The concept of wireless radio-frequency (RF) energy
transmission dates back to the 1880s. With current trends on wireless
technology, RF energy harvesting is a promising solution of the limitation of
regular batteries.
Energy harvesting is the process
of using ambient energy, such as solar, wind, vibrational, and electromagnetic
wave energy, then convert them into electrical energy. This kind of strategy
makes use of environmental resources in order to replenish power. Examples of
energy harvesting , in a large scale, are windmills that is used to provide
power in some areas. With the continuing progress of wireless technology, there
is a move to power up small devices, say mobile devices and sensor nodes, using
these ambient energies.
Some of the energies like solar
and wind are weather-dependent solutions. Vibrational energy requires
mechanical movements. RF, having the least power density, is a promising source
of energy for low-voltage devices. RF is
a frequency portion on the electromagnetic spectrum that is usually in the
ultra high frequency band. This can be from communication devices like TVs,
cellphones, and Wireless Fidelity (WiFi) signals. These devices are almost in
every corner of our lives.
Now why bother find a way to
replace regular batteries? Progress. Batteries are limitations to the overall
area of devices, which is one of the metric to consider in designing things.
Another reason is that there is limited amount of power in batteries for
wireless devices. For sensor networks, each sensor node may need to have
individual batteries. Replacing each and everyone of it makes it costly. In the
medical field, battery-powered devices may not be accessible easily.
So how does RF energy harvesting works? The
system involves the receiving antenna and the rectifier. An electromagnetic
wave like RF wave induces electric current upon contact in a conductor such as
the antenna. This will be the RF input power of the system. This signal is in
alternating current (AC) form but devices uses direct current (DC) signal. In
order to convert this AC signal into DC signal, a rectifier is used. The
rectifier is made up of devices such as a diode or a transistor which should allow only one way flow of
current. An energy storage device such as a capacitor is then needed to store
the charge for future use. The harvester includes a matching network before the
antenna and rectifier in order to improve the efficiency of the power
conversion. A voltage regulator that conditions the power before the load, the device to be powered up, is also
included.
At present, RF energy harvesting
is a solution for low-voltage devices but since technology is developing, who
knows if in the near future it is already a solution for a larger scale power
problems.
[Allen SHamir Clavio]
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