1 Introduction
Global energy demand has increased significantly. It is estimated that the global energy consumption in 2040 will be about 28% higher than that in 2015 [1]. The limited traditional energy will be difficult to meet the demand in the near future, and the development and utilization of renewable energy will become more urgent. It is predicted that at least 60% of the global energy structure before 2030 must be renewable energy, so as to meet the energy demand of a planet with a population of nine billion. This means that we must invest in renewable energy wisely; otherwise, the future will be unsustainable [2,3].
The ocean, which accounts for 71% of the earth’s area, is a treasure house of green energy, and wave energy is ubiquitous in the vast ocean, which is regarded as one of the most promising marine energy sources [4–8]. Wave energy has the advantages of large energy storage, high energy density, and little impact on the environment. At the same time, the ocean environment is complex and changeable, and the ocean wave energy is highly variable in time and space, which also acquires wave energy face many challenges [9–13]. Thanks to the joint efforts of researchers all over the world, various types of wave power generation devices have been developed [14–18].
The research of wave energy and its conversion device has a long history. Masuda began to study wave energy technology in 1940, and Salter published an article on wave energy in Nature in 1974 [19]. Since then, various wave power generation devices have come out one after another. According to the wave energy capture mode, there are three kinds of wave power generation devices: oscillating water column type, oscillating float type, and pendulum type wave power generation device [20]. The oscillating water column wave power generation device converts the motion of waves into the motion of air to generate electricity. The problems of this kind of device are low conversion efficiency, difficult construction, and high cost. The oscillating wave power generation device is developed on the basis of the oscillating water column wave power generation device, which converts the up-and-down motion of the floating body under the action of waves into hydraulic pressure or mechanical energy to generate electricity. This kind of device has entered a mature application stage in the United States and Britain, and its main problem is that it is easily damaged. Pendulum wave power generation device converts wave energy into kinetic energy of pendulum shaft and then generates electricity. The main issues are poor reliability and complex maintenance.
To sum up, although some progress has been made in the development technology of wave energy at present, compared with other mature renewable energy technologies, the wave energy conversion technology is still in the primary stage. Also the efficiency of energy capture and conversion and the stability of power generation have always been the primary problems faced by the development of wave power generation plant [2,14].
Nanogenerator is a technical device that converts mechanical energy or thermal energy caused by slight physical changes into electrical energy. There are currently three typical technical paths for nanogenerators: piezoelectric [21–31], triboelectric [32–36], and pyroelectric [37–41]. Among them, piezoelectric nanogenerator (PENG) and triboelectric nanogenerator (TENG) can realize the conversion from mechanical energy to electrical energy, while pyroelectric nanogenerator converts harvested thermal energy whose temperature fluctuates with time into electrical energy.
The researcher started to study the scientific principle of using nanostructures to convert mechanical energy into electrical energy in 2005 [42–44], and the first PENG was manufactured in 2006 [45]. When the vertically grown zinc oxide nanowires are bent with the tip of the Atomic Force Microscope probe, due to the combined action of semiconductor effect and piezoelectric effect, the zinc oxide nanowires emit electric charges, and the conversion of mechanical energy into electric energy is successful on the nano scale [46,47], and the power generation efficiency of the generator can reach 17%–30%.
In 2012, the first TENG was developed, which can use the coupling effect of triboelectric charging and electrostatic induction to convert mechanical energy into electrical energy [48,49]. In the process of harvesting energy from the generator, two materials with different triboelectric polarities generate triboelectric charges on their surfaces after contacting each other. Once the two materials are out of contact, a potential difference will be generated between the electrodes corresponding to the two materials, thus forming current output in the external circuit. What is exciting is that the TENG has superior output and can realize voltage output of up to several thousand volts.
TENG can harvest different forms of mechanical energy, including human motion [50–62], vibration [63–68], rotation [69–72], rolling [73], wind energy [74–80], acoustic energy [81–89], raindrops [90], and water waves [91–102]. The electric energy generated by TENG can act as an energy source of various chemical catalytic reactions [103–107], personal electronic products [108–114], precision medical devices [115–124]. In addition, by analyzing the relationship between various factors (mechanical movement and environmental conditions) and the electrical signal output, the TENG can directly act as a self-powered sensor [125–131] to detect physical parameters such as movement direction [9,132–136], movement displacement [137,138], movement speed [139], metal ions [140], humidity [141], ethanol [142], ultraviolet light intensity [143–147], contact pressure [148–150], temperature [151,152].
It should also be noted that TENG has a wide working frequency range. Even for fluctuations with frequency lower than 5 Hz, TENG still has a high output efficiency, which can just make up for the defect of electromagnetic generator (EMG) output at low frequency. And TENG can provide stable output under the current and wave with high variability in time and space [153–157]. Moreover, the TENG does not contain magnets or coils, which is made of cheap polymer, so the manufacturing cost is low. Therefore, it is very suitable for harvesting blue energy. Under some special circumstances, the maximum output energy density of the TENG can reach 500 W·m–2, and total energy conversion efficiency up to 85%. The TENG network structure is adopted to harvest marine “Blue Energy”, and it is estimated that 1.15 MW of electric energy can be generated on a sea surface with an area of 1 km2.
This paper summarizes the progress of TENG in harvesting blue energy in recent years. This article focuses on various structures of TENG in obtaining blue energy. In the first part of the article, the basic principle of TENG is expounded. In the second part, four working modes of TENG are introduced. In the third part, three major applications of TENG are introduced, and the application of TENG in blue energy is mainly discussed. The fourth part presents the networking strategy of large-scale TENG. Finally, we have briefly discussed TENG technology’s prospects and challenges for the future development in blue energy. This paper summarizes some application results of TENG in blue energy harvesting. Based on Maxwell displacement current, the essence of the nanogenerator is analyzed. Then, the different working structures and application fields of TENG are summarized, among which the application of TENG in blue energy is comprehensively described. Also, the large-scale networking strategy of TENG is further discussed. Finally, we briefly summarized the development potential and existing problems of TENG in the field of blue energy.
2 Theoretical basis of nanogenerator
Maxwell’s equations are a set of partial differential equations established by the British physicist James Clerk Maxwell in the 19th century to describe the relationship between electric and magnetic fields and the density of charge and current [158]. It consists of four equations: Gauss’ law, which describes how electric charges generate electric fields; Gauss’ law for magnetism, which describes the absence of magnetic monopoles; the current law, which describes how electric currents and time-varying fields generate magnetic fields; and Faraday’s law of electromagnetic induction, which describes how time-varying magnetic fields generate electric fields. Maxwell’s equations, as they are often called, are mostly in their differential form. Maxwell’s equations in the differential form are expressed as follows:
In the formula, is the Hamiltonian operator; ρ is the bulk density of free charge; H is the strength of the magnetic field; J is the conduction current density; D is the electrical displacement; E is the intensity of the electric field; B represents the magnetic flux density. The second term of Eq. (1) is what we call Maxwell displacement current:
2.1 Maxwell displacement current
It is worth noting that the above equations are not complete when there is a medium, but for an isotropic medium, there is:
where P stands for the density of the polarization field; ε represents the dielectric constant of the dielectric; ε0 represents the vacuum permittivity.
Another expression of Maxwell displacement current can be obtained by substituting Eq. (6) into Eq. (5):
The second term Ps in the equation is the polarization current, which is caused by the polarization field generated by the static charge on the surface of the medium with a surface polarization charge. It is the fundamental theoretical basis of the nanogenerator [2,159], and the working principle of nanogenerator based on this theory is shown in Fig.1.
2.2 Fundamental principles of PENG
When some dielectrics deform under the action of external forces in a certain direction, polarization will occur inside them [160–164]. PENGs drive the flow of electrons in the external circuit by using piezoelectric polarization charges and the electric field generated by them that changes with time [165–169]. The theoretical basis is the polarization current in Maxwell displacement current. As shown in Fig.1(a), the two ends of the piezoelectric material are covered with two electrodes respectively. When the two ends of the material are deformed by external forces, polarization charges will be generated, resulting in potential difference [170,171]. To balance this potential difference, electrons move directionally between two electrodes. Suppose σp(z) represents the polarization charge density of the material, the charge density of the free electrons in the electrode is σ(t), and z is the thickness of the material. For anisotropic piezoelectric materials, assuming that the temperature (T), the stress (X) and the electric field (E) change slightly, dT, dX, and dE, respectively, and the stress and electric field in the initial state are zero, the following linear state equation of elastic dielectric can be obtained.
where x is the strain, α represents the coefficient of thermal expansion, s represents the coefficient of elastic flexibility, d represents the piezoelectric constant, p represents the thermoelectric coefficient, c represents the specific heat of the material, s represents the entropy, and ρ represents the density of the medium. Superscripts indicate the variables that remain constant during the response.
During the working process, the piezoelectric body inevitably heats up, which is challenging to maintain the isothermal condition, but the heat exchange can usually be ignored, which satisfies the adiabatic condition. Therefore, to study the properties of the piezoelectric body under the adiabatic condition, we first discuss the stress and electric field as independent variables and obtain the linear equation of state as follows:
In the adiabatic condition, it can be concluded that:
When strain X and electric field E are independent variables, the corresponding equation is:
where cij is Elastic Modulus, emi is the piezoelectric stress constant. For the polarization field, there are:
The displacement current generated by dielectric polarization is:
It can be seen from Eq. (19) that the output current of the PENG is a positive correlation with the change speed of deformation.
The displacement current represents the change rate of the electric field. In the absence of an applied electric field, the displacement field is equal to the polarization field of the medium: Dz = Pz = σp(z), and the displacement current can be expressed as:
The characteristics of the PENG are analogous to those of a parallel plate capacitor. The external circuit current is generated by the flow of electrons driven by an electric field generated by a polarized charge. The open circuit voltage is VOC = zσp(z)/ε, and the output current equation of the PENG is:
where A represents the electrode area and R represents the external load.
2.3 The fundamental principle of TENG
Triboelectric is one of the most common phenomena in nature; whether combing hair, dressing or walking, driving can be encountered. However, triboelectric energy is difficult to harvest and use, so it is often ignored. The team, led by professor Wang of the Georgia Institute of Technology, developed a transparent, flexible triboelectric generator that can successfully convert friction into usable electricity using flexible polymer materials [172,173].
