Electrostatic MEMS vibration energy harvester for HVAC applications with impact-based frequency up-conversion

This paper reports on electrostatic MEMS vibration energy harvesters with gap- closing interdigitated electrodes, designed for and tested on HVAC air ducts. The harvesters were fabricated on SOI wafers with 200 mu m device layer using a custom microfabrication process. Designs with aspects ratio (electrodes' gap versus depth) of 10 and 20 were implemented, while the overall footprint was approximately 1 cm x 1 cm in both cases. In order to enhance the power output, a dual- level physical stopper system was designed to control the minimum gap between the electrodes, which is a key parameter in the conversion process. The dual- level stopper utilizes cantilever beams to absorb a portion of the impact energy as the electrodes approach the impact point, and a film of parylene with nanometer thickness deposited on the electrode sidewalls. The parylene layer defines the absolute minimum gap and provides electrical insulation. The fabricated devices were first tested on a vibration shaker to characterize the resonant behavior. Devices with aspect ratio 10 were found to exhibit frequency up- conversion, which enhances the amount of converted power. Devices with both aspect ratios were found to exhibits spring hardening due to impact with the stoppers and spring softening behavior at increasing voltage bias. The highest power measured on shaker table for sinusoidal vibrations was 3.13 mu W (includes enhancement due to frequency up- conversion driven by impact) for aspect ratio 10, and 0.166 mu W for aspect ratio 20. The corresponding dimensional figure- of- merit, defined as the power output normalized to vibration acceleration and frequency, squared voltage and device mass, was in the range of 10 . 10(-8) m V-2 for both devices, about an order of magnitude higher than state- of- the- art. Testing was carried out on HVAC air duct vibrating with an RMS acceleration of 155 mg(RMS), a primary frequency of 60 Hz and a PSD of 7.15 . 10(-2) g(2) Hz(-1). The peak power measured was 118nW with a PSD of 1.91 x 10(-9) W Hz(-1) at 120 Hz (four times the primary frequency of 60 Hz), some of the highest values reported to date.