Mechanical energy conversion systems for triboelectric nanogenerators: Kinematic and vibrational designs

Author’s Accepted Manuscript Mechanical Energy Conversion Systems for Triboelectric Nanogenerators: Kinematic and Vibrational Designs Wook Kim, Divij Bhatia, Shinkyu Jeong, Dukhyun Choi www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(18)30860-7 https://doi.org/10.1016/j.nanoen.2018.11.056 NANOEN3212

To appear in: Nano Energy Received date: 1 October 2018 Revised date: 7 November 2018 Accepted date: 17 November 2018 Cite this article as: Wook Kim, Divij Bhatia, Shinkyu Jeong and Dukhyun Choi, Mechanical Energy Conversion Systems for Triboelectric Nanogenerators: Kinematic and Vibrational Designs, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.11.056 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Mechanical Energy Conversion Systems for Triboelectric Nanogenerators: Kinematic and Vibrational Designs Wook Kim1, Divij Bhatia1, Shinkyu Jeong*, Dukhyun Choi* Department of Mechanical Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-Si, Gyeonggi-do 17104, South Korea. *

[email protected]`

[email protected]

ABSTRACT Triboelectric nanogenerators (TENGs) represent a promising next-generation renewable energy technology. So far, TENGs have been successfully used as highly sensitive and self-powered internet of things (IoT) sensors and portable/wearable power sources owing to their various merits, such as their light weight, freedom of material selection, low cost, and high-power conversion. The ability to take advantage of diverse mechanical input sources is another significant advantage of TENGs. However, the irregular magnitudes and frequencies of input sources are critical limitations that currently prevent utilizing TENGs in industrial or practical applications. In this review, we focus on mechanical energy conversion systems (MECS) for the regular or controlled operation of TENGs; to do this, we employ kinematics or vibrational theory. Once we control the mechanical operation of TENGs, we can predict the power production from 1

These authors equally contributed to this work.

these devices. Furthermore, mechanical frequency matching can greatly reduce power loss from electrical circuits. Motion control from, rotational to linear movement, can effectively provide high-frequency operation of contact-separation mode TENGs, enabling us to obtain sustainable and high-performance TENGs. Finally, resonant system designs for TENGs can produce the maximum output power. Thus, we discuss how to account for damping effects or non-linear impacts when designing a resonant system with TENGs. Finally, this review offers an effective way to avoid wasting irregular mechanical input sources for TENGs, making the practical commercialization of TENGs more feasible.

Graphical abstract We report a comprehensive review of researches on recent progress of kinematic and vibrational designs for triboelectric nanogenerators (TENGs). TENGs are one of promising energy harvesters using mechanical energies such as wind, wave, vibration, and human motion in our nature. However, the mechanical energies are normally irregular due to the variable environments. Thus, the output electrical power from TENGs shows low-quality with irregular, unstable, unpredictable, and high-loss characteristics. Mechanical energy conversion systems can be utilized to overcome those limitations related to input/output energies. We briefly introduce theories of kinematics and vibration then examine the mechanically designed TENGs. This review thus offers the effective way to control irregular mechanical input sources for the commercialization of TENGs.

Keywords: Triboelectric nanogenerators (TENGs); Kinematics; Motion control; Vibration; Resonant system

1. Introduction Exhaustion of resources and global warming have become increasingly critical issues in our modern, energy-consuming society since the 21st century industrial revolution. Additionally, low-income countries struggle to establish power plants due to the high cost and long construction periods of these structures. Thus, environmentally friendly and low-cost power production systems should be developed. To solve these problems, renewable energy systems using solar [1-3], thermal [4-6], and wind [7-10] energies have been developed; however, each of these still demonstrates shortcomings. For example, although photovoltaic cells are one of the

cleanest, most applicable, and promising alternative energy solutions, they are limited when solar energy is not readily available, such as at night or when it is raining or snowing. Another energy issue is related to the fourth industrial revolution, which is leading to a new technical paradigm based on the internet of things (IoT) [11]. To improve society’s quality of life and protect the environment, we need to establish sustainable and maintenance-free operation of micro/nanosystems through innovative and mobile energy harvesting technologies. Abundant mechanical energies, such as wind, wave, vibrations, and even body movements, exist in our environment, but they are almost always wasted. By utilizing these wasted mechanical energies, we can effectively produce electricity via piezoelectric [12], electrostatic [13], electrochemical [14], magnetostrictive [15], electromagnetic [16], and triboelectric [17] energy harvesting technologies. Due to a variety of mechanical energy sources, we could produce electrical energy anytime and anywhere, creating various mechanical and selfpowered sensors [17]. Furthermore, some mechanical energy harvesters could offer unique properties, such as flexibility [18], light weight [19], easy processability [20], cost effectiveness [20], and environmental friendliness [21]. Owing to these merits, we can develop new macroand micro-power plants. However, many of these mechanical energy harvesters suffer from irregular mechanical sources with low input forces and variable frequencies [22,23], yielding too-low and irregular output power making it difficult to predict power production. Through good understanding of mechanical energy conversion systems (MECS), we can overcome this problem, and make the input forces work in our favor. Most mechanical movements can be controlled by MECS via kinematics or vibrational design. Kinematics describes the motion of points, bodies, and systems [24,25]. Using kinematic designs, we can increase the working frequency from low input frequencies and change the

direction of motion (e.g., from rotational motion to linear motion). It is well known that two or more gears working in a sequence (i.e., a gear train) can change the speed, torque, and direction of a power source. Whereas high-speed rotational energies can be transformed into linear motion by using cam-follower based mechanical systems. In the case of environmental vibrations, we can design and develop ocean-energy harvesters; often referred to as blue energy [26], and various vibrations from cars, trains, and body movements can be used for powering self-powered IoT systems [16,27,28]. However, mismatching designs between the input vibrational frequency and system resonant frequency lead to low output power or no power generation. In other words, well-designed resonant systems can provide optimal speed and amplitude motion, thereby yielding higher output power. Furthermore, by introducing nonlinearities into the vibration motion dynamics of the generator, its frequency bandwidth coverage can be broadened in order to deal with the variability in environmental input vibration frequencies. Thus through good understanding and design of kinematic and vibrational energy conversion systems integrated with the energy harvester, the output performance can be improved. In this review, we briefly introduce kinematics and vibrational theory for mechanical energy conversion. As a mechanical energy harvester, we highlight triboelectric nanogenerators (TENGs), which have great potential to produce electrical energy by harvesting environmental mechanical energies. We briefly introduce the fundamentals for TENGs, such as the working mechanism and control parameters. Particularly, we separately review kinematic energy conversion systems and vibrational energy conversion systems for TENGs. Fig. 1 illustrates that by employing MECS, we can produce regular, predictable, and controlled output power from TENGs. By making the power output predictable, despite the irregular mechanical inputs, we can reliably design circuit systems which is essential for practical applications of TENGs. Our goal

for this focused review is to improve the understanding of MECS design and integration with the TENG, to obtain improved and controlled output power despite the irregular nature of mechanical input sources.

2. Fundamental Theory 2.1 Kinematics As mentioned above, kinematics describes the geometry of motion of points and bodies by using mathematical analysis, as shown in Fig. 2a [24,25]. In other words, the systematic interaction of bodies can be defined by kinematic analysis. When connected bodies interact via external or internal changes, displacement of the bodies is observed, and this geometrical change can be expressed mathematically via kinematic analysis. Based on kinematic analysis, various kinematic components have been invented and applied to daily lives. For example, there are many kinematic components in vehicles, such as belts, gear trains or boxes, slider-cranks, and so on. Kinematic components can be independently utilized or systematically combined by a mechanical design. Properly designed kinematic systems transfer mechanical energy or modify the characteristics of mechanical energy, such as its direction, velocity, or force, as described in Fig. 2b. Fig. 3a shows the basic concept of the gear mechanism. The gear mechanism is based on linear velocity conservation at the contact position of gears [24,29]. Therefore, this concept can be described as [24]: (1)

Here, r is the radius of the pitch circles and ω is the angular velocity of the gears. This equation means that the angular velocity ratio is equal to the gear ratio, and the angular velocity linearly increases as the gear ratio increases. The biggest advantage is that the angular velocity can be controlled by applying gear trains of different sizes. The size difference of gears is called the gear ratio. The gear ratio is the same as the teeth ratio and diameter ratio of gears. Moreover, the gear mechanism can be utilized to change mechanical motion by using a rack gear. A rack gear is a linearly shaped gear that modifies linear motion into rotational motion, or vice versa. Thus, this gear can change the angular velocity, rotation direction, and mechanical motion. Fig. 3b depicts the cam-follower mechanism, which is able to change rotational motion into linear reciprocal motion [24,30-32]. A cam system is composed of a cam and a follower. Usually, the cam is an eccentrically supported circle or an asymmetrically shaped rotator that accepts rotational energy. The follower is part of the motion converter that converts rotational motion into linear reciprocal motion. Therefore, the linear displacement form can be utilized to provide a linear mechanical load. Mathematical descriptions are utilized to design the motion of the cam and cam follower, such as the constant velocity, constant acceleration, simple harmonic motion, and cycloidal motion. In these motions, the simple harmonic motion is widely considered because it is simple to design. The displacement (s), velocity (v), and acceleration (a) of a follower in a simple harmonic motion can be expressed as [24,30]: ( ) ( )

( )

(

( (

(

)) )

)

(2)

(3)

(4)

Here, θ is the rotary angle of the cam, β is the range of the rotary angle corresponding to the motion event, and h is the stroke of the motion event of the follower. As described in Eqs. (2-4), the motion of the follower is a function of the rotary angle of the cam. The designers can use these kinds of equations to design cam-follower systems for this purpose. Fig. 3c shows the kinematic structure of the slider-crank mechanism. The slider-crank also converts rotational motion into linear motion [24,33,34]. The key components are the crank, connecting rod, and slider. One end of the crank is fixed by a pin and the other end is connected with a connecting rod. The connecting rod connects the crank and slider. The slider generates a reciprocal linear motion according to the rotation of the crank. As mentioned above, this kinematic analysis is based on the geometrical change of the crank and connecting rod. The displacement (s) of the slider can be described as [24,33]:

