Figures of merit of passive daytime radiative cooling materials

Passive daytime radiative cooling (PDRC) materials represent an emerging technology that can provide sub-ambient cooling by dissipating heat as radiation through the long-wave infrared transparency window of the atmosphere. As such, they hold promise to alleviate our growing cooling needs and could find application in a broad range of areas. An increasing number of PDRC materials and applications is reported and tested each year. The fast-paced progress in this field also creates higher demand for reliable and universal methods for comparing the performance of novel materials and predicting their cooling abilities in different environments. However, clear figures of merit and standardised testing methods to evaluate real-world cooling performance are still lacking, so that the cooling performances of various novel PDRC materials presented in literature often cannot be compared. In this work, we review and discuss these issues from the specific viewpoint of the European Partnership on Metrology project PaRaMetriC, which aims at developing a metrological framework to classify and compare these materials.