Piezoelectric Energy Harvesting for Civil Engineering Applications

Abstract

This work embarks on an exploration of piezoelectric energy harvesting (PEH), seeking to unravel its potential and practicality. PEH has emerged as a promising technology in the field of civil engineering, offering a sustainable approach to generating energy from ambient mechanical vibrations. We will explore the applications and advancements of PEH within the realm of civil engineering, focusing on publications, especially from the years 2020 to 2024. The purpose of this study is to thoroughly examine the potential and practicality of PEH in civil engineering applications. It delves into the fundamental principles of energy conversion and explores its use in various areas, such as roadways, railways, bridges, buildings, ocean wave-based energy harvesting, structural health monitoring, and even extraterrestrial settings. Despite the potential benefits of PEH in these domains, there are significant challenges that need to be addressed. These challenges include inefficient energy conversion, limitations in scalability, concerns regarding durability, and issues with integration. This review article aims to address these existing challenges and the research gap in the piezoelectric field.

1. Introduction

The broad application and the wide adaptability of piezoelectric materials have found their novel structural application in many fields of civil engineering. Due to this promising future, any deep understanding or important progress in mechanical−electrical coupling and other characteristics of piezoelectric materials and transducers will greatly promote civil engineering structures to show better performance under diverse environmental loadings. One major advantage of PEH is its versatility in design, as it can be configured in different structures, such as cantilever, ring, or windmill designs, to efficiently capture energy from dynamic vibrations, like that from railways and pavements [1]. Additionally, piezoelectric materials, like PZT and PVDF, which have high longitudinal piezoelectric coefficients, are commonly used in energy harvesting systems due to their ability to convert mechanical stress and dynamic vibrations into electrical energy effectively [2]. Nechibvute et al. [3] provided a concise review of piezoelectric microgenerators and nanogenerators for powering wireless sensors, addressing the limitations of finite power sources in remote or hazardous locations within wireless sensor networks (WSNs). Briscoe and Dunn [4] focused on the development and application of nanostructured piezoelectric materials intended for energy harvesting. They discussed the expansion of this field, primarily focusing on zinc oxide, but also exploring other materials like lead zirconate titanate (PZT) and barium titanate. Caliò et al. [5] explored materials, configurations, and operating modes to enhance efficiency. The key findings included the potential of $d_{15}$ materials for efficiency, $d_{33}$ materials for higher voltage output, and $d_{31}$ materials for simpler fabrication. Bowen et al. [6] covered piezoelectric and related materials for energy harvesting. Elahi et al. [7] investigated PEH mechanisms, including vibration-based. The study recommended improved aerodynamic models for aero-elastic harvesters and explored the underdeveloped underwater-based PEH. Yang et al. [8] reviewed high-efficiency PEH methodologies and their practical uses, emphasizing the attainment of elevated power output and expanded operational frequency ranges. Sharma and Baredar [9] analyzed low-frequency vibration energy harvesting utilizing piezoelectric devices and included experimental validation. Feng et al. [10] introduced a honeycomb triboelectric nanogenerator (H-TENG) for real-time vibration measurement in downhole drilling tools, showcasing its adaptability in varied drilling environments. In Safaei et al. [11], an extensive review was conducted on developments in PEH in terms of new materials, advanced transducers, and innovative applications. Sarker et al. [12] reviewed PEH systems, emphasizing optimization techniques for enhanced performance. The review primarily focused on vibration-based piezoelectric systems, proposing optimization methods to address performance challenges. Moreover, the paper suggested the use of specific technologies, like dSPACE DS1104 controller boards and high-performance AC-DC rectifiers, for better performance in PEH systems. Covaci and Gontean [13] aimed to assess PEH methods and applications, emphasizing the direct piezoelectric effect’s potential for self-powered systems, like IoT. Ghazanfarian et al. [14] provided a review, identifying gaps and overlaps in existing review papers. They summarized recommendations for future research directions and highlighted topics for missing review papers.

There are two types of PEH. Self-powered PEH can generate electricity solely from ambient mechanical vibrations, without the need for an external power source [15]. Conversely, external power PEH relies on an external force or power source to induce mechanical vibrations, which are subsequently converted into electricity using piezoelectric materials [16]. This paper focuses on self-powered piezoelectric energy harvesting, addressing a key gap in existing research, which has largely overlooked its applications in civil engineering. While many review papers discuss PEH in general, this study uniquely emphasizes its potential within civil engineering—a domain where its adoption remains mainly underexplored. By offering valuable insights and a more comprehensive understanding of PEH across various civil engineering fields, this paper highlights its potential to transform energy efficiency and sustainability in infrastructure development. In Section 3, how the PEH technology has been utilized in diverse civil engineering applications will be explored. The challenges and future perspectives of PEH technology will be explored in Section 4. Lastly, Section 5 will present concluding remarks.

2. Fundamentals of Piezoelectric Energy Harvesting

2.1. Direct Piezoelectric Effect

The direct piezoelectric effect is a fundamental principle in piezoelectricity, a phenomenon observed in certain materials that possess a non-centrosymmetric crystalline structure, such as certain ceramics and crystals. When these materials are subjected to mechanical stress, they generate an electric charge or voltage across their surfaces. This effect is a direct conversion of mechanical energy into electrical energy and is the cornerstone of PEH [17]. A key characteristic of the direct piezoelectric effect is the generation of electrical polarization within the material due to the rearrangement of charged ions or atoms reacting to an applied mechanical stress. This polarization results in the separation of positive and negative charges, creating an electric potential across the material [18,19]. As an example of this, the alignment of domains in piezoelectric ceramics, such as barium titanate and PZT, through the application of a direct current (DC) electric field, and leading to permanent polarization, is shown in Figure 1 [20].

In civil engineering applications, the direct piezoelectric effect can be harnessed for structural health monitoring, where piezoelectric sensors embedded within a building, bridge, or other infrastructure capture mechanical vibrations and deformations.

2.2. Resonance and Frequency Matching

Piezoelectric resonance occurs when the frequency applied to a piezoelectric material matches its mechanical resonance frequency. This resonance frequency is determined by Equation (1)

$$f_0={\displaystyle \frac{1}{2t}} \sqrt{{\displaystyle \frac{K}{\rho }}}$$

(1)

where $t$ represents the thickness of the piezoelectric material, $f_0$ is the natural mechanical resonance frequency of first order, $K$ stands for the stiffness constant of the material, and $\rho$ denotes the density of the material [21].

Piezoelectric components exhibit their greatest displacement and, consequently, achieve optimal transmission performance when operating at the series resonance frequency. However, the maximum sensitivity to received voltage occurs at the parallel resonance frequency, which is higher than the series resonance frequency. If a piezoelectric element is employed at its series resonance for emitting ultrasonic waves with maximum efficiency, it can only detect echoes with limited sensitivity. To use the piezoelectric transducer as both a transmitter and receiver effectively, it is necessary to align the series and parallel resonance frequencies [22]. This was studied by Lin [23], where the impact of a parallel matching inductor on high-power piezoelectric transducers was analyzed. It was found that increasing the parallel matching inductor resulted in an unchanged resonance frequency, a decreased anti-resonance frequency, and a reduced effective electromechanical coupling coefficient. Additionally, the electro-acoustic efficiency was greater at resonance than at anti-resonance, as shown in Figure 2, and the electric quality factor decreased with an increased matching inductor.

The electro-acoustic efficiency of a piezoelectric transducer mentioned above is the capacity of conversion of electric power to acoustic power. It is expressed as the proportion of the acoustic power output to the electrical power input, as shown in Equation (2):

$$\eta =P/P_i$$

(2)

where $\eta$, $P,$ and $P_i$ are the electro-acoustic efficiency, the radiated acoustical power, and the input electric power, respectively.

A significant amount of power is typically lost in harvesters when flipping the output voltage across the inner capacitor and resistor of the piezoelectric material. The Synchronized Switch Harvesting on Inductor technique has emerged as one of the most efficient methods for harvesting this power loss. In the parallel SSHI technique, Eltamaly and Addoweesh [24] introduced a new, simple, and efficient circuit by flipping the PEH terminal voltage through an external inductor.

2.3. Piezoelectric Materials

This is a class of materials with unique properties that allow them to generate electric charges when subjected to mechanical stress or deformation. These harvesters typically consist of piezoelectric elements strategically placed within a device or structure to capture vibrations, movements, or mechanical stress. Advanced designs often incorporate optimized geometries and materials to maximize energy conversion efficiency [24].

• Characteristics of piezoelectric materials

Piezoelectric materials exhibit a non-centrosymmetric crystalline structure, which means that the centers of positive and negative charges within the material do not coincide. This lack of symmetry is fundamental to their piezoelectric properties [25]. When a force is applied to a piezoelectric material, it induces a mechanical deformation in the material. This deformation causes a shift in the positions of charged ions xc or atoms within the material, creating an electric polarization (Figure 3) [26].

