Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> This chapter provides the introductory information on piezoelectric energy harvesting covering various aspects such as modeling, selection of materials, vibration harvesting device design using bulk and MEMS approach, and energy harvesting circuits. All these characteristics are illustrated through selective examples. A simple step-by-step procedure is presented to design the cantilever beam based energy harvester by incorporating piezoelectric material at maximum stress points in first and second resonance modes. Suitable piezoelectric material for vibration energy harvesting is characterized by the large magnitude of product of the piezoelectric voltage constant (g) and the piezoelectric strain constant (d) given as (d· g). The condition for obtaining large magnitude of d·g has been shown to be as |d| =en, where e is the permittivity of the material and n is a material parameter having lower limit of 0.5. The material can be in the form of polycrystalline ceramics, textured ceramics, thin films, and polymers. A brief coverage of various material systems is provided in all these categories. Using these materials different transducer structures can be fabricated depending upon the desired frequency and vibration amplitude such as multilayer, MFC, bimorph, amplified piezoelectric actuator, QuickPack, rainbow, cymbal, and moonie. The concept of multimodal energy harvesting is introduced at the end of the chapter. This concept provides the opportunity for further enhancement of power density by combining two different energy-harvesting schemes in one system such that one assists the other. <s> BIB001 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> Energy harvesting techniques using piezoelectric materials have been rapidly developed in the world. In our previous work, one kind of kinetic energy harvester using a piezoelectric material was proposed and developed; the harvester was called Flexible Piezoelectric Device (FPED). The FPED was applied to wave and current energy, wind energy and vibration energy. In this study, improving the FPED, a new type of wind energy harvester was proposed and developed to generate electric power from broad band of wind conditions, especially low wind and breeze. The energy harvester consists of a piezoelectric material and a thin soft material, which is Flexible Piezoelectric Sheet (FPS) for wind energy harvesting. The theoretical approach was provided using a classical modal analysis technique. The electric performance of the FPS in several wind conditions was revealed and some important design parameters of the FPS were shown based on experimental results. In this study, improving the FPED, a new type of wind energy harvester was proposed and developed to generate electric power from broad band of wind conditions, especially low wind. The energy harvester consists of a piezoelectric material and a thin soft material as substrate structure, which is Flexible Piezoelectric Sheet (FPS), for wind energy harvesting. To optimize basic structure and size of the FPS, theoretical approach was provided using a classical modal analysis technique. The electric performance of the FPS in several wind conditions was revealed and some important design parameters of the FPS were shown based on experimental results. <s> BIB002 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> Abstract A low frequency wideband response cantilever nano generator (NG) was designed for energy harvesting from ambient vibration in natural environment. The textured ZnO nanowires, acting as energy converters, were grown directly on titanium foil to form a composite that can work at resonance mode (around several Hz) after properly modulated. By adding a proper amplitude limiter at one side of the cantilever, the vibration can be modulated from linear mode to piecewise linear mode. As a result, the response bandwidth of single cantilever can be broadened at least 200% compared with that of a simple cantilever device without a limiter. A series of cantilevers with gradient changed resonance frequencies and partially overlapped bandwidth between each other can be integrated to further widen the bandwidth. Such resonance mode NGs can provide efficient energy conversion even at low frequencies. <s> BIB003 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> Abstract A huge number of piezoelectric materials have been demonstrated since the discovery of piezoelectricity over a century ago. The use of nanostructured piezoelectrics, however, is a relatively recent development, and the understanding of the nanoscale size effects on ferro- and piezoelectricity is still being formulated. Despite this, the application of piezoelectric nanostructures to energy harvesting has expanded rapidly in the last decade leading to a huge range of reported devices [96,97] . Most studies focus on zinc oxide, as its nanostructures are formed