TENGs use the principle of triboelectrification and use some friction materials that are easy to electrify to rub (or contact with each other) each other to generate an equal amount of different charges on their surfaces [174]. Then, the separation technology is utilized to separate the electric charges generated by the friction of the friction layer that is responsible to produce a potential difference, and the electric current can be produced through an external circuit.
Here, we use the most basic model (contact-separation mode) of the TENG to illustrate its original theory. As shown in Fig.1(b), the dielectric constants of two dielectrics (polymer films) are ε1 and ε2, respectively. The thickness is d1 and d2. Surface charge density σc(t) increases with continuous contact between the two polymers until saturation. The electric field created by the friction charge drives electrons to flow between two electrodes, causing free electrons to accumulate in the electrodes σI(z,t). In electrostatic equilibrium, there is no charge inside the conductor, so the charge can only be distributed on the surface of the two polymer films. When the edge effect is not considered, these charges can be assumed to be uniformly distributed. According to Gauss’ law, E1 = σI(z,t)/ε1 and E2 = σI(z,t)/ε2 are the two electric fields of dielectric in the middle of the two thin films of the air gap, E0 = (σI(z,t) – σc)/ε0 respectively, The relative potential difference between the two electrodes is:
When the two electrodes are directly connected without load, V = 0 is obtained. This condition can be obtained by substituting into Eq. (22):
According to Eq. (23), it can be obtained that the displacement current inside the TENG is:
The first term of Eq. (24) indicates that the displacement current is related to the surface charge density of two polymer films and the contact separation velocity of the two polymer films. The second term indicates that the displacement current is relevant to the change speed of surface charge density of the two polymer films. Where, the charge density σc on the two polymer films reaches saturation when the two polymer films touch and separate more than 10 times, and the value of the second term in the formula becomes zero.
Similar to the PENG, the external circuit current of the TENG is generated by the flow of electrons driven by the electric field generated by the triboelectric charge, and its output current equation is as follows:
3 Working mode of TENGs
The working modes of TENG mainly include vertical contact-separation, lateral sliding, single-electrode, and freestanding triboelectric-layer, as shown in Fig.2.
3.1 Vertical contact-separation mode
Before the start of work, the upper and lower parts of TENG are separated. Since the surfaces of both friction materials are uncharged, the potential between the two electrodes is equal (Fig.3(I)). At the beginning of work, the upper and lower parts of TENG are squeezed together by an external force, and the surfaces of the two friction layers come into contact. According to the principle of friction electrification, charge transfer will occur at the contact surface of the two friction materials [27,177]. Because polyimide’s ability to gain electrons is higher than that of a polymethyl methacrylate (PMMA), the rubbed PMMA surface and Kapton surface generate the same amount of different charges, respectively (Fig.3(II)) [178,179]. Compared with the parallel plate capacitor, it can be seen that there is no potential difference at this time because the distance between the two friction layers is zero. With the disappearance of the external force or the reversal of its direction, the gap begins to appear between the two friction layers, accompanied by the potential difference (Fig.3(III, IV)). At the same time, electrons will flow to balance this potential difference. Until the external force acts on TENG again, the gap between the two friction layers decreases (Fig.3(V)), and the electrons flow reversely until they return to state II (Fig.3(II)).
3.2 Lateral sliding mode
This section describes the second basic operating mode of the TENG, the lateral sliding mode [180–183], as shown in Fig.4. Similar to the TENG with vertical contact-separation mode, the basic principle of the TENG with the lateral sliding mode is also the coupling of triboelectrification and electrostatic induction. The difference is that the lateral sliding mode is triboelectrification with lateral sliding friction instead of contact and separation.
In the initial state, two media materials are stacked together. Based on the principle of triboelectrification, the contact part of the two materials has the same amount of different charges (Fig.4(I)). According to the triboelectric sequence, the surface of nylon film is positively charged, while that of polytetrafluoroethylene (PTFE) film is negatively charged. When the upper part is pushed by external forces horizontally parallel to the surface of the two polymer films, the upper part slides outward, resulting in a smaller overlapping area of the two polymer films (Fig.4(II)). This allows the surface charge between the separated parts to create a potential difference that drives the electrons from the lower electrode to the upper one. As the upper part continues to slide outward, the transfer of charge will continue until the upper part stops sliding or is completely separated from the lower part (Fig.4(III)). Subsequently, the upper part is pushed by external forces in the opposite direction to the previous one and slides back to the initial position (Fig.4(IV)). As the contact area increases, electrons flow from the top electrode to the bottom one until the polymer film returns to the overlap state.
3.3 Single-electrode mode
The two kinds of TENGs introduced in the previous subsections 3.1 and 3.2 need to connect the electrodes with polymer materials. However, in daily life, such as walking, driving, and other behaviors, we all need to exercise freely instead of being fixed in a certain place. To harvest these forms of mechanical energy, the connection of wires is a big obstacle. In order to effectively harvest mechanical energy in free motion, two new working modes of TENGs are proposed: single-electrode mode [185,186] and freestanding triboelectric-layer mode [187,188]. This section briefly explains the working principle of the simplest single electrode mode TENG (Fig.5), single electrode mode TENG with vertical contact-separation structure [189–191].
The working principle of single-electrode mode TENGs is also based on the coupling effect of triboelectrification and electrostatic induction. Its working principle is very similar to that of the first two TENGs. In the initial state, the high molecular material fluorinated ethylene propylene (FEP) and aluminum (aluminum is both a friction layer and an electrode) are overlapped together (Fig.5(I)). According to the principle of triboelectrification, the contact parts of the two materials have the same amount of dissimilar charges. According to the triboelectric sequence, the surface of Al is positively charged, and the surface of FEP is negatively charged. When the motion separates the two friction materials (Fig.5(II)), a potential difference is generated between the Cu electrode and the Al electrode, thus driving electrons to flow from the former to the latter until the distance between the two friction materials no longer increases (Fig.5(III)). Subsequently, the movement causes the FEP to move back, and the decrease in the gap between the two friction materials causes the reverse flow of electrons to balance the change of potential difference (Fig.5(IV)).
3.4 Freestanding triboelectric-layer mode
As shown in Fig.6, FEP film is used as a sliding friction material, and Al is both a friction material and electrode. Before starting the operation, the FEP film overlaps with the left electrode. According to the principle of triboelectrification, the contact parts of the two materials produce the same amount of different charges. According to the triboelectric sequence, the left Al electrode surface bears positive charges, the FEP surface bears negative charges. Similar to the working principle of the horizontal sliding mode, with the application of horizontal force to the right, the FEP slides toward the right electrode, and the current flows from the left electrode to the right electrode. When FEP completely overlaps with the right electrode, the positive charge in the left electrode entirely flows into the right electrode. Subsequently, FEP slides toward the left electrode under the force opposite to the previous direction. The current flows from the right electrode to the left electrode until it overlaps with the left Al electrode again, forming a complete power generation cycle.
4 Three applications of TENGs
TENGs are believed to be applicable to various places where periodic kinetic energy exists. On the macro scale, such as wind and ocean waves; on the microscopic scale, there are muscle activity, heartbeat, lung respiration, arterial contraction. At present, TENGs have three main application fields (Fig.7): the first is micro/nano-energy, which is mainly used for self-power supply of micro-small equipment [196]; the second is the self-powered sensor [127,197,198], including health detection, environmental monitoring, Internet of Things, etc.; the third is blue energy [199–201], which can harvest ocean wave energy at low frequencies.
4.1 Micro/nano-energy
Energy is the fundamental cornerstone for the progress of human civilization and the decisive factor for global technological change [202–206]. Today, human society has begun to enter the era of big data, cloud computing, and the Internet of Things [207–211]. In addition to traditional “big” energy sources such as electric energy and fossil energy, human beings have also begun to explore “small” energy sources such as micro-nano energy to solve the self-power supply of small devices in the Internet of Things era [212–214].
TENGs are energy harvesting devices based on nano-scale triboelectric effects, which can effectively convert low-frequency and disordered mechanical energy into electrical energy [215,216]. Nanotechnologists have established a relatively perfect principle and technical system of obtaining random mechanical energy from the human living space and living bodies to drive microminiature equipment.
As a micro/nano-energy source, the TENG can be used wherever there is mechanical movement. (1) Self-powered system. TENG can act as an independent, maintenance-free power supply to drive electronic devices and systems [186,197,217,218]. TENG can directly drive portable devices under some special circumstances. In general, it is necessary to rectify and store the generated AC electrical pulse first [219], and then use the stored energy to drive traditional electronic devices, such as transistors, laser diodes, and small liquid crystal displays. (2) Wearable devices. In daily life, human movement produces a large amount of mechanical energy. However, due to the inconvenience of collection and the scattered energy, almost all of this energy is wasted. TENG can effectively harvest this energy to charge wearable devices such as smart watches, smart bracelets, and Google Glasses [220–226]. This will greatly improve the convenience of people’s lives and expand the application range of wearable electronic products. (3) Electrochemical process power supply. TENG can act as a direct power supply to promote electrochemical processes, such as sewage degradation, electrodeposition, electrolysis of water, purification, dust removal. (4) Implantable medical device. In the future medical field, all kinds of biomedical electronic systems, especially microelectronic devices used in health care, need a safe and reliable energy supply [227,228]. However, ordinary batteries are bulky, so there will be potential dangers in biomedical applications. TENG can directly convert mechanical energy generated by limb swing, muscle contraction/stretching, heart/lung movement, and blood flow into electrical energy, and is considered as an ideal energy source for biomedical electronic devices. In addition, TENGs can also be directly used as electrical stimulation sources to treat various diseases.
4.2 Self-powered sensor
According to the previous chapters (theoretical basis), the output voltage of the TENG is proportional to the distance between the two friction materials in a certain range, and the current is positively related to the contact and separation speed of the two materials. Under certain parameters of the device itself, its output signal mainly depends on external mechanical stimulation such as displacement, speed, force, frequency, and environmental factors such as humidity. Using this characteristic, the TENG is fully qualified for the role of a sensor. At the same time, TENG, as a sensor, has the advantages that the traditional sensors do not have-without an external power supply. TENG itself can generate electrical signals and self-supply, which has important research significance and application value to complete the permanent and stable work of the sensing system [130,229–236]. There are two kinds of self-powered sensors based on TENG: one is the TENGs themselves as sensors to detect mechanical motion; the other is to realize self-powered sensing of gas composition, illumination intensity, solution concentration, etc., in combination with traditional nanomaterial-based sensors.
