Development and Investigation of Evacuated Windows Based on Monolithic Silica Xerogel Spacers

The objective of the project is to develop and investigate insulating glazings based on evacuated monolithic silica xerogel spacers. Since the starting date January 1, 1994 the project has been closely connected to the parallel project "Development and Investigation of Evacuated Windows based on Monolithic Silica Aerogel Spacers" contract JOU2-CT92-0192.

Low density monolithic silica xerogel and monolithic silica aerogel are almost identical, but while the aerogel requires a supercritical drying process, xerogels is dried at atmospheric pressure. Ambient pressure drying is possible due to a chemical strengthening of the material prior to drying. This process results in an increased density of xerogels compared to aerogels which leads to a slightly higher thermal conductivity with typical values of 0.030 W/(m K) measured in air at atmospheric pressure.

If evacuated below 50-100 hPa the thermal conductivity will be approximately 0.013 W/(m K) which is approximately 33% of the value for commonly used insulation materials, e.g. mineral wool. Monolithic silica xerogel is a highly porous material (pore volume up to 90%) with a solar transmittance of 50% (thickness = 20 mm).

However, if the silica xerogel is not made hydrophobic it has to be protected against liquid water, that will demolish the pore structure of the material due to the surface tensions. For the application in window glazings the protection against liquid water is formed by placing the xerogel in between two sheets of glass and sealing the rim.

The project has been carried out as a co-operation between institutes and companies in Denmark, France, Germany, Norway and Sweden. The project is divided into three tasks - 1) improvement of the production process and the quality of the material, 2) characterization of the thermal and physical parameters and 3) application for insulating glazings.Scientific developments have made it possible to prepare low density monolithic silica xerogels, only from about 1990, and developments in both the production process as well as size of the samples are necessary for a commercial use of the material.The improvement of the production process has as the main goals to improve the optical quality and the thermal conductivity of the monolithic silica xerogel by decreasing the density.

Secondary an increase of the sample size should be achieved primarily by means of an optimisation of the drying process. Different precursors, solvents and catalysts are used to improve the optical quality.To demonstrate and evaluate the improvements of the material (task 1) it was characterized with respect to the relevant physical properties, especially optical transmission, imaging and modulation transfer as well as thermal conductivity.

Furthermore, properties necessary for the application (task 3) were investigated: Thermal expansion, elastic modulus and long term (inelastic) creep as well as water vapour adsorption and hence condensation risk.The thermal properties make the monolithic silica xerogel a well suited material for insulating glazings.

Using the material as spacer between two layers of glass with a vacuum tight sealing of the rim combined with an internal gas pressure below 50-100 hPa result in an insulating glazing having a heat loss coefficient comparable with that of the surrounding walls, but at the same time offers a large solar heat gain possibility.

However, the rim seal is the crucial point as it has to be airtight and vapour tight, but it may not become a serious thermal bridge that destroys the total performance of the glazing. Development of an airtight and vapour tight rim seal with negligible thermal bridge effect is the main goal in task 3 of the project.

The process developed in the project leads to xerogels with a density of 220 kg/m3 and a sample size of 0.10 × 0.10 m2 having a solar energy transmittance of approximately 50% (20 mm thickness). This result represents the best known monolithic low density xerogels with a reasobale size at the time. Further increase of the sample size is required for use in window glazing systems.

For an increase of sample size further studies of the drying process is required.For the characterization of the material, available measurement techniques were used and even improved. For the optical investigations on imaging and modulation transfer new techniques were developed. The most important results are the reduction in density from approximately 450 kg/m3 to 180 kg/m3 which leads to lower thermal conductivity, and a solar transmittance of 50% that combined with the low thermal conductivity offers good possibilities for production of energy efficient windows.

For the xerogel window system it is necessary to have the xerogel sufficiently dried, if not hydrophobic xerogels are used, because residual water vapour adsorbed in the material will cause condensation at the cold side and intolerable long term creep.A small scale laboratory assembled glazing (0.15 × 0.15 m2, density = 490 kg/m3, thickness = 10 mm) has been made.

The development of a rim seal based on a laminated plastic foil has been carried out in close co-operation with the aerogel project due to shortage of large xerogel samples. Within the xerogel project a thermally improved wooden frame has been developed which has been used for measurements on aerogel windows to evaluate the effect on total heat loss coefficients.

The calculated centre U-value of a xerogel glazing (xerogel density = 250 kg/m3, thickness = 20 mm) is 0.54 W/(m2 K) which result in energy savings of 2220 MJ/year (615 kWh/year) for a typical new Mediterranean single family house and 8980 MJ/year (2500 kWh/year) for a typical new Danish single family house, if the standard glazings are exchanged with xerogel glazings.