21 Ways of Harvesting Energy

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As  an  alternative technology, highly efficient  miniaturized  energy harvesters  using  thin-film structures can be developed. A lot of researchers are working for harvesting energy in different forms like 1. Solar energy 2. Vibration energy 3. Magnetic energy 4. Thermal energy etc and many more. In the present module, we discuss in detail about the Solar energy harvesting using ferroelectric thin films, piezoelectric energy harvesting and magnetic energy harvesting.

 

1)      Ferroelectric photovoltaic effect :-

 

Researchers have been striving to open different corridors for the production of renewable energy with the evolving demands of the inexhaustible energy and clean fuel sources. With Earth receiving plenty of continual solar energy everywhere from the Sun which is the primary source of the viable clean energy, momentous attempts have been done on gathering light energy through photovoltaic effects (PV) in which light energy is converted into electricity. Photovoltaic effect involves three primary processes : (1) electron-hole pairs creation under illumination of light (2) breaking away of electrons and holes, and (3) the migration of electrons and holes to build the net electric current in a specific direction. The charge disassociation commences ensuing four mechanisms : (1) granularity (2) in-built non-centrosymmetry in bulk material (3) the heterojunction (such as schottky barrier) as in conventional silicon (Si) based solar cells (4) built-in electric field originating from polarization . The electric field existing in the space charge region of p-n junction or heterojunction seperates the charge carriers in the conventional Si-based solar cells. On the other hand in a ferroelectric thin film, electric polarization give rise to a net internal electric field throughtout its bulk region without being absolutely cancelled by screening charges. Thus, PV effects are not constrained to any particular region and they can be achieved without creating complexities. Moreover diffusion do not restricts the charge propagation and output photovoltage is not confined by energy barrier. The exceptional and superior edge of the ferroelectric photovoltaic effect (FPV) (i) high output voltage (as big above bandgap), (ii) polarization controlled PV response have been intriguing many researches in this field [18,20,]. FPV effect has been presented in several perovskite oxides, such as BiFeO3 (BFO), Pb(Zr,Ti)O3 (PZT), BaTiO3 (BTO) and the elemental mechanisms have been mentioned which include : (i) internal electric field (depolarization field) effect generated by the polarization, (ii) metal-ferroelectric interface (schottky-ohmic barrier) effect, (iii) bulk PV effect (BPVE) [19,20]. At the same time, the photovoltaic efficiency (light to power conversion ability) has been curbed by small current densities (of the order of 10-9 A/m2) and large band gaps (typically ~ 3.5 eV) of these materials. Therefore there is a high need of finding an alternative route to tailor the properties of these ferroelectric materials so that the high photovoltages which can be easily accomplished with modest light intensities can be obtained and can become a boon for a broad range of applications, where low electrical power is recommended. Inspite of low conversion efficiency, FPV can be used to empower future solar cells with much new capabilities.

 

2)  Magnetoelectric effect :-

 

Layered multiferroic materials are candidates for the next generation multifunctional devices. In these structures, the interaction between ferroelectric and ferromagnetic layers produce new coupled magnetoelectric (ME) effect. The ME effect is defined as an induced electric polarization (P) of a material with an applied magnetic field (H) or vice versa (Direct ME and converse ME). The ME effect comes from the local exchange between internal orderly magnetic structure and ferroelectric sub-lattice. Unfortunately, single phase materials exhibit weak ME effect, which explains their limited application. Magnetoelectric in layered structures provide an alternative, exhibiting higher ME effect due to mechanical coupling between piezoelectric and ferromagnetic layers.

The principle of ME effect in the composite system is that the magnetic-field-induced strain in the magnetostrictive component is transferred to piezoelectric component through elastic coupling, resulting ina piezo-induced voltage and vice versa. It means that in the piezolagnetic/piezoelectric composites, in presence of an applied magnetic field, piezomagnetic particles change their shape due to magnetostriction effect, and this strain is passed to the piezoelectric phase, resulting in an electric polarization change as shown in figure 2.