4.2.1 Self-powered mechanical sensor
Since the energy source of TENGs is mechanical energy, physical quantities related to mechanical motion can be detected as long as the structural design of TENG is improved (Fig.8). Luo et al. [135] have designed a wood-based TENG (Fig.8(a)), which can realize the functions of speed sensing, path tracking, and distribution statistics. This design has opened up a new path for intelligent sports equipment. Zhang et al. [237] have designed a water level sensor (Fig.8(b)), which can measure water level information quickly and accurately and detect the dynamic draught of ships. Xiao et al. [238] have designed a honeycomb-shaped inspired TENG (Fig.8(c)), which can effectively monitor the running state of the engine and accurately obtain its running frequency.
Fig.8 Self-powered mechanical sensor. (a) Wood-based TENGs for self-powered sensing. Reprinted with permission from Ref. [135], copyright 2019, Springer Nature. (b) Self-powered distributed water level sensors. Reprinted with permission from Ref. [237], copyright 2019, Wiley-VCH Verlag GmbH. (c) Honeycomb structure inspired TENG. Reprinted with permission from Ref. [238], copyright 2019, Wiley-VCH Verlag GmbH. |
4.2.2 Self-powered sensor combined with nanomaterials
TENG has a simple structure and a light device, and can be skillfully combined with nano-sensing materials to realize self-powered detection in other physical, chemical, and biological aspects besides mechanical physical quantities (Fig.9). Lee et al. [239] prepared a high output TENG based on highly transparent polyimide by introducing functional groups into the main chain (Fig.9(a)), which can clearly identify H2, CO, and NO2 in gas samples. Qian et al. [240] manufactured a high-performance biocompatible cellulose based TENG by full printing method (Fig.9(b)). Due to its high water absorption, it can be used as a high-sensitivity humidity sensor with a response ratio as high as 5:1. Based on the impedance matching effect of TENGs, Chen et al. [241] designed a self-powered online ion concentration monitor in water (Fig.9(c)), which can illuminate an alarm light-emitting diode (LED) with only 1 × 10–5 mol·L–1 ion concentration change and has good reliability.
Fig.9 Self-powered sensors combined with nanomaterials. (a) The high output TENG recognizes H2, CO, and NO2 gases. Reprinted with permission from Ref. [239], copyright 2019, Wiley-VCH Verlag GmbH. (b) Biocompatible cellulose based TENGs. Reprinted with permission from Ref. [240], copyright 2019, Elsevier. (c) Self-powered online ion concentration monitor in water. Reprinted with permission from Ref. [241], copyright 2019, Elsevier. |
4.3 Blue energy
It should also be noted that TENG has a wide working frequency range. Even for fluctuations with frequency lower than 5 Hz, TENG still has a high output efficiency, and stable output power can be obtained under the conditions of slow water flow and random waves, which can just make up for the defect of EMG output at low frequency. Therefore, TENGs are very suitable for harvesting blue energy [33]. At present, a large number of scientific researchers have devoted themselves to this research field one after another. TENGs that convert water energy into electrical energy are divided into four categories at this stage. Tab.1 summarizes some representative articles on these four types of devices.
Tab.1 Representative articles on four types of devices for harvesting water wave energy |
| Types of TENG operational modes | Maximum power density | Ref. |
|---|---|---|
| Liquid–solid contact | ||
| Adaptive solid–liquid interfacing TENG | 147.1 mW·m–2 | [248] |
| Amphiphobic hydraulic TENG | 245.2 mW·m–2 | [249] |
| Liquid-metal-based TENG | 6.7 W·m–2 | [250] |
| Water-tube TENG | 16.6 W·m–3 | [251] |
| Hydrokinetics energy | ||
| Waterwheel hybrid generator | 165.3 W·m–2 | [256] |
| Multi-layered disk TENG | 42.6 W·m–2 | [260] |
| Dual-mode | ||
| Dual-mode TENG | 1.69 W·m–2 | [142] |
| Leaf-mimic rain energy harvester | 82.66 W·m–2 | [259] |
| Hybridized TENG | 0.62 W·m–2 | [263] |
| Fully enclosed | ||
| Open-book-like TENG | 7.45 W·m–3 | [265] |
| TENG with spring-assisted linkage structure | 9.559 W·m–3 | [266] |
| Elliptical cylindrical structure | 17.6 W·m–3 | [267] |
| Nodding duck structure TENG | 4 W·m–3 | [268] |
| Integrated TENG array | 13.23 W·m–3 | [281] |
| High power density tower-like TENG | 10.6 W·m–3 | [284] |
4.3.1 TENG based on liquid–solid contact
The contact charging behavior between the lotus leaf surface and water droplets (Fig.10) has been confirmed [252]. In order to take advantage of this characteristic, natural lotus leaf-TENG (LL-TENG) was manufactured by depositing metal on the back of the surface of the natural lotus leaf. Fig.10(a) shows the manufacturing process and working principle of LL-TENG. When water drops on the blade surface, liquid drops contact with the top of LL-TENG. Due to the difference in electron gain and loss ability, the contact surface of LL-TENG generates negative charges and the water drops generate positive charges. When the droplet rolls on the LL-TENG surface, charges will be continuously generated until the first droplet is separated from the contact surface. For balancing the potential difference, electrons flow from electrode 1 to electrode 2 during the water droplet rolling from electrode 1 to electrode 2. As the water droplets roll over electrode 2, electrons flow back from electrode 2 to electrode 1, thereby generating an alternating current. To enhance the power generation efficiency, the FEP film imprinted with the surface morphology of the natural lotus leaf is used to replace the natural lotus leaf, and the new TENG is named LL-PATERN. LL-PATERN has a much higher output performance than LL-TENG, and the electric output generated by 30 µL deionized water droplets is sufficient to illuminate 30 LEDs simultaneously. And LL-PATERN has a good self-cleaning effect because it imitates the superhydrophobic surface of natural lotus leaves.
Fig.10 TENG based on liquid−solid contact. (a) TENG inspired by lotus leaves. <i> The manufacturing process of LL-TENG. <ii> The working mechanism of LL-TENG. Reprinted with permission from Ref. [252], copyright 2017, Elsevier. (b) Buoy-like TENG. (c) The structure of the liquid−solid-contact TENG (LS TENG). Reprinted with permission from Ref. [253], copyright 2018, Wiley-VCH Verlag GmbH. |
A buoy-shaped LS TENG (Fig.10(b)) has been developed to harvest various types of water wave energy such as vibration and rotation [253]. LS TENG consists of a plurality of annular TENG units with PTFE and water as friction materials connected in parallel (Fig.10(c)). When LS TENG sways up and down, water and PTFE film rub and induce charges on their contact surfaces. The potential difference between the PTFE back electrode and the device bottom electrode causes electrons to flow between the two electrodes, thus generating alternating currents. Similarly, when TENG shakes horizontally or rotates, the shaking of liquid in the device will also cause TENG to generate alternating currents.
A sliding freestanding mode TENG based on an energy harvesting water wheel (ww-TENG) (Fig.11) can be directly used to generate electricity using low-speed flowing rivers [254]. The ww-TENG consists of a water wheel, a shaft for supporting and fixing the water wheel, blades for draining water, alternating carbon paper electrodes, and hydrophobic fluorine coating (Fig.11(a)).
When the device is partially submerged in the water, the blades drive the water wheel to rotate under the push of the water flow. The first stage: according to the triboelectric sequence, the fluorine coating in the submerged part is negatively charged, and the water belt is positively charged, assuming that the entire fluorine layer is fully negatively charged, at this time it is in an electrostatic equilibrium state and there is no current signal (Fig.11(b)). The second stage: the red electrode starts to submerge, because the positive charge in the water shields the negative charge on the fluorine coating, the electrostatic balance is destroyed, and current flows from the red electrode to the blue one. The third stage: the red electrode completely submerges in the water and rotates in the water. Because there is no further shielding effect, the device is in an electrostatic equilibrium state again and there is no current signal. The fourth stage: the blue electrode starts to submerge, the same principle as the second stage, current flows from the blue electrode to the red one. So far, stages 2 to 4 form a complete discharge cycle.
A water–solid TENG (W-TENG) based on a superhydrophobic surface (Fig.12) can capture the energy of water flow and repair itself when the surface is damaged [255]. As shown in Fig.12(a), the superhydrophobic surface is obtained by adhering polyvinylidene fluoride (PVDF) nanospheres to the microporous surface of double-sided adhesive tape, and then modifying the friction electrode surface with fluorinated alkyl silane (Fig.12(b) shows the scanning electron microscope (SEM) image of the superhydrophobic surface).
The water inside the device will reciprocate under the impetus of waves, which makes W-TENG convert the mechanical vibration energy around it into electric energy (Fig.12(c)). The working process is shown in Fig.12(d). In a calm state, the surface of the water filled in the equipment is positively charged due to contact with the PVDF superhydrophobic surface, and the PVDF surface is negatively charged. At this time, it is in electrostatic equilibrium, and no current is generated. When the device is stressed and vibrated, the water in the device will accumulate to one side, which leads to break the electrostatic balance fail. Thus generating current from the left electrode to the right one; subsequently, the water in the device returns to the initial state, generating a current in the opposite direction to the previous one. When the water in the device accumulates to the other side over the initial state, the current will be generated in the same principle as the above process. Then the water returns to its initial state again, thus completing a completed power generation cycle. According to experiments, when water flows through the W-TENGs array at a speed of 0.3 m·s–1, a current output of 20 μA can be obtained, and more than 30 commercial LEDs can be easily lit. Furthermore, the superhydrophobic property of the damaged friction electrode can be self-repaired by transferring fluorinated alkyl silane to the upper surface.
4.3.2 TENG hydrokinetics energy harvesting
This kind of TENG, which uses the principle of lateral sliding TENG to generate electricity, can drive itself to rotate through the flow of fluid, thus generating electric energy [256]. However, this type of structure is mostly used for wind energy harvesting and less for water flow.
A multi-layer disk TENG (Fig.13) can be rotated by fluid flow to realize effective electric output [257]. The multi-layer disc TENG is composed of a rotor and a stator (Fig.13(a)), and a D-shaped shaft (Fig.13(b)) is adopted to restrain the rotor, so that the multi-layer rotor can move in strict synchronization. An aluminum film is deposited on the rotor as both a friction layer and an electrode layer, and silver nanoparticles (Fig.13(c)) are coated on top of the aluminum film to improve the triboelectrification strength [258]. An aluminum film is deposited on the rotor as an electrode layer, and a PTFE film is coated on the aluminum film as a friction layer.
