Categories
Materials Energy

322: Smart textile, Radiative heat transfer

Problem:

Many textiles are made for social etiquette and aesthetic purposes, but the pressing threat of global warming has created demand for innovative textiles that help to better cool the person who wears them.

Solution:

Stanford engineers have developed a low-cost, plastic-based textile that, if woven into clothing, could cool the body far more efficiently than is possible with the natural or synthetic fabrics in clothes worn today – and without air conditioning. If you can cool the person rather than the building where they work or live, that will save energy.

Photon-to-cooling phenomenon relies on the atmospheric transparency window to dissipate heat from the earth into outer space, which is an energy-saving cooling technique.

The emissivity of aluminized Polymethylpentene (PMP) thin films as selected by the Stanford team matches well to the atmospheric transparency window so as to minimize parasitic heat losses.
This new material works by allowing the body to discharge heat in two ways that would make the wearer feel nearly 4° F cooler than if they wore cotton clothing.

The material cools by letting perspiration evaporate through the material, something ordinary fabrics already do. But the Stanford material provides a second, revolutionary cooling mechanism: allowing heat that the body emits as infrared radiation to pass through the plastic textile.

First, they found a variant of polyethylene commonly used in battery making that has a specific nanostructure that is opaque to visible light yet is transparent to infrared radiation, which could let body heat escape. This provided a base material that was opaque to visible light for the sake of modesty but thermally transparent for purposes of energy efficiency.

They then modified the industrial polyethylene by treating it with benign chemicals to enable water vapor molecules to evaporate through nanopores in the plastic, said postdoctoral scholar and team member Po-Chun Hsu, allowing the plastic to breathe like a natural fiber.

To test the cooling potential of their three-ply construct versus a cotton fabric of comparable thickness, they placed a small swatch of each material on a surface that was as warm as bare skin and measured how much heat each material trapped.

The comparison showed that the cotton fabric made the skin surface 3.6 F warmer than their cooling textile. The researchers said this difference means that a person dressed in their new material might feel less inclined to turn on a fan or air conditioner.

The researchers are continuing their work on several fronts, including adding more colours, textures and cloth-like characteristics to their material. Adapting a material already mass produced for the battery industry could make it easier to create products.

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