Energy Harvesting from Water Flow by Using Piezoelectric Materials

As a promising energy‐harvesting technique, an increasing number of researchers seek to exploit the piezoelectric effect to power electronic devices by harvesting the energy associated with water flow. In this emerging field, a variety of research themes attract interest for investigation; these include selection of the excitation mechanism, oscillation structure, piezoelectric material, power management interface circuit, and application. Since there has been no comprehensive review to date with respect to the harvesting of water flow using piezoelectric materials, herein relevant work in the last 25 years is reviewed. To ensure that key aspects of the water‐flow energy harvester are overviewed, they are discussed in the context of energy‐flow theory, which includes the three stages of energy extraction, energy conversion, and energy transfer. The development of each energy‐flow process is reviewed in detail and combined with meta‐analysis of the published literature. Correlations between the harvesting processes and their contribution to the overall energy‐harvesting performance are illustrated, and directions for future research are also proposed. In this review, a comprehensive understanding of water‐flow piezoelectric energy harvesting is provided and it is aimed to guide future research and the development of piezoelectric harvesters for water‐flow‐powered devices is promoted.

the threshold operation voltage of any diodes used for rectifying an AC into DC, so that the output of the harvester is ineffective.

Electrostatic Energy Harvesters
An electrostatic energy harvester (ESEH) typically contains a parallel-plate capacitor with an elastic dielectric layer, which is then connected to an external power source.The capacitor is first pre-charged by an external power source in a high-capacitance configuration, as shown in Figure 1b-i.The external power source is then removed, and the capacitor is switched to a low-capacitance configuration by an external force or a displacive motion, see Figure 1b-ii.With a constant charge and reduced capacitance, the voltage across the capacitor increases to a level higher than the charging voltage, thereby inducing a current in the external circuit.An ESEH can be scaled down to micrometer or millimeter dimensions [11] and exhibit a high output voltage (typically of 10-10 3 V), moderate power density (typically > 10 À3 μW mm À3 ) [12][13][14] and a reduced dependence of the output on frequency (even at 2 Hz [13] ) and device volume. [15]owever, one disadvantage of an EHEH is that an additional power source is necessary to provide the initial pre-charging. [9]

Triboelectric Energy Harvesters
In a triboelectric energy harvester (TEH), specific materials become electrically charged after making contact with a different material, see Figure 1c.The electric field associated with the surface electrostatic charges affects the electric charge in an adjacent electrode via electrostatic induction, which can produce a current in an external circuit.With its high output voltage, typically between 10 2 and 10 3 V, even at low-excitation frequencies, [7] and ease of down scaling to millimeter dimensions, [16,17] TEH devices have significant potential in harvesting energy from a Figure 1.Working mechanisms of a) electromagnetic energy harvesters (EMEH) (Reproduced with permission. [142]Copyright 2014, John Wiley and Sons, Wood); b) electrostatic energy harvesters (ESEH): (i) high-capacitance configuration and (ii) low-capacitance configuration: [143] c) triboelectric energy harvesters (TEH): (i) separation configuration and (ii) contact configuration.Reproduced with permission. [144]Copyright 2020, CellPress; and d) piezoelectric energy harvesters (PEH): (i) compression configuration and (ii) tension configuration.Reproduced with permission. [144]Copyright 2020, CellPress.e) Number of published outputs on water-flow piezoelectric energy harvesters since 2000 with Web of Science as the database.
variety of mechanical excitation, in which water flow has been proven to be an available candidate. [18,19]However, three challenges remain associated with TEH.First, its significant internal impedance leads to a low output current and thus, a moderate power density, typically > 10 À2 μW mm À3 . [15,20]Second, their lifetime can be reduced by wear of the materials due to the forces and heat generated by friction. [21]The third issue is the impact of humidity, [15] a common factor in water-based applications, which can significantly influence the friction-charge generating rate, [22] and thus, the harvester's output. [23] 4

. Piezoelectric Energy Harvesters
Piezoelectric materials exhibit a change in polarization in response to a varying mechanical load, e.g., a fluctuating force, see Figure 1d, so that electrical charge is produced.By connecting the piezoelectric material to an external circuit, an alternating current can be induced.As with TEHs, PEHs are also promising for harvesting ambient mechanical energy due to their high output voltage, typically > 1-10 V even at a frequency below 5 Hz, [24,25] and ease of down scaling to millimeter or centimeter dimensions. [26]Moreover, a PEH is less affected by humidity, [27] and can exhibit a long lifetime under a cyclic load, even in extreme conditions with a high force excitation [28] or high cycle times. [29]Similar to TEHs, a low output current is a central characteristic of PEHs; therefore, the power density is limited, typically > 10 À4 μW mm À3 , [20] due to the high internal impedance of piezoelectric materials.
The features of the aforementioned four electromechanical energy-harvesting techniques are summarized in Table 1.EMEHs are specifically suitable for intense water flow with a large availability of space, such as hydropower stations.However, the space available for electronic devices in water flow is generally smaller, meaning that both the input energy around the EMEH and its volume can be limited.In this case, the advantages of an EMEH are limited, leading to a poor output capability.ESEHs are not ideally suited as an individual power supply for electronic devices due to their reliance on an additional power source.However, there may be a benefit from integrating an ESEH with other harvesting mechanisms as a hybrid power supply to enhance the output voltage.For a TEH, the low humidity and water tolerance of TEHs, alongside challenges with longevity arising from wear are not ideally suited as a power source in or around the water flow.Compared with other energy harvesters, a PEH has relatively low power density, but also have no obvious disadvantages for this particular application.As a result, PEHs are a promising option as a remote power supply for electronic devices in water environments.
The number of publications on water-flow PEHs since 2000 is displayed in Figure 1e, where Web of Science has been used as the database.It can be seen that this area has been attracting increasing attention since 2010 due to the rapid development of internet of things (IoT) in water environments and their need for a power supply. [30]Among these research outputs, a number of studies have been carried out that focus on the different aspects of water-flow PEHs; these include 1) the excitation mechanism to allow the water flow to deliver a fluid force, 2) the harvester structure used to establish a mechanical oscillation from the fluid force, 3) the piezoelectric materials used to generate electrical energy form the oscillations, 4) the interface circuit employed for energy transfer between the piezoelectric material and the external load, and 5) the application and devices that were deployed in the water flow.In contrast, only a limited number of relevant literature reviews have emerged in the area, which did not specifically focus on PEHs for water flow.Naqvi et al. [31] and Hamlehdar et al. [32] examined some general excitation mechanisms for PEHs in fluid flow including air and water flow, and Zhu et al. [33] reviewed their oscillation structures.Due to the contrasting characteristics of air flow and water flow, e.g., different density, viscosity, flow direction, velocity, and environment, a more targeted investigation into water-flow PEHs is needed.Mariello et al. [34] conducted a review of fluid energy nanogenerators, including TEHs and PEHs with micro-or nanostructures.In contrast to conventional energy harvesters, nanogenerators operate at a smaller scale and are designed for microscale power generation applications.Sezer et al. [35] provided a review of the materials and applications of general PEHs, in which the waterflow PEH was involved while the discussion is brief.

Energy harvesters
Pros Cons EMEH • High power density (>10 À1 μW mm À3 ) at high excitation frequency (>5 Hz) and with a large coil volume (cm in size) • Low power density at low frequency and with small coil volume • Low output voltage (<0.5 V) for rectifying from AC to DC at low-excitation frequency and with small coil volume.

ESEH
• High output voltage (>10-10 3 V), even at low frequency (<5 Hz) • Reliance on an additional power source • Ease of down scaling (mm in size) • Moderate power density (>10 À3 μW mm À3 ) TEH • High output voltage (>10 2 -10 3 V), even at low frequency (<5 Hz) • Low lifetime (significant output decay in ≈10 5 cycles) • Ease of down scaling (mm in size) • Moderate power density (>10 À2 μW mm À3 ) • Poor tolerance to humidity PEH • High output voltage (>1-10 V), even at low frequency (<5 Hz) • Low power density (>10 À4 μW mm À3 ) • Ease of down scaling (mm in size) • Limited selections of materials Therefore, to date there has been no comprehensive and specific review on the development of all the aspects of PEHs in a water flow.This article aims to review the overall development of water-flow-based PEHs in the last 25 years, including their excitation mechanisms, oscillation structures, piezoelectric materials, external circuits, and applications.38] In this regard, as shown in Figure 2, a mechanical energy input is initially extracted from an external mechanical excitation, namely energy extraction.The extracted mechanical energy (E mec,in ) is influenced by the amplitude and frequency of the external fluid force and the mechanical impedance of the oscillation structure.The second process is energy conversion, in which the extracted mechanical energy from a water flow is converted into electrical energy (E ele,sto ).In this process, the properties of the piezoelectric and its working mode play a significant role in the conversion efficiency.In the third process, the generated electrical is transferred, via an interface circuit, to an external load as a power supply for electronic devices, namely energy transfer.In this case, the transfer efficiency is dependent on the electrical impedance of the harvester and the connected circuit.A detailed analysis on PEH by the energy-flow model can be seen in ref. [38].The initial process of energy extraction is now reviewed.

Mechanical Energy in Water Flow
The water flow is the volume of water flowing with both a direction and a velocity, which is commonly observed in rivers, oceans, and water pipes.The Bernoulli equation was proposed to describe the conservation of mechanical energy in a fluid, in which all forms of mechanical energy in the fluid are involved: where P water is the pressure in the water flow, ρ water is the water density, g is the standard acceleration due to gravity, h water is the height of the water flow and v water is the flow velocity.It can be seen that the mechanical energy per unit volume of water flow includes the energy associated with pressure (P water ), the kinetic energy ) and the potential energy (ρ water gh water ), respectively.As shown in Table 2, a number of water-flow PEHs have been reported since 2000, where Web of Science was used as a database and all the publications selected clearly report complete experimental data.It can be seen that kinetic energy is the dominant ambient energy in water flow since it is a large quantity in all the flow types.
In contrast, pressure and potential energy are only abundant for enclosed flow and vertical flow, respectively.Although the distribution of energy in water flow is complex, researchers have proposed a number of excitation mechanisms and successfully extracted mechanical energy from a variety of water environments.

