The upcoming generation of wearables is expected to be lighter and more flexible. Future devices may also be equipped with an advanced power source, which could be a portable, long-lasting battery or even a generator. Indeed, as Professor Byung Jin Cho of The Korea Advanced Institute of Science and Technology (KAIST) notes, supplying power in a stable and reliable manner is one of the “most critical issues” for the successful commercialization of wearable devices.
As such, Cho, a professor of electrical engineering, developed a glass fabric-based thermoelectric (TE) generator that is extremely light, flexible and even produces electricity from the heat of the human body. In fact, the TE generator is so flexible that the (allowable) bending radius of the generator is as low as 20 mm. Effectively, this means there are no changes in performance – even if the generator bends upward and downward for up to 120 cycles.
To date, two types of TE generators have been developed based either on organic or inorganic materials. The organic-based TE generators use polymers that are highly flexible and compatible with human skin, ideal for wearable electronics. The polymers, however, have a low power output. In contrast, inorganic-based TE generators produce a high electrical energy, but they are heavy rigid and bulky.
Essentially, Professor Cho came up with a new concept and design technique to build a flexible TE generator that minimizes thermal energy loss, yet maximizes power output. His team synthesized liquid-like pastes of n-type (Bi2Te3) and p-type (Sb2Te3) TE materials – printing them onto a glass fabric by applying a screen printing technique. The pastes permeated through the meshes of the fabric and formed films of TE materials in a range of thickness of several hundreds of microns. As a result, hundreds of TE material dots (in combination of n and p types) were printed and well-arranged on a specific area of the glass fabric.
According to Cho, the TE generator boasts a self-sustaining structure, eliminating thick external substrates (typically made of ceramic or alumina) that hold inorganic TE materials. These extraneous substrates have traditionally consumed a significant portion of thermal energy, causing low output power.
“For our case, the glass fabric itself serves as the upper and lower substrates of a TE generator, keeping the inorganic TE materials in between,” said Cho. “This is quite a revolutionary approach to design a generator. In so doing, we were able to significantly reduce the weight of our generator (~0.13g/cm2), which is an essential element for wearable electronics.”
When using KAIST’s TE generator (with a size of 10 cm x 10 cm) for a wearable wristband device, the device is capable of producing approximately 40 mW electric power based on the temperature difference of 31 °F between human skin and the surrounding air.
“Our technology presents an easy and simple way of fabricating an extremely flexible, light, and high-performance TE generator. We expect that this technology will find further applications in scale-up systems such as automobiles, factories, aircrafts and vessels where we see abundant thermal energy being wasted,” added Cho.
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