 

Thus, the magnetoelectricity in the composite system is a product property and needs biphasic surrounding to exhibit the complex behaviour. The primary ME composites materials become magnetized when placed in an electric field and electrically when placed in a magnetic field.

 

The term ‘magnetoelectric coefficient’ is the voltage generated in the piezoelectric due to the applied magnetic field per piezoelectric thickness, and has the units of V/(cm Oe). The ME device can be operated in four configuration which are as follows:

 

1.      L-L mode (longitudinal-longitudinal) – magnetic field and electric field are along the surface

 

2.      L-T mode (longitudinal-transverse)- magnetic field is along the surface and electric field is perpendicular to the surface

 

3.      T-L mode (transverse-longitudinal)- magnetic field is perpendicular and electric field is along the surface of the system

 

4.      T-T mode (transverse-transverse)- magnetic field and electric field are perpendicular to the surface.

 

One of the advantages of magnetoelectric devices is that it can be used as energy harvesting device as the input source of energy which can be taken from the nature.

 

3)   Piezoelectric cantilevers :-

 

The vast majority of piezoelectric energy harvesting devices uses a cantilever beam structure. A cantilever beam, by definition, is a beam with a support at only one end, and is often referred to as a ‘fixed free’ beam. When the generator is subjected to vibrations in the vertical direction, the support structure will move up and down in sync with the external acceleration. The vibration of the beam is induced by its own inertia; since the beam is not perfectly rigid, it tends to deflect when the base support is moving up and down (Figure 3). Typically, a proof mass is added to the free end of the beam to increase the deflection amount. This lowers the resonant frequency of the beam and increases the deflection of the beam as it vibrates. The larger deflection leads to more stress, strain, and consequently a higher output voltage and power. Hence, by adding the variable proof mass, the resonance frequency of the cantilever can be tuned. The cantilevers can be classified in two categories: 1) Unimorph and 2) bimorph. When the beam has only a piezoelectric layer attached to a substarte layer, the device is known as a unimorph. When a substrate material is sandwiched between two piezoelectric materials, the device is known as a bimorph.

The operation of cantilever benders is relatively simple. If one layer is in compression, the other layer is in tension. The stress in one layer affects the stress in the other layer. For example, in a piezoelectric unimorph when an electric filed is applied to the piezoelectric layer, the piezoelectric layer expands or contracts where as the non-piezoelectric material is not affected by the electric field. This causes the cantilever to bend. The opposite also occur when the beam undergoes bending from an applied force from an external vibration source. This bending causes a charge to be generated between the electrodes of the piezoelectric layer. In this situation, energy can be easily harvested from the electrodes.

 

4    Cantilever (Details) :

 

A structural element anchored at one end to a support and subjected to load transverse to its axis at the other end is known as a cantilever. A cantilever is classified under category of beam. In general, a beam can be either free from any axial force or the effect of this force may be negligible. Usually, a beam is considered in horizontal direction and load in vertical direction. The load can be of two types, 1) Concentrated load and 2) Distributed load. The concentrated load is assumed to act at a particular point, though in practice it may be distributed over a small area. On the other hand, distributed load is one which is spread over the length of the cantilever. However, the rate of loading may be uniform or may vary from one point to another. There are different types of supports for beam which are as follows:

 

1.      Roller support: In case of roller support, a beam rests on a sliding surface like a roller or a flat surface (Figure 4). The roller support can sustain a force normal to its surface as the possible movement on the supporting surface does not allow any resistance in that direction. Therefore, the reaction (R) along the rolling surface is zero and it is present only normal to the surface.

 

2. Hinged Support: In case of hinged support, the possibility of translation displacement of the beam is zero, however, rotation is possible. In this, there can be reactions in vertical (R) as well as in horizontal direction (H) (Figure 5).

3.  Fixed or encastre or built in support: A built-in to a rigid support which does not allow any type of movement or rotation is known as fixed or encastre or built-in support. A fixed support exerts a fixed moment (M) and a reaction (R) on the beam (Figure 6).

