Progress in passive daytime radiative cooling: A review from optical mechanism, performance test, and application

By exploiting the 3 K coldness of outer space as heat sink of terrestrial thermal radiation, passive daytime radiative cooling (PDRC) can achieve sub-ambient temperatures without any energy consumption, and thus exhibiting extraordinary application potentials. By pursuing the dual-band (solar and atmosphere window) optical properties to approach ideal 100 %, PDRC can maximize dissipating long wavelength infrared radiation to space and minimize absorbing sunlight simultaneously. PDRC technology can reach146 W/m2 theoretical cooling power and up to 120 W/m2 during application. This article focuses on the fundamental physics mechanism of radiative transfer and natural radiative cooling phenomena. Methodologies of infrared absorption functional group selection, optical band gap selection for solar reflection, photon and phonon enhanced resonance by micro-structure were discussed in detail to give a comprehensive instructing strategy of radiative cooling power improvement. Although PDRC technology can achieve sum-ambient temperature cooling without consuming any energy, its drawbacks such as single function of cooling and high whiteness requirement needed to hinder its large-scale application. This article also outlines the current primary applications of PDRC technology, along with summarizing the challenges and potential opportunities it encounters in practical implementation. This paper aims to provide a comprehensive overview on the fundamental physical mechanisms, methods for enhancing its power output, as well as the current state and future developments of PDRC applications, providing potential guidance for reducing greenhouse gas emissions and energy consumption.