Electrical Power Transfer with “No Wires” - Energy Harvesting Autonomous Sensor Systems - page 181

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Electrical P ower Transfer with “No Wires”
In this chapter, the concept of electrical power transfer with “no wires” is
introduced with a similar objective as energy harvesting, that is, to enable
remote charging of low-power electronic devices. Wireless power transfer
(WPT) is the process by which a system delivers electrical energy from a
power source to the load without any connecting wires. Almost 100 years
ago, before the advent of the electric grid, the genius inventor Nikola Tesla
envisioned a future when huge towers would radiate energy directly into our
homes for consumption [147]. But only over the past 5 years has there been
aremarkable interest in researching commercially viable and safe methods
of WPT, mainly due to the surge in the use of low-power electronic devices
like laptops, netbooks, smart phones, wireless sensor nodes, and more that
require regular charging and battery maintenance. It is obvious that WPT
products can increase convenience and quality of life, but what is not so
obvious are the environmental and economic benefits that this technology
may have to offer. There is great potential in using WPT to directly power
devices such as clocks and remote controllers, which would drastically reduce
the 6 billion batteries being disposed of every year around the world that are
a source of groundwater contamination and producers of toxic waste when
burnt in incinerators. In other cases where wiring is too expensive, hazardous,
or impossible, WPT may be the only enabling technology.
Presently, several WPT techniques are being pursued, which can be catego-
rized in terms of their underlying power transfer mechanism to understand
the implications for range, adaptation, and efficiency. Far-field WPT is one of
the emerging techniques that uses propagating electromagnetic (EM) waves
to transfer energy. This method has been successfully used to power radio-
frequency identification (RFID) tags; which have no batteries and an operat-
ing range of about 10 m [148-149]. One of the drawbacks with far-field WPT
approaches is the inherent trade-off between directionality and transmission
efficiency. There are many examples of RF and microwave systems that use
lasers or high-gain antennas to transfer power over kilometre distances at
efficiencies of over 90% [150-151]. These systems suffer from the need for so-
phisticated tracking and alignment equipment to maintain an uninterruptible
line of sight (point-to-point) connection in unstructured and dynamic envi-
ronments. Alternatively, RF broadcast methods, which transmit power in an
omnidirectional pattern, allow for power transfer anywhere in the coverage
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