Thermal and Solar-Optical Properties of Silica Aerogel for Use in Insulated Windows

Silica aerogel is a porous insulating material that is transparent to solar radiation. To understand its insulating performance in a window system, it is necessary to first study component heat transfer paths. Aerogels absorption coefficient, a measure of the attenuation of radiation heat transfer, was determined over the spectral range 1-200 µm. Although radiation heat transfer is negligible over much of this region, there is a transmission window between 3-6 pm. At ambient temperatures, for aerogel thicknesses of 0.5-5.0 cm, radiation heat transfer through an unmodified aerogel window is less than 15% of the total heat flux. For evacuated or high-temperature furnace windows, this contribution can be over 50%. Thermal radiative transfer can be somewhat decreased by allowing the aerogel to absorb moisture, but solar transmission and optical clarity are sacrificed. Absorption of water vapor over time causes irreversible structural changes that increase scattering in the solar spectrum. Aerogels thermal performance can be improved by replacing the pore gas with one of lower conductivity or by evacuating the aerogel to pressures below 0.1 atm. A hypothetical evacuated aerogel window has a calculated UValue of ≈ 0.5 W/m²-K for a gap spacing of 12.5 mm, which is four times better than currently available low-emissivity gas-filled units of similar size.