Direct-current, long-lasting and highly efficient electret energy harvesting from ultra-low-frequency motions using toothed clutch mechanism

Generally, most wearable energy harvesters generate only a small amount of electrical energy from the low-frequency motions of human body and immediately stop working when the activity ends, resulting in low energy extraction efficiency. Here, we propose an electret rotatory energy harvester (e-REH) with a mechanical tooth-clutch transmission system, converting the ultra-low-frequency irregular human motion (< 0.1 Hz) into a high-speed and long-lasting rotation for boosted energy conversion efficiency. The tooth-clutch transmission system is designed with a compact integration of a linear-to-rotation unit, a tooth-clutch transmission unit and an energy storage unit to generate continuous, stable and highly efficient electrical output. Triggered by impulse excitation of gentle hand tapping, the flywheel can continuously rotate for about 45 s inertially, and the maximum speed of the flywheel can reach about 1500 rpm, indicating the frequency is increased up to 4500 times from 0.022 Hz to 100 Hz. In addition, by introducing the multi-layer phase-shift electrode design, a constant direct-current output with a crest factor down to 1.09 is obtained by the phase coupling effect, avoiding the complicated signal processing circuit. Experimental results show the fabricated e-REH can produce an overall coupling current of 22.1 μA and average output power of 3.95 mW. Thanks to the lightweight, compact and high-efficient transmission design, the e-REH is further integrated into a shoe for wearable energy harvesting and self-powered underground environmental monitoring applications. The research is of considerable significance to the practical applications of e-REH in powering sensors and building self-powered wearable sensor networks.