Analysis of power and efficiency of piezoelectric vibration energy harvesters through an impedance plot

Through an impedance plot obtained from the equivalent circuit modeling of linear energy harvesters, this article provides explanations of their system behaviors such as power and efficiency. The impedance plot shows the tuning impedance and the matched source impedance in the same graph, providing a visualization of their relationship as the system parameters are changed or tuned. Using this relationship, the power characteristics of the system are clearly explained. In addition, the impedance plot is connected to the structural effects in stiffness and damping due to energy harvesting. Two types of efficiency are defined in terms of the electrically induced damping: the energy conversion efficiency and conventional energy efficiency. Using the impedance plot, it is shown that the maximum power and maximum efficiency are achieved almost simultaneously for weakly coupled systems. However, for strongly coupled systems, they cannot be achieved at the same time due to the significant reduction in structural energy associated with high efficiency. Although many relationships discussed in this article are reasonably understood in the research community, a deeper and more direct understanding of these relationships are offered by this article with the aid of this graphical and intuitive approach.