Episode 16: Thermoelectrics enables clothes with adjustable temperature

Sophia Chen of MRS Bulletin interviews Renkun Chen of the University of California, San Diego about his flexible thermoelectric devices that can provide personalized cooling and heating effects in clothing. Read the article in Science Advances.
Transcript

Transcript
SOPHIA CHEN: If you’ve ever had to pay an air conditioning bill during the summer, you know how expensive it gets. Renkun Chen is a mechanical engineer at UCSD with an energy-saving idea: clothes with adjustable temperature. He and his team have designed and fabricated a material you can wear that directly cools the skin. 

RENKUN CHEN: Instead of having a centralized air conditioning system in a building, where you need to cool down a large volume of space for building occupants, we use our system to cool down a much smaller volume at a personal level. By doing so, we can save energy by at least an order of magnitude. 

SC: The power consumption per person of a conventional AC system is a few kilowatts, he says. Whereas personalized cooling, like a temperature-regulating outfit, uses tens of watts. Chen isn’t the first to invent clothes that directly cool your skin. For example, you can buy shirts right now that circulate icy water to cool you off. But his team’s design uses a thermoelectric material, which cools via a distinctive mechanism known as the Peltier effect, which creates cooling by passing an electric current between the junction of a semiconductor and metal. When you reverse the current, you create a heating effect. This can achieve much subtler temperature control than the wearables that are commercially available. Chen’s device can cool and heat. 

RC: It’s really like the thermostat in the air conditioning system. You can really set the skin temperature. 

SC: The highest performing thermoelectric materials are rigid, so Chen’s team needed to configure these materials to make a flexible, wearable device. They used two different commercially available thermoelectric materials. These materials consist of two bismuth telluride alloys: a p-type semiconductor alloyed with antimony, and an n-type semiconductor alloyed with selenium. Both alloys are connected to metal electrodes, and they create a cooling effect by making an electric current flow from the metal to the p-type material, or from the n-type material to the metal. Reversing the direction of the current causes heating. To make their system flexible, Chen and his team made these alloys into pillars and sandwiched them between two sheets of Ecoflex, a flexible silicone rubber. 

RC: Even though the pillars by themselves are rigid, the entire device is flexible because of the overall architecture. 

SC: They wanted the entire layer of each sheet of Ecoflex to keep at a uniform temperature. So to achieve this, they embedded aluminum nitride particles to increase its thermal conductivity. They also kept a 4 mm air gap between the two sheets for insulation. When the ambient temperature was between 22°C and 36°C, they could maintain the wearer’s skin temperature at 32°C, which they defined as a condition of thermal comfort. Chen wants to develop this into a therapeutic device for people who have medical conditions that make it difficult for them to regulate their skin temperature. 

RC: There are patients who are very sensitive or prone to overheating with certain health conditions like multiple sclerosis, or people who are genetically not able to sweat, they are prone to overheating. There are certain occupations, outdoor construction workers or fire fighters, and people who are doing outdoor activities, like athletes for example. For this kind of application, I think our device will also provide good thermal comfort solution.