Fig.13 Multi-layer disk TENG. (a) The structure of the multi-layer disk TENG; (b) D-shaped shaft; (c) the SEM image of the Ag nanoparticles coated on the Al electrode; (d) the working principle of the multi-layer disk TENG; (e) the detailed structural design of the energy harvesting system; (f) the short-circuit current of the energy harvesting system; (g) the photo of lighting 100 commercial LEDs with the multi-layered disk TENG. Reprinted with permission from Ref. [257], copyright 2014, Elsevier. |
During operation, the rotation of the rotor causes the overlapping area of the friction layer to change periodically, results in the periodic change of potential difference, which leads to a the periodic current (Fig.13(d)). When the device is placed in the water flow, the rotor of TENG is connected with the water turbine through the D-shaped shaft, which can realize synchronous rotation with the water turbine (Fig.13(e)). Testing with a standard household faucet, the water from the faucet can drive TENG to generate a short-circuit current with a peak value of 20 mA·m–2 (Fig.13(f)) and easily illuminate 100 commercial LEDs (Fig.13(g)).
4.3.3 Dual-mode TENG
While the flowing water has mechanical energy, it is bound to produce electrostatic energy, because the contact between water and the surrounding medium will generate frictional charges during the water flowing process [259].
A water wheel type hybrid TENG can capture two different forms of energy in water flow (Fig.14), which includes two parts: water-TENG and disk-TENG [260]. Among them, the water-TENG part can harvest the electrostatic energy in the water flow. The disk-TENG part can harvest mechanical energy in water flow.
The water-TENG part consists of eight vanes, each of which can be regarded as a single-electrode TENG (Fig.14(a)). When the positively charged water flow reaches the blade 1, a positively charged water film will cover the PTFE film surface of the blade, resulting in a positive potential difference between the electrode on the back of the blade and the ground, thus driving electrons to flow from the ground to the electrode (Fig.14(b)). The impact of the water flow will cause the wheels to rotate, and then the water flow will contact the blade 2, this process is the same as the above process. When the wheel rotates further and the water film slides off the surface of blade 1, a negative potential difference is generated between the electrode on the back of the blade and the ground, causing the electrons to flow from the electrode to the ground. With the continuous rotation of the wheel, the water flow will contact different blades in turn, continuously generating alternating current.
The disk-TENG consists of two 16-segment disks with PTFE and polyethylene terephthalate (PET) as friction layers and Cu as electrode layers (Fig.14(c)). In the initial stage, the two disks completely overlap, and the potential difference between the electrodes of the two disks is zero. When the blade receives the impact of water flow, it drives the back disk to rotate, the two disks are separated, resulting in a potential difference between the two electrodes, which drives the flow of electrons. Continue to rotate, the two disks start to contact the next segment. Similarly, the electron flow is driven to flow reversely until the two disks completely overlap again, and then the next cycle starts. Under the impact of 54 mL·s–1 water flow, 20 commercial LEDs can be lit at the same time, and a 4.7 μF commercial capacitor can be charged to 13 V within 326 s.
A hybrid TENG composed of water-TENG and contact-TENG (Fig.15) has been designed to simultaneously capture flowing water’s electrostatic and mechanical energies [142]. Water-TENG is composed of Cu electrode in the upper part of the device, PET film, and top TiO2 layer, wherein the layered micro/nano-structured TiO2 layer makes water-TENG superhydrophobic (Fig.15(a)). Contact-TENG consists of four parts: Cu electrode and PTFE film in the upper part of the device, SiO2 film and Cu electrode in the lower part of the device.
The working mechanism of water-TENG is shown in Fig.15(b). When the positively charged water drops fall on the TiO2 layer, a potential difference will be generated between the water drop and the ground, resulting in electrons flowing from the ground to the Cu electrode at the top. As the water drops fall off the TiO2 layer, a potential difference opposite to that before will be generated, driving electrons to flow back to the ground. The working mechanism of contact-TENG is shown in Fig.15(c). The impact of water droplets on the top of the device causes the upper half of the device to move downward, causing the PTFE film to contact the SiO2 film thus the PTFE film surface is negatively charged, while the SiO2 film surface is negatively charged.
As the force exerted by water drops on the device weakens or disappears, the PTFE film and the SiO2 film gradually separate, resulting in two potential differences with opposite directions between the two electrodes and the ground. The negative potential difference drives electrons to the earth, while the positive potential difference drives electrons to electrode 2. The impact of the next drop of water gradually shortens the distance between the PTFE film and the silica film until they contact again. Similarly, two potential differences with opposite directions are generated between the two electrodes and the ground. A positive potential difference drives electrons to the earth, while a negative potential difference drives electrons to electrode 1. This dual-output mode dramatically increases the output of TENG. Under the flow rate of 40 mL·s–1, the total instantaneous output power density of two output terminals 2 of dual-mode TENG can reach 1.69 W·m–2.
4.3.4 Fully enclosed TENG for harvesting water wave energy
Seawater can easily corrode polymer films, thus greatly reducing the output performance of TENG. Therefore, before harvesting water wave energy through TENG, it is necessary to package it [261,262]. Due to the totally enclosed structural design, this kind of TENG is not easy to corrode by seawater and can work in various harsh environments in the sea [99,263–268]. Various basic TENG structures for harvesting blue energy.
4.3.4.1 Rolling structure
RF-TENG: A rolling-spherical freestanding-triboelectric-layer based nanogenerator (RF-TENG) (Fig.16) can effectively collect wave energy [269]. The prepared RF-TENG is encapsulated in a spherical shell with nylon ball and Kapton film as friction materials and Al as an electrode (Fig.16(a) and 16(b)). TENG can easily float on the water surface because of its completely closed structure, light weight, and low density of spherical shell. In which two arc-fixed electrodes are connected to the back of the Kapton film. Once the impact of water waves is received, the nylon balls in the ball shell will roll in the shell, alternately rolling over the two electrodes, generating an alternating current. At the water wave frequency of 1.43 Hz, RF-TENG can light up more than 70 LEDs and charge four supercapacitors to 1.8 V in one hour, in which the stored energy can run the electronic thermometer for 20 min [270].
Fig.16 TENGs with spherical structure. (a) The physical picture of RF-TENG; (b) schematic diagram of RF-TENG structure; (c) the working mechanism of RF-TENG; (d) simulated change of RF-TENG potential in working state. Reprinted with permission from Ref. [270], copyright 2018, Elsevier. (e) Structural design of soft contact spherical TENG (SS-TENG); (f) the physical picture of SS-TENG; (g) the working mechanism of SS-TENG; (h) simulated change of SS-TENG potential in working state. Reprinted with permission from Ref. [271], copyright 2015, Wiley-VCH Verlag GmbH. |
SS-TENG: The softer the contact surface, the more sufficient the contact between the two objects, which is conducive to improving the output of TENG [272]. Based on this principle, SS-TENG has been developed (Fig.16(e) and Fig.16(f)) [271]. The working mechanism of SS-TENG and RF-TENG is the same (Fig.16(g) and Fig.16(h)), and the structure is similar. The difference between SS-TENG and RF-TENG is that the flexible silicone rubber ball filled with liquid is used instead of the hard nylon/PTFE ball. Due to the improvement of contact, the maximum transfer charge of SS-TENG is 10 times higher than that of RF-TENG. It is meaningful to study that this may not be the optimal output of SS-TENG, because the output of SS-TENG may be further improved by selecting contact balls with different solution filling amounts, different softness, different sizes, and different materials.
EC-TENG (elliptical cylindrical structure TENG): although the above-mentioned rolling spherical structure can harvest wave energy, once the device capsizes due to excessive waves, the output will be greatly reduced. Tan et al. [267] developed an EC-TENG (Fig.17) with elliptical cylindrical swing structure (Fig.17(a) and 17(b)), which can be divided into internal rolling TENG (Fig.17(d)) and external V-shaped TENG (Fig.17(c)) with four corners, which can effectively avoid overturning. Under the action of water waves, the steel/PTFE rod rolls back and forth in the inner shell (Fig.17(e)), which enables the outer TENG to complete the contact and separation movement (Fig.17(f)) while realizing the output of the inner TENG. Under the simulated water wave environment (frequency 1.6 Hz), the designed EC-TENG can light up 400 LEDs at the same time.
Fig.17 Structural design of EC-TENG. (a) Schematic diagram of the overall structure of EC-TENG. Structural schematic diagram of (b) EC-TENG, (c) V-shaped TENG, and (d) internal TENG. Working principle of (e) internal TENG and (f) external TENG. Reprinted with permission from Ref. [267], copyright 2022, Springer Nature. |
4.3.4.2 Spring-assisted structure
The spring can store elastic potential energy. Theoretically, when integrated with the spring, the short and low-frequency force on TENG will be converted into high-frequency and continuous vibration [273]. According to this principle, spring-assisted TENG (Fig.18) has been developed [274]. A single TENG consists of two Cu electrodes and a PTFE membrane, in which the PTFE membrane is pre-charged (Fig.18(c)) to improve the overall output of TENG [275–279]. Two TENGs are connected by springs and packaged by acrylic plates to form a basic spring-assisted TENG (Fig.18(a) and Fig.18(b)).
Fig.18 (a) Schematic diagram of spring-assisted TENG; (b) the physical picture of spring-assisted TENG; (c) pre-charging process of PTFE film; (d) operating mechanism of spring-assisted TENG; (e) integrated spring-assisted TENG device; (f) wiring diagram of integrated spring-assisted TENG. Reprinted with permission from Ref. [274], copyright 2016, Elsevier. |
When working, under the impetus of an external force, the PTFE film comes in contact with the TENG electrode (Fig.18(d)). Because the external force is intermittent, after the force subsides, the PTFE and TENG electrode will be separated again, so the cyclic contact and separation will generate periodic current. Due to the spring’s existence, the energy stored by the impact of the spring is released during the intermission of external force, which makes TENG continue to make contact and separation movements [280]. This makes the original single-cycle power generation become multi-cycle power generation, which greatly improves energy utilisation rate. By selecting suitable spring parameters, the conversion efficiency of TENG can be increased by 150.3%. In addition, multiple spring-assisted TENG can be integrated to further improve the catch of wave energy. Four spring-assisted TENG connected in parallel via rectifier bridge can easily light up 70 LEDs, and can charge the capacitor with a capacity of 1 μF to 30 V within 100 s (Fig.18(e) and Fig.18(f)).
