Vacuum glazing panels are already being sold in Japan in sizes of up to 2.4 × 1.35 m, under the trade name Spacia. Now they are set to break into the UK market.
They differ from conventional double glazing because the air between the glass panes is replaced with a vacuum. This produces the same thermal insulation as standard double-glazed units but at a fraction of the thickness.
The vacuum glazing system consists of two sheets of glass separated by an evacuated space that can be as small as a one-tenth of a millimetre; this means that the whole double-glazed unit can be as thin as 6-8 mm.
In a standard double-glazed unit, a gap this small would conduct heat but here “the glazing works in the same way as a vacuum flask – the vacuum eliminates the heat flow between the two glass plates,” says the product’s inventor, professor Richard Collins of the University of Sydney.
Collins says the system will affect refurbishment most. He thinks that with units this thin, it will be possible to replace an ordinary piece of single glazing with a vacuum-glazed unit without having to change the window frame.
The units are claimed to have higher levels of heat insulation than cost-comparable double glazing. “Vacuum glazing can achieve levels of thermal insulation of less than 1 W/m2°C at a cost not too different from that of high quality double glazing,” says Collins.
Even without low-emittance coatings on the glass, heat conductance through the units is similar to, or less than, some uncoated models of traditional double glazing (see table opposite). With emittance coatings its performance is even often better.
All the initial design work was carried out by Professor Collins and his team at Sydney University. In 1994, Nippon Sheet Glass saw the concept’s potential and entered into a licensing agreement with the university. The two teams then collaborated in developing the pilot production plant in Kyoto.
In production, air is sucked out of the glazing unit through a small tube in the corner of one pane of glass. This is melted and closed at the end of the process.
The challenge for the team developing the glazing was to come up with a solution that would keep the two sheets of glass apart. As the air is sucked from between the panes, the air pressure outside the glass pushes the panes together. If the panes were to touch heat would flow through the two sheets of glass as though they were a single sheet.
The solution is regularly spaced metal or ceramic support pillars that hold the two panes of glass apart. The use of pillars strikes a balance between stressing the glass and allowing heat to flow through it. The pillars have to be strong enough to maintain the gap, but they also conduct heat across the vacuum, so their diameter has to be kept to a minimum. Keeping the pillars small also means they are practically invisible.
In fact, the pillars can have a diameter as small as 0.25 mm – making them all but imperceptible. They are typically spaced 20 to 25 mm apart. By preventing the glass bowing inwards, the pillars also stop any stresses forming near the edges of the glass.
The design of the seal around the edge of the unit is also critical, if the vacuum is to be maintained. The team had to find a material that expanded and contracted at exactly the same rate as the soda lime glass. Solder glass fitted the bill exactly, and being a type of glass, it will not deteriorate like the seals on some double-glazed units.
One of the problems with using glass to seal the unit is that, like the pillars, it bridges the gap between the two sheets of glass. This means that far more heat passes through the panel’s edge than through its centre.
“It probably limits the useful application of vacuum glazing to windows 0.5 × 0.5 m,” says Collins, “although the heat flow can be reduced by recessing the edge seal into a thermally insulating window frame.” Another problem is that, because the glass panes are so close together, reflected light waves from both faces combine to form what are known as interference fringes under fluorescent light. But, according to Collins, “they are quite difficult to see, and do not constitute a significant difficulty”.
The team is now developing a hybrid high-performance glazed unit for countries with harsh climates. This unit will be similar to a conventional air-filled double-glazed unit, but with a vacuum unit replacing the single outer pane of glass.
Currently, the units are only manufactured in six production plants run by Nippon Sheet Glazing. However, the company plans to launch another three plants in the near future, and is holding discussions to extend licensing agreements to other countries. So, keep an eye out for a panel coming to a building near you …