relatively easily using low temperature methods [98] , unlike many ferroelectrics which require high temperature processing. In addition, the nanostructures are crystallographically aligned and non-ferroelectric, and therefore do not require poling. However, more recently other well-known materials have been investigated for nanostructured energy harvesters including lead zirconate titanate (PZT) and barium titanate, with the potential for higher power outputs due to their higher piezoelectric coefficients. This review summarises the work to date on nanostructured piezoelectric energy harvesters, commonly called nanogenerators, starting with early reports of piezoelectric output from single strained ZnO nanorods, and moving through the use of nanorod arrays, flexible substrates and alternative materials and nanostructures. Applications that have been demonstrated for devices are summarised, and the future prospects for this field are considered. Before embarking on the development of any nanostructured energy harvesters a thorough understanding of piezoelectric and ferroelectric materials – and the broader energy harvesting field – is essential, for which there are a number of suitable reviews and textbooks [1–4] . <s> BIB004 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> Abstract A novel wind power harvester using piezoelectric effect is developed. This engineering structure device employs a wind turbine that extracts kinetic energy from the wind and converts it into the rotational motion of a shaft. The rotational shaft is connected to a Scotch yoke mechanism that is used for converting the rotational motion into linear vibrations of two piezoelectricity-levers through springs. A mathematic model is developed to calculate the root mean square value of the generated electric power. The influences of some practical considerations, such as the rotational speed of the wind turbine and the stiffness of the springs, on the root mean square of the generated power are discussed. The research results show that a power up to 150 W can be harvested for a piezoelectric wind turbine with a radius of blades of 1 m at the wind speed of 7.2 m/s, and the designed angular velocity of 50 rad/s. <s> BIB005 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> Abstract Energy harvesting using piezoelectric materials can be realised by periodic external force. Piezoelectric material directly converts strain energy into electric power to capture a wasted ambient kinetic energy. This recovered energy can be used for operating wireless sensors, such as those found in environmental monitoring, mechanical sensing and structural diagnostic. In our previous work, a flexible piezoelectric device, FPED, was proposed and developed as an energy harvester for generating electric power from flow-induced vibration in ocean and wind environments. In this study a FPED with a painted piezoelectric layer, highly durable in order to withstand extreme bending and weathering caused by waves and currents, is proposed and developed by spray coating for use as an ocean energy harvester. A numerical method is developed to predict electro-fluid–structure interactions and to evaluate electrical performance and mechanical behaviours of the painted FPED. Additionally, validation of the numerical model is provided through several experimental tests. This study also investigates the relationship between the stiffness of the painted FPED and the vibrated frequency, as well as determining their influence on the electrical performance. Finally, the outcomes from a field test, conducted in real ocean space, is presented to provide information on electrical performance, mechanical behaviours and durability of painted FPEDs. The paper shows that a painted FPED is a useful and robust energy harvester for generating electric power from harsh environments. <s> BIB006 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> This data file contains all the the results used in the compilation of the tables and charts presented in the associated paper: ::: Almusallam, Ahmed et al. (2017) Flexible piezoelectric nano-composite films for kinetic energy harvesting from textiles. Nano Energy, 33, 146-156. <s> BIB007 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> Abstract Piezoelectric MEMS energy harvesters based on thin films are compact and cost-effective microgenerators for scavenging environmental vibrations. This technology is promising for the replacement of electrochemical batteries in low power autonomous sensors and microdevices capturing vibrations in the μW-mW range. Most of piezoelectric MEMS devices, reported in the last few years, exhibit low generated power/voltage and are not suitable for practical applications. This work reviews the current status of MEMS energy harvesters based on piezoelectric thin films, highlighting approaches/strategies to face the two main challenges to be addressed for high performance