# Research Progress in Fluid Energy Collection Based on Friction Nanogenerators
## 2 Recent Developments in Triboelectric Nanogenerator-Based Fluid Energy Harvesting Systems
### Wave-Based Energy Harvesting Device
#### 2.2.3 Pendulum Structure Energy Harvesting Device  
The potential energy of the mechanical structure is disposed of by the elastic potential energy used by the spring, and the gravitational potential energy can also be collected and \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**TENG** & \begin{tabular}{c} **Operating** \\ **Frequency** \\ \end{tabular} & \
\begin{tabular}{c} **Short-Circuit** \\ **Current** \\ \end{tabular} &
\begin{tabular}{c} **Open-Circuit** \\ **Voltage** \\ \end{tabular} & **Power** & **Ref.** \\ \hline EC-TENG & 0.25–25 Hz & 2 \(\mu\)A & 63 V & 90 \(\mu\)W & [64] \\ TENG & & & 4 V & 0.9 \(\mu\)W & [65] \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Performance of DC output wave energy collection devices based on cylindrical structure in recent years. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**TENG** & \begin{tabular}{c} **Operating** \\ **Frequency** \\ \end{tabular} & \
\begin{tabular}{c} **Short-Circuit** \\ **Current** \\ \end{tabular} &
\begin{tabular}{c} **Open-Circuit** \\ **Voltage** \\ \end{tabular} & **Power/Power** \\ \hline WS-TENG & 5–57 Hz & 17.04 \(\mu\)A & 116 V & 0.0626 mW & [60] \\ WLM-TENG & 1 Hz & 30 \(\mu\)A & 3 kV & 50 mW & [61] \\ multifunctional & 2 Hz & 24 \(\mu\)A & 490 V & 15 mW & [62] \\ TENG & 0.33 Hz & 7.5 \(\mu\)A & 400 V & 6.67 mW/m\({}^{3}\) & [63] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Performance of AC output wave energy collection devices based on cylindrical structure in recent years. Figure 9: (**a**) EC-TENG structure. Reprinted with permission from Ref. [64]. Copyright 2022, the Authors. Published by Springer Nature. (**b**) TENG structure designed by [PERSON] et al. Reprinted with permission from Ref. [65]. Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. utilized. Using the high entropy of the movement of the mechanism, the mechanism collects a certain amount of energy and continuously releases it, and the pendulum structure comes into being. Research has always been needed to develop communication systems in the ocean, and the Arctic Ocean presents particular difficulties for satellite communication power supply. [PERSON] et al. have studied an Arctic-TENG to power satellite communication systems in the Arctic Ocean. Arctic-TENG is designed to serve low-temperature environments and has inferior electrical performance at room temperature to low temperatures. Using an extra-underwater wave simulator at 0.2 Hz, the peak power density of the Arctic-TENG reaches 21.4 W/m\({}^{3}\), and one Arctic-TENG can transmit 540 bytes of data per day for one year [66]. [PERSON] et al. investigated an isotropic triboelectric-electromagnetic-hybrid nanogenerator (iTEHG) based on a guide liquid, as shown in Figure 10a. Its structural design integrates TENG and EMG. The inclined dish panel is inlaid with unique concentric circular electrode pairs that guide the movement of the liquid by gravity, as shown in Figure 10b. The solid-liquid coupling of iTENG as the friction working interface prolongs the service life of iTENG [67]. The tumbler-style pendulum design collects waves and is insensitive to the direction of the incident wave. In order to obtain omnidirectional wave energy, [PERSON] et al. proposed an active resonance triboelectric  
Figure 10: **(a)** iTEHG working scenario and structure, **(b)** working principle. (i) In the initial state, DI water is in contact with the bottom FEP, and the electrode has no charge; (ii) the movement of the device makes DI water move to the right FEP, breaking the original charge balance, and the charge is transported to the middle electrode through the external charge electrode; (iii) DI water returns to its original position, the original charge balance between DI water and FEP is realized, and the charge of the middle electrode is small; (iv) similarly, the charge is transported to the middle electrode again. Reprinted with permission from Ref. [67]. Copyright 2023, the Author(s). Published by Elsevier Ltd. (c) AR-TENG typical pendulum working structure.