Improving Skylight Geometry for Daytime Passive Radiative Cooling

Åbo Akademi University (ÅAU) is researching a passive radiative cooling (PRC) skylight window prototype utilizing greenhouse gases (GHGs) that interact strongly with thermal radiation. The first prototype achieved 100 W/m2 passive cooling using two ZnS windows, one at the bottom and one at the top, both transparent to long-wave (LW) infrared, and a central window. The aim of this ongoing work is to improve the skylight design by utilizing computational fluid dynamics (CFD) software (Ansys Fluent). The objective of this design improvement is to eliminate the usage of central window used in the earlier design. In this improved design, sections of ZnS glass are positioned symmetrically, at the top and at the bottom. The remaining window is composed of conventional window glass, while the side walls are made of wood. Another objective entails using various greenhouse gases, such as CO2 and NH3, inside the skylight and subsequently calculating the transmittive radiative fluxes within the atmospheric window (8–14 μm) wavelength range, followed by a comparative analysis with using air. Thus far, the radiative heat fluxes achieved with the new skylight design are as follows: 85.5 W/m2 when CO2 is used as the participating medium, 83.0 W/m2 with air, and 88.5 W/m2 when NH3 is used. Additionally, temperatures of the ZnS Cleartran glasses give a calculated lowering of approximately 3 to 4 ℃ in comparison to the ambient temperature. The ultimate aim is to develop a transparent PRC skylight with a net cooling capacity >> 100 W/m2 without moving parts also during daytime.