devices, namely generated power and frequency bandwidth. The paper introduces the theoretical principles and the main figures of merit of energy conversion in piezoelectric thin films and devices. After an overview on piezoelectric thin films for energy harvesting applications, highlighting their key properties, the manuscript reports a comprehensive survey on the state of the art for this device technology. The last section summarizes the review, highlighting key issues to be addressed and providing an insight into the future outlook to realize devices for practical applications. <s> BIB008 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> The paper describes an experimental study of wind energy harvesting by self-sustained oscillations (flutter) of a flexible piezoelectric membrane fixed in a novel orientation called the “inverted flag”. We conducted parametric studies to evaluate the influence of geometrical parameters of the flag on the flapping behavior and the resulting energy output. We have demonstrated the capability for inducing aero-elastic flutter in a desired wind velocity range by simply tuning the geometrical parameters of the flag. A peak electrical power of ∼5.0mW/cm3 occurred at a wind velocity of 9m/s. Our devices showed sustained power generation (∼0.4mW/cm3) even in low-wind speed regimes (∼3.5m/s) suitable for ambient wind energy harvesting. We also conducted outdoor experiments and harvested ambient wind energy to power a temperature sensor without employing a battery for energy storage. Moreover, a self-aligning mechanism to compensate for changing wind directions was incorporated and resulted in an increase in the temperature sensor data output by more than 20 times. These findings open new opportunities for self-powered devices using ambient wind energy with fluctuating conditions and low speed regimes. <s> BIB009 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> Abstract In our times, the importance of energy efficiency is known by anyone. Besides, it is possible to reclaim the energy consumed by means of the developed technology. In this study, it is aimed to reclaim the energy transferred to the ground while people are walking in their daily lives by using piezoelectric materials, which convert mechanical energy into electrical energy. Having designed a sole to serve this goal, different piezoelectric materials are placed into the sole. Its behaviors under human weight are observed using computer software. For this reason, parametric analyses were carried out using 50, 60, 70, 80, and 90 kg, PZT-5H and PZT-8 piezoelectric ceramics and frames made of steel and aluminum materials as holding bodies of piezoelectric ceramics as human bodies. As a result of the analysis, a system of PZT-5H piezoelectric ceramic with a steel frame integrated into a human shoe of a weight of 90 kg used, showing that 0.4% of the applied force can be harvested to 1.43 mW of electrical power. <s> BIB010 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> Abstract This study focuses on the conceptual development, analytical modeling and experimental wind tunnel testing of a high efficiency energy harvesting (EH) device, based on piezoelectric materials, with an application for the sustainability of smart buildings. The device, harvests the airflow inside Heating, Ventilation and Air Conditioning (HVAC) systems, using a piezoelectric component and an appropriate customizable aerodynamic appendix (“fin”) that takes advantage of specific airflow phenomena (vortex shedding and galloping), and can be implemented for optimizing the energy consumption inside buildings. Different relevant aspects are explored in wind tunnel testing, after a thoughtful investigation using analytical methods. Two different configurations for the fin (circular and T-section) are compared. Aspects of the electrical modeling for the EH circuit are provided (including issues on the EH induced damping), and the effective energy harvesting potential of the working prototype device in close-to-real conditions is assessed. <s> BIB011 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> Abstract Piezoelectric energy harvesters have been studied extensively because they show considerable promise for use in both military and commercial applications. In this work, research was conducted into the core component of a piezoelectric energy harvester, i.e., the piezoelectric cantilever. The phenomenon where the piezoelectric cantilever has a very low output when no substrate layer is present can be explained using neutral plane theory. The relationship between the neutral plane's position and the piezoelectric cantilever’s output power can be studied in depth through analysis of this phenomenon. A mathematical model of a vibrating piezoelectric cantilever is established and the neutral plane position and open-circuit voltage of a vibrating piezoelectric cantilever are deduced based on Euler-Bernoulli beam