Rolling Spherical Triboelectric Nanogenerators (RS-TENG) under Low-Frequency Ocean Wave Action

Abstract

Triboelectric nanogenerators (TENG), which convert mechanical energy (such as ocean waves) from the surrounding environment into electrical energy, have been identified as a green energy alternative for addressing the environmental issues resulting from the use of traditional energy resources. In this experimental design, we propose rolling spherical triboelectric nanogenerators (RS-TENG) for collecting energy from low-frequency ocean wave action. Copper and aluminum were used to create a spherical frame which functions as the electrode. In addition, different sizes of spherical dielectric (SD1, SD2, SD3, and SD4) were developed in order to compare the dielectric effect on output performance. This design places several electrodes on each side of the spherical structure such that the dielectric layers are able to move with the slightest oscillation and generate electrical energy. The performance of the RS-TENG was experimentally investigated, with the results indicating that the spherical dielectrics significantly impact energy harvesting performance. On the other hand, the triboelectric materials (i.e., copper and aluminum) play a less important role. The copper RS-TENG with the largest spherical dielectrics is the most efficient structure, with a maximum output of 12.75 V in open-circuit and a peak power of approximately 455 nW.

1. Introduction

Innovative technologies have been developed to extract mechanical energy from the environment and convert it into electrical energy, including electromagnetic, electrostatic and piezoelectric methods [1,2,3,4,5,6,7,8,9,10,11]. Ocean wave action makes up a significant part of the ocean’s energy, and is estimated to be around 2–3 TW along coastlines worldwide [6]. In recent years, triboelectric nanogenerators (TENG) with the benefits of high power density, light weight, etc., have been developed as a green energy technique for energy harvesting under various application scenarios [12,13,14]. TENG technology advancements have resulted in a wide range of applications, including self-powered biomedical devices (e.g., electronic skins, mechanical communication systems, and wearable systems), self-powered engineering devices (e.g., temperature fluctuation and structural vibration sensors), and various other sensing devices (e.g., motion vector sensing, wearable sensors, driving monitoring, fish bladder film-based position monitoring, self-functional healthcare and monitoring socks, and human–machine interfacing) [15,16,17,18,19]. Triboelectric nanogenerators (TENG) [20] were proposed in 2012 as a new kind of energy harvesting approach. For the first time, in 2014 TENG was utilized to gather wave energy [21]. TENG has a number of advantages over electromagnetic (EMG) generation, including higher power density (up to 3200 W/m²), lower weight, increased productivity (up to 70%) and lower production costs [6]. It is a significant advance in the attempt to collect low-frequency ocean energy. [22,23,24]. Different types of TENG have been constructed and used to harvest ocean energy. [25,26,27,28]. For example, multi-mode magnetic TENG energy harvesting systems were developed using the magnetic oscillation method to oscillate the dielectric capsules. Steel spring connectors were used to increase the oscillation frequency and achieve higher power output [10]. In addition, a spring-assisted TENG was reported to harvest water wave energy by using a spring to increase the power density [29]. Due to the advantages of a light weight, low resistance and ease of integration, a rolling spherical TENG with or without connected networks has been developed to collect ocean wave energy [30,31,32,33,34,35,36,37]. However, the previously-constructed spherical TENG was designed with lever arms or springs to trigger the energy materials and generate electrical power, which critically limits their practical applicability. One available alternative in ocean applications is to change the functioning structure of the TENG by designing rolling spherical versions without assistance to increase the output voltage of mechanical energy from ocean waves and convert it to electric power.

Here, we propose a rolling spherical TENG (RS-TENG) which utilizes a rolling structure and a freestanding triboelectric layer-based process to harvest energy from low-frequency ocean wave action. The RS-TENG is designed by contacting a copper or aluminum film in an enclosed spherical shell with rolling silicon rubber balls, which are light, inexpensive, and capable of floating on the ocean’s surface. Taking advantage of the spherical structures in its the freestanding mode, this RS-TENG system can harvest electrical energy under low-frequency and low-amplitude oscillations. The conducted experiments and results presented here indicate that by increasing the size of the spherical dielectric, output voltage was increased. Therefore, RS-TENG achieved efficient results with SD4. In addition, by increasing the oscillation the spherical dielectric was able to rotate more, allowing the output voltage to be further increased.