( )

{

[

√

(

) ]}

(5)

Here, R is the length of the crank, θ is the rotation angle of the crank, and L is the length of the connecting rod. Since the crank is experiencing rotational motion, the displacement of the slider is a function of the rotation angle of the crank. The displacement can be controlled by altering the length ratio of the crank and connecting rod, as shown in Eq. (5). This description is valid when the centers of the crank and slider are on the same axis. Fig. 3d shows a schematic image of a rectangular flywheel and its moment of inertia. The important aspect of a flywheel is its moment of inertia. The moment of inertia is also known as the rotational inertia and is a property of any object that is in rotation. This means that stopped or rotated objects tend to maintain their status, if possible. Based on this concept, flywheels are

utilized to store and stabilize rotational energy [24]. Flywheels have a symmetric shape and relatively large mass, which increase the moment of inertia. Furthermore, the rotation axis is through the center of a flywheel. The stored rotation energy increases proportionally with the square of the angular velocity, which can be expressed as [24]:

(6) Here, I is the moment of inertia, ω is the angular velocity, and E is the stored rotational energy in a flywheel. The moment of inertia of a flywheel increases proportionally as its mass (M) increases. Therefore, the stored rotational energy can be controlled by changing the mass and angular velocity. As mentioned above, the stored rotational energy is released constantly according to the moment of inertia of a flywheel over time. However, it is important to note that the required energy for starting rotation increases due to the rotational resistance increase caused by the moment of inertia of a flywheel. Therefore, designers can utilize kinematic analysis and components, but they should also consider changes in the operation requirements. These kinematic components can be used individually or systematically with other components. Based on the kinematic analysis and components, numerous kinematic systems have been invented and used for specific purposes such as a solar tracking system [35], shaft docking system [36], dual actuator unit [37], and mechanical control system for energy harvesting [38].

2.2 Vibration systems with impact non-linearity Fig. 4a illustrates a generic vibration spring-mass-damper system and its analysis using a free-body diagram [39]. Vibration energy harvester systems can have non-linear dynamics due to impact non-linearity. The illustrated free-body diagram of the vibrating mass represents this

impact state. Fig. 4b broadly summarizes the types and functions of vibration systems. The three primary types of vibration systems are based on helical coil springs, cantilever beam springs, and fixed-fixed beam springs [40]. Vibration systems have harmonic modes occurring at different frequencies, and these modes affect the dynamics of the structure. The figure illustrates one vibration mode shape for each of the three primary types of vibration systems [41-43]. Based on the application, vibration systems can be designed to achieve resonance control, operable frequency bandwidth control, or damping control. Fig. 5 shows the parametric dependence of the frequency response characteristics of a vibration system with impact non-linearity. Fig. 5a shows a lumped schematic model of a generic vibration system with top and bottom contacts. The middle movable mass, m, is held by a spring of stiffness, k0, and air damping constant, c0. The bottom and top contact materials are represented as having stiffness values k1 and k2 and damping constants c1 and c2, respectively. The separation gaps between the mass and contact materials are d1 and d2, respectively. The fixed support, to which the mass and contact materials are indirectly and directly attached, is under the influence of sinusoidal acceleration: y(t) = Ysint. The dynamic motion of the movable mass is described by z(t). Under a no-impact state (d1 > z > -d2), the movable mass is in free vibration; however, when the mass contacts the top material (z >= d1), the overall stiffness that restricts m becomes k0+k1 and damping increases to c0+c1. Similarly, when the mass makes contact with the bottom material (z <= -d2), the overall stiffness that restricts m becomes k0+k2 and damping increases to c0+c2. The dynamic behavior of mass m can be modeled as [44]:

̈

̈ (

̇ ) ̇

(

̈ ) ̈

(7) .

̈

(

) ̇

(

) ̈

.

Bhatia et al. [41] simulated the above model using MATLAB/Simulink for only bottom-side impact (and no top-side impact) to obtain the frequency response characteristics of a vibration system with impact non-linearity, as shown in Fig. 5b. The flattening of the frequency response due to non-linearity is referred to as stiffness hardening or stiffening behavior and results in widening of the operable frequency range or bandwidth [41,45-47]. The resonance and impact frequency equations for the movable mass are given as:

√ .,

√ .,

√ .

(8)

Bhatia et al. [41] studied the effects of changing the stiffness (k0) and mass (m) on the frequency response characteristics of a vibration system with impact, as shown in Fig. 5c and Fig. 5d, respectively. As shown in Eq. (8), increasing the mass shifts the frequency response to the left while increasing the stiffness shifts the frequency response to the right. Liu et al. [44] studied the effects of damping and the gap distance for the bottom-impact mode described in Eq. (7). If the damping ratios ξ0, ξ1, and ξ2 are defined as: (

)

.,

(

)

.,

(

)

.,

(9)

then the frequency bandwidth decreases for the bottom-impact case when the stopper damping ratio ξ2 is increased. This shows that a lower stopper damping ratio will lead to a wider frequency bandwidth. The influence of the gap distance on the frequency response was also studied by Liu et al. [44]. Reducing the gap distance resulted in broadening of the bandwidth at the expense of reducing the mass motion amplitude. In summary, the vibration under impact

non-linearity shows stiffness hardening or stiffening under impact, and the important design parameters for vibration TENGs are the mass, stiffness, damping, and gap distance. Mass design depends on the material density,  and volume, V: .

(10)

Additionally, the spring stiffness design depends on the type of spring, as discussed previously in Fig. 4. In the case of a helical coil spring, if G is the shear modulus of the material, d is the coil wire diameter, D is the mean coil diameter, and n is the number of active turns, then the coil stiffness is given as [40]: (11)

.

In the case of a cantilever beam spring, if E is the Young’s modulus of the material, l is the length of the beam, and I is the area moment of inertia of the beam cross section, then the beam stiffness is given as [40]: .

(12)

In the case of a fixed-fixed beam spring with a load at the middle, the beam stiffness is given as [40]: .

(13)

Damping is a material property that is associated with internal frictional heating, which depends on the size, shape, and dynamic stress distributions [48]. Additionally, frictional damping can be associated with the structure and design. The gap distance between the vibrating mass and stopper can depend on the length of the helical coil spring [41] but is freely adjustable for cantilever-beam springs and fixed-fixed beam springs.

Fig. 5e shows a design protocol flowchart for impact vibration TENGs, as described by Bhatia et al. [41]. Once the target frequency is determined, the TENG is designed to operate in resonance with the input. In order to accomplish this, practical values of the spring stiffness and mass are estimated based on Eq. (8). The gap distance is then estimated to provide the appropriate output level and a broad bandwidth. Then, the TENG material thicknesses are estimated. Based on these estimates, the spring is designed using stiffness Eq. (11), (12), or (13) and the mass is prepared based on Eq. (10). Finally, the device is assembled based on the specifications and tested. If the output is unsatisfactory, the design protocol is repeated from the appropriate step onwards. This design protocol is expanded by Bhatia et. al. [49] providing the choice of single-impact or double-impact vibration TENG design with the same target frequency.

3. Principles of TENGs 3.1 Working mechanisms of TENGs TENGs operate on the principles of contact electrification and charge induction. Fig. 6 illustrates the four fundamental modes of TENG operation: vertical contact-separation mode, lateral sliding mode, single-electrode mode, and freestanding triboelectric-layer mode [17]. Depending on the applications, the appropriate TENG mode design can be adopted. As shown in Fig. 6a, the vertical contact-separation mode is associated with the top-down motion of the moving TENG layer [50]. After static charges are induced on the dielectric surfaces by rubbing, pressing and releasing forces result in charge flow between the electrodes in opposite directions and through the external load. The lateral sliding mode is shown in Fig. 6b; here, the moving TENG layer moves sideways instead of from the top-down [51]. Once the top and bottom TENG

layers no longer overlap, due to motion towards the right, charge flow occurs between the electrodes and through the external load. Again, when motion occurs towards the left, charge flows in the opposite direction. However, this mode is not ideal in terms of its durability. In the case of materials with a high friction coefficient, material deterioration is quite rapid [52]. Furthermore, material coatings can easily deteriorate due to friction if they are not secured [53]. Fig. 6c shows the single-electrode mode, which can accommodate both top-down and sideways motion [54]. Here, there is a single dielectric with an electrode connected to a reference ground. The top layer is not externally connected to the bottom layers, and it is free to move in any direction. Charge flow only occurs between the electrode and reference ground, depending on the location of the top layer. Thus, this mode is ideal for touch-sensing applications or water energy harvesting [54,55]. Similarly, in the freestanding triboelectric-layer mode (as shown in Fig. 6d), the top layer is free to move in any direction [56]. Freestanding mode TENG operation can be contact or non-contact [57]. As shown in the figure, the motion of the dielectric layer determines how the charges flow between the two planar electrodes and through the external load. Additionally, electrodes need not be planar. They may adopt a vertical–facing arrangement, with a freely moving middle dielectric layer [58]. This TENG mode, with a planar electrode arrangement, is popular for high-frequency rotation energy conversion and flowing water and wind energy harvesting [59-61]. High-frequency energy conversion is also possible with the vertical-electrode arrangement [62]. Theoretical representation of TENGs that accounts for non-linear impact dynamics has been attempted by Ibrahim et al. [39]. They further developed Eq. (7) to estimate the voltage and power output of an electrostatic or triboelectric generator. If q(t) is the amount of charges transferred between two electrodes, S is the electrode area, r is the dielectric constant, and o is

the vacuum permittivity, then the electrostatic force under a no-impact state can be reduced to [39]: ( )

.

(14)

Eq. (14) was added to Eq. (7) for the no-impact state (d1 > z > -d2). If td is the thickness of the dielectric and is the surface charge density on the dielectric, then the fundamental V-Q-x relationship for TENGs (originally derived by Niu et al. [63]) is:

( )

( )

(

(

( )))

(

( ))

.