A beam with one end fixed and the other end free is called cantilever (Figure 6). There is vertical reaction (R) and moment (M) at the fixed end and is called fixed moment. In the present chapter, cantilever beam is made which is supported from one end (fixed support) and free from other ends. In this case, cantilever beam transfers the load to the rigid support where it manages the moment of force and shear stress [Duan et al. (2014)].

 

Shear Force

 

Shear force is one of the most important parameters in case of cantilever. It is an unbalanced vertical force on one side (other than fixed support) of the cantilever beam and is the sum of all the normal forces [Duan et al. (2014)]. In other words, it represents the tendency of either portion of the cantilever to slide or shear laterally relative to the other. Shear force is considered positive when the resultant of the forces to the left of a section is upwards or to the right downwards.

 

Bending Moment

 

Bending moment is another parameter of interest while understanding the theory behind cantilever. Bending moment at some section of a beam is defined as the algebraic sum of the moments about the section of all the forces on one side of the section [Duan et al. (2014)].

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 REFERENCES

 

  1. Solar Cells. chemistryexplained.com
  2. http://solarbotics.net/starting/200202_solar_cells/200202_solar_cell_use.html
  3. “Photovoltaic System Pricing Trends – Historical, Recent, and Near-Term Projections, 2014 Edition” (PDF). NREL. 22 September 2014. p. 4.
  4. Fiebig, M. (2005). “Revival of the magnetoelectric effect”. Journal of Physics D: Applied Physics. 38: R123
  5. ETH Zürich, Laboratory for Multifunctional Ferroic Materials, “Magnetoelectric Effect”.
  6. L. Landau & E. Lifshitz, “Electrodynamics of continuous media”, Pergamon press, 1960
  7. Dzyaloshinskii, I. (1960). “On the Magneto-Electrical Effect in Antiferromagnets”(PDF). Zh. Exp. Teor. Fiz. 37: 881.
  8. Astrov, D. (1960). “The magnetoelectric effect in antiferromagnetics” (PDF). Sov. Phys. JETP. 11: 708.
  9. Spaldin, Nicola A.; Fiebig, Manfred (2005-07-15). “The Renaissance of Magnetoelectric Multiferroics”. Science. 309 (5733): 391–392.
  10. Nan, C.W.; Bichurin, M.I.; Dong, Shuxiang; Viehland, D.; Srinivasan, G. (2008). “Multiferroic magnetoelectric composites: Historical perspective, status, and future directions”. J. App. Phys. 103: 031101.
  11. P. Curie J. Physique, 3ième série III (1894)
  12. P. Debye, Z. Phys. 36, 300 (1926)
  13. Spaldin, Nicola A.; Cheong, Sang-Wook; Ramesh, Ramamoorthy (2010). “Multiferroics: Past, present, and future”. Physics Today.
  14. R.J. Wilfinger, P. H. Bardell and D. S. Chhabra: The resonistor a frequency selective device utilizing the mechanical resonance of a silicon substrate, IBM J. 12, 113-118 (1968)
  15. P. M. Kosaka, J. Tamayo, J. J. Ruiz, S. Puertas, E. Polo, V. Grazu, J. M. de la Fuente and M. Calleja: Tackling reproducibility in microcantilever biosensors: a statistical approach for sensitive and specific end-point detection of immunoreactions, Analyst 138, 863-872 (2013)
  16. A. R. Salmon, M. J. Capener, J. J. Baumberg and S. R. Elliott: Rapid microcantilever-thickness determination by optical interferometry, Measurement Science and Technology 25, 015202 (2014)
  17. P. C. Fletcher, Y. Xu, P. Gopinath, J. Williams, B. W. Alphenaar, R. D. Bradshaw, R. S. Keynton, “Piezoresistive Geometry for Maximizing Microcantilever Array Sensitivity,” presented at the IEEE Sensors, Lecce, Italy, 2008.