Is triboelectricity the next harvesting source?

-February 10, 2014

We're always looking for a new source of energy to harvest, and why not? It's often free (ignoring the up-front cost), convenient, and solves many practical installation/replacement issues. But before the energy can even reach the harvesting electronics and load, there are two front-end problems to resolve: finding a consistent physical phenomenon available for harvesting, and having a reasonably efficient and reliable transducer to actually capture its energy (such as a piezo-based component for vibration, or blades for airflow, or a water-diving vehicle that uses temperature gradients ).

Among the well-known potential sources (sorry about that pun) are solar radiation, of course, along with temperature differences, sound, vibration, airflow, and motion. There's one more possible source that is truly ubiquitous but has been hard to capture: static electricity due to friction (it often appears as ESD — electrostatic discharge — and is considered a "bad thing"). But some recent work at the Georgia Institute of Technology may change that.

According to the Georgia Tech press release, a team including that school has developed inexpensive, flexible polymer materials that are very good at developing a charge through rubbing, then holding it until it can be extracted as current flow. While static electricity is not a new development, it's tricky to generate, hold, and extract (think of the Leyden jar).

The image shows how power is generated by triboelectric generation, by sliding two materials together and then creating a gap between them. (Credit: Inertia Films)

This is certainly interesting, but how much harvestable energy is there via this mechanism? Here's where the analysis gets a little tricky.

The press release cites a power output as high as 300 W, which is pretty significant. But energy harvesting is about energy on the capture side, while it is about power delivery on the load side. You collect energy in random "dribbles" when you can, but you have to spend it as power (the rate at which the energy is expended) because any real load requires some minimum threshold of power to function. That 300 W figure may translate into barely meaningful energy levels, and the lower the energy level available to be harvested, the harder it is to actually collect it with acceptable losses.

Still, the Georgia Tech work sounds impressive and intriguing. They claim a volume power density of more than 400 kW per cubic meter, along with efficiency of greater than 50%. How this translates into real-world applications is hard to say, but is certainly worth keeping an eye on.

The researchers also say that their materials can be used to harvest energy from contact with flowing water. That certainly opens up new opportunities since there are many "hidden" flow sources, such as sinks and faucets, which perhaps could be tapped as well.

It will be interesting to check back in a few years and see what has become of this source. Is there enough energy available to make it viable for the electronics which must extract it effectively? Will the material and associated physical embodiment of the transducer be reliable and cost-effective? Will it become a mass-market source, or a highly specialized one such as the thermoelectric generator (TEG) which uses thermocouples to extract energy from a heat source?

Are there any other "untapped" harvesting sources you'd like to see explored? Any reasons you think they have not been exploited thus far?

Related blogs on energy harvesting:
Clever harvesting scheme takes a deep dive, literally
An energy-harvesting scheme that is nearly useless?
Enough with the solar-power "magic"—please!
Nothing new about energy harvesting
Remember: voltage is not power (but we still need it)

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