CFD Study on Selection of Low GWP Participating Gas for a Passive Cooling Skylight

Passive cooling and air-conditioning methods are being developed for both night-time and daytime cooling of buildings. A passive cooling skylight under development at Åbo Akademi demonstrated a night-time passive cooling effect of ~ 100 W/m ². This depends strongly on the gas used inside the skylight, picking up (long wavelength, LW) thermal radiation via a lower window and after a natural convection transfer inside the skylight, releasing the heat to the sky via an upper window. Proof-of-concept work utilised air, carbon dioxide, ammonia and, for best results, pentafluoro ethane, HFC-125. The 2016 Kigali Amendment to the 1986 Montreal Protocol on HFCs necessitates using a low-global warming potential (GWP) alternative for HFC-125: future refrigeration installations cannot contain a GWP > 150 gas under European regulation. The key gas property is high emissivity/absorption in the LW range 8–14 µm, the “atmospheric window”, thus HFC-152a or HFC-41 could replace HFC-125. CFD simulations (Ansys Fluent 2024 R1) were used to calculate the passive cooling heat fluxes, temperatures, convection flow fields, and transported heat inside the skylight, comparing gases. Results show that HFC-152a (117.8 W/m ²) and slightly less so HFC-41 (115.4 W/m ²), both with a GWP < 150, can match the performance achieved earlier with HFC-125 (117.3 W/m ²).