Daytime radiative cooler using porous TiO₂: new approach

Passive daytime radiative cooling remains a topic of intense interest that has gained great attention recently, mainly due to its important applications, such as electronic heat dissipation, solar cells, and photo-thermal technologies. In order to achieve the daytime radiative cooling for thermal management applications, a new smart shield should be designed to have both a low absorptivity in the solar range of 0.3-2.5 µm and high emissivity in the atmospheric transparency windows of 8-13 µm and 16-26 µm. In this work, we propose and analyze a new, inexpensive, and highly scalable double-layer coating to reach an efficient passive daytime radiative cooling. Double-layer coatings consisting of a top TiO₂ porous adsorbent layer and a bottom providing excellent transmittance in the solar spectrum are achieved properly by borosilicate-crown glass (BK7). We have shown that the proposed device preserves an average absorptivity/emissivity well below 7% in the entire solar spectrum and almost 95% in the atmospheric transparency windows. Besides, our design allows for sub-ambient temperature drops of 45° and diurnal cooling power output of 133W/m², even in the case of solar irradiance up to 1000W/m². The cooling performance persists under high humidity, even when taking the non-radiative heat exchange progress into account. The results revealed that the proposed design can be easily applied to a large area and promotes a significant step towards achieving large scale application in solar cells and related systems.