# Fluid Flow-Based Vibration Energy Harvesters: A Critical Review of State-of-the-Art Technologies
## 3 Karman Vortex Street-Based Energy Harvesting
### Karman Vortex Street-Based TE-FBEHs  
[PERSON] et al. [59] suggested a VIV-based TE energy harvester (TEEH) that enabled efficient energy harvesting from wind at low speeds. The harvester comprised a cylinder, springs and friction layers, as shown in Figure 5a. The cylinder was attached to springs and a polyaniline friction layer, which allowed it to move up and down. Polytetrafluoroethylene (PTFE) was another friction layer that was fixed at the lower section. With a wind speed of 2.78 m/s, an average power of 392.72 uW and power density of 96.79 mW/m\({}^{2}\) were obtained. [PERSON] et al. [60] developed a VIV-TEEH, featuring a wind vane along an internal power generation unit, external frame, four springs, square cylinder and circular turntable. The internal power generation unit consisted of PTFE balls, a honeycomb frame and two copper electrodes. Unlike most prior TEEHs, the bouncing PTFE balls were entirely bundled in a square cylinder. The unique design isolated the contact electrification process from the surrounding environment while preventing frictional wear in traditional triboelectric devices. [PERSON] et al. [61] reported a novel design of TEEH based on VIVs that captures the kinetic energy of a low-speed water flow. The harvester was primarily made of a cylinder and a cantilever beam containing two nanogenerator units, as depicted in Figure 5b. The maximum voltage of 174 V occurred at 2 Hz of vortex resonance (flow speed of 0.5 m/s), with a power output of 2.5 mW at 1 M\(\Omega\) resistance. Recently, a unique flag-type TEEH with a Y-type bluff body (Figure 5c) has been reported in [62] for harvesting wind energy. The Y-type bluff body amplified the vibration of vortices shedding. The mounting position, design dimension and windward angle of the Y-type bluff body were explored to improve flag vibration and triboelectric power output. At a wind velocity of 6 m/s, the maximum output power was 2.9 uW with 8 M\(\Omega\) load resistance. A Y-shaped bluff body offers substantial potential for powering numerous sensor nodes across a broad range of wind speeds. [PERSON] et al. [63] developed a rolling-based TEEH, which can efficiently harvest wind energy from all directions, utilizing a circular guide vane and a deformable rolling cylinder for effective triboelectrification, as shown in Figure 5d. The airfoil-shaped guide vane consistently shed rotating vortices which eventually increased the speed beyond the actual input speed. At 8 m/s wind speed, an RMS voltage of 120 V and RMS power of 1.94 mW at 40 M\(\Omega\) were achieved. Table 2 represents the VIV-based energy harvesters discussed above, the output of which ranges from several uW to few mW at different wind speeds. The output generated in [48; 54] was higher due to additional elements contributing to the vortex generation and thereby the vibration amplitude of the energy harvester. Additionally, the TEEHs [61; 63] produced more output power, mainly due to structural parameters like springs and the shape of the wind vane, increasing the vortices shedding and eventually enhancing the actual wind speed. These features make their use more favorable in varying environments and thereby increase the range at which energy can be collected.