Outdoor daytime passive radiative cooling by transparent skylights in a Nordic climate

Rising cooling demands motivate passive alternatives to air conditioning. This study evaluates a transparent skylight window for daytime passive radiative cooling (PRC) under outdoor summer conditions in Turku, Finland. A heated box held a two-sided skylight window (0.72 m × 0.225 m × 0.08 m). The skylight consists of standard window glass (SWG) and circular Cleartran® ZnS inserts (CG). The inserts (Ø 0.10 m) were positioned diagonally in the lower window facing indoors and in the upper window facing the sky. The upper SWG carried a 3 M solar-control thin film (TF, ∼65% solar reflectance). The sky facing CG surfaces were covered with dispersed TiO ₂–SiO ₂ nanoparticles (NPs, ≈30% areal coverage). The skylight gas volume contained air, CO ₂, or air/NH ₃ (2:1). A pyrgeometer provided local weather data as long-wave (LW) irradiance (L↓). Overall performance is reported as the sky-directed net LW radiative heat flux, split into Q _CG (CG insert region, with and without NPs) and Q _SWG+TF (SWG region covered by TF). The SWG + TF surface is effectively opaque in the LW range (4–14 µm). On a clear summer day (14:00–20:00), Q _CG increases 38.5 → 54.0 W/m ² for air (+40.3% with NPs), 45.8 → 63.0 W/m ² for CO ₂ (+37.6%), and 63.5 → 66.0 W/m ² for air/NH ₃ (+3.6%). NPs increase thermal radiation via CG. Because NH ₃ strongly participates in the 8–14 µm atmospheric window, the additional benefit from NPs is smaller, even though the total radiative flux is highest.