(15)

If R is defined as the load resistance connected to a TENG, Eq. (15) can be modified based on Ohms law as:

̇( )

( )

(

(

( )))

(

( ))

.

(16)

Ibrahim et al. [39] solved the modified vibration impact equation and Eq. (16) by using a “shooting method” to obtain periodic solutions for voltage and power frequency responses.

3.2 Mechanical variables of TENGs Fig. 7 shows the parameters related to TENG development. These variables can be arranged into four different categories: 1) material, 2) geometry, 3) environment, and 4) input source.

Since the triboelectric behavior is deeply related to surface charge transfer and

electrostatic induction, parameter control related to the material is the most important part of TENG development. Fig. 7a-i shows the effect of relative permittivity on TENG performance [64]. After contact electrification between tribo-pair materials, electrostatic induction progresses

in the TENG device. The degree of electrostatic induction is affected by the relative permittivity of the tribo-materials. The polarization increases with an increase in the relative permittivity, while also enhancing the tribo-capacitance. Therefore, the triboelectric performance can be enhanced by controlling the relative permittivity of the tribo-materials. Another important parameter is the surface charge density. Surface charges provide TENG devices with an electric field and determine the TENG performance. The TENG output performance proportionally increases with improvements in the surface charge density, as shown in Fig. 7a-ii [65]. The surface charge density can be modified by using plasma [66-68], corona discharge [69,70], and ionized gas injection [65]. The geometrical variables, such as surface morphology, material thickness, contact area, and gap distance, must be considered and determined to design TENG devices efficiently. Fig. 7b-i shows the effect of surface morphology on the output voltage [71]. The surface morphology can increase the surface area for triboelectrification. Therefore, the surface charge density on a non-planar surface can be increased relative to a flat surface and the output performance of TENGs can be enhanced. Increasing the contact area also increases the total amount of surface charge and enhances the TENG output performance, as shown in Fig. 7b-ii [72]. However, it is important to find the optimal contact area based on the systematic specifications, which are determined by the application. Additionally, the thickness of tribo-materials is related to the capacitive behavior of TENGs and should be considered during TENG development. As mentioned above, the capacitance of TENGs is important to enhance the output performance; this is related to the thickness of the tribo-materials. The capacitance proportionally increases with decreasing thickness. Therefore, the TENG performance can be enhanced by using a thin film, as shown in Fig. 7b-iii [73]. Fig. 7b-iv shows the relationship of the ratio between the

surface charge density and triboelectric charge density with gap distance [74]. An optimized gap distance is required to prevent interference from electric fields, which disturb triboelectric charge movement. The geometric parameters significantly influence TENG performance. Thus, the geometrical design should be considered during TENG development. Environmental parameters, such as humidity and temperature, affect the surface charge density of TENGs, as shown in Figs. 7c-i and 7c-ii. The humidity can disperse the surface charges on tribo-materials by forming a water layer, which is electrically conductive [75]. Thus, the surface charge decreases linearly with increasing relative humidity, as shown in Fig. 7c-i. The environment temperature also affects the surface charge state based on the electron thermionic emission [76]. At high temperatures, thermal energy excites the transferred electrons on tribo-materials. The excited electrons easily avoid the potential barrier. This means that it is difficult for tribo-materials to maintain a constant surface charge density. Therefore, the surface charge density drops with an increase in the environment temperature, as described in Fig. 7c-ii. Thus, researchers should consider the environment and prepare appropriate countermeasures, such as packaging. Mechanical parameters, such as the contact frequency and force, are related to the level of mechanical energy produced by the environment and affect TENG performance. The contact frequency influences the short-circuit current. The definition of the short-circuit current of a TENG is the variation rate of the charge [77]. Since the amount of charge is fixed in an ideal environment, the short-circuit current can be enhanced linearly by shortening the generation time (i.e., contact and separation), as shown in Fig. 7d-i [74]. A higher contact force can compensate for imperfect contacts across the contact area. As the contact force increases, therefore, the actual contact area increases and surface charges become uniformly distributed across the entire area.

Based on this surface charge uniformity, the output voltage and current can be improved, as shown in Fig. 7d-ii [41,78]. However, the mechanical energies are not constant in practical settings due to variations in the surroundings, such as the weather, population, and geometries. Therefore, the actual efficiency of TENG devices is not high enough, compared to ideal conditions. To overcome this mechanical limitation, TENG systems should be considered with mechanical subsystems. Mechanical subsystems can be designed through kinematic and vibrational analyses. As mentioned above, kinematic design can be used to modify the velocity, frequency, direction, and mechanical force. Vibrational design is based on the resonance behavior of a TENG device under a certain input frequency. Therefore, efficient mechanical energy harvesting is possible, and the application area can be expanded by utilizing kinematic- and resonance-based TENG design.

4. Kinematic Energy Conversion Systems for TENGs In this section, we review various kinematic energy conversion systems for TENGs that have been proposed by previous researchers. This review divides these systems into four different categories: TENGs based on 1) gear trains, 2) cam-followers, 3) trusses, and 4) spiral springs. Previously, kinematic systems were integrated with mass generation, such as power plants that utilize blue energy. This review focuses on the kinematic systems used in TENGs and shows the importance of mechanical systems for TENG development.

4.1 Gear train-integrated TENG systems The performance of rotational motion-based TENGs can be enhanced by increasing the angular velocity. As described above, the gear mechanism can modify the angular velocity, rotation direction, and motion. In particular, the angular velocity can be enhanced by changing the gear ratio of the gear train. To enhance the angular velocity of TENG devices, therefore, the input mechanical energy should be transferred from a large gear that accepts input mechanical energy to a small gear that is connected to a TENG device. Based on this basic concept, many gear train-integrated TENG systems have been presented, as shown in Fig. 8. Fig. 8a is a TENG system based on a simple spur gear train and slider-crank mechanism [79]. In this work, three different gear trains were utilized to compare the effect of angular velocity on TENG performance. As mentioned above, a large spur gear was connected to the input energy source and a small spur gear was connected to the crank. The slider-crank mechanism can convert rotational motion into linear reciprocal motion. Therefore, the TENG mechanism is a vertical contact-separation mode. The peak power density dramatically enhanced the gear ratio, by more than a factor of six, as shown in Fig. 8b. Both the current and generation rates were enhanced due to the shortened contact and separation times. Therefore, the TENG performance during the same period can be noticeably enhanced. The gear sets can be utilized in water-based TENGs, as presented in Fig. 8c [60]. This work also used a spur gear train to enhance the angular velocity. The designed gear ratio was two and the gears were connected in the manner proposed by Kim et al. Due to the chosen gear ratio and gear connection, the angular velocity of the TENG device was two times faster than the angular velocity of the handle. Fig. 8d shows the output performance results of a water–based rotating TENG system under a handle rotation speed of 150 rpm or 2.5 Hz. The measured rotation speed of the TENG device was about 5 Hz. The same

effect of the angular velocity was observed, indicating the positive effects of the gear-based TENG system. The gear train can have multiple types of gears, such as spur, rack, bevel, helical, and so on. When combining different gear types, the kinetic motion can be modified by the gear train. A motion–convertible, gear train-based TENG system was proposed by Tcho et al., as shown in Fig. 8e [80]. In this system, the linear mechanical motion can be changed to rotational motion. A rack gear was utilized to accept the linear energy. The input linear energy was transferred to the spur gear and then finally transformed into rotational energy by the bevel gear. Compared to TENGs based on the vertical contact-separation mode, the output power of the designed TENG system is enhanced by more than 153 times with the same input linear mechanical load, as shown in Fig. 8f. The gear mechanism-integrated TENG system can modify the angular velocity by altering the gear ratio and change the mechanical motion by using different gear types. Due to the angular velocity improvement, the generation rate is increased and the current is enhanced by shortening the contact-separation time.

4.2 TENG systems based on the cam-follower mechanism As mentioned above, mechanical motion conversion is the key point of the cam-follower system. The cam-follower mechanism can be utilized to make a TENG based on the vertical contact-separation mode with rotational energy, as shown in Fig. 9a [52]. As mentioned above, this basic system is composed of a cam and TENG device attached to a follower. The important point of cam-based TENGs (C-TENGs) is the design of the follower. A bumper spring is required to ensure clear contact and separation of the TENG device, as described in Fig. 9b. Without the bumper spring, the TENG device can become stuck during contact. However, the stiffness ratio of the spacer and bumper spring needs to be considered because the stiffness ratio

affects the contact-separation velocity of the follower. The resultant current equation, provided by Lee et al., is described in Eq. (17) [52].

( ) (

( ))

⁄

(17)

Here, S is the contact area, σ is the static tribo-charges of the tribo-surfaces caused by electrostatic and contact electrification, εr and d are the relative dielectric constant and thickness of the negative tribo-material, x(t) is the distance between the negative tribo-material and the bottom electrode, and r and ω are the distance between the cam shaft and the nose and the angular velocity. Furthermore, ks and kb are the stiffness values of the spacer and bumper spring. The cam system could apply to environments that have high-speed rotational motion. The tribo-materials in TENGs based on the conventional lateral sliding mode can be degraded by friction and wear. This material degradation can reduce the output performance of TENG devices, as shown in Fig. 9c. To increase the electricity generation, the number of noses and devices can be controlled, as shown in Fig. 9d. With multiple noses and followers, the generation cycle is stable and reliable. A practical application of a TENG system, using wind, was proposed by Dudem et al. (Fig. 9e) [81]. This TENG system can stably operate at relatively low wind speeds, from 3 to 4 m/s, as shown in Fig. 9f. The cam system acts as a mechanical motion converter and improves the lifetime of the TENG device by taking advantage of the vertical contact-separation mode. If the TENG system is required to operate in the vertical contact-separation mode with rotational energy, the cam system provides the proper kinematic components for this situation.