Excitation Mechanisms in Water Flow
The first stage to extract mechanical energy from water flow is to ensure that the water flow applies an alternating fluid force to the harvester.A variety of excitation mechanisms have been investigated and are now discussed.

Vortex-Induced Vibration
As a widely used excitation mechanism, vortex-induced vibration (VIV) has been used to excite 21% of the water-flow piezoelectric energy harvesters (PEHs) as shown in Table 2.The mechanism is illustrated in Figure 3a-i; when an elastic bluff body is located in a steady water flow, the flow will be separated at the front surface of the bluff body.Alternating vortices are then able to form around the bluff body when both the body size and the flow velocity are appropriate, see Figure 3a-ii, iii.These vortices are then shed to the wake periodically, thereby applying an alternating force on each side of the bluff body so that it oscillates transversely to the flow when the frequency of the fluid force is close to the resonant frequency of the bluff body. [39]For a water-flow PEH excited by VIV, the reported oscillation is typically in the frequency range of 1.5-6.5 Hz, [40] which is in the frequency range between the alternatively formed vortices in the water flow and the resonant frequency of the oscillation structure.Since VIV is induced by a condition of resonance, the oscillation can be excited in low-speed water flow, i.e., at a low threshold flow velocity, typically > 0.1 m s À1 , and can operate in a range of flow velocities, namely the lock-in range, with a broad width of typically 0.35-0.6m s À1 .With a further increase in the flow velocity, the bluff body cannot be effectively excited by the water flow, and can even remain stationary.In ref. [39], a detailed analytical model was provided to understand the VIV approach.

Galloping Mode
Galloping was used in 7% of the water-flow PEHs as shown in Table 2, and its mechanism is shown in Figure 3b-i.For a bluff  [38] (F 0 sinðωtÞ = fluctuating external force, E mec,in = mechanical energy extracted from the water flow, E mec,in = electrical energy stored in the piezoelectric material of the harvester, which is converted from, E mec,in E ele,out = electrical energy transferred to the external loading, and P out = power output of the harvester).Based on the previous work (ref.[51]), a multi-winding configuration of piezoelectric layers was proposed, and the effects of the serial and parallel electrical connection were studied.
Table 2.   Table 2. body that is elastically mounted in a water flow, once the stabilizing effect is overcome by the destabilizing effect of the fluid force, a small transverse displacement can occur; see Figure 3bii.This small motion leads to a fluid force in the same direction, if the cross-section geometry of the bluff body is appropriate, such as a tri-prism or semicylinder, as shown in Figure 3b-iii.Such a fluid force can lead to a further displacement of the bluff body and consequently a stronger fluid force than that generated by a shedding vortex.In this case, an oscillatory motion (transverse to the flow) is developed which increases in amplitude until the energy dissipated per cycle is balanced by the energy input from the water flow.For water-flow PEHs that are excited by a galloping mode, the flow velocity needs to exceed the threshold speed of at least 0.3 m s À1 to overcome the stabilizing effect. [41]he reported oscillation is typically in the frequency range of 1.6-5 Hz. [42] In contrast to VIV, on raising the flow velocity, the amplitude of the galloping-induced oscillations grows monotonously, which allows the harvester to be excited at a high flow velocity of 4 m s À1 . [43,44]An analytical model of galloping mode has been developed and can be seen in ref. [42].

Wake-Induced Vibration
As the most widely used excitation mechanism, wake-induced vibration (WIV) has been used in 36% of the water-flow PEHs as shown in Table 2. Figure 3c-i illustrates the WIV mode of operation.If an elastically mounted bluff body is placed in a wake flow where the vortices alternately shed from the upstream bluff body, the downstream bluff body can be excited to oscillate via WIV.The most plausible explanation for this phenomenon is related with the interaction between the vortices in the wake and the vortices shed from the downstream bluff body. [45]The vortices shed from the downstream bluff body can be placed closer to the bluff body, as shown in Figure 3c-ii, or partially absorbed by the vortices street from the upstream; see Figure 3c-iii.As a result, both the intensity and the position of the vortices shed from the bluff body change which can apply a fluctuating fluid force to the bluff body and sustain the oscillation.The lowest threshold flow velocity of the PEHs in water flow excited by WIV was experimentally observed at 0.04 m s À1 in ref. [46].With an increase in the flow velocity, the peak oscillation amplitude of WIV increases monotonically without a peak or decay, similar to the galloping mode.Therefore, WIV has also been described as wake-induced galloping. [47]Moreover, the oscillation frequency also increases with the flow velocity [48] with a typical oscillation frequency of 1-8 Hz. [49] As a complex excitation mechanism, models of WIV are at a relatively early stage compared to other modes.

Turbulence-Induced Vibration
In Table 2, 9% of the water-flow PEHs used turbulence-induced vibration (TIV) as the excitation mechanism.In a turbulent flow, the speed of the fluid at a point is continuously changing in both magnitude and direction, as shown in Figure 3d-i.This turbulence can be exploited to create a fluctuating force on an oscillation structure located in the water flow to induce TIV.To create turbulence, an obstacle is typically placed in the water flow, such Continued the previous work , [57]   the Qiqi structure was modified with a pair of blades to increase the rotational speed.
as a nozzle, [50] see Figure 3d-ii, a ring-shaped bluff body [51] or a pipe with a large number of orifices. [52]The water-flow PEHs excited by TIV exhibit a wide range of oscillation frequencies of 10-160 Hz, with no threshold flow velocity.Typically, a large amount of energy dissipation occurs in the water flow during TIV since the turbulence is obtained by an interference with the flow.In ref. [50], this process was quantified by a water pressure drop of 53%.Compared to VIV, galloping, and WIV, the relevant factors of TIV are more complex, which leads to an additional element of randomness to its oscillation behavior, thereby making it difficult to simulate, predict, and optimize.

Cavity-Flow-Induced Vibration
Cavity-flow-induced vibration (CIV) is an excitation mechanism in which the fluid flow in a cavity can be used to generate a fluctuating fluid force and vibrate an oscillatory structure that is mounted within a cavity.This mechanism was first proposed in 1965 [53] and was the first and only time used in water-flow PEHs in 2022 by Thangamani et al. [54] In their work, as shown in Figure 3e, above the bottom wall of a water channel where a rectangular cavity is embedded, vortical disturbances are shed from the cavity leading edge and convect downstream and impinge at the aft wall.As shown in insert figure in Figure 3e, this impingement creates a high-pressure zone in front of the aft wall to generate a pressure wave that propagates in the upstream direction.To harvest the fluctuating fluid force, a cantilever beam was fixed on the aft wall of the cavity that was oscillated by CIV.Their experimental results showed that the oscillation frequency was 228 Hz at a flow velocity of 30 m s À1 .For the water-flow harvester excited by CIV, one challenge is the relatively high threshold flow velocity of ≈30 m s À1 .To address this challenge, additional research is needed to study and optimize the parameters of the cavity and the oscillation structure.

Blocking
As a commonly observed phenomenon, blocking has been widely used as the excitation mechanism in 30% of the water-flow PEHs as shown in Table 2, in which the water flow is blocked by an object which in turn applies a force to the object.This force can be used to excite oscillations without a reliance on vortices or turbulence.In ref. [55], fast water flow from a tap (7.5 L min À1 ) was completely blocked by a flexible film that was mounted at an angle of 45°to the flow direction, see Figure 3f.As a result, the film oscillated at a frequency of ≈20 Hz due to the impact force of the water flow.For low levels of water flow with a small amount of mechanical energy, the blocking excitation mechanism continued to operate; in ref. [56], for example, raindrops were caught by the free tip of the flexible cantilever beam.Raindrops were formed with a diameter of 5.5 mm and fell from a height of 1.5 m at a frequency of nine drops per second, thereby oscillating the flexible cantilever beam at a frequency of 10 Hz.In ref. [57], a different strategy was proposed to extract the small energy associated with raindrops, in which the free tip of the cantilever beam was connected to a container with an eccentric mass tip at the bottom.When raindrops (1.28 L min À1 ) fell into the container, the container was increasingly slanted and periodically poured out the water (every 33 s) thereby leading to an elastic springback of the cantilever beam, leading to oscillation for a period of seconds at a frequency of 2.1 Hz.
In other research, the water flow was not blocked by the oscillation structure but by a rotating structure, such as a propeller [58] or a wheel, [5,59] see Figure 3g, which was driven by the water flow to rotate.Simultaneously, the angular kinetic energy of the rotatable structure was transferred to a cantilever beam by rods, plates, or magnets [5] that were vertically attached to the rotation axis.Each of the elements excited the cantilever beam once per turn as the propeller or wheel rotated.Such a rotatable structure enabled the force to be applied to the oscillation structure with a concentrated force amplitude acting along a single direction.It was observed that water-flow PEHs excited by the blocking mechanism with a rotatable structure oscillated at a frequency of 23-27 Hz.The introduction of a rotating structure led to additional frictional losses, which resulted in a high threshold flow velocity of 1-2 m s À1 being required.
The blocking excitation mechanisms can also operate when the pressure fluctuations are enclosed in water pipes, in which the hollow part of the pipe wall is blocked by a flexible film.In this case, the pressure fluctuation of the water flow inside the pipe can apply a fluctuating force to the film, thereby inducing an oscillation, see Figure 3h.The amplitude of the pressure fluctuation ranged from 0.35 to 110 kPa, which originated from the water pump at the inlet of the pipe [60][61][62] or air bubbles mixed in the water flow. [63,64]The water-flow energy harvester excited by blocking with pressure fluctuation exhibited a wide range of oscillation frequency of 20-450 Hz, with no significant threshold flow velocity.

