Selective thermal emitters for high-performance all-day radiative cooling

Abstract

Passive radiative coolers (PRCs), which pump excess heat to cold exterior space via thermal radiation, have emerged as a promising energy-free technology in cooling buildings, thermal power plants, and photovoltaics. However, designing a ‘daytime’ PRC is challenging due to the simultaneous requirement of high reflectance in the solar spectral regime (0.3–2.5 µm) and high emissivity in the atmospheric transmittance window (8–13 µm). Here, we present a large-area compatible and lithography-free nanoscale multilayer design of daytime PRC based on two pairs of tandem silicon dioxide–aluminium nitride dielectric layer cascaded to a silver ground metal placed over a silicon substrate. We theoretically achieve near-perfect reflectance (97.3%) over the solar spectral regime while maintaining high emissivity (80%) in the atmospheric transmittance window. During the daytime under direct sunlight, the cooling power of the proposed structure is reported to be 115 with a temperature reduction up to 15 K below the ambient temperature, when the effect of convection and conductive heat transfer is considered. Our design is polarization-independent and angle-insensitive up to 70 degrees of angle of incidence. An excellent match between our theoretical and simulation results validates our findings. The fabrication tolerance study reveals that the cooling performance of our robust design is unlikely to degrade much during experimental realization. The figure of merit calculation indicates that our PRC can outperform recently reported daytime PRCs.