Potential passive cooling methods based on radiation controls in buildings

Buildings inevitably absorb solar (thermal) radiation through the envelope, i.e., window, roof and wall, whose characteristics, in terms of material, thickness, area etc., affects the performance of space cooling, thereby inducing energy wastage by air-conditioning system to maintain satisfied indoor thermal comfort. Metropolises with congested-built architectures and humid climates consume tremendous energy in space cooling, contributing to massive carbon emissions. Space cooling has been the most soaring electricity end-user and its increasing carbon emission amplifies global warming, which in turn increases occupants’ dependence on artificial cooling, especially in hot climates. The urgency of the climate crisis has put passive cooling technologies, which can efficiently manage heat transfer of buildings, to the forefront of research. Thermochromic smart windows, daytime radiative coolers and reflective paints are three prominent technologies that have drawn industries’ attention. These technologies respond to incident sunlight and thermal radiation differently, compared to conventional building envelope, and can passively mitigate solar radiation absorbed at the building envelope, reducing electricity consumption for air-conditioning. This review article comprehensively discusses the characteristics of the three passive energy-efficient techniques that can be integrated with building façade (window, roof and wall), and their feasibility in building cooling applications through thermal analyses. Through this review, the ideal spectral properties of these technologies will be distinguished. Although the properties of these technologies have been abundantly explored, the research on their long-term cooling performance under the influence of weatherability and aging remains scarce. Further, their cost-effectiveness is essential for realizing commercialization and reducing hesitancy in adopting these novel passive cooling technologies for taking a significant leap towards carbon neutrality.