TY - BOOK
T1 - Development and Investigation of Evacuated Windows Based on
Monolithic Silica Xerogel Spacers
AU - Schultz, Jørgen Munthe
AU - Svendsen, Sv Aa Højgaard
A2 - Jensen, Karsten Ingerslev
PY - 1996
Y1 - 1996
N2 - 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.
AB - 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.
M3 - Book
BT - Development and Investigation of Evacuated Windows Based on
Monolithic Silica Xerogel Spacers
PB - European Commission
CY - Brussels
ER -