Vibration Based Piezoelectric Energy Harvesting - A Review

In this digital race, electronic equipment has been integrated into human beings as a part of their body. Some electronic equipment is connected by wires, while some are self-powered by batteries. Today the ultra-low-power smart electronic gadgets and smart wireless sensor devices need an unlimited battery for enhancing the performance. In a remote area such as forests and hill areas, conventional charging methods of batteries by wire is not possible. Supplying power through wires is difficult. To overcome this, a sustainable solution is energy harvesting. The renewable sources for energy harvesting are light, heat, wind, tidal, motion, and vibration. Researchers have more interest in harvesting energy through mechanical vibration due to its abundant availability. This paper reviews the work about piezoelectric crystals and their role in energy harvesting, simulation software used, energy harvesting circuits and storage devices.


Introduction
The process of collecting functioning or unwanted energy from the sources of natural and human-made things from the environment could be called energy harvesting. This system is used to charge batteries for self-powered portable devices or wireless sensor networks [1]. Non-renewable energy has a great impact on the environment, but piezoelectric material is renewable. The piezoelectric material could able to convert mechanical energy into electrical energy. It received the most attention of researchers due to the direct conversion of applied stress into electricity. Due to the compactness and smartness of piezoelectric material, it has a major role in portable electronic applications. The applications are power generation in a highway [2] and human bodies are to monitor breathing, blood flow and power pacemaker [3,4]. The portable devices such as wireless sensor networks in automotive, railways, aerospace, temperature and humidity sensing systems can be powered using piezoelectric materials [5]. This paper focuses on the review of literature related to piezoelectric materials and their role in energy harvesting, simulation software used, energy harvesting circuits and storage devices [6].

Sources of Energy Harvesting
Harvesting of energy can be broadly classified into two types, Micro and Macro energy harvesting. In micro harvesting the generation of power in m/μ volts, but in macro harvesting the production of power in kilo/mega volts [7]. [8,9]The energy sources for micro harvesting are vibration, motion, heat, etc. The  Table 1 shows a detailed summary of the sources of energy harvesting [7,8].

Piezoelectric Effect
In 1880 Pierre Curie and Jacques discovered the direct piezoelectric effect. When the mechanical strain is applied to a piezoelectric material, it tends to electrically polarize. The applied strain was proportional to polarization. Later in 1881 Jacques and Curies discovered inverse piezoelectric effect, in which applied polarization to the piezoelectric material can deform its size. [10] Figure 1 illustrates the direct and inverse piezoelectric effects. Figure 1. (a) Shows the piezoelectric material under no-load conditions. If an external force compresses the piezoelectric material then there is a change in dipole moment, which causes the same polarity poling voltage to appear between the two nodes as shown in Figure  1 (b). If the material is stretched, then piezoelectric material will produce the opposite polarity as shown in Figure 1 (c). When the opposite polarity of poling voltage is applied to the piezoelectric material then it will shorten as shown in Figure 1 (d). When the same polarity of poling voltage is applied to the piezoelectric material then it will expand as shown in Figure 1 (e). The piezoelectric material will expand and shorten by applying the alternative voltage and the frequency of material is based on the applying voltage as shown in Figure 1 (f).

Sources of Mechanical Vibration
When energy hits a medium, it can cause vibrations, the common sources of vibration are drilling machine, milling machine, bearing bed testing machine, automotive, household things, public transports, etc. [11]. Table 2 shows the sources of vibration with their acceleration and frequency levels [12,13].  [14]. Anton and Sodano suggested that piezoelectric materials can be arranged in different ways through altering the electrode pattern, stress direction, changing the direction of poling and varying the piezoelectric materials [15]. They also reviewed that the selection of materials has a major influence on harvesting functions and performance. PZT is highly brittle and PVDF is flexible. Lee [17]. Sarker et al. developed a MEMS-based energy harvester using C0MS0L Multiphysics software. The comparison between PZT and PVDF was done by varying the size of the micro cantilever beam. The generated voltage for PZT is 0.4V and for PVDF is 0.2V. From the simulation results, they have concluded that an increase in the length of a cantilever beam can decrease the resonant frequency [18]. Hassan et al. converted ultra-low sound frequency into electrical energy using a piezoelectric material. In this experiment, PZT was used to harvest energy from a loudspeaker at different distances and then converted into electrical energy. They have found that 27mV was produced at 80 dB of sound at the frequency of 62 Hz at 100 mm distance using coupling mode 31. Using coupling mode 33, authors obtained a generation of 90 mV at 103 dB of sound at 374 Hz. Finally, they concluded that an increase in the distance could lead to decrease in voltage generation [19]. Cook-Chennault et al. 2008 reviewed that generative and non-generative power supply systems for Microelectromechanical system (MEMS), by using piezoelectric energy harvesting system and found that PZT performance is not significantly influenced by temperature [20]. M.Taware and Deshmukh reviewed that PZT with a thickness of 0.28mm on the length of 11 mm beam with proof mass at the end can produce 375 μW at 120Hz. The generated power is used to demonstrate a radio transceiver [21]. [22,23] The summary of various piezoelectric materials is shown in Table 3.

