A Pragmatic and High-Performance Radiative Cooling Coating with Near-Ideal Selective Emissive Spectrum for Passive Cooling

Abstract

Radiative cooling is a passive cooling technology that can cool a space without any external energy by reflecting sunlight and radiating heat to the universe. Current reported radiative cooling techniques can present good outside test results, however, manufacturing an efficient radiative material which can be applied to the market for large-scale application is still a huge challenge. Here, an effective radiative cooling coating with a near-ideal selective emissive spectrum is prepared based on the molecular vibrations of SiOx, mica, rare earth silicate, and molybdate functional nanoparticles. The radiative cooling coating can theoretically cool 45 °C below the ambient temperature in the nighttime. Polyethylene terephthalate (PET) aluminized film was selected as the coating substrate for its flexibility, low cost, and extensive production. As opposed to the usual investigations that measure the substrate temperature, the radiative cooling coating was made into a cubic box to test its space cooling performance on a rooftop. Results showed that a temperature reduction of 4 ± 0.5 °C was obtained in the nighttime and 1 ± 0.2 °C was achieved in the daytime. Furthermore, the radiative cooling coating is resistant to weathering, fouling, and ultraviolet radiation, and is capable of self-cleaning due to its hydrophobicity. This practical coating may have a significant impact on global energy consumption.

Photonic structures in radiative cooling

“…Regarding the applications where there is a large cooling power at a temperature near the ambient or largely reduced sub-ambient temperature, the surface of the radiative cooler can be designed as two types: a broadband emitter with unity emissivity at all IR wavelengths and a selective emitter with unity emissivity only within the wavelength range of the atmospheric window (Fig. 2f (left)) 30 . The surface with the broadband emitter should have high emissivity at all IR wavelengths to achieve a large cooling power.…”