|Fig. 1: A thermoelectric couple (T1 and T2) depicting the Seebeck process. (Source: Wikimedia Commons)|
Wearable thermoelectric generators (TEGs) harness the temperature difference between a body and the surrounding air to generate electricity.  Today, researchers are developing wearable TEGs that power long-term health monitoring devices like devices that record heart health.  In addition to long-term health monitoring devices, TEGs are also used in automobiles to utilize vehicles' waste heat and, in doing so, enhance fuel efficiency and reduce greenhouse gas emissions. 
Many researchers have presented different designs for wearable TEGs. A promising design of wearable TEGs includes a thermoelectric (TE) plate, two polydimenthylsiloxane (PDMS) plates, two semiconductors, and aluminum oxide ceramic heads.  The TEG has a heat spreader attached across one side of it. The thermoelectric plate is sandwiched between two PDMS plates. The PDMS plates act as insulators and reduce the heat lost during the transfer of heat from the heat spreader to the TEG.  The semiconductors include a n-type (negative) and a p-type (positive) to form a thermoelectric pair.  The TE element is sandwiched on the two remaining sides by aluminum oxide ceramic heads. Lastly, a copper heat spreader is attached to the bottom of the TEG to enhance dissipation of heat and cooling in the device. The heat spreader also improves the ability for the technology to be worn on various parts of the body. 
TEGs utilize the Seebeck effect (as depicted in Fig. 1) to power a voltage difference between the semi-conductors and thus allow current to flow.  More specifically, the Seebeck effect uses the temperature difference between the two semiconductors to produce a voltage difference between the semiconductors. Voltage difference results in current flow across the semiconductors, which is harnessed to provide electrical power.
Wearable TEGs will generate more electricity if there is a greater temperature difference between a body and the surrounding air. Given this fact, researchers tested wearable TEGs on different parts of the body and while the body was performing different activities.  In a specific study, researchers tested the electricity produced by a wearable TEG on different body parts including the wrist, upper arm, chest, and on a shirt. They tested these specific locations while a person was sitting and walking. The researchers of this study concluded that the highest amount of electricity produced was 20 μW/cm2 and was measured when the TEG was attached to the upper arm of a walking person.  For context, 20 μW/cm2 is a small amount of power and can be expressed as 0.000020 Watts/cm2. Given that one study found the average circumference of the male upper arm to be 28 cm, if TEGs were wrapped around the upper arm so that they spanned an area with a length of 28 cm and a height of 5 cm, they would cover a total of 140 cm2.  With the finding that TEGs produce 0.000020 Watts/cm2, 0.0028 Watts of power could be harnessed over 140 cm2. This is found with the follow multiplication: 0.000020 Watts/cm2 × 140 cm2 = 0.0028 Watts.
This is still a small amount of energy. For comparison, a study found that the average refrigerator requires 3,000 watts per day.  This suggests that the amount of power gained through TEGs is minimal in powering any appliances commonly used today. The purpose of TEGs, however, is not to necessarily power everyday appliances, but instead to power health monitoring devices. The study assessing power harnessed through TEG placement on different parts of the body concluded that the power output is adequate to run small environmental and health monitoring devices including an ozone sensor or an electrocardiogram. 
The success of wearable TEGs negates the need for batteries in powering wearable devices. Reducing the use of batteries is beneficial as it reduces the use of hazardous materials that are required in producing batteries. Additionally, successful wearable TEGs provide a consistent energy source since body heat is an inherent characteristic of the human body. It is important to note, however, that the amount of power currently being harnessed with TEGs is quite small. While it may be able to power small environmental or health monitoring devices, the overall power output is quite low.
© Elle Billman. 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.
 M. Stevens, "Human Body Heat as a Source for Thermoelectric Energy Generation," Physics 240, Stanford University, Fall 2016.
 V. Leonov, "Thermoelectric Energy Harvesting of Human Body Heat for Wearable Sensors," IEEE Sens. J. 13, (2013)
 Y. Shi et al., "Design and Fabrication of Wearable Thermoelectric Generator Device for Heat Harvesting," IEEE Robot. Automat. Lett. 3, 373 (2018).
 M. Hyland et al. "Wearable Thermoelectric Generators For Human Body Heat Harvesting," Appl. Energy 182, 518 (2016).
 N. Brito et al., "Relationship between Mid-Upper Arm Circumference and Body Mass Index in Inpatients," PLOS One 11, e0160480 (2016).
 S. Mudie et al. "Electricity Use in the Commercial Kitchen," Int. J. Low Carbon Tech. 11, 66 (2013).