Corrugation-like Triboelectric Nanogenerators Integrated Buoys for Wave Energy Harvesting

Marine buoys, as the nodes of the marine monitoring system, play a vital role in monitoring marine activities, which can collect hydro-meteorological information for marine scientific research, offshore oil development, and construction of marine ports. However, sustainable and cost-effective power supply for marine buoys equipped with electrical devices is still a challenge. In this paper, the corrugation-like triboelectric nanogenerator (C-TENG) integrated with marine buoys was proposed, aiming at harvesting wave energy and powering marine buoys. The C-TENG consisting of corrugated plates, corrugation-like channels, aluminum electrodes, barrier sheets, flat plates, and polytetrafluoroethylene (PTFE) pellets, was capable of effectively converting the energy harnessed from waves into electricity. As a result of the ocean wave’s excitement, the sliding contact between the PTFE pellets and the aluminum electrode produced electron transfer via the triboelectrification effect and electrostatic induction. With the unique corrugation-like channels, the output of the devices was in the same phase, avoiding the electrical loss resulting from rectification. Furthermore, the power density of the 5 layers parallel-connected C-TENG units reached 21.95 W/m3, which was approximately 300% higher than the previous tower-like TENG. The experimental results prove that the proposed C-TENG may provide a possible solution for wave energy harvesting and marine buoy powering.


Introduction
In recent years, marine information monitoring system plays a significant role in marine environment development, oil and gas resources development, and scientific research [1].The marine information monitoring system consists of a large number of ocean buoys, radar, ships, satellites, and shore-based monitoring stations [2].Especially, as the nodes of the marine monitoring system, marine buoys, equipped with various types of sensors and electronic devices, are important tools for hydro-meteorological information obtaining [3].However, large-scale deployment of marine buoys requires an economical and stable power supply.Until now, marine buoys are mainly powered by a combination of solar energy and batteries, which is gradually unable to meet the working requirement of the buoys [4].Consequently, there is an urgent demand for a stable, sustainable, and cost-effective power supply to the marine buoys.
Wave energy, as clean and renewable high-grade energy, is widely distributed in the environment where marine buoys work [5].Besides, wave energy is more stable, predictable, and has a higher power density compared to traditional clean energy sources [6], which may provide an economical and reliable solution for marine buoy powering.Currently, the exploitation of wave energy is mainly based on traditional electromagnetic generators (EMG), such as oscillating water columns, buoyancy pendulums, and oscillating floating body wave energy harvesting devices [7].However, they are not suitable for marine buoys due to their large mass, large size, high cost, and difficult installation.Meanwhile, the irregularity and low-frequency features of ocean waves limit the development of EMG devices for efficient wave energy harvesting [8].
A triboelectric nanogenerator (TENG) [9], which operates on the principle of the triboelectric effect and electrostatic induction coupling, has shown promise for capturing irregular mechanical energy and converting it into electricity.Meanwhile, TENG has significant advantages over traditional electromagnetic generators in low-frequency wave (<5 Hz) energy harvesting [10].Previously, some TENG-based wave energy collection studies have been carried out.For example, Zhang et al. [11] created triboelectric nanogenerators based on sea snake structures, consisting of multiple-segmented units connected by spring, which can capture wave energy effectively, yet cannot capture wave energy in arbitrary directions.Xu et al. [12] designed a tower-shaped triboelectric nanogenerator with a 3D printed arc surface and nylon dielectric film with pair of attached aluminum electrodes that can harvest wave energy in arbitrary directions while having low power density and spatial utilization.Wang et al. [13] introduced a triboelectric nanogenerator composed of polytetrafluoroethylene (PTFE) pellets and a pair of aluminum electrodes arranged in a sandwich-like structure, which has higher power density and space utilization, yet it is difficult to guarantee that the output of S-TENG is in the same phase, resulting in power losses.Therefore, new technologies for ocean wave energy harvesting are still a research topic that requires to be explored.
This paper proposed the integration of a corrugation-like triboelectric nanogenerator (C-TENG) with buoys for the purpose of harvesting wave energy and supplying power to marine buoys.The C-TENG consisted of corrugated plates, corrugation-like channels, aluminum electrodes, barrier sheets, flat plates, and polytetrafluoroethylene (PTFE) pellets.Especially, the aluminum electrodes were attached in pairs to the surface of the corrugated plates and the flat plates.Under the excitation of wave motions, the PTFE pellets could roll freely on the surface of aluminum electrodes to convert mechanical energy into electrical power via the triboelectrification effect and electrostatic induction.With the aid of the corrugation-like channels, the PTFE pellets roll in the same direction and the output of the devices was in the same phase, avoiding the electrical consumption caused by rectification devices.The analysis of experimental results revealed that the output of the C-TENG increased proportionally with the number of units.Moreover, the five layers of parallel-connected C-TENG units' power density reached 21.95 W/m 3 , indicating the potential for large-scale integration of the devices.Thus, the proposed C-TENG might provide a feasible solution for blue energy harvesting and has a broad application prospect in the fields of marine environmental monitoring.