4.3.4.3 Air-driven membrane structure
Although the above work has achieved the harvesting of water wave energy, the space utilization rate is still relatively low, so it is particularly important to improve the power density of TENG’s harvesting of water wave energy. Based on this consideration, an integrated TENG array device based on air-driven membrane structure (Fig.19) has been developed (Fig.19(a) and Fig.19(b)) [281].
Fig.19 Air-driven membrane structure TENG. (a) The structural sketch of the device; (b) the physical drawing of the device; (c) nanostructures on the surface of PTFE; (d) the exploded schematic diagram of the device; (e) the sectional structure of the device; (f) the working mechanism of the device. Reprinted with permission from Ref. [281], copyright 2016, Elsevier. (g) Dielectric elastomer capacitor (DEC), electret electrostatic voltage source, and charge pump circuit; (h) DENG’s (dielectric elastomer nanogenerator) photos in the original state (illustration) and stretched state; (i) the photo of DENG-based wave energy harvester; (j) working diagram of the wave energy harvester. Reprinted with permission from Ref. [282], copyright 2022, Wiley-VCH Verlag GmbH. |
A single TENG unit consists of a Cu electrode, an Al electrode, and a PTFE film, wherein the surface of the PTFE film is prepared with nanostructures (Fig.19(c)) to enhance the contact with the Cu electrode (Fig.19(d)) [283]. The flexible membrane and propylene separator string together a plurality of TENGs of TENG array, and divide the whole shell into upper and lower air chambers (Fig.19(e)). The fluctuation of water waves will break the balance of the air pressure in the upper and lower air chambers, and make the TENG units of the upper and lower parts periodically contact and separate, resulting in periodic current (Fig.19(f)). At the frequency of 2.9 Hz, TENG with pneumatic membrane structure can achieve short-circuit current of 187 mA and power density of 13.23 W·m–3, and can easily light up 600 LEDs in a real water wave environment.
Xu et al. [282] designed a DENG, which consists of a DEC, an electret electrostatic voltage source (EEVS), and a charge pump circuit (Fig.19(g) and Fig.19(h)). Dielectric elastomer is sandwiched between two flexible grease electrodes to form DEC. The EEVS is used to inject charge into DEC. The stretchability of the dielectric elastomer makes DENG deform to a certain extent, thus changing the capacitance of DEC, thus resulting in voltage change. Periodic stretching will periodically reverse the current, so the output current of DENG is an alternating current. The charge pump circuit is connected in parallel with DEC to boost the voltage output of DEC. In addition, by integrating DENG with a chamber with a water column, wave energy can be converted into electric energy, which can generate enough electric energy to charge a commercial capacitor and power a remote temperature reading module (Fig.19(i) and Fig.19(j)).
4.3.4.4 Multilayer structure
Recently, TENG’s structural design for wave energy harvesting has gradually changed from the initial liquid–solid contact mode to the fully enclosed structure, because the fully enclosed structure is easy to float on the sea and can harvest multi-directional wave energy. However, the friction energy loss of a single TENG unit is large, and the space utilization rate is low. Based on the above problems, some multi-layer and multi-unit TENG have been developed to harvest water wave energy.
A high power density TENG based on a tower-like structure (Fig.20) could harvest wave energy from any direction [284]. The tower-like TENG (T-TENG) consists of several units made of PTFE balls and three-dimensional (3D) printed arc surfaces coated with fused bonded reticulated nylon film (Fig.20(a) and Fig.20(b)) [285]. The nylon film is connected to the Al electrode by hot pressing.
Fig.20 The multilayer TENG. (a) T-TENG schematic design composed of multiple units; (b) the detailed structure of each TENG unit; (c) kinetic analysis sketch of PTFE spherical arc surface; (d) the working mechanism of the T-TENG; (e) the potential distribution of T-TENG simulated by COMSOL during working. Reprinted with permission from Ref. [284], copyright 2019, American Chemical Society. Structure design and working mechanism of nodding duck structure multi-track freestanding TENG (NDM-FTENG). (d) Schematic diagram of the overall structure of nodding duck and (e) internal multi-track FTENG unit; (f) the working mechanism of the single-track FTENG unit. Reprinted with permission from Ref. [268], copyright 2021, American Chemical Society. |
Similar to the working mechanism of RF-TENG, PTFE balls periodically roll to generate alternating current when TENG is working (subject to external forces such as water waves), which can effectively convert the energy of arbitrary direction wave into electric energy (Fig.20(c)). In the absence of a rectifier parallel connection, it is found that the power density of T-TENG increases by 10 times to about 10.6 W·m–3 with the increase of the number of units in the block (from 1 to 10). In a water wave box, five parallel T-TENG units can light up 540 LEDs and charge 100 μF capacitors to them within several minutes. By connecting more T-TENG blocks, a T-TENG network can be formed, thus providing innovative and effective methods for large-scale blue energy harvesting.
Liu et al. [268] developed an NDM-FTENG (Fig.20(d)). Similar to the working principle of the tower structure, nylon balls roll on an arc-shaped dielectric composite film, which can convert low-frequency ocean wave energy into electric energy (Fig.20(f)). In order to reduce the energy loss caused by collision, NDM-FTENG added a track structure based on the tower structure (Fig.20(e)). By studying the number of tracks and the connection mode of each track unit, the synchronous movement of nylon balls was realized, thus achieving stable and efficient power output (NDM-FTENG can output an open circuit voltage of 507 V and an instantaneous power density of 4 W·m–3 at the oscillation frequency of 0.21 Hz and the swing amplitude of 120, and can light 320 LEDs). Similarly, NDM-FTENG can also be connected in parallel to form a large-scale wave energy harvesting network to enhance the power generation capacity.
4.3.4.5 Underwater structure
Most published TENG for harvesting wave energy are designed to harvest energy from ocean surface waves. Their structures make them float on the water’s surface, and they can hardly obtain underwater energy. Although most marine applications are surface applications, providing renewable power for underwater applications should not be neglected.
Wang et al. [286] designed a flexible seaweed-like TENG (S-TENG) (Fig.21) inspired by seaweed (Fig.21(a)). The FEP rubber coated with conductive ink and PET coated with conductive ink were sealed in two layers of PTFE film. The existence of a sponge introduces an air gap between the two friction layers, which ensures that the electrification behavior of friction contact can proceed well (Fig.21(b) and Fig.21(c)). In addition, by simply connecting nine S-TENG in parallel, the thermometer or 30 LEDs can be successfully lit under the simulated water wave vibration, which proves that it can be effectively applied to various marine applications (Fig.21(d)).
Fig.21 TENGs for underwater energy harvesting. (a) The structural diagram of S-TENG; (b) S-TENG’s (i) top view and (ii) side view, and (iii) enlarged view of material surface morphology on the side view; (c) the working mechanism of S-TENG; (d) multi-application idea of S-TENG, illustrated by (i) thermometer and (ii) 30 LEDs powered by nine parallel S-TENG. Reprinted with permission from Ref. [286], copyright 2021, American Chemical Society. (e) The structural diagram and (f) physical drawing of UF-TENG (underwater flag-shaped TENG); (g) schematic diagram of ocean current energy harvesting by UF-TENG. Reprinted with permission from Ref. [287], copyright 2021, Elsevier. |
In the same year, Zhang et al. [287] designed a UF-TENG through a similar principle. UF-TENG is made of two PET films coated with conductive ink and a PTFE film stuck together by PTFE waterproof tape, which can effectively harvest the energy of water current (ocean current, eddy current, etc.) (Fig.21(e–g)).
4.3.4.6 Hybrid structure
Besides the above basic structure, researchers have also developed some hybrid nanogenerators composed of different types of generators [7,16,288–291]. By combining the advantages of multiple generators, the hybrid nanogenerator can not only operate over a wide range of frequencies, but also harvest various forms of ocean energy, including waves, currents, and tides.
(1) Ship-shaped hybridized nanogenerator
A ship-shaped hybrid nanogenerator (SHNG) consisting of three TENG and one EMG (Fig.22) can work in freestanding rolling and contact-separation modes at the same time [292]. SHNG mainly includes three parts, namely contact out TENG (CS-TENG) and EMG between the outside of the cabin and the inside of the hull, and independent rolling mode TENG (FR-TENG) inside the cabin (Fig.22(a)) [293]. The hull of SHNG is made by 3D printing technology, which is light in weight, well-sealed, and can float on the water surface (Fig.22(e)) [272]. The FR-TENG consists of a silicon film, a grating electrode (Fig.22(b)), and a separate roller (Fig.22(d)). CS-TENG consists of a flexible magnet (Fig.22(c)), a silicon film, a copper electrode, and a magnet at both ends of a separate roller (Fig.22(d)). EMG consists of copper coils embedded in the outer hull of the inner hull and magnets at both ends of the separate rollers shown in Fig.22(d).
Fig.22 The SHNG. (a) The overall design of SHNG; (b) schematic diagram of FR-TENG stator bottom; (c) schematic diagram of CS-TENG between the cabin and the hull; (d) schematic diagram of the rolling roller; (e) photos of SHNG; (f) CS-TENG and EMG working mechanism schematic diagram; (g) FR-TENG working mechanism schematic diagram. Reprinted with permission from Ref. [292], copyright 2018, Elsevier. |
When SHNG is excited by the water wave, independent rollers roll back and forth on the silicon film at the bottom of the ship, thus causing the FR-TENG to generate a periodic current (Fig.22(g)). The movement of the roller causes the magnets at both ends of the roller to attract the flexible magnet as shown in Fig.22(c) periodically, which results in the periodic contact between the copper electrode and the silicon film, thus generating alternating current (Fig.22(f)). At the same time, the magnets at both ends of the roller cause the periodic change of magnetic flux in the copper coil, thus generating alternating current (Fig.22(f)). The SHNG can produce a peak power of up to 850 μW due to the combination of multiple structural designs. Meanwhile, when charging the capacitor, SHNG has a higher charging voltage and charging speed than the single EMG and TENG. In addition, SHNG is capable of lighting 55 LEDs while meeting the power requirements of a digital temperature hygrometer.
