Electret-based energy harvesters: A review

Extracting a few microwatts to a few milliwatts of power from sporadic sources of energy in our surrounding, e.g. wind gusts, living body motions, or vibrations of civil structures can provide the required electricity to drive various self-sustained small-scale systems like off-grid sensor/actuator networks, lab-on-a-chip electronics, etc. In this regard, electrostatic energy harvesters offer attractive solutions due to the features such as portability, versatility, longevity, and robustness. Electret-based harvesters are a family of electrostatic energy scavenging devices characterized by their high-voltage output, broad operational bandwidth, acceptable efficiency and compactness, and realizable structural flexibility. A layer of electret material is the electrostatic counterparts of a magnet. A charged electret film has permanent electric charges at the surface or co-oriented electric dipoles inside. If placed in the space between the two electrode sides of a capacitive structure, it electrostatically charges the capacitor. By externally connecting the two sides of the capacitor through a load, if the geometry of the capacitor alternates due to an external stimulus, an alternating current flows into the closed circuit and powers the load. Given the growing research interest in electret-based harvesters, a more recent tabular survey on this topic seemed inevitable. In this review, we delve into the principles of electret-based energy harvesting, different types of electret materials and their fabrication processes, as well as design, modeling and performance evaluation of the harvesters. We also shed a light on the main challenges in their design, fabrication and operation along with the respective remedies proposed so far. Our surveys reveal a strong focus on efficiency enhancement, structural flexibility, device miniaturization and lifetime prolongation, robustness in harsh environments, and simplification of fabrication processes in most of the research works in the presented literature.