4.3 Truss structure-based energy harvesting system The link connection and its relative displacement can be utilized to produce a desired movement. In this design, the length and fixed position of links affect the relative movement. This link connection-based kinematic system can provide an effective structure for efficient mechanical energy utilization. Based on this concept, a truss structure–based, triboelectricpiezoelectric hybrid nanogenerator was proposed by Li et al., as shown in Fig. 10a [82]. Key components of this system include the 1) triboelectric patch, 2) proof mass, 3) linear spring, 4) truss structure, 5) piezoelectric patch, and 6) piezo plate holder. Linear mechanical energy (i.e., vibration) is induced at the bottom of the triboelectric patch. Due to the vertical displacement, the triboelectric patch and mass move in the vertical direction. When the mass compresses the links, the truss structure horizontally stretches and a tensile load forms in the piezoelectric patch. In the opposite case, a compressive load forms in the piezoelectric patch. Therefore, a bending load is induced in the piezoelectric patch during a single stroke. Vertical linear energy operates the TENG and horizontal linear energy operates the piezoelectric nanogenerator (PNG). Fig. 10b shows the output performance of the truss-based energy harvesting system. The experiment results were obtained with an amplitude of 2.87 mm and an excitation frequency of 11 Hz. As shown in the results, both PNGs and TENGs work properly with a synchronized working frequency. Fig. 10c shows another TENG application based on a rhombic grid (i.e., another truss structure) [83]. The principle kinematic mechanism is the same as the one in the kinematic system shown in Fig. 10a. Link-based TENG systems can be designed with multiple link connections for effective mechanical energy harvesting. Fig. 10d shows the effects of using a multiple-link structure. The enhancement is not linearly increased due to changes in the total

number of unit cells (Ntotal). Since two contact places exist in a unit cell, the total number of unit cells in one TENG can be expressed as [83]: (18) Here, n is the number of unit cells along the edge length. Thus, the number of contact sites dramatically increases as the number of unit cells increases. Furthermore, all unit cells operate simultaneously, and the generation cycle is almost matched. Therefore, the triboelectric output performance is noticeably improved, as shown in Fig. 10d. It is important to note that a designer should consider the proper way to induce input mechanical energy in the link system. Since the link system can be deformed by linear energy, the input transfer module should deliver or produce enough linear energy to generate displacement. The link system can be applied when the TENG system requires a specific mechanical load

4.4 Spiral spring-based TENG system Mechanical energy irregularities exist due to differences in the environment. Because of these irregularities, mechanical energy harvesters produce irregular electrical output. To constantly provide power or regularly use electricity from TENGs, the mechanical energy should regularly excite mechanical energy harvesters. To overcome this problem, a spiral spring-based TENG system, which is called a mechanical frequency regulator TENG (MFR-TENG), was proposed by Bhatia et al. (Fig. 11a) [84]. The key elements of this system are a pawl ratchet, mainspring (i.e., spiral spring), stopper, gear train, and flywheel. Input mechanical energy turns the handle and winds up the mainspring. When input mechanical energy is irregularly induced or

disappears, the mainspring can release the stored mechanical energy. To prevent this situation, the pawl-ratchet mechanism was installed between the handle and mainspring. The pawl-ratchet mechanism limits the rotation direction and prevents relaxation of the mainspring, as shown in Fig. 11b. The stored mechanical energy can be released by removing the stopper. After removing the stopper, the mainspring releases the stored mechanical energy. The stored mechanical energy rotates the gear train, cam, and flywheel attached to the shaft, as shown in Fig. 11c. The flywheel can control the mainspring relaxation time by changing the torque. Since the follower (i.e., the TENG device) is a vibration system composed of a mass and a spring, the effective rotation conditions should be considered when exciting the spring in the follower. To determine these effective rotation conditions, the torque balance of components in the MFRTENG should be considered. The angular velocity (ω) will change based on the mechanical components, which can be described as [84]:

√

(

∫

∫

)

(19)

The angular velocity can increase as the mainspring torque (Tmainspring) increases, but it decreases as the follower torque (Tfollower), gear ratio number (Ng), and total moment of inertia of the kinematic components (Itotal) increase. The TENG output frequency (fTENG) can be written as [84]:

(20) The TENG output frequency will proportionally increase as the number of cam noses (Nc) and angular velocity increase. Therefore, the output frequency can be controlled by adjusting the mainspring torque, gear ratio, flywheel mass, flywheel dimensions, and number of cam noses.

The operation time (Tr) is a function of the total number of spiral spring windings (θr), and the frequency of the rotating cam shaft is defined as [84]:

(21) It is interesting to note that the output frequency and operation time can be controlled by changing the kinematic components in the frequency regulator, as shown in Fig. 11d. The controllable frequency range is especially wide, going from 10 Hz to over 50 Hz. Frequency matching is important when a user wants to utilize a commercialized electric component, such as industrial transformers. Commercialized transformers require a specific AC frequency range from 50 to 60 Hz. As shown in Fig. 11e, when the AC frequency is not satisfied, the transformer cannot perform efficiently. In the opposite case, the efficiency of the transformer increases and the current is enhanced by more than a factor of five. Other types of spiral spring based TENG designs have also been proposed in the literature, where one TENG layer is attached to the fixed cylindrical housing and the other TENG layer is rolled up in the shape of the spiral spring and pulled in order to initiate interaction between the two TENG layers and generate electricity [8587]. A TENG system based on multiple kinematic mechanisms can operate reliably and overcome limitations imposed by the environment. However, designers and/or manufacturers should consider the mechanical interactions between kinematic systems to properly operate various TENG systems in different application areas.

5. Vibrational Energy Conversion Systems for TENGs In this section, various TENG vibration system designs found in the literature are reviewed. As mentioned previously, most vibration system designs are based on helical coil springs, cantilever beam springs, or fixed-fixed beam springs. Although other novel vibration TENG designs have been reported in the literature, such as ball-based structures [88,89], sandwiched elastic wavy structures [90,91], and elastic multi-unit TENG structures [92], this review only focuses on the three primary vibration structures (described in Fig. 4b) and the effect of design parameters on the TENG frequency response outputs.

5.1 Helical coil spring-based TENG systems Fig. 12a shows the voltage output frequency response of a typical vibration TENG where the movable mass is supported by four helical coil springs (inset schematic) [93]. The figure also illustrates a method for determining the working bandwidth of the TENG by measuring the frequency range at half maximum open-circuit voltage. This is also referred to as full-width at half-maximum (FWHM) [41]. The stiffness of each of the four helical coil springs was 112 N/m and the moving mass (m) value was 56.8 grams, which corresponded to a theoretical resonance frequency of 14.1 Hz based on Eq. (8), close enough to the experimentally observed resonance at 14.5 Hz. The working bandwidth was measured to be 13.4 Hz at an input acceleration of (1/50) G m/s2. Fig. 12b shows a method for experimentally determining the damping of the vibration TENG. Considering the time response output at 16 Hz, if the output voltage decays from V at time t1 to V/2 at time t2, then T1/2 is t2-t1, and the damping coefficient could be estimated by 2mln(2)/T1/2 as shown in the figure. By this method, the damping coefficient of the vibration

TENG was experimentally estimated by the authors to be 0.34. Estimating the damping coefficient is necessary for vibration TENG simulation and optimization. Fig. 12c and Fig. 12d show a multi-directional implementation of a helical coil springbased vibration TENG [94]. In the out-of-plane motion, the TENG operates in the verticalcontact mode; alternatively, in the in-plane motion, the TENG operates in the lateral sliding mode. The moving mass value was reported to be 25 grams, and the resonance frequency was reported to be 36 Hz. This suggests that, based on Eq. (8), the equivalent stiffness of the vibration system was 1279 N/m. However, since the lateral or oblique stiffness of a helical coil spring can differ from its translational stiffness, the stiffness per spring cannot be assumed from this value. Experiments were conducted at an input acceleration of 6 m/s2. The frequency response output for out-of-plane motion in Fig. 12c shows a broad FWHM bandwidth of 75 Hz due to the impact non-linearity. The frequency response output for in-plane motion in Fig. 12d shows a narrower FWHM bandwidth of 13 Hz due to the impact non-linearity. Although the inplane sliding has friction-based non-linearity, there is no impact non-linearity in the motion dynamics; thus, the frequency response showed a narrower bandwidth relative to the out-ofplane motion. Fig. 12e shows a proof of concept of a tandem TENG system; here, four individual vibration TENGs (VTENGs) were designed explicitly to operate at a specific resonant frequency and then assembled in a vertical-stack arrangement [41]. Each VTENG was designed by following the design protocol described in Fig. 5g. The selected target frequencies were 20 Hz, 27 Hz, 34 Hz, and 40 Hz. The mass value for all VTENGs was restricted to 6.5 grams. Then, according to Eq. (8), the required equivalent stiffness values were determined to be 100 N/m, 190 N/m, 295 N/m, and 410 N/m, respectively. Four equivalent helical coil springs were used in

each VTENG to support the moving mass. Thus, the required stiffness values per spring were 25 N/m, 47.5 N/m, 73.75 N/m, and 102.5 N/m for each VTENG, respectively. A gap distance of 1 mm was maintained between the tribo-active layers, and frequency response experiments were conducted at an input acceleration of 0.5 G m/s2. Fig. 12f shows the voltage output results for the strategically designed tandem TENG. The results showed that each VTENG operated at the desired target frequency with a broad enough bandwidth to make the complete system reliably operable within the frequency range from 20 Hz to 40 Hz. Such a tandem TENG system can accommodate irregularities in the vibration input source frequencies and can also be used under multiple vibration input sources.

