# Fluid Flow-Based Vibration Energy Harvesters: A Critical Review of State-of-the-Art Technologies
## 7 Enhancement Methods to Optimize and Increase the Efficiency of FBEHs
### Multi-Transduction Mechanisms: Hybrid Flow Energy Harvesters (HEHEHs)  
Researchers focusing on expanding the frequency range of FEHs are also working to increase the voltage and power levels of the harvester. Hybrid flow energy harvesting [182] is one of the techniques which combines numerous transduction mechanisms, i.e., EM, PE and TE or simultaneously extracts two or more ambient energies. Multi-transduction techniques integrated into a single unit can create more power across a wider band of frequencies, increasing its adaptability and efficiency in obtaining energy from various sources. This method offers novel opportunities for more adaptable and effective FEHs. Combining PE and EM harvesting mechanisms concurrently benefits from the advantages of both. According to tests conducted by [PERSON] et al. [183] on the hybrid piezoelectric-electromagnetic energy harvesters, the output power of a hybrid energy harvester is greater than that of a single piezoelectric or electromagnetic energy harvester. [PERSON] et al. [184] developed a new piezoelectric-electromagnetic hybrid flow energy harvester (HEEH) based on VIVs in water flow. The experimental findings demonstrate that the HFEH outperforms the single transduction energy harvesters in terms of power generation. Under the combined impacts of the dual transduction, the maximum power of 16.55 mW was attained when the attached resistances were 400 k\(\Omega\) and 2.2 k\(\Omega\) for the piezoelectric and electromagnetic parts, respectively. A high-efficiency energy harvesting windmill with the capacity for power delivery and quick charging was proposed by [PERSON] et al. [185]. The reported harvester was based on three conversion mechanisms: piezoelectric, electromagnetic and triboelectric conversion. [PERSON] and [PERSON] [186] investigated that the positioning of a magnet on a piezo-cantilever beam has a large impact on the synchronization region and power generation. [PERSON] et al. [187] proposed a galloping energy harvester utilizing piezoelectric and triboelectric transduction. The PEH would begin operations in the low wind zone. As the wind speed increased, the TEEH would begin to work along the PEH. The TEEH safeguards the piezoelectric material from damage in high wind speeds and improves the performance in terms of increased output broadening the working speed range. Total power generated by the device was 2.3 times that of the PEH alone; for the harvester a power of 0.24 mW at 14 m/s was reported. A magnetic coupling force was used by [PERSON] et al. [188] to combine a flutter-based PE-FEBH and a linear spring-type electromagnetic energy harvester, which significantly decreased the cut-in wind speed and improved harvesting performance. [PERSON] et al. [189] proposed a PE-EM hybrid energy harvester. They used a cylindrical blunt body fixed to the end of a piezoelectric cantilever beam and embedded the magnetic coil inside the cylindrical blunt body, suggesting a more efficient and compact system. [PERSON] et al. [190] reported a flutter HEH with outstanding performance. The nonlinear behavior of the harvester was analyzed. Compared with a conventional flutter-based FEH, the device produced more power and had a lower cut-in wind speed. At a wind speed of 6.70 m/s, the piezoelectric and electromagnetic components had output powers of 1.35 mW and 36.63 mW, respectively. Along with other nonlinearities, the HEH can be utilized in multistable configurations; for example, [PERSON] et al. [191] proposed a magnetically coupled hybrid galloping energy harvester. The bistable nonlinear device comprised PE and EM components. The higher the degree of coupling, the more dominant were the nonlinear features. The results demonstrate that the onset wind speed and output power are superior to those of the conventional galloping PE-FBEHs. When the wind speed was 11 m/s, the onset wind velocity was decreased by 28% and the output power was enhanced by 136%. Table 7 summarizes some of the HFEHs.