Reflective coatings on windows block the hot infrared rays from sunshine so offices don't heat up so much.
Alas, a hot-and-cold climate thwarts the advantage of reflective glass. Although reflective window coatings do their job in summer, they also keep the sun's warmth out in winter. In other words, although you pay less for air conditioning, you'll pay more for heat.
Ivan Parkin, a professor of inorganic chemistry at the University College London in the United Kingdom, has found a smarter solution: intelligent glass. He has created glass that reflects heat (actually infrared light) when the weather is hot but lets heat pass through when there's a chill in the air. Applied to a modern glass-faced office building, the coated glass could cut air-conditioning bills by 25 percent to 50 percent, Parkin estimates, without a wintertime penalty.
The trick to making glass smarter was to coat it with vanadium dioxide, a material with a remarkable property: At low temperatures, it acts like a metal. At high temperatures, it becomes a semiconductor.
"What happens is that it undergoes a phase change," said Parkin. "At higher temperatures the electrons that are bound together go into conduction bands, and the material becomes more reflective in the infrared." In other words, when the material becomes a semiconductor, it reflects radiant heat.
"Theoretically the maximum reflectivity you can get is 50 percent. In our studies so far we get about 30 percent," said Parkin. That's enough to pare energy bills.
This property of vanadium dioxide is well known, so naturally it was the first coating material Parkin considered. "It's the only material that can be used realistically," he noted.
Unfortunately, the remarkable switching ability of vanadium dioxide has a drawback that previously has made it unsuitable for windows. The switch between states occurs at a high temperature, about 70 degrees Celsius. That's nearly 160 degrees Fahrenheit. Said Parkin, "People would be roasted by then."
Parkin reasoned that he could alter the switch temperature of vanadium dioxide by doping it, adding a trace amount of another material. When he tried tungsten, he got the desired effect -- a substantially reduced switching temperature. Better still, the switching temperature was easy to control by adjusting the amount of tungsten used.
"We have established a linear relationship between the added tungsten and the temperature at which the material switches states," Parkin said. "With 1 percent tungsten, it switches at 50 degrees Celsius; with 2 percent, at 30 degrees; with 3 percent, at 10 degrees. We can reduce it down to zero."
Inventing something by itself is not enough. You've got to be able to make it practically and affordably, and Parkin scores on both counts.
The coating can be applied using a standard industrial process called Atmospheric Pressure Chemical Vapor Deposition, or APCVD.
"It is directly compatible with glass manufacture," Parkin said. "Although it is not commercially available at the moment, it could be made."
Moreover, the APCVD technology he used for adding the coating is proven and inexpensive, so intelligent glass would impose little additional price penalty over other reflective glasses.
"It should be very cheap on an industrial scale," Parkin said. "I would envisage it would add about 20 percent to the cost of the glass. But you would soon get your money back in air-conditioning costs."
The vanadium dioxide coating does have one other drawback. Simply put, it's not pretty.
"Its yellow color is not aesthetically pleasing," Parkin said. "But it may be possible to change the color by adding other doping. Or you could add a color suppression layer."
Parkin admits that the coating is not yet ready for prime time. So far, he's produced only laboratory samples measuring about 2 by 6 inches.
Industrial plants cast glass in sheets that may be 10 to 12 feet wide, so he must scale up his process. The University College London has not yet offered the intelligent glass technology for commercialization, although patents are being prepared. Actual windows made from the vanadium dioxide glass are still years off.