theory. Calculations and finite element simulation results show that the neutral plane's position in the piezoelectric cantilever is dependent on the elastic moduli and thicknesses of the piezoelectric and substrate layers and that the output of the piezoelectric cantilever is also closely related to the neutral plane's position. To study how the neutral plane's position affects the output power of the piezoelectric cantilever, polyvinylidene fluoride (PVDF) piezoelectric cantilevers with different substrate layers but the same piezoelectric layer were fabricated and the output powers of these layers were measured. Experimental results show that the composite piezoelectric cantilever’s performance is enhanced in terms of its output voltage, average power and power density as the neutral plane is gradually moved further away from the mid-plane of the piezoelectric layer. <s> BIB012 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> Abstract A typical linear piezoelectric energy harvester (PEH) is represented by a unimorph or bimorph cantilever beam. To improve the efficiency of linear PEHs, classical strategies involve the increase of the beam length, tapering or adding additional cantilever beams to the free end. In this work we discuss the design of novel type of composite linear multi-layer piezoelectric energy harvester (MPEH). MPEHs here consist of carbon fibre laminates used as conducting layers, and glass fibre laminas as insulating components. We develop first a electromechanical model of the MPEH with parallel connection of PZT layers based on Euler-Bernoulli beam theory. The voltage and beam motion equations are obtained for harmonic excitations at arbitrary frequencies, and the coupling effect can be obtained from the response of the system. A direct comparison between MPEH and PEH configurations is performed both from the simulation (analytical and numerical) and experimental point of views. The experiments agree well with the model developed, and show that a MPEH configuration with the same flexural stiffness of a PEH can generate up to 1.98–2.5 times higher voltage output than a typical piezoelectric energy harvester with the same load resistance. <s> BIB013 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> Abstract In this work, piezoelectric energy harvesting via friction-induced vibration is investigated experimentally and numerically. A test setup which is able to generate friction-induced vibration and simultaneously harvest vibration energy is created. The experimental results verify the feasibility of energy harvesting via friction-induced vibration. They suggest that there is a critical driving speed for the friction system to generate strongest friction-induced vibration and output highest voltage; a larger normal load is beneficial for producing stronger vibration and outputting higher voltage; the external electric resistance has little effect on the vibration of the friction system, instead it will modify the output voltage amplitude within limits. To further understand the experimental findings, both the complex eigenvalue analysis and implicit dynamic analysis are performed in ABAQUS. The complex eigenvalue analysis further confirms the feasibility of energy harvesting by means of friction-induced vibration, and shows that the vibration in both tangential and normal directions can be harvested. The implicit dynamic analysis verifies the effect of driving speed and normal load on the system dynamics and harvested energy. Accordingly, a two-degree-of-freedom friction system model is proposed to qualitatively characterise the effect of external electric resistance on the system dynamics and harvested energy. This investigation offers quite a new way of harvesting vibration energy. <s> BIB014 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> Abstract Designing a wideband vibration based piezoelectric energy harvester is a challenging problem and is currently being investigated by many researchers. In this work, a new piezoelectric energy harvester is proposed to enhance broadband frequency using two beam structures which have variable cross section and tapered cavity. The wider operating frequency range is achieved by bringing the second mode frequency closer to the first mode maintaining the mass ratio between bigger beam and smaller beam. In addition, the enhancement in the magnitude of the generated output voltage is achieved by introducing the tapered cavity. Therefore, the proposed harvester provides enhanced output voltage and power in first mode as well as second mode of the resonance frequency. In order to accomplish this target, the harvester is analytically modeled using Euler Bernoulli beam theory with Bessel functions and the effectiveness is experimentally demonstrated. It is shown the magnitude of the generated voltage is increased as the cavity thickness increases. It is also validated that the generated voltage and harvested