A review of clear sky radiative cooling developments and applications in renewable power systems and passive building cooling

Although nocturnal radiative cooling has been known for centuries, providing sub-ambient radiative cooling during daytime was a challenge until recent years. Recent advances in nano-fabrication technologies, have made it possible to manufacture structures with tailored radiative properties for various energy applications like daytime clear sky radiative cooling. It has been shown that photonic and plasmonic selective emitters can be tuned efficiently to emit heat through clear sky to the outer space and cool terrestrial objects providing passive cooling. There is a renewed interest in clear sky radiative cooling among researchers. Providing continuous day and night sub-ambient cooling and dissipation of low grade heat from renewable power systems without use of water or external energy under direct sunlight and other applications have made clear sky radiative cooling a hot research topic. This paper reviews relevant publications on clear-sky radiative cooling methods. An overview of radiative cooling fundamentals and a detailed literature survey of published studies on selective emitter structures for daytime and nighttime cooling purposes is presented. Furthermore, a detailed energy analysis is performed identifying key performance indicators and evaluating the cooling performance under various conditions. Findings from studies that have used empirical equations for numerical energy analysis and selective emitter structure designs for daytime and nighttime applications are summarized in tables for easy comparison.