A team of researchers at Stanford have developed a reversible fabric that, without expending energy or effort, maintains skin at a comfortable temperature regardless of the weather.
The team, led by Yi Cui, professor of materials science and engineering, designed a double-sided fabric based on the same material as ordinary kitchen wrap. Their fabric can either cool or warm the wearer, based on which side faces out. Details of the work were published on November 10 in Science Advances.
This project was born out of Cui’s interest in energy efficiency and his knowledge in exploiting nanoscale materials. He thought if people could be more comfortable in a variety of temperatures, they could save energy on central heating and air conditioning.
“Why do you need to cool and heat the whole building? Why don’t you cool and heat individual people?” asked Cui.
In the United States, 13% of all of the energy consumed is just dedicated to indoor temperature control. But for every 1 °C (1.8 °F) that a thermostat is turned down, a building can save an enormous 10 % of its heating energy, and the reverse is correct for cooling. Regulating temperature controls by just a few degrees has key effects on energy consumption.
Cooling kitchen wrap
Human bodies have many ways of adjusting the body’s temperature. When it is cold, the hairs on the skin stand erect to trap warm air. Sooner or later, shivering may occur so as to produce more radiant heat in the muscles. When it is hot, heat is released as infrared radiation from the skin, and in case one is still warm, then sweat starts. Water evaporating away from the body carries a large quantity of heat with it. However, those mechanisms just help within a few degrees. Get outside the temperature range to which the bodies can adjust, and one reaches for the dial on the air conditioning or heating.
In 2016, the team reported a first step toward a solution: fabric that allowed the heat from the body to pass through, cooling the skin. Although they were inspired by transparent, water-impermeable kitchen wrap, their new material was breathable, opaque, and retained its ability to transfer infrared radiation away from the body. Compared to a cotton sample, their fabric maintained artificial skin 2 °C cooler in a laboratory test – possibly sufficient to stop a person from ever reaching for the building thermostat or a fan. The team’s first textile could save a building crowded with workers 20 to 30% of their total energy budget.
“Right around when we figured out cooling, then came the question: Can you do heating?” said postdoctoral fellow Po-Chun Hsu, who was first author on the latest paper. It was a predominantly chilly winter, and he was traveling to a conference in Minneapolis with a carry-on bag containing many coats. Could he develop a piece of clothing that would serve him both in a crowded warm conference room and on the frosty street?
Hsu realized that regulating radiation could function both ways. He stacked two layers of material with varying abilities to discharge heat energy, and then sandwiched them between layers of their cooling polyethylene.
On one side, a copper coating captures heat between a polyethylene layer and the skin; on the other, a carbon coating discharges heat under another layer of polyethylene. When worn with the copper layer facing out, the material captures heat and warms the skin on cool days. When the carbon layer faces out, it discharges heat, keeping the wearer cool.
Combined, the sandwiched material can raise a person’s range of comfortable temperatures beyond 10 °F, and Hsu envisages that the potential range is much larger – nearing 25 °F. With inhabitants wearing a textile like that, buildings in certain climates might never need central heating or air conditioning at all.
The white-colored fabric is not quite wearable still, the team said.
“Ideally, when we get to the stuff you want to wear on skin, we’ll need to make it into a fiber woven structure,” said Cui. Woven textiles are stronger, more comfortable, more elastic, and look a lot like regular clothing. But good news: They have already begun testing to make sure their fabric can be machine washed.
From my perspective, this work really highlights the significant opportunities in combining thermal engineering concepts with nanophotonic structures for creating novel functionalities.
Shanhui Fan, a professor of electrical engineering
The team’s goals are to develop an easily manufactured, practical textile that people could use to save large quantities of energy all across the globe. And they do not halt there – Cui, Hsu and Fan visualize clothing with medical devices and even entertainment printed directly into the fabric.
“I think we are only seeing the beginning of many creative ideas that can come out of such combinations,” Fan said.
Cui is also professor of photon science at the SLAC National Accelerator Laboratory and a member of Stanford Bio-X, the Precourt Institute for Energy and the Stanford Neurosciences Institute. Fan is also an affiliate of the Precourt Institute for Energy. Other Stanford researchers who contributed to the study are postdoctoral fellows Chong Liu, Alex Y. Song, Jin Xie, Kai Liu and Lili Cai; graduate students Ze Zhang, Yucan Peng, Chun-Lan Wu and Shang Zhai; senior research engineer Peter B. Catrysse; and Arun Majumdar, a professor of mechanical engineering and of photon science and co-director of the Precourt Institute for Energy.
The research received funding from the Advanced Research Projects Agency–Energy, U.S. Department of Energy.