Wave Motion
Both transverse and longitudinal waves exist in an ocean environment, in which the water particle vibrates perpendicular and parallel respectively to the propagation direction of the wave.These two waves lead to wave motion, and this unique excitation mechanism in the ocean can be used to apply an alternating heaving and pitching forces on an object located in the ocean.In Table 2, wave motion was used to excite 9% of the water-flow PEHs as shown in Table 2.The key to the wave motion excitation mechanism on the surface of the ocean is to establish relative motions between the different elements within the oscillation structure.As an example, shown in Figure 3i, Na et al. [24] installed a percussion bar inside a buoy, and when the buoy was moved by waves, the bar was able to slip forward and backward along a bearing and impact on cantilever beams that were fixed vertically at the two ends to induce an oscillation.Another form of oscillation structure can be seen in ref. [65], in which a cantilever beam with a mass tip was vertically mounted onto a floating boat, see Figure 3j.When the boat was excited by wave motion, there was a relative motion between the boat and the tip mass, thereby inducing inertial forces on the cantilever beam to produce oscillations.Although the wave frequency was as low as 0.5 Hz, the oscillation frequency of the harvester reached 8.3 Hz.For a PEH located below the ocean surface, the wave motion excitation mechanism can be combined with the range of excitation mechanisms described earlier namely VIV, [66] galloping, [67] WIV, [26,68] and blocking. [69]Nevertheless, since the flow conditions in the ocean are always changing, [70] the harvester must be able to exhibit some degree of directional adaptivity [71] and frequency adaptivity. [72]

Oscillation Structures
To enable the harvester to respond to the fluid force provided by the water flow, researchers have proposed a variety of oscillation structures, which include the 1) cantilever beam, 2) flapping eel, and 3) compression structures in film, plate, and bar shapes, which are now discussed.

Cantilever Beam
Due to their excellent adaptivity to the excitation mechanisms discussed earlier, cantilever beams have been widely used in 65% of the water-flow PEHs as shown in Table 2.A typical cantilever beam consists of a substrate layer (typically a metal) and piezoelectric layer(s), which are usually piezoelectric ceramics or composites.As shown in Figure 4a, one tip of the cantilever beam is fixed, while the other tip is free to move and subject to the excitation force.If the excitation mechanism requires a bluff body, it can be connected to the free tip of the cantilever beam.During the application of an excitation force, the cantilever beam Figure 4. Schematic of different oscillation structures: a) cantilever beam, [146] b) flapping eel: (i) typical configuration, (ii) various mode shapes, and (iii) the corresponding strain energy (Reproduced with permission. [73,75]Copyright, 2018, Springer) and compression structures in c) film shape (Reproduced with permission. [63]Copyright, 2017, American Chemical Society), d) disk shape (Reproduced with permission. [27]Copyright, 2021, Elsevier), and e) bar shape (Reproduced with permission. [62]Copyright, 2013, IOP Publishing).Development of energy extraction of piezoelectric energy harvesters (PEHs) for water flow: f ) available flow velocity with publication year and g) oscillation frequency with publication year.h) Finite-element analysis on the influence of the harvester on the downstream flow (Reproduced with permission. [48]Copyright, 2021, MPDI).
oscillates and the resulting bending stress can be transferred to the associated piezoelectric layer(s).

Flapping Eel
The flapping eel is an oscillation structure that is used mainly for the WIV excitation mechanism and has been used in 20% of existing research on water-flow piezoelectric energy harvesting, see Table 2.As shown in Figure 4b-i, the flapping eel typically consists of a substrate layer (typically a polymer) and piezoelectric layer(s), which are typically piezoelectric polymers.One tip of the flapping eel is fixed behind a bluff body and the other tip is free to move.It therefore has a similar structure to a cantilever beam, so that the flapping eel can be regarded as a specific case of a cantilever beam, but with a high length to thickness ratio.The high length-thickness ratio leads to a low bending stiffness of the eel so that it can readily flap in a water flow with various mode shapes, i.e., with several bends occurring over the flapping eel as shown in Figure 4b-ii.It has been highlighted that the greater the level of bending, the higher the strain energy in the eel and power output, see Figure 4b-iii. [73]Moreover, different deformation modes also exhibited different natural frequencies [74] and energy-conversion efficiencies. [75]However, multiple bends in the piezoelectric can result in charge cancellation effects, since opposite bending strains can occur on the same side of the eel, which reduces the generated current and reduces the energyconversion efficiency. [75]

Compression Structures
The fluid force produced by water flow can also be received by a compression structure, which has been used in 15% of the research to date on water-flow PEHs, mainly with respect to the blocking excitation mechanism, see Table 2.The core component of such a structure is a piezoelectric material that is subject to the fluid force so that the piezoelectric element can be compressed.According to the shape of the piezoelectric material, compression structures can be classified into film shaped, [63] disk shaped, [27] and bar shaped, [62] as shown in Figure 4c-e, respectively.Nevertheless, their working mechanisms remain the same: with the application of a compressive fluid force and an associated elastic restoring force a small deflection/oscillation along the force direction is established.To improve performance of the harvester, an additional functional layer can be attached to the piezoelectric material to reinforce the fixture, thereby increasing its water resistance or amplifying the stress.

Parameters of the Energy-Extraction Process
The evaluation of the performance of water-flow PEHs in the energy-extraction process is complex as a range of parameters are involved, which are now discussed.

Effective Flow Velocity
The effective flow velocity is an important parameter that determines the performance water-flow PEHs in the energy-extraction process and how the harvester can effectively oscillate to extract mechanical energy from a water flow to generate power.In Figure 4f, the effective flow velocity in published research is collected, where it can be seen that the majority of harvesters operate at a flow velocity of 0.2-1 m s À1 .In addition, the range of effective flow velocities of water-flow PEHs has broadened from ≈1 to 0.05-30 m s À1 in the last 25 years, indicating the improved adaptivity of water-flow PEHs to different water-flow conditions.Interestingly, the majority of these studies have focused on reducing the initial effective flow velocity, namely the threshold flow velocity, which is of important considering the advantage of PEHs for harvesting a low-speed water-flow environment, e.g., river surfaces and the ocean. [76]For a harvester deployed in an environment with varying flow velocity, e.g., water pipes onand off-peak hours, the range of the effective flow velocity, i.e., the effective flow velocity bandwidth, is also of significance; however, due to inconsistent definitions and characterization methods of the effective flow velocity bandwidth in different studies, an effective comparison of the effective flow velocity bandwidth in those work is difficult to provide from the current literature.

Oscillation Behaviors
The oscillation behavior of water-flow PEHs includes the oscillation frequency and amplitude.For a harvester in constant waterflow conditions, both a high oscillation frequency and amplitude are desirable, which provide a large amount of mechanical energy.Figure 4g summarizes the oscillation frequency of water-flow PEHs reported to date.The minimum oscillation frequency is constant and above 0.5 Hz, while the maximum oscillation frequency has significantly increased to hundreds of Hz with increasing publication year.Compared to the oscillation frequency, the reported oscillation amplitude for water-flow PEHs for a range of oscillation structures are more difficult to collect and compare; for example, the amplitude of compression structures are typically too small to measure, while that of flapping eels differs with the testing position of the eel.
To improve the aforementioned properties, research can be mainly classified into 1) tailoring the fluid force applied to the harvester and 2) optimizing the mechanical impedance of the harvester, which is dependent on the forcing frequency, mass, stiffness, and damping coefficient of the oscillating structure. [38][82][83] In addition, the force applied to the harvester can also be tailored using magnets.Since 2020, a number of emerging studies have aimed to integrate magnetic materials into the oscillation structure to study a range of magnet parameters, including magnet polarity, [25,84,85] the number of magnets, [5] and magnet position. [25,85]In terms of optimizing the mechanical impedance of the harvester, the harvester impedance describes how much it resists the motion caused by the fluid force that is applied to the harvester.In ref. [38], by reducing the mechanical impedance of a disk-based compression structure by ≈80%, the extracted energy significantly increased by 560%.For water-flow PEHs, the mechanical impedance could be tuned by the density/mass of oscillation structure [86] and the geometric configuration, such as length, [54] width, [87] thickness, [58] and shape. [87]

Other Parameters of the Energy-Extraction Process
To better evaluate the performance of the harvester during the energy-extraction process, new parameters are expected to be studied in future; these include the strain energy stored in the oscillation structure and the influence of the harvester on the downstream flow.As an outcome of the energy-extraction process, the stored strain energy indicates the capability of the harvester for energy extraction and is of significance for analysis of the harvesting processes and efficiency; this has been the focus for only a limited numbers of outputs in terms of analytical models. [73,75]Moreover, it is not desirable for the harvester to have significant influence on the downstream flow and the water environment.A range of parameters should be taken into consideration, including the reduction in water-flow velocity and water pressure.In ref. [48], a detailed case of analyzing the aforementioned parameters was conducted by finite-element analysis, see Figure 4h.

