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The need for electrical power supply to remote
installations without the use of diesel generating set or
without installing power transmission line has spurred
on interest on piezoelectric energy harvesting, or the
extraction of electrical energy using a vibrating
piezoelectric device.
Installation of power transmission line in inhospitable
terrain and its maintenance are costly. Use of DG set
produces sound signature, which can be easily picked
up by the enemy in case of defence installation. It also
produces pollution, which is not to the liking of
astrophysicists.
Piezoelectric
power
generators
overcome all these problems.
Piezoelectricity, discovered by Curie brothers in 1880,
originated from the Greek word “piezenin”, meaning, to
press.
The original meaning of the word “piezoelectric” implies
“Pressure electricity’ –the generation of electric field
from applied pressure. This definition ignores the fact
that the process is reversible, thus allowing the
generation of mechanical motion by applying a field.
Piezoelectricity is observed if a stress is applied to a
solid, for example, by bending twisting or squeezing it.
The phenomenon of generation of a voltage under
mechanical stress is referred to as the direct
piezoelectric effect, and the mechanical strain produced
in the crystal under electric stress is called the converse
piezoelectric effect.
The necessary condition for the piezoelectric effect is the
absence of a center of symmetry in the crystal structure. Such
an effect is not fond in crystals with a center of symmetry. Of the
32 crystals classes 21 lack a center of symmetry, and with the
exceptions of one class, all of these are piezoelectric.
If lead zircon ate titan ate, a piezoceramic, is placed between two
electrodes and a pressure causing a reduction of only 1/20th of
one millimeter is applied, a 100,000-volt potential is produced.
The basic equations of piezoelectricity are:
P = d x stress and E = strain/d
Where,
P = Polarization,
E = electric field generated and
D = piezoelectric coefficient in metres per volt.
MAKING
• The piezoelectric axis is then the axis of
polarization. If the polycrystalline material is poled
as it is cooled through its curie point, the domains in
the crystals are aligned in the direction of the strong
electric field. In this way, a piezoelectric material of
required size, shape and piezoelectric qualities can
be made within limits.
• In a given crystal, the axis of polarization depends
upon the type of stress. There is no crystal class in
which the piezoelectric polarization is confined to a
single axis. In several crystal classes, however, it is
confined to a plane. Hydrostatic pressure produces
a piezoelectric polarization in the crystals of those
ten classes that show piezoelectricity, in addition to
piezoelectricity.
For understanding the
mechanism of generation
of
piezoelectricity
the
crystal structure of unit
cell of tetragonal barium
titan ate (BaTiO3) as
shown on fig may be
referred.
The positive ‘Ti’ ion, surrounded by an almost regular octahedron of
negative oxygen ions, is not located at the centre of the octahedron, and
is some what displaced along the Z- axis. This structure already has a
dipole moment or spontaneous polarization, in the absence of externally
applied stress. When the crystal is mechanically compressed in XY
plane or is elongated along Z axis, the additional polarization associated
with the deformation is the piezoelectric polarization, which generates
electric field.
PVDF
• . In 1961 polyvinylidene fluoride, a piezoelectric
plastic was invented. It is one of the most widely
used piezopolymer from which substantial electricity
can be generated. It is cheap and physically quite
strong.
• In 2001 researchers found that PVDF becomes
supersensitive to pressure when impregnated with
very small quantity of nanotubes, thus PVDF with its
inherent superior mechanical properties when
upgraded with nano-technology produces a new
generation of piezopolymer, which are durable and
can generate large quantity of electricity
economically.
Although a number of polymers possess piezoelectric
properties, none match the magnitude of the effects in
polyvinylidene fluoride (PVDF), which is the most widely studied
and commercially used piezoelectric polymer. PVDF has been
commercially available since 1965. Substantial piezoelectricity
can be permanently induced by heating stretched films of PVDF
to about 1000Cfollowed by cooling to ambient temperature with a
strong DC electric field (about 300kVcm-1) applied. This
treatment is called “Polling”. Such polarization, attributed to
redistribution of electronic or ionic charges within the solids or
injected from electrodes, characteristically vanishes on
exceeding some polarization temperature, Tp. The effect in PVDF
is totally different in that the induced polarization is thermally
reversible and polarizations current are, produced on either
heating or cooling.
When a sheet of PVDF is compressed or stretched, an electric
charge is generated and collected on the surfaces. The PVDF
sheet is metallized on both sides which acts as electrodes
PHYSICAL PROPERTIES OF PVDF

