We are constantly in motion. We walk around, we turn around, and we run around. Whenever we move, we exert energy. What if we could harness that energy and turn it into electricity? We'd have a renewable source of energy, one that doesn't pollute the environment. Better yet, we could charge our bodily electronics (e.g., cellphones) at no cost.
Dr. Zhong Wang at the School of Materials Science and Engineering, Georgia Institute of Technology, is leading a research project that could someday harvest mechanical energy from your motions and turn it into electrical energy for small systems. The research project is based on the development of a triboelectric nanogenerator (TENG), which creates electric energy from movement.
"The triboelectric effect has been around for centuries," said Dr. Wang in a phone interview last week. The triboelectric effect is based on having friction between two materials. When that occurs, an electrical charge from one material transfers to the other. That's what happens when you walk across a carpet on a dry day. When you move the two layers apart to form a gap, you create a voltage where the charge density is related to the size of the gap. When an electrode is connected to the materials and to a load, current will flow.
Figure 1 shows the sequence. Figure 2 shows that the output can be connected directly to a load or rectified prior to connection to a load.
Figure 1
When the materials are compressed and released, they generate a voltage that,
when connected to a load, produces current.
Source: "Nanoscale Triboelectric-Effect-Enabled Energy Conversion for Sustainably Powering Portable Electronics" by Sihong Wang, Long Lin, and Zhong Lin Wang, NANO Letters, ACS Publications.
The TENG device consists of two "plates." The lower plate is made of aluminum while the upper plate consists of polydimethylsiloxane (PDMS), a sheet of Kapton, and a 500nm SiO2 film applied at 250°C. The difference in thermal expansion between the Kapton and the SiO2 film creates an arch when cooled to room temperature. The arch, whose maximum height is about 0.5mm, compresses under mechanical force and then returns to its original shape when the force is removed.
Figure 2
The output of the triboelectric device can connect directly to a load, or it can be rectified through a diode bridge (a and c). A loss of power to the load occurs because of losses in the rectifying diodes (b and d).
Source: "Nanoscale Triboelectric-Effect-Enabled Energy Conversion for Sustainably Powering Portable Electronics" by Sihong Wang, Long Lin, and Zhong Lin Wang, NANO Letters, ACS Publications.
Dr. Wang noted that the triboelectric effect has never before been used to create electric energy from human motion. The energy is a few milliwatts, enough to drive several LEDs. It's inexpensive, scaleable, and "green." The latest published paper,"
Nanoscale Triboelectric-Effect-Enabled Energy Conversion for Sustainably Powering Portable Electronics," documents 280V at about 130µA, but the research team has now created devices that generate around 1000V. Because the voltage is so high, contact resistance isn't a problem in transferring the energy to a useful load.
I can imagine devices that you can wear, perhaps in your shoes, that can generate electrical energy with every step. "Give us a couple of months and we'll report that data," said Dr. Wang. He has seen in his lab where using footsteps can energize 600 LEDs. That's enough to power a cellphone. You just need a wire from your shoe to your phone.
Developing products that use triboelectric motion to generate useful electrical energy will clearly be some years away. More research is needed, followed by the development of manufacturable TENGs into everyday products. Because of the high voltages, these products will need to protect users from possible shock. That may require the same kinds of protection devices used to protect circuits for ESD events.
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