Energy Conversion of Water-Flow PEHs
In Section 2, we reviewed how water-flow PEHs extract mechanical energy from water flow.In the second energy-harvesting process, energy conversion, the mechanical energy input is converted into electrical energy by the piezoelectric material, in which the working mode and the material type plays a significant role.The nature of the piezoelectric effect is that an externally applied stress changes the polarization of material, which generates charge on the surface of the material to produce a current in a circuit.The most commonly used materials in PEHs are ferroelectric materials.Their piezoelectric coefficients tend to be one or two orders of magnitude higher than non-ferroelectric piezoelectric materials, e.g., quartz. [88]Moreover, ferroelectrics are easier to process than non-ferroelectric piezoelectrics as their piezoelectric properties can be induced by a poling process to yield a net spontaneous polarization and thus, a single crystal or highly textured form is unnecessary.After poling, a remnant polarization can be retained in the material, see Figure 5a, and opposite charges concentrate at the two ends to achieve charge neutrality.As illustrated in Section 1.4, when no stress is applied to the material, the charge state is stable, and no current will be induced.However, when an external stress is applied to the material it changes its polarization, see Figure 5b, and the charge balance at the electrodes changes, thereby producing charge and inducing an electrical current in the external circuit.and c) different working modes.Specific cases of piezoelectric energy harvesters (PEHs) in water flow: d) d 31 mode, [24] e) d 33 mode, [38] and d 15 mode employed: f ) a spiral-shaped beam (Reproduced with permission. [65]Copyright, 2016, AIP Publishing) and g) a vibrating beam connected with an eccentric cylinder (Reproduced with permission. [91]Copyright, 2017, MDPI).
The relationship between the external stress and the change in polarization can be described by Equation (2): in which D i is the polarization change in the direction, i σ j is the stress applied to the material in the direction, j and d ij is the piezoelectric charge coefficient.For, d ij the first subscript, i relates to the polarization axis (i.e., the direction in which a ferroelectric has been poled) and the second subscript, j relates to the direction in which the external stress is applied.According to the stress direction and poling direction, the piezoelectric material can typically operate via three different working modes: d 33 /longitudinal mode, d 31 /transverse mode, and d 15 /shear mode, see Figure 5c.To illustrate the stress direction and the electrode position more clearly, a Cartesian coordinate system is also provided in Figure 5c.Typically, the direction of positive polarization is made parallel to the z axis, i.e., 3-direction, which is considered in the following discussion.For the d 33 mode, the piezoelectric material is subjected to a compressive stress in the 3-direction, and the electrodes are perpendicular to the 3-direction.When the material is compressed, there is a decrease in polarization in the 3-direction.Correspondingly, charges are generated at the electrodes parallel to the planes 1-2.The d 31 mode is operated by the application of a force in a direction perpendicular to the polarization direction.A lateral compressive stress is applied to the material, which results in longitudinal elongation leading to an increase in polarization.Unlike the d 33 or d 31 modes, for the d 15 -shear mode, electrodes are positioned parallel to the poling direction; for example, in Figure 5c-d 33 , which are attached perpendicular to the 1-direction.The material is subjected to a shear stress in the planes 1-3, i.e., the arrow 5. Since a shear strain is applied to the material, the polarization in the 1-direction increases.
In practical applications for water-flow PEHs, the d 31 mode has the widest applicability with all the oscillation structures including cantilever beams, [25] flapping eels, [77] and compression structures. [61]Figure 5d shows how this working mode operates in the most commonly used oscillation structure, cantilever beams, in which the poling direction was along the thickness direction of the beam, perpendicular to the bending stress caused by the bending moment along the length direction.However, for the ferroelectric materials commonly employed for water-flow piezoelectric harvesters, the value of d 31 is typically lower than that of d 33 and d 15 see Table 3.As a result, with a constant amplitude of applied stress, the piezoelectric response of the d 31 mode is the lowest in terms of charge generation.The d 33 mode tends to have a larger piezoelectric response than the d 31 mode and can be operated with compression structures [89] and cantilever beams. [90]To operate the d 33 mode in a cantilever beam, the poling direction needs to be the same as the stress caused by the bending moment, namely the length direction, as shown in Figure 5e.The d 15 coefficient is typically higher than either the d 31 or, d 33 and therefore the d 15 mode has potential for the best piezoelectric response.However, since the d 15 mode requires a shear stress, it has only been investigated in a limited number of cantilever beams configurations, e.g., a cantilever beam in spiral shape [65] and a cantilever beam connected with an eccentric cylinder bluff body, [91] see Figure 5d,e, respectively.

Piezoelectric Materials
At the heart of a PEH, the piezoelectric material has a significant influence on the overall energy-harvesting process.In this subsection, the three commonly employed piezoelectric materials for water-flow PEHs are reviewed, including lead zirconate titanate (PZT) ceramics, the macrofiber composites (MFCs) and polyvinylidene fluoride (PVDF) polymers.

PZT Ceramics
The PZT-based ferroelectric ceramics are the most common piezoelectric materials used in water-flow PEHs.As shown in Table 2, PZT has been used in 51% of studies on water-flow PEHs as shown in Table 2 and combined with, d 31 , d 33 and d 15 modes.PZT has a cubic structure above the Curie point of 150-350 °C, [92] with Pb 2þ and O 2À situated at the corners of the unit cell and the surface centers, respectively  (see Figure 6a-i); the center of the unit cell is occupied by Ti 4À or Zr 4À randomly.The cubic structure is symmetric until the temperature is reduced below its Curie point, where there is a phase transformation to a tetragonal or rhombohedral, see Figure 6a-ii, in which a shift in the position of Ti 4À /Zr 4À occurs.
Correspondingly, the unit cell is non-centrosymmetric, thereby forming a dipole, and piezoelectricity in a bulk material can be obtained after the material is polarized to align the dipoles in a common direction, as outlined in Section 3.1.As shown in Table 3, among the three types of piezoelectric materials, diagram with a range of Zr/Ti ratios (Reproduced with permission. [147]Copyright, 2019, IOP Publishing; Reproduced with permission. [148]Copyright, 2018, AIP Publishing); b) MFC: (i) the multilayer structure and (ii) the electric field distribution around the interdigitated electrodes while poling (Reproduced with permission. [37]Copyright, 2018, John Wiley and Sons; Reproduced with permission. [149]Copyright, 2015, Society of Photo-Optical Instrumentation Engineers); and c) PVDF (Reproduced with permission. [150]Copyright, 2012, Elsevier).d) Composites of ZnO nanoparticles and PVDF: i) ZnO nanoparticles and ii) composites (Reproduced with permission. [99]Copyright, 2020, Elsevier); (e) AlN-based PEH with a flapping eel in micrometer scale thickness.(Reproduced with permission. [103]Copyright, 2021, Elsevier).
PZT ceramics typically exhibits highest piezoelectric charge coefficients while due to the nature of ceramics material, PZT ceramics also shows brittleness and high Young's modulus, which can limit its application.

MFC
A solution to reducing the high Young's modulus of PZT materials is to use MFCs, which was first proposed by NASA in 1996. [93]MFCs have been used in 18% of the research on water-flow PEHs as shown in Table 2 and used for d 31 mode.As shown in Figure 6b-i, the MFC consists of rectangular piezoelectric ceramic fibers, which are embedded in an epoxy matrix and sandwiched between structural epoxy and electrode layers.
The epoxy matrix reduces the Young's modulus of the material and can inhibit crack propagation in the piezoelectric phase.For a d 33 -mode MFC, interdigitated electrodes are used to separate each PZT fibers into a number of sections and connect them in parallel, see Figure 6b-ii.For a d 31 -mode MFC, the PZT fibres can be directly covered by a large single-electrode layer.In comparison to PZT ceramics, the Young's modulus of an PZT-based MFC is almost half and the piezoelectric charge coefficients are slightly reduced, see Table 3.Interestingly, for the water-flow PEH used in ref. [94], by using a single piezoelectric phase Pb(In 0.5 Nb 0.5 )O 3 -Pb(Mg 0.33 Nb 0.67 )O 3 -PbTiO 3 as the MFC fiber, the fabricated MFC exhibited a high d 33 of 792 pC N À1 .

PVDF
As a ferroelectric polymer, PVDF is naturally flexible and relatively low cost, and therefore favorable for piezoelectric energy harvesting. [95]Among the range of potential phases of PVDF, the β phase exhibits the best ferroelectric and piezoelectric properties. [96]The structure of a single chain of β-phase PVDF is illustrated in Figure 6c, in which an all-trans conformation can be observed.The side of the molecules where the hydrogen atoms are tightly packed exhibits a positive charge, while the other side with fluorine atoms is negatively charged.In β-PVDF, there are two such polymer chains in each unit cell and, since they point in opposite directions, the net dipole moment is zero until a sufficiently large poling electric field is applied to align the switchable ferroelectric dipoles.As shown in Table 3, the Young's modulus of PVDF is only of several gigapascal, which is much lower than that of ferroelectric ceramics and MFCs, thereby leading to excellent flexibility.In addition, the permittivity (ε σ 33 ) of the material and the piezoelectric coefficient (d ij ) of PVDF is low.As shown in Table 2, PVDF has been used in 29% of research to date on waterflow PEHs and demonstrated good compatibility with d 31 mode.

Non-Ferroelectric Piezoelectric Materials
In addition to the aforementioned ferroelectric materials, researchers have also investigated the energy-harvesting performance of non-ferroelectric piezoelectric materials, including zinc oxide (ZnO) and aluminum nitride (AlN).While these materials have relatively low piezoelectric charge coefficients compared to ferroelectric materials, as shown in Table 2, their unique advantages provide them with potential for use in water-flow PEH.In recent years, due to the widely developed method for ZnO nanoparticle preparation, [97] this material has been successfully used as a second phase to fabricated ZnO@PVDF composites, see Figure 6d. [98]The fabricated composite exhibited ≈400% higher piezoelectric voltage output compared to pure PVDF. [99]In addition, ZnO nanoparticles can also tune the Young's modulus the PVDF-based composites from 2 to 30 GPa, [100,101] which can be used to tailor the mechanical impedance of the harvester for specific flow conditions.For AlN, due to its ability to be deposited as thin micrometer-scale films on a soft substrate, [102] researchers have successfully fabricated an AlN-based flapping eel harvester with a ≈30 μm thickness, as shown in Figure 6e. [103]Such an extremely thin oscillation structure provides a solution to significantly reduce the mechanical impedance of the harvester, which is beneficial for the energy-extraction process.

Parameters of the Energy-Conversion Process
The piezoelectric element is the core part of the energyconversion process.To evaluate the performance of the piezoelectric element on the harvesting process, a range of parameters should be taken into consideration, which are now discussed.

