Waterborne coatings with sub-ambient cooling under direct sunlight–part I: Optical properties and cooling effect measurements

Abstract The conventional way to fabricate current sub-ambient daytime radiative cooling (SDRC) systems is to maximally suppress the cooler's solar absorption and substantially improve its selective emissivity only in the atmospheric transparency window. This greatly limits the selection of materials and their practical applications. In this paper, an alternative method, which combines sunlight-triggered fluorescent cooling, particle scattering, and broadband infrared radiation, was presented for creating serial eco-friendly coatings with the SDRC effect with conventional cost-effective building materials. The presence of conventional fluorescent pigments in the coating composition increased the effective direct-normal solar reflectance via sunlight-excited fluorescent emissions to the level required for SDRC. When applied to various substrates, the serial coatings exhibited distinct SDRC effects under direct and strong sunlight in typical temperate monsoon climate regions such as Beijing. The observed sub-ambient temperature reduction of the coating with yellow–green fluorescence over an aluminium plate was 3.5 °C under a peak solar intensity of 996 W/m2, yielding a cooling power of 45.7 W/m2 under a peak solar intensity of 981 W/m2. Compared with the conventional cool white coatings, the large-scale application of the coating to urban surfaces is predicted to reduce a city's air temperature by approximately 1 °C and thereby decrease the Heat Island Intensity. Additionally, the method proposed in this paper was generic and expanded the SDRC design paradigm, thereby promoting SDRC as a viable energy-efficient technology.