5.2 Cantilever beam spring-based TENG systems Fig. 13a shows a cantilever beam spring-based vibration TENG with double impact in its motion dynamics [95]. The main bodies of the cantilevers were made out of beryllium-copper alloy foil. The middle cantilever beam had a suspended mass of 12.29 grams. The length of the beam was 65 mm, and its cross-sectional dimensions were 28 mm x 0.2 mm. The resonance frequency of the cantilever beam was experimentally determined to be 3.7 Hz. The authors studied the effect of changing the values of the tip mass and length of the middle cantilever beam on its resonance frequency, as shown in Fig. 13b. With the beam length fixed at 65 mm, decreasing the tip mass from 15 grams to 12.9 grams to 10 grams slightly increased the resonance frequency. Additionally, with the tip mass fixed at 12.9 grams, decreasing the beam length from 75 mm to 65 mm to 55 mm significantly increased the resonance frequency. As demonstrated by Eq. (12), decreasing the beam length significantly increased the cantilever spring stiffness. This, in turn, increased the resonance frequency based on Eq. (8). Furthermore,

from Eq. (8), decreasing the mass should also increase the resonance frequency. Thus, the experimental results are consistent with the vibration TENG design theory laid out in Section 2. Han et al. [96] fabricated three cantilevers with different resonant frequencies, as shown in Fig. 13c. However, only the longest of the cantilevers was equipped with a TENG. The material used for the cantilevers was the piezoelectric polyvinylidene fluoride (PVDF), which has a low Young’s modulus of 2500 MPa. A tip mass of 0.0225 grams was used in all three cantilevers. Once again, based on Eqs. (8) and (12), the cantilever beam resonant frequency is proportional to l-3/2, where l is the cantilever beam length. Based on this relationship, the three cantilevers were designed to operate at 15 Hz, 30 Hz, and 45 Hz. The cross-sectional areas of all three beams were maintained at 5 mm x 100 m, while their required lengths were calculated to be 25 mm, 16 mm, and 12 mm, respectively. The authors conducted frequency response experiments to verify their design. The frequencies at which the piezoelectric output voltages were highest corresponded to the target design frequencies. Experiments were conducted at an input acceleration of 1 G m/s2. Fig. 13d shows the TENG voltage output at 15 Hz, which corresponds to the resonant frequency of the longest cantilever beam. Fig. 13e shows an interacting dual resonant cantilever beam structure [47]. When one cantilever beam is vibrating in resonance, it collides with the other cantilever beam’s tip mass, resulting in strong mechanical coupling between the vibrating cantilevers. Strategic design of these two cantilevers can greatly broaden the overall bandwidth of the vibration system. The TENG dielectric surface was corona charged to a voltage of -950 V to form an electret. Both cantilever beams had a length of 5.0 cm and a cross-sectional area of 1.5 cm x 0.2 cm. The tip mass was used to tune the resonant frequencies of the cantilever beams based on Eq. (8). The tip masses of the bottom and top beams were 1.14 grams and 0.83 grams, respectively. Thus, the

resonant frequencies of the two cantilever beams were 37 Hz and 45 Hz. A gap distance of 0.2 cm was maintained between the two cantilever beams. Fig. 13f shows the frequency response results for the dual resonant cantilever beam structure. A frequency sweep was conducted at input accelerations of 1.3 m/s2 to 9.3 m/s2. At an input acceleration of 2.0 m/s2, the frequency response showed a stiffening phenomenon due to impact, but overlap between the frequency responses of the two cantilever beams was only observed at input accelerations of 5.3 m/s2 and above. The dual resonant cantilever beam structure is another version of a tandem TENG system.

5.3 Fixed-fixed beam spring-based TENG systems Fig. 14a shows a vibration TENG based on a fixed-fixed beam spring [97]. The paper layer has holes punched in it, which allows free motion of air through the cavities. The incident acoustic waves interact with the polytetrafluoroethylene (PTFE) fixed-fixed beam, inducing vibrational motion. The PTFE beam has a Young’s modulus of 440 MPa and a thickness of 0.025 mm with a 100-nm-thick copper coating. The simulated resonant modes of the 0.025-mmthick vibrating PTFE beam occur at 75 Hz, 169 Hz, and 299 Hz, as shown in the figure. However, based on experimental results, the resonant modes were reported to occur in multiples of ~80 Hz. Fig. 14b shows that, by varying the thickness of the paper layer and the corresponding hole thickness, the bandwidth of the output frequency response can be changed. Under a constant sound pressure of 120 dB and at an optimal thickness of 2.4 mm, the resonant behavior was shown to be the sharpest; this occurred at a frequency close to 240 Hz. Alternatively, at a thickness of 0.25 mm, the frequency response was flatter with a higher bandwidth due to the activation of multiple resonant modes. Furthermore, there is a tradeoff between the output level and bandwidth, i.e., the narrower the response, the higher the output level (and vice versa).

Fig. 14c shows a TENG based on a frequency–selective, fixed-fixed beam structure [98]. The shape of the vibrating PTFE film was trapezoidal, which implies that the length of the fixedfixed beams gradually changed from 30 mm (at the bottom) to 10 mm (at the top). This changes the stiffness of the beam according to Eq. (13), which also influences the resonant frequency at that point. The thickness of the vibrating PTFE beam was 0.02 mm. The distance between each adjacent electrode was 10 mm, with electrode number one at the bottom (higher length) and electrode number nine at the top (smaller length). Fig. 14d shows the relationship between the electrode point and the beam resonance frequency. Based on other data in the manuscript, it was estimated that the experiments were conducted under an input pressure of ~80 dB. At electrode number two (inset), the experimental TENG output frequency response shows a maximum peak at 200 Hz. Alternatively, at electrode number eight (inset), the maximum peak occurs at 1850 Hz. Thus, as the beam length decreases, the resonant frequency increases due to an increase in the beam stiffness. This type of frequency–selective, beam structure-based TENG can be used in bionic cochlear basilar membrane sensors. Yang et al. [99] proposed an acoustic energy harvesting TENG with a fixed-fixed beam structure, as shown in Fig. 14e. The PTFE film incident to the acoustic pressure had a prestressed, curved shape. Similar to the work described in Fig. 14a and Fig. 14b, the electrode layer (a flat aluminum layer in this case) contains punched holes to enable airflow. In order to demonstrate the application of their sensor as a self-powered microphone, the authors used a design approach to obtain a broadened working bandwidth, as shown in Fig. 14f. The dimensions of the TENGs were 11 mm x 11 mm x 11 mm, 9 mm x 9 mm x 9 mm, 7 mm x 7 mm x 7 mm, and 5 mm x 5 mm x 5 mm. This was done to obtain resonance frequencies of 350 Hz, 650 Hz, 110 Hz, and 1400 Hz, respectively. A larger structure (i.e., a larger length) results in a

lower stiffness (based on Eq. (13)), which produces a lower resonance frequency (based on Eq. (8)). The total working bandwidth of the tandem TENG design ranged from 10 Hz to 1700 Hz and showed sufficient overlap in the frequency responses from individual TENGs.

6. Perspectives and Conclusions Since their invention in 2012, TENGs have been widely investigated as micropower systems, self-powered sensors, and security monitoring systems due to the high degree of freedom for material selection, large number of operating sources, high power-to-weight ratio, and various device structures. Recently, they were further applied to human-adaptive, selfpowered IoT electronics for the fourth industrial revolution and even macro-scale power sources by using abundant tide and ocean waves (i.e., blue energy). Further, we can expand the applications of TENGs to new markets, such as personal/industrial/military drones, where extra energy is necessary to augment the usable time of batteries. So far, many researchers have focused on new material designs, surface treatments, and geometrical effects to improve TENGs. Although these studies are very important, improving mechanical system designs for irregular input energy conversion may offer the best chance to achieve commercialization of TENGs. Hence, this review systematically explored kinematic and vibrational energy conversion systems for TENGs from their fundamental theory to their different use cases. In the case of kinematic energy conversion systems, the mechanical components could be categorized as (i) gear-train integrated TENGs, (ii) cam-follower integrated TENGs, (iii) truss structure integrated TENGs, and (iv) spiral spring based TENG systems. In the case of vibrational energy conversion systems, the different structures were categorized by (i) helical coil spring based TENGs, (ii)

cantilever beam spring based TENGs, and (iii) fixed-fixed beam spring based TENGs. The references that were detailed were strategically selected in order to understand solutions to the different design issues depending on the use case. The complete energy harvesting system consists of the input energy source, the MECS integrated with the TENG and the load circuit to be powered. The mechanical energies in our environment are irregular in input behavior, while the load circuit to be powered requires regulated and predictable output. By employing MECS based on kinematics and vibrational designs, we can improve the energy conversion performance of the TENG as well as obtain controlled output from it. Although the total system size might increase and there could be reduction in efficiency, in order to avoid significant losses in scavenged power at the load circuit stage, the use of MECS becomes unavoidable. Thus, further research in the optimization of these systems and their integration with the TENG can lead to practical realization of TENG based complete energy harvesting solutions. It is our hope that this review enlightens the reader on the importance of optimized MECS design to improve and control the output from TENGs. Furthermore, based on the fundamental functions of MECS, other different types of mechanical energy harvesters besides the TENG, such as PENGs and EMGs could utilize such MECS to produce high-quality optimized output power.

Acknowledgments W. Kim and D. Bhatia equally contributed to this work. This work was supported by a grant from the Kyung Hee University in 2016 (20160600).

References [1]

B. Tian, T.J. Kempa, C.M. Lieber, Single nanowire photovoltaics, Chem. Soc. Rev. 38 (2009) 16-24.

[2]

J. Chen, Y. Huang, N. Zhang, H. Zou, R. Liu, C. Tao, X. Fan, Z.L. Wang, Micro-cable structured textile for simultaneously harvesting solar and mechanical energy, Nat. Energy 1 (2016) 16138.

[3]

N. Zhang, J. Chen, Y. Huang, W. Guo, J. Yang, J. Du, X. Fan, C. Tao, A wearable all‐ solid photovoltaic textile, Adv. Mater. 28 (2016) 263-269.

[4]

X. Niu, J. Yu, S. Wang, Experimental study on low-temperature waste heat thermoelectric generator, J. Power Sources 188 (2009) 621-626.

[5]

Y. Zi, L. Lin, J. Wang, S. Wang, J. Chen, X. Fan, P.K. Yang, F. Yi, Z.L. Wang, Triboelectric–pyroelectric–piezoelectric hybrid cell for high‐efficiency energy‐ harvesting and self‐powered sensing, Adv. Mater. 27 (2015) 2340-2347.