power from the analytical model agree well with the measured results in wide frequency. <s> BIB015 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Piezoelectric Energy Harvesting <s> Abstract Recent advances in the field of microelectromechanical systems (MEMS) have generated great interest in the substitution of inorganic microcantilevers by organic ones, due to their low cost, high flexibility and a simplified fabrication by means of printing methods. Here, we present the integration of electrostrictive nanocomposites into organic microcantilever resonators specifically designed for mechanical energy harvesting from ambient vibrations. Strain sensitive nanocomposite materials composed of reduced graphene oxide (rGO) dispersed in polydimethylsiloxane (PDMS) are integrated into all-organic MEMS by means of an innovative low-cost and environment friendly process by combining printing techniques and xurography. Static tests of the electrostrictive nanocomposite with 3.7 wt% rGO show good performances with variations of capacitance that exceeds 4% for strain values lower than 0.55% as the microcantilever is bent. The results in dynamic mode suggest that the organic MEMS meet the requirements for vibration energy harvesting. With an applied sinusoidal acceleration (amplitude 0.5 g, frequency 15 Hz) a power density of 6 μW/cm 3 is achieved using a primitive circuit. <s> BIB016
Piezoelectricity, also known as the piezoelectric effect, is a phenomenon converting mechanical energy into electrical energy due to the inherent polarization characteristics of certain crystals BIB001 . Figure 11 shows its working principle that the positive and negative charges are produced on two opposite surfaces of the dielectric when the dielectric is deformed by an external force. Piezoelectric ceramics and polymers are two common representative piezoelectric materials  . Piezoelectric generators can convert vibration energy to electrical energy. Piezoceramics have high conversion efficiency leading to the extensive applications in industry and research fields, but they are rigid with low ductility and toughness. Turkmen et al. BIB010 designed a cymbal type energy harvester with two different piezoelectric ceramics, PZT-5H and PZT-8, and showed that PZT-5H is most suitable for this design. Additionally, some designers have developed a piezoelectric-based electrical model and a prototype device with a cantilever beam structure to power a self-sustaining wireless sensor in heating, ventilation and air conditioning systems BIB011 . In BIB005 , the kinetic energy of wind, extracted by a wind turbine, is successively converted into the rotational motion of a shaft and linear vibrations with a Scotch yoke mechanism BIB005 . Then, the vibrations are converted into electricity by the piezoelectric (PZT4)-lever design. The schematic diagram of this piezoelectric wind turbine is described in Figure 12 . Conversely, piezopolymers have been successfully utilized in the field of sensors, especially for biomedical sensors, because they are soft, deformable and lightweight, but their conversion efficiency is relatively poor compared with piezoceramics. Three types of portable piezoelectric devices Piezoelectric ceramics and polymers are two common representative piezoelectric materials  . Piezoelectric generators can convert vibration energy to electrical energy. Piezoceramics have high conversion efficiency leading to the extensive applications in industry and research fields, but they are rigid with low ductility and toughness. Turkmen et al. BIB010 designed a cymbal type energy harvester with two different piezoelectric ceramics, PZT-5H and PZT-8, and showed that PZT-5H is most suitable for this design. Additionally, some designers have developed a piezoelectric-based electrical model and a prototype device with a cantilever beam structure to power a self-sustaining wireless sensor in heating, ventilation and air conditioning systems BIB011 . In BIB005 , the kinetic energy of wind, extracted by a wind turbine, is successively converted into the rotational motion of a shaft and linear vibrations with a Scotch yoke mechanism BIB005 . Then, the vibrations are converted into electricity by the piezoelectric (PZT4)-lever design. The schematic diagram of this piezoelectric wind turbine is described in Figure 12 . Piezoelectricity, also known as the piezoelectric effect, is a phenomenon converting mechanical energy into electrical energy due to the inherent polarization characteristics of certain crystals BIB001 . Figure 11 shows its working principle that the positive and negative charges are produced on two opposite surfaces of the dielectric when the dielectric is deformed by an external force. Piezoelectric ceramics and polymers are two common representative piezoelectric materials  . Piezoelectric generators can convert vibration energy to electrical energy. Piezoceramics have high conversion efficiency leading