Though winter is approaching for folks living in the northern hemisphere, it's starting to warm up for those south of the equator. As stuffy offices get even more stuffy in rising temperatures, aircon units will very likely be switched on to make working a mite more comfortable. But aircons use a lot of power. Engineers have created a see-through coating for windows that's reported to reflect up to 70 percent of heat coming in from the sun.

MIT says that air conditioners account for some 6 percent of all electricity produced in the US, costing billions of dollars. The team estimates that its heat-rejecting film could reduce a building's aircon (and energy) costs by as much as 10 percent if all exterior-facing windows were coated with the film.

Thanks to microparticles embedded within, the film remains pretty much see-through up to temperatures of 32° C (89° F), but anything above that will result in the phase-changing material shrinking to give the film a frosted glass look, limiting the amount of heat allowed through.

Work on the project began last year when MIT's Nicholas Fang started collaborating with researchers from the University of Hong Kong on ways to reduce energy consumption of buildings, particularly during hot and balmy summer months.

"It turns out that for every square meter, about 500 watts of energy in the form of heat are brought in by sunlight through a window," said Fang.

The research team looked at different phase-changing materials as a way to stem incoming heat and settled on a material made from poly (N-isopropylacrylamide)-2-Aminoethylmethacrylate hydrochloride microparticles, which was modified to more effectively reject heat. When temperatures reach 85° F (29.4° C) or more, teeny water-filled spheres shrink, effectively squeezing out the liquid to form tight bundles of fibers. In this state, the film turns translucent rather than transparent.

To test the heat-rejecting properties of the film, 12 x 12 inch sheets of glass were coated with a microparticle solution and when light from a solar simulator was shone through, the film changed its state to give a frosty appearance. The researchers measured the heat coming through the other side and found that the film "was able to reject 70 percent of the heat produced by the lamp."

In another experiment, a small calorimetric chamber was coated with the film. Without the film, temperatures within the chamber can get to 102° F, but that reduced to 93° F with the coating applied.

The research team is looking at adjusting the formula to see if improvements can be gained, as well finding out if applying the coating differently could increase its efficiency.

A paper detailing this research has been published in the journal Joule.

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