# Fluid Flow-Based Vibration Energy Harvesters: A Critical Review of State-of-the-Art Technologies
## 5 Flutter Energy Harvesting
### Flutter-Based TEEHs  
Triboelectric energy harvesters utilizing flutter provide a novel technique to harvest energy from ambient air flows. The performance of flutter-based harvesters is influenced by the wind direction and variations in airflow patterns. [PERSON] et al. [119] reported a freestanding, lightweight woven TEEH (W-TEEH) that can capture wind energy from arbitrary directions. The interlaced interactions between Kapton film and a conductive fabric under wind-induced  
Figure 9: Flutter EM-FBEHs. (**a**) Cantilever-based EMEH [118]. (**b**) Design of wind belt EMEH reproduced with permission from [115] Elsevier, 2016.  
flapping of the flag ensures the functioning of the W-TEEH flag. This combined contact electrification and electrostatic induction effect work together to generate current. At a wind speed of 22 m/s, about 40 V and 30 \(\upmu\)A were the maximum open circuit voltage and current produced, respectively. At a resistance of 6.5 M\(\Omega\), the output peak power density reached a maximum of 135 mW/kg. Broadband airflow energy captured using TEEHs operated by aerodynamic flutter was reported in [120]. The thin, free-standing Al foil electrodes that constituted the flutter membrane's unit component were covered on both sides with electro-spun poly (vinyl chloride) nanofiber-structured mats, which offered useful tribo-surfaces designed to increase the friction area. Under mild airflow, a single flutter-membrane-based TEEH could generate up to 0.33 \(\upmu\)W of triboelectric power. [PERSON] et al. [121] utilized the polarization of a ferroelectric BaTiO3 material in their developed TEEH. A maximum output power of 67 mW was possible with a load resistance of 18 M\(\Omega\) at a 14 m/s wind speed. Although flutter-based TEEHs have potential for use with self-powered devices, practically all the described devices are based on parallel structures, which have the flaw of having insufficient triboelectric surface contact. In [122], an angle-shaped TEEH was introduced to solve this issue. A fluorinated ethylene propylene (FEP) film was sandwiched between two Al layers that had been layered to form an angle and were connected on one side to create the device. For the purposes of contact electrification and electrostatic induction, the FEP film could closely contact the Al layers. At an optimum resistance of 4 M\(\Omega\), the peak power obtained was 0.82 mW (equivalent to a power density of 26 mW/m\({}^{2}\)) when the flow velocity was 25 m/s. This research could provide a substantial and important solution for natural weak wind energy harvesting in practical applications. In [123], a leaf-TEEH was developed using fresh leaves, live leaves, dry leaves, and leaf powders as frictional materials to capture wind and contact mechanical energy from nature. All these leaves were inexpensive and environmentally friendly. The authors used fresh leaves and dried leaves that had fallen as the frictional materials of the harvester for energy harvesting in a green way in nature, which could power an electronic watch and several LEDs when the wind speed was 7 m/s. A maximum power of 17.9 mW was obtained at a load resistance of 11 M\(\Omega\). A wind actuated venturi TEEH [124] transforms ambient energy to produce a periodic vibration of a flag film. A low-pressure zone in the throat region of the venturi system causes the flag film to flutter at high frequencies between the electrodes when air travels through it. By displacing the charge between the electrodes, this flag flapping results in a faster charge transfer efficiency. This charge movement causes variation in the electrode potential which enhances the triboelectrification process. The harvester can deliver an optimum RMS power of 1.5 mW with maximum output power density of 2850 mW/m\({}^{2}\). The study in [125] proposed a flag-type TEEH that is humidity resistant and multi-directional to harvest wind energy and also measure the wind speed. The device was optimized and exhibited higher power generation when bending stiffness and mass ratio were decreased.