to the extensive applications in industry and research fields, but they are rigid with low ductility and toughness. Turkmen et al. BIB010 designed a cymbal type energy harvester with two different piezoelectric ceramics, PZT-5H and PZT-8, and showed that PZT-5H is most suitable for this design. Additionally, some designers have developed a piezoelectric-based electrical model and a prototype device with a cantilever beam structure to power a self-sustaining wireless sensor in heating, ventilation and air conditioning systems BIB011 . In BIB005 , the kinetic energy of wind, extracted by a wind turbine, is successively converted into the rotational motion of a shaft and linear vibrations with a Scotch yoke mechanism BIB005 . Then, the vibrations are converted into electricity by the piezoelectric (PZT4)-lever design. The schematic diagram of this piezoelectric wind turbine is described in Figure 12 . Conversely, piezopolymers have been successfully utilized in the field of sensors, especially for biomedical sensors, because they are soft, deformable and lightweight, but their conversion efficiency is relatively poor compared with piezoceramics. Three types of portable piezoelectric devices Conversely, piezopolymers have been successfully utilized in the field of sensors, especially for biomedical sensors, because they are soft, deformable and lightweight, but their conversion efficiency is relatively poor compared with piezoceramics. Three types of portable piezoelectric devices designed with flexible piezoelectric materials by Mutsuda et al. BIB006 BIB002 can harvest energy from ambient kinetic energy, such as water, wind and other mechanical vibrations. In order to create energy harvesters with high flexibility and efficiency, piezoelectric composites made of piezoceramics and piezopolymers have had rapid development from the 1970s BIB007 BIB012 . Presently, generators made by piezoelectric composites have a wide range of applications in medical, sensing, measurement and other fields. For example, some researchers have improved a piezoelectric generator with the linear cantilever beam (made by an innovative type of multilayer composite) instead of the lengthening beam to power WSNs in BIB013 . The power response of the system is up to 2.66 mW/g at a frequency of about 90 Hz, which has significant advantages (about 2.5 times higher) over the classical piezoelectric energy harvester. Wang et al. BIB014 designed a generator with composite materials to harvest friction induced vibration energy, which offers a new approach to scavenge the wasted vibration energy. To further increase the overall harvesting efficiency of piezoelectric generators, Todaro et al. BIB008 makes a survey on the current status of micro piezoelectric energy harvesters and focused on two primary challenges, the frequency bandwidth and the conversion efficiency. These challenges also attract the attention of other engineers. For example, a piezoelectric energy harvester is designed and fabricated with two beam structures to bring the second mode frequency closer to the first one with the assistance of a variable cross-section and a tapered cavity to broaden the resonance bands BIB015 . Orrego et al. BIB009 has fixed a flexible piezoelectric membrane with self-oscillations in an innovative inverted flag to scavenge wind energy for supporting self-powered devices. For achieving high efficiency of energy harvesting, nanomaterials have been investigated. Briscoe et al. BIB004 have made a survey of the development process of nanomaterials and nanostructures. Developers from the Chinese Academy of Sciences have designed a nanogenerator with an extra wide response bandwidth of a single cantilever by adding a limiter at one side of the cantilever, and further widened the bandwidth through combining a series of cantilevers with different response bandwidths BIB003 . Additionally, an innovative MEMS-based energy harvester is explored and fabricated by the integration of strain sensitive nanocomposite materials and MEMS devices in BIB016 . A potentially fatal flaw of piezoelectric materials is that the performance of materials will gradually degrade, leading to lower conversion efficiency and lower power output with the time. Wireless condition monitoring systems powered by the converted electricity are prone to failure because of the insufficient power supplied. The few suitable materials and the complicated fabrication process are the key problems that need to be solved before piezoelectric generators become an efficient commercial product. Furthermore, the improvement of piezoelectric generator structures is a tremendous challenge aiming at selecting its resonant frequency in the frequency band with energy concentration or working at a wide resonance bandwidth. In a word, the piezoelectric EH technique has a bright and promising future in machine condition monitoring if the aforementioned challenges can be addressed.