# 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
(vi) The PTFE vibration plate moves upwards until it comes into contact with PVA, and the electronegativity of the two materials is not consistent, resulting in contact charges; (vii) When PTFE vibrators leave PVA, the charge balance of an equal amount of contact charge is disrupted, relying on the external circuit to transport the charge, causing the negative charge in the external circuit to move. (c) (i) WBF-TENG structural diagram. (ii) When the air flow passes through the inlet, the blown film vibrates and generates friction charges due to the electronegativity difference of materials. (d) (i) Without airflow excitation, there are equivalent heterogeneous charges on the contact surfaces of aluminum and FEP; (ii) when there is airflow excitation and aluminum and FEP are separated, the difference of electronegativity causes the friction charge of aluminum to flow to the steel; (iii) aluminum is completely separated from FEP, and all charges are transferred to steel; (iv) aluminum is in contact with FEP, and the charge of steel is transferred back to aluminum.Reprinted with permission from Ref. [42]. Copyright 2022, Wiley-VCH GmbH. (**e.f**) Flutter working principle of LL-TENG. Reprinted with permission from Ref. [43]. Copyright 2023, Wiley-VCH GmbH. (**g**) Realizing the WG-TENG structure using wake propagation. Reprinted with permission from Ref. [44]. Copyright 2022, Elsevier. (**g**) W-TENG structure (after adding channels) that collects weak airflow by adding a shroud. Reprinted with permission from Ref. [45]. Copyright 2022, Elsevier. outer cylindrical shell is made of PET embedded with aluminum film, and the inner cylindrical shell is made of PTFE embedded with copper film. Its structure is shown in Figure 7a. Due to the Coulomb attraction between aluminum and PTFE, the self-suspended structure of the thin shell is formed. When the turbulent shell is disturbed at a certain wind speed, as shown in Figure 7b, the change in the relative distance between the two charged layers enables the energy collection element to pass through the triboelectric energy conversion mechanism. And thanks to the cylindrical structure, the incident directivity factor of the energy harvesting device does not need to be overly clear [46]. Inspired by a new type of caliopsis, [PERSON] et al. designed a TENG (C-TENG) that harvests energy or can be used as a self-powered sensor. When the frequency is 5 Hz and the wind speed is 154 m/s, the open circuit voltage (_Voc_) of a single C-TENG can reach 61.7 V and the short circuit current (_Isc_) can reach 0.341 A; when the load resistance is 100 M\(\Omega\), the peak value of a single C-TENG is reached. The output power is 11.57 mW/m\({}^{2}\), as shown in Figure 7c,d, respectively, showing its structural design and working cycle [47]. [PERSON] proposed a TENG structure with a four-way curved dielectric film, as shown in Figure 7e. An aluminum layer is inserted into the dielectric film to improve the electrostatic induction, thereby improving the triboelectric performance, and good wind resistance can be seen in Figure 7f [48]. Inspired by the tree structure, [PERSON] et al. designed a supercell tree consisting of leaf cells 1 and stem cells 2 as shown in Figure 7g. With an input wind speed of 11 m/s, it can produce a _Voc_ of 330 V, an Isc of 59.6 uA, a matching resistance of 60 M\(\Omega\), and an output power of 3.6 mW [49]. In the design of TENGs for flutter structures, flutter plates have become a key factor in collecting wind speed thresholds for this energy harvester due to their inherent natural frequency and allowable swing. In addition, a reasonable air duct design can improve the response characteristics of this device, and the continuously decreasing wind speed threshold is its manifestation. Figure 7: (**a**) Structural diagram of the self-suspended shell-based triboelectric nanogenerator and (**b**) working condition using the turbulent current of the shell. Reprinted with permission from Ref. [46]. Copyright 2022, Elsevier.