Abstract

Vacuum Glazing is a new generation of thermally insulating window. The design of vacuum glazing is based on the principle of operation of the Dewar flask and is similar to a double glazing unit with an evacuated volume. Vacuum glazing consists of two glass sheets hermetically sealed together around the edges. The glass sheets are separated by a very narrow vacuum space and small supporting materials. High levels of thermal insulation are achieved by evacuating the space to a very low pressure. This low pressure greatly reduces the conduction and convection of gas within the space. Therefore, heat transfer through vacuum glazing is significantly lower as compared with double glazing unit with inert gas. In the development of vacuum glazing, many designs and techniques have been investigated in order to produce vacuum glazing with high insulating properties. Various quality control techniques have been developed to monitor the performance of vacuum glazing with regard to thermal conductance and internal pressure.

Introduction

In buildings, windows are relatively less insulating than the other building structures such as walls. Therefore, they play an important role in energy consumption. The insulation of windows is one of the crucial factors in the design of energy efficient buildings.

The insulating performance of window can be improved by using double or multiple glazing which involve two or more glass sheets separated by air filled spaces. The heat transport between the glass sheets is affected by radiation between the glass surfaces, and conduction and convection within the space. Conduction can be reduced either by filling the cavities with inert gases such as argon, or by increasing the separation of the glass sheets. However, increasing the space, which is about 10 mm wide for a typical double glazing, also increases the convection. If there are no gas molecules, conduction and convection can be eliminated. Therefore, it is possible to achieve high levels of insulation in a double glazing by evacuating the gas from the space between the two glass sheets. This leads to the concept of vacuum glazing. There are obvious advantages from the development of vacuum glazing. High levels of insulation could be achieved from the combination of vacuum and low emittance coatings. As the insulating property of an evacuated space is independent of its space width, good insulation should be achievable in a structure with a very narrow vacuum space. Consequently, the thickness of a vacuum glazing could be reduced to almost the same as that of two glass sheets. Table 1 shows the typical thermal conductance of different window designs. It shows that the conductance of vacuum glazing is about 7 times lower than that of a single glass sheet. The performance of triple glazing with a thickness of 15 mm is about the same as vacuum glazing. However, the thickness of vacuum glazing is only 6 mm which makes this device an ideal candidate for retrofit market.

Scientists, engineers and developers have spent many years trying to design a window that could utilize the advantages of the high insulation value of a vacuum and the small thickness of vacuum glazing. However, they encountered a lot of challenges and problems in the construction of the device.

Development of vacuum glazing

Since vacuum glazing requires an evacuated space between the glass sheets, it is essential to make a leak free seal around the edges of the two glass sheets. The high permeability of all polymers and plastics makes them unsuitable for this purpose. Secondly, in order to reduce radiative heat flow between the glass sheets, it is necessary to develop low emittance coatings which are vacuum compatible and able to withstand high temperature outgassing process. Thirdly, the internal pressure of the device must be sufficiently low (< 10-3 Torr) for gas conduction to be negligible. It is also important to develop supporting material which can prevent the touching of the glass sheets under the influence of atmospheric pressure.

In 1989, the vacuum glazing group of the University of Sydney [1] made the first report of the successful production of a vacuum glazing. It consists of two flat sheets of glass which are sealed together hermetically around the edges and separated by a narrow space. Due to atmospheric pressure, the glass sheets are kept apart by small supporting materials. Figure 1 shows the schematic diagram of a sample of vacuum glazing. The glazing is made from two sheets of soda-lime glass, typically 3 - 4 mm thick. The edge seal around the glazing periphery is made from solder glass [2]. The sample is pumped through a small pumpout tube. Due to the narrow gap (~0.2 mm) between the glass sheets and the small dimension of the pumpout tube, it seems that pumping might take an unacceptably long time in terms of commercial production. However, our results indicated that the time required for evacuation of vacuum glazing is determined by outgassing from the glass surfaces, rather than by pumpdown considerations [3]. After the high temperature outgassing and pumping process, the sample is sealed by melting the end of the pumpout tube.

Quality control of vacuum glazing

The success of vacuum glazing depends on its insulation performance and the vacuum stability. Extensive research has been performed related to these aspects. The performance of vacuum glazing is affected by conductance of the device, which is governed by pillar conduction, radiation and gas conductance. Its vacuum stability could be affected by leaks at the edge seal or the pumpout tube, or by the release of gases from the internal glass surfaces. Outgassing could also be increased by exposing the glazing to high temperatures.

After a vacuum glazing is made, it is important to determine whether a vacuum exists within the glazing. In addition, it is essential to demonstrate that the internal pressure of a vacuum glazing remains low (<10-3 Torr) over a long period of time. Therefore, techniques were developed to evaluate the performance of vacuum glazing and its stability.

The guarded hot plate apparatus have been developed for measuring local heat flow through vacuum glazing [4]. It provides very accurate local measurements of heat flow through vacuum glazing. This device has been used to validate theoretical models for the different heat flow processes through the glazing due to radiation, gas conduction, and thermal conduction through the pillars. The guarded hot plate apparatus has also proven useful to demonstrate that the internal vacuum in the glazing is stable over long periods of time.

A method of measuring the conductance of vacuum glazing when the sample is at high temperatures was developed - transient technique [5]. The method is very rapid, easy to apply and gives highly reproducible results. This method has proved to be useful in the study of outgassing processes in vacuum glazing, and has found extensive use as a quality assurance tool during the manufacture of these devices.

The stability of vacuum glazing was studied in two areas: thermal and optical. The thermal stability of the device was investigated by using accelerated ageing experiments in which samples were held at elevated temperatures. The results indicated that outgassing of vacuum glazing at high temperatures involves diffusion of water molecules within the bulk of the glass, and adsorption / desorption of gas molecules on the internal glass surfaces [6]. Since the main application of vacuum glazing is in window, we have studied the outgassing of the device when it is exposed to sunlight. Optical ageing experiments were performed to study the change in pressure of the sample when it was stored outdoors.

The study showed that optical outgassing processes are different from that of thermal outgassing and the gas molecules released are not water, but carbon dioxide.

Conclusion

Samples of vacuum glazing with a low internal pressure have been successfully developed by the research group at the University of Sydney. They showed negligible gaseous conduction. The thickness of the vacuum glazing is almost as thick as two pieces of glass because of the 0.2 mm evacuated gap. Quality control of the vacuum glazing has been performed on a regularly basis by using guarded hot plate apparatus and transient technique. When samples are made by a high temperature process, there is no change in the pressure of vacuum glazing for up to a decade.

References

[1] S. J. Robinson and R. E. Collins, ISES Solar

World Congress, International Solar Energy

Society, Kobe, Japan (1989).

[2] F. Rosebury, Handbook of Electron Tube and

Vacuum Techniques, Addison-Wesley, Reading,

MA (1965).

[3] N. Ng and R. E. Collins, Evacuation and

outgassing of vacuum glazing, J. of Vacuum

Science and Technology A, 18, 2549 (2000)

[4] C.J. Dey, T.M. Simko, R.E. Collins and Q-C.

Zhang. Rev. Sci. Instr. 69, 2939 (1998).

[5] G.M. Turner and R.E. Collins. Int. J. Heat Mass

Transfer 40, 1437 (1997).

[6] M. Lenzen and R. E. Collins, J. Vac. Sci.

Technol. A 17, 1002 (1999).