Introduction - Energy Harvesting Autonomous Sensor Systems - page 29

at 1 Hz) stimulates and up to 800 W/cm 3 from machine-induced stim-

uli (2 nm motion at 2.5 kHz). Many meso and microscale EH generators
have been developed in the last 5 years [38]; however, there is a lack of ad-
equate ultralow-power management circuit to condition their micropower
generation.
In another VEH research work, Paradiso et al. [52] have successfully demon-
strated a piezoelectric element with a resonantly matched transformer and
conditioning electronics that, when struck by a button, generate 1 mJ at 3
V per 15 N push, enough power to run a digital encoder and a radio that
can transmit over 15 m. However, this system requires a large transformer
to step up the output voltage generated by the piezoelectric element. The
efficiency of the transformer is limited by flux leakage and core saturation
when the primary current peaks. Taking an interesting turn, assuming an av-
erage blood pressure of 100 mmHg (normal desired blood pressure is 120/80
above atmospheric pressure), a resting heart rate of 60 beats per minute, and
a heart stroke volume of 70 ml passing through the aorta per beat [53], then
the power generated is about 0.93 W. Ramsay et al. [54] found that when
the blood pressure is exposed to a piezoelectric generator, the generator can
generate power on the order of microwatts when the load applied changes
continuously and milliwatts as the load applied changes intermittently. How-
ever, harnessing power from blood pressure would only limit the application
domains to wearable microsensors.

1.3.3.4 Wind Energy Harvesting System
Like any of the commonly available renewable energy sources, wind energy
harvesting (WEH) has been widely researched for high-power applications
where large wind turbine generators (WTGs) are used for supplying power to
remote loads and grid-connected applications [55, 56]. According to a study
by the National Renewable Energy Laboratory (NREL) [57], wind energy
is the fastest-growing electricity-generating technology in the world. In the
past 10 years, global installations of wind energy systems have grown at least
10-fold, from a total capacity of 2.8 gigawatts (GW) in 1993 to almost 40 GW
at the close of 2003 [58]. In spite of this continuing success of WEH at a
large scale, there have been very few attempts on the development of small-
scale WEH, those that are miniaturized in size and highly portable, to power
small autonomous sensors deployed in remote locations for sensing or even
to endure long-term exposure to a hostile environment such as a forest fire.
Although very few research works are reported in the literature on small-scale
WEHs, some efforts to generate power at a very small scale have been made
recently. Park et al. presented a MPPT for a small windmill [35]. The WEH
system, as illustrated in Figure 1.20a , exploits the near-linear relationship
between the wind speed and the rotating frequency of the WTG's rotor to
force the WTG to work in its MPPs.
Similarly, a small-scale WTG has been presented by Holmes et al. [59], and
the power densities harvested from air velocity are quite promising. Later,