Wireless Power Transfer via Strongly Coupled Magnetic Resonances

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introduction
In the early 20th century, before the electricalwire
grid, Nikola Tesla (1) devoted much
effort toward schemes to transport power wirelessly.
However, typical embodiments (e.g., Tesla
coils) involved undesirably large electric fields.
The past decade has witnessed a surge in the use
of autonomous electronic devices (laptops, cell
phones, robots, PDAs, etc.). As a consequence,
interest in wireless power has reemerged (2–4).
Radiative transfer (5), although 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, and unidirectional
radiation requires an uninterrupted line of sight
and sophisticated tracking mechanisms. A recent
theoretical paper (6) presented a detailed analysis
of the feasibility of using resonant objects coupled
through the tails of their nonradiative fields
for midrange energy transfer (7). Intuitively, two
resonant objects of the same resonant frequency
tend to exchange energy efficiently, while dissipating
relatively little energy in extraneous offresonant
objects. In systems of coupled resonances
(e.g., acoustic, electromagnetic, magnetic, nuclear),
there is often a general “strongly coupled”
regime of operation (8). If one can operate in that
regime in a given system, the energy transfer is
expected to be very efficient. Midrange power
transfer implemented in this way can be nearly
omnidirectional and efficient, irrespective of the
geometry of the surrounding space, with low interference
and losses into environmental objects (6).
The above considerations apply irrespective
of the physical nature of the resonances. Here,
we focus on one particular physical embodiment:



Measurement of the efficiency. The maximum
theoretical efficiency depends only on
the parameter k/[(LSLD)1/2] = k/G, which is
greater than 1 even for D = 2.4 m (8 times the
radius of the coils) (Fig. 3). Thus, we operate
in the strongly coupled regime throughout the
entire range of distances probed