Evacuation of the glazing can be performed either during assembly in a vacuum chamber or afterwards through stubs in the glass pane or rim seal. The diffusion coefficient of monolithic silica aerogel is in the range of 10-5 — 10-6 m2/s [8], [9] and it is the governing parameter with respect to evacuation time. Therefore the evacuation should take place at least from one surface of the aerogel in which case only the aerogel thickness determines the evacuation time and not the actual glazing area.

a) Foil wrapped around aerogel edge

Polystyrene

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Butyl

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From the thermal analyses use of laminated plastic foils developed for vacuum insulation purposes seems as the most suitable solution for the rim seal in aerogel glazing. The big challenge was how to make an airtight connection between the foil and the glass panes. From a thermal point of view, the total rim seal thickness should be as thin as possible which can be obtained by the option shown in Fig. 3a, where the foil is wrapped around the aerogel edges. A butyl sealant is applied between the glass panes and the foil before evacuation of the aerogel glazing. During evacuation the atmospheric pressure will press the glass panes against the aerogel and the butyl sealant making a firm and airtight joint between the foil and the glass panes. The principle is in this way "self tightening”. This principle was developed in a previous European project [12].

The drawback is that the glazing will not be flat due to the additional thickness along the glazing perimeter, which is an aesthetical problem.

Furthermore, the bending of the glass panes results in tensile stresses at the glass edges and tempered glass is required to avoid breakage. This makes the aerogel glazing considerably more expensive. Also the process of wrapping the foil around the aerogel edges is difficult due to the fragility of the aerogel.

In the HILIT project [4] the drawbacks have been overcome by development of a principle, where the foil is folded around rods of polystyrene as shown in Fig. 3b. The height of the polystyrene is a few millimetres lower than the aerogel thickness making room for the butyl sealant. The polystyrene spacer is required as support for the foil and has compression strength large enough to ensure the necessary compression of the butyl sealant when the aerogel glazing is evacuated. By proper choice of the polystyrene dimension a flat glazing is achieved. Furthermore the handling of the foil and application of the butyl sealants can be done without touching the aerogel edges. The drawbacks are an increased thermal bridge effect of the rim seal solution and a more difficult corner solution with enhanced risk of leakages.

Prototypes

The investigations and developments described in the previous sections have been implemented in the laboratory in a process for making prototypes of aerogel glazing. The prototypes were primarily made for testing of thermal and optical properties and for testing the assembly process at a pre-industrial scale.

The core element is the Aerogel Glazing Evacuation Apparatus (AGEA) developed in a national Danish project [10]. The AGEA is a vacuum chamber that makes it possible to evacuate and assemble the aerogel glazing in few minutes as the evacuation takes place mainly from the top aerogel surface.

The rim seal is made as the method shown in Fig. 3b with polystyrene rods wrapped in the Mylar® 250 RSBL300 foil [6]. The process is as follows:

• The heat treated aerogel (T = 425 °C) is placed on the lower glass pane

• A butyl sealant strip is applied to one side of the polystyrene rods with foil, which are placed along the aerogel edges with the butyl facing the lower glass and pressed slightly in position.

• The corners are joined with butyl sealant and a butyl sealant strip is applied on top of the polystyrene rods.

• The top glass pane is centred in the vacuum chamber and small self-adhesive metal disk are placed on the top glass pane opposite to electro magnets in the vacuum chamber lid. The lid is closed and the magnets are activated in which way the upper glass is fixed to the lid in the right position.

• The vacuum chamber lid is opened and the lower glazing with aerogel and rim solution is placed in the vacuum chamber.

• The vacuum chamber lid is closed and the evacuation started. The evacuation is continued until a pressure of approximately 1 hPa has been maintained for 5 minutes. Total evacuation time is approximately 30 minutes.

• The upper glass pane is lowered and pressed firmly against the aerogel and rim seal solution to make an airtight connection between glass panes and rim seal.

• The chamber is gently vented and the atmospheric pressure further compresses the glazing securing the complete compression of the butyl sealing between the foil and the glass panes.

Upper glass fixed to lid of vacuum chamber by means of electromagnets Figure 4. An aerogel glazing sample before evacuation and assembly as well as after.

Lower glass + aerogel + rim seal in vacuum chamber

Final aerogel glazing.

Results

Several prototypes have been made during the HILIT project [4] based on aerogel sheets made by Airglass AB following the optimised aerogel elaboration process developed

during the project on the basis of the previously patented route [11]. The aerogel thickness was 15 mm + 1.

The centre U-values of the optimised glazing prototypes have been measured by means of a hot plate apparatus. The average centre U-value is found to be 0.66 W/m2K + 0.03, which with the average aerogel thickness of 14.8 mm correspond to an estimated thermal conductivity of 0.010 W/mK.

This indirect determined thermal conductivity is in accordance with the measured material properties at a pressure level of 1-10 hPa.

Figure 5. Four optimised aerogel prototypes mounted in a test frame for guarded hotbox measurements.

higher.

