Size optimization of metamaterial structure for elastic layer of a piezoelectric vibration energy harvester

Abstract Vibration energy harvesters are attracting attention as sensor node power sources for the Internet of Things society because they are maintenance-free. Vibration energy harvesters, which generate power efficiently by utilizing structural resonance, have difficulty in reducing the size of the device and matching low-frequency bands. In this study, a piezoelectric vibrational energy harvester (PVEH) is proposed with mechanical metamaterials for the elastic layer, which has a low resonance frequency and high-power output. We designed the dimensions of the metamaterial structure for the elastic layer of the PVEH, and the bending stiffness was varied by changing the size of the periodic structure of the metamaterial structure to adjust the device’s performance. Finite element analysis was performed for four metamaterial PVEHs with different periodic structure sizes. The metamaterial PVEH with the smallest structure had a 16% decrease in resonance frequency and 100% increase in power generation compared to the metamaterial PVEH with the largest structure. The proposed PVEHs were fabricated by photolithography, and their performance was evaluated by vibration tests. In a sinusoidal excitation with an acceleration of 0.2 G, assuming that the vibration is caused by human body motion, the metamaterial PVEH with the smallest structure shows a resonance frequency of 32 Hz and a maximum output power of 1.3 μW. Compared to the conventional flat plate-type PVEH, the resonance frequency is reduced by 48% and the power output is 3.2 times higher in the proposed PVEH. An increase in flexibility due to metamaterial structures has an advantage that there is no need to change materials or processing processes as compared with the case of using the low stiffness substrate.