# 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
#### 2.2.1 Cylindrical Structure AC Output Device  
In energy harvesting devices using water as the medium, the columnar structure is widely used. The collection of wave energy in ocean waves has the characteristics of low amplitude and randomness. The cylindrical structure is easy to adapt to this kind of random energy, its dynamic characteristics are excellent, and the stability is good. [PERSON] et al. proposed a wheel-shaped triboelectric nanogenerator (WS-TENGs), the appearance of which is shown in Figure 8a. As shown in Figure 8b, the cylindrical structure of the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**TENG** & \begin{tabular}{c} **Operating Wind** \\ **Speed Range** \\ \end{tabular} & \begin{tabular}{c} **Short-Circuit** \\ **Current** \\ \end{tabular} & \
\begin{tabular}{c} **Open-Circuit** \\ **Voltage** \\ \end{tabular} &
\begin{tabular}{c} **Power/Power** \\ **Density** \\ \end{tabular} & **Ref.** \\ \hline PTFE-based flexible wind-driven & 10 m/s & 29.72 \(\mu\)A & 695.18 V & 1.74 mW & [41] \\ TENG & 3–4 m/s & 6.9 \(\mu\)A & 97.9 V & 1.98 mW/m\({}^{2}\) & [42] \\ LL-TENG & 0.2–2.5 m/s & 7.9 \(\mu\)A & 338 V & 2 mW & [43] \\ WG-TENG & 1–8.1 m/s & 3.4 \(\mu\)A & 175 V & 149 mW/m\({}^{2}\) & [44] \\ W-TENG structure after adding & 0.4–15 m/s & & 150 V & 0.6 mW & [45] \\ channels & & & & & \\ Self-suspended shell-based & 0.3–10 m/s & 119.8 V & & 8.43 mW/m\({}^{2}\) & [46] \\ triboelectric nanogenerator & 15–17.1 m/s & 0.341 A & 61.7 V & 11.57 mW/m\({}^{2}\) & [47] \\ C-TENG and CIA-TENG & 3–9 m/s & 348 \(\mu\)A & 233 V & 46.1 mW/m\({}^{2}\) & [48] \\ Tree-structured triboelectric & 4–17 m/s & 59.6 \(\mu\)A & 330 V & 3.6 mW & [49] \\ nanogenerator & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Performance of wind energy harvesting devices based on flutter structure in recent years. WS-TENGs is equipped with an impeller on the cylinder and a TENG on the side, which is agitated by a wave to turn the TENG [60]. [PERSON] et al. (Figure 8c) proposed a TENG (WLM-TENG) with an _Isc_ up to 30 \(\upmu\)A unidirectional rotating wave-driven cylindrical linkage mechanism with an output power of 50 mW [61]. In the field of energy harvesting, a large number of studies on nanogenerators can only collect the energy of a single fluid medium, such as wind energy or wave energy, and it is rare to collect the energy of two fluids at the same time. [PERSON] et al. proposed three types of energy TENG (multifunctional TENG) that can be used to collect them, including water waves, air flow, and water flow, as shown in Figure 8d. When the wind blows the TENG to a rotation speed of 200 rpm, Voc reaches 490 V and Isc reaches 24 \(\upmu\)A; 3 Hz; at a motion frequency of 100 V, _Isc_ reaches 2.7 \(\upmu\)A [62]. As shown in Figure 8e, [PERSON] et al. used a magnet to store potential energy and immediately release it to generate a frequency-doubled cylindrical TENG (FMC-TENG) with high-frequency kinetic energy. As shown in Figure 8f, the bottom of the FMC-TENG is equipped with two permanent magnets. The mutually exclusive polarity between the permanent magnets is used to further store potential energy and increase the flipping ability of the internal rotor. When the excitation wave frequency is as low as 0.33 Hz, the peak power density reaches 6.67 mW/m\({}^{3}\)[63]. In recent years, the performance of some AC output wave energy harvesting devices based on cylindrical structures has been summarized in Table 5.  
Figure 8: (**a**) WS-TENG Working Group Network and (**b**) individual WS-TENG diagrams. Reprinted with permission from Ref. [60]. Copyright 2022, Wiley-VCH GmbH. (**c**) (i) WLM-TENG (ii) junction of linkage-cylindrical structure and (iii) cylindrical TENG module. Reprinted with permission from Ref. [61]. Copyright 2022, Wiley-VCH GmbH. (**d**) Diagram of the structure of [PERSON]’s pain. Reprinted with permission from Ref. [62]. Copyright 2017, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (**e**) Schematic diagram of a single FMC-TENG structure and (**f**) FMC-TENG motion in waves.