Optically Transparent – Thermally Insulating Silica Aerogels for Solar Thermal Insulation

Event Date: March 22, 2018
Authors: A.A. Gunay, H. Kim, N. Nagarajan, M. Lopez, R. Kantharaj, A. Alsaati, A. Marconnet, A. Lenert, and N. Miljkovic
Journal: ACS Applied Materials & Interfaces
Paper URL: Link Full Text
ACS Applied Materials & Interfaces, vol. 10, no. 15, pp. 12603-12611, 2018. DOI: 10.1021/acsami.7b18856

Rooftop solar thermal collectors have the potential to meet residential heating demands if deployed efficiently at low solar irradiance (i.e., 1 sun). The efficiency of solar thermal collectors depends on their ability to absorb incoming solar energy and minimize thermal losses. Most techniques utilize a vacuum gap between the solar absorber and the surroundings to eliminate conduction and convection losses, in combination with surface coatings to minimize re-radiation losses. Here, we present an alternative approach that operates at atmospheric pressure with simple, black, absorbing surfaces. Due to their high transmission to solar radiation and low transmission to thermal radiation, silica based aerogels coated on black surfaces have the potential to act as simple and inexpensive solar thermal collectors. To demonstrate their heat trapping properties, we fabricated tetramethyl - orthosilicate (TMOS) based silica aerogels. A hydrophilic aerogel with a thickness of 1 cm exhibited a solar-averaged transmission of 76% and thermally-averaged transmission of  ≈1% (at 100 °C). To minimize unwanted solar absorption by O-H groups, we functionalized the aerogel to be hydrophobic, resulting in a solar-averaged transmission of 88%. To provide a deeper understanding of the link between aerogel properties and overall efficiency, we developed a coupled radiative-conductive heat transfer model and used it to predict solar thermal performance. Instantaneous solar thermal efficiencies approaching 55% at 1 sun and 80 were predicted. This study sheds light on the applicability of silica aerogels on black coatings for solar thermal collectors and offers design priorities for next generation solar thermal aerogels.