(2) Hybridized ocean wave nanogenerator (How-NG)
A How-NG based on honeycomb-like three electrodes includes TENG and EMG (Fig.23), which can work in the in-plane sliding mode and the vertical contact-separate mode [294]. The TENG works in both in-plane sliding mode and vertical contact-separation mode, corresponding to the capture of the two types of wave energy, kinetic energy and potential energy of ocean surface waves, respectively. The in-plane sliding mode can harvest the kinetic energy of waves, while the vertical contact out mode can harvest the potential energy of waves [295].
Fig.23 The How-NG. (a) The design of the How-NG schematic diagram; (b) structure of TENG electrodes; (c) the Cu coils of EMG; (d) the nanostructure of PTFE surface; (e) photos of the How-NG; (f–h) simulated change of How-NG potential in working state. Reprinted with permission from Ref. [294], copyright 2018, Elsevier. |
How-NG is composed of two parts (Fig.23(a)). The lower part is a honeycomb electrode at the bottom, covered by PTFE film and copper coil at the bottom of the electrode (Fig.23(b)), and the upper part is a movable magnet with aluminum film at the bottom suspended by three springs spaced 120 degrees from each other (Fig.23(c)). The actual picture of How-NG is shown in Fig.23(e). TENG consists of aluminum film attached to the bottom of a removable magnet, a honeycomb electrode at the bottom, and PTFE film covered over the electrode. EMG consists of movable magnet and copper coil attached to the base of the electrode. In addition, PTFE was treated by reactive ion etching to improve its surface roughness, and the SEM image after treatment was shown in Fig.23(d).
As shown in Fig.23(b), the electrodes are divided into three categories: A, B and C, and distinguished by different colors to illustrate their working principles. TENG’s work procedure is shown in Fig.23(f). When How-NG is moving horizontally under the action of water waves, the magnet slides with the movement of the device. It is assumed that the initial position of the magnet is above electrode A, since there are non-homonymous electrodes around each electrode (Fig.23(b)), when the magnet is sliding under the force of waves, a potential difference will be generated between the two non-homonymous electrodes, resulting in the flow of electrons due to the combined action of electric and electrostatic induction based on friction. Then, due to the force of the spring and the reverse force of the water wave, causing the electrons to flow in the opposite direction, producing a reverse current. In addition, if How-NG moves vertically under the action of water waves, the aluminum film and PTFE film will periodically contact, thus generating a periodic current. At the same time, whether the magnet moves horizontally or vertically, it will lead to a periodic change in the magnetic flux of the copper coil, resulting in a periodic current in the coil.
Due to its unique electrode structure, How-NG can not only harvest the kinetic energy of waves in almost all directions, but also effectively harvest potential energy. In a real water wave environment, How-NG can charge a 4.7 μF commercial capacitor to 2 V in 23 seconds. In addition, by integrating two How-NG units, 108 LEDs can be lit at the same time, which indicates that the How-NG has the potential for large-scale integration to obtain marine energy.
(3) Multifunctional hybrid power unit
A multifunctional hybrid power unit (Fig.24) includes three parts: contact-separate mode TENGs (CS-TENG), freestanding sliding mode EMGs (FS-EMG), and commercial water-proof silicon based solar cells (WS-SCs), which can harvest wave energy in multiple directions and harvest solar energy at the same time [193].
Fig.24 Hybrid power unit device structure. (a) Structure diagram of functional parts of the hybrid power unit; (b) CS-TENG unit; (c) SEM images of polymer nanowires in PTFE membrane (scale 1 microns); (d) FS-EMG unit; (e) tilt view photo of a hybrid power unit; the lower right corner illustration is the photo of the bottom of the unit; (f) the working mechanism of the hybrid power unit. Reprinted with permission from Ref. [193], copyright 2017, Elsevier. |
The overall structure of the device is shown in Fig.24(a). The CS-TENG includes an upper layer of aluminum film and a lower layer of PTFE film (Fig.24(b)), and a nanowire structure is fabricated on the surface of the PTFE film to improve the surface roughness (Fig.24(c)) [296,297]. The structure of FS-EMG is shown in Fig.24(d), consisting of a sandwich structure composed of magnets at the top of the CS-TENG, copper coils at the bottom of the CS-TENG, and magnets on a sliding acrylic plate at the bottom of the device. WS-SCs are located on top of the box that encapsulates CS-TENG. The actual picture of the multifunctional hybrid power unit is shown in Fig.24(e).
The working process of the multi-function hybrid power unit can be disassembled into three parts (Fig.24(f)). The working process of CS-TENG includes two parts. The magnets on the top of the CS-TENG move up and down when subjected to the force of the waves, causing the CS-TENG to continuously contact and separate, thus generating alternating currents. At the same time, the sliding acrylic plate at the bottom of the device slides left and right, and the periodic attraction of the upper and lower magnets also results in CS-TENG constant contact separation, which generates alternating currents. For FS-EMG, the sliding acrylic plate at the bottom of the device will slide back and forth when subjected to the force of waves, resulting in the constant change of the relative position of the two magnets, which results in the periodic change of the magnetic flux through the Cu coil. According to Lenz’s law, the Cu coil will generate a periodic current. WS-SCs are located on the top of the CS-TENG box and can work with sun exposure.
The device combines CS-TENG, FS-EMG, and WS-SCs. CS-TENG can effectively harvest low-frequency water wave energy, while FS-EMG can harvest higher-frequency wave energy. Moreover, there is an abundant light on the sea so that WS-SCs can harvest solar energy very well. Experiments have shown that the multifunctional hybrid has a good output in different weather conditions, which can directly power commercial supercapacitors or drive LEDs. In addition, the number of CS-TENG, FS-EMG, and WS-SCs of the device can be self-regulated according to actual needs, which has a very important reference function for the hybrid power unit in blue energy harvesting.
(4) Hybridized water wave energy harvester (H-WWEH)
Zheng et al. [298] proposed an H-WWEH (Fig.25) with swinging magnetic structure with permanent magnets as mass blocks, which mainly includes TENG and EMG (Fig.25(a) and 25(b)). The TENG consists of a swinging friction layer, bottom friction layer and Tai Chi shaped electrode; the EMG consists of a swinging permanent magnet and a bottom copper coil (Fig.25(c)). When the device is excited by water waves, the swing friction layer swings periodically, and the TENG module generates alternating current output (Fig.25(d)); with the periodic oscillation of the oscillating friction layer, the permanent magnet also oscillates, so that the copper coil periodically cuts the magnetic induction line, and therefore the EMG module induces current (Fig.25(e)). Through optimization design (the distance between two friction layers is 3 mm, TENG and EMG are connected in parallel), in the simulated water environment (1.75 Hz), the output voltage and current of TENG and EMG can reach 90 V and 0.61 μA, 5.3 V and 6.4 mA, respectively. In addition, H-WWEH can also be used for marine environmental monitoring. Once the wave height exceeds the trigger threshold, and the inclination switch is turned on to recognize early warning of self-powered wireless wave height (Fig.25(b)).
Fig.25 H-WWEH. (a) The photo of H-WWEH; (b) schematic diagram of self-powered wireless water level alarm system; (c) the detailed structural diagram of H-WWEH; (d) the working principle of TENG; (e) the change of magnetic induction intensity during EMG operation. Reprinted with permission from Ref. [298], copyright 2021, Elsevier. |
5 Networking strategy of large-scale TENGs
The output voltage of TENG is high and the current is low, which makes it difficult to directly supply power to microminiature electronic devices [8,114,282,287,299–304]. Therefore, necessary energy storage units are needed [136,305–313]. The output power of a single TENG unit is relatively low [112,114,278,314–320], which cannot meet the demand of high power and requires large-scale integration. Moreover, the energy of TENG comes from the mechanical movement in the environment, and its output has great randomness, so the power management module (PMM) is needed to control the output of TENG. Moreover, effective power management methods are even more essential for large-scale integration. Fortunately, TENG has developed rapidly in recent years and has made great progress in large-scale application, energy storage [321–326], and power management [327–332].
5.1 Components (or arrays or networks) of large-scale TENGs
A pendulum inspired TENG (P-TENG) (Fig.26) is expected to realize TENG’s large-scale energy harvesting [333], and its structure is shown in Fig.26(a–c). The electrode consists of an inner circular electrode and an outer annular electrode, and a layer of PTFE film covers the surface of the electrode layer. The triboelectric materials are respectively copper films serving as pendulums and PTEF strips uniformly fixed on the edge of cambered acrylic. Among them, there is a gap of 1 mm between the pendulum and the PTFE film covered by the electrode layer, which not only ensures the freedom of the pendulum, but also reduces the material loss.
Fig.26 Pendulum inspired TENGs. (a) The overall structure of the P-TENG; (b) physical picture of the pendulum; (c) physical picture of the P-TENG; (d) schematic illustrations of the triboelectrification process; (e) the working principle of the P-TENG; (f) the rectification circuit for the array; (g) the P-TENG array to power a thermometer; (h) the practical power supply application of P-TENG; (i) large-scale TENG network scenario diagram. Reprinted with permission from Ref. [333], copyright 2019, Elsevier. |
The pendulum swings back and forth when stimulated by an external force, generating frictional charges when in contact with PTFE strips (Fig.26(d) and 26(e)). Then, the swing of the pendulum with positive charges will cause a varying potential difference between the two electrodes, thus generating alternating current. Large-scale energy can be harvested by rectifying a plurality of P-TENG units and then forming an array in parallel (Fig.26(f)). As shown in Fig.26(g), a 2 × 3 P-TENG array is tested, which can charge a 100 F capacitor to 3.1 V within 780 s, and the stored electricity can be used by thermometer for temperature detection (Fig.26(h)). It can be expected that when thousands of units are connected in parallel, there will be very considerable output, Fig.26(i) is a schematic diagram of this idea.
A low-cost, durable, and large-area fabric-based TENG (FB-TENG) (Fig.27) manufactured by melt-blowing technology has a good application in large-area energy harvesting and self-powered sensing [334]. The friction layer of FB-TENG is polypropylene non-woven fabrics (PP-NWF) and commercial polyamide 66 fabrics (PA-66-F). The PP-NWF is made of melt-blown PP particles by a melt-blown machine (FCN-2) at 200 °C. The schematic manufacturing process is shown in Fig.27(a).