• Common piezoelectric materials

A variety of piezoelectric materials are utilized, each offering distinct advantages and drawbacks:

• Crystals: Crystalline piezoelectric materials, including quartz, tourmaline, and Rochelle’s salt, have specific attributes. Quartz, valued for its stiffness, durability, and resistance to high temperatures, is less ideal for high-frequency excitation and structural control [27]. Nonetheless, it has found application in the concrete industry [28]. Tourmaline and crystalline tourmaline are prized for their high piezoelectric voltage coefficient [29]. Rochelle’s salt, a synthetically produced material, is notable for its chemical sensitivity and resistance to adverse environmental conditions, boasting a very high piezoelectric constant [30].
• Ceramics: PZT and barium titanate are prominent piezoceramic materials commonly employed in civil engineering [31]. PZT is renowned for its strength, sensitivity, and high electromechanical coupling coefficients, making it well-suited for structural health monitoring and energy harvesting [32]. Barium titanate serves various electronic elements and can be environmentally friendly when lead-free variants are used [33,34].
• Polymers: Polyvinylidene difluoride (PVDF) is a flexible and robust polymer featuring piezoelectric properties [35]. It is favored for applications involving intricate and sizable shapes and finds utility across diverse industries, including aerospace [36]. PVDF offers advantages over ceramic piezoelectric materials, such as cost-effectiveness, resilience, and resistance to harsh environmental conditions [37,38].

The advantages and disadvantages of some of the most used materials are represented in

Table 1. Advantages and disadvantages of piezoelectric materials.

Material	Advantages	Disadvantages	References
Quartz crystal	Excellent electrical resistivity Ultra-high mechanical Q * High-temperature stability. High stiffness High safety, durability, and longevity	Low piezoelectric coefficient Not likely to work in high-frequency excitation Not usable in damage detection and structural control	[3,30]
Rochelle’s salt	Strong piezoelectric effect Not affected by adverse environmental conditions	Hygroscopic Strong temperature-dependent behavior High chemical sensitivity	[3,30]
Tourmaline	Mechanically strong High piezoelectric voltage coefficient	Low piezoelectric activity Not used and very high frequency Low energy production	[3,39]
PVDF	High flexibility, temperature, and chemical corrosion resistivity Low acoustic impedance Higher range of frequency than PZT More suitable for applications with large and complex shapes	Low piezoelectric coefficient Low Curie temperature Difficulty in material stretching and polarization	[3,40]
MFC	High performance and flexibility Durability	Temperature Sensitivity Environmental impact	[2]
Barium titanate	High ferroelectric capacitance material Is an insulator in the pure form No negative impact on the environment	Low Curie temperature Low piezoelectric effect Lower sensitivity	[3,40]
PZT	High E-M (electromechanical) coupling, range of frequency and energy production Low electricity loss High Curie temperature, spontaneous polarization	High density Less mechanically stable Negative impact on the environment	[3,30]

* Q is a dimensionless parameter (factor) to describe or understand how underdamped a material is, or the dissipation of electrical or electromagnetic energy [41].

.

The selection of a piezoelectric material hinges on the civil engineering application and the desired characteristics, including sensitivity, durability, and environmental resilience. Natural materials often have stable properties, while synthetic materials can be engineered to meet specific needs.

2.4. Fabrication of PEH Used in Civil Engineering

Researchers have explored various fabrication techniques and materials to enhance these harvesters’ performance and applicability. Tu et al. [42] introduced precipitation-printed high-β phase polyvinylidene fluoride for energy harvesting, showcasing new additive manufacturing methods beyond thin films. Pei et al. [43] combined solid-state shear milling and 3D printing for high-performance harvesters. Jeong et al. [44] demonstrated a flexible harvester using lead-free piezoceramic thin film, achieving high piezo response and output performance. Park et al. [45] utilized additive manufacturing to create a flexible ceramic–elastomer composite harvester, highlighting fabrication versatility. Lozano Montero et al. [46] presented a printing-based process for flexible modules, emphasizing the electrode structure’s impact on performance. In civil engineering, Wu et al. [47] explored a hybrid piezoelectric and electromagnetic harvester for low-frequency sloshing environments, providing stable power for microsensors. Zhang et al. [48] discussed piezoelectric harvesting in bridge systems, emphasizing the importance of harvester design for sustainable power in infrastructure.

These references highlight advancements in fabricating PEH for civil engineering, showcasing diverse materials, techniques, and design considerations to enhance efficiency and applicability.

2.5. Methodologies, Design and Modeling

Several methodologies have been explored to optimize piezoelectric transducer design and placement. Analytical models, like those by Kim et al. [49] and Tzou and Tseng [50], determine optimal properties and configurations for maximizing power generation and vibration control. Finite-element modeling is widely used to predict electrical outputs, with examples including the work of Erturk and Inman [51] using ANSYS for beam systems. Experimental methods often involve mounting piezoelectric devices on small-scale structures, like cantilever beams excited by shakers, as done by Kim et al. [49] and Ibrahim et al. [52]. Field tests implement harvesters on full-scale structures, such as Cahill et al. [53] on bridge installations and Bian et al. [54] with seismic tests on concrete beams.

Some important key methodologies for PEH research include:

• Analytical Modeling: Erturk and Inman [55] developed distributed parameter electromechanical models for PEH integrated with slender structural elements, like beams and plates, optimizing power output for different excitation frequencies and electrical loads. Roundy and Wright [56] presented a coupled modeling approach combining mechanical elements modeled by distributed parameter equations and electrical elements by simple circuit equivalents, predicting steady-state responses for piezoelectric vibration-based energy harvesters. Triplett and Quinn [57] derived an analytical piezoelectric harvester model specifically for base-excited cantilever beams, enabling predictions of maximum power output versus resistance for given vibration amplitudes and frequencies.
• Finite-element Modeling: Badel et al. [58] used ATILA finite-element software to model a reinforced concrete beam with piezoelectric patches, predicting stored electrical energy under dynamic loading. Shen et al. [59] presented a finite-element framework for modeling piezoelectric composites for structural health monitoring and energy harvesting in concrete structures, accounting for anisotropic piezoelectric properties. De Marqui Jr et al. [60] compared analytical and finite-element numerical predictions versus experiments for a uni-morph PEH, finding that FEM accurately captured coupled strain and electrical response.
• Other Approaches: Qiu et al. [61] used wavelet analysis to predict dynamic responses and electricity generation in PEH from fluid–structure interactions. Dwivedi et al. [62] applied neural network models to predict power generation from piezoelectric cantilevers based on training datasets, enabling rapid optimization for different designs and loading conditions. Shu and Lien [63] developed lumped parameter models using electrical equivalents of inductors, resistors, and transformers to model a PEH.

2.6. Mechanisms and Fatigue

Key methodologies for PEH mechanisms in civil engineering applications include cantilever mechanisms, beam and plate mechanisms, trapezoidal mechanisms, cymbal mechanisms, and integration into host civil infrastructure. Cantilever configurations are common, with Kim et al. [49] investigating piezoelectric composite cantilevers with interdigitated electrodes, Peigney and Siegert [64] exploring geometries, like trapezoidal and circular beams, and Cahill et al. [53] implementing harvesters inside highway bridge box beams to generate power from vehicle-induced vibrations. Beam and plate mechanisms involve integrating piezo patches and layers with structural elements, as seen in Tzou et al. [65] with a rectangular plate model, and Dai et al. [66], who embedded piezoelectric layers within beams and plates for energy harvesting and structural control. Trapezoidal mechanisms, such as those proposed by Quinn et al. [67] and fabricated by Li et al. [68], concentrate strain near the fixed end to maximize power generation. Cymbal mechanisms use an oval piezo disk coupled to a metal cap through a proof mass, with Daniels et al. [69] designing a cymbal-based harvester built into reinforced concrete roadways to generate power from vehicle vibrations. An example is shown in Figure 4 [70]. Host civil infrastructure integration involves embedding piezoelectric materials throughout structural components, with Dishvoli et al. [71] developing piezo-reinforced concrete beams and columns, and Gweon et al. [72] proposing smart bridge systems for PEH and structural control.

Fatigue considerations span the bulk piezoceramic material, the interfacial bonding region, and impacts on surrounding structural fatigue resistance. Piezoelectric ceramics, like PZT, can degrade after extended cyclic loading. Jiang et al. [73] identified domain reorientation and defect accumulation mechanisms, and Uprety et al. [74] investigated fatigue damage in macro-fiber composite actuators. The interface between piezo devices and the structural substrate is also prone to fatigue, as Kitagawa et al. [75] observed in debonding and electrode cracking in PZT patches bonded to aluminum beams, and Nijesh et al. [76] studied PZT–epoxy interfaces, developing models to predict debonding rates. The integration of piezoelectric components can influence the fatigue life of host structures. Lin et al. [77] showed effective structural health monitoring with piezo wafer sensors on cracked aluminum plates, while Downey et al. [78] modeled the effects of piezo patch actuators on fatigue crack growth rates in aluminum specimens. Experimental methodologies for fatigue testing include Zanuy et al.’s [79] high-cycle loading on piezo-integrated concrete beams, Smith and Kar-Nayaran’s [37] vibration fatigue tests on lead-free piezo cantilevers, and Avvari et al.’s [80] resonant fatigue testing using piezo stacks to generate high-frequency excitations.

