Aerogel: future of glazing?
Note:
units are presented in International Units (SI), non-SI units are
presented in italicized blue between
brackets.
1 ft²·°F h/Btu ≈
0.1761 K m²/W,
or 1 K m²/W ≈ 5.6745 ft²·°F
h/Btu
A paper published in the
journal of Solar
Energy Materials & Solar Cells (volume 89, pages 275-285)
describes the results of a thermal analysis of monolithic silica
aerogel based glazing. Current window manufacturing industry has provided
us with single-glazed or multi-glazed windows with either high or low
insulation-to-optical transmittance ratios. So if a highly insulated
window is desired, one could opt for a triple-glazing, low-e window
with an R-value between 0.88 and 1.0 K°m2/W
[5 – 6 F°ft²h
/Btu]. Such choice would result in a window
having visible transmittance (VT) reduced to 50% and a solar heat
gain coefficient (SHGC) of 0.40. Conversely, if high solar heat gain
is desired, a window with an R-value of 0.17 K°m2/W
[less than 1 F°ft²h
/Btu] would be needed providing a VT of 75%
and a SHGC of 0.76. These options are not well suited for poleward
climates where both a high SHGC and high R-value are
desired. Aerogel windows may finally help overcome this limitation.
Silica
aerogel was first "extracted" in the 1930s at the College
of the Pacific (California, USA) from hydrogel by removing the water
component by use of alcohol substitution. The developer of the
aerogel, Steven S. Kistler, was later hired by Monsanto Corporation
where the aerogel was produced and marketed. Aerogel did not benefit
from a huge market until the 80's where modified versions of the
product evolved in Europe then later in the USA. One of the niches
being carved out by aerogels is in the fenestration industry. Silica
aerogel is a very low-density silica-based solid (2-3 mg/cm3)
composed of 99.8% air. It has high compressive strength, very low
thermal conductivity (0.01 W/mK°),
and under recent manufacturing techniques transmits 76% of the
visible light (for a 15 mm thick slab of aerogel).
In
the journal of Solar Energy Materials & Solar Cells, J.M. Schultz
et al. test the thermal characteristics of a window built
around a layer of aerogel 15 mm ( 0.59 inches) thick. Because of
aerogel's poor tensile strength (i.e. it is brittle and will readily
fragment when bent), the authors of the study sandwich the sheet of
aerogel with 2 thin layers of low-iron glass glazes. The low-iron
content of the glass optimizes the daylight/solar transmittance of
the window assembly. The air is evacuated from the assembly down to a
pressure of 5 hPa, and the rim is sealed with a laminated plastic
foil (Mylar 250 RSBL300) and butyl sealant providing low thermal heat
transfer across the rim of the window assembly. R-value at the center
of the window was measured to be 1.51 K°m2/W
[8.57 F°ft²h
/Btu] and 1.388 K°m2/W
[7.88 ft² F°ft²h
/Btu] for a 120x120 cm window assembly with 4 aerogel
glazing (55x55 cm each) joined in a frame whose material is not defined in
the article. The authors state that such windows installed on a
typical Danish home (i.e. one meeting modern energy efficient
standards) would provide energy savings of 16% over similar homes
using argon-filled triple glazed windows.
This
is promising technology with many positive potential for the PACCS
community. Future manufacturing technology should help improve solar
heat gain (higher VT) allowing for thicker aerogel glazing, hence
higher R-values. Could aerogel someday be incorporated into SIP systems
where plywood would be substituted with a transparent plastic allowing for
translucent walls?
At the time of this writing, it's not known when
such windows will become readily available to the general public...
hopefully soon ;-)
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A simulation comparing the thermal response between a triple-glazed and
an aerogel equipped structure follows. The
structure is modeled with low internal reflectance (i.e. high
solar radiation absorption) and highly insulated walls and roof.
Typical Mean Year ((TMY2) meteorological data for Portland, Maine (USA) is used.
Internal temperatures for the structure using triple-glazed
windows (blue line), aerogel windows (red line) and external temperatures
(green line) are plotted for 2 dates: January 14th (a
clear day) and February 1st (a partly cloudy day).
Y-axis represents temperatures in degrees Fahrenheit and the
x-axis represents military time. The structure has no internal
source of heat, solar radiation is the sole source of heat.
Disclaimer: these
simulations are provided for educational purposes only. The results presented have
not been validated.
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Model used for simulation. All three openings are glazed and facing the equator.
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Air temperature
profiles within and outside the structure using TMY2 meteorological data for Portland, Maine (US)
for January 14. Y-axis represents temperature in °F. X-axis
represents military time in hours.
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These examples show that for a clear sunny day with an outside
air temperature below 0 °F for
most of the day, the aerogel windows increase the internal
temperature of the structure by an additional 10 degrees over the
triple-glazed windows equipped structure at solar noon.
When the sky is partly cloudy, the solar heat gain is a bit more modest as expected
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Air temperature
profiles within and outside the structure using TMY2 meteorological data for Portland, Maine (US)
for February 1. Y-axis represents temperature in °F. X-axis represents military time in hours.
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References:
Last Modified on Monday, October 05, 2009
