# 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
Reprinted with permission from Ref. [68]. Copyright 2021 Elsevier Inc. (d) TS-TENG structure diagram. Reprinted with permission from Ref. [69]. Copyright 2019, Elsevier Ltd. (e) Schematic diagram of PS-TENG structure. Reprinted with permission from Ref. [70]. Copyright, 2019 Elsevier Ltd. (f,g) SC-TENG outline diagram and structure, motion schematic diagram. Reprinted with permission from Ref. [71]. Copyright, 2022 Elsevier Ltd. (h) Structure diagram of SSR-TENG. Reprinted with permission from Ref. [72]. Copyright, 2023 Wiley-VCH GmbH. nanogenerator (AR-TENG) system made of a single pendulum, tumbler, and flexible ring to optimize the super resonant performance, as shown in Figure 10c. The design of active resonance can reduce the wave frequency threshold while increasing the natural frequency output, improving the energy harvesting efficiency [68]. [PERSON] et al. proposed a toroidal structure TENG (TS-TENG). Similarly, TS-TENG is insensitive to wave directivity and can collect wave energy at different angles of incidence, as shown in Figure 10d [69]. [PERSON] et al. proposed a pendulum structure PS-TENG, as shown in Figure 10e. The charge output density of TS-TENG can exceed the output of the spherical-shell TENG structure by 13.2 times, reaching 4622 \(\upmu\)m\({}^{3}\). In low-frequency water waves below 0.5 Hz, a peak power density of 14.71 W/m\({}^{3}\) and an average power density of 31.05 W/m\({}^{3}\) were obtained [70]. In order to reduce frictional losses, [PERSON] et al. designed a synchronous contact separation triboelectric nanogenerator (SC-Teng) with intermittent sliding frictional self-excitation, as shown in Figure 10f. The structure is shown in Figure 10g, and the self-charge can be supplemented by self-rotation, providing the highest possible charge density at the friction-coupled interface moment in the contact separation mode [71]. As shown in Figure 10h, [PERSON] et al. proposed a swinging self-regulating triboelectric nanogenerator (SSR-Teng), which can make it possible to use the broken low-frequency wave energy by adjusting the oscillation frequency and resonance effect. As shown on the right side of Figure 10, the friction coupling interface of the SSR-TENG designed by [PERSON] et al. is designed in such a way that the cone surface of the table is coupled with the inner surface of the peripheral cylinder to form a contact-separation mode. The round table is fastened to the base by a spring, and the center of gravity of the round table is adjusted. The center of gravity and spring selection affect the energy recovery performance of the SSR-TENG. SSR-Teng outputs a peak power of 0.14 mW over a wave height range of 6-11 cm [72]. A feasible solution for improving the output performance is imitating the design concept of the initial pendulum model, including increasing the number of pendulums and adding magnets and coils. Increasing the number of pendulums staggered the initial motion phase of the pendulum and prolonged its motion time. By adding magnets and coils, the activation of charge transfer can be achieved by utilizing the principle of magnetic induction lines being cut. In recent years, the performance of some energy harvesting devices based on pendulum structures has been summarized in Table 7 below.