3. PEH Applications

3.1. Key Publications on PEH for Civil Engineering in the Last Decade

Recent publications have made significant progress in implementing PEH technology in civil engineering infrastructure. For instance, Yang et al. [81], Wang et al. [82], Yuan et al. [83] and Song [84] have successfully integrated PEHs into existing roads by grooving them, showcasing the feasibility of this approach in energy generation from traffic-induced vibrations. Similarly, Hong et al. [85] innovatively incorporated PEHs into asphalt pavements by cutting the asphalt, demonstrating another practical application of PEH technology in civil engineering. Moreover, Zhang et al. [86] introduced PEH in cement concrete pavements by excavating the existing pavement and placing the PEH within the trench, offering an alternative method for energy harvesting in infrastructure projects. Additionally, the development of flexible PEH has been a focus of recent research. Key publications by Hong et al. [85] and Song et al. [84,87] have explored the potential of flexible PEHs, highlighting their versatility and applicability in various settings. Furthermore, Jasim et al. [88] proposed a layered polarization method to optimize the geometry of PEH, contributing to the advancement of efficient energy harvesting technologies in civil engineering applications. On the other hand, rigid PEH has also been extensively studied. Works by Khalili et al. [89] have provided insights into the design and implementation of rigid PEH, expanding the options available for energy harvesting solutions in civil infrastructure. Additionally, research by Cho et al. [90] and Jung et al. [91] has focused on designing flexible PEH, further enhancing the understanding and practical applications of flexible energy harvesters in civil engineering projects. Overall, the advancements in PEH technology, as evidenced by recent key publications, offer promising opportunities for sustainable energy generation and integration within civil engineering structures [92].

3.2. PEH Applications in Roadways

As indicated in the reviewed literature, the integration of PEH with modern roadway infrastructures can provide a viable source of power for low-power wireless sensor networks, traffic lights, and indicators. The advancements in piezoelectric materials and power conversion techniques, as highlighted by Bogatsis [93], underscore the potential for efficient energy generation in transportation systems. Furthermore, the ongoing research on lead-free materials for PEH [94] signals a shift towards environmentally friendly and high-performance systems suitable for various applications.

There are two primary strategies for capturing energy from traffic systems using piezoelectric harvesters. The first approach involves utilizing an autonomous structure, such as a bimorph equipped with a tip mass, securely affixed to road surfaces [52], as shown in Figure 5 [95]. The added mass increases the inertia of the system, making it more responsive to vibrations of various frequencies and amplitudes. This maximizes the efficiency of energy conversion and minimizes energy losses within the system [96]. In the other approach, piezoelectric transducers are directly integrated into the road pavements to harness the mechanical strain and kinetic energy generated by vehicular loads and gravitational forces [97], as shown in Figure 6 [81]. However, a significant practical challenge arises due to the high laying temperature of standard highway-grade asphalt. The temperature at which regular highway asphalt is laid can be quite similar to the Curie temperature of piezoelectric materials [86]. For example, the PZT material has a Curie temperature of around 350 $\mathrm{℃}$, yet its highest suggested operational temperature ranges between 150 and 250$℃$ [98]. As noted in Zhang’s study [86], piezoelectric devices are typically shielded by concrete blocks, marble cubes, or mortar components to protect them from external damage.

Noh [99] proposed the innovative concept of using PEH integrated into noise-barrier walls along highways to generate electricity from the vibrations induced by passing vehicles and wind. Isacsson and Lu [100] introduced a highly innovative concept, developing an intelligent asphalt compound by integrating piezoelectric rubber constituents into traditional asphalt mixtures used for road pavements.

When the vehicle $m_2$ applies pressure to the piezoelectric module $m_1$ positioned beneath the road surface, it generates an alternating current (AC) signal along the top and bottom sides of the module using the road spring-damper $k$ and $c$. This AC signal is subsequently directed to the rectifier, storing the collected energy temporarily in a storage device. To analyze the harvester’s performance, a data acquisition system (DAQ) and a computer are required to capture the signals and examine the behavior of the harvester, as shown in Figure 7 [101].

Cao et al. [102] aimed to measure the energy output of piezoelectric transducers under different vehicle loads and speeds and to approximate the daily energy generation of piezoelectric pavements (Figure 8). The results indicated that larger loads and higher vehicle speeds led to increased energy production. Under daily traffic volumes, a single piezoelectric transducer could produce $963.99 \mathrm{J}$ of energy, and a $100$-m piezoelectric pavement had the potential to generate energy equal to the total power of $35$ mobile phone batteries.

Qabur and Alshammari [103], reported a maximum power output of $1.68 \mathrm{m}\mathrm{W}$ per piezoelectric element under a vehicle load of $1000 \mathrm{N}$. The researchers utilized a piezoelectric composite material made of lead-free KNN (potassium sodium niobate)-based ceramics, which exhibited relatively high piezoelectric coefficients and energy conversion efficiency. In contrast, another study by Zhao et al. [104] achieved a significantly lower maximum power output of $0.02 \mathrm{m}\mathrm{W}$ per piezoelectric element, despite using a similar vehicle load. It was found that the transducer’s energy conversion efficiency decreases as it is placed within asphalt pavement. This reduction is directly related to the depth at which it is buried and the modulus of the surface course. The key factors that contributed to the disparities in power output between the two studies can be attributed to the design of the piezoelectric harvesters and the specific conditions of the roadway environment. Wu et al. [105] employed a strategic structural design that maximized the strain experienced by the piezoelectric material, leading to a higher power amount.

Furthermore, Wale et al. [106] reported that PEH installed on highways with high traffic volumes could generate up to $400 \mathrm{k}\mathrm{W}\mathrm{h}$ of electrical energy per year per kilometer of road. The system was tested on a highway with heavy traffic flow, which provided continuous and consistent vibrations from vehicles. The authors found that the energy output increased linearly with traffic volume and vehicle weight. Wang et al. [107] analyzed various studies on PEH. They reported that the lowest power output observed was around $1.2 \mathrm{W}$ for a piezoelectric harvester installed on an urban road with frequent stops and starts. Light vehicles in urban areas were also found to exert lower forces on the road surface, resulting in reduced strain on the piezoelectric materials and lower energy conversion.

Factors such as material selection, harvester design, traffic conditions, and road surface characteristics play crucial roles in maximizing energy output and ensuring the durability and longevity of the PEH systems. Continuous and high-volume traffic flow, coupled with smooth road surfaces and optimized harvester designs, can maximize energy generation and improve the overall performance of these systems. Jiang [108] compared the performance of PZT and PVDF-based harvesters and found that PZT exhibited higher energy conversion efficiency but was more brittle and susceptible to damage from heavy vehicles. The results showed that smoother surfaces, like asphalt and concrete, provided better energy outputs compared to unpaved roads with uneven surfaces. Mehmood et al. [109] investigated different geometric shapes and arrangements of piezoelectric elements, concluding that stacked configurations and cylindrical shapes exhibited better performance in terms of energy harvesting and load distribution. Zhu et al. [110] explored the use of piezoelectric cantilever beams integrated into the road surface. They found that adjusting the beam length and thickness could enhance energy harvesting efficiency, but excessive deformation could lead to fatigue and failure.

According to the various strategies that can be implemented to improve the efficiency of power extraction, for SSH (synchronized switch harvesting) designs, research done by De Fazio et al. [111] encourages further exploration and potential implementation of the SSH technique in PEH from roadways. They provide numerical simulations and experimental validation, demonstrating the effectiveness of their approach in improving the power output and efficiency of the PEH. Lefeuvre et al. [112] focus on the application of the SSH technique for Rayleigh-distributed vibrations, which are commonly encountered in roadway environments.

SSH rectification methods are rooted in a non-linear treatment of the generated piezoelectric voltage, resulting in the generation of piezoelectric forces at resonance that counteracts the speed and solicitation forces [112], as shown in Figure 9 [17], where $V_p$ is the capacitor voltage, $SW$ is the switch, $L$ is the inductor, and $C_{rect}$ is the rectifier.

Silveira et al. [113] researched the maximum power point tracking techniques in PEH from roadways. MPPT techniques facilitate the adjustment of the input impedance of the conditioning section to match that of the transducer, thereby enhancing power extraction efficiency [82]. This method is explained in Figure 10, where $I_p$ is the magnitude of PE current, $C_p$ is the capacitor, and $R_p$ is the resistor.

Table 2. PEH Applications in roadways: A Literature Review.