[6]

Y. Yang, H. Zhang, Z.H. Lin, Y. Liu, J. Chen, Z. Lin, Y.S. Zhou, C.P. Wong, Z.L. Wang, A hybrid energy cell for self-powered water splitting, Energy Environ. Sci. 6 (2013) 2429-2434.

[7]

B. Zhang, J. Chen, L. Jin, W. Deng, L. Zhang, H. Zhang, M. Zhu, W. Yang, Z.L. Wang, Rotating-disk-based hybridized electromagnetic–triboelectric nanogenerator for sustainably powering wireless traffic volume sensors, ACS Nano 10 (2016) 62416247.

[8]

L. Zhang, B. Zhang, J. Chen, L. Jin, W. Deng, J. Tang, H. Zhang, H. Pan, M. Zhu, W. Yang, Z.L. Wang, Lawn structured triboelectric nanogenerators for scavenging sweeping wind energy on rooftops, Adv. Mater. 28 (2016) 1650-1656.

[9]

Y. Yang, G. Zhu, H. Zhang, J. Chen, X. Zhong, Z.H. Lin, Y. Su, P. Bai, X. Wen, Z.L. Wang, Triboelectric nanogenerator for harvesting wind energy and as self-powered wind vector sensor system, ACS Nano 7 (2013) 9461-9468.

[10] J. Chen, J. Yang, H. Guo, Z. Li, L. Zheng, Y. Su, Z. Wen, X. Fan, Z.L. Wang, Automatic mode transition enabled robust triboelectric nanogenerators, ACS Nano 9 (2015) 12334-12343. [11] K. Schwab, The Fourth Industrial Revolution, Penguin Books Limited, 2017.

[12] S. Priya, Advances in energy harvesting using low profile piezoelectric transducers, J. Electroceramics 19 (2007) 167-184. [13] Y. Lu, E. O’Riordan, F. Cottone, S. Boisseau, D. Galayko, E. Blokhina, F. Marty, P. Basset, A batch-fabricated electret-biased wideband MEMS vibration energy harvester with frequency-up conversion behavior powering a UHF wireless sensor node, J. Micromech. Microeng. 26 (2016) 124004. [14] S. Kim, S.J. Choi, K. Zhao, H. Yang, G. Gobbi, S. Zhang, J. Li, Electrochemically driven mechanical energy harvesting, Nat. Commun. 7 (2016) 10146. [15] L. Wang, F.G. Yuan, Vibration energy harvesting by magnetostrictive material, Smart Mater. Struct. 17 (2008) 045009. [16] P.D. Mitcheson, E.M. Yeatman, G.K. Rao, A.S. Holmes, T.C. Green, Energy harvesting from human and machine motion for wireless electronic devices, Proc. IEEE 96 (2008) 1457-1486. [17] Z.L. Wang, L. Lin, J. Chen, S. Niu, Y. Zi, Triboelectric Nanogenerators, Springer, 2016. [18] Y. Lee, S.H. Cha, Y.W. Kim, D. Choi, J.Y. Sun, Transparent and attachable ionic communicators based on self-cleanable triboelectric nanogenerators, Nat. Commun. 9 (2018) 1804. [19] S.W. Chen, X. Cao, N. Wang, L. Ma, H.R. Zhu, M. Willander, Y. Jie, Z.L. Wang, An ultrathin flexible single‐electrode triboelectric‐nanogenerator for mechanical energy harvesting and instantaneous force sensing, Adv. Energy Mater. 7 (2017) 1601255. [20] G. Khandelwal, A. Chandrasekhar, N.R. Alluri, V. Vivekananthan, N.P.M.J. Raj, S.J. Kim, Trash to energy: A facile, robust and cheap approach for mitigating environment pollutant using household triboelectric nanogenerator, Appl. Energy 219 (2018) 338349. [21] W. Xu, L.B. Huang, M.C. Wong, L. Chen, G. Bai, J. Hao, Environmentally friendly hydrogel‐based triboelectric nanogenerators for versatile energy harvesting and self‐ powered sensors, Adv. Energy Mater. 7 (2017) 1601529. [22] S. Roundy, P.K. Wright, J. Rabaey, A study of low level vibrations as a power source for wireless sensor nodes, Comput. Commun. 26 (2003) 1131-1144.

[23] Z.L. Wang, J. Chen, L. Lin, Progress in triboelectric nanogenerators as a new energy technology and self-powered sensors, Energy Environ. Sci. 8 (2015) 2250-2282. [24] R.L. Norton, Design of Machinery, second ed., McGraw-Hill, New York, 1992. [25] E.T. Whittaker, A Treatise on The Analytical Dynamics of Particles & Rigid Bodies, fourth ed., Cambridge University Press, New York, 1947. [26] Z.L. Wang, T. Jiang, L. Xu, Toward the blue energy dream by triboelectric nanogenerator networks, Nano Energy 39 (2017) 9-23. [27] J. Lee, B. Choi, Development of a piezoelectric energy harvesting system for implementing wireless sensors on the tires, Energy Convers. Manag. 78 (2014) 32-38. [28] L. Jin, W. Deng, Y. Su, Z. Xu, H. Meng, B. Wang, H. Zhang, B. Zhang, L. Zhang, X. Xiao, M. Zhu, W. Yang, Self-powered wireless smart sensor based on maglev porous nanogenerator for train monitoring system, Nano energy 38 (2017) 185-192. [29] S.Y.T. Lang, Graph-theoretic modelling of epicyclic gear systems, Mech. Mach. Theory 40 (2005) 511-529. [30] T.L. Dresner, P. Barkan, New methods for the dynamic analysis of flexible singleinput and multi-input cam-follower system, J. Mech. Des. 117 (1995) 150-155. [31] N. Sateesh, C.S.P. Rao, T.A.J. Reddy, Optimisation of cam-follower motion using Bsplines, Int. J. Comput. Integ. M. 22 (2009) 515-523. [32] H. Qiu, C.-J. Lin, Z.-Y. Li, H. Ozaki, J. Wang, Y. Yue, A universal optimal approach to cam curve design and its applications, Mech. Mach. Theory 40 (2005) 669-692. [33] F. Ahmad, A.L. Hitam, K. Hudha, H. Jamaluddin, Position tracking of slider crank mechanism using PID controller optimized by Ziegler Nichol’s method, J. Mech. Eng. Technol. 3 (2011) 27-41. [34] J.-L. Ha, R.-F. Fung, K.-Y. Chen, S.-C. Hsien, Dynamic modeling and identification of a slider-crank mechanism, J. Sound Vib. 289 (2006) 1019-1044. [35] M. Neagoe, M.M. Vatasescu, R.G. Saulescu, N. Creanga, On new high performance systems with linear actuators for diurnal orientation of PV platforms, Appl. Mech. Mater. 162 (2012) 214-223. [36] P.M. Moubarak, P. Ben-Tzvi, On the dual-rod slider rocker mechanism and its applications to tristate rigid active docking, J. Mech. Robot. 5 (2013) 011010.

[37] B.-S. Kim, J.-B. Song, J.-J. Park, A serial-type dual actuator unit with planetary gear train: basic design and applications, IEEE-ASME T. Mech. 15 (2010) 108-116. [38] X. Zhang, Z. Zhang, H. Pan, W. Salman, Y. Yuan, Y. Liu, A portable high-efficiency electromagnetic energy harvesting system using supercapacitors for renewable energy applications in railrods, Energ. Convers. Manage. 118 (2016) 287-294. [39] A. Ibrahim, A. Ramani, S. Towfighian, Experimental and theoretical investigation of an impact vibration harvester with triboelectric transduction, J. Sound Vib. 416 (2018) 111-124. [40] S.S. Rao, Mechanical vibrations, Pearson Prentice Hall, 2004. [41] D. Bhatia, W. Kim, S. Lee, S.W. Kim, D. Choi, Tandem triboelectric nanogenerators for optimally scavenging mechanical energy with broadband vibration frequencies, Nano Energy 33 (2017) 515-521. [42] S. Wang, X. Mu, Y. Yang, C. Sun, A.Y. Gu, Z.L. Wang, Flow‐driven triboelectric generator for directly powering a wireless sensor node, Adv. Mater. 27 (2015) 240-248. [43] S. Wang, X. Mu, X. Wang, A.Y. Gu, Z.L. Wang, Y. Yang, Elasto-aerodynamicsdriven triboelectric nanogenerator for scavenging air-flow energy, ACS Nano 9 (2015) 9554-9563. [44] H. Liu, C. Lee, T. Kobayashi, C.J. Tay, C. Quan, Investigation of a MEMS piezoelectric

energy

harvester

system

with

a

frequency-widened-bandwidth

mechanism introduced by mechanical stoppers, Smart Mater. Struct. 21 (2012) 035005. [45] H.T. Li, Z. Yang, J. Zu, W.Y. Qin, Distributed parameter model and experimental validation of a compressive-mode energy harvester under harmonic excitations, AIP Adv. 6 (2016) 085310. [46] K. Suhaimi, R. Ramlan, A. Putra, A combined softening and hardening mechanism for low frequency human motion energy harvesting application, Adv. Acoust. Vib. 2014 (2014) 217032. [47] Y. Zhang, T. Wang, A. Zhang, Z. Peng, D. Luo, R. Chen, F. Wang, Electrostatic energy harvesting device with dual resonant structure for wideband random vibration sources at low frequency, Rev. Sci. Instrum. 87 (2016) 125001. [48] C.W. Bert, Material damping: An introductory review of mathematic measures and experimental technique, J. Sound Vib. 29 (1973) 129-153.

