Called triboelectric generation, the harvester in this case relies not on rubbing, but aluminium foil being touched against a film of fluorinated ethylene propylene, then moved away again – dubbed ‘contact-separation’ operation. A thin copper coating evaporated onto the back of the polymer acts as the second electrode.
Using Kapton tape as a substrate, a zig-zag folded structure (see drawing) was developed to stack 10 or 15 generating cells together so that they can be activated by the same force, while being electrically connected in series. The springy Kapton provides separation force when pressure is removed.
Total size is 5.7 x 5.2 x 1.6cm for a 10 layer generator weighing 30g. From this, somewhere around 2uC of charge is available at around 700V each time the generator is physically cycled.
To match this high-voltage high-impedance source to a low-voltage low-impedance load, a two-stage power converter was developed.
Once the capacitor reaches a threshold voltage (~200V, determined by the required input impedance), stage two is activated.
Stage two is a fly-back down-converter, where the capacitor is switched across the primary of a coupled inductor, transferring the capacitor’s energy into a magnetic field.
A control circuit turns off the primary switch when the capacitor is empty and simultaneously turns on a secondary-side switch which flips the inductive energy into an output reservoir capacitor.
“By matching the impedance of the storage device to that of the triboelectric generators, the system can boost energy efficiency from just 1% to as much as 60%,” said the university, which got its 1% figure from missing out the converter – theoretically connecting the rectified generator directly to a 1V load.
Despite control logic and switches being self-powered, efficiency is 90%. Minimum input power is 100uW, said the university.
According to ‘A universal self-charging system driven by random biomechanical energy for sustainable operation of mobile electronics‘, a paper in Nature Communications, the generator is not affected by humidity and there are few sign of wear after nearly 200,000 operations – the latter due to using contact-separation mode instead of rubbing mode.
The research team has reported four modes (plus hybrids) to operate triboelectric generators:
- Vertical contact-separation
- Lateral sliding
- Free-standing triboelectric-layer