A new “smart” window system that can inexpensively change from summer to winter modes, darkening to save air conditioning costs on scorching days and returning to crystal clarity in the winter to capture free heat from the sun, scientists from Soongsil University, Seoul report.

“Smart” windows that reflect sunlight away from buildings in summer and switch back to full transparency in winter already are available. But they have many drawbacks, including high cost, rapid deterioration in performance, and manufacturing processes that involve potentially toxic substances. So, the researchers set out to develop a smart window that overcomes these drawbacks.

They discovered that using a polymer, “counterions,” and a solvent such as methanol was an inexpensive and less harsh way to make a stable, robust smart window. It has the added advantage of being extremely tunable — quickly and easily switching from 100% opaque to almost completely clear in seconds.

“To our knowledge, such extreme optical switching behavior is unprecedented among established smart windows,” the authors state. “This type of light control system may provide a new option for saving on heating, cooling and lighting costs through managing the light transmitted into the interior of a house.”

Ref.: Chang Hwan Lee, et al., Counterion-Induced Reversibly Switchable Transparency in Smart Windows, ACS Nano, 2011; [DOI: 10.1021/nn202328y]