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Posted: Sep 14, 2011
Nanogenerator harvests waste energy from a rotating tire
(Nanowerk Spotlight) Harvesting unexploited energy in the living environment to power small electronic devices and systems is increasingly attracting the attention of research groups around the world. As device sizes shrink to the micro- and even nanoscale, power consumption also decreases to ever lower levels, i.e. microwatts to milliwatts range. And as research findings have shown over the past few years, it is entirely possible to drive such minuscule devices by directly scavenging energy from their working environment (see for instance: "Nanotechnology converts heartbeat and breathing into electricity").
This self-powered technology makes the periodic battery replacement or recharging no longer necessary; a fact that makes this technology so attractive for portable or inaccessible devices (think implants like pacemakers).
"Mechanical energy is a very conventional energy source in our living environment, such as the vibration of a bridge, friction in mechanical transmission system, deformation in the tires of moving cars or bicycles, even heartbeat and breathing," Zhong Lin Wang, Distinguished Professor and Director, Center for Nanostructure Characterization at Georgia Tech, explains to Nanowerk. "This form of energy is normally wasted, but it becomes particularly important when other sources of energy such as sunlight or thermal gradient are not available. Nanogenerators are designed to transfer this kind of mechanical energy into electric energy by means of the piezoelectric effect."
In their most recent work, Wang's group has integrated nanogenerators into the inner surface of tires, demonstrating the possibility of energy harvesting from the motion of automobiles.
a) Shape change of the tire during the vehicle's movement. b) Experiment setup. A tire was caught between two boards to simulate the tire's deformation at the position where touching or detouching the road surface takes place. c) Sketch map of the nanogenerator's construction, which is a cantilever structure with five layers. d) A photograph showing that a nanogenerator was fixed on the inner surface of a tire using adhesive tape. (Reprinted with permission from Wiley-VCH Verlag)
For their experiments, the team mounted a bicycle tire in a setup so that it could be squeezed and released periodically to simulate the conditions that occur at the position where touching or detouching of the road surface takes place. The nanogenerator, which they taped to the inner surface of the tire, was designed with a free-cantilever beam structure and consisted of five layers: a flexible polyester substrate, zinc oxide nanowire textured films on its top and bottom surfaces, and electrodes on the surfaces. Earlier this year, we described the fabrication and working mechanism of this type of nanogenerator in a Nanowerk Spotlight: "First self-powered nanosystem with wireless data transmission".
Each time the tire was squeezed, the nanogenerator generated an electric pulse. Under their experimental conditions, the researchers measured output voltage and current of the nanogenerator of 1.5 V and 25 nA, respectively. The nanogenerator directly lighted a small LCD screen with the scavenged energy.
According to Wang, the effective working area of our nanogenerator is about 1.5 cm × 0.5 cm, and the maximum output power density approaches 70 µW/cm3.
Compared to the results reported in the above-mentioned Nanowerk Spotlight, the performance was somewhat degenerated under these conditions. As Wang explains, "this is because rubber is a plastic material, which absorbs some of the mechanical energy, and the strain rate in this material is smaller compared to a rigid material under the same trigger conditions. Despite this, the performance is still very good."
"While we demonstrated the nanogenerator's potential to work as a self-powered tire pressure sensor and speed detector in mobile vehicles, we also showed the possibility of further integration and scale-up by two nanogenerators connected in parallel" he says.