Fig.27 FB-TENG. (a) The manufacturing process of PP-NWF; (b) the working principle of FB-TENG; (c) photograph of the self-powered pedestrian volume harvester; (d) schematic diagram of the self-powered pedestrian volume collector (SP-PVC); (e) response and recovery time of the proposed SP-PVC. Reprinted with permission from Ref. [334], copyright 2019, Elsevier. |
The working mechanism of FB-TENG is shown in Fig.27(b). Before the start of work, the two triboelectric layers are separated from each other, the surfaces are uncharged, and no current is generated in the circuit. Once the two triboelectric layers come in contact under the action of external force, according to the triboelectric sequence, the surface of PP-NWF is positively charged, and the surface of PA-66-F is negatively charged. When the pressure is released, the gap between the two friction layers increases, and current flows between the back electrodes of the two friction layers. When FB-TENG is subjected to external force again, a potential difference opposite to that before is generated between the two electrodes, and the current flow is opposite to that before. So far, a complete power generation cycle has been completed.
A 400 mm × 800 mm large-area SP-PVC was fabricated with FB-TENG to record pedestrian flow in a specific period of time (Fig.27(c) and 27(d)). As shown in Fig.27(e), with ISC as the output signal of SP-PVC, the measured response time is 44.4 ms and the recovery time is 84.5 ms, which indicates that the response and recovery time are short enough to harvest pedestrian flow information.
An encapsulated TENG with magnetic joints (Fig.28) has the characteristics of self-assembly and easy reconfiguration, which provides a possible way for large-scale utilization of wave energy [335]. Each TENG unit is formed by tightly packaging electrode balls welded by 13 transverse electrode plates and four longitudinal electrode plates with PP shells (Fig.28(a)). The front and back sides of the substrate are printed with copper layers by electronic printing. There are many spaces filled with FEP pellets in the electrode balls welded. The working principle of TENG is shown in Fig.28(b). When TENG moves under the action of external force, the FEP pellets roll between the electrode plates. According to the triboelectric sequence, the surface of FEP pellets is negatively charged, and the surface of the Cu electrode is positively charged. Periodic changes in the number of FEP pellets corresponding to the electrodes on both sides will cause electrons to flow periodically between the electrodes on both sides, thus generating alternating currents.
Fig.28 A macro self-assembly network of encapsulated TENGs. (a) The overall structure of the TENG unit; (b) the working principle of the TENG unit; (c) the self-assembling process of self-adaptive magnetic joints (SAM-joints, without the limit block); (d) schematic diagram of horizontal movement between TENG units; (e) schematic diagram of vertical movement between TENG units; (f) TENG network self-assembly process. Reprinted with permission from Ref. [335], copyright 2019, Elsevier. |
In order to realize large-scale application and better adapt to various marine environments, a SAM-joint is installed on the outer shell of the TENG unit. SAM-joint consists of rotatable spherical magnet and limiting block. Among them, the application of the rotatable spherical magnet avoids assembly errors. Regardless of the initial orientation of the magnets, once the distance between the two SAM-joints is close enough, the two spherical magnets will automatically rotate by mutual force to adjust their positions until the two magnets are attracted together in opposite directions (Fig.28(c)). At the same time, the use of the limiting block can greatly limit the rotation of the connected TENG unit in the horizontal direction (Fig.28(d)), but has little influence on the rotation movement in the vertical direction (Fig.28(e)).
In order to verify the self-assembly behavior of the TENG unit in actual water environment, 16 model balls with 4 SAM-joints are used for testing. The testing shows that the TENG unit with SAM-joints has well self-assembly performance (Fig.28(f)). Subsequently, a 4 × 9 array consisting of 2 × 9 TENG units and 2 × 9 model balls of four SAM-joints was used to test the water wave energy harvesting performance of the TENG network. The 18 TENG units connected in parallel can easily light up 300 LEDs, and when charging the capacitor, it can make the capacitor supply power to the thermometer and keep the voltage of the capacitor rising continuously.
5.2 Charging and modular storage
A self-charging power supply device (Fig.29) has been developed by integrating MXene-based electrochemical micro-supercapacitor (MSC) with TENG [336]. The manufacturing process of the solid MSC is shown in Fig.29(a). Firstly, the pre-patterned substrate is laser cut, then the active substance pattern is formed by MXene spraying, and finally the cross-linked gel electrolyte is coated on the whole equipment.
Fig.29 MXene-based electrochemical MSC with TENG. (a) The manufacturing process of the solid MSC; (b) the overall structure of the device (Illustration is a photo of the device worn on the arm); (c) the equivalent circuit diagram of the device; (d) charging curve of the MSC charged by TENGs at various frequencies. (e) self-discharge measurement; (f) four series-connected charged MSCs supply power for the commercial hygrometers. Reprinted with permission from Ref. [336], copyright 2018, Elsevier. |
The designed TENG is a single electrode mode, with silicone and human skin as two friction layers and carbon fiber as the electrode layer. The self-charging power supply device is made by using silicone rubber as packaging material to integrate single-electrode mode TENG, rectifier, and MXene-based solid-state MSC into a monolithic device. The structural schematic diagram and equivalent circuit diagram of the device are shown in Fig.29(b) and 29(c), respectively. The obtained self-charging power supply device can be wound on the wrist like a wristband (Fig.29(b)) without any discomfort. Experiments show that when the motion frequency is 10 Hz, TENG can charge MSC to 0.11 V within 200 s (Fig.29(d)), while the self-discharge rate of MSC is only 0.1 V·h–1 (Fig.29(e)). Four series-connected charged MSCs can supply power for the hygrometer (Fig.29(f)).
A self-charging lithium-ion battery (Fig.30) has been designed based on the power generation principle of TENG [337]. The device consists of two flexible lithium-ion batteries and an FEP film between the two lithium ion batteries (Fig.30(a) and Fig.30(b)). Fig.30(c) is a schematic diagram of the manufactured flexible lithium ion battery. LiMn2O4 nanowires on an aluminum electrode manufactured by an electrospinning method are used as cathodes of the lithium ion battery, C nanowires on a copper electrode are used as anodes of the lithium ion battery, PP is used as a diaphragm, and two layers of aluminum plastic (PA\Al\polyethylene (PE)) are used as encapsulation layers. The FEP film fixed at both ends and the PA of the two flexible lithium ion battery packaging layers are used as friction materials (Fig.30(d)). When the FEP film vibrates between the two PA layers due to wind force, alternating current will be generated.
Fig.30 Self-charging Li-ion battery based on the generation principle of TENG. (a) Photograph of the self-charging Li-ion battery; (b) FEP film between two flexible lithium ion batteries and two lithium ion batteries; (c) schematic diagram of the manufactured flexible lithium ion battery; (d) schematic illustrations of the self-charging Li-ion battery; (e) the charging circuit structure of lithium ion battery. Reprinted with permission from Ref. [337], copyright 2017, Wiley-VCH Verlag GmbH. |
In order to meet the charging requirements of the lithium ion battery, a circuit structure is designed as shown in Fig.30(e). The transformer is used to reduce the voltage signal of TENG, the rectifier converts alternating current into direct current, and four switches are used to control the charging behavior of the lithium ion battery (if S1 and S2 are on, S3 and S4 are closed, the top lithium ion battery is charged; while S3 and S4 are on and S1 and S2 are closed, the lithium ion battery at the bottom is charged). After using the transformer, the maximum output power is about 0.7 mW. At the wind speed of 10 m·s–1, the lithium-ion battery can be charged from 1.5 to 3.5 V within three minutes, and the voltage above 1.5 V can be maintained for about 142 s with a constant discharge current of 0.1 mA.
An all-solid-state sodium-ion battery (Fig.31) that can be integrated with TENG has been designed [338]. The device consists of a rotating TENG, a rectifier bridge, and an all-solid-state sodium ion battery (Fig.31(a)). The designed all-solid-state sodium-ion battery uses a hexagonal P2 structure Na0.67Ni0.23Mg0.1Mn0.67O2 on aluminum foil as cathode, metal sodium as anode, and Na-SPE (solid polymer electrolyte) polymer as an electrolyte membrane. The rotary TENG consists of a stator and a rotor (Fig.31(a) and 31(b)), both of which are plated with radially arranged copper electrodes, wherein the copper electrode of the stator is also coated with a Kapton film, which together with the copper on the rotor serves as the friction layer of the TENG.
Fig.31 An all-solid-state sodium ion battery that can be integrated with TENG. (a) The all-solid-state sodium ion battery storage is integrated with TENG; (b) the partial enlargement of TENG; (c) the charging and discharging curves of the all-solid-state sodium ion battery; (d) the discharge curve of the all-solid-state sodium ion battery charged. Reprinted with permission from Ref. [338], copyright 2017, Wiley-VCH Verlag GmbH. |
Comparing the charging and discharging curves of all-solid-state sodium ion battery under constant current charging (Fig.31(c)) and TENG charging (Fig.31(d)), under the two charging modes of constant current and TENG, the discharge capacity of the all-solid-state sodium ion battery is the same at a current density of 96 mA·g–1, which is 40 mAh·g–1. When the rotating speed of TENG is 800 r·min–1 and the all-solid-state sodium ion battery discharges at a current density of 5 mA·g–1, the energy conversion efficiency reaches 62.3%, which indicates that the all-solid-state sodium ion battery can effectively harvest mechanical energy harvested from TENG.
5.3 Power management
A TENG with unidirectional switch (TENG-UDS) and passive power management circuit (PMC) (Fig.32) can maximize the external output without being affected by the load [339]. TENG-UDS includes a freestanding triboelectric-layer mode TENG and a UDS. The structural schematic diagram is shown in Fig.32(a). PTFE is a friction layer of freestanding triboelectric-layer mode TENG, and copper is both a friction layer and an electrode layer. The UDS consists of two moving contacts that can slide with the PTFE layer and four fixed spring contacts.
