Recent Advances in Fluorescence-Based Colored Passive Daytime Radiative Cooling for Heat Mitigation

Abstract

Passive daytime radiative coolers (PDRCs) with exceptionally high solar reflectance and emissivity in the atmospheric window can provide sub-ambient cooling while reducing buildings’ cooling energy demand. However, glare and esthetic issues limit their application to high-rise buildings while may increase the building’s heating energy needs. Passive colored radiative coolers (PCRCs), based on fluorescent materials, convert part of the absorbed UV and visible solar radiation into emitted light, providing color and reducing the thermal balance of the materials and the potential visual annoyance. This article investigates the state of the art on the PCRC based on fluorescent technologies. Seven articles presenting different combinations of PDRC technologies with fluorescent components to create PCRCs of various colors are presented and analyzed in detail. Quantum dots and phosphors embedded in polymer matrices and combined with reflecting and emitting layers were used as the fluorescent layer of the seven developed green, red, yellow, and yellow–green films. The proposed PCRCs are characterized by very significant differences in cooling performance, although most presented sub-ambient surface temperatures. Their cooling potential is comparatively investigated in terms of the testing climatic conditions and their optical characteristics. The potential increase of their surface temperature, caused by the addition of the fluorescent component, is analyzed through comparisons between the proposed PCRCs and the corresponding white PDRCs without the fluorescent component. The average temperature difference of the green, red, yellow, and yellow–green films against the reference PDRCs is found to be 0.66 °C, 2.6 °C, 1.7 °C and 1.4 °C, respectively. A relevant decreasing trend, but not statistically significant, is observed between the temperature increase caused by the fluorescent additives and the corresponding photoluminescence quantum yield.