Reference	Piezoelectric Energy Harvester			Method Based On	$$\begin{gathered} \mathbf{Power} \\ \mathbf{Output} \\ \left(\mathbf{m}\mathbf{W}/\mathbf{c}{\mathbf{m}}^3\right) \end{gathered}$$
	Materials	Shape	Dimensions
[86]	PZT-5H	Square (Plate)	$10 \mathrm{c}\mathrm{m}\times 10 \mathrm{c}\mathrm{m}\times 1 \mathrm{c}\mathrm{m}$	Theoretical approach based on Kirchhoff plate theory, dynamic load using sine series expansion, Fourier transform, and Cauchy’s residue theorem.	$0.473$
[115]	PZT-5H	Disc	$24 \mathrm{p}\mathrm{c}\mathrm{s}$  $\mathrm{diameter} 20.2 \mathrm{c}\mathrm{m}\times 1 \mathrm{c}\mathrm{m}$	Experimental and numerical approaches based on field tests and finite-element analysis.	$0.565$
[101]	PZT-5H	Disc	$8 \mathrm{p}\mathrm{c}\mathrm{s}$ diameter $5 \mathrm{c}\mathrm{m}\times 0.2 \mathrm{c}\mathrm{m}$	Numerical approach for optimization.	$1.592$
[116]	PZT-5H	Disc	$16 \mathrm{p}\mathrm{c}\mathrm{s}$  $\mathrm{diameter} 2.5 \mathrm{c}\mathrm{m}\times 0.2 \mathrm{c}\mathrm{m}$	Experimental approach based on field tests.	$0.199$
[117]	PZT-5H	Rectangle (Plate)	Multilayer $3 \mathrm{c}\mathrm{m}\times 2 \mathrm{c}\mathrm{m}\times 0.2 \mathrm{c}\mathrm{m}$	Numerical approach using finite-element analysis.	$1.111$
[118]	PZT	Square (Plate)	$8 \mathrm{c}\mathrm{m}\times 8 \mathrm{c}\mathrm{m}\times 3 \mathrm{c}\mathrm{m}$	Experimental and numerical approaches based on laboratory tests and finite-element analysis.	$1.349$
[119]	Various Model	Cuboid	$600 \mathrm{c}\mathrm{m}\times 900 \mathrm{c}\mathrm{m}\times 100 \mathrm{c}\mathrm{m}$	Numerical approach using finite-element analysis.	$0.022$
[104]	PZT	Rectangle	$3 \mathrm{c}\mathrm{m}\times 2 \mathrm{c}\mathrm{m}\times 0.7 \mathrm{c}\mathrm{m}$	Experimental and numerical approaches based on laboratory tests and finite-element analysis.	$0.001$
[83]	Polypropylene (PP)	Square (Plate)	$15-20 \mathrm{c}\mathrm{m}\times 15-20 \mathrm{c}\mathrm{m}\times 3-4 \mathrm{c}\mathrm{m}$	Experimental and numerical approaches based on laboratory tests and finite-element analysis.	$0.152$
[90]	PZT-PZNN	Rectangle (Plate)	$80 \mathrm{p}\mathrm{c}\mathrm{s}\times 3.8 \mathrm{c}\mathrm{m}\times 3.8 \mathrm{c}\mathrm{m}\times 0.2 \mathrm{c}\mathrm{m}$	Experimental approach based on field tests.	$0.164$
[120]	PZT-5H	Rectangle (Plate)	$10 \mathrm{p}\mathrm{c}\mathrm{s}\times 52 \mathrm{m}\mathrm{m}\times 36 \mathrm{m}\mathrm{m}\times 0.1 \mathrm{m}\mathrm{m}$	Experimental and numerical approaches based on laboratory tests and finite-element analysis.	$1.584$
[121]	PZT-5H	Disc	diameter $20 \mathrm{c}\mathrm{m}\times 2.32 \mathrm{c}\mathrm{m}$	Experimental and numerical approaches based on laboratory tests and finite-element analysis.	$0.041$
[122]	Microfiber Composite	Rectangle (Plate)	$8.5 \mathrm{c}\mathrm{m}\times 2.8 \mathrm{c}\mathrm{m}\times 0.1 \mathrm{c}\mathrm{m}$	Experimental approach based on laboratory tests.	$1.597$
[107]	PZT-5H	Square (Plate)	diameter $15 \mathrm{c}\mathrm{m}\times 3 \mathrm{c}\mathrm{m}$	Theoretical approach based on electrical theory.	$0.074$
[87]	PZT-PZNM	Rectangle (Plate)	$48 \mathrm{p}\mathrm{c}\mathrm{s}\times 3.8 \mathrm{c}\mathrm{m}\times 3.8 \mathrm{c}\mathrm{m}\times 0.02 \mathrm{c}\mathrm{m}$	Experimental approach based on frequency matching and impedance matching.	$0.111$
[123]	PZ-EHPS	Cylinder	$6 \mathrm{p}\mathrm{c}\mathrm{s}$ diameter $6.5 \mathrm{c}\mathrm{m}\times 18 \mathrm{c}\mathrm{m}$	Numerical approach using MatLab and SolidWorks.	$0.204$
[124]	PZT-5H	Square (Plate)	$58 \mathrm{p}\mathrm{c}\mathrm{s}$  $\mathrm{diameter} 1.14 \mathrm{c}\mathrm{m}\times 1.06 \mathrm{c}\mathrm{m}$	Theoretical, experimental, and numerical approaches using a three-degree-of-freedom electromechanical model, material testing system, and finite-element analysis.	$0.082$

presents a summary of the main contributions and methodologies employed in the application of PEH in roads as documented in the literature.

Despite these various studies, challenges, such as cost, durability, and scalability, still need to be addressed to fully realize the potential of PEH in roadways. Future re-search should focus on optimizing the design and materials used, as well as exploring innovative integration methods to enhance the overall performance and practicality of this technology. Additionally, field testing and real-world implementation will be cru-cial in further evaluating the viability and impact of PEH systems on a larger scale [125].

3.3. PEH Applications in Railways

Railways have a huge capacity to transport people and goods at a low price, making them the main chosen means of transportation in many countries. As the speed, passenger capacity and freight volume of trains increase, researchers are particularly interested in studying the extraction of piezoelectric energy from them. Many early studies have been made on this topic; for example, in 2006, Nuffer and Bein [126] emphasized the potential of harvesting energy from railway vibrations, estimating that a single passing train could generate enough energy to power trackside equipment for several hours. Nelson et al. [127] in 2008 also stated that on-board piezoelectric VEHs are effective in supplying power for low-voltage sensor nodes.

There are many innovative studies in PEH applications in railways. Cleante et al. [128] in 2018, presented a novel design incorporating multiple piezoelectric elements in a single harvester, which could generate power from both vertical and lateral train-induced vibrations. Local variations in rail acceleration were harnessed by a rail-borne “seismic” energy harvester to produce electrical energy [129]. Song [84] presented a finite-element model for simulating PEH in railway environments, which can aid in device optimization and placement strategies.

Wang et al. [130] created a prototype capable of harvesting 28 Watts, while Mishra et al. [131] analyzed two piezoelectric systems with outputs of only a few milliwatts. The high-output system used a multi-layer stack configuration, efficiently converting compressive forces into electrical energy, and employed a mechanical motion rectifier (MMR) to increase force on the piezoelectric elements. In contrast, the low-output system used a single cantilever beam without force amplification, resulting in less efficient energy harvesting. Wang’s harvester was placed directly under the rail for maximum force capture, while Mishra’s was near a concrete sleeper, which dispersed much of the force. The significant power output difference is due to factors like piezoelectric configuration, force amplification, harvester placement, material selection, power management, and system optimization. The high-output study demonstrated the importance of a comprehensive approach to maximize energy harvesting for railway applications.

The bulk of railway energy harvesting systems in use today focus on piezoelectric and electromagnetic harvesters and take advantage of the motion’s peak–valley characteristics [132]. The harvesters are designed to capture higher-intensity vibrations compared to roads, resulting from heavier loads and faster speeds [133]. For heavy-duty freight railroads, a vibration energy harvesting system based on track energy-recycling technology was considered [134]. Through a track fixture, the harvester was installed beneath the rail track as shown in Figure 11. The power storage module, the generator module, and the vibration conversion module were all part of the TVEH (track vibration energy harvester) system. The electricity was used to power low-power sensors around the heavy-duty freight railroad system, including strain gauges, tilt sensors, gyroscope sensors, and wheel sensors, after it was rectified and stabilized.

Qi et al. [135] in 2022 stated that there are still some major challenges and concerns for practical application regarding cost, sustainability, stability, and comparison of consumed and obtained energy. The above difficulties have not been solved yet or are concentrated in the existing literature and should be given high attention in future research on training VEH. However, since motion amplification or rectifiers can be specifically designed to address these issues, motion-driven electromagnetic harvesters appear to hold the greatest promise for resolving these problems [136]. The railway applications with different PEH methods and outputs are shown in

Table 3. PEH Applications in railways: A Literature Review.

