Coloured Radiative Cooling Materials in the Built Environment Parallel the Cooling Benefits of White Conventional Surfaces and Balanced Winter Performance

Urban surfaces absorb heat, raising ambient temperature and contributing to urban overheating, greenhouse gas emissions, environmental pollution, energy hikes, and heat-related health issues. Passive Daytime Radiative Coolers (PDRC), available in white or silver colours, are characterised by very high solar reflectance and emissivity in the atmospheric window and offer sub-ambient cooling. Nevertheless, optical annoyance and aesthetic issues limit their application to high-rise buildings. Winter overcooling induced by the highly reflected PDRC application is another concern, mainly in heating-dominated climates. To address these issues, an alternative approach dubbed fluorescent-based Passive Coloured Radiative Coolers (PCRC) offers an additional fluorescence-based cooling mechanism. By absorbing solar radiation and then reemitting light, they provide colour, avoiding absorbing pigments and reducing thermal balance and visual discomfort. Three fluorescent- based prototype PCRCs in orange, green and red colours were developed using quantum dots and fluorescent dyes and tested under the hot desert climatic conditions of Alice Springs (Australia) in the autumn and the humid climatic conditions of Sydney (Australia) in the winter to assess their year-round thermal performance. Overall, the best cooling performance was observed for the orange-fluorescent prototype PCRC. Under dry-hot conditions in Alice Springs, this prototype achieved the same surface temperature as highly reflective corresponding PDRCs and 2.7 degrees C higher cooling than the conventional white cooling roofing membrane during the daytime. During the night, all PCRCs were 7-8 degrees C below the ambient temperature. Under the humid-cold Sydney's winter conditions, the orange-fluorescent film yielded improved insulation, roughly 4.5 degrees C higher than the white reference. Under cold conditions, higher PCRC surface temperatures were attributed to the lower IR transmittance of polymers in which fluorescent dyes or quantum dots were embedded. These results are encouraging, as implementing such PCRCs may support achieving a balanced performance during both summer and winter and expanding the use of radiative coolers in urban environments by addressing optical annoyance and aesthetic issues.