Four of the optimised prototypes have been used for a test window (Figure 5) measuring 1.2 by 1.2 m2 designed for hotbox measurements of the overall U — value. The well-insulated framing system is made only for fixation of the four glazings and will not withstand exposure to real climate for longer periods. The measured overall U-value of the glazing is deduced from the measurements by subtracting the heat loss through the framing system. The result of an average total U-value of 0.72 W/m2K + 0.04 compared to the average centre value of 0.66 W/m2K confirms the very small thermal bridge effect of the developed rim seal solution.

The average direct solar energy transmittance for the four glazings was measured to 73% + 2. The total solar energy transmittance, the g-value, has not been measured but would be a few %-points

Application

For a new single family house in a Danish climate the annual energy savings amounts to about 2300 kWh/a (-16%) if conventional argon-filled triple glazing, (U-value = 0.5 W/m2K, total solar energy transmittance = 0.4) is replaced with aerogel glazing (U-value = 0.5 W/m2K, total solar energy transmittance = 0.75). For a low-energy house the savings are reduced to 1600 kWh/a, but in this case it corresponds to 25% of the annual heating demand. A high solar transmittance may result in high indoor temperatures during summertime even in colder climates and solar shading and enhanced venting may be needed.

However, the optical quality of aerogel glazing is not at the same level as conventional glazing units especially not if exposed to non-perpendicular direct radiation where some diffusion of the radiation in the aerogel occurs and makes the outlook hazy. But the optical quality has been improved considerably thanks to the research carried out as part of the European projects [4], [5], [12] to a level where almost no disturbance in the view through is present if shielded against direct radiation. This makes aerogel glazing an excellent option for improved daylight utilization combined with a fair outlook by placing large areas of aerogel glazing in north facades. Due to the very good insulation properties and the high solar and daylight transmittance this can be done without energy loss or even with energy gain (Figure 1), which cannot be obtained with any other known glazing or daylight component options. Furthermore, the daylight will be at a more constant as well as pleasant level during the daytime compared to a south orientation and the excess temperature problems will be reduced considerably.

So the application of aerogel glazings in new buildings will offer the possibility of increase the north facing glazing area and decrease the south facing one. Hereby, the capital cost for overheating prevention, e. g. shading devices, air conditioning, enhanced venting, etc., can be greatly reduced.

Despite the promising results already achieved, the research is still focusing on further improvement of the optical quality through detailed studies of the aerogel process and a post heat treatment aiming at an optical quality comparable to ordinary glass.

Conclusions

Within the present European projects HILIT/HILIT+ transparent and insulating plane monolithic silica aerogel tiles are elaborated at large-scale on the basis of a previously patented synthesis route.

Evacuated aerogel glazings of approximately 55 by 55 cm2 have been made — the size dictated by the production plant dimensions at Airglass AB.

A rim seal has been developed with the required barrier properties against atmospheric air and water vapour to ensure a theoretical lifetime of the glazing of about 30 years and with a limited thermal bridge effect. The rim seal is dimensioned so a completely flat glazing is obtained making it possible to use non-tempered glass.

The final assembling and evacuation takes place in a vacuum chamber. The evacuation time is approximately 30 minutes resulting in a final pressure in the aerogel of 5 hPa.

The solar and daylight transmittance of the aerogel glazing is optimised by means of low — iron glass covers with an anti reflection coating. The optical quality has reached a level with minimal disturbance in the view through except if exposed to direct non-perpendicular radiation where diffusion of the light becomes significant.

The measured centre U-value is 0.66 W/m2K. Including the thermal losses in rim seal an overall U-value for the 55 by 55 cm2 glazing is found to 0.72 W/m2K deduced from the hotbox measurements on a window with 4 aerogel glazings joined in an interim frame.

The direct solar transmittance is measured at laboratory conditions to more than 75%, making the aerogel glazings developed in this project superior to other highly insulating glazings on the market with respect to energetic performance in northern European or equivalent climates. The total solar energy transmittance, the g-value, has not been measured but will be 1 — 2 %-point higher than the direct solar transmittance.

Within the frame of the present HILIT+ project, current studies are aiming at optimising further the elaboration process by decreasing duration of supercritical drying of the aerogel panes.

Acknowledgements

This work was funded in part by the European Commission. The authors would like to thank the participants in the two projects on which this work is based:

P. Achard, A. Rigacci & Y. Masmoudi, Ecole des Mines de Paris, France; L. Gullberg & G. Petermann, Airglass AB, Sweden; M. Ryden, Air Liquide Gas AB, Sweden; B. Chevalier, CSTB, France; P. Nitz & W. Platzer, Fraunhofer ISE, Germany; B. Sunden, LTH, Sweden;

M.-A. Einarsrud, E. Nilsen & R. A. Strom, NTNU, Norway; M. Durant, D. Valette & P.-A Bonnardel, PCAS, France; S. Bauthier & G. M. Pajonk, Universite Claude Bernard Lyon 1, France.

The AGEA has been developed and build with support from the Danish Energy Agency.