Vacuum Insulating Glass – Past, Present and Prognosis

ld of the manufacturing process, as normally occurs in the early stages of any new production technology.

Some changes were major, and were a direct consequence of the earlier limited understanding of important technical issues. At one stage, the entire post-assembly part of the production process was redesigned and rebuilt, reducing the number of heating steps in the process from two to one (Figure 4).

Figure 4 Single step manufacturing process
Figure 4 Single step manufacturing process

This became possible through the development of a method for making a high temperature vacuum seal around the pump out tube using a demountable all-metal evacuation cup ([12] and Figure 5). Although very costly and time consuming, these changes greatly improved the viability of the technology by halving the post-assembly manufacturing time and significantly simplifying the manufacturing process.

Figure 5 All-metal evacuation cup
Figure 5 All-metal evacuation cup

A feature of this commercial development was the effectiveness of the interactions between NSG and the University. Both parties in the collaboration were completely open to each other about the information that was generated and the issues that needed to be addressed. The University’s research program continued to provide new information of relevance to the product development at NSG.

Much of the University’s work was stimulated by issues that emerged from the production process. For its part, NSG was unshakable in its resolve to make the project succeed, even when significant technical challenges arose that required major changes in production processes.

In hindsight, the decision by NSG to commercialise a VIG product must be regarded as very courageous. NSG committed to this goal at a time when many other companies took the view that commercial development of the VIG concept was too risky and costly.

As it turned out, the costs involved were much larger than originally anticipated, and the technological challenges were also much greater. Despite concerns about the viability on this enterprise, NSG chose to work these issues through, and over time this technology has become one of the company’s premium products.

The demonstration of the technical and commercial feasibility of VIG stimulated several other organisations to commence work on VIG, leading to many publications and patents.

 

2001 to present

The past 16 years have been a period of consolidation of VIG technology. Several million VIG units have been manufactured by NSG, and have shown excellent reliability in many types of building, Several major research studies have been undertaken at other academic institutions, in government laboratories and by other companies. VIG products made by other manufacturers are in the market or under development. There has been extensive relevant publication and patenting.

Current commercially available VIGs using annealed glass can have centre-of-glazing Uvalues as low as 0.6 W m-2 K-1 in a structure 10 mm thick. VIGs are also being used in hybrid glazings and laminated assemblies. The International Standards Organisation (ISO) is developing Standards for this technology.

This work is being undertaken by Working Group 10 (Glass in building – Product considerations – Vacuum glass) of ISO Technical Committee 160, Sub-committee 1. Part 1 of a draft Standard for measurement of the thermal insulating properties of VIG is in the process of ratification. Work has commenced on Part 2 relating to temperatureinduced effects in VIG.

 

Future prospects

Although VIG is now a well established technology, there is considerable scope for further development on many aspects of its design, manufacture and marketing. Several of these are listed below. Those currently under active development or consideration are indicated by an asterisk (*)..

 

Design

  • VIG made with tempered glass* offers the prospect of higher levels of thermal insulation, and broader areas of application. A major challenge for this design approach is the development of a relatively low temperature edge seal. Departures from planarity of the tempered glass sheets need to be accommodated so that the mechanical load due to atmospheric pressure is uniformly distributed over the pillars.
  • Performance improvements are also likely with innovative pillar designs, including pillars made from thermally insulating materials*, high strength materials*, and by melting small areas on the glass sheets*. Pillars are under development with low friction bearing surfaces*, and different geometries*, that facilitate a small amount of relative lateral movement of the glass sheets.
  • It is likely that the pillar separation in current VIGs with annealed glass can be increased*, resulting in improved thermal insulation,
  • VIGs with a flexible edge seal* would experience very low stresses and bending under temperature differentials. However, in such designs the pillars must slide repetitively across the glass. Because of the high stresses in the glass sheets near the pillars, the author considers it unlikely that this type of edge seal will prove viable.

 

Production technology

  • There are many alternative possibilities for making the edge seal, including low temperature solder glass *, lead free solder glass*, and metal*. All currently available polymer materials are far too permeable for this application. In the author’s opinion, this is unlikely to change.
  • There are also many different design possibilities for the evacuation port of the device*.
  • VIG production technology is sufficiently well understood that a continuous manufacturing process can now be implemented*. Although relatively capital intensive, such an approach would enable larger numbers of VIGs to be made, and this could result in significant cost reductions in the product.
  • In principle, it is possible to form the edge seal in the VIG within a highly evacuated space, eliminating the need for a pump out port. Challenges in this approach include bubbling of the molten solder glass, vacuum degradation due to outgassing of the hot internal surfaces after sealing, and avoiding bending of the glass sheets in the edge region.

 

Conclusion

The history of VIG is quite unusual for a technological development. There was an extraordinarily long period (75 years) between the initial patent describing the concept and its first realisation in the laboratory. This was followed by a relatively short period (8 years) of research, development and technology transfer, leading to the launch of the first commercial VIG product.

Over the subsequent 20 years, the development of the technology has continued, although most currently available products have much in common with early commercial designs. In addition, although sales of VIG have steadily increased and the product has exhibited high reliability in practical installations, only a few manufacturers are currently active in the field.

NSG is in the process of completing a significant expansion of its VIG manufacturing capability. In addition, there is currently a high level of interest in the technology, both at the research level, and in product development programs by other manufacturers.

It is therefore not unreasonable to expect that the next few years will see more manufacturers entering the VIG market, and substantially increased sales volumes. Should this occur, new and possibly better performing VIG products made using different processes will become available, and the cost of VIG will decrease.

The capital investment for a VIG manufacturing facility will always be greater than for a conventional IG plant of comparable capacity. The unit area cost of VIG is therefore always likely to be greater than for conventional IG. In large volume manufacture, however, materials cost should dominate, and the cost differential between the two technologies need not necessarily be large.

The capability of VIG to achieve high levels of thermal insulation, the high reliability afforded by the hermetic edge seal, and the very small thickness of the structure, are likely to make VIG technology an increasingly attractive choice in the market for high performance thermally insulating glazing.

 

Acknowledgments

VIG would not exist as it is today without the many contributions by the author’s students at the University of Sydney, and his colleagues in the University’s School of Physics, at NSG, and in other research laboratories. The author dedicates this paper to Stephen Robinson, whose student project led to the first practical VIG samples, and Hideo Kawahara, whose vision, dedication and tenacity enabled this technology to be commercialised successfully. Sadly, neither is alive today to reflect on the significance of their achievements.

 

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