Piezoelectric Energy Harvesting (PEH) | Piezoelectricity | ScienceMonk

“Piezoelectric energy harvesting has the potential of integration in many human-powered applications, and thus, they provide a viable solution to our increasing energy demands.”



The energy crisis is one of the major problems faced by the world over the past few decades. Energy consumption all over the world has increased rapidly at a very alarming rate due to overpopulation, excessive use of non-renewable energy sources, outdated and inadequate infrastructure of power generating equipment, wastage of available energy sources like fuel, electricity, etc.

To overcome the demand rate, energy harvesting from alternative sources of energy is gaining much importance, and extensive research is going on to come up with some viable solution to the energy crisis problem.



What is Energy Harvesting?

“Energy harvesting or, energy scavenging is the process of extracting electrical energy from external sources like solar, thermal, mechanical, wind, etc. and storing it for small, autonomous wireless devices, like those used in wearable electronics and wireless sensor networks. Energy generation from waste or idle power such as from environmental sources, industrial machines, vehicles, human, mechanical activities, etc. can contribute to the world’s energy demands”.



 

Energy harvesting materials and systems have emerged as a prominent research area and continue to grow at a rapid rate. Some of the favourite ambient energy sources suitable for energy harvesting applications include solar, thermal, chemical, vibration, radiofrequency, acoustic waves, and temperature gradients.

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Solar devices provide high energy density in comparison to the other sources, but it has drawbacks of interrupted and uneven supply, low conversion efficiency, etc. Significant research efforts have been carried out in the field of power generation using stress, strain, and vibrations.



An inertial mechanism is used to harvest the vibration energy in which the vibration is coupled to a proof mass and then extracted by damping the mechanical motion of the mass. Piezoelectric, electromagnetic and electrostatic vibrations give rise to three types of vibration-based energy harvesters in which piezoelectric ones have higher energy density for all practical applications [1].

Special consideration is given on the design of energy harvesters intended for converting mechanical energy from a host system to electrical energy for direct consumption or storage for later use.



What is Piezoelectric Energy Harvesting?

Piezoelectric energy harvesting (PEH) works on the principle of piezoelectricity, a phenomenon shown by a specific class of materials called piezoelectric materials. Pierre Curie and Jacques Curie discovered it in 1880 while studying the effects of pressure on the single crystal of quartz (SiO2). The word piezoelectricity means electricity resulting from mechanical pressure.

Piezoelectric Energy Harvesting

“It is the accumulation of electric charges on the faces of certain crystals when they are subjected to mechanical stress and result in an induced electric field. It is also known as direct piezoelectricity. Conversely, when an electric field is applied on a piezoelectric material, a mechanical strain is produced in the material. It is known as inverse piezoelectricity. So, the piezoelectric effect is a reversible process”.



Piezoelectricity

Piezoelectricity is a result of the linear electromechanical interaction between the mechanical and electrical stresses in the non-centrosymmetric crystalline materials, i.e., materials lacking the centre of symmetry.

Out of 32 crystal point groups, only 21 are non-centrosymmetric and in these 21, except point group 432 in cubic symmetry, 20-exhibit piezoelectric effect. Out of these 20 non-centrosymmetric point groups, ten are polar, and rest 10 are non-polar. The crystals belonging to polar groups possess a unique polar axis, and they show spontaneous polarization without the application of mechanical stress. The non-polar crystals show net polarization only under the application of mechanical stress.

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Many piezoelectric materials have been used in energy harvesting applications with satisfactory results. Commercially available piezoelectric materials are now available as ceramics and polymers that can be manufactured into a variety of convenient shapes and sizes.

Some of the commonly used ones are Lead Zirconate Titanate [PbZr1-xTixO3, (PZT)], Polyvinylidene Fluoride (PVDF), Lead Zirconate Nickel Niobate [Pb{(Zr1-xNix)1/3Nb2/3}O3, (PZNN)], Aluminum Nitride (AlN), etc.  PZT, a stiff and brittle piezoceramic, is most widely used in energy harvesting applications. PVDF is a piezopolymer, which provides flexibility to the system.



Recent decades have seen an advancement in developing new types of energy harvesters for efficient energy generation and use. Piezoelectric energy harvesters have been employed in many applications for harnessing the mechanical disturbances in the surroundings.

Railway platforms, footpaths, highways, gyms, etc. are some of the places where these harvesters can be used to generate electrical energy using the great mechanical vibrations caused by human activities.

Due to the flexibility in the selection of the material, these harvesters have a full-size range: macroscale to microscale and their miniaturization enhances their use to power up the smart wearables and electronics.



References:

[1]    S Roundy, PK Wright (2004) A piezoelectric vibration based generator for wireless electronics. Smart materials and structures 13:1131.

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