Electrical Power Transfer with “No Wires” - Energy Harvesting Autonomous Sensor Systems - page 182

area. In this case, mobility is maintained, but end-to-end efficiency is lost since
power density decreases with a 1

r 2 dependence, resulting in received power
levels many orders of magnitude less than what is transmitted [152]. In order
to provide power comparable to a typical wall-mounted direct current (DC)
power adapter, the system would violate RF safety regulations [153] or have
to use a large number of transmitters, resulting in an impractical implemen-
tation. Therefore, far-field techniques are most suitable for very-low-power
applications unless they are used in less-regulated environments, such as
military endeavours or space exploration.
According to Low et al. [153], inductive coupling, which is nonradiative
and near field, has been one of the leading candidates in achieving WPT at
power levels ranging from several microwatts to several kilowatts. Its op-
erating range is limited as power delivery and efficiency degrade rapidly
with increasing distance between the transmitting and receiving units. In-
ductive coupling, which does not rely on propagating EM waves, operates
at distances less than a wavelength of the signal being transmitted [154]. Ap-
plications include rechargeable toothbrushes and the recently proliferating
“power” surfaces [155]. These techniques can be very efficient but are lim-
ited to transmission distances of about a centimetre. Alternatively, near-field
RFID pushes the limit on distance by sacrificing efficiency. Near-field tags
have a range of tens of centimetres, but only receive power in the microwatt
range with 1% to 2% transmission efficiency [148]. Previously demonstrated
magnetically coupled resonators used for WPT [156-158] have shown the
potential to deliver power with more efficiency than far-field approaches
and at longer ranges than traditional inductively coupled schemes. However,
this prior work is limited to a fixed distance and orientation, with efficiency
falling off rapidly when the receiver is moved away from its optimal oper-
ating point. This chapter explores both types of near-field WPT techniques
for remote charging of low-power electronic devices : (1) inductively coupled
WPT from power lines (see Section 6.1) and (2) strongly coupled WPT with
magnetic resonances (see Section 6.2).

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6.1 Inductively Coupled Power Transfer from Power Lines
Inductive coupling has considerably increased in the last 10 years because it
permits the supply of power to electronic circuits remotely and also provides
a means for exchange of data between two sensor nodes. Today in the in-
dustry, the most important part of inductive coupling applications is related
to RFID [159]. RFID is used for contactless smart cards or to sort and locate
travelling objects (e.g., postal parcels). In many buildings, door keys have
been replaced by the use of EM badges. However, existing technologies that
utilize near-field wireless power transmission such as RFID tags generally op-
erate only over limited distances with very low efficiency. Other than RFID
application, power transmission and remote sensing by inductive coupling