by Bryan Bergeron, Editor
The combination of unstable oil
prices, pressure to identify
green alternatives to fossil fuel,
and a poor economy has renewed
interest in economical alternatives to
petroleum-based energy sources.
While coal, wind, and solar hold
promise, one of the most
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technologically intriguing approaches
to powering electronic devices is
energy harvesting, which involves
siphoning off relatively small
amounts of energy from an ongoing
The poster child for mechanical
energy harvesting is the perpetual,
self-wound wristwatch. Walking
and other normal body movement
is typically sufficient to keep
a self-wound watch going.
Furthermore, unless the wearer has
a sedentary job and lifestyle, there
is no need to consciously shake
the watch. Reclaiming ambient
energy is almost like getting energy
for nothing. In this regard, energy
harvesting is different from manually
operating self-charging devices
such as flashlights that must be
shaken vigorously for a minute
or more, emergency radios with
built-in generators that must
There have been numerous
demonstrations of energy harvesting
using piezoelectrics, ranging from
sidewalks and backpacks to combat
boots. Piezoelectrics harvest energy
by converting an applied stress to
an electrical charge. In the case of
sidewalks, the stress generated by
the movement of pedestrians is
converted to an electric charge.
Similarly, piezoelectric backpack
straps and piezoelectric elements in
the heels of combat boots convert
the bouncing and pounding to
useful energy by charging a supercap
The high cost of installation and
maintenance and the relatively small
return on investment have kept
energy harvesting either in the lab or
in highly publicized (but short-lived)
demonstration projects. What's
been lacking is an affordable,
general-purpose energy harvesting
technology that can be applied to a
variety of application areas with a