Piezoelectric Energy-Harvesting Figure of Merit
The efficiency of the energy-conversion process is significantly influenced by the properties of the piezoelectric material, which is described by the electromechanical coupling coefficient (k 2 ij ) as where, Y, d ij and ε σ 33 are the Young's modulus, piezoelectric charge coefficient, and permittivity at constant stress of the piezoelectric material, respectively.While a high Y is beneficial for the conversion efficiency, it also increases the mechanical impedance of the oscillation structure, thereby reducing the extracted mechanical energy from a water flow; for example, the mechanical impedance of off-resonance compression structure of PEH is approximately proportional to 1=Y. [38]Therefore, considering the opposing effects of Y on the energy-harvesting performance, a more sensible evaluation of the energy-harvesting performance of the piezoelectric material is based on the piezoelectric energyharvesting figure of merit (FoM ij ): [104] The physical meaning of this index is the energy density generated by the piezoelectric material for an applied stress [104] and, for water-flow PEHs, a high FoM ij is desirable.Figure 7a  modes, it could provide a higher FoM ij compared to the MFC or PVDF.In addition, it is worth noting that while some research has been undertaken to improve the FoM ij of the piezoelectric material used in water-flow harvesters, e.g., doping techniques, [59,105] there have been no significant improvements in the FoM ij in the last 25 years.Considering the range of novel techniques that are being employed to enhance the FoM ij in wider piezoelectric harvesting applications, e.g., single piezoelectric crystal with FoM 33 of 68-10 3 Â 10 À12 m 2 N À1 , [106] porous piezoelectric ceramics with FoM 33 of 36-161 Â 10 À12 m 2 N À1 [107] and multiple-layer piezoelectric composites with FoM 33 of 65-100 Â 10 À12 m 2 N À1 , [108] future opportunities are expected to exploit these new materials in water-flow PEHs to provide a potential improvement in the FoM ij .

Piezoelectric Material Volume
The volume of the piezoelectric material can be considered a parameter to describe the size of the harvester and the applicability of the harvester to different scales of water environments.This parameter should be a balance between the energy density and general output of the piezoelectric material due to their opposite dependence trends on the material volume.In Figure 7b, the material volume parameter is displayed with publication year.The large size range of PZT ceramics and PVDF can be seen ranging from 10 mm 3 to 10 cm 3 , which presents the well-developed applicability of water-flow PEHs to millimeterand centimeter-scale water environments.Moreover, interestingly, in most of the studies on water-flow PEHs study, the piezoelectric material volume was ≈0.3 cm 3 .

Other Parameters of the Energy-Conversion Process
In addition to the aforementioned two parameters of the piezoelectric material used in water-flow PEHs, there are other properties should be taken into consideration with respect to the energy-conversion process; these include the Young's modulus, density, service life, and use of lead-free materials.The Young's modulus and density play a significant role in the stiffness and mass of the piezoelectric material, and therefore the mechanical impedance of the oscillation structure.As shown in Table 3, due to the limited range of piezoelectric materials studied to date in this area, the ranges of the Young's moduli and densities are large.This may be addressed by the use of novel piezoelectric composites, [109] where the mechanical impedance of oscillation structures can be better tailored, which is beneficial to the energy-extraction process.For water-flow PEHs, the service time is typically a long period, counted by years.Therefore, as a parameter describing the durability of the harvester, the service life of the piezoelectric material is of significance, which involves the mechanical fatigue limit.A fatigue fracture of a PZTceramics-based cantilever beam was reported in ref. [50] after ≈3 Â 10 8 vibrations.Such a service life can only support a harvester with an oscillation frequency of ≈10 Hz to operate for 1 year, which can be considered insufficient for water-flow PEHs; therefore, close control of lifetime and stress applied in required.The wide use of lead-based PZT ceramics and PZTbased MFC is another challenge for water-flow PEHs since lead is an element that is harmful to the environment.Governments are increasingly restricting its use in the manufacture of electronic products; for example, the EU's Restriction of Hazardous Substances directive has restricted lead and its compounds in electrical and electronic products.The limit is 0.1% by weight in all homogeneous material in a product. [110]ortunately, material researchers have made effort to develop high-performance lead-free alternatives to PZT ceramics, for example, (K 0.5 Na 0.5 )NbO 3 [111]   and (Ba 0.85 Ca 0.15 )(Zr 0.1 Ti 0.9 ) O 3 . [112]However, to date, none have been used in water-flow PEHs and is an area worthy of further study.

Energy Transfer of Water-Flow PEHs
The electrical energy generated by the piezoelectric element does not have any practical value until the harvester is connected to an external load.Therefore, the final energy-harvesting process, namely energy transfer, relates to the transfer of the electrical energy to the external load.For water-flow PEHs, the energy transfer has been realized by a range of interface circuits connecting the harvester and the external load, including purely resistive circuits and energy-harvesting circuits, which are now discussed.

Purely Resistive Circuits
The most commonly used interface circuit of water-flow PEHs is purely resistive circuit, as shown in Figure 8a, in which the harvester is directly connected to a load resistor (R load ).To measure the voltage across the load (V out ), a measurement device, such as an oscilloscope or an electrometer (R meas ), is typically connected to the R load in parallel.Since the piezoelectric material is typically capacitive in nature, the harvester can be considered an AC current source (I in ) with a capacitor (C piezo ) connected in parallel and the equivalent circuit is shown in Figure 8b.As a result, the generated I in simultaneously charges the C piezo and flows through the R load establishing V out across the, R load see Figure 8c.The power output (P out ) can be calculated with the measured V out and the known R load by With constant, I in P out can be maximized by implementing impedance match as shown in Figure 8d, in which the P out first increased with the R load and then decreased.The maximum P out can be correlated to the optimum loading resistance: R opt,load j purely resistive circuits ¼ 1 ωC piezo (6)   at which the R load equaled the capacitive reactance of the, C piezo and therefore the C piezo and the R load shared the same current.In this case, power output can be maximized as Logically, one of the two main insufficiencies of purely resistive circuits is that a proportion of the I in is used to charge the C piezo without providing any P out .The other main challenge of the purely resistive circuit is its limited practical use, [113] since the provided V out is AC, which makes it difficult to operate as a useful power source for sensing, data transmission, or controlling electronic devices.

Standard Energy-Harvesting Circuits
A solution to address the problem of the AC output is provided by standard energy-harvesting (SEH) circuits, which has been used in some studies related to water-flow PEHs.As shown in Figure 8e, the SEH circuit consists of a full-wave bridge rectifier of four diodes, a filter capacitor (C filt ), and an external load (R load ).  and d) the impedance matching process. [65]Standard energy-harvesting circuit: e) circuit diagram and f ) voltage and current flow in the circuit (Reproduced with permission. [114]Copyright, 2021, Elsevier) and g) output voltage with time (Reproduced with permission. [57]Copyright, 2019, Elsevier).h) Numbers of studies with resistor or capacitor as the loading with years.
The rectifier enables the AC current generated by the harvester to flow through the R load in a consistent direction all the time, see i c .
The filter capacitor is used to decrease the ripple voltage across the R load .As a result, a DC output is obtained.The voltage and current flow of a PEH connected to an SEH circuit are shown in Figure 8f.When the voltage accumulated across the harvester (V piezo , see the black line) is exceeding a threshold voltage, i.e., the sum of the voltage drop of the bridge rectifier (V d ) and the output voltage (V out , see the red line), a current is generated in the circuit (i c , see the blue line) and flows through the bridge rectifier to provide output.As a result, the C filt is charged and the V out increases.When the V piezo falls below the threshold voltage, namely, V d þ V out the interface circuit becomes an open-circuit and thus, there is no i c .In ref. [114], the accumulation of V out in half-cycle of the harvester's vibration is analytically modeled as and the maximum P out of SEH circuits can be obtained by impedance match when where Q is the charge generated by the harvester in the half-cycle harvester's vibration.With V out accumulating, ⋅V out becomes smaller and smaller, which is consistent with the experimental data as shown in Figure 8e.In the work of ref. [57], a water-flow PEH was used to harvest a vertical water flow.Due to its oscillation structure, there was a long interval of around 20 s between each excitation of the harvester, and therefore the ⋅V out of each vibration cycle could be clearly distinguished.However, SEH circuits still have a limitation of a purely resistive circuit in that the current generated by the harvester has to charge the piezoelectric material, i.e., the C piezo .In addition, due to the presence of a threshold voltage, the accumulated voltage across the harvester that is below the threshold level is not harvested as marked by the red area in Figure 8e.This electrical energy will be lost in the next half-cycle of vibration and thus, the available output of SEH circuits is further reduced.In ref. [115], it was claimed that the maximum power output of a PEH with an SEH circuit was ≈36% lower than a PEH operating with a purely resistive external circuit.

Practical Use of the Harvester's Output
The practical use of the harvester's output, i.e., DC output, seems to attract increasing research attention.This trend could be seen by the increasing number of the studies using capacitor as the external load.As shown in Figure 8h, the number of the waterflow PEHs combined with a capacitor load began to increase since 2019, although it remains smaller than that of a harvester with resistive loading.With more SEH circuits used in waterflow PEHs to provide a practical output, the magnitude of the DC output is also believed to be of significance in future.
Currently, considering a large proportion of the generated electricity in SEH circuits has to charge the piezoelectric material, or is wasted due to the existence of a threshold voltage, there should be a considerable improvement space of the DC output of waterflow PEHs.This may be inspired by more efficient interface circuits for PEHs that introduce an inductor and switch into the circuit, such as synchronized charge extraction (SCE) circuits [116] and synchronized switch harvesting on inductor (SSHI) circuits; [117] more complex circuits can be seen in ref. [118], where this is an area of future study for water-flow harvesting.

