A fab way to keep cool

Here's news for all those dreaming of air-conditioned clothes. Researchers at The University of Chicago have designed a fabric that cools the wearer's body. The layered fabric engineered by Po-Chun Hsu, an Assistant Professor at the university's Pritzker School of Molecular Engineering, and his team can also be used in buildings, automobile interiors and food storage. The scientists reported their work in a recent issue of Science (bit.ly/coolfab).

The material blocks thermal radiation from buildings and paved roads while beaming infrared heat rays into outer space. The material — called spectrally selective hierarchical fabric — has three layers. The top layer is made of a plastic material called polymethylpentene (PMP), which does not absorb or reflect any radiation, but emits a narrow band of infrared radiation into the sky. The middle layer of silver nanowires reflects radiation. The innermost layer is from clothing materials such as wool or cotton.

"The working principle is to engineer the fabric's spectral channels to block the thermal radiation from the buildings while maintaining the channel to emit heat to the cold sky," says Hsu. While the PMP layer does this, the nanowires block the building's thermal radiation and thus prevent it from reaching the innermost layer, he explains.

Passive or radiative cooling materials are already commercially available. Athletes engaged in outdoor sports often wear such fabrics for thermal comfort. But these materials are designed to block only solar radiation. It is assumed that the material would be oriented horizontally to the sky like a solar panel on a rooftop rather than the vertical orientation of cloth. Typically, only about 3% of clothing — headgear, cloth on the shoulders and shoe coverings — directly faces the Sun.

Further, the Sun is just one source of heat in an urban setting. Thermal radiation from buildings, paved roads and the ground raises temperatures. This is because of a phenomenon called the urban heat island effect in which urban areas experience higher temperatures than neighbouring rural areas.

But designing materials that take care of heat emanating from different sources — sunlight as well as thermal radiation — can be challenging. Hsu and colleagues discovered the importance of selecting a specific range in the electromagnetic spectrum. In tests conducted in hot Arizona, the material was found to be 2.3°Celsius cooler than the current radiative cooling fabric used for outdoor endurance sports and 8.9°C cooler than the silk commonly used for making shirts and dresses.

"The team has been able to make an engineered design that has a very high technology readiness level, which is certainly important for practical adoption," says Aaswath Raman, Associate Professor of Materials Science and Engineering at the UCLA Samueli School of Engineering. Raman's team was the first to work out the underlying principles used in the current work and identified materials that could be put to use for making such fabric. That study was posted on the preprint server Arxiv.org in 2020.

Passive cooling, where energy requirements are very little or nil, is emerging as an area of research, largely because of the increasing awareness that cooling a person directly is more efficient than cooling the air with, say, air conditioners (Technologies to cool the world). Currently, air conditioning and fans account for nearly 10% of global electricity use, according to the International Energy Agency.

In 2023, taking inspiration from the layered structure of the human skin, researchers led by Fuqiang Wang of the Harbin Institute of Technology, China, developed a clothing material that could keep its wearers cooler by 12°C than the surroundings. The material, their report in ACS Photonics (bit.ly/skin-fabric) stated, was made of cotton and polyester fibre and had dermis- and epidermis-like layers, with pores akin to those found on the skin, and an ability to reflect sunlight and infrared radiation.

The Chicago team's new material, when used in building facades or cars, can lower internal temperatures and reduce the cost and carbon impact of air conditioning. The material may also be used to transport perishable foods, cutting down the use of refrigeration, the scientists say.

See also:

Technologies to cool the world