A string-suspended and driven rotor for efficient ultra-low frequency mechanical energy harvesting

•A fundamentally different rotor structure (string-driven rotor) is proposed.•The string-driven rotor features extremely simple structure.•High electric outputs are enabled by the energy harvester designed with the rotor.•Sufficient energy is converted from human motions to drive various small electronics.•The string-driven rotor can be integrated with various transduction mechanisms. The abundant mechanical energy distributed in ambient environments features ultra-low frequency. Implementation of self-sustained low-power electronic systems is largely dependent on the efficient exploitation of the ubiquitous ultra-low frequency mechanical energy. This paper reports a fundamentally different approach for constructing high-output rotational energy harvesters on the basis of a novel string-suspended and driven rotor, which uses only two strings to suspend and actuate a rotor. Distinguished from the conventional energy harvesters that convert various mechanical motions to vibrations of a mechanical oscillator with high resonant frequency, the proposed approach converts vibrations or linear reciprocating motions to either rapid rotation motion or high-frequency small twisting vibration of a rotor, making the harvester well suited for harnessing ultra-low frequency mechanical motions. We demonstrate the superior performance of this approach by applying it to electromagnetic energy harvesting from ultra-low frequency vibrations and human body motions. Under a periodical excitation with amplitude of 23 mm, the fabricated electromagnetic energy harvester achieves 12.3 mW power with a matched resistive load (25 Ω) at 2.6 Hz. With human body motion as the energy source, the harvester can sustainably drive as well as charge various small electronic devices, manifesting the promising potential of the proposed approach for capturing ultra-low frequency mechanical energy as a practical power source.