Biomedical Engineering Reference
In-Depth Information
of the energy harvesters by volume. Figure 3.1 is mainly dominated by
electromagnetic and piezoelectric transduction generators at frequen-
cies close to 1 Hz, making them preferable over electrostatic genera-
tors. Filled symbols show the trend that larger devices are able to
produce a higher power output. Devices operating at 1 Hz were
reported to produce from less than 1
μ
W up to 7 W of power, at the
expense of the generator ' s size. Figure 3.2 describes better this trend
where larger devices were reported to produce a larger power.
According to this chart, devices on the order of 1 cm 3 were reported to
produce from less than 1
Wupto B 1 mW for generators operating
at frequencies close to walking. Figure 3.3 summarizes that generators
operating at 1 Hz presented power densities between B 1
μ
W/cm 3 and
μ
B 1 mW/cm 3 .
The shaded area in Figure 3.1 represents the range of frequencies
commonly associated with body motion activities. Those generators that
are found inside this region are potential candidates for portable devices
and/or biomedical applications. Although piezoelectric generators pre-
sented a relatively large power output, a percentage of them were
designed to be placed inside shoes which might not be the most desirable
location. The clear-shaded area in Figure 3.2 corresponds to the zone
for miniature systems with energy harvesters
than 1 cm 3
smaller
(1
1 mW power output). The designs from von Buren and Troster
(2007) and Renaud et al. (2005) are suitable solutions for portable or
implantable devices in that size range, although power output is below
50
μ
W
W, whereas the Feenstra et al. (2008) device produced a higher out-
put but it is designed to be placed in backpack straps. The darker
shaded region encompasses devices with volumes between 1 and 10 cm 3 ,
which is populated with a larger variety of energy harvesters.
Generators in this region can be selected according to the application
constraints since power output goes from less than 1
μ
W to several milli-
watts. Devices with volumes over 10 cm 3 seem to be more appropriate
for portable or wearable systems because of the larger dimensions.
μ
Figure 3.3 better describes the energy generation panorama for gen-
erators operating at frequencies between 1 and 3 Hz. The chart does
not include all the reviewed generators because not all publications
provided the power output or the device ' s volume to estimate the
power density values. It can be appreciated that generators are capable
of producing power densities as high as 1 mW/cm 3
for devices with a
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