Cooling with colour: Passive-Coloured Radiative Coolers for energy-efficient temperature regulation in adverse climatic conditions

Abstract

Passive Daytime Radiative Cooling (PDRC) is a high-performance strategy to mitigate urban overheating by combining high solar reflectance and strong thermal emission, particularly within the Atmospheric Window wavelength range. However, several intrinsic challenges, such as glare, aesthetics, and winter overcooling, limit its widespread application. This study reports on the development and cooling performance of Passive Coloured Radiative Coolers (PCRCs) with a threefold heat-rejection mechanism: moderately high solar reflectance, high infrared emissivity, and sunlight-excited fluorescence. The objective was to create PCRCs with reduced reflectivity to diminish glare and aesthetic concerns, while the incorporation of fluorescence offsets the cooling decrease caused by lower reflectance. The development of PCRCs sought consistent performance throughout the year, reducing the winter heating penalty. Seven PCRCs–Green, Red, Orange, Reddish-orange, and Purple–were developed and tested in two climate zones with unfavourable conditions for radiative cooling: Sydney and Alice Springs, Australia, characterised by high humidity and dust concentrations, respectively. The developed PCRCs consistently maintained lower surface temperatures than their coloured non-fluorescent counterparts and the highly reflective white references. All PCRCs–except purple–outperformed the white reference, maintaining surface temperatures up to 5.4 °C lower in Sydney and 4.0 °C lower in Alice Springs. These findings highlight the potential of PCRCs to reduce urban surface temperatures and cooling energy demand and underline their role in advancing sustainable urban design. By addressing PDRCs’ limitations, PCRCs could facilitate the adoption of radiative cooling technologies in urban environments, supporting energy policy objectives and promoting resilient urban planning strategies aimed at combating climate change and urban overheating.