Air-conditioning uses a staggering 5 percent of all the electricity produced in the U.S. That’s a vast amount of resources that, literally, go up in hot air every year. So even small advances that help people keep cool can have an important effect.
Today, Jonathan Tong and pals at MIT reveal their approach to the problem—design fabrics that allow heat to radiate away from the body more effectively.
Not only are most clothes opaque to visible light, they are also opaque to infrared. This traps infrared radiation, causing the body to heat up. That’s handy when it’s cold but not so good when it’s hot.
So Tong and co have come up with a solution. Design a material that is opaque to visible light but transparent in the infrared.
Their approach is straightforward. Tong and co say the human body emits heat primarily in the mid- and far-infrared, so their fabric should be designed to be transparent in those parts of the spectrum.
To get a sense of the challenge, they have measured the transmission characteristics of ordinary clothing materials such as cotton and polyester fabrics that together account for almost 80 percent of all textile fiber production. These materials transmit only about 1 percent of the infrared light that hits them.
By contrast, cotton and polyester transmit between 30 and 40 percent of visible light, so they are more transparent in the visual range than they are in the infrared. The team says these materials appear opaque because they reflect more light than they transmit and the human eye is only sensitive enough to pick up the reflected light. (Indeed, it is usually easy to see through these materials if you hold them up to your eyes.)
There are two reasons why cotton and polyester are so opaque in the infrared. The first is that the molecules bend and rotate at many infrared frequencies and so strongly absorb light at these frequencies. And because any fabric is many thousands of molecules thick, almost all infrared light ends up being absorbed.
Second, the fibers themselves are about the same size as the wavelength of infrared light and so scatter it effectively. This makes the materials reflect infrared light strongly. That’s why these materials often appear white in infrared images.
That sets up two challenges for Tong and co. Their new materials must be significantly less absorbent of infrared light and they must be made of fibers that do not scatter it as strongly.
Two artificial materials immediately come to mind. The first is polyethylene which is made from repeating ethylene monomers that concatenate to form molecules of different lengths. This has very few vibrational and rotational modes at infrared frequencies and so is much less absorbent.
The second is nylon which has a similar molecular structure to polyethylene with the addition of an amide chemical group. This additional group makes nylon more absorbent in the infrared, although not nearly as absorbent as cotton or polyester at the crucial 10-micrometer range where the human body radiates most of its heat.
Tong and co have another trick up their sleeve. They point out that making the fibers smaller will make them scatter and reflect infrared light less strongly. So more infrared light will pass through.
This has an additional benefit. Nylon and polyester transmit visible light quite well but reducing the size of the fibers to that of visible light will cause the fibers to scatter it more strongly. That should ensure these materials look opaque to the human eye.
The team goes on to calculate the effect that wearing a nylon or polyester fabric made from fibers that are one micrometer in diameter and woven into yarn that is 30 micrometers thick. These calculations suggest that the increased infrared transmittance would provide the equivalent of at least 23 watts of cooling. Crucially these materials should still be opaque at visible wavelengths.
“Infrared transparent visibly opaque fabric provides a simple solution to reduce the energy consumption of HVAC systems by enabling higher temperature set points during the summer,” they conclude.
There are some caveats, of course. The first is that these materials would have to be dyed for aesthetic purposes before they can be widely used in retail garments. The effect that these dyes would have on infrared transmittance is not yet clear.
Water porosity is another factor. This has an important influence on comfort because much body cooling comes from the evaporation of sweat. If water cannot pass through these materials, they are likely to be uncomfortable to wear.
And wearability will be an important in itself. These materials will need to be soft to the touch and comfortable to wear, factors that are hard to gauge from a study like this. Then there is durability and washability—important factors in many purchase decisions.
Finally, there is the question of privacy. Infrared cameras are becoming increasingly cheap and common and anyone wearing these fabrics will appear more or less naked to these cameras. Tong and co make no mention of this in their paper but it is likely to be an important consideration in a world where embarrassing images can spread around the world in the blink of an eye.
One solution might be to blend the material with other more common ones but again the impact of this in infrared transmittance needs to be carefully determined. Clearly more work is needed on all these fronts.
Nevertheless, this is an interesting take on an important problem that could have significant impact on energy usage in places where air-conditioning is widespread.
Ref: arxiv.org/abs/1507.04269 : Infrared-Transparent Visible-Opaque Fabrics for Wearable Personal Thermal Management
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