Adaptivity of the Maximum Power Output
In addition to the practical use and amount of the energyharvesting circuit's output, the adaptivity of the maximum power output to varying vibration frequencies is also of significance for water-flow PEHs since the input of water flow is often not constant.As shown in Equation ( 6) and ( 9), the optimum resistance corresponds to a particular vibration frequency.In this case, if there is an increase or decrease in the oscillation frequency of the harvester, the energy-transfer efficiency of the interface circuit will decrease.To address this issue, a maximum power point tracker [119] could be integrated in the interface circuit in future.

Applications of PEHs in Water Flow
We have so far discussed how the electrical energy has been transferred to the external load, to power electronic devices within a water environment.In this section, the applications are now discussed.

Application Prospects
Currently, electronic devices that operate in a water flow are generally powered by batteries, whose fabrication costs are typically much lower than that of a PEH. [51]Moreover, a water-flow PEH typically cannot provide a power output as high as batteries.Therefore, it is necessary to first clarify the boundary between the application of water-flow PEHs and batteries to judge whether the harvester can act as a power source to replace the battery.To define the potential application, it is helpful to reference the five criteria proposed by Keddis et al. [51] for the use of piezo-powered sensing systems in water-flow-related applications; which includes the following: 1) no battery replacements are possible; 2) it is not possible to use a power grid or batteries; 3) an ability for simultaneous use of a single external circuit by a number of harvesting units; 4) an ability to deliver a constant supply of electrical energy; and 5) an ability to simultaneously act as both a PEH and a sensor (multifunctionality).
For criterion (i), battery replacements are taken into consideration, which means that the service term of the device is longer than the battery life.Criterion (ii) refers to applications in which the replacement and maintenance of the power supply are challenging and inconvenient, so that the use of batteries is costly.This is relevant in remote or inaccessible water-flow regions, which includes remote rivers or pipelines that are buried deep underground, or when the device is difficult to trace, such as being placed in an ocean or mounted on a swimming fish.Criteria (iii) and (iv) both focus on the power output of the PEH.For criterion (iii), the importance of the potential for integration of a number of PEHs is to enhance the total power output, while criterion (iv) emphasizes the significance of the stability of the harvester power output.These two criteria indicate that the PEH should be able to power the device to operate normally.Criterion (v) is a special case where the piezoelectric effect is simultaneously used by both sensing and energy harvesting.Therefore, it is not applicable to all the piezo-powered devices and will be separately discussed in Section 5.2.With the previous discussion, three premises can be undertaken to judge whether a PEH in water flow is needed as a power source: 1) long-term, where the service life of the electronic device is too long (typically measured in years) to be supplied by a single set of batteries; 2) inaccessible, where poor accessibility of the device makes the replacement and maintenance of the power supply difficult and expensive; and 3) powerable, where the power output of the water-flow PEH should be sufficiently large and stable to allow the connected electronic device to operate continuously.
As a result, based on the long-term, inaccessible, and powerable (LIP) principle, the application scenarios for water-flow PEHs are constructed, as shown in Figure 9. Water-flow PEHs should be used in remote or difficult to access water environments.Furthermore, it should ideally support electronic devices with a low power consumption such as sensing, data communication, and control devices such as valves.Finally, the whole piezo-powered system should be targeted toward long-term service.As a result, water-flow PEHs can be correlated to five water-based environments including rivers, pipes, oceans, aquatic animals, and vertical flow, for three functions: sensing, such as 1) climate measurement, 2) water quality monitoring, and 3) pipeline leakage detection; data transmission, such as 4) flood alarms and 5) electronic fish tags; and control, such as 6) water management.

Application Devices
With a clear approach to determine whether a water-flow PEH is needed as a power source, we will now discuss electronic devices that either have been, or can be integrated, within a piezopowered system.In addition, other relevant self-powered systems using energy-harvesting techniques, e.g., electromagnetic energy harvesting, and different ambient mechanical energy sources, are also discussed due to their importance and relevance to this topic.

Sensors
A sensor is a device that measures physical input from the environment and converts it into an output signal that can be interpreted by either a human or a machine.It is worth noting that since piezoelectric materials can convert a physical input into an electrical signal via the piezoelectric effect, they can also operate as sensors without the need for an external power source.For example, in ref. [5], since there was a correlation between the rate of water flow and the force applied to the piezoelectric material, the generated voltage could be used to estimate the flow rate.Similarly, in ref. [63], the output voltage of piezoelectric film reflected the pressure change within a water pipe.These piezoelectric sensors can be considered as a self-powered sensing device, which are specifically suitable for measuring hydrodynamic information regarding the water flow, e.g., flow rate and pressure.However, since piezoelectric effect cannot be used to sense other physical signals such as the presence of chemicals, pH, or pollutant, an independent sensor will be required.In ref. [57], to measure the climate change near the water environment, a thermometer-hygrometer was integrated with a PEH, which consisted of a PZT cantilever beam with dimensions of 60 Â 20 Â 0.7 mm 3 .The harvester was tested in the laboratory using a vertical water flow as the energy source.The electrical output of the harvester was first stored in a capacitor (470 μF, 25 V).Initially, the charging rate was relatively high at 42 μW and then gradually reduced to 10 μW, where the voltage across the capacitor gradually increased.When the voltage reached 1.5-1.8V, the sensor was manually switched on and data was obtained within 3 s with an electrical energy consumption of ≈0.1 μWh.Based on the experimental results, it was demonstrated that, for the piezo-powered water-flow climate measuring system, it was necessary to alternatively switch the sensor on and off to enable the capacitor to be charged for 8-34 s between each measurement, namely a charging to working time ratio of 3-11 was required.It is of interest to note that since many piezoelectric materials, such as the ferroelectric ceramics and polymers, are also pyroelectric, there may be potential to use the material as a form of thermal sensor, but this has yet to be investigated.
Researchers have attempted to fabricate a piezo-powered flow meter located in a pipeline to detect leakage [120,121] ; this issue has been claimed to lead to a 15-25% loss of drinking water each year across the world. [122]In this case, a disk-shaped compression structure based on PZT with dimensions of 46 Â 6.4 Â 0.25 mm 3 was attached to a pipeline buried in the ground and used to harvest energy from the vibration of the pipeline surface induced by the fluid flow.By optimization of various parameters, including the vibration frequency, the tip mass attached to the film and the location of the film on the pipeline, the root mean square (RMS) power output reached 4 μW. [121]The energy consumption of electronic flow meters to obtain a single data point varies with the working mechanism of the sensor, with approximately ≈0.2 μWh required for Hall effect or magnetic-based sensors and ≈14 μWh for ultrasonic-based sensors. [123]Thus, the charging time for every measurement is calculated to be 3 min to 7 h, which is potentially acceptable considering the requirement adopted by most UK water companies that log a data point per 15 min. [124]ater quality monitoring is another application with ample scope for piezoelectric powered sensors.Within the European Union's 7th framework program, a project entitled "GOLDFISH -ENLARGED" was formed, whose aim was to build a monitoring system that can rapidly detect contaminating substances in watercourses.To realize this target, a piezo-powered water quality monitoring system was fabricated. [125,126]The harvester consisted of a PVDF-based flapping eel with dimensions of 621 Â 194 Â 0.45 mm 3 , and the device was tested in a channel flow with a flow velocity of 0.5-2.0m s À1 .The experimental results indicated that the time-average output power was ≈10 mW at a maximum.In their river-flow field experiments, a pollution sensor was arranged in the Coello river, Ibague, Colombia, whose energy consumption was ≈0.42 mWh per measurement, with the sensor required to measure the data every 15 s.Therefore, it can be estimated that the required timeaverage power output of the harvester should be ≈100 mW, 10 times higher than the practical output.As a result, significant improvement of the power output of the harvester, or a reduction in required power is needed.

