# Fluid Flow-Based Vibration Energy Harvesters: A Critical Review of State-of-the-Art Technologies
## 8 Rotary Wind Energy Harvesters
The design approach focused on low-frequency wind speed to increase electrical output. The plucking units were designed to reduce the number of strips plucked per time, resulting in a lower start-up wind speed. The rotational EMEH was crucial to the design. The piezoelectric materials were distributed in a circular pattern along the inner area of the outer shell. The TEEH was incorporated at the interface between the outer layer of the inner case and the inner layer of the device. At a 3.5 m/s wind speed, the PE, EM and TE components produced maximum electrical powers of 121 uW, 191 uW and 168 nW, respectively. The study in [210] designed a low wind-speed piezoelectric windmill type RWEH with five blades, and a magnetic force was utilized to stimulate the piezoelectric cantilever beam. A shaft carrying the hub (holding the magnets) translated the shaft's angular motion into a vibration of the piezoelectric cantilever. The harvester produced powers of 1.06 and 2.21 mW at wind speeds of 2 and 5 m/s, respectively. [PERSON] et al. [211] developed a high-performance hybrid RWEH by coupling EM and TE mechanisms for ocean wind energy harvesting. By introducing an energy management circuit along the electromagnetic component, the charging efficiency of the TEEH was enhanced by 15 times. The all-in-one device structure can be installed on a lighthouse or the deck of a cruise ship to easily gather oncoming ocean wind energy. The energy harvester was made up of two freestanding mode TEEHs, top and lateral, and a rotary-structured bottom EMEH to fully utilize and harvest ocean wind energy. Most contemporary wind-induced vibration energy harvesters are unidirectional and insensitive to changing wind direction. To address the challenge, [PERSON] et al. [212] offered an orientation-adaptive EMEH. The EH consists of a rotating bluff body, an elastic beam, a cylinder, a round-shaped magnet and coils array, as shown in Figure 14d. When the wind blows in any direction, the design structure can turn adaptively, exciting the cylinder sleeve and causing VIVs. It vibrates perpendicular to the wind, causing the permanent magnet to oscillate back and forth above the wound coil array, allowing the coils to cut magnetic induction lines and produce electric current. It was observed when the beam stiffness was large, increasing the rotatory inertia of the rotatable bluff body maximized the EMEH's output performance. Experiments revealed that an output power of 0.673 mW was obtained at a wind speed of 8 m/s. [PERSON] et al. [213] reported a TE-based RWEH with soft-contact operating mode, which was developed with tunable contact areas by utilizing the consistent thermal response of Nickel titanium (NiTi) shape memory alloy (SMA) to air/wind temperature. The device demonstrated promising application as a power source with an average power density of 140 mW/m\({}^{2}\) at the wind speed of 12 m/s. This was demonstrated as a power source for online monitoring sensors, self-powered wind speed sensing and airflow temperature monitoring. Table 9 represents various wind driven RWEHs operating on PE, EM, TE or multiple transduction techniques. These energy harvesters can be widely utilized in renewable energy systems, including wind turbines and hydroelectric generators. These EHs have a simple and compact design which can help to reduce maintenance requirements. In [207], EM- and TE-based RWEHs can be enclosed in a compact structure. Additionally, optimizing the TE materials ensures increasingly consistent and sustainable sources of energy over time. It is worth mentioning that EM RWEHs usually produce larger output power than clearly realized in Table 9. The fundamental advantage of RWEHs is their ability to operate effectively in multiple directions, which allows them to efficiently gather energy from a variety of sources, including vibrations and rotations, independent of the direction of input wind motion.