Micro-energy harvesting from conductive droplets

May 30, 2012 //By Julien Happich
Micro-energy harvesting from conductive droplets
In a paper recently published in the Applied Physics Letters, researchers from the department of Micro and Nano Systems Technology (IMST) from the Vestfold University College, Norway, presented a non-resonant electrostatic energy harvester.

Relying on conductive droplets sliding along the surface of a micro-fabricated electret film, this non-resonant approach could generate electrical power from low frequency vibrations, explains the paper.

When a charged droplet slides across the interdigital electrodes situated below the charged electret film, it causes a capacitance variation that is used to generate electric power. A prototype of the fluidic energy harvester demonstrated a peak output power at 0.18uW with a single droplet having a diameter of 1.2mm and sliding on a 2-um thick electret film. This electrostatic transduction which is motion-induced is possible thanks to the bias provided by the internal fixed charge polarization of the electret film.

While traditional electrostatic energy harvesters often rely on spring-mass configurations optimised to operate in resonance at specific frequencies, this new type of energy harvesters broadens the frequency range in which energy can be harvested, including the low frequency sources such as human body motion.

Using a fluid droplet as a proof mass is also interesting because it avoids wear and can potentially display very low friction if a sufficient contact angle is achieved, explains the paper. The team used mercury droplets sliding on a thin charged polytetrafluoroethylene (PTFE) electret with interdigital electrodes (IDEs) patterned beneath, but it is also investigating the use of ionic liquid marbles with hydrophobic properties. When the droplet is located at the center of the two adjacent electrodes, the capacitance reaches its peak value as the droplet overlaps the two electrode faces, then it falls to one tenth of this value when the droplet is centered on one of the electrode fingers – see figure 1.


Fig. 1: The variable capacitance induced by a conductive droplet sliding on a charged film (the electret) beneath which are patterned thin electrodes. (a) and (b) indicates different droplet position versus the interdigital electrodes; (c) is the equivalent circuit.

There are embedded electric charges in the dielectric film, so the device

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