Sandwich-structured two-dimensional MEMS electret power generator for low-level ambient vibrational energy harvesting

This paper presents a novel sandwich-structured electret-based power generator (SEPG) for two-dimensional (2D) low-level ambient kinetic energy harvesting. The generator is based on a symmetrical resonator that consists of a double-sided electrode-patterned seismic mass suspended by four-folded parallel beam flexures. The experimental analysis shows that it is able to scavenge kinetic energy from arbitrary in-plane directions by combining its two primary vibration modes at the frequencies of 121 Hz and 125 Hz. Sandwich structure is adopted in the proposed device that has two separate capacitive circuits 180° out-of-phase with each other to be integrated into a single seismic mass system. This configuration has its merits in that both the vertical pull-in electrostatic force as well as the horizontal damping force can be reduced. With the present prototype, an overall output power of 0.12 μW is obtained for the two capacitive parts at an acceleration of 0.2 g at 125 Hz, corresponding to a normalized power density of 5.56 μW cm^-3 g^-2. The results show that the generator can potentially offer an intriguing alternative for scavenging low-level ambient energy from 2D vibration sources.