Coupling Modes
The coupling modes of piezoelectric material with a substrate could affect the performance of energy harvester. Several authors investigated [15,[26][27][28] that 33 mode and 31 mode are the most influencing coupling modes if piezoelectric material in energy harvesting. When the applied force is perpendicular to the poling direction then it is called as mode 31. When the applied force is same as the poling direction then it is called as mode 33. Figure 2. Shows the coupling modes of piezoelectric materials. Mode 31 is commonly employed in the harvesting of energy because, a small force at lower vibration levels, it can produce efficient results than mode 33.

Various Geometries of Energy Harvester
Energy harvesting through a piezoelectric material can improve by optimization of piezoelectric geometry. The rectangular cantilever beam is the most commonly used geometrical configurations. Mateu and Moll analytically compared the rectangular and triangular (the small end is free and the large end is clamped) cantilever beams. It was proven that the triangular cantilever beam can produce a maximum deflection and higher strain equated to a rectangular cantilever beam with the same base and height [29]. When a piezoelectric material is subjected to higher strain and deflection, the output generated is also high. The authors concluded that the rectangular cantilever beam produces less compared triangular cantilever beam. S. Roundy [30][31][32]. They also presented that the trapezoidal beam was able to generate twice the energy as that of a rectangular cantilever beam. Baker 2005 proved that the triangular trapezoidal cantilever beam generated 30% more energy than the rectangular cantilever beam, through experimental studies [33]. Rami Reddy et al. modelled and experimentally compared the rectangular beam with and without a cavity. The cavity of the substrate tends to shift the neutral axis of energy harvester away from the surface which increases the strain and voltage [34]. They found that the beam with a cavity can produce 75% more voltage than the beam without a cavity. Reddy et al. experimentally compared the trapezoidal and rectangular cavity beam with the conventional rectangular beam. From the analytical and experimental model, they found that the trapezoidal cavity beam produces 97.5% and 108% more voltage compared with the rectangular cavity beam and the beam without cavity [35]. Usharani et al. designed a wideband energy harvester which can vary the resonant frequency range by varying the length of the beam by introducing step section in the beam. From the experiment, they concluded that step thickness of 2 mm beam reach resonance condition at 5.52 Hz while the beam without step section reach resonance at 21.24 Hz and the voltage is also 4.55 times higher compared to the beam without step section [36]. J. Park et al. designed the piezoelectric energy harvester which can be vibrated by any rotary motion of mechanical devices. From the experiment, authors conclude that their design can produce 160% more power than the tapered beam [37]. S P Matova et al. numerically and experimentally analyzed the effect of size of tapered beams. From their study, they showed that the beam with long and slender taper has a positive effect on power performance and the beam with short and wide does not affect power generation [38]. Hosseini and Hamedi developed the formula to calculate the resonance frequency of V-shaped trapezoidal beam using the Rayleigh-Ritz method and they validate their work using ABAQUS simulation [39]. Goldschmidtboeing and Woias considered the different beam shapes and sizes for piezoelectric energy harvester using the RR method and they validate the result experimentally. Finally, the authors conclude that the shapes have an only small influence on energy but have more effect on excitation amplitude and the triangular shaped with a tip mass performed well compared to rectangular beam [40]. Salmani et al. have developed and proved the mathematical modelling for exponentially tapered piezoelectric energy harvester. While tapering the beam, the natural frequency of the beam increases due to the thinner substrate layer and adding a tip mass at the free end. Table 4 shows a summary of several piezoelectric geometries investigated [41].