Data Transmission
The data obtained by the sensor cannot be used unless it can be transmitted to a terminal (e.g., mobile phone, computer, or cloud server) so that it can be recorded, processed, and analyzed.Therefore, data transmission devices that have been integrated with water-flow piezoelectric powered systems are now discussed; relevant self-powered data transmission devices with a range of energy source environments and harvesters are also discussed.
Among the range of available data transmission techniques, wireless communication, i.e., radio frequency communication, is commonly used due to its low cost and flexible device location.In piezo-powered water quality monitoring systems for the "GOLDFISH -ENLARGED" project, [126] the data was transmitted from the sensor to the terminal device, via the GSM communication (Global System for Mobile, i.e., 2G cellular networks).GSM was selected due to its long-range communication of tens of kilometers, so that sensors in the whole Coello river (111.6 km in length) could be readily covered by several gateways installed along the riverbank.Researchers designed the GSM communication module to operate for 2 min every hour and the experimental results showed that each communication process consumed ≈0.5 Wh of electrical energy.Therefore, the average power consumption of the GSM module could be estimated to be ≈500 mW, which is 50 times higher than the power output of the PEH used in the Coello river of ≈10 mW.
A power level of 500 mW can be considered as high, and even when using a standard 12 V-7 Ah battery (≈1000 cm 3 in volume) as the power source, it will run out of power in one week.As a result, energy-saving data communication techniques must be considered.[129] This has been demonstrated by integrating a Bluetooth module (HC-06) and a flow meter with an EMEH in a pipeline. [130]The experimental result indicated that when the flow velocity exceeded ≈19 m s À1 , the flow velocity could be monitored and the data could be transmitted via Bluetooth to a smartphone.For the Bluetooth module, the peak power consumption occurred under paring mode reaching ≈132 mW; however, during the normal and sleeping modes, the power consumption was reduced by 80% and 95%, respectively.To further reduce power consumption, BLE was proposed in 2011.
Unlike Bluetooth, BLE devices do not operate constantly, but regularly remain asleep.In addition, BLE is designed to handle fewer data compared to Bluetooth.Although this is not ideal for communicating via a phone, it is suitable for applications that periodically exchange small amounts of data, such as the applications discussed here.Researchers have integrated a BLE (Texas Instruments CC2541) with a piezo-powered temperature monitoring system. [131]The mechanical energy input source was a speaker that simulated the vibration of a real bridge.The BLE module was set to transmit the measured temperature to the terminal device after every 5 s of sleep.The experimental data indicated that the average power consumption of the whole piezopowered temperature monitoring system was as low as 121 μW.
As a novel communication concept, a low-power wide-area network (LPWAN) was proposed in early 2013, which can be facilitated by a number of communication technologies, e.g., narrowband-internet of things (NB-IoT), long range communication (LoRa), and Sigfox. [132]Unlike SRW communication, LPWAN communication techniques sacrifice communication speed, rather than communication range, for low energy consumption.This strategy makes them appropriate for applications that transmit a small amount data per day over several to tens of kilometers.Using this method, a hybrid-powered forest fire monitoring system was fabricated in which a piezoelectricelectromagnet hybrid energy harvester was used to convert wind energy to power a CO concentration sensor and an NB-IoT module (M5311, Longmain Co., Ltd.) whose data was transmitted to the base station and cloud data center thousands of miles away from the forest. [129]The NB-IoT module was able to provide an uplink speed of ≈15.6 kb s À1 and was set to transmit data every 13 min.Approximately, 1.25 mWh of energy was consumed for each communication cycle, giving an average power consumption of 5.8 mW.
The LoRa technique adopted a more aggressive energy-saving strategy by further reducing the transmission speed.Orfei et al. [133] fabricated a piezo-powered monitoring system to observe the asphalt condition on bridges, whose input energy was extracted from the bridge vibration produced by the passing vehicles.The system contained a temperature sensor, water sensor, and a LoRa module (RN2483).The RN2483 module was able to cover a communication range of 5 km (in urban areas) to 15 km (in suburban areas), with an uplink speed of 0.8 kb s À1 .In their experimental results, the LoRa module required an average power consumption as low as 11.3 μW, which could be attributed to the extremely low charging to working ratio: transmitting 8 byte data within 10 ms after every 3.5 h sleeping.
The energy consumption of mobile cellular communication techniques with required powers of >100 mW, e.g., 2G, is too high for PEHs.Therefore, a trade-off must be made between the communication rate/range and the power consumption, giving rise to SRW communication and LPWAN.SRW is appropriate for a wide range of communication rates (several bits per second to several megabits per second) within a short range (below 300 m).In contrast, LPWAN is appropriate for communication over a long range (tens to hundreds of kilometers) with low requirement for the communication rates (several bits per second to hundreds of kilobits per second).In comparison to the power consumption of mobile cellular network transmitters of hundreds of milliwatts to several watts, the power consumption of SRW and LPWAN was significantly reduced from tens of microwatts to tens of milliwatts.

Control
A control function means that the device can conduct some specific actions, where a typical control device in water environments can be a valve, which has potential in IoT for water supply management. [134]In ref. [135], a solenoid valve was integrated with a LoRa transceiver and a microcontroller in a water tank to establish a remote water supply management system.This system was designed to be powered by a hybrid energy-harvesting system using solar power and water-flow energy.When the valve was operating, a DC voltage of 12 V and a current of 500 mA were required.The charging time and sleeping time ratio was set to 600 and thus, the time-average power consumption of the valve was estimated to be 10 mW.In addition, the power consumption of the transceiver and the microcontroller should also be taken into consideration, where the time-average power requirement of the remote water management system reached ≈46 mW.In comparison to sensing and data transmission devices, control devices are harder to produce due to its more complex configuration and also the higher power consumption.As a result, no work has been done to date in this emerging area to integrate control devices into water-flow PEHs.

Parameters of Applications
Although the LIP principle that was outlined in Section 5.1 provides a blueprint for the diverse applications of water-flow PEHs, not all the mentioned electronic devices are currently powerable.As discussed in Section 5.2, only simple sensors have been successfully powered by water-flow PEHs to date.It therefore remains challenging for the harvester to power electronic devices with high power requirements and short charging times.The feasibility and operational performance of a water-flow PEH self-powered system is determined by the relationship between the power requirement of the device and the power output of the harvester, which are now discussed.

Power Requirement of Devices
Due to the presence of a capacitor [136] or a battery [66] that is integrated within the interface circuit as the external loading, see Figure 8e, the generated electricity can be stored until there is sufficient electrical energy to power any connected devices.In this case, the power requirement of the device can be considered as a time-average power requirement.Based on the different device types and charging to working time ratios of 5-3600, i.e., assuming that the electronic device works for 1 s in a single cycle after every charging for 5-3600 s (3 h), the time-average power requirement of the application devices ranged from microwatt to watt level, see Figure 10a.For simple devices, such as thermometers, hygrometers, and BLE modules, their power consumption is as low as ≈10 À2 μW.For other common electronic devices given in Figure 9, including location and chemical sensors and data communication devices, their power consumption reaches ≈10 À1 mW.For the other complex devices such as chemical sensors, satellite communication modules, and controlling devices, a time-average power consumption of ≈10 2 mW was required.For water-flow PEHs powered multifunctional systems, such as sensing þ data communication þ control, a higher power requirement will be needed.
In the previous discussion in Section 5.2, the power consumption of particular electronic devices has been significantly reduced due to the development of novel techniques, e.g., BLE and LoRa for data communication.A more general overview of how the power consumption of devices decreased with increasing year can be seen in Figure 10b,c, which displays the evolution of the power consumption of gas sensors [137] which included 93% of the harvesters as shown in Table 2. Figure 11 displays the P norm with publication years, in which an exponential increase in P norm can be observed with an annual growth rate of ≈70%.Considering the slow development in the harvester's FoM ij as shown in Figure 7a and the fixed interface circuit configuration, the improvement in P norm should be mainly due to the progress in the energy-extraction process, namely more efficient excitation mechanisms and oscillation structures as discussed in Section 2.
Therefore, in the application of water-flow PEHs, the gap between the blueprint and the actual progress can be due to the 1-2 orders of magnitude gap between the power requirement of any devices and the power output of harvesters.As an inspiration to future work, the power requirements for data transmission and device operation have been decreasing exponentially, while the power output has been increasing exponentially.In future, as the power requirement of devices and the power output of harvesters converge, the future application of waterflow PEHs is therefore foreseeable.

Challenges and Future Directions
Based on the previous discussion, three major challenges can be identified for water-flow PEHs: 1) to develop a sufficient output, 2) to deliver a stable output, and 3) to be environmentally friendly.There are several pathways to address these challenges, which can help guide the future research and are now discussed.

Sufficient Power Output
Delivery of a sufficient power output presents a match between the power requirement of the application devices and the power output of the water-flow PEHs.To date, there is a gap between the output and requirements of approximately one to two orders of magnitude.Therefore, new techniques in sensing, data transmission, and control devices are expected to continue reducing their power consumption.As a research direction for water-flow PEHs, the power output of the harvesters has been gradually increased in the last 25 years and this trend is believed to continue in future.A higher output can be achieved by improving the whole energy flow, namely the energy-extraction, energyconversion, and energy-transfer processes.In the energyextraction process, a number of designs have been developed to improve the harvester's power output; these include optimization of the excitation mechanism, tailoring the mechanical impedance of oscillation structure and exploiting magneticforce-enhanced oscillations.In more recent studies, the emerging combination of two or more of the aforementioned designs has shown potential for significantly enhancing the energy extracted from water flow.For example, with a synergy between WIV optimization and magnetic force enhancement, the oscillation amplitude of the harvester was improved by 11 times. [85]In the energy-conversion process, the progress of the energy density of the piezoelectric element was limited, which could be seen in the slow development of the piezoelectric energy-harvesting FoM ij in the last 25 years, see Figure 7a.As a result, there is scope of significant improvement in the FoM ij of materials used for water-flow PEHs, in particular on considering that material researchers have developed a range of novel techniques to enhance the FoM ij of piezoelectric materials by 2-20 times, including single piezoelectric crystals, [106] porous piezoelectric ceramics [107] and multiple-layer piezoelectric composites. [108]or the energy-transfer process, both a DC output and a highenergy-transfer efficiency are of significance.In the last 5 years, SEH circuits have been integrated with water-flow PEHs to provide a DC output.However, no work has been undertaken to date to improve their energy-transfer efficiency.Successful cases in piezoelectric generators for other applications can be learnt from, in which an inductor and switch were cleverly introduced into the circuit cleverly; these include SCE circuits [116] and SSHI circuits [139] where the energy-transfer efficiency was increased to 400%-900%.
In addition to improving the individual energy-harvesting processes, it is also worth noting the correlation between each process.For example, in the energy-extraction process, the oscillation frequency can affect the electrical impedance of the energy-transfer process, and the oscillation structure determines the working modes the piezoelectric element.In the energyconversion process, the Young's modulus and permittivity of the piezoelectric element can affect the mechanical impedance of the oscillation structure and the electrical impedance of the interface circuit, respectively.In the energy-transfer process, due to the effect of the electrical load on the mechanical impedance of the oscillation structure, the interface circuit configuration can also affect the oscillation behavior of the harvester. [140]herefore, isolated studies on a single energy-harvesting process are not desired and a holistic approach is needed.Another pathway to increase the power output would be the use of hybrid energy-harvesting technologies, in which a PEH can be coupled with other energy harvesters.Therefore, collaboration across disciplines including hydrodynamics, material science, and electronic engineering should be encouraged, and a comprehensive evaluation of the energy-harvesting process in the whole energy flow is expected.

