Creating a vacuum between panes of glass seals windows against heat and cold more effectively. Up to now, the best insulation properties are delivered by heavy insulating glass units with triple or even quadruple glazing. The skalVIG research project, which is coordinated by the Fraunhofer Institute for Mechanics of Materials IWM, aims to demonstrate that excellent insulation properties can also be achieved with less material – thanks to a vacuum between just two panes of glass.
Windows have to meet a tough set of requirements. In winter, they should let sunshine in to heat the building, but not in summer. In winter, they should keep heat inside the building – but in summer, they should keep it out. These requirements are currently best met by triple or quadruple glazing.
“However, thick multiple glazing becomes a problem when windows are not permanently sealed, but instead need to be opened and closed as needed. In that case, both the wall and the window fittings have to bear a heavy weight. This means that, particularly when renovating old buildings, windows with good insulation properties are often too heavy,” explains Tobias Rist, Group Manager of the Glass Forming and Machining group at the Fraunhofer IWM. Rist thinks less is more: while expensive noble gases such as argon are currently used for insulation between panes of glass, he is investigating vacuum gaps between the panes as a solution.
Vacuum-insulated glass (VIG) is less than 1 cm thick – around a quarter of the thickness of triple-glazed insulated glass units. The skalVIG project now wants to bring this technology to market maturity by developing highly scalable, environmentally friendly production processes and demonstrating the viability of VIG technology.
Scientists to build on previous VIG research
The idea of creating a vacuum between panes of glass is not new: “We have already acquired considerable technical expertise through previous projects here at our institute,” explains Rist. One of the challenges the researchers faced was the atmospheric pressure on the glass panes, which amounts to approximately 10 tonnes per square metre.
During the precursor project VIG-S, which ran from 2013 to 2015, the researchers used tiny metal cylinders as spacers in the gap – a solution that worked well. However, there is one last problem: “The challenge is to maintain the vacuum permanently,” says Rist. A prototype from the VIG-S project demonstrated that this is possible.
“We now want to make the process steps more robust, so that they can serve as a model for industrial manufacturing,” he explains. For this reason, a number of industry partners are participating in the current project, with skalVIG aiming to cover the entire process chain: LILA (“Laser Integration / Laser Applikation”) will focus on welding technology, TS Elino on facilities for the soldering process, and RJ Lasertechnik on innovative welding methods under vacuum conditions. Meanwhile, HEGLA will develop solutions for automated spacer placement and contribute glass handling expertise, while TMP Fenster + Türen will produce demonstration objects.
Complex edge seal
To seal the edge of double panes, researchers previously used glass solder. While the solder is effective, the disadvantage is that it becomes rigid. Large temperature differences between the two sides of a window cause mechanical stress, which could lead to cracks. This would allow air to enter the gap, which would severely limit the insulation effect.
Comparing insulating glass
To address this difficulty, the project partners intend to manufacture a flexible edge seal using a metal ribbon. This is possible at comparatively low temperatures of less than 250 °C, which ensures that the stress in the glass panes does not decrease under high temperatures during manufacturing. If the project succeeds, this would be a major leap in the field of insulation.
A window’s insulation capabilities are measured by the “U-value”. According to the German Energy Saving Regulation (EnEV) introduced in 2014, windows can have a maximum U-value of 1.3 W/m²K. VIG could achieve U-values of 0.4 W/m²K – a figure that is currently only met by quadruple-glazed windows filled with noble gas. These are more than 5 cm thick and weigh around 40 kg per square metre.
VIG, on the other hand, would only weigh half as much. Rist is therefore also focusing his attention on older buildings whose structure is not designed for thick, heavy windows. Such windows would also be environmentally friendly as they require less material and are easier to transport. Work on the skalVIG project began in July, and is scheduled to run for three years.