A nonresonant triboelectric-electromagnetic energy harvester via a vibro-impact mechanism for low-frequency multi-directional excitations

A majority of current studies about non-resonant energy harvesters for low-frequency energy are based on pure experiment without theoretical modeling. In this work, a novel hybrid triboelectric-electromagnetic harvester in form of a rolling magnet inside a casing is proposed for scavenging low-frequency energy from multi-directional excitations, which includes a number of sub triboelectric energy harvesters (sub-TEHs) and two sub electromagnetic energy harvesters (sub-EEHs). Both the mechanical model and electrical model are established for a horizontal excitation, a rocking excitation and a vertical excitation. A velocity-dependent coefficient of restitution for impact and an impact-velocity-dependent charge density are obtained via experiment, which improve the theoretical model and result in better agreement between theoretical and experimental results. Structural response and electric outputs under different types of external excitations are investigated theoretically. It is found that the harvester has a good sensitivity to small-angle ultra-low-frequency rocking excitations, which is beneficial to low-frequency energy harvesting. When tested on a shake table at 4 Hz, a sub-TEH and a sub-EEH can generate peak powers of 212.1 µW and 57.8 mW, respectively. An electronic clock is able to work continuously when powered by the harvester shaken by hand. The harvester is also tested inside a handbag carried by a walking human for biomechanical energy and on a floating structure for wave energy, which exhibits good performance. This work presents a novel design with a capability of harvesting low-frequency energy under different external excitations, which contributes to the development and applications of triboelectric and electromagnetic energy harvesting.