Stable Power Output
A need for a stable power output indicates that the output of the piezoelectric water-flow harvester can be maintained at a specific level for a range of conditions, in which two factors are important, namely the flow velocity sensitivity and the service life of the harvester.Typically, the flow velocity is not constant in a water environment, which can affect the energy-extraction and energytransfer processes.For common excitation mechanisms in the energy-extraction process such as VIV and WIV, a variation in flow velocity leads to variation in the oscillation frequency.However, the resonance frequency of a specific oscillation structure is typically constant.As a result, an increase or decrease in the flow velocity can decrease the oscillation amplitude and the extracted energy in each vibration cycle.To enable the harvester to be less sensitive to varying flow velocities, magnetic-forceenhanced oscillation techniques have attracted research attention since 2020.To date, this technique has been studied in two aspects: 1) providing a nonlinear force to attenuate the change in mechanical impedance of an oscillating structure around the resonance frequency and 2) making the oscillation structure multi-stable to exhibit several resonance frequencies.Due to the lack of an index parameter to describe the sensitivity of an oscillation structure to varying flow velocities, progress to date in different studies is hard to evaluate and compare.A varying oscillation frequency can also make electrical impedance matching in the energy-transfer process complex, since the optimum load resistance typically corresponds to a specific oscillation frequency.To address this issue, a maximum power point tracking technique will be useful, although no work to date has used it in the interface circuit of a water-flow PEH.
The service life indicates how long the water-flow PEHs can operate.This property is important since water-flow energy harvesters are typically designed to operate for long periods, over many years; however, relevant research in this important area is rare.Based on the limited available experimental data, the service life could be limited by two factors: 1) failure of the oscillation structure and piezoelectric material due to fatigue [50] and 2) a decrease of the FoM ij of piezoelectric materials with time due to the decay of piezoelectric charge coefficient or depolarization at high stress, [141] in which more work is expected to be undertaken in the future to provide more data and an understanding of factors influencing lifetime.

Environmental Friendliness
The need for environmental friendliness implies that both the harvesting process and the fabrication of the harvester should not be harmful to the environment, which is significant for the ecosystem.To date, little research has been undertaken in this area.In the harvesting process, since the energy of the water flow is extracted, the downstream flow can be affected.It has been proven by both experimental and modeling methods that a flow velocity reduction and a pressure drop will occur downstream to the harvester.This can obstruct the normal water flow and can have an impact on the water environment, especially the water flow with initially low energy intensity, e.g., rivers with low flow velocity.Nevertheless, to date, the understanding of how water-flow PEHs influence the water environment is limited, which should be due to the lack of effective characterization methods of the influence process and the lack of models of the energy-extraction efficiency.
For the fabrication process of water-flow PEHs, an urgent issue to address is the widespread use of lead-based piezoelectric material, which is harmful to the environment and its use has been restricted by many governments in recent years.Inspiringly, material researchers have made effort to develop a range of high-performance lead-free alternatives to lead-based piezoelectric materials.Nevertheless, to date, none of them has been used in water-flow PEHs, and it is an area worthy of further study in future.
Addressing the issues and challenges described is believed to provide a route for the development of PEHs to extract energy from a water flow to deliver power to electronic devices that are used in a water environment, such as water sensors, data communication devices, and control devices, such as valves.

Figure 2 .
Figure 2. Schematic of energy-flow model in water-flow piezoelectric energy harvesters[38] (F 0 sinðωtÞ = fluctuating external force, E mec,in = mechanical energy extracted from the water flow, E mec,in = electrical energy stored in the piezoelectric material of the harvester, which is converted from, E mec,in E ele,out = electrical energy transferred to the external loading, and P out = power output of the harvester).
macrofiber composite, PVDF = poly(vinylidene fluoride), VIV = vortex-induced vibration, WIV = wake-induced vibration, TIV = turbulence-induced vibration, CIV = cavity-flow-induced vibration).shape of the beam was studied to tune the strain distribution.Magnets were used to provide nonlinear force on the oscillation structure.mounted in side-by-side arrangement and the effects of the distance between them were studied.cross section of the bluff body was made into an elliptic shape with different curvatures, which was proven to enhance the shed vortices.array was proposed, where 4 PHEs were arranged in a vertical plane in a pipe with an angle of 90°between each of two PEHs.A magnetic coupler was fixed at the center of the array to enhance the oscillation.
effects on the maximum power output; initial vibration velocity and half-power bandwidth were studied.
was elastically mounted, which was claimed to minimize the damping effect of the water flow on the oscillation structure in the wake.
of the whole pipe caused by the turbulence inside the pipe was used as the mechanical excitation.Piezoelectric films were mounted at the pipe surface, and the effects of the distance from the film to the pump were studied.turbulence was formed by a nozzle in the pipe.The effects of the flow rate and the nozzle size were studied.A lifetime test was conducted.A pressure from the inlet of 517 kPa to the outlet of 241 kPa was observed.water pressure in the pipe was generated by a pulse pump.The upper wall of the pipe was replaced by a flexible diaphragm.The piezoelectric film was connected to the diaphragm to response to the pressure fluctuation in the pipe.body was placed in the pipe to induce vortices, resulting in pressure fluctuation.The upper wall of the pipe in the downstream was replaced by a flexible nanofibers (PVDF NFs) were prepared by electrospinning method.The as-synthesized PVDF NFs exhibited polarized β-phase structure and high aspect ratio, and the fabricated PEH was highly flexible.The pressure fluctuation was realized by alternately pumping air and liquid flow into the pipe.was used to provide pressure fluctuation.Studied the effects of the pressure condition and the area of the interface subjecting to the water flow.subjected to a rocking motion at 2.5 Hz resulting in traveling wave motion for the test.The composite showed good shaped elastic beam was developed, which can scavenge vibration energy from arbitrary directions.The PEH was mounted onto a wooden buoy which was placed in a basin filled work attempted to propose a noncontact PEH for low-frequency ocean waves.A pendulum-shaped structure was connected with a buoy to respond to wave motions.The structure was coupled with piezoelectric cantilever beams by magnets.-ball-shaped buoy, a percussion bar was swayed by waves and sloped back and forth between a pair of piezoelectric beams, hitting and bending them.A motor-based wave-making apparatus was used to form waves in the water tank.conducted in a water flume using a flap-type wave maker to generate waves.Effects of 1) wave conditions and 2) the distances from the piezoelectric eel to the bluff body and the water surface, respectively, were studied.sinusoidal wave generator was used to produce waves in the water channel.The influences of 1) the piezoelectric beam's orientation, 2) the distance beneath the water surface, 3) the added tip mass of the beam, and 4) wave conditions were studied.transducers were bonded to the piezoelectric disc for stress amplification and the stacking arrangement.
consisted of 300 disks connected to a DC-DC bulk converter to match the impedance.The output power was used to charge a 480 Wh battery within shaped cantilever beam was proposed for energy harvesting at the interface between air and ocean waves.was integrated with the piezoelectric cantilever beam, which provided the PEH with all-around multidirectional sensitivity in ocean flow.water droplets were produced by a syringe and used as the external mechanical excitation.The effects of 1) droplet diameter, 2) release height, 3) drip frequency, and 4tip of the beam was connected to a container with an eccentric mass tip at the bottom, i.e., a Qiqi structure.With water continuously flowing into the container, it will periodically rotate (every 33 s) and pour out the water causing a force fluctuation.The PEH was used to charge a 470 μF capacitor to 2.9 V after 7working cycles of around 240 s.
piezoelectric composite was fabricated by a uniformly dense dispersion of few layers black phosphorous nanosheets in PDMS matrix, which exhibited a high piezoelectric coefficient and flexibility.

Figure 5 .
Figure 5. Illustration of the piezoelectric effect in piezoelectric/ferroelectric materials: a) poled material without external force and b) with external force,and c) different working modes.Specific cases of piezoelectric energy harvesters (PEHs) in water flow: d) d 31 mode,[24] e) d 33 mode,[38] and d 15 mode employed: f ) a spiral-shaped beam (Reproduced with permission.[65]Copyright, 2016, AIP Publishing) and g) a vibrating beam connected with an eccentric cylinder (Reproduced with permission.[91]Copyright, 2017, MDPI).

Figure 7 .
Figure 7. a) Piezoelectric energy-harvesting figure of merit (FoM ij ) of piezoelectric materials used in water-flow piezoelectric energy harvesting.b) Volume of the piezoelectric materials used in water-flow piezoelectric energy harvesters (PEHs).

Figure 8 .
Figure 8. Purely resistive external circuit: circuit diagrams of a) external loading and b) equivalent current source of the harvester; c) output voltage with timeand d) the impedance matching process.[65]Standard energy-harvesting circuit: e) circuit diagram and f ) voltage and current flow in the circuit (Reproduced with permission.[114]Copyright, 2021, Elsevier) and g) output voltage with time (Reproduced with permission.[57]Copyright, 2019, Elsevier).h) Numbers of studies with resistor or capacitor as the loading with years.

Figure 9 .
Figure 9. Applications of water-flow piezoelectric energy harvesters (PEHs) based on the long-term, inaccessible, and powerable "LIP" principle at five different water environments powering a range of devices for three functions including sensing, e.g., i) climate measurement, ii) water quality monitoring, iii) pipeline leakage detection; data transmission, e.g., iv) flood alarms and v) electronic fish tags; and controlling, e.g., vi) water supply management.(SRW: short-range wireless communications such as Bluetooth; LPWAN: low-power wide-area network such as NB-IoT).

Figure 11 .
Figure 11.The normalized power density of piezoelectric harvesters in water flow with publication years.

Table 2 .
Summary of water-flow piezoelectric energy harvesters (PEHs) reported since 2000, using web of science as the database (PZT = lead zirconate titanate, MFC =
shows the development of the FoM ij of different piezoelectric materials used in water-flow PEHs with publication year.Interestingly, the value of the FoM ij was primarily dependent on the working mode (d 33 , d 31 or)d 15 rather than the material type.As indicated by three different coloured bands, the value of shear mode FoM 15 was typically higher than FoM 33 than.FoM 31 As a result, due to excellent compatibility of PZT ceramics to, d 31 , d 33 and d 15