Reference	Piezoelectric Energy Harvester			Method Based On	Power Output
	Materials	Shape	Dimensions
[137]	Not mentioned	Shaft	Not mentioned	Experimental and numerical approaches based on bench tests and finite-element analysis.	$7.47 \mathrm{W}$
[138]	PZT-5A	Rectangle	$50.8 \mathrm{m}\mathrm{m}\times 31.8 \mathrm{m}\times 0.26 \mathrm{m}\mathrm{m}$	Theoretical and experimental approaches based on infinite Euler–Beornulli beam and patch-type and stack-type harvesters.	$0.19 \mathrm{m}\mathrm{W}/\mathrm{g}$
[132]	Not mentioned	Shaft	Not mentioned	Theoretical and experimental approaches based on mechanical motion rectifier and laboratory tests.	$11.4 \mathrm{W}$
[139]	Not mentioned	Shaft	Not mentioned	Experimental approach based on Speed Driven Adaptive (SDA) technique.	$7.3 \mathrm{W}$
[129]	PZT	Cantilever	$20 \mathrm{c}\mathrm{m}\times 17 \mathrm{c}\mathrm{m}\times 8 \mathrm{c}\mathrm{m}$	Experimental and numerical approaches based on laboratory tests, modeling, and simulations.	$1 \mathrm{m}\mathrm{W}/\mathrm{g}$
[131]	PZT	Rectangle	$58 \mathrm{m}\mathrm{m}\times 15 \mathrm{m}\mathrm{m}\times 1.8 \mathrm{m}\mathrm{m}$	Experimental approach based on laboratory tests.	$0.12 \mathrm{m}\mathrm{W}/\mathrm{g}$
[134]	Not mentioned	Cantilever	Not mentioned	Experimental approach based on field tests.	$7.44 \mathrm{W}$
[140]	Not mentioned	Ring	4 pcs diameter $12 \mathrm{c}\mathrm{m}\times 1 \mathrm{c}\mathrm{m}$	Numerical approach based on finite-element analysis.	$0.56 \mathrm{m}\mathrm{W}/\mathrm{g}$
[141]	Not mentioned	Sphere	diameter $2.5 \mathrm{c}\mathrm{m}$	Experimental approach based on laboratory tests.	$0.40 \mathrm{m}\mathrm{W}/\mathrm{g}$
[142]	PZT	Rectangle	$2.8 \mathrm{c}\mathrm{m}\times 2.8 \mathrm{c}\mathrm{m}\times 2 \mathrm{m}\mathrm{m}$	Experimental approach based on laboratory tests.	$0.21 \mathrm{m}\mathrm{W}/\mathrm{g}$

.

3.4. PEH Applications in Bridges

For many years, bridges have been primarily seen as a means of transportation. However, beneath their seemingly still structure, there exists a realm of continuous movement and unexplored possibilities. Bridges and piezoelectricity have found their correlation a long time ago. By 1923, piezoelectric accelerometers found application in bridges and even aircraft [143]. In 2023, Lu et al. [144] proposed an innovative omnidirectional piezoelectric–electromagnetic hybrid wave energy harvester (HWEH) based on a pendulum structure for self-powered sensors in sea-crossing bridges. Test results showed that the output voltage of the piezoelectric module can reach $17.4 \mathrm{V}$, and its power density can reach $0.27 \mathrm{W}/{\mathrm{m}}^2$. Cámara-Molina et al. in 2024 [145] introduced an innovative approach to optimize the performance of energy harvesters installed on railway bridges. The energy harvester utilized in this study is a cantilever bimorph beam with a mass located at the tip and load resistance. Additive manufacturing techniques were employed to create the substructure, which was constructed using PAHT-CF15 (High-Temperature Polyamide carbon fiber reinforcement). Field tests demonstrated that energy harvesters designed with the new tuning strategy were able to generate an increase of up to 300% in energy output.

Peigney and Siegert [64] explored the viability of energy harvesting from traffic-induced vibrations in bridge structures, with a specific focus on a pre-stressed concrete highway bridge as a case study. The findings indicate that, despite the relatively low-frequency and small-amplitude nature of the bridge vibrations, it is feasible to generate an average power output of approximately $0.013 \mathrm{m}\mathrm{W}$, accompanied by a controlled voltage range between $1.8$ and $3.6 \mathrm{V}$. Zhang et al. [48] conducted a study on piezoelectric-based energy harvesting on bridges, specifically focusing on four representative concrete slab-on-girder bridges. By simulating different scenarios involving bridge–vehicle interactions, the researchers made significant discoveries. They identified dominant vibration frequencies ranging from $2.6 \mathrm{H}\mathrm{z}$ to $6.5 \mathrm{H}\mathrm{z}$, regardless of whether the vehicles passed by individually or continuously. Additionally, they observed that the average power output at the midpoint of girder $1$ varied between $10 \mathrm{m}\mathrm{W}$ and $40 \mathrm{m}\mathrm{W}$, depending on the road conditions and vehicle speed. The researchers also determined the optimal vehicle speeds for achieving maximum power output, which ranged from 41 km/h to 105 km/h. These optimal speeds were influenced by factors such as the length of the bridge span and the condition of the road. Song [84] investigated the practicality of utilizing a PEH on a railway bridge by employing a finite-element model. By incorporating electromechanical equations, the analysis evaluated the efficiency of the device in relation to vibrations caused by passing trains. The findings revealed that the developed finite-element model accurately corresponded to established solutions and successfully examined the dynamic train load, bridge response, and voltage output properties. This study showcased the considerable potential for generating substantial amounts of energy, reaching up to 205.5 V for a single-span bridge with a tip mass. In Figure 12, three different types of PEH implied in bridges are illustrated where $x$ is the position of the studied structure. The harvester utilizes a proof mass to modify the resonant frequency wt of the PEH. Under base excitation level $Y$, the beam or membrane experiences a significantly amplified displacement during vibration (Equation (3)). As a result of this deflection, strain occurs within the piezoelectric material, leading to the creation of charge across the piezoelectric layer [146].

$$y\left(t\right)=Y\mathit{sin}\left(wt\right)$$

(3)

Galchev et al. [147] utilized an inertial micropower generation system to harness vibrations caused by traffic on bridges. The mechanical harvester, known as a non-resonant parametric frequency increased generator (PFIG), was specifically designed for this purpose. The testing was conducted on the primary section of the bridge. The electromagnetic transducer employed in the system does not involve complex magnetic circuits or geometry, and it was employed to scavenge the extremely low-amplitude, low-frequency, and irregular vibrations found on bridges. The resulting device produced a maximum power of $57 \mathsf{\mu }\mathrm{W}$, with the generator’s internal volume measuring $43 \mathrm{c}{\mathrm{m}}^3$. Consequently, the power output is calculated to be $0.001 \mathrm{m}\mathrm{W}/\mathrm{c}{\mathrm{m}}^3$, which is considerably low. In another research paper, a novel and effective dynamic-magnified PEH (D-M PEH) has been suggested by Sheng et al. [148], achieving an impressive energy output of 0.55 W when subjected to a harmonic acceleration excitation of unit amplitude and low frequency, the experimental setup is shown in Figure 13. This output power surpasses the performance of previously documented PEH utilized on railway bridges. The harvester is a cube with volume $300 {\mathrm{c}\mathrm{m}}^3$, and the calculated power output is $1.833 \mathrm{m}\mathrm{W}/{\mathrm{c}\mathrm{m}}^3$.

Considering not only these two papers, the power output for PEH applications depends on many factors, such as piezoelectric coefficient, optimization design of the harvester, traffic conditions, harvester’s shape and dimensions, placement strategy, materials, etc. This variability highlights the ongoing challenges and opportunities in this field for improving and standardizing PEH systems for bridge applications. Bridge applications with different PEH methods and outputs are explained in

Table 4. PEH Applications in bridges: A Literature Review.

Reference	Piezoelectric Energy Harvester			Method Based On	Power Output (mW/cm3)
	Materials	Shape	Dimensions
[149]	PVDF	Beam	$30 \mathrm{m}\times 8 \mathrm{c}\mathrm{m}\times 10 \mathrm{c}\mathrm{m}$	Numerical approach using the method of integral transformations and method of Laplace–Carson.	$0.121$
[147]	PZT	Cantilever	$1.8 \mathrm{c}\mathrm{m}\times 1.8 \mathrm{c}\mathrm{m}\times 20 \mathrm{c}\mathrm{m}$	Experimental approach based on field tests.	$0.001$
[146]	PVDF	Cubic	$2.74 \mathrm{c}{\mathrm{m}}^3$	Experimental and numerical approaches based on laboratory tests, modeling, and simulations.	$0.072$
[64]	PZT	2 × bimorph patches	$4 \mathrm{c}\mathrm{m}\times 22 \mathrm{c}\mathrm{m}\times 0.08 \mathrm{c}\mathrm{m}$	Experimental and numerical approaches based on laboratory tests and simulations.	$0.013$
[150]	Microfiber Composite (MFC)	Rectangle (plate)	$85 \mathrm{m}\mathrm{m}\times 28 \mathrm{m}\mathrm{m}\times 0.3 \mathrm{m}\mathrm{m}$	Experimental approach based on field tests.	$0.110$
[151]	Microfiber Composite (MFC)	Rectangle (plate)	$37 \mathrm{m}\mathrm{m}\times 17 \mathrm{m}\mathrm{m}\times 0.18 \mathrm{m}\mathrm{m}$	Experimental and numerical approaches using MOSFET device and finite-element analyses.	$0.018$
[152]	PVDF	Rectangle (plate)	$5 \mathrm{m}\mathrm{m}\times 5 \mathrm{m}\mathrm{m}\times 0.25 \mathrm{m}\mathrm{m}$	Experimental approach based on potential energy, restoring force, and stiffness analyses.	$0.389$
[153]	PZT	Rectangle (plate)	$165 \mathrm{m}\mathrm{m}\times 17 \mathrm{m}\mathrm{m}\times 0.6 \mathrm{m}\mathrm{m}$	Numerical approach based on finite-element analyses and the Automatic Resonance Tuning (ART) technique.	$0.811$
[148]	PVDF	Cube	$300 \mathrm{c}{\mathrm{m}}^3$	Theoretical and experimental approaches based on harvesting efficiency and laboratory tests.	$0.787$
[154]	PZT5A	Rectangle (plate)	$50.8 \mathrm{m}\mathrm{m}\times 31.8 \mathrm{m}\mathrm{m}\times 0.26 \mathrm{m}\mathrm{m}$	Theoretical approach based on Kirchhoff–Love plate theory and isogeometric analysis.	$0.093$

.

