.

Fig. 34: VIG exhibits at glasstec. The support pillars in the inter-pane cavity are only perceptible close up and do not hinder visibility.
© Glaser

Fig. 35a: Schematic structure of vacuum-insulated glass: The standard assembly consists of two 3- to 4-mm-thick float glass panes, whereby one pane is coated with a heat reflecting layer (low-ε coating). The cavity between the panes is less than 1 mm.
© ZAE Bayern

Fig. 35b: Classification of vacuum glazing in the current window market (based on various manufacturer specifications)
© BINE Informationsdienst
14 / 17

Vacuum-insulated glass

In thermal terms, glazing still forms the weak point in buildings. The heat transfer coefficient of highly insulating triple-glazing systems of between 0.5 and 0.7 W/(m²K) is still five times higher than for opaque facades designed to the passive house standard. Insulation glass with a vacuum in the inter-pane cavity would not only provide better insulation but would also be slimmer and lighter. Researchers have already developed prototype glazing systems and subjected them to detailed testing. Work is now being carried out to make the products and production technology ready for mass production.

A vacuum in the inter-pane cavity instead of inert gases – this idea could help glazing systems make a considerable leap forward in terms of their development. Whereas the transition from double-glazing to triple-glazing improves the insulation by having a greater system thickness – more helps more –, vacuum-insulated glass (VIG) provides a qualitative improvement: the elimination of gaseous thermal conduction in the inter-pane cavity. This enables Ug values of 0.5 W/(m²K) to be also achieved with double-glazed assemblies with system thicknesses less than 10 mm. Vacuum glazing would therefore not only be considerably slimmer and lighter, itwould also provide two to three times more insulation than conventional insulation glass. To realise this, the gas pressure in the interpane cavity has to be evacuated to a level less than 10-3 mbar, i.e. a millionth of the atmospheric pressure.

Technical challenges

In order to produce vacuum glazing that is competitive in Europe, the researchers had to develop a sufficiently airtight, thermally stable edge seal as well as visually, thermally and mechanically suitable support pillars that can absorb the atmospheric pressure on the inter-pane cavity.

  • Edge seal

Not only must the edge seal remain airtight throughout the window’s service life, it must also absorb any changes in size caused by the considerable temperature deviations between the outer and inner pane that occur as a result of the excellent thermal insulation. Although glass is in itself well suited, it is too rigid, which would cause considerable mechanical stresses even with low temperature differences. The developers were able to solve this problem, however, by using thin metal foil. Its elasticity balances out the temperature-related stresses. It is soldered to the glass pane and then welded in a vacuum chamber on both sides so that it is gas-tight. This assembly achieves Ug values of 0.5 W/(m²K).

  • Support pillars

To prevent the pressure of the ambient atmosphere from pressing the panes together, they have to be kept apart by using tiny support pillars in a regular grid. The size of the support pillars, the spacing between them and the thermal conductivity of the materials used influence the overall thermal loss from the vacuum glass. Whereas small support pillarswith compact surface areas are better in terms of the appearance and thermal performance of the vacuum glass, the opposite is the case as far as the mechanical performance is concerned. Stainless steel cylinders positioned in a roughly 30 x 30mm grid have been determined to be the best compromise. With a 0.5 mm diameter, the support pillars are only perceptible when seen against a low-contrast background and when less than one metre away. A special treatment enables the cylinder surfaces to reflect the incident solar radiation in a diffuse manner, thus preventing glare.

Vacuum glazing for all cases

Windows, facades or roofing for new and old buildings – these will be the main application areas for VIG, but its use in solar collectors, vehicles and refrigerators is also conceivable. In addition to standard designs made of float glass, it is also possible to produce safety glazing using toughened or laminated safety glass as well as thermal insulation and solar control glazing. Although VIG is comparable to conventional double-glazed thermal insulation glass units in terms of the energy transmittance and light transmission, its simultaneous excellent thermal insulation enables higher solar gain.

Still being researched

Gas-tight sample panes have already been produced at a laboratory scale and their mechanical stability has been confirmed. The work is now focussing on developing the joint and production processes for mass-producing VIG systems. The production in a vacuum chamber eliminates the need for evacuation pieces that still disrupt visibility with the Asian glazing systems. A demonstration plant for testing the individual process stages is now in operation. New testing procedures have been developed to confirm that the maximum resilience and durability of the VIG test panes are comparable with conventional glazing systems. With large-scale production technology, VIG glazing is expected to cost around 100 euros per square metre in the long term, i.e. no more than the price for conventional triple glazing. The fully developed production technology is planned to be available by the end of 2012.

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