Thermoelectric generation using body heat

Dr. Neil Canter, Contributing Editor | TLT Tech Beat December 2016

A new device takes advantage of the temperature difference between the human body and the outside air to produce electricity.

 

KEY CONCEPTS
• A new device using thermoelectric generation generates power by using body heat.
• The success of this device is due to the use of a heat spreader that creates the temperature difference needed to produce the power.
• Positioning the device in the right area on the body is important in maximizing the power produced.
RESEARCH IS ONGOING TO DETERMINE WAYS TO UTILIZE energy that is often lost in the ongoing effort to improve efficiency. This process is continuing not only with large machinery such as internal combustion engines but also on a small scale involving the human body.

In a previous TLT article, mechanical energy produced during the human motion of walking is converted into electricity through a technology called reverse electrowetting (1). The energy is converted by inserting a dielectric-film-coated electrode that interacts with a liquid droplet in a shoe. Power generation can reach one watt per shoe if the individual takes one step per second.

Another approach to convert energy into a useful form is thermoelectric generation. Haywood Hunter, undergraduate student at North Carolina State University in Raleigh, N.C., says, “Thermoelectric generation (TEG) takes advantage of the temperature difference between two objects to generate power. In our work, we are applying the difference in temperature between the human body and the surrounding air to produce electricity. The hot side is the skin of the human body while normal air flow is the cold side. The difference in temperature can create microwatts of power.”

TEG is well suited for use in powering devices worn by an individual to monitor health and environmental factors. Daryoosh Vashaee, associate professor of electrical and computer engineering at North Carolina State University, says, “Normally there is a 10- to 15-degree temperature difference between the core body temperature and ambient temperature. However, only a small fraction of this will drop across the TEG due to the poor dissipation condition on top of the TEG. As a result, in work we are doing using the human body, there is a temperature difference of only about one degree, which leaves us with not much room to develop an effective device.”

An alternative option to TEG is to use batteries, but Vashaee indicates there are issues with this power-generating device. He says, “The power density of a small battery is high enough to generate several milliwatts. But the challenges in using batteries are their durability and the hassle with having to replace them.”

This is what is so advantageous about using TEG. However, Vashaee points out that there are limitations with this power-generation technique. He says, “TEG has not been able to generate sufficient power to be used with wearable devices. Part of the problem is that TEG is by nature a low-voltage, high-current approach. A need exists to boost the voltage produced from millivolts to volts.”

A new TEG device has now been developed that can generate more power and will be comfortable to wear.

HEAT SPREADER
Vashaee, Hunter and Melissa Hyland, graduate student at North Carolina State University, have developed a new type of wearable TEG that is more effective at using body heat to generate power. In contrast to previous TEG devices that use heat sinks, this approach uses a component known as a heat spreader. 

Hyland explains, “The heat spreader is a thermally conductive material that is spread across the skin and absorbs heat from the body. It is soldered to the bottom of the TEG device. The heat spreader conducts the heat through a bismuth telluride TEG device. It also dissipates the heat to the ambient environment creating the temperature difference needed to generate electricity. In contrast to the heat sink, the heat spreader conforms to the body making it much more comfortable for the wearer.”

Two heat-spreader materials, carbon film and copper, are evaluated by the researchers. While the carbon film has a higher thermal conductivity, copper is selected because it is more durable. The researchers report that the carbon heat spreader ripped more easily, leading to a loss of heat transfer, while the copper material is more malleable. 

The researchers then placed the TEG device at several body locations and in a t-shirt to determine where it generated the most power. The locations chosen included the wrist, upper arm and the chest. 

A maximum power generation of 20 microwatts per square centimeter is measured when the device is used on the upper arm while the individual is walking at a rate of 1.1 meters per second. Lower power figures are seen with the wrist due to either the rougher skin or lower body temperature. The chest also shows lower power figures, probably due to the lower air flow as compared to the upper arm, which leads to a lower temperature difference with the body heat. 

The most convenient location for the TEG device is the t-shirt, but it produces the lowest level of power. Figure 1 shows the positioning of the TEG device on a t-shirt and an upper arm.


Figure 1. A thermoelectric generation device placed on a t-shirt (left) and on the upper arm (right) uses body heat to generate power. (Figure courtesy of North Carolina State University.)

Vashaee says, “We are focusing on working to produce a more efficient TEG device with nanocomposites. We also would like to take this concept to the industry to scale up the device for use in consumer applications.”

 Additional information on this research can be found in a recent paper (2) or by contacting Vashaee at dvashae@ncsu.edu. 
REFERENCES
1. Canter, N. (2011), “Powering your cell phone as you walk,” TLT, 67 (11), pp. 6-7.
2. Hyland, M., Hunter, H., Liu, J., Veety, E. and Vashaee, D. (2016), “Wearable thermoelectric generators for human body heat harvesting,” Applied Energy, 182, pp. 518-524.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.