Piezo-Electricity

INTRODUCTION

Imagine the world when the world is without power. It is horrible to imagine. In the present era scientific community is looking for different sources of energy. The present sources of energy (conventional) are depleting at unimaginable rate & is last for few years only.
Some non-conventional sources are there but still we have to look for other alternative sources, as they are uneconomical & inefficient .The power of the wind and the tide have been harnessed- now the football of trudging shoppers is to become the latest source of emission free energy. Imagine we can produce electricity by our foot step and here is come…
“Piezoelectricity”

BASIC PRINCIPLE[1]

Piezoelectricity is the ability of some materials (notably crystals and certain ceramics, including bone) to generate an electric potential in response to applied mechanical stress. This may take the form of a separation of electric charges across the crystal lattice. If the material is not short-circuited, the applied charge induces a voltage across the material.

Abstract Decreasing size and power requirements of wearable microelectronics make it possible to replace batteries with systems that capture energy from the user's environment. Unobtrusive devices developed at the MIT Media Lab scavenge electricity from the forces exerted on a shoe during walking: a flexible piezoelectric foil stave to harness sole-bending energy and a reinforced PZT dimorph to capture heel-strike energy.[2]

Many materials, both natural & man made, exhibit the piezoelectric effect. Some of the examples of piezoelectric are as follows
Naturally-occurring crystals
• berlinite (AlPO4), a rare phosphate mineral that is structurally identical to quartz
• cane sugar
• quartz
• Rochelle salt
• topaz
• tourmaline-group minerals
Man-made crystals
• gallium orthophosphate (GaPO4), a quartz analogic crystal
• Langasite (La3Ga5SiO14), a quartz analogic cryst

Mechanism

In a piezoelectric crystal, the positive and negative electric charge are separated, but symmetrically distributed, so that the crystal overall is electrically neutral. Each of these sites forms an electric dipole and dipoles near each other tend to be aligned in regions called Weiss domains. The domains are usually randomly oriented, but can be aligned during poling (not the same as magnetic poling), a process by which a strong electric field is applied across the material, usually at elevated temperatures.
When a mechanical stress is applied, this symmetry is disturbed, and the charge asymmetry generates a voltage across the material. For example, a 1 cm cube of quartz with 2 kN of correctly applied force upon it, can produce a voltage of 12,500 V. [3]



PIEZO-TECHNOLOGY AS GENERATION OF ELECTRICITY

Piezo electric concept was given in 1880 by the brothers Pierre Curie and Jacques Curie. This concept was in picture for a long time ago, but still it is used for electronic frequency generator, microbalance, device for sonar, ultrasonic submarine detector, hydrophone etc.
It was first used as mini voltage generator during post World War I by US engineers into soldier boot[3] for illuminating torches etc.
We can use this technology on mass scale for the generation of commercial electricity as….
Under floor generator powered by “heel strike” and can be designed by engineers may soon be installed at public places like supermarket, stations etc.. The technology could use the foot step of pedestrian to power thousand of light bulb at shopping centers.
Its work by using the pressure of feet on the floor to compress the pad underneath, “pad” is a composite of “piezoelectric crystal” through mini turbine that generate electricity which is stored in battery. It has been calculated that 34,000 traveler passing through every hour could power 6,500 light bulb (as stated in TIMES TREND Kolkata edition).
In addition to floor’s technology could also be installed beneath railway lines, on road bridges to exploit the energy of passing train and vehicle. In this process generator convert energy from train passing above into electricity. Miniature generator (piezoelectric crystal made pad) would be installed beneath the surface of the stair use by the visitor. We can use this concept everywhere there is a source of pressure.
ECONOMY OF PIEZO-TECHNOLOGY AS GENERATION OF ELECTRICITY
The one of the material used for this purpose is QUARTZ. This is most abundant mineral on earth crust. But still it is not economical. There are some manmade crystals as stated above which have piezoelectric property can be use for this purpose as it is economical [4].
LIMITATIONS
The electrical characteristics of a piezoelectric power generator are investigated under quasi-static (duration >100 ms) and dynamic (stress duration <10 ms) stress applications. The electromechanical model of piezoelectric generator is presented and used to explain the effects of the two stress conditions. A computer simulation of the piezoelectric generator is used to compare the theoretical and experimental results. The simulation predicts that a quasi-static stress will produce a bidirectional generator output voltage, and a dynamic stress will produce a unidirectional output voltage. The simulation also predicts that, when equal stresses are applied to the generator, the dynamic stress will generate a 10X higher output voltage than the quasi-static stress, contradicting results reported by other investigators. [5]
It is clear from the above discussion that it produces large voltage (which is difficult to handle) but due to large impedence current produce is less hence low power output. So new advance MEMS technology used to generate more current.


FERENCES:
[1] IEEE journal
[2] T. Starner, "Human Powered Wearable Computing," IBM Systems J., vol. 35, no. 3, 1996, pp. 618-629.
[3] Wikipedia
[4] http:\ www.sciencedirect.com
[5] IEEE journal of Chok Keawboonchuay and Thomas G. Engel


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