2.3 PEH Systems
2.3.1 Base excited PEH systems
PEH Systems scavenge energy from the ambient vibrations for energy harvesting.
Generally, the harvester is a cantilever beam with piezoelectric patch subjected to external excitation from the ambient vibration. External excitation can be broadly classifies into deterministic and stochastic type [45]. Among them the deterministic type is widely explored. Within the vast deterministic excitations, harmonic type has
piezoelectric patch tip mass
substrate
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Figure 2.1: Different configurations of conventional cantilever beam based piezo- electric energy harvesters (PEH) with base excitation (a) PEH system I (b) PEH
system II (c) PEH system III (d) PEH system IV
been mostly used. The mechanical energy associated with harmonic base excitation is expressed either in displacement or forced form. The nature of vibration may be linear if the amplitude of excitation is very small and the system is away from the resonance conditions. Further, it may be nonlinear depending on the amplitude and frequency of the external excitation and the resonance conditions. In this section the work related to linear and nonlinear energy harvesters has been discussed.
2.3.1.1 Linear PEH systems
The earlier works [28, 46–51] give the direction for the development of the vibration based energy harvesters. Later on linear vibration based energy harvesters have been
explored, where near resonance condition is essential for effective energy harvesting in harmonically base excited systems.
Roundy et al. [16] presented a basic linear electromechanical model of a conventional energy harvester with a tip mass. They have developed a prototype of size 1 cm3 and generated a power density of 70 µW/cm3. Also, they have shown that with an optimized configuration, maximum power of 250 µW can be achieved with an acceleration of 2.5 m/s2 and a frequency 120 Hz. Sodano et al. [52] theoretically and experimentally investigated a cantilever based linear PEH system.
Further, many researchers have attempted to increase the power and bandwidth of the linear harvester using different methods, which include strain magnification [53], geometric alteration [54, 55] and an array of harvesters having nearly spaced frequencies [56, 57]. One may find the details of early developments in the field of linear vibration based harvesters in the review work of Anton and Sodano [7] and Cook-Chennault et al. [58]. In the following section, the nonlinear PEH systems are discussed.
2.3.1.2 Nonlinear PEH systems
The limitations of linear vibration based harvesters are their limited frequency band- width and few resonance conditions. This leads to shift the research focus on nonlin- ear vibration based harvesters [10, 59–64]. The term bandwidth defined as the range of frequency that is available for energy transduction. Wider bandwidth means a higher range of ambient frequency over which energy transfer is possible. The source of nonlinearity in the dynamic systems may be of material, geometric or inertial type.
Nonlinear stretching or large curvature causes the geometric nonlinearity and con- centrated or distributed mass imparts inertial type nonlinearity in the system. As discussed in the introduction chapter one may observe multiple resonance conditions due to external excitation and internal resonance due to nearness of the natural fre- quencies. In these systems one may observe both stable and unstable fixed point and periodic responses with multiple stable equilibrium points and many bifurcation points. Also, quasiperiodic and chaotic responses [65] may be observed in this type of system. A nonlinear vibration based harvesting system brings several advantages in the power harvesting capabilities as the ambient vibration contains wide range
of frequencies. Hence these nonlinear vibration based energy harvesters outperform the linear harvesters.
Geiyer and Kauffman [66] induced chaotic nonlinear phenomena in a bistable piezo- magnetoelastic energy harvester to increase bandwidth and power output. Chaos has been observed by Cao et al. [67] in piezomagnetoelastic energy harvester under the low frequency of excitation when a nonlinear magnetic force is applied. A hybrid bistable piezoelectric beam energy harvester with internal resonance due to movable magnet is presented by Yang and Towfighian [68]. An axially loaded, tunable and clamped-clamped piezoelectric energy harvester (PEH) is presented by Masana and Daqaq [62] where an improved steady state response, power and frequency band is achieved by imparting nonlinearity by axial load. Similarly, Harne and Wang [69]
developed a biologically inspired harvesting system by using a dynamic compressive axial suspension. The system comprised of a simply supported beam in horizontal position with piezoelectric patches. The axial compression helps in maximizing the beam bending and enhances the average output power harvested.
Kecik and Borowiec [70] presented an autoparametric energy harvester which con- sists of a pendulum and a nonlinear oscillator, where they observed that the chaotic motion of the pendulum generates the highest power as compared to swinging and rotation. A coupled nonlinear vibro-impact beam system for energy harvesting is analyzed by Vijayan et al. [71]. The power generated is found to be sensitive with beam thickness ratio and clearance between beams. A higher power is observed due to dissimilar mode shapes of the interacting modes. Borowiec [72] analysed the nonlinear dynamics of vertical beam harvester with tip mass under the harmonic excitation with an added stochastic component. It has been observed that in non- linear piezoelectric based harvesters the power output is significantly higher than those of the linear harvesters.
A cantilever beam with a tip mass is considered as an energy harvester by Rezaei et al. [73]. An improved resonance bandwidth is observed by applying a nonlinear cubic restoring force. Scapolan et al. [74] analysed the problem of energy harvesting as a parametrically excited electromechanical oscillator with nonlinear cubic damp- ing to extend the dynamic range of the harvester. Tang and Li [75] developed a two stage vibration energy harvester. When the second stage of higher frequency is driven by first stage of low frequency under higher acceleration, the first stage
exhibits chaotic characteristics. Friswell et al. [64] proposed a base excited vertical cantilever beam with a tip mass for energy harvesting considering cubic and inertial nonlinearity. Here an improved bandwidth is achieved as compared to linear config- uration. Litak et al. [76] examined the regular and chaotic responses of a vertical beam and a tip mass based vibrational energy harvester by varying the tip mass. A similar model is evaluated under various excitation forces such as purely harmonic, purely random, and combinations of harmonic and random by Bilgen et al. [77].
Apart from the base excitation, the flow energy is also utilized as an energy source to develop PEH systems. The next subsection describes the flow induced PEH systems.