Power for Remote Installation

8/8/2019 Power for Remote Installation

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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 spurredon 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 alsoproduces 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 ³piezenin´, meaning, to

press.The original meaning of the word ³piezoelectric´ implies

³Pressure electricity¶ ±the generation of electric field

from applied pressure. This definition ignores the fact

that the process is reversible, thus allowing thegeneration 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 poledas it is cooled through its curie point, the domains inthe crystals are aligned in the direction of the strong

electric field. In this way, a piezoelectric material of required size, shape and piezoelectric qualities canbe made within limits.

In a given crystal, the axis of polarization dependsupon the type of stress. There is no crystal class in

which the piezoelectric polarization is confined to asingle axis. In several crystal classes, however, it isconfined to a plane. Hydrostatic pressure producesa piezoelectric polarization in the crystals of thoseten classes that show piezoelectricity, in addition to

piezoelectricity.




For understanding the

mechanism of generationof piezoelectricity the

crystal structure of unit

cell of tetragonal barium

titan ate (BaTiO3) as

shown on fig may bereferred.

The positive µTi¶ 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 piezoelectricplastic was invented. It is one of the most widelyused piezopolymer from which substantial electricitycan be generated. It is cheap and physically quitestrong.

In 2001 researchers found that PVDF becomessupersensitive to pressure when impregnated withvery small quantity of nanotubes, thus PVDF with its

inherent superior mechanical properties whenupgraded with nano-technology produces a newgeneration of piezopolymer, which are durable andcan generate large quantity of electricityeconomically.




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 beencommercially 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 ³Polling´. Such polarization, attributed toredistribution 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. a







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 unitadjoined. 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 withmechanical 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. Byassuming 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, thechange 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 highbreakdown voltage rating, a custom wound inductor with inductanceof about 10mH, a Schottky diode, and a filter capacitor. The voltageacross the current sense resistor is amplified with a precisionoperational amplifier and then sampled by the A/D converter on thecontroller card.

The controller card then generates the PWM signal at the calculatedduty cycle that is fed to a high side MOSFET driver. The driver ispowered by an external DC power supply.

The flexibility of the controller allows the energy harvesting circuit tobe 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 affectcontroller operation. The DC-DC converter with this control algorithmharvests energy at over four times the rate of direct charging withoutconverter.




ADVANTAGES

Low maintenance

Easy replacement of equipment

Good efficiency

Easy installation




APPLICATION

Light house

Defence observation post

Astronomical observatory

Aeroplane & ship






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Power for Remote Installation