[49] D. Bhatia, D. Choi, Vibration analysis and design methodology for single-impact and double-impact triboelectric generators, IET Conference Proceedings (2015) 4-4. [50] G. Zhu, C Pan, W. Guo, C.Y. Chen, Y. Zhou, R. Yu, Z.L. Wang, Triboelectricgenerator-driven pulse electrodeposition for micropatterning, Nano Lett. 12 (2012) 4960-4965. [51] G. Zhu, J. Chen, Y. Liu, P. Bai, Y.S. Zhou, Q. Jing, C. Pan, Z.L. Wang, Linear-Grating Triboelectric Generator Based on Sliding Electrification, Nano Lett. 13 (2013) 22822289. [52] Y. Lee, W. Kim, D. Bhatia, H.J. Hwang, S. Lee, D. Choi, Cam-based sustainable triboelectric nanogenerators with a resolution-free 3D-printed system, Nano Energy 38 (2017) 326-334. [53] K.N. Kim, Y.K. Jung, J. Chun, B.U. Ye, M. Gu, E. Seo, S. Kim, S.W. Kim, B.S. Kim, J.M. Baik, Surface dipole enhanced instantaneous charge pair generation in triboelectric nanogenerator, Nano Energy 26 (2016) 360-370. [54] Y. Yang, H. Zhang, Z.H. Lin, Y.S. Zhou, Q. Jing, Y. Su, J. Yang, J. Chen, C. Hu, Z.L. Wang, Human skin based triboelectric nanogenerators for harvesting biomechanical energy and as self-powered active tactile sensor system, ACS Nano 7 (2013) 92139222. [55] S. Lee, J. Chung, D.Y. Kim, J.Y. Jung, S.H. Lee, S. Lee, Cylindrical water triboelectric nanogenerator via controlling geometrical shape of anodized aluminum for enhanced electrostatic induction, ACS Appl. Mater. Interfaces 8 (2016) 25014-25018. [56] L. Lin, S. Wang, S. Niu, C. Liu, Y. Xie, Z.L. Wang, Noncontact free-rotating disk triboelectric nanogenerator as a sustainable energy harvester and self-powered mechanical sensor, ACS Appl. Mater. Interfaces 6 (2014) 3031-3038. [57] S. Wang, Y. Xie, S. Niu, L. Lin, Z.L. Wang, Freestanding triboelectric‐layer‐based nanogenerators for harvesting energy from a moving object or human motion in contact and non‐contact modes, Adv. Mater. 26 (2014) 2818-2824. [58] S. Wang, S. Niu, J. Yang, L. Lin, Z.L. Wang, Quantitative measurements of vibration amplitude using a contact-mode freestanding triboelectric nanogenerator, ACS Nano 8 (2014) 12004-12013.

[59] G. Zhu, J. Chen, T. Zhang, Q. Jing, Z.L. Wang, Radial-arrayed rotary electrification for high performance triboelectric generator, Nat. Commun. 5 (2014) 3426. [60] T. Kim, J. Chung, D.Y. Kim, J.H. Moon, S. Lee, M. Cho, S.H. Lee, S. Lee, Design and optimization of rotating triboelectric nanogenerator by water electrification and inertia, Nano Energy 27 (2016) 340-351. [61] H. Yong, J. Chung, D. Choi, D. Jung, M. Cho, S. Lee, Highly reliable wind-rolling triboelectric nanogenerator operating in a wide wind speed range, Sci. Rep. 6 (2016) 33977. [62] K. Zhao, Z.L. Wang, Y. Yang, Self-powered wireless smart sensor node enabled by an ultrastable,

highly

efficient,

and

superhydrophobic-surface-based

triboelectric

nanogenerator, ACS Nano 10 (2016) 9044-9052. [63] S. Niu, S. Wang, L. Lin, Y. Liu, Y.S. Zhou, Y. Hu, Z.L. Wang, Theoretical study of contact-mode triboelectric nanogenerators as an effective power source, Energy Environ. Sci. 6 (2013) 3576-3583. [64] J. Chen, H. Guo, X. He, G. Liu, Y. Xi, H. Shi, C. Hu, Enhancing performance of triboelectric nanogenerator by filling high dielectric nanoparticles into sponge PDMS film, ACS Appl. Mater. Interfaces 8 (2016) 736-744. [65] S. Wang, Y. Xie, S. Niu, L. Lin, C. Liu, Y.S. Zhou, Z.L. Wang, Maximum surface charge density for triboelectric nanogenerators achieved by ionized-air injection: methodology and theoretical understanding, Adv. Mater. 26 (2014) 6720-6728. [66] X.-S. Zhang, M.-D. Han, R.-X. Wang, B. Meng, F.-Y. Zhu, X.-M. Sun, W. Hu, W. Wang, Z.-H. Li, H.-X. Zhang, High-performance triboelectric nanogenerator with enhanced energy density based on single-step fluorocarbon plasma treatment, Nano Energy 4 (2014) 123-131. [67] X. Cheng, B. Meng, X. Chen, M. Han, H. Chen, Z. Su, M. Shi, H. Zhang, Single-step fluorocarbon plasma treatment-induced wrinkle structure for high-performance triboelectric nanogenerator, Small 12 (2016) 229-236. [68] H.Y. Li, L. Su, S.Y. Kuang, C.F. Pan, G. Zhu, Z.L. Wang, Significant enhancement of triboelectric charge density by fluorinated surface modification in nanoscale for converting mechanical energy, Adv. Funct. Mater. 25 (2015) 5691-5697.

[69] J.J. Shao, W. Tang, T. Jiang, X.Y. Chen, L. Xu, B.D. Chen, T. Zhou, C.R. Deng, Z.L. Wang, A multi-dielectric-layered triboelectric nanogenerator as energized by corona discharge, Nanoscale 9 (2017) 9668-9675. [70] G. Cheng, H. Zheng, F. Yang, L. Zhao, M. Zheng, J. Yang, H. Qin, Z. Du, Z.L. Wang, Managing and maximizing the output power of a triboelectric nanogenerator by controlled tip-electrode air-discharging and application for UV sensing, Nano Energy 44 (2018) 208-216. [71] F.-R. Fan, L. Lin, G. Zhu, W. Wu, R. Zhang, Z.L. Wang, Transparent triboelectric nanogenerators and self-powered pressure sensors based on micropatterned plastic films, Nano Lett. 12 (2012) 3109-3114. [72] F. Xing, Y. Jie, X. Cao, T. Li, N. Wang, Natural triboelectric nanogenerator based on soles for harvesting low-frequency walking energy, Nano Energy 42 (2017) 138-142. [73] Y. Feng, Y. Zheng, G. Zhang, D. Wang, F. Zhou, W. Liu, A new protocol toward high output TENG with polyimide as charge storage layer, Nano Energy 38 (2017) 467-476. [74] S. Wang, L. Lin, Z.L. Wang, Nanoscale triboelectric-effect-enabled energy conversion for sustainably powering portable electronics, Nano Lett. 12 (2012) 6339-6346. [75] V. Nguyen, R. Yang, Effect of humidity and pressure on the triboelectric nanogenerator, Nano Energy 2 (2013) 604-608. [76] C. Xu, Y. Zi, A.C. Wang, H. Zou, Y. Dai, X. He, P. Wang, Y.-C. Wang, P. Feng, D. Li, Z.L. Wang, On the electron-transfer mechanism in the contact-electrification effect, Adv. Mater. 30 (2018) 1706790. [77] Y. Zi, S. Niu, J. Wang, Z. Wen, W. Tang, Z.L. Wang, Standards and figure-of-merits for quantifying the performance of triboelectric nanogenerators, Nat. Commun. 6 (2015) 8376. [78] J. Chun, B.U. Ye, J.W. Lee, D. Choi, C.-Y. Kang, S.-W. Kim, Z.L. Wang, J.M. Baik, Boosted output performance of triboelectric nanogenerator via electric double layer effect, Nat. Commun. 7 (2016) 12985. [79] W. Kim, H.J. Hwang, D. Bhatia, Y. Lee, J.M. Baik, D. Choi, Kinematic design for high performance triboelectric nanogenerators with enhanced working frequency, Nano Energy 21 (2016) 19-25.

[80] I.-W. Tcho, S.-B. Jeon, S.-J. Park, W.-G. Kim, I.K. Jin, J.-K. Han, D. Kim, Y.-K. Choi, Disk-based triboelectric nanogenerator operated by rotational force converted from linear force by a gear system, Nano Energy 50 (2018) 489-496. [81] B. Dudem, N.D. Huynh, W. Kim, D.H. Kim, H.J. Hwang, D. Choi, J.S. Yu, Nanopillar-array architectured PDMS-based triboelectric nanogenerator integrated with a windmill model for effective wind energy harvesting, Nano Energy 42 (2017) 269-281. [82] Z. Li, Z. Saadatnia, Z. Yang, H. Naguib, A hybrid piezoelectric-triboelectric generator for low-frequency and broad-bandwidth energy harvesting, Energ. Convers. Manage. 174 (2018) 188-197. [83] W. Yang, J. Chen, G. Zhu, J. Yang, P. Bai, Y. Su, Q. Jing, X. Cao, Z.L. Wang, Harvesting energy from the natural vibration of human walking, ACS Nano 7 (2013) 11317-11324. [84] D. Bhatia, J. Lee, H.J. Hwang, J.M. Baik, S. Kim, D. Choi, Design of mechanical frequency regulator for predictable uniform power from triboelectric nanogenerators, Adv. Energy Mater. 8 (2018) 1702667. [85] H. Moon, J. Chung, B. Kim, H. Yong, T. Kim, S. Lee, S. Lee, Stack/flutter-driven selfretracting triboelectric nanogenerator for portable electronics, Nano Energy 31 (2017) 525-532. [86] B. Kim, J. Chung, H. Moon, D. Kim, S. Lee, Elastic spiral triboelectric nanogenerator as a self-charging case for portable electronics, Nano Energy 50 (2018) 133-139. [87] J. Chung, H. Yong, H. Moon, S.T. Choi, D. Bhatia, D. Choi, D. Kim, S. Lee, Capacitor-Integrated Triboelectric Nanogenerator Based on Metal-Metal Contact for Current Amplification, Adv. Energy Mater. 8 (2018) 1703024. [88] H. Zhang, Y. Yang, Y. Su, J. Chen, K. Adams, S. Lee, C. Hu, Z.L. Wang, Triboelectric nanogenerator for harvesting vibration energy in full space and as self‐powered acceleration sensor, Adv. Funct. Mater. 24 (2014) 1401-1407. [89] Z.L. Wang, New wave power, Nature 542 (2017) 159. [90] X. Wen, W. Yang, Q. Jing, Z.L. Wang, Harvesting broadband kinetic impact energy from mechanical triggering/vibration and water waves, ACS Nano 8 (2014) 7405-7412.