Specific gravity: 1.75 -1.80;

melting point: 154-1840 C;

water absorption: 0.04-0.06%;

tensile strength at break: 36-56 Mpa;

elongation at break: 25-500%,

hardness shores D: 70-82;

low temperature embrittlement; -62 to 640 C.
Electrical Properties of PVDF
(with out nanotubes impregnation)

Volume resistivity: 2x1014 ohm-cm;

Dielectric constant at 60 Hzs: 8.40 pm/V

Piezoelectric stress constant: 0.23V/ (m. pa)
Nanotechnology is a new generation of technology of building
devices whose dimensions range from atoms up to 100 nanometers
with programmed precision. Nano is a prefix meaning dwarfed. It
is a prefix representing 10-9 which is one-billionth of the unit
adjoined. Nanotubes are tiny tubes of carbon about 10,000 times
thinner than a human hair. These consist of rolled up sheets of
monolayer or multilayer carbon atoms bonded together in
hexagon.
However only in 1991 nanotechnology was filtering into academic
and government circles as something worth thinking about and
intensive research work started. Nanotubes are over 50 times
stronger than steel wire and only a quarter as dense.
In 2001, a group of researchers in USA discovered that
polynimylidene fluoride, a piezoelectric plastic becomes three
times as sensitive to pressure when nanotubes are sprinkled in. just
addition of one nanotubes for every 8000 strands of PVDF is
enough to produce such super sensitivity.
A vibrating piezoelectric element can be considered as sinusoidal current
source at a particular time (t), ip (t) in parallel with its internal electrode
capacitance Cp. The magnitude of the polarization current Ip varies with
mechanical excitation level of the piezoelectric element.
These waveforms can be divided into two intervals. In interval 1, denoted as
u, the polarization current is chagrin the electrode capacitance of the
piezoelectric element. During this time all diodes are reverse biased and no
current flows to the output.
At the end of the commutation interval,
interval 2 begins, and output current flows
to the capacitor Crect and the load. By
assuming Crect >> CP, the majority of the
current will be delivered as output current.
The peak out put power occurs when Vrect
IP/2UCP or one half the peak open circuit
voltage of the piezoelectric element.
The magnitude of the polarization current
IP generated by the piezoelectric
transducer, and hence the optimal rectifier
voltage, may not be constant as it depends
upon the vibration level exciting the
piezoelectric element. This creates the need
for flexibility in the circuit. i.e., the ability
to adjust the output voltage of the rectifier
to achieve maximum power transfer.
The control algorithm is based upon the sign of a rate of change of the duty cycle. In
practice, the duty cycle continuously changes. Once the controller is stabilized, the
change of duty cycle amounts to small perturbations about the optimal operating
point. The control board includes a floating point digital converter for sampling
measurements, and pulse width modulated signal outputs for controlling the converter.
The sign of the quotient, ∂I/∂D, is used by a 0-threshold block to increment the duty
cycle by a set rate of 20 mill percent/s.
The duty cycle is then filtered and used to generate the PWM signal for the driver
circuitry of the step-down converter. The additional filtering of the PWM signal is
necessary to slow the rate of change of the duty cycle so the change in current can be
measured and evaluated. Without the LPF, the controller is prone to duty cycle
oscillations, as the perturbing signal reacts faster than the finite settling time of the
battery current signal.
DC/DC CONVERTER
• The step-down converter consists of a MOSFET switch with a high
breakdown voltage rating, a custom wound inductor with inductance
of about 10mH, a Schottky diode, and a filter capacitor. The voltage
across the current sense resistor is amplified with a precision
operational amplifier and then sampled by the A/D converter on the
controller card.
• The controller card then generates the PWM signal at the calculated
duty cycle that is fed to a high side MOSFET driver. The driver is
powered by an external DC power supply.
• The flexibility of the controller allows the energy harvesting circuit to
be used on any vibrating device regardless of excitation frequency.
Also external parameters, such as device placement, level of
mechanical vibrations or type of piezoelectric devices, will not affect
controller operation. The DC-DC converter with this control algorithm
harvests energy at over four times the rate of direct charging without
converter.
ADVANTAGES
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Low maintenance
Easy replacement of equipment
Good efficiency
Easy installation
APPLICATION
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Light house
Defence observation post
Astronomical observatory
Aeroplane & ship