Subambient passive radiative cooling effects of barium sulfate and calcium carbonate paints under Malaysia's tropical climate

Global cooling requirements are increasing at an unprecedented rate due to rapid urbanization and population growth, further aggravating climate concerns. Passive radiative cooling is a unique phenomenon that can be utilized to reduce global cooling, energy consumption and alleviate the urban heat island effect. Paints can act as passive cooling devices that are able to reflect incoming sunlight and emit radiation in the atmospheric window (8–13 μm), where it propagates directly into deep space without any interference. In this work, we have successfully fabricated and tested two different types of cooling paints, consisting of BaSO4 and CaCO3 as their respective pigments under Malaysia's tropical climate. Different types of binders, solvents, and pigment concentrations were tested to obtain the most optimum cooling paint configuration. Field test results proved that both cooling paints were able to achieve remarkable subambient temperatures throughout the entire day, even under direct solar irradiation. The BaSO4 cooling paint was able to achieve subambient temperature reductions of up to −6.1 °C and a mean net cooling power of 71.0 W m−2 while the CaCO3 cooling paint achieved a maximum subambient temperature reduction of −6.0 °C and a mean net cooling power of 69.9 W m−2. Both paints were able to significantly outperform commercial white paint on a variety of different surfaces, in terms of cooling performance. The hindering effects of various climate conditions including humidity levels and local wind speeds on the overall cooling performance of both the paints were also investigated.