Design
Fig. 1(a) indicates the potential application of the C-TENG integrated marine buoys.Specifically, the buoys equipped with multiple C-TENG are capable of supplying power to their own sensors and electrical devices.Fig. 1(b) demonstrates the diagrammatic representation of the units responsible for generating power, which consists of five layers in the same direction of stacking and enclosed with an acrylic sealing shell.Besides, the number of C-TENG units in parallel connection can be adjusted to meet the various electrical power requirement.Fig. 1(c) indicated a detailed view of the C-TENG structure, consisting of 3D printed polylactic acid (PLA) corrugated plates (length of 115 mm, width of 120.5 mm, thickness of 2 mm, and height of 16 mm), PLA barrier sheets (120.5 mm in length, 16 mm in width, and thickness of 2 mm), PLA flat plates (120.5 mm in length, 120.5 mm in width, and thickness of 2 mm), PTFE pellets (diameter of 10.5 mm), and aluminum electrodes (length of 185 mm and width of 54 mm) attached in pairs to the surface of PLA corrugated plates and PLA plates.Fig. 1(d).depicts the operating principle of C-TENG.PTFE pellets, which possess electret features, move freely across the surfaces of the aluminum electrodes, driven by wave motion.Upon contact, the pellets become negatively charged, and these charges persist on the PTFE surface.As a result of the electrostatic induction, when the negatively charged pellets roll from the left section to the right section, positive charges will be generated on the right part of the aluminum electrodes, and electrons will transfer from the right electrodes to the left electrodes through the external circuit.When PTFE pellets roll back, the electrons on the left side electrodes will transfer to the right side electrodes through the external circuit.Up to this point, an entire electrical power cycle is generated.

Experiments and Results
Fig. 2(a) shows the physical components of the experiment setup.To more clearly describe the experimental results, we define amplitude and frequency as A (50 mm < A < 130 mm) and f (0.4 Hz < f < 2.0 Hz).In order to achieve optimal performance of the C-TENG unit, depicted in Fig. 2(b) and Fig. 2(c), an investigation was conducted on the impact of PTFE ball count on output performance.The results demonstrated that the C-TENG unit achieved its optimal output when utilizing five PTFE balls.This is because the total diameter of the five PTFE balls is the half-length of the channel, and the effective displacement of the PTFE balls is maximized.Furthermore, according to the principle of the triboelectrification effect, the greater difference in the electrical properties between the two materials used as contact tribological materials, the greater the tribological charge generated, and the greater output of the electric energy in the external circuit.As indicated in Fig. 2(d-g), we use aluminum, copper, and nickel-copper alloys as electrode materials to measure their influences on the output property of the C-TENG units at various frequencies and amplitudes, respectively.The transferred charge and short-circuit current changing trends were analyzed in conjunction with the experimental results.Notably, the output performance of the aluminum and Ni-Cu alloy-based C-TENG units were found to be similar, while significantly outperforming the copper-based counterpart.The aluminum-based C-TENG is used as the dielectric material based on cost considerations.Fig. 3(a) indicated the effect of frequency on the current signal.Under the condition of A = 130 mm, as f grows from 0.4 to 2 Hz, the current signals grow from 0.5 to 2.6 µA.This is because as the frequency increases, there are more contact separations and the charge transfer rate becomes faster, resulting in an increased output current signal.Fig. 3(b) indicates the transferred charge remains constant as the f increases from 0.4 to 2 Hz under the condition of A = 130 mm.The effect of amplitude on the performance of C-TENG is shown in Fig. 3(c-d).With the A increasing from 50 to 130 mm, the current signal grew from 0.8 to 2.6 µA, as shown in Fig. 3(c).The contact separation area increases with the motion amplitude, resulting in an increased output current signal.Fig. 3(d  To harvest wave energy efficiently, multiple C-TENG units are parallel connected without a rectifier since the output of C-TENG is in the same phase.Also, all units are stacked in the same direction and on the same block.As indicated in Fig. 4(a-b), with f = 2 Hz and A = 130 mm, when the stacked C-TENG units increase from one to five layers, the output current grows from 2.64 to 12.19 µA and the transferred charge grows from 207 to 930 nC, which indicates the feasibility of large-scale stacked C-TENG to harvest wave energy.Figure 4(c) presented the experimental results of the current signal and C-TENG units' power density across a range of external loads.Notably, optimal power density, reaching 21.95 W/m3, is achieved when the external resistance matches the internal resistance of the stacked units, which is 500 MΩ.The aforementioned value represents a threefold increase compared to the power density of the previously suggested tower-like TENG.
Fig. 4(d) depicts a schematic diagram that defines the directional load acting on the C-TENG as the angle between the wave and the electrode.The effect of directional load on the C-TENG's output property is indicated in Fig. 4(e) and Fig. 4(f).Obviously, the maximum electrical output (Isc = 11.7 µA, Qsc = 930 nC) occurs when the angle is 0°, where the acceleration direction of the rolling spheres is parallel to the direction of aluminum electrodes, and the effective displacement of the PTFE pellets is maximum.As the angle gradually rises to 90° from 0°, the output of the C-TENGs reduces due to the reduction of the effective displacement of the PTFE pellets in the corrugation-like channels.The output property of C-TENG units is symmetric with angle, revealing the potential of C-TENG units to sense wave direction.In addition, Fig. 4(g) shows the five-layers connected C-TENG charging for the capacitors with different resistance, which further validates the potential of C-TENG in marine buoy powering.

