The HILIT AEROGEL WINDOW project with participants from Denmark (coordinator), France, Germany, Norway and Sweden, was formulated in order to develop a safe and clean production of monolithic silica aerogel based on supercritical CO2 drying of the gels, to study the process parameters and to transfer the results from lab- to mid- and finally to large-scale making of 60 by 60 cm2 in a pre-industrial plant. The large samples forms the basis for assembly of evacuated aerogel glazings optimised with respect to thermal and optical properties.
The production process development and transfer to pre-industrial fabrication of aerogels has succeeded in all details. A pilot plant for precursor elaboration has been established and precursors of required amount and quality has been delivered to all partners. Studies at lab-scale have identified the important parameters for optimising the mixing of the chemicals, which is the mixing rate and the HF (catalyst) flow rate. A mixing reactor have been designed and successfully transferred to large-scale application. A wet gel strengthening process has been developed and optimised at laboratory-scale and transferred at mid-scale with success (concerning monolithicity). A direct supercritical CO2 drying loop has been developed at mid-scale, successfully transferred and re-adapted at large-scale. At large scale a complete CO2 loop has been build including CO2 regaining by separation of CO2 from the solvent. The CO2 has been reused for several batch runs. The up scaling required invention of several technical solutions related to moulding and handling of the large gels. Despite the efforts only aerogels with a thickness up to 15 mm have been produced with a good reproducibility. The thermal conductivity is approximately 0.015 W/mK at atmospheric pressure and 0.010 W/mK at 10 hPa. The optical properties have been improved compared to previous aerogels thanks to the process and the smooth surfaces obtained and a heat treatment of the dried aerogel.
A rim seal solution that offers the required air and moisture tightness without leading to severe thermal bridge effects has been developed as well as an assembly process including heat treatment and evacuation in a vacuum chamber. The centre U-value is measured for several prototype glazings to 0.68 W/m2K, which is somewhat higher than the targeted value of 0.4 W/m2K due to the thinner aerogel sheets available. The overall U-value including the thermal bridge effect of the rim seal solution is measured to 0.74 W/m2K. The solar energy transmittance is measured to 76% thanks to the use of low-iron glass with an anti reflective coating.