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| Fig. 1: A miniaturized thermoelectric generator (left) and a thermoelectric generator powering an LED strip using heat from a camp stove (right) are shown. (Source: Wikimedia Commons |
The human body has the potential to be a great portable power source, generating on average 58.2 W/m2 at resting metabolic rate, which is given off as waste heat. [1] Tapping even a small percentage of this could be enough to enable a new class of low-power wearable devices that do not require batteries. This would reduce device profile and complexity, remove the need to regularly charge or swap batteries, and allow devices to be directly integrated into clothing. Such wearables could be used for long term monitoring of vitals and chronic diseases, glucose sensors, hearing prostheses, and accelerometers for rehabilitation. [1,2] A promising technology to harvest waste heat from skin is the thermoelectric generator (see Fig. 1). [1]
Thermoelectric power generation is not a new concept, having been used for decades in extreme environments such as space, where low power but long operating times are necessary. [3] Voyager I and II used radioisotope thermoelectric generators (TEGs) to convert heat given off by radioactive decay to generate hundreds of Watts of operating power, crucial since solar power drops to less than 1 W/m2 past Pluto. [4] Another promising use of TEGs is in waste heat recovery, with applications such as boosting fuel efficiency of cars using heat from exhaust gases to harvest hundreds of Watts. [4] They have a long lifespan, no moving parts, produce no noise, and allow for direct energy generation (as opposed to converting heat power to mechanical power, and then turning a generator to create electricity). [4]
In the context of wearables, thermoelectric generators pose an advantage over other power sources. Mechanical energy conversion requires the user to be somewhat active, which is often not the case for elderly or bed-ridden patients. Photovoltaics suffer when the user is in the dark, such as indoors or at night. TEGs however will generate constant power as long as there is a difference in skin to ambient temperature, which is typical for people in comfortable temperatures. [1]
Thermoelectric generators take advantage of the Seebeck effect illustrated in Fig. 2 to generate electrical power from a heat source. When two dissimilar materials (typically metals/semiconductors), an n-type and a p-type, have each of their junctions at two different temperatures, charge carriers (electrons and holes, respectively) move to the cold ends. This creates an electric field in both of the materials that is proportional to the temperature difference, and if connected to a circuit, current flows. [3]
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| Fig. 2: A schematic showing electric current generated by the Seebeck effect. (Source: Wikimedia Commons) |
A TEG can be constructed by putting a heat sink on one end of the n-type and p-type legs, and heat source on the other. For a thermoelectric material to be good, it must have a high Seebeck coefficient and electrical conductivity but low thermal conductivity so that the temperature gradient producing voltage can be maintained across the generator. [3] In recent decades, much research has gone into creating materials with these properties, and while conversion efficiency has been approximately 15%, recent research is pushing it to 20% and beyond. [3]
Humans generate heat as a side effect of metabolism, and to maintain core body temperature at approximately 37°C. On average, this amounts to 58.2 W/m2 of heat generated, though not all of it exits through the skin, as some is lost through exhalation and perspiration. [1] Heat from the skin is transferred to the lower temperature surroundings by convection and radiative transfer at rates of 1-10 mW/cm2 throughout the body. The rate depends on the part of the body, with muscle acting as an insulator, and arteries having the highest heat transfer. Clothing also obstructs heat transfer, so the average over the whole body is approximately 5 mW/cm2. Optimal areas can be targeted such as the radial artery in the wrist which has a heat flow of about 25 mW/cm2 at room temperature. [1]
There are challenges to converting skin heat to electricity, especially since devices must be flexible, thin, and non- toxic. Thin TEGs have much lower conversion efficiency than those with large heat sinks. [1] However, many useful wearable electronics have very low power consumption, in the microwatt range. A flexible wristband covered in TEG modules was shown to be enough to capture accelerometer data from a user. [2] Thin TEGs have been integrated into textiles such as shirts to harvest lower power on the trunk while taking advantage of a larger surface area. [1]
Thermoelectric generators are a promising approach to powering wearable electronics using waste human heat. [1] Improvements to the technology are allowing for more efficient power conversion, and thinner devices. [2, 3] Furthermore, companies are creating organic and flexible TEGs with roll-by-roll manufacture methods, dramatically increasing availability and lowering costs. [4] These will allow them to conform to large and arbitrary shaped surfaces, including human skin.
© Alex Gruebele. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] V. Leonov, "Human Machine and Thermoelectric Energy Scavenging for Wearable Devices", ISNR Renewable Energy 2011, 785380 (2011).
[2] Y. Wang, et al., "Wearable Thermoelectric Generator to Harvest Body Heat for Powering a Miniaturized Accelerometer," Appl. Energy 215, 690 (2018).
[3] X. Zhang et al., "Thermoelectric Materials: Energy Conversion Between Heat and Electricity," J. Materiomics 1, 92 (2015).
[4] D. Champier, "Thermoelectric Generators: A Review of Applications," Energy Convers. Manag. 140, 167 (2017).
