16-03-2021 | By Robin Mitchell
Recently, researchers have developed a prototype of a wearable thermoelectric generator that can generate electricity from the warmth of human skin. How do thermoelectric generators work, how does the device developed by the researchers work, and is such a device truly practical?
Thermoelectric generators are devices that produce an EMF when exposed to a temperature gradient. For a thermoelectric generator to work, specific materials have to be chosen with high electrical conductivity and low thermal conductivity.
The need for a low thermal conductivity arises because the generator requires a thermal gradient. Thus, heat from the hot side must not transfer too quickly to the cool side, thereby reducing the temperature gradient, thus dropping the efficiency.
One of the biggest drawbacks behind TEGs is their low efficiency, resulting in a large size. TEGs also suffer from difficulties in creating an efficient temperature difference across the device. As the hot side is heated, the cool side must be cooled, and traditional air cooling can struggle.
However, TEGs can be highly practical in niche applications that require a reliable power source for extremely long times. Such an example is Radioisotope Thermoelectric Generators, or RTGs, which are used in spacecraft. These power devices use plutonium pellets which generate large amounts of heat as a result of radioactive decay. The cooling side of RTGs utilises large black fins which radiate heat from the cool side.
The popularity of wearable devices such as sports watches continues to increase, but the use of power sources that require constant charging can make them inconvenient. Battery life can be increased if such devices are reduced in complexity, reducing their capabilities.
Recently, researchers from the University of Colorado have created a wearable TEG that can generate electricity from the human body. According to the researchers, their device can produce 1V per square centimetre of skin, but details regarding the current output of the TEG is unclear.
The device, which has been built into a large ring, could be expanded into a wrist-worn device to provide wearable electronics with a permanent power source that requires no charging. Furthermore, the device is self-healing, meaning that if it is torn, it can be pinched together, starting a healing process after a few minutes.
While the device built by the researchers does indeed work, there is a reason why using TEGs to produce power are often reserved for niche applications; TEGs are incredibly inefficient and produce minimal amounts of power. While a TEG could be used to power a smartphone, a large temperature difference of around 40 degrees C would be needed. Such setups can be done experimentally at home, but often use a heat source such as a candle and ice for the cooling side resulting in a complex setup.
However, if large amounts of energy cannot be extracted from such TEGs, the alternative solution is to develop electronics that use less energy. As the size of transistors is reduced, the total energy consumed per transistor is also reduced. Therefore, if future wearable electronics can get energy consumption into the microwatts while remaining functional, such wearable TEGs could become practical.
Overall, the TEG developed by the research team could become commercially viable, but this would need electronics to significantly reduce power consumption while also improving the efficiency of any wearable TEG.