Magnetic Frequency Tuning of a Shape Memory Alloy Thermoelectric Vibration Energy Harvester

This paper discusses the frequency tuning and dynamics of a novel energy harvester that converts thermal energy into mechanical vibrations of two polyvinylidene fluoride (PVDF) piezoelectric cantilevers that generate electrical energy through a shape memory alloy (SMA) filament. The vibration frequency is tuned by two ferromagnetic masses symmetrically arranged on the SMA filament and interacting with fixed NdFeB permanent magnets. The SMA filament moves transversely due to its own longitudinal temperature contractions and stretches due to a constant temperature heater. Temperature differences above the heater cause periodic changes in the length of the SMA filament, resulting in self-excited oscillations of the masses in the vertical and horizontal directions. An experimental setup was created to study the harvester by measuring the mass displacements and electrical voltages generated by the piezoelectric cantilevers. Data on the dependence of the output voltage and power on the load resistance of the consumer were obtained. The experimental results have been validated by a multiphysical dynamic model that considers the relationships between the mechanical, thermal, magnetic and electrical domains. Research shows that permanent magnets increase the frequency of bending vibrations from 8.3 Hz to 9.2 Hz. which for a heater with a constant temperature of 70 °C, increases the output power from 1.9 µW to 8.18 µW. A special feature of the proposed energy harvester design is the ability to operate at cryogenic temperatures.