We investigated the dependence of output power, frequency band width, and resonance frequency on the initial air gap for electret-based out-of-plane vibration energy harvesters, both numerically and experimentally. In this investigation, the external acceleration and surface charge densities of the electret were held constant. The numerical investigation predicted the following results: (1) an optimum value exists in the initial air gap to maximize the output power; and (2) enhanced electrostatic forces with decreasing the initial air gap emphasize the soft spring effect, which widens the frequency band width and lowers the resonance frequency. The experimental results showed behaviour consistent with the numerical predictions. The maximum output power in experiment was 4.0 μW at the optimum initial air gap of 0.43 mm when the external acceleration and the frequency were 4.9 m s-2 and 102 Hz, respectively. With reducing the initial air gap to 0.28 mm, the frequency band width increased to 17 Hz, a 2.6-fold increase over the optimum initial air gap. The peak output power at the initial air gap of 0.28 mm was 2.7 μW, when the external acceleration and frequency were 4.9 m s-2 and 96 Hz, respectively.
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