# Research Progress in Fluid Energy Collection Based on Friction Nanogenerators
## 2 Recent Developments in Triboelectric Nanogenerator-Based Fluid Energy Harvesting Systems
### Wind Energy Collection
#### 2.1.4 Energy Harvesting Device Based on Flutter Structure  
The effect of humidity on nanogenerators in wind energy harvesting environments is a significant field of research. As a result, TENG's output performance suffers and its service life is drastically lowered. Triboelectric charges were generated between a hydrophobic polytetrafluoroethylene (PTFE) flexible film and a polyvinyl alcohol (PVA) film, as shown by [PERSON] et al. Figure 6a,b illustrate the design and operation of the device, respectively. At 95% RH (relative humidity), the optimized short-circuit current is 29.72 \(\mu\)A, the output voltage is 695.18 V, and the optimized power is 1.74 mW [41]. Different from previous studies, which focused on unilateral fixation of the windward side of the flutter film, [PERSON] et al. proposed a wind-driven bidirectional flutter triboelectric nanogenator with bilateral fixation (WBF-TENG). The membrane fixed on both sides provides two stabilization lines, which can effectively receive airflow from more directions. Its structure is shown in Figure 6[42]. In order to further reduce the wind speed response threshold of the flutter structure TENG, increasing the windward surface is a feasible solution. A triboelectric nanogenator (TENG) with a response threshold of 0.2 m/s was suggested by [PERSON] et al., as illustrated in Figure 6d. As can be seen in Figure 6e[43], the flexible blades, vein-bearing plates, and counterweights that make up the foliate structure enhance the frontal area while also providing the generator with a precisely calibrated damping ratio. [PERSON] and colleagues created a wake-driven triboelectric nanogenerator (WG-TENG) based on the sweeping spoiler. The WG-TENG consists of two flutter plates and a spoiler column and shell in the center, as seen in Figure 6f[44]. [PERSON] et al. used the principle of aerodynamics to optimize the airway structure and designed a flutter structure nanogenerator with an inlet channel (W-TENG) to successfully gather the dispersed weak airflow flow energy, as shown in Figure 6g. The opening speed threshold of W-TENG is reduced to 0.4 m/s, and the average output voltage is 6.1 V [45]. Researchers have developed several efficient solutions to the problem of capturing erratic airflow. Figure 5: Output device based on a rotating self-adjusting structure. (**a**) (**i**) SA-TENG structural design (**ii**) Self-adjusting torque structure. (**b–d**) Digital image of each part of SA-TENG. (**b**) photos of of Self-adjusting torque structure, (**c**) photos of SA-TENG rotor, (**d**) photos of SA-TENG stator. (**e**) SA-TENG working mode switching under different wind speeds. Reprinted with permission from Ref. [39]. Copyright 2022, Elsevier. (**f**) SM-TENG structure and (**g**) SM-TENG sliding and contact separation mode switching principle. Reprinted with permission from Ref. [40]. Copyright 2023, Elsevier. Figure 6: **(a)** PTFE-based flexible wind-driven TENG and (**b**) working cycle diagram. (i) Under the influence of airflow, PTFE flutter blade moves upward; (ii) the PTFE sheet contacts the upper plate, and the electrons in PVA are transferred to PTFE due to their electronegativity differences, resulting in positive charges on the PVA surface and negative charges on the PTFE surface; (iii) the potential difference between the top copper electrodes can drive the negative friction charge from PTFE to the copper electrode surface; (iv) similarly, due to the influence of air flow, PTFE approaches the lower PVA film and generates a positive charge on the PVA surface, while PTFE generates a negative charge on the surface; (v) When the middle friction layer of PTFE moves upward, the negative friction charge will be transferred from PTFE to the copper electrode surface at the bottom of Teng, resulting in a potential difference between PTFE and the top copper surface, and driving the negative friction charge from the top copper surface to PTFE. Reprinted with permission from Ref. [41]. Copyright 2021, Elsevier.