A review on design improvements and techniques for mechanical energy harvesting using piezoelectric and electromagnetic schemes

Energy harvesting has gained a growing attention and has seen great advancements during the last decades. Among energy sources, mechanical energy is one of the most investigated forms due to its abundance, accessibility and ubiquity in the environment, in addition to multiple possible transduction types. Several reviewing articles have been published to organize and categorize these works. Nonetheless, some concepts are missing in most reviews and some improvement strategies have been overlooked such as non-resonant and multi-directional systems, etc. We present in this paper an exhaustive reviewing study of the state-of-the-art of mechanical harvester systems, enclosing main different existing improvement techniques and their design concept, particularly for piezoelectric and electromagnetic (EM) transductions. Accordingly, we propose a new generic categorization approach based on the improvement aspect of the harvester, which includes techniques for widening operating frequency, conceiving a non-resonant system and multidirectional harvester. In the category of widening operating frequency, three approaches are conceivable allowing the improvement of the system frequency response, which are: frequency tuning, multi-frequency and non-linear system. For non-resonant systems, possible approaches consist in Frequency-up conversion and free moving mass. The last improvement allows multiplying the directions to harvest more energy from arbitrary movements. Last section is interested in the applicability of the presented techniques under different conditions and their compatibility with MEMS technology.