2. Results and Discussion

2.1. Structure of the RS-TENG

The design principles of the RS-TENG are illustrated in Figure 1. Figure 1a demonstrates the RS-TENG oscillation scenarios in the copper frame, which are compared to the experimental observations captured by a high-speed camera (see Supplementary Materials Video S1). Figure 1b demonstrates the use of the RS-TENG for ocean wave action with low-frequency and low-amplitude oscillations. The oscillations cause the spherical dielectric in the spherical frame to roll. The oscillation response of the copper RS-TENG with the high-speed camera is provided in Supplementary Materials Video S1. In Supplementary Materials Video S2, the energy harvesting performance of the copper (Cu) RS- TENG is illustrated. Supplementary Materials Section 1 details the equipment and principle of the RS-TENG used in the experiments.

2.2. Output Performance and Setup of Experiment of the Copper and Aluminum RS-TENG

The RS-TENG were made with copper (Cu) and aluminum (Al) for the various spherical dielectrics. Figure 2a presents the design details of the copper (Cu) and aluminum (Al) RS-TENG. The electrode frame and spherical dielectric were the components made from copper (Cu) and aluminum (Al). The space between each layer was set to 1 mm. In this structure, 3D printing with PLA (Polylactic Acid) materials was used to create spherical frames. Figure 2b shows the manufacturing process of spherical silicon rubber as a spherical dielectric. A high-speed centrifuge was used to create silicone rubber spheres with diameters of 14 mm, 20 mm, 24 mm and 27 mm. The prototypes used in this study as well as the frames and molds of the rolling silicon rubber balls were 3D printed from of PLA (Polymaker-Tough) material using a 3D printer (Ultimaker-S3, Ultimaker Inc., Utrecht, The Netherlands) with a maximum printing size of 223 mm × 223 mm × 205 mm and a maximum printing velocity of 50 mm/s. Figure 2b shows the manufacturing process of the rolling silicon rubber balls with various dimensions as the spherical dielectric. The molds had inner dimensions of ${\mathrm{d}}_{\mathrm{SD}1}=14\mathrm{mm}$, ${\mathrm{d}}_{\mathrm{SD}2}=20\mathrm{mm}$; ${\mathrm{d}}_{\mathrm{SD}3}=24\mathrm{mm}$, and ${\mathrm{d}}_{\mathrm{SD}4}=27$ mm, and outer dimensions of ${\mathrm{d}}_{\mathrm{SD}1}=16\mathrm{mm}$, ${\mathrm{d}}_{\mathrm{SD}2}=22\mathrm{mm}$; ${\mathrm{d}}_{\mathrm{SD}3}=26\mathrm{mm}$, and ${\mathrm{d}}_{\mathrm{SD}4}=29\mathrm{mm}$, and were also 3D printed from PLA material using the 3D printer. The high-speed centrifuge thoroughly mixed the silicon rubber materials for ten minutes to make spherical dielectric parts. In addition, copper and aluminum tape were added to the frame with a 1 mm gap between each layer to make the electrode parts. The RS-TENG experimental setup is depicted in Figure 2c. A shaking machine was used to generate frequencies of 1.1 Hz, 2.4 Hz and 4.2 Hz for 6 s; voltage signals were collected using a digital oscilloscope. In the RS-TENG design, there are two electrode terminals on the inner surface of the hemisphere. Electrode (Cu/Al) contacts the external load. Layers (4) and (8) were selected as one electrode terminal, and the remaining layers served as another electrode terminal.

