PIEZOELECTRIC ENERGY HARVESTING FOR SUPPLYING POWER TO REMOTE INSTALLATION
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 • Low maintenance • Easy replacement of equipment • Good efficiency • Easy installation
APPLICATION • Light house • Defence observation post • Astronomical observatory • Aeroplane & ship