A new “smart” window system has the unprecedented ability to 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 are reporting.
The Korean study appears in the journal ACS Nano.
Authors Ho Sun Lim, Jeong Ho Cho, Jooyong Kim and Chang Hwan Lee point out that the drive for energy conservation has fostered efforts to develop new types of window glass for everything from skylights and windows in houses to conference room walls in offices.
“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, so-called “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.”
The report adds: “One of the most important issues of the 21st century is the looming fact that the world will be faced with a serious scarcity of energy.
“To address the fast-approaching energy crisis, the past decade has seen a large number of research efforts that have struggled to develop renewable energy sources from natural resources, such as sunlight, wind, rain, tides, biomass, and geothermal heat.
“Moreover, current interest has been concentrated on ways to effectively use our finite energy resources by improving energy efficiency, operating at lower rates of energy consumption, storing energy, and conserving energy.
“Recently, in this context, smart windows, which can tailor light transmittance in response to environmental circumstance, are particularly intriguing as a potential alternative to economizing on energy.
“They can lead to practical applications, such as house roofs, skylights, architectural or vehicle windows, and interior partitions. Since smart windows can appropriately modulate heat transfer from the transmitted sunlight in a house, they can moderately suppress unnecessary energy usage through air conditioning or heating.
“For example, smart windows can prevent the inside of a building from becoming overheated by reflecting away a large fraction of the incident sunlight in summer.
“Alternatively, they can help keep a room warm by absorbing the sun’s heat in winter. In principal, optical switching of smart windows has been achieved to control light propagation or absorption either by aligning liquid crystals, by dispersing suspended particles, or by stimulating the oxidation–reduction transduction of chromophores in response to light irradiation, electric charge, or temperature changes.
“However, until now, the numerous technologies developed not only have been chemically unstable, prohibiting their use in long-term switching applications but have been accompanied by the use of expensive special equipment and complicated harsh processing conditions.”
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