the current work, we focus on one particular physical embodiment: magnetic resonances [10], meaning that the interaction between the objects occurs predominantly through the magnetic fields they generate. Magnetic resonances are particularly suitable for everyday use because biological tissue and most common materials do not interact strongly with magnetic fields, which helps make the system safer and more efficient. We were able to identify the strongly coupled regime in the system of two coupled magnetic resonances by exploring non-radiative (nearfield) magnetic resonant induction at MHz frequencies. At first glance, such power transfer is reminiscent of the usual magnetic induction [11]; however, note that the usual non-resonant induction is very inefficient unless the two coils share a core with high magnetic permeability or are very close to each other. Moreover, operating on resonance is necessary but not sufficient to achieve good efficiency at mid-range distances. Indeed, Tesla's pioneering work made extensive use of resonant induction, and many technologies available today (e.g., radio receivers, RFID tags, and cochlear implants [12]) also rely on resonance, yet their efficiencies are not very good at mid range distances. Operation in the strong-coupling regime, for which resonance is a precondition, is what makes the power transfer efficient.

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