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voltage regulator [37]. The thermoelectric module of the wristwatch is re-
corded to yield 60
W/cm 2 ata5Ktemperature gradient with 10 TEGs
coupled together in series [41]. Similarly, Leonov et al. [39] have considered
TEH through thermoelectric power generation from body heat to power wire-
less sensor nodes as shown in Figure 1.18b . The average power generation
at daytime of about 250
W/cm 2 with a tem-
perature difference of 10 K, which is better than solar panels in many indoor
situations, especially considering the TEG power is also available at night.
However, these systems do not consider proper matching between the source
and the load to ensure MPP operation.
In other TEH research, both Stevens [42] and Lawrence et al. [43] consider
the system design aspects for solar-TEH via thermoelectric conversion that
exploits the natural temperature difference between the ground and air. Later,
Sodano et al. [40] presented a solar-TEH system placed in a greenhouse with
a solar concentrator as seen in Figure 1.18c . The solar-TEH system uses a
TEG to recharge a NiMH nickel metal hydride battery. At an estimated
W corresponds to about 20
T of
25 K, the harvested energy was able to recharge an 80-mAh battery in 3.3 min.
The authors have demonstrated that a TEG may be used for solar energy
conversion as an alternative to photovoltaic devices. However, like before,
there are few discussions on the power management aspects of the solar-TEH
system.
1.3.3.3 Vibration Energy Harvesting System
The first important virtue of random mechanical vibrations as a potential
energy source is that they are present almost everywhere. Mechanical vibra-
tions occur in many environments (e.g., buildings, transports, terrains, human
activities, industrial environments, military devices, and so on). Their charac-
teristics are various: spectral shape from low to high frequency and amplitude
and time duration manifold depending on the surroundings. Theory and ex-
periments from many research works show that the power density that can
be converted from vibrations is about 300
W/cm 3 [38]. Devices that convert
mechanical motion into electricity can be categorized in electromagnetic, elec-
trostatic, and piezoelectric converters [44, 45]. In the case of electromagnetic
converters, a coil oscillates in a static magnetic field and induces a voltage.
In electrostatic converters, an electric charge on variable capacitor plates cre-
ates a voltage if the plates are moved. Piezoelectric converters finally exploit
the ability of some materials like crystals or ceramics to generate an electric
potential in response to mechanical stress. A prominent example for the em-
ployment of vibrational harvesters is in the watch industry, where vibrational
energy converters have been used with success to power wristwatches.
Shenck et al. presented a piezoelectric-powered RFID (radio-frequency
identification) system [46] for shoes, as illustrated in Figure 1.19a , that har-
vests energy from human walking activity. The developed shoe inserts are
capable of generating around 10 mW of power under normal walking condi-
tions. This shows that mechanical vibration from human activity is another
promising renewable energy source worth investing effort to investigate.
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