Energy source for energy harvesters is presentEnergy source for energy harvesters is present

harvesting is the
process by which energy is derived from external sources
captured, and stored for small, wireless autonomous devices, like those used
in wearable
electronics and
wireless sensor
networks. Energy
harvesters provide a very small amount of power for low-energy electronics.
While the input fuel to some large-scale generation costs resources the energy
source for energy harvesters is present as ambient background.


A number of energy harvesters utilizing piezoelectric technology have
been proposed with various mechanisms of energy conversion. In the study some authors
purposed a piezoelectric energy harvester to harvest energy from vibration. 6
Afterward, they have designed an energy harvester with piezo ceramic thin films
mounted on a cantilever beam. 1?W of average power has been generated from this
170?m × 260?m beam-shaped energy harvester. 7

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The cantilever beam transfers the vertical force to the mounted thin
films. The thin films are deformed transversely and the electric potential is
created. Such method of energy conversion was named as 31-mode. Another method
in which the piezo ceramic generates electrical energy from the force applied
along its direction of polarization. 7


A vibration is applied as an input on to a piezoelectric
material as shown in Figure 2.4. This causes mechanical force to converted to
electrical voltage. 8 Lead-zirconate-titanate (PZT) is material that will
always be used for generating the power


Figure 2.8: Equivalent circuit for energy harversting.



LM represents the mechanical

CM the mechanical stiffness

RM is the mechanical losses.

The mechanical part and
electrical part is connected together by a transformer which converts pressure
to voltage. On the electrical side, Cp is the plate capacitance of the PZT.  The whole circuit can be converted to the
electrical system at resonance


There is another
characteristics of this source is that it outputs is very low for the low
levels of input vibration in ambient secifications. 8 This low output voltage
makes it hard to develop a very efficient circuit for diode rectifier.


A piezoelectric
harvester is normally shown where a resistor and capacitor will be connected in
parallel with current source. The power output by the piezoelectric harvester cannot
be used directly to the loads such as radio. 8 The voltage and current output
by the harvester needs to be  designed
and changed into a form which can be used for the loads. 9


The power
conditioning and converting circuits should also be able to acheive the maximum
power available from the piezoelectric energy harvester. Commonly used analogue
and digital circuits are needed to regulate the supply voltage to operate. Since
a sinusoidal current  is the output of
the harvester, it must be connected into rectifier before it can be used for
any other applications like power circuits.