Fractal-inspired multi-frequency structures for piezoelectric harvesting of ambient kinetic energy

Energy harvesting devices capable of converting freely-available ambient energy into electrical energy have received significant attention recently. Ambient kinetic energy is particularly attractive for conversion since it is almost ubiquitous and easily accessible. Piezoelectric energy harvesting devices are promising due to their simple configuration and high conversion efficiency. This paper studies multifrequency structures for piezoelectric energy harvesting of ambient kinetic energy, inspired by fractal geometry. Identifying such structures that are simple and efficient is challenging. We propose four fractal-inspired structures and we examine them at both micro and macroscales. We calculate their frequency response up to 100 Hz with computational modeling, and we also examine the effect of the fractal geometry iteration level. We use a cantilever plate example as a reference to validate computational results against analytical ones. A quantitative criterion to assess the harvesting efficiency of the proposed structures is introduced using the bending strain associated with each mode shape. Results show that a large number of eigenfrequencies is obtained, evenly distributed below 100 Hz, particularly in the macroscale. In addition, the iteration level of the fractal geometry affects the number and distribution of eigenfrequencies in the range of interest. Comparison with a conventional batch of cantilevers of the same size as the proposed structures shows noticeable improvement in electric charge generation.