3.5. PEH Applications in Buildings

There are numerous possibilities for harnessing vibrations generated by sources within buildings. These opportunities are present in various areas, including floors, ceilings, windows, air ducts, household appliances, staircases, internal machinery and HVAC systems [155]. In their papers, Hobbs and Hu [156] and McGarry and Knight [157], since 2011, have proposed the innovative idea of designing PEH that mimics the biomechanics and aeroelastic behavior of tree branches and leaves (i.e., in green buildings).

The feasibility of utilizing PEH in the central hub building of Macquarie University was thoroughly investigated by Li and Strezov [158]. A specially optimized tile was tested, covering approximately 3.1% of the high-traffic areas. The results indicated an annual energy potential of $1.1 \mathrm{M}\mathrm{W}\mathrm{h}/\mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r}$, as depicted in Figure 14. These findings underscore the importance of considering high-traffic zones and tile orientation for effective energy optimization. A piezoelectric harvester was designed by Li Xie et al. [159], featuring a series of interconnected piezoelectric generators connected by a shared shaft. This innovative design was driven by a linking rod attached to a cantilever on the roof of the building, as illustrated in Figure 15. Through numerical simulations, it was demonstrated that practical factors, such as the length ratio of the cantilever, mass ratio of the proof mass, and flexural rigidity ratio, could influence the root mean square (RMS) of the generated electric power and energy harvesting efficiency. Notably, under specific building parameters, the RMS reached an impressive $432.21 \mathrm{M}\mathrm{W}$. The potential of employing piezoelectric tiles for renewable energy generation at Kuala Lumpur International Airport (KLIA) was explored by Chew et al. [160]. Specifically, the focus was on harnessing kinetic energy from crowd movement at the main entrances. The study revealed that, by utilizing 48 piezoelectric tiles arranged as shown in Figure 16, at the entrance gates, a daily energy harvest of $0.589 \mathrm{k}\mathrm{W}\mathrm{h}$ could be achieved. Furthermore, deploying $1000$ piezoelectric tiles at a crowd hot spot resulted in a substantial daily energy generation of $130.03 \mathrm{k}\mathrm{W}\mathrm{h}$, sufficient to power 26 bulbs continuously for an entire month.

Elhalwagy et al. [161] showed that a $40 \mathrm{c}\mathrm{m}\times 40 \mathrm{c}\mathrm{m}$ square harvester developed by Waynergy produced $7.88 \mathrm{m}\mathrm{W}/\mathrm{c}{\mathrm{m}}^3/\text{tile per day}$. The experiment was conducted in a busy pedestrian area, such as a train station or subway station. The study also showed that, even in a small area such as an apartment, the Waynergy tile would produce $1.15 \mathrm{m}\mathrm{W}/\mathrm{c}{\mathrm{m}}^3/\text{tile per day}$. In another study, Kale et al. [162] used a square PZT converter with dimensions of $100 \mathrm{c}\mathrm{m}\times 100 \mathrm{c}\mathrm{m}$ and a series-parallel combination circuit. They came up with a result of $0.78 \mathrm{m}\mathrm{W}/\mathrm{c}{\mathrm{m}}^3/\text{tile per day}$, which is much lower than previous work. Therefore, piezoelectric technology usage is influenced by several key factors, including material, shape, output power per step, battery storage capacity, design, cost, accessibility of charging facilities, shape, user volume, dimensions, distribution of high-frequency walking areas, and the approach to maximizing energy savings. Building applications with different PEH methods and outputs are explained in

Table 5. PEH Applications in buildings: A Literature Review.

Reference	Piezoelectric Energy Harvester			Method Based On	Power Output (mW/cm3)
	Materials	Shape	Dimensions
[163]	1. PZT 2. SEF	1. Square Tiles 2. Square Tiles	1. $90\times 90\times 2.5 \mathrm{c}\mathrm{m}$ 2. $75\times 7\times 20 \mathrm{c}\mathrm{m}$	Experimental and numerical approaches based on laboratory tests and simulations.	$25 \mathrm{p}\mathrm{e}\mathrm{r} \mathrm{d}\mathrm{a}\mathrm{y}$ $2.4 \mathrm{p}\mathrm{e}\mathrm{r} \mathrm{d}\mathrm{a}\mathrm{y}$
[160]	PZT	Square Tiles	$48 \mathrm{p}\mathrm{c}\mathrm{s} \times 1.5 \mathrm{f}\mathrm{t} \times$ $1.5 \mathrm{f}\mathrm{t}\times 0.06 \mathrm{f}\mathrm{t}$	Numerical approaches based on simulations.	$93 \mathrm{p}\mathrm{e}\mathrm{r} \mathrm{d}\mathrm{a}\mathrm{y}$
[158]	Pavegen	Triangular Tiles	$1820 \mathrm{p}\mathrm{c}\mathrm{s} \times 50 \mathrm{c}\mathrm{m} \times$ $50 \mathrm{c}\mathrm{m}\times 50 \mathrm{c}\mathrm{m}\times 9 \mathrm{c}\mathrm{m}$	Experimental approach based on field tests.	$403 \mathrm{p}\mathrm{e}\mathrm{r} \mathrm{d}\mathrm{a}\mathrm{y}$
[161]	Waynergy	Square Tiles	$8 \mathrm{p}\mathrm{c}\mathrm{s}\times 40 \mathrm{c}\mathrm{m}\times 40 \mathrm{c}\mathrm{m}$	Experimental and numerical approaches based on field tests and simulations.	$63 \mathrm{p}\mathrm{e}\mathrm{r} \mathrm{d}\mathrm{a}\mathrm{y}$
[164]	PZT-4	Cantilever	$10 \mathrm{m}\times 3 \mathrm{m}\times 0.2 \mathrm{m}$	Theoretical and numerical approaches based on sinusoidal wave seismic motion and simulations.	$130 \mathrm{p}\mathrm{e}\mathrm{r} \mathrm{d}\mathrm{a}\mathrm{y}$
[165]	PZT-5H	Beam	$20 \mathrm{p}\mathrm{c}\mathrm{s} \times 5.44 \mathrm{c}\mathrm{m} \times$ $2.24 \mathrm{c}\mathrm{m}\times 0.46 \mathrm{m}\mathrm{m}$	Theoretical and experimental approaches based on Fourier transform and laboratory tests.	$60.2$
[155]	1. Piezoelectric fiber composite bimorph 2. Mide Volture harvester	1. Cantilever 2. Cantilever	1. $130 \mathrm{m}\mathrm{m}\times 10 \mathrm{m}\mathrm{m}\times 1 \mathrm{m}\mathrm{m}$ 2. $3.2 \mathrm{i}\mathrm{n}\times 1.3 \mathrm{i}\mathrm{n}\times 0.03 \mathrm{i}\mathrm{n}$	Numerical approach based on simulations, vibration, and airflow-driven energy harvesting method	$48.5$
[166]	Ceramic P-876K015	T-shape	$100 \mathrm{c}\mathrm{m}\times 18.5 \mathrm{c}\mathrm{m}\times 3.1 \mathrm{c}\mathrm{m}$	Experimental and numerical approaches based on laboratory tests and modeling energy simulation.	$69.8$
[162]	PZT	Square Tiles	$29 \mathrm{p}\mathrm{c}\mathrm{s}\times 1 \mathrm{m} \times$ $1 \mathrm{m}\times 12 \mathrm{m}\mathrm{m}$	Numerical approach based on linear relation review.	$22.7$
[167]	Piezo crystals	Square Tiles	$36 \mathrm{p}\mathrm{c}\mathrm{s} \times 40.2 \mathrm{c}\mathrm{m} \times$ $40.2\times 8 \mathrm{c}\mathrm{m}$	Experimental approach using harvester prototype implementation.	$41.3$
[168]	1. Thiol + PVDF 2. PVDF 3. PZT	1. Spheric 2. Spheric 3. Spheric	1. $\mathrm{r}=2.37 \mathrm{c}\mathrm{m}$ 2. $\mathrm{r}=2.37 \mathrm{c}\mathrm{m}$ 3. $\mathrm{r}=2.37 \mathrm{c}\mathrm{m}$	Experimental and numerical approaches using harvester prototype implementation and simulations.	$2.7$ $3.9$ $3.9$

.