[91] L.M. Zhang, C.B. Han, T. Jiang, T. Zhou, X.H. Li, C. Zhang, Z.L. Wang, Multilayer wavy-structured robust triboelectric nanogenerator for harvesting water wave energy, Nano Energy 22 (2016) 87-94. [92] X. Wang, S. Niu, F. Yi, Y. Yin, C. Hao, K. Dai, Y. Zhang, Z. You, Z.L. Wang, Harvesting ambient vibration energy over a wide frequency range for self-powered electronics, ACS Nano 11 (2017) 1728-1735. [93] J. Chen, G. Zhu, W. Yang, Q. Jing, P. Bai, Y. Yang, T.C. Hou, Z.L. Wang, Harmonic‐ resonator‐based triboelectric nanogenerator as a sustainable power source and a self‐ powered active vibration sensor, Adv. Mater. 25 (2013) 6094-6099. [94] J. Yang, J. Chen, Y. Yang, H. Zhang, W. Yang, P. Bai, Y. Su, Z.L. Wang, Broadband vibrational energy harvesting based on a triboelectric nanogenerator, Adv. Energy Mater. 4 (2014) 1301322. [95] W. Yang, J. Chen, G. Zhu, X. Wen, P. Bai, Y. Su, Y. Lin, Z.L. Wang, Harvesting vibration energy by a triple-cantilever based triboelectric nanogenerator, Nano Research, 6 (2013) 880-886. [96] M. Han, X. Zhang, W. Liu, X. Sun, X. Peng, H.X. Zhang, Low-frequency wide-band hybrid energy harvester based on piezoelectric and triboelectric mechanism, Sci. China Technol. Sc. 56 (2013) 1835-1841. [97] X. Fan, J. Chen, J. Yang, P. Bai, Z. Li, Z.L. Wang, Ultrathin, rollable, paper-based triboelectric nanogenerator for acoustic energy harvesting and self-powered sound recording, ACS Nano 9 (2015) 4236-4243. [98] Y. Liu, Y. Zhu, J. Liu, Y. Zhang, J. Liu, J. Zhai, Design of bionic cochlear basilar membrane acoustic sensor for frequency selectivity based on film triboelectric nanogenerator, Nanoscale Res. Lett. 13 (2018) 191. [99] J. Yang, J. Chen, Y. Liu, W. Yang, Y. Su, Z.L. Wang, Triboelectrification-based organic film nanogenerator for acoustic energy harvesting and self-powered active acoustic sensing, ACS Nano 8 (2014) 2649-2657.

Vitae

Wook Kim received the M.S. degree in mechanical engineering from Kyung Hee University, Yongin, South Korea, in 2016 and he is currently pursuing his Ph.D. degree in mechanical engineering under the supervision of Prof. Dukhyun Choi, Kyung Hee University, Yongin, Republic of Korea. His major research focuses on the mechanical system design based triboelectric nanogenerators for efficient energy harvesting and self-powered systems.

Divij Bhatia has a Ph.D. in Mechanical Engineering from the Kyung Hee University, South Korea, and an M.S. in Electrical Engineering from the Pennsylvania State University, USA. He is currently a postdoctoral researcher under Prof. Dukhyun Choi at Kyung Hee University. He is interested in renewable energy system design, optimization and implementation.

Shinkyu Jeong received his Ph.D. in Department of Aeronautics and Aerospace from Tohoku University in 1999. From 2001 to 2004, he was a research associate at Japan Aerospace Exploration Agency (JAXA). From 2004 to 2013, he was assistant Professor in Tohoku University. Since 2013, he is a Professor in Department of Mechanical Engineering at Kyung Hee University. His research interests include Mars Exploration Airplane, Development of Near Future Aircraft, Construction of Turbulent Wind Environment Database. Details can be found at: http://adol.khu.ac.kr.

Dukhyun Choi received his Ph.D. in Mechanical Engineering from Pohang University of Science and Technology (Postech) in 2006. From 2006 to 2008, he was a postdoctoral fellow with Prof. Luke P. Lee at UC Berkeley. Dr. Choi moved to Samsung Advanced Institute of Technology (SAIT) as a research staff. Since 2010, he is a Professor in Department of Mechanical Engineering at Kyung Hee University. His research interests include Energy Harvester, Plasmonics, Hybrid Photovoltaics, Flexible Electronics, and Water Splitting. Details can be found at: http://dchoi.khu.ac.kr.

Fig. 1. Schematic describing the conceptual advantages of using MECS integrated with TENGs.

Fig. 2. (a) Concept of kinematic analysis with a three-link system. (b) Usage of a kinematic system and examples of kinematic components.

Fig. 3. Kinematic models for the (a) gear, (b) cam follower, (c) slider-crank mechanism, and (d) rectangular flywheel with the equation for the moment of inertia.

Fig. 4. (a) Concept of vibrational system dynamic analysis with impact non-linearity. (b) Usage of vibrational systems and examples of three basic vibrational structures [39,41-43]. Reproduced with permission from Elsevier [39, 41], Wiley [42], and American Chemical Society [43].

Fig. 5. (a) Piecewise model for vibration system with stoppers on two sides. Amplitude frequency response simulation showing (b) stiffening phenomena due to impact, (c) the left-shift of the resonance frequency position with increasing mass, and (d) right-shift of the resonance frequency position with increasing stiffness. (e) Resonance design protocol for vibration systems [41]. Reproduced with permission from Elsevier [41]

Fig. 6. Four different modes of operation of TENGs: (a) vertical contact-separation mode, (b) lateral sliding mode, (c) single-electrode mode, and (d) freestanding triboelectric-layer mode [50,51,54,56]. Reproduced with permission from American Chemical Society [50, 51, 54, 56].

Fig. 7. TENG performance changes with various parameters: (a) material characteristics related to i) relative permittivity and ii) surface charge density; (b) geometries, including i) surface morphology, ii) contact area, iii) thickness of dielectrics, and iv) gap distance; (c) environmental conditions, including i) relative humidity and ii) temperature; and (d) level of input energies that determine i) contact frequency and ii) contact force [41,64,65,71-76,78]. Reproduced with permission from Elsevier [41,72,73,75], American Chemical Society [64,71,74], Wiley [65,76], and Springer Nature [78].

Fig. 8. Schematics of (a) TENG system based on the gear and slider-crank mechanism and (b) its power density changes with various gear ratios. Schematics of (c) water-based TENG system using a gear train and (d) its output voltage with different device connections. (e) Schematic of gear–based, motion-convertible TENG system. (f) Comparing the output power before and after converting linear motion to rotational motion [60,79,80]. Reproduced with permission from Elsevier [60,79,80].

Fig. 9. (a) Schematics of TENG system based on a cam-follower mechanism and (b) the follower structure. (c) Mechanical durability of TENGs based on the vertical contact-separation and lateral sliding mode. (d) Output current of cam-based TENG with varying numbers of cam noses. Schematic of (e) cam-based windmill TENG system and (f) its output current at various wind speeds [52,81]. Reproduced with permission from Elsevier [52,81].

Fig. 10. (a) Kinematic analysis of truss structure-based PE-TE hybrid generator. (b) Output performance of PNG and TENG using a truss structure with an input frequency of 11 Hz. Schematics of (c) rhombic grid-structured TENG and (d) its output power change with varying numbers of units [82,83]. Reproduced with permission from Elsevier [82] and American Chemical Society [83].

Fig. 11. (a) Schematic of MFR-TENG system. (b) Input energy-storage part of the MFR-TENG system. (c) Energy-release part of the MFR-TENG system. (d) Regulated output voltages of the MFR-TENG at 10, 20, 30, 40, and 50 Hz frequencies. (e) Comparing the output TENG current with the transformer current output at different TENG frequencies [84]. Reproduced with permission from Wiley [84].

Fig. 12. Helical coil spring-based vibrational energy conversion systems for TENG. Method for experimentally determining the vibration TENG (a) bandwidth and (b) damping. Voltage and current output frequency responses for multi-directional TENG with (c) out-of-plane motion and (d) in-plane motion. (e) Tandem TENG system design with a vertical stack arrangement. (f) Voltage output frequency response of strategically designed tandem TENGs [41,93,94]. Reproduced with permission from Elsevier [41] and Wiley [93,94].

Fig. 13. Cantilever beam spring-based vibrational energy conversion systems for TENG. (a) Schematic of double-impact cantilever TENG with a tip mass, and (b) the corresponding output frequency response at different cantilever lengths and tip masses. (c) Schematic of cantilever beams explicitly designed to operate at different frequencies by tuning their lengths, and (d) TENG output of the longest cantilever beam at a resonance frequency of 15 Hz. (e) Schematic of dual resonant structure cantilever beams with non-linear impact, and (f) power output frequency response of the harvester under different input accelerations [47,95,96]. Reproduced with permission from American Institute of Physics [47] and Springer Nature [95,96].

Fig. 14. Fixed-fixed beam spring-based vibrational energy conversion systems for TENG. (a) Illustration of PTFE membrane vibration frequency modes, and (b) current output frequency response at different paper thicknesses with holes punched in the paper. (c) Schematic of a frequency-selective TENG with a vibrating PTFE membrane attached to nine electrodes and its operation. (d) Results showing the relationship between the electrode number and local resonance frequency of the PTFE membrane. (e) Schematic of an acoustic energy harvesting singular TENG and (f) the output frequency response of four such explicitly designed TENGs [97-99]. Reproduced with permission from American Chemical Society [97,99] and Springer Nature [98].

Highlights 

This review summarizes the recent progress in kinematic and vibrational TENG systems research.



The fundamental kinematics and vibrational theory for mechanical energy conversion is briefly introduced.



Mechanical energy conversion systems that can provide regular, stable, predictable, and low-loss output power from TENGs are discussed.

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Effective ways to control the irregular mechanical input energies are suggested for the practical commercialization of TENGs.