Conclusion
In this paper, a corrugation-like TENG integrated with marine buoys for wave energy harvesting and marine buoy powering was designed.The C-TENG combines the TENG principle with a unique corrugated structure, which gives it many advantageous features, such as larger power density, higher space utilization, and lower power loss.Furthermore, the simulated wave motions with a range of amplitude, frequency, and incident angles are utilized for investigating the performance of C-TENG and stacked C-TENG units in terms of their output.The power density of five layers of stacked C-TENG units reaches 21.95 W/m3, which is 300% higher than the previous work.Through the experimental research and analysis, the experimental data have validated the proposed C-TENG for harvesting wave energy and powering marine buoys.Generally, the C-TENG presents a novel and simple energy supply method for constructing distributed marine monitoring systems.

Fig. 1 (
Fig.1(d).depicts the operating principle of C-TENG.PTFE pellets, which possess electret features, move freely across the surfaces of the aluminum electrodes, driven by wave motion.Upon contact, the pellets become negatively charged, and these charges persist on the PTFE surface.As a result of the electrostatic induction, when the negatively charged pellets roll from the left section to the right section, positive charges will be generated on the right part of the aluminum electrodes, and electrons will transfer from the right electrodes to the left electrodes through the external circuit.When PTFE pellets roll back, the electrons on the left side electrodes will transfer to the right side electrodes through the external circuit.Up to this point, an entire electrical power cycle is generated.Fig.1(e) illustrated the operational principle and potential-voltage distribution of the C-TENG, as demonstrated using COMSOL Multiphysics.
) demonstrates the transferred charge increases from 95 to 220 nC as the A increases from 50 to 130 mm in the case of f = 2 Hz.The experimental data reveals the ability and potential of C-TENG to capture wave energy across a range of amplitudes and frequencies.

Fig. 2 (
Fig.2 (a) Diagrammatic representation of the experimental platform.(b).The impact of the PTFE ball counts on the amount of transferred charge Qsc.(c).Influence of the PTFE ball counts on the current signal.Influence of electrode materials on the current signal (d) and the amount of transferred charge Qsc (e) with a range of wave frequencies.Influence of electrode materials on the current signal (f) and the amount of transferred charge (g) with various wave amplitudes.

Fig. 4
Fig.4 Output performance of five layers connected C-TENGs.(a) Current signal and (b) Transferred charge obtained from one-layer to five-layer C-TENGs.(c) Output current and power density for five layers connected C-TENGs under different resistances.(d)Schematic diagram of the angle formed between the wave and the electrode.(e) Influence of directional loads on the short-circuit Isc.(f) Influence of directional loads on the transferred charge Qsc.(g) Schematic of five layers connected C-TENGs charging for different capacitors.