3.6. PEH Applications in Ocean Waves

Piezoelectric sensors have also been explored for monitoring and characterizing ocean waves, owing to their ability to generate and detect mechanical vibrations. While the application in this domain is not as extensively researched as structural health monitoring, several studies have investigated the potential of piezoelectric sensors for ocean wave measurements. This issue began to be studied many years ago. For example, Burns [169] in 1987 created a floating harvester to capture energy from the movements of water particles. The wave energy harvester consisted of a semi-submerged plate that was mechanically connected to a piezoelectric element that vibrated up and down in response to surface waves. In this manner, the piezoelectric effect induced mechanical strain in the piezoelectric plate, which was then utilized to generate electricity. Taylor et al. [170], in 2001 created an Eel generator that uses piezoelectric polymers to transform the mechanical flow energy found in rivers and oceans into electrical power. Zurkinden et al. [171], in 2007 used PVDF to create a generator that transforms ocean wave energy into electrical power, simulating the motion of sea plants under the force of waves. Mutsada et al. [172] created a method for utilizing a Flexible Piezo Electric Device (FPED) made of PVDF and elastic materials to harvest electrical energy from ocean power sources.

Viet et al. [173] developed a novel Piezoelectric Wave Energy Harvester composed of a cylinder shape case 0.8 m in diameter and 20 m in height. It was tested in wave height 2 m and wave frequency 8 s. The power output was $7.46 \mathrm{m}\mathrm{W}/\mathrm{c}{\mathrm{m}}^3$. An energy harvester utilizing the piezoelectric effect has been created by Viet et al. [174] to collect energy from intermediate and deep-water waves. The shape of the harvester was a cuboid with dimensions $1 \mathrm{m}\times 1 \mathrm{m}\times 0.5 \mathrm{m}$ and it was tested in wave height 1 m and wave frequency 6 s. The power output was $1.02 \mathrm{m}\mathrm{W}/\mathrm{c}{\mathrm{m}}^3$.

The energy harvested by ocean waves depends on wave height and period, piezoelectric coefficient, material stiffness, structural shape, and dimensions, for example, in Karim and Awal [175] to investigate the possibilities and connections between the significant wave height and the piezoelectric device’s energy generation. It was discovered that the generated voltage increased as wave height increased and significantly decreased as water depth increased. The work of Kargar and Hao [176] PEH was included in a thorough atlas, categorized according to piezoelectric configurations [177].

In conclusion, the literature reveals that maximizing power output from piezoelectric ocean wave energy harvesters requires a multidisciplinary approach. Optimal performance depends on carefully balancing material selection, device design, and electrical circuitry while considering the specific characteristics of the deployment environment. However, challenges remain in scaling up these technologies and improving their long-term durability in marine environments. Future research directions should include developing more efficient piezoelectric materials, optimizing device designs for specific wave conditions, and creating integrated systems that combine multiple energy harvesting technologies. The research carried out within the domain of ocean wave based PEH and the energy generated is summarized in

Table 6. PEH Applications in ocean waves: A Literature Review.

Reference	Piezoelectric Energy Harvester			Method Based On	Power Output (mW/cm3)
	Materials	Shape	Dimensions
[170]	PVDF	Eel	$5 \mathrm{p}\mathrm{c}\mathrm{s}\times 132 \mathrm{c}\mathrm{m}\times 15 \mathrm{c}\mathrm{m}\times 0.4 \mathrm{m}\mathrm{m}$	Experimental and numerical approaches using a utility acoustic modem and simulations.	$2.53$
[164]	PZT4	Cantilever	$10 \mathrm{p}\mathrm{c}\mathrm{s}\times 10 \mathrm{c}\mathrm{m} \times$ $8 \mathrm{c}\mathrm{m}\times 1 \mathrm{c}\mathrm{m}$	Numerical simulation using a mathematical model.	$3.38$
[178]	Not mentioned	Double beam	$2 \mathrm{p}\mathrm{c}\mathrm{s}\times 100 \mathrm{c}\mathrm{m} \times$ $40 \mathrm{c}\mathrm{m}\times 1 \mathrm{c}\mathrm{m}$	Numerical approach based on finite-element analyses.	$3.38$
[179]	Not mentioned	Buoy	diameter $5 \mathrm{c}\mathrm{m}\times 91 \mathrm{c}\mathrm{m}$	Theoretical and experimental approaches based on the transfer of energy between two systems and prototypical design.	$0.01 \mathrm{p}\mathrm{e}\mathrm{r} \mathrm{h}\mathrm{o}\mathrm{u}\mathrm{r}$
[174]	PVDF	Cuboid	$1 \mathrm{m}\times 1 \mathrm{m}\times 0.5 \mathrm{m}$	Theoretical and numerical approaches based on the Lagrangian–Euler method and a mathematical model.	$1.02$
[180]	PVDF	Sheet	$14 \mathrm{c}\mathrm{m}\times 3 \mathrm{c}\mathrm{m}\times 0.8 \mathrm{m}\mathrm{m}$	Experimental approach using flexible piezoelectric devices.	$2.98$
[173]	Not mentioned	Cylinder	diameter $0.8 \mathrm{m}\times 20 \mathrm{m}$	Numerical approach using a mathematical model.	$7.46$
[181]	Not mentioned	Patches	$10 \mathrm{p}\mathrm{c}\mathrm{s}\times 100 \mathrm{c}\mathrm{m} \times$ $10 \mathrm{c}\mathrm{m}\times 1 \mathrm{c}\mathrm{m}$	Numerical approach using computational fluid dynamics software Ansys Fluent 14.0.	$2.11$
[182]	Not mentioned	Patches	$12 \mathrm{p}\mathrm{c}\mathrm{s}\times 100 \mathrm{c}\mathrm{m} \times$ $10 \mathrm{c}\mathrm{m}\times 1 \mathrm{c}\mathrm{m}$	Theoretical and numerical approach based on Airy linear wave theory and a mathematical model.	$2.50$
[183]	Not mentioned	Cantilever	diameter $5 \mathrm{c}\mathrm{m}\times 100 \mathrm{c}\mathrm{m}$	Theoretical and numerical approach based on JONSWAP wave theory and MatLab software.	$0.87 \mathrm{p}\mathrm{e}\mathrm{r}$ $\mathrm{w}\mathrm{a}\mathrm{v}\mathrm{e} \mathrm{h}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}$

.

3.7. PEH Applications in Structural Health Monitoring

Structural health monitoring (SHM) using piezoelectric sensors has been an active area of research, with numerous studies exploring the potential of these sensors for detecting and locating damage in various structures. Piezoelectric sensors offer many benefits when compared to alternative monitoring methods or sensors, including compact design, low weight, affordable price, multiple format availability, high sensitivity, and more [184]. One of the most widely studied applications of piezoelectric sensors in SHM is the use of guided wave propagation methods, such as the electromechanical impedance (EMI) technique and wave propagation-based methods.

• The EMI technique involves monitoring the electrical impedance of piezoelectric sensors bonded to the structure. Several research papers have explored the use of the EMI technique for damage detection in various structures, including composite materials, concrete structures, and metallic structures. For example, Annamdas et al. [185] demonstrated the use of the EMI technique for detecting and locating damage in reinforced concrete structures using surface-bonded piezoelectric sensors. They developed a damage index based on the impedance signatures and showed its effectiveness in identifying and locating simulated damage scenarios.
• Wave propagation-based methods involve exciting, guided waves in the structure using piezoelectric actuators and receiving the waves using piezoelectric sensors. The presence and location of damage can be inferred from the changes in the wave characteristics, such as wave velocity, amplitude, and mode conversion. Giurgiutiu et al. [186] investigated the use of piezoelectric wafer active sensors (PWAS) for detecting and locating damage in thin-walled structures using guided wave propagation methods. They developed algorithms for damage detection and localization based on the time-of-flight and amplitude analysis of the received signals. Raghavan and Cesnik [187] explored the use of piezoelectric sensors for damage detection in composite plates using guided wave propagation. They developed a damage metric based on the changes in the wave signals and demonstrated its effectiveness in detecting and locating various types of damage, including delamination and impact damage.
• In addition to the sensing techniques, researchers have also focused on developing advanced Signal Processing and Damage Identification Algorithms to enhance the accuracy and reliability of piezoelectric sensor-based SHM systems. Janeliukstis et al. [188] proposed a damage identification algorithm based on wavelet transform and Bayesian inference for piezoelectric sensor-based SHM of plate-like structures as shown in Figure 17. Gharibnezhad et al. [189] developed a damage detection algorithm based on the principal component analysis (PCA) of the piezoelectric sensor signals.
• Acoustic emission (AE) is defined as a term for the brief elastic stress waves that result from the energy released when a material undergoes microstructural changes [190]. Vibration is transmitted to the PZT inside the transducer through the wear plate when the transducers are pressed up against the material’s surface. The PZT element produces an electric signal when it vibrates.
• Piezo-floating-gates (PFG); a self-powered mechanical strain monitoring sensor was introduced by Salehi et al. [191]. It was based on the impact-ionized hot electron injection principle driven by piezoelectricity, and the floating gate serves as a non-volatile memory. The physics of hot electron injection and piezoelectric power harvesting are combined in this sensing technology to sense, compute, and store mechanical usage statistics.

Figure 17. Numerical modeling of an aluminum plate indicating the damaged region. Reproduced with permission from [188], Elsevier 2016.