Various Simulation Software used in Energy Harvesting
Saadon and Sidek designed and reviewed the different types of cantilever beam excited at various resonance frequencies. The geometry of the cantilever beam, vibration frequency, and centre level of resonance frequency was simulated using ANSYS [28]. Sarker et al. developed a micro cantilever beam using C0SM0L Multiphysics. From the various analysis, they found the optimum placement of PZT and silicon mass on the beam [18]. Diyana et al. modelled and simulated comb-shaped piezoelectric beam structure using C0MS0L Multiphysics. The piezoelectric patches and structure were modelled in C0MS0L Multiphysics, MATLAB was used to make mathematical works. The natural frequency of beams was compared using both software. The power output of piezo with and without tip mass was simulated using C0MS0L Multiphysics [44]. Boban et al. optimized energy harvesting by piezo transduction mechanism. NI-PXI workstation was interlinked with LabVIEW software to measure and monitor the output voltage. They have harvested noise and converted it into electrical energy [45]. Prabhakar and Krishna analyzed rectangular cantilever beams at different conditions using Modal analysis in ANSYS. The comparison of resonant frequency for different beam mounding methods was made, and the results had only slight variations compared to experimental work [46]. Motter et al. developed non-controlled rectifier circuits for energy harvesting using a piezoelectric transducer. All tests are performed using MATLAB Simulink setup. The use of the software is to ensure the accuracy of numerical simulations and experimental verifications [47]. Nandish and Hosamani used Multisim software to simulate and analyze the results of the hardware circuit. The simulated result has a close agreement with experimental work [48]. Kundu and Nemade developed a mathematical model using MATLAB and FEM model using C0MS0L Multiphysics. Finally, the short circuit resonance frequency for C0MS0L Multiphysics and MATLAB are 100.0 Hz and 99.80Hz [48,49]. E.L.Pradeesh & S.Udhayakumar used C0SM0L Multiphysics to analysis the optimal location of piezoelectric material over the length of the beam. The effect of proof mass material and shapes were also analyzed. From the numerical analysis, they observed that piezoelectric material located near to the fixed produced more power compared to other locations [50].

Energy Harvesting Circuit
The piezoelectric materials can produce AC voltage as an output [51]. The generation of AC voltage is about in μ / m volts, so the AC has to be changed into DC for storage or direct use of electronic equipment [23,[52][53][54][55]. Figure 3. Shows a Full wave bridge rectifier circuit, which is used to convert AC to DC.

Figure 3: Full Wave Bridge Rectifier Circuit
The basic circuit of energy harvesting is shown in Figure 4, ambient vibration is caused by surrounding environments and a piezoelectric element which is used to harvest the energy from vibration [56].
Step up circuit is used to boost the power of piezoelectric [57]. The signal conditioning circuit consists of several Dedicated Integrated Chips to provide a regulated voltage for the storage of power in the storage unit [58]. generated from piezoelectric has some ripples, this could be avoided using rectification and filtering. Rectifier circuit converts AC to DC and the regulator circuit removes ripples and maintains the same value of DC. They use MCP73862 charge management controllers used to charge Lithium-ion batteries [59]. Linear Corporation Technology of USA developed a device LTC 3588 series for standalone energy harvesting which is shown in Figure 5. It is completely optimized energy harvesting solution for high impedance sources of piezoelectric elements. It can be interfaced with a piezoelectric element, storage device, load and microcontroller. By using this unit the rectifier circuit, step-up circuit and signal conditioning unit can be eliminated. This unit can also be used to operate ultra-low-power electronic equipment without batteries [60].

Storage Devices
Storage devices are used to store and power the electronic equipments, the storage devices are a capacitor [61,62] and the battery. Typically, batteries are in different types, Nickel Metal Hydride battery (NiMH) and Lithium-Ion battery (Li-ion) are used in piezoelectric harvesting. Sodano et al. compared Nickel Metal Hydride battery and a capacitor for storage of power from PZT. They found that the charging and discharging rates of the capacitor are quick and the capacitor can't store much power like batteries.
ICMEEP 2020 IOP Conf. Series: Materials Science and Engineering 995 (2020) 012007 IOP Publishing doi:10.1088/1757-899X/995/1/012007 9 Sodano et al. use a Nickel Metal Hydride battery instead of Lithium-Ion batteries because NiMH has a high current charge density and Li-ion batteries need a voltage regulator or charge controller [17]. Guan and Liao investigated that the super capacitor has high efficiency of 95 %, while Li-ion batteries have 92 % of efficiency and NiMH batteries have only 65% of efficiency. The lifecycle of the super capacitor is unlimited, but batteries can withstand only 300-1000 life cycles. The self-discharge rate of Lithium-ion batteries about 95 % in 30 days. The Nickel Metal Hydride battery has a self-discharge rate of 70% in 30 days and super capacitor can withstand only 65% in 30 days [63]. The conclusion was that a super capacitor can perform better than the batteries in the terms of longer lifetime, charging and discharging efficiencies, but it has poor withstand capacity compared to batteries.

Conclusion
Vibration based piezoelectric energy harvesting was reviewed in this paper. It is one of the lasting power source solutions for transportable microelectronic devices by harvesting vibration. The piezoelectric energy harvester is clean and greener technologies can protect the environment. This system is less maintenance and cost-efficient to generate power. However, the real application of energy harvester is still limited and it does not use commercially by consumer electronic equipment. The limitation can overcome by improving geometric configuration, selection of multiple piezoelectric patches, development of efficient smart circuits and selection of better storage devices. Vibration is everywhere around us and we look forward that self-powered smart devices will exist in our life shortly.