Windows can save energy too: Significantly less heat escapes through vacuum-insulated panes than through conventional double-glazed windows. A first prototype will be on show at Glasstec 2006 in Düsseldorf (hall 11, stand F72).
Poorly insulated windows act as thermal bridges: They let the heat out. Double-glazed windows are much better at keeping the heat indoors, but even they allow more than three times more heat to escape than well-insulated walls. Working in a consortium of research institutes and commercial partners, scientists at the Fraunhofer Institute for Silicate Research ISC in Würzburg have now put an end to the heat loss. ISC team leader Bernhard Durschang reveals how it is done: “We have replaced the air between the two panes with a vacuum. That gives the window a thermal conductivity of 0.5 watt per meter per kelvin – a really excellent value.” By comparison, the thermal conductivity of a wall is 0.3, and that of a conventional double-glazed window about 1 watt per meter per kelvin.
It is not easily possible to extract the air from a double-glazed window. Certain precautions must be taken, otherwise the glass panes would move together and form another thermal bridge. To prevent this from happening, the researchers insert spacers at four-centimeter intervals, each one not more than a millimeter thick. Since transparent spacers may produce reflections when backlit, they are given different colors depending where the window is located: lighter colors for dormer windows, darker colors at ground level. A prototype of the vacuum window will be on show for the first time in Hall 11, stand F72, at the Glasstec trade fair in Düsseldorf from October 24 to 28.
The engineers use shatterproof glass to ensure that the vacuum windows remain stable for a long time. Glass panes of this type are heated during the manufacturing process to about 650 degrees Celsius, then rapidly cooled again. Given that insulating coatings are not always able to withstand such high temperatures, the production conditions need to be simulated and optimized. Researchers from the Fraunhofer Institute for Surface Engineering and Thin Films IST in Braunschweig will be at Glasstec to demonstrate the new software they have developed: “The RIG-VM simulation environment allows us for the first time to simulate optical spectra simultaneously with X-ray reflection,” says Bernd Szyszka, head of the large-area coating department at the IST. “This combination gives us precise information about reflective properties and coating thicknesses.” The researchers were thus able to produce glass coatings that can withstand temperatures up to 650 degrees Celsius without damage.