A built-in charging electret rotational energy harvester for kinetic energy at ultra-low frequency

In harsh environments, extreme temperature or humidity may lead to the decay in the surface potential of electrets in open structures. When electret materials in energy harvesters need to be charged to maintain polarization, disassembling harvesters or separating electret films from harvesters is generally an unavoidable procedure, which complicates the charging process and increases maintenance costs. At present, there is no ideal built-in charging method for electret materials in non-resonant structures that can balance low cost, safety, and simple operation. Herein, we propose a built-in charging electret rotational energy harvester (BIC-EREH), which can achieve the contact charging of the fluorinated ethylene propylene (FEP) film using a gap-adjustable platform and interdigital electrodes inside the harvester. After charging, the surface potential of the FEP film can reach over −1200 V. The interdigital electrodes in the BIC-EREH are not only used to recharge the FEP film but also serve as a component of an electret transducer to participate in electrical energy conversion. Based on a pawl-ratchet clutch driver, the harvester can output uninterruptedly even under ultra-low frequency excitation. Under one excitation with a velocity of 200 mm/s, the harvester can continuously output for about 23 s with a peak power of 98.2 μW, generating electric energy of about 0.4 mJ. Over a broad bandwidth (0.1–2 Hz), the harvester still has a stable output power. Furthermore, the BIC-EREH has high stability in a high humidity environment and the attenuated output performance can be perfectly restored with an average output power of 85.4 μW after built-in charging. Finally, the harvester is successfully applied as the power supply to drive a wireless sensor node. The study has considerable significance for sustainable power supply of electret energy harvesters in wireless sensor nodes.