Vibration Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 109

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Vibration E nergy Harvesting System

Many environments, such as highways, railways, and so on, are subjected to
ambient vibration energy that is not commonly used. To use these ambient
vibrations as a power source, many researchers have successfully built and
tested three basic methods for generating electrical energy from this vibration
energy source: electromagnetic induction [48], electrostatic generation [97],
and piezoelectric materials [98]. While each of these techniques can provide
a useful amount of energy, piezoelectric materials have received the most
attention due to their ability to directly convert applied strain energy into
usable electric energy and the ease at which they can be integrated into a sys-
tem [98]. Unlike the electrostatic and electromagnetic approaches, which re-
quire a complex “two-part” design (the two plates of the variable capacitor in
the electrostatic configuration, the coil and the magnet in the electromagnetic
one), the piezoelectric approach is relatively simpler in design and implemen-
tation. Plus, Roundy et al. [99] demonstrated that the piezoelectric type has
the highest energy density. Based on these positive findings, the piezoelec-
tric approach has been employed in this vibration energy harvesting (VEH)
research for powering the electrical load.
Piezoelectricity is the ability of some materials (i.e., crystals) to convert me-
chanical energy into electrical energy and the inverse [100]. When an external
force mechanically strains the piezoelectric material, the material becomes
electrically polarized, and the degree of polarization is proportional to the
applied strain. The opposite effect is also possible: When the piezoelectric
material is subjected to an external electrical field, it is deformed. The relation-
ships between the applied force and the subsequent response of a piezoelectric
material depend on three factors [101]: (1) the dimensions and geometry of the
material, (2) the piezoelectric properties of the material, and (3) the directions
of the mechanical or electrical excitation. The first relationship is straightfor-
ward. As for the second relationship, the behaviour of piezoelectricity can be
modelled with the following constituent equations:

X E

D = dX + (4.1)

s E X

x = + dE (4.2)

Based on the describing electromechanical expression of a piezoelectric
material expressed in Equation 4.1 , the electrical displacement D relates with

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