Harvesting vibration energy with a few drops of liquid

Most contraptions for harvesting energy from vibrations use either a spring mass or a piezoelectric material. But a new idea from researchers in Norway is to let external motion slide a droplet of mercury along a special electret film to generate power.

The fluid droplet works as a proof mass that slides along a structure that is basically a capacitor. The capacitor is fabricated from a sheet of electret material with interdigital electrodes patterned beneath. The electret material is polytetrafluoroethylene (PTFE). The droplet slides back and forth on the surface of the PTFE.

The droplet is either mercury or some other conductive material. It is sized so that it can overlap two electrodes. As the droplet rolls over the electret material, charges on the surface of the droplet get redistributed, though as a whole it is still charge neutral. Fringing fields from this action cause not only a variation in the capacitance of the structure, but also an open-circuit voltage that depends on the position of the droplet.

In experiments at Vestfold University College in Norway, researchers put together such a circuit with a 2-μm-thick PTFE film and 500-μm-wide fingers forming the capacitor. The device worked with a 16 MΩ load.

The output voltage depends on the diameter of the droplet. Using droplets with diameters of about a millimeter, researchers measured peak output power of about 0.18 μW and a mean output for one traversal of the device of 7.78 nW.

Of course, things are just getting started, researchers say. They want to scale up the device with multiple droplets and channels. They also need to study how output varies with electret film thickness, droplet properties, electrode geometry, and other factors.

To brush up on what an electret is, try this:

The researchers published their work in Applied Physics Letters:

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