Figure 3 shows the effect of the frequency (1.1 Hz, 2.4 Hz and 4.2 Hz) performance on the RS-TENG. The output performance of the copper (Cu) RS-TENG with SD1 in the open circuit (OC) is shown in Figure 3a. The maximum voltage of $V_{\max }=7.25\mathrm{V}$ was obtained at a speed of 4.2 Hz, and the minimum voltage of $V_{\min }=0.75\mathrm{V}$ was obtained at a speed of 1.1 Hz. On the contrary, the output of the copper (Cu) RS-TENG with SD1 in the open circuit at the loading frequency of 2.4 Hz was 3.75 V. The voltage of the aluminum (Al) RS-TENG with SD1 is shown in Figure 3b. A similar voltage output pattern was achieved at a speed of 4.2 Hz, with $V_{\max }$ = 5 V as the maximum output and $V_{\min }$ = 0.75 V at a speed of 1.1 Hz. At a loading frequency of 2.4 Hz, the output voltage of the aluminum (Al) RS-TENG with SD1 in the OC was 3.5 V. The output performance of the copper (Cu) RS-TENG with SD2 in the open circuit is shown in Figure 3c, with the maximum output performance being $V_{\max }$ = 10.5 V at 4.2 Hz and $V_{\min }$ = 1.3 V at 1.1 Hz. The output performance of the copper (Cu) RS-TENG with SD2 in the open circuit at a loading frequency of 2.4 Hz was 7.75 V. The aluminum (Al) RS-TENG voltage with SD2 is shown in Figure 3d. A similar output performance pattern was obtained, with the $V_{\max }$ = 6.25 V at a speed of 4.2 Hz and $V_{\min }$ = 1.1 V at a speed of 1.1 Hz. In addition, the open-circuit output voltage of the aluminum (Al) RS-TENG with SD2 at a speed of 2.4 Hz was 5 V. The output of the copper (Cu) RS-TENG with SD3 is shown in Figure 3e, with $V_{\max }$ = 11.75 V at 4.2 Hz and $V_{\min }$ = 1.3 V at 1.1 Hz. Figure 3f presents the voltage of the aluminum (Al) RS-TENG with SD3; a similar output performance pattern was achieved, with $V_{\max }=11.5\mathrm{V}$ at the frequency of 4.2 Hz and $V_{\min }=1.2\mathrm{V}$ at 1.1 Hz. Figure 3g demonstrates the output performance of the copper (Cu) RS-TENG with SD4; the maximum voltage was $V_{\max }=12.75\mathrm{V}$ under 4.2 Hz and the minimum was $V_{\min }=1.7\mathrm{V}$ under 1.1 Hz. The output of the aluminum (Al) RS-TENG with SD4 is shown in Figure 3h, with the maximum voltage being $V_{\max }$ = 11.25 V at 4.2 Hz and $V_{\min }$ = 1.4 V at 1.1 Hz.

Figure 4a,b compares the voltage of (a) copper (Cu) and (b) aluminum (Al) in SD 1, SD2, SD3 and SD4 with various frequencies. Figure 4c shows the series of triboelectric materials. The triboelectric effect is the basic strategy for exploring these materials. The fundamental idea behind the triboelectric effect is to construct a triboelectric series based on the charge point when any two materials come into contact through rubbing or pressing. The materials listed in the upper area of the triboelectric group are positively charged, while those introduced in the lower section are negatively charged [38]. The triboelectric series is widely accepted as a primary reference for selecting appropriate materials for TENG. When a triboelectric charge is formed by friction between two different materials, the triboelectric series can predict which side would be negatively or positively charged. In this study, the rolling silicon rubber balls are designed to serve as the dielectric component in the RS-TENG.