Fig.32 TENG-UDS and passive PMC. (a) The overall structure of the TENG-UDS; (b) the working principle of the TENG-UDS; (c) the correlation between voltage peak and resistance values of TENG-UDS and TENG without switches (TENG-WOS); (d) the correlation between output energy and resistance values of TENG-UDS and TENG-WOS; (e) the working mechanism of the passive PMC for TENG-UDS. Reprinted with permission from Ref. [339], copyright 2018, Wiley-VCH Verlag GmbH. |
When the PTFE layer slides to the right, the moving contact contacts the spring contact on the right, the circuit conducts, and current flows through the load from top to bottom (Fig.32(b)). Then the PTFE layer slides to the left, the current flowing through the load from top to bottom is also generated. Unlike the conventional TENG without switches (TENG-WOS), the voltage peak Vp and the half-cycle output energy E of TENG-UDS are independent of the resistor R (Fig.32(c) and Fig.32(d)). According to this characteristic, a simple passive PMC composed of a capacitor, an inductor, and a diode is designed (Fig.32(e)). The working process of the passive PMC is divided into two stages. In the first stage, electric energy generated by TENG-UDS is converted into magnetic energy and stored in inductor L. In the second stage, the magnetic energy in L is converted into electric energy and stored in capacitor C2. Theoretical calculation shows that the theoretical energy storage efficiency of passive PMC can reach 75.8%. In the actual experiment, the measured energy storage efficiency can reach 48.0%. TENG-UDS can drive the electronic watch to work continuously at a working frequency of 1 Hz.
A self-powered electrostatic vibrator switch was combined with a voltage transformer to form a power management system (Fig.33) which can manage and optimize the output of TNEG [340]. Taking the TENG structure with an electrostatic vibrator switch (TENG-EVS) based on the traditional lateral sliding mode TENG as an example, the working mechanism of the TENG-EVS is briefly explained (Fig.33(a)). When the PTFE film slides to the right, a potential difference will be generated between the copper electrodes on the left and right sides (Fig.33(b)). Since the switch is not closed, the electric charge cannot flow between the electrodes on both sides, and the potential difference cannot be balanced. Moreover, with the sliding of the PTFE membrane, the potential difference between the two electrodes also increases until the electrostatic attraction force at both ends of the switch’s electrostatic vibrator is sufficient to close the switch. After the circuit is turned on, the positive charge flows from the left electrode to the right electrode until the potential difference is balanced. At this time, the electrostatic attraction force at both ends of the switch of the electrostatic vibrator disappears and the switch is disconnected. Similarly, sliding PTFE to the left will generate a pulse current opposite to that before.
Fig.33 The TENG structure with an electrostatic vibrator switch. (a) The structure diagram of the TENG-EVS; (b) the working principle of the TENG-EVS; (c) the structure diagram of the rotation disk mode TENG-EVS; (d) the schematic diagram of the powering circuit configuration; (e) the voltage curve of a 47 μF capacitor, as charged by four different powering circuit configurations, and (f) the corresponding curves of the stored energy in the capacitor. Reprinted with permission from Ref. [340], copyright 2018, Elsevier. |
The peak output current of TENG-EVS is much higher than that of traditional unswitched TENG, and under the condition of frequency of 17.2 Hz and the load resistance of 0.1 MΩ, the peak output current (7.2 mA) of TENG-EVS can reach 4186 times that of traditional unswitched TENG. The peak output power (5.13 W) is 1.8 × 107 times that of the conventional unswitched TENG. In order to maximize the output energy, the rotation disk mode TENG is further adopted instead of the lateral sliding mode TENG to increase the frequency (Fig.33(c)). For a clear understanding of the actual performance of TENG-EVS, a transformer is added to the circuit (Fig.33(d)), and then the 47 μF capacitor is charged with four configurations, namely, TENG-EVS with transformer, TENG-EVS without transformer, traditional TENG with transformer, and traditional TENG without transformer. The results show that the voltage and energy storage of the capacitor after 15 s charging of the TENG-EVS with transformer are much higher than those of the other three configurations, 10.2 V and 2.43 mJ respectively (among which the voltage and energy storage of the capacitor after 15 s charging of the traditional TENG with transformer are only 0.24 V and 1.35 μJ) (Fig.33(e) and Fig.33(f)).
A TENG network integrated with a PMM (Fig.34), can effectively utilize wave energy [295]. A Z-shaped multi-layer TENG structure is formed by taking FEP as a friction layer, Al as a friction layer and an electrode layer, and a Kapton film as a substrate, and four rigid springs are fixed around to enable TENG to better work in a contact-separate mode (Fig.34(a)). Finally, the obtained TENG of the multi-layer structure is packaged into a spherical shell. When the spherical TENG unit is subjected to the force of water waves, Al and FEP surfaces of a plurality of TENG units of a multi-layer structure TENG are in contact with each other to generate frictional charges (Fig.34(b)). With the help of the spring force, the two contact surfaces are separated, and the potential difference between the upper and lower electrodes drives the electrons flow. Then periodic contact produces periodic current.
Fig.34 TENG network integrated with PMM. (a) Schematic diagram of Z-shaped multi-layer TENG; (b) the working mechanism of the spherical TENG; (c) the connection mode of TENG network; (d) photograph of the TENG networks; (e) the power management mechanism; (f) the output voltage of TENG network of PMM with different resistances; (g) charging curve of 10 mF capacitor with or without PMM; (h) practical application of TENG network with PMM. Reprinted with permission from Ref. [295], copyright 2019, Wiley-VCH Verlag GmbH. |
According to the method shown in Fig.34(c), seven spherical TENG are connected by cable ties to form a hexagonal TENG network. Fig.34(d) is a photograph of the TENG network. Since the movements of the seven spherical TENG are not completely synchronized, in order to avoid the mutual influence between the currents generated by the seven spherical TENG, the seven spherical TENG are first connected to seven rectifiers respectively, and then connected in series. To improve the power supply’s efficiency, a PMM is designed to manage the power harvested by the TENG network. The PMM is a Buck circuit composed of a switch (Metal-Oxide-Semiconductor Field-Effect Transistor), a diode, an inductor, and a capacitor (Fig.34(e)). Through the integrated PMM, the voltage of resistors with different resistance values can reach a stable state in a very short time (Fig.34(f)). The TENG network charges the 10 mF capacitor with management module and the TENG network without management module respectively (Fig.34(g)). After 60 s, the TENG network with management module charges the capacitor voltage to 0.345 V, while the TENG network without management module only charges the capacitor voltage to 0.035 V, with a 96-fold difference in stored energy. In addition, the TENG network integrated with the PMM can drive the digital thermometer to work continuously for a long time under the lateral sine wave motion of 1.5 Hz (Fig.34(h)).
5.4 Economic benefit evaluation
Based on triboelectrification and electrostatic induction effects, TENG has the ability to harvest energy released by almost all moving objects in nature and convert it into electric energy. Some people think that TENG is clean and pollution-free, the materials used are cheap, and the production cost is low. It is a new green and clean energy. Of course, there are also some people put forward that the base and friction layer of most friction nanogenerators are based on synthetic polymer materials, and are skeptical about their sustainability, environmental protection, low cost, biocompatibility, and degradability.
Ahmed et al. [341] took two different representatives TENG modules as examples to compare and discuss the life cycle, performance efficiency, production cost, environmental impact, and other aspects with the existing energy harvesting technology (Fig.35). The two selected TENG modules are shown in Fig.35(a) and Fig.35(b), Module A is a micro-grating TENG (MG-TENG) based on thin films, and Module B is a TENG based on two radially arranged fine electrodes. Fig.35(c) shows the comparison results of two TENG modules and the other eight energy harvesting technologies in terms of Eco-indicator 99. Among them, the two TENG modules have the lowest harmful effects on human health, resources, and ecosystem quality. Fig.35(d) shows the comparison between two TENG modules and seven photovoltaic technologies in terms of energy payback period (EPBP). Module A’s EPBP is shorter than all photovoltaic technologies, and the EPBP of Module B is also shorter than most photovoltaic technologies. Fig.35(e) compares CO2 emission factors. Module A is the lowest, but Module B has a higher CO2 emission factor. Finally, the Levelized cost of energy (LCOE) of two kinds of TENG and existing energy sources is compared. As shown in Fig.35(f), the LOCE of Module A is the lowest, and the LCOE of Module B is equivalent to hydropower.
Fig.35 Economic benefit analysis of two TENG modules. (a) Structural diagram of the TENG Module A; (b) structural diagram of the TENG Module B; (c) the comparison between two TENG modules and other eight energy harvesting technologies in terms of Eco-indicator 99; (d) the comparison between two TENG modules and seven photovoltaic technologies in terms of EPBP; (e) the comparison of CO2 emission factors; (f) the comparison of LCOEs. Reprinted with permission from Ref. [341], copyright 2017, The Royal Society of Chemistry. |
After the above comparison, TENGs have higher environmental friendliness and lower cost. It is worth noting that module A has the best performance in all comparisons. But in fact, the performance efficiency of the TENG module is very sensitive [302,318,342]. As its service life increases, its service efficiency will be greatly reduced, far lower than other technologies [343]. Therefore, once the problems of efficiency and longevity of TENG are solved, TENG technology is expected to become one of the most environmentally friendly and economical energy harvesting technologies.
6 Summary and perspective
As one of the three renewable energy sources provided by nature for free, blue energy has a fatal temptation to human development. However, due to the limitations of current technology, only relying on traditional EMG, the water wave energy utilisation rate is very low. TENG has an entirely different working mechanism from EMG, and has been developed rapidly in recent years due to its incomparable advantages in low-frequency energy harvesting. Based on the working principle, this paper reviews the latest progress of TENG in Marine energy harvesting, and proves the potential of TENG in wave energy harvesting. Although TENG’s performance to capture water wave energy is gradually improving, it is even expected that TENG will be able to make significant contributions not only to power small electronic devices, but also to power the grid in the near future. However, the TENG technique is still not directly applicable to the actual Marine environment. There are many problems for the TENG to become a major driver of blue energy dreams in the future.
First, the performance of nanogenerator materials needs to be further improved. For TENG applications in the marine environment, frequent replacement of TENG devices is impossible, so good durability of nanogenerator materials is required. The amount of charge on the surface of the triboelectric layer determines the amount of energy we can obtain from the TENG, which needs to be further enhanced to meet the requirements better. Second, storage and management systems for large TENG networks need to be further optimized. Although existing TENG management systems are already highly efficient, future hybrid energy use requires a storage and management system that can be used for different mechanisms of electrical energy. Finally, there is the issue of energy transport between land and sea. The large-scale use of TENG devices in the ocean requires careful planning of the location and size of the TENG network to minimize the impact of ocean waves on power lines and to minimize the impact of the TENG network on marine transportation, marine life, and related marine industries. The development and utilization of blue energy is a huge project. Still, once the dream of “blue energy” is realized, the world energy pattern will fundamentally change, which will surely benefit generations.