Smart windows block heat not light
Coated glass reflects infrared rays on hot days.
Chemists know how to be cool. A team of researchers has created a window coating that reflects the Sun's heat without filtering out visible light.
They say that the glass could be a huge money saver. As architects use more and more glass in buildings, our offices are becoming greenhouses and air-conditioning bills are soaring. But conventional window tints are often unpopular because they block out both heat and light.
At lower temperatures, the new coating allows through both visible and infrared rays, just like normal glass. But once it heats up above 29°C, its atomic structure changes and it begins to reflect heat. This makes it more versatile than other coatings, which reflect heat all the time.
"It will take considerably less energy to keep a room cool," says Troy Manning, the chemist who invented the coating with Ivan Parkin at University College London.
Their veneer is made from vanadium dioxide mixed with just 1.9% tungsten metal. Scientists already knew that when they added other metals to vanadium dioxide it began to reflect heat above about 70°C.
The research identifies just the right blend of chemicals so that the material starts reflecting heat at exactly 29°C. "Nobody's been able to do that before, and we use really simple off-the-shelf chemicals," says Manning, who is now based at Liverpool University, UK. Their research is published in the latest edition of Journal of Materials Chemistry1.
Glass tends to heat up relatively easily, adds Manning, so the chemical transition that triggers the heat-reflecting properties might happen when the ambient temperature is around 25°C, which would be just right for a balmy summer's day. But in winter the glass will not reach its transition temperature, allowing all the Sun's heat to flood through when it is most needed.
The chemists coated their windows by heating a sheet of glass to about 550°C. Then they made a steamy vapour of vanadium oxytrichloride and tungsten hexachloride, both common laboratory chemicals. As the molecules passed over the glass, they reacted together to form a thin film of vanadium dioxide that contained just a hint of tungsten.
University of Liverpool, UK
Unlike many other coating methods, the process works at atmospheric pressure, which should make commercialization of the process much easier, he adds.
The process is so flexible that they have been able to make the transition temperature go all the way down to 5°C, so it may be possible to make different glasses for different climates, each set to a different temperature.
It might take five years before the new windows make it from laboratory to production line, says Simon Hurst, a research technologist at the glass manufacturing business Pilkington in Ormskirk, UK. "But we're very interested in the potential of glass films like this," he says, "and a smart coating is obviously better than a passive one that reflects heat all the time."
The only stumbling block is that the coating is coloured yellow. The team hope to add other chemicals to the film that will tone down the colour, perhaps to a misty grey, says Manning. But he hopes that there may be a niche market already waiting for their glass: "I've been told that a lot of Japanese architects like to use coloured windows," he says.
- Manning T. D. & Parkin I. P. J. Mater. Chem. ,14. Article (2004).
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