Figure 5a,b shows the power and voltage of the copper (Cu) and aluminum (Al) RS-TENG, respectively, along with the electrical resistance with spherical dielectric 1 (SD1) at the frequencies of 1.1 Hz, 2.4 Hz and 4.2 Hz. A peak power of approximately 136.9 $\mathrm{nW}$ and peak voltage of 4.5 $\mathrm{V}$ were obtained for the copper (Cu) RS-TENG, and a peak power of 146.3 $\mathrm{nW}$ and peak voltage of 4.5 $\mathrm{V}$ were obtained for the aluminum (Al) RS-TENG, with electrical resistance of 100 KΩ to 1 GΩ. Figure 5c,d depicts the power and voltage of the copper and aluminum (Al) RS-TENG, respectively, with spherical dielectric 2 (SD2) at the frequencies of 1.1 Hz, 2.4 Hz and 4.2 Hz. Peak power and voltage were obtained as 253 $\mathrm{nW}$ and 7 $\mathrm{V}$ for the copper (Cu) RS-TENG and 220 $\mathrm{nW}$ and 7.5 $\mathrm{V}$ for the aluminum (Al) RS-TENG, with an electrical resistance of 100 KΩ to 1 GΩ. Figure 5e,f presents the power and voltage of the copper (Cu) and aluminum (Al) RS-TENG with spherical dielectric 3 (SD3); the peak power and voltage are, respectively, 330 nW and 9 V for the former and 273 nW and 8 V for the latter. Figure 5g,h provides the power and voltage of the copper (Cu) and aluminum (Al) RS-TENG with spherical dielectric 4 (SD4), with the peak power and voltage being 455 nW and 9 V and 338 nW and 9 V, respectively. There are three ways of writing an equation for power. In this study, the power was obtained as

$$P=\frac{v^2}{R}$$

(1)

where $\nu$ and $R$ refer to the voltage and resistor.

In this work, rolling silicon rubber balls with a diameter of 27 mm (SD4) in the copper setup proved to be optimal based on output power and voltage. In fact, with an increasing surface of the dielectric, voltage and power increase. In Furthermore, the performance of the copper setup was more efficient than aluminium. In addition to conductivity, the electrode material significantly influences the difference between copper and aluminum TENG. Choosing materials with an enormous energy gap would theoretically result in a higher output voltage. According to the triboelectric series, because silicone rubber is one of the TENG materials in both circumstances, aluminum should provide a larger output than copper. However, we must first consider the conductivity and resistivity of aluminum versus copper. Copper, in general, is a highly conductive metal with low resistance (1.68 × 10⁻⁸). On the other hand, aluminum has poorer conductivity than copper and a resistance (2.65 × 10⁻⁸) that is nearly 1.5 times that of copper. In other words, with the same size electrodes of both aluminum and copper, aluminum produces a weaker or lower output than copper. Furthermore, the higher output produced by the aluminum–silicone rubber arrangement will be lower than that produced by the copper–silicone rubber design. In fact, with increasing surface contact between the electrode and dielectric, output performance will be increased. There are certain factors which generally affect TENG power generation and transmission, such as the motion parameters (including frequency, amplitude and contact and noncontact motion regimes). In addition, using a sonic array configuration can enhance the performance of the RS-TENG through better design and optimization of the structure of RS-TENG, as will be considered in future studies [39].

3. Conclusions

This design presents a design for rolling spherical triboelectric nanogenerators (RS-TENG) to collect low-frequency ocean wave energy. The nanogenerators were fabricated with copper (Cu) or aluminum (Al) structures and four different sizes of spherical dielectrics (i.e., SD1, SD2, SD3 and SD4). The electrical output performance of the RS-TENG was evaluated through experiments. The results showed that the spherical dielectrics (i.e., SD 1, 2, 3, and 4) had a significant impact on energy harvesting performance, while the triboelectric materials tended to have only a slight effect. The copper (Cu) RS-TENG with SD4 was the most efficient design, with a maximum (OC) voltage of 12.75 V and peak power of approximately 455 nW. The RS-TENG’s innovative use of several electrodes of copper (Cu) or aluminum (Al) in each side of the spherical structure caused the spherical dielectric to move with even the slightest oscillation in order to produce energy.