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Posted: Nov 14, 2016

A fully stretchable energy harvester for thermal waste

(Nanowerk Spotlight) Thermoelectric generators (TEGs) promise a cheap and pragmatic way to obtain energy out of waste heat. Most electronic devices develop heat that can become detrimental to performance. Rather than cooling these systems, a better solution would be to convert the excess heat to electricity by exploiting the thermoelectric effect, where a thermal gradient induces the movement of charge carriers.
A fundamental mechanism to make use of a heat source is by following the Peltier-Seebeck effect, which states that a difference of temperature can be transformed into a difference of electric potential and vice versa. The means to produce such transformation is through special materials – known as thermoelectric (TE) materials – interconnected in a particular manner across the temperature difference. The higher the temperature difference, the higher will be the electric potential (voltage) and power output.
Researchers now have demonstrated a fully stretchable energy harvester for thermal waste, which is very simple to fabricate and uses inexpensive substrate materials such polymers or paper.
The team, led by Muhammad Mustafa Hussain, an Associate Professor of Electrical Engineering at King Abdullah University of Science and Technology (KAUST), has published their findings in Nano Energy ("Stretchable Helical Architecture Inorganic-Organic Hetero Thermoelectric Generator").
fabrication of stretchable helical thermoelectric generator
3D schematics describing the fabrication process of the stretchable helical thermoelectric generator; (a) a PMMA hard mold is prepared with a laser cutter, then (b) PDMS is applied on top to form (c) a soft mold. (d) Next the OSTE mix is prepared, applied to the PDMS mold and cured under UV light. (e) The OSTE-spirals are then carefully removed from the soft-mold and finally (f) the TE materials are deposited sequentially. (g) Digital photographs of un-stretched OSTE-based TEG with 3 TE-pairs and (c) stretched to a 100% strain. (Scale bars are 1 cm). (© Elsevier)
"The biggest challenge for us was the rigid nature of commonly used inorganic thermoelectric materials," Hussain tells Nanowerk. "The great novelty of our work was to effectively integrate the high-performance of inorganic TE materials (Bi2Te3/Sb2Te3) with the mechanical advantages of affordable organic materials (polymers, paper) and the use of innovative geometries that can be inherently stretched (spirals, helixes)."
He adds that these results represent an important milestone in the power generation component of wearable and bio-integrated technologies, whose market is rapidly and continuously growing.
Thanks to the inherent flexibility, stretchability and very thin body of the presented devices, they can easily be stacked or folded to achieve much higher power densities, and thus reach more beneficial power values. As a result of the devices' high elongation capacity due to the helical design, the TEG's cold side can move further from the heat source, thus causing a larger temperature difference along the TEG and leading to a higher power generation.
Furthermore, due to its mechanical advantages, these devices have a great potential to be used on wearable or even bio-integrated systems, or in highly mobile and mechanically demanding applications such robotics and cybernetics.
"Our main achievement was to be able to adjust/stretch the length of the TE materials and thus increase the temperature difference across the device," says Jhonathan P. Rojas, now an associate professor at King Fahd University of Petroleum & Minerals, and the paper's first author. "Because the intrinsic rigidity of commonly used, high-performance, inorganic TE materials, stretchability was achieved thanks to the mechanical properties of the structural substrate material, which mechanically supports the active TE material."
These substrates consist of inexpensive, flexible organic materials (polymers, paper) with the additional functionality of being shaped with geometries that can be inherently stretched such as spirals or helixes. Moreover, the final fabrication scheme is very simple which might lead to very affordable implementations.
There have been only a few demonstrations of stretch-capable thermoelectric generators. A recent work, in fact, showed a device with high foldability and stretchability but its generated power was severely affected by the stretching process.
"With our much simpler approach, we not only demonstrated functionality and mechanical flexibility and stretchability, but also the generated power was found to increase under stretching conditions given the appropriate circumstances – the substrate material must have a low enough thermal conductivity as in the case of the used paper," Hussain points out.
Although the present device delivers relatively low power, the researchers note that this can be greatly improved through adequate optimization of materials and processes.
Additionally, some performance degradation was observed after the device was subjected to continuous mechanical stress. A suitable approach to reduce this issue is to develop a good packaging arrangement, to not only reduce the applied stress and strain, by locating the TE materials at the neutral mechanical plane, but also to protect the films from diverse environmental conditions.
Finally, out-of-the-box folding and stacking schemes, through origami/kirigami techniques, can also be used to further increase power density.
The will address these issues in the next phase of their work.
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