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“Smart windows” will adjust to the weather outside

Photonics Spectra
Dec 2011
Marie Freebody, Contributing Editor, marie.freebody@photonics.com

Home or office windows soon could be made to darken to shield us from a scorching summer sun, then made to switch back to transparent in winter to take advantage of light and heat available from the sun on those shorter days.

A team of Korean scientists has developed a smart window that exhibits a record optical-switching behavior of >90 percent – which means that the window can change from transparent to opaque and back again.

Such windows could save billions of dollars in heating, cooling and lighting costs and could be used in a wide range of applications, including roofs, vehicles and architectural windows.

“Smart windows can prevent the inside of a building from becoming overheated by reflecting away a large fraction of the incident sunlight in summer,” said Ho Sun Lim, a senior researcher in the Electronic Materials and Device Research Center at Korea Electronics Technology Institute in Gyeonggi-do. “Alternatively, they can help keep a room warm by absorbing the sun’s heat in winter. This technology may provide a new platform for efficiently conserving on energy usage in the interior of buildings.”


A new smart-window approach could allow switching of transmittance from opaque (left) to transparent (right), corresponding to a reversible ion-exchange mechanism in a polymer that responds to a chemical solution. Images courtesy of Ho Sun Lim, Korea Electronics Technology Institute.

Unlike the photochromic lenses often found in light-reactive spectacles – which absorb incident light with specific wavelengths – the smart window system scatters light through the formation of microporous structures. This results in extremely tunable transparency, which can convey either high transmittance (transparency around 90.9 percent) or complete blockage (transparency around 0 percent) of incident light.

Interest in the energy-saving potential of smart windows is nothing new and includes research carried out on electrochromic windows, which can vary in transmittance by about 40 percent in response to an applied voltage.

But Lim and colleagues wanted to focus on developing windows with a much higher switchable transmittance, something involving the application of specific chemicals.

They achieved this using a reversible ion-exchange mechanism in a polymer that responds to a chemical solution. A window based on polyelectrolytes is immersed in solutions containing thiocyanate (SCN) ions or bis(trifluoromethane)sulfonimide (TFSI) ions, respectively.


This schematic representation shows the reversible optical switching of the smart window as a result of ion exchange.

“Since polyelectrolytes coordinated with SCN ions fully swell in methanol, smart windows become highly transparent,” Lim said. “But when replaced with TFSI ions, the glass lost its optical transparency due to the immense scattering of incident light of micron-sized surface structures.”

And because the ion exchange is reversible, the optical switching property also is reversible.

Although the new smart-window system has many advantages – unprecedented optical switching behavior – and the use of polymers offers an alternative that is cheaper to manufacture than what is now available on the market – it also has some major technical hurdles to overcome. The drawback to controlling its optical transparency by direct exchange of chemical solutions means that a practical window in the home or office is still a long way off – but not impossible, Lim said.

Currently, the size of the smart window depends on the coating techniques of polymer solutions. If the polymers are coated uniformly on a large enough area, the application could be used in a smart-window system with the optical-switching behavior triggered by spraying the window with a chemical solution.

The group recognizes the limitations, however, and is developing smart windows that not only respond to temperature or electric fields, but that also have optical switching properties of >90 percent.


GLOSSARY
transparency
An image affixed to a transparent photographic film or plate by photographic, printing or chemical methods. It may be viewed by transmitted light.
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