High temperature stability of transparent silica aerogels for solar thermal applications

Solar thermal energy systems combined with low-cost thermal storage provide a sustainable, dispatchable source of renewable energy. One approach to increase the attractiveness of these systems is to use high-performing solar transparent, thermally insulating silica aerogel to significantly increase efficiency. Several past works have proposed using these ultra-nanoporous materials to reduce thermal losses in the receiver, but only recently have aerogels reached the high solar transparency necessary to be considered for concentrated solar applications (>97%). However, the durability and stability of optically transparent silica aerogels at the operating conditions of solar-thermal receivers has not been examined. Here, we investigate the high temperature stability of transparent silica aerogel for use in concentrated solar thermal energy applications. Transparent samples (visible transmission >95% at 4 mm thickness) were annealed for several months at 400, 600, and 800 °C to investigate the relative change in nanostructure, solar transparency, and effective thermal conductivity. Results showed that at 400 and 600 °C, the temperature-dependent changes reach a plateau within 30 days of continuous annealing, but at 800 °C, samples are structurally unstable. A simple receiver efficiency model was used to show stable performance at 400 and 600 °C temperatures, even after months of exposure. This work validates that transparent silica aerogels can be used in solar thermal receivers below 800 °C, yielding appreciable increases in efficiency for solar energy harvesting operation.