Ultra-low frequency vibration energy harvesting: Mechanisms, enhancement techniques, and scaling laws

With the increasing demand for clean and sustainable energy, self-powered technologies based on environmental vibrations have attracted a lot of attention recently. Natural vibration sources, exemplified by ocean waves, tall buildings swaying, flow-induced vibrations, human motions, etc, offer abundant energy which usually exhibits ultra-low frequency characteristics. This paper provides a state-of-the-art review on ultra-low frequency vibration energy harvesters and corresponding enhancement methods. Existing designs, underlying principles and associated energy conversion mechanisms are reviewed. Due to the low energy conversion efficiency when the harvesters are directly excited by ultra-low frequency vibrations, enhancement methods are essential. Various issues like principle-based structural optimization, transducer property improvement and combination of hybrid harvesters are discussed. Scaling laws of harvesters, existing challenges and future prospects are presented in view of promoting the development of ultra-low frequency vibration energy harvesters.