Study of an Ultra-Low-Frequency Inertial Vibration Energy Harvester with a Frequency Up-Conversion Approach.

For ultra-low-frequency vibration applications, this study focuses on a piezoelectric energy harvesting system with a spring mass system, utilizing magnetic plucking to up-convert the frequency. The proposed spring mass system includes a spring, a magnet mass with a guide rail, and a fixed pulley. The spring mass system responds to external ultra-low-frequency excitation and transfers the vibration to the piezoelectric cantilever beam through the magnets, achieving frequency up-conversion. The theoretical model of the designed piezoelectric energy harvesting system is established, and the effects of magnetic forces and potential energy between the magnets are analyzed. Numerical analysis and experimental studies demonstrate that the proposed piezoelectric energy harvesting system can efficiently achieve frequency up-conversion and generate a higher output power under the conditions of sinusoidal excitation at a frequency of 1 Hz and an amplitude of 40 mm. The system exhibits its highest power output with a magnetic distance of 15 mm, resulting in a maximum output power of 57.35 μW. Finally, to verify the performance of the designed energy harvester in low-velocity water flow, a series of underwater experiments were carried out. The results show that the designed harvester can generate an output power of 23.73 μW with optimal resistance of 250 kΩ at a flow rate of 0.371 m/s. The designed structure is well suited for energy harvesting in flow-induced vibration in low-velocity water flow.