Effects of electrical loads containing non-resistive components on piezoelectric energy harvesting

This study investigates the fundamental effects of electrical loads containing non-resistive components (e.g., rectifiers and capacitors) on piezoelectric energy harvesting performance. Theoretical, numerical and experimental studies have been carried out to investigate three types of electrical loads, namely (I) a rectifier followed by a resistor, (II) a rectifier followed by a regulating capacitor and a resistor, and (III) a simple charging circuit consisting of a rectifier and a capacitor. It is shown that device performance based on pure-resistive loads cannot be generalized to applications involving non-resistive components, i.e., rectifiers and capacitors. Results from cases (I) and (II) show that the rectifier voltage drop leads to a decrease in the power delivered to the load resistance and to an increase in the natural frequency of the device but does not change the optimal resistance corresponding to a driving frequency. The regulating capacitor, however, results in increases in both the optimal load resistance and the natural frequency of the device. Therefore, tuning the natural frequency of a piezoelectric device is possible through an adjustable regulating capacitor with an appropriate rectifier. From case (III), it has been found that a larger storing capacitor, with a low rectifier voltage drop, improves the piezoelectric energy harvesting performance.