Figure 17. Numerical modeling of an aluminum plate indicating the damaged region. Reproduced with permission from [188], Elsevier 2016.

Feng and Liang [192] divided piezoelectric-based monitoring technology into two categories: passive identification, which consists of passive sensing and the acoustic emission method, and active identification, which consists of active sensing and the electromechanical impedance (EMI) method. Practical applications, such as steel, composite, and especially concrete structures under the influence of earthquake disasters and geological hazards, were examined for every technique. Smart aggregates (Figure 18) can also be used to track and assess the humidity inside concrete buildings using the humidity index or to evaluate the performance of different crack repairs of building civil structures materials using the repair index, even for underwater structures [193]. Wang et al. [194] studied a high-power gadget for industrial use that scavenges the AC magnetic field via a power-line cable using a magneto-mechanical PEH. This generator successfully powered a wireless sensor network (WSN) integrated with an Internet of Things (IoT) device, including a temperature sensor deployed in a thermal power plant. According to the findings, the magneto-mechanical piezoelectric energy harvester (MPEH) can satisfy the needs of self-powered monitoring systems in the presence of a small magnetic field.

The need for monitoring systems keeps growing because of the higher standards for safety. Seeking energy harvesting for self-powered monitoring or other track-side electrical systems is a useful strategy to lower operation and maintenance costs in situations where access to electrical power supplies is not cost-effective [195]. The field continues to evolve, with ongoing research efforts focused on improving the sensitivity and reliability of piezoelectric sensor-based SHM systems, developing new signal processing techniques, and exploring applications in various structural domains.

3.8. PEH in Extraterrestrial Applications

PEH in extraterrestrial environments is an intriguing concept that has not been extensively explored in published research. However, research papers on this topic started many years ago. Rogallo and Neuman in 1965 [196] introduced an innovative concept proposing to use the impact of micrometeoroids and space debris to mechanically deform piezoelectric cantilevers, generating electrical pulses from the high-velocity impacts to power sensor systems during long-duration deep space travel. Another innovative concept was introduced by Bowen et al. [197], who introduced a novel hybrid harvesting approach. It coupled the piezoelectric effect with the pyroelectric effect by using cycled temperature fluctuations to induce strain and electrical dipoles in special piezo-pyroelectric materials. Theoretically, this could be utilized in the high-pressure environment of Venus.

In 2019 [198], the potential of energy harvesting from a space tether system attached to the Moon’s surface was explored, and piezoelectric materials played a significant role in this process. These piezoelectric materials were integrated into the tether system to help convert mechanical vibrations resulting from the tether’s mechanical damping into usable electrical energy. This allowed the tether system to generate electricity for lunar infrastructure. By including piezoelectric technology in the tether system, the study demonstrated a method for utilizing this energy harvesting approach in a lunar context, enhancing its viability for future missions [198].

Jin et al. [199] showed that a piezoelectric nanogenerator (ML-PENG) based on multilayer polyvinylidene fluoride (PVDF) films can produce significant power output to long-term UV irradiation, large temperature or extremely low temperature and pressure fluctuations, and excessive mechanical vibrations. This appears to be a reliable and economical energy source in harsh and hard-to-reach environments, such as the Moon and Mars. Wang et al. [200] filled the research gap concerning piezoelectric ultrasonic drill performance in extreme temperature environments, shown in Figure 19, and provided valuable insights for future extraterrestrial applications. They proposed an ultrasonic drill driven by a single-crystal piezoelectric transducer in China’s asteroid exploration mission. Results indicated a linear decrease in the transducer’s resonant frequency with rising temperature, and the piezoelectric ultrasonic drill showed consistent drilling performance between −175 °C and 100 °C, with low drilling pressure and power requirements.

A novel ultrasonic drill [201], energized by piezoelectric ceramics, was engineered to facilitate lightweight, low-powered rock drilling on celestial bodies with weak gravitational fields, shown in Figure 20. The purpose of the ultrasonic horn was to increase the elastic wave amplitude in the piezoelectric transducer. The transducer’s maximum vibration amplitude occurred at the horn’s end. A vibration-impact system was formed by the drill stem, impact block, and horn. The impact blocked functions as an energy transfer device, converting the mechanical vibration at an ultrasonic frequency at the horn’s end to low-frequency vibration in the drill stem. The implementation of this ultrasonic drill enhances the potential for utilization of piezoelectric energy during future planetary exploration missions.

While there are not many published research papers specifically focusing on PEH in extraterrestrial environments, this topic presents an interesting area for future exploration. Interdisciplinary research combining expertise in PEH and space engineering could potentially lead to innovative solutions for harnessing energy from mechanical vibrations in space exploration missions or extraterrestrial settlements.

4. Challenges and Future Perspective

In future smart cities, motion and vibration sources could be exploited for energy harvesting. With the aid of PEH, the kinetic energy from everything can be directly converted to electrical energy [202]. Nonetheless, the numerous issues that are currently related to this new technology, such as the effective design of the energy harvesting modules, energy transfer and storage devices, etc., still require a great deal of research [203]. Cost competitiveness is another issue that needs to be resolved in relation to other traditional energy sources because PEH technology is still in its infancy [204]. Before moving on to practical applications, PEH must be highly biocompatible and environmentally friendly but, because lead-containing materials are biologically toxic, the Restriction of Hazardous Substances Directive has placed strict limitations on their use [205]. Calautit et al. [206] indicated that piezoelectric technologies have several drawbacks (Figure 21). The main issue has been power or energy loss, which is mostly the result of inefficient integrated circuits and componentry. The efficient storage of energy obtained from electric energy in an energy storage system has also proven to be a technical challenge; this results in low battery current leakage [207]. Ensuring long-term durability and reliability in harsh environmental conditions, such as exposure to temperature extremes and moisture, remains a challenge that requires ongoing research and innovation [208]. Moreover, coordinating the harmonization of the generated energy with the grid for power distribution and addressing regulatory and safety standards are other challenges that civil engineers must navigate when implementing PEH systems [209].

The thickness and material composition of the piezoelectric layers play a crucial role in determining the effectiveness of energy generation. Future research directions could focus on developing advanced modeling techniques to optimize the design of PEH in civil engineering, as well as exploring the integration of other renewable energy sources to create hybrid energy systems. By addressing these gaps in knowledge, researchers can work towards improving the efficiency and practicality of utilizing PEH technology in sustainable development [210]. In assessing the efficacy of PEH, storage technologies, and their management, most studies have employed diverse research methods, including theoretical frameworks, numerical models, and laboratory experiments. Moreover, simulations have frequently complemented either field tests or lab trials to validate findings. However, due to substantial costs associated with setup and field trials, there has been a limitation in research endeavors in this domain. Notably, our review, as is shown in

Table 7. The overall percentage of studied research papers.

	Theoretical Approach	Experimental Approach	Numerical Approach	Field Test
Roadways	12%	40%	36%	12%
Railways	13%	60%	20%	7%
Bridges	13%	40%	33%	13%
Buildings	11%	37%	42%	11%
Ocean Waves	27%	20%	53%	0%
Total	15%	39%	37%	9%

, reveals that only 9% of research papers included field testing, due to the challenges and costs associated with real-world implementation. However, they are more prevalent in areas like roadways where access to existing infrastructure is easier, whereas 37% relied on numerical methodologies, more than one-third of the research efforts on average. Theoretical approaches generally account for a small percentage; however, they remain important for developing new concepts and models. To authenticate novel concepts, conducting additional field tests using standardized and realistic protocols becomes imperative. This distribution reflects the current state of research in PEH for civil engineering applications, balancing theoretical work with practical experimentation and real-world testing. The focus on numerical approaches and laboratory tests indicates a strong emphasis on optimizing designs before moving to more costly field implementations.

5. Concluding Remarks

This article aims to offer a concise overview of pivotal studies in the field, acknowledging the challenge of summarizing the breadth of published works. Serving as a valuable resource, this review caters to both current and prospective researchers interested in PEH. There are some less explored areas of PEH applications in civil engineering that authors recommend for future research, such as harvesting energy from rivers or underground water pipes; energy harvesting from environmental vibrations; intelligent infrastructure with energy harvesting capabilities; adaptive materials and structures; urban energy harvesting, etc. Our recommendations to future researchers or any other interested group are to invest in research to develop more efficient and durable piezoelectric materials that can withstand the harsh conditions typical in civil engineering applications; to explore the integration of piezoelectric harvesters with other energy harvesting technologies (e.g., solar, thermal) to create more robust and efficient energy systems; to develop industry standards for testing and implementing PEH systems in civil infrastructure to facilitate wider adoption and comparability of different solutions; to concentrate on applications where the unique advantages of piezoelectric systems (e.g., harvesting from vibrations, self-powered sensing) provide clear benefits over alternative technologies; to enhance numerical models and simulation techniques to better predict the performance of piezoelectric systems in complex civil engineering environments, reducing the need for costly field trials; to conduct comprehensive life cycle assessments to better understand the long-term environmental and economic impacts of PEH systems in civil engineering applications; and to strengthen cooperation between civil engineers, materials scientists, and electrical engineers to develop more integrated and efficient PEH solutions.