Sustainable building design increasingly relies on passive solar thermal systems to reduce the carbon footprint of space heating. Among these technologies, building-integrated solar air collectors mounted on facades or roofs offer a cost-effective solution for fresh air preheating. However, the performance of natural convection collectors is inherently limited by the delicate balance between buoyancy-driven flow and heat loss through the glazing. This study presents a detailed three-dimensional numerical investigation into the thermal behavior of a passive solar air collector, comparing conventional glass covers with monolithic silica aerogel as a transparent insulation material.
The numerical model was developed within the Ansys Fluent environment, focusing on the thermosiphon effect, where the flow is entirely driven by buoyancy forces rather than external mechanical power. To accurately capture the physics of natural convection, the Boussinesq approximation or ideal gas density variations were employed in conjunction with the gravitational vector. A multi-band radiation modeling approach was implemented to decouple the incoming short-wave solar radiation from the long-wave thermal radiation emitted by the absorber. This allowed for a realistic assessment of the "greenhouse effect" and the radiative shielding provided by the aerogel layer in a vertical or inclined mounting configuration.
The findings reveal that monolithic aerogel acts as a transformative component for passive systems. In natural convection collectors, flow velocities are typically low, which usually leads to high stagnation temperatures and increased top heat losses in glass-covered units. The integration of aerogel, with its exceptionally low thermal conductivity and high infrared absorption, effectively suppresses these losses. Numerical results indicate that the aerogel-integrated collector maintains a significantly higher thermal equilibrium, leading to improved absorber surface temperatures and enhanced air delivery temperatures into the building interior. While the porous structure of the aerogel introduces a minor penalty in solar transmittance, the resultant enhancement in the stack effect—driven by higher internal temperatures—compensates for this by maintaining a stable and warm airflow. This research demonstrates that aerogel-based glazing is a superior alternative for the next generation of building-integrated passive solar systems, where thermal retention is the primary driver of overall seasonal efficiency.
