Optimization of Out-of-Plane Vibration Energy Harvesters Employing Electret

This paper investigated the dependence of out-of-plane electret-based vibration energy harvesters' output power, frequency bandwidth, and resonance frequency on surface charge density and load resistance. As the external acceleration amplitude and electret size were held constant, the following results are predicted by the numerical investigation: (1) An optimum value exists in the surface charge density to maximize the output power. As the surface charge density is increasing, electrostatic forces are enhanced, which emphasizes that the soft spring effect widens the frequency bandwidth and lowers the resonance frequency. (2) Different surface charge densities correspond to different optimum initial air gaps, resonance frequency, and optimum load resistance. (3)With the attenuation of the surface potential, the output power, frequency drift, and frequency bandwidth decreased. (4) An optimum value exists in the load resistance to maximize the output power. As the load resistance is decreasing, electrostatic force is enhanced, which lowers the resonance frequency. (5) A maximum frequency bandwidth exists with further load resistance increasing, and the initial air gap is smaller, the greater the frequency bandwidth.