Efficient Thin Polymer Coating as a Selective Thermal Emitter for Passive Daytime Radiative Cooling

Abstract

Radiative cooling to subambient temperatures can be efficiently achieved through spectrally selective emission, which until now has only been realized by using complex nanoengineered structures. Here, a simple dip-coated planar polymer emitter derived from polysilazane, which exhibits strong selective emissivity in the atmospheric transparency window of 8–13 μm, is demonstrated. The 5 μm thin silicon oxycarbonitride coating has an emissivity of 0.86 in this spectral range because of alignment of the frequencies of bond vibrations arising from the polymer. Furthermore, atmospheric heat absorption is suppressed due to its low emissivity outside the atmospheric transparency window. The reported structure with the highly transparent polymer and underlying silver mirror reflects 97% of the incoming solar irradiation. A temperature reduction of 6.8 °C below ambient temperature was achieved by the structure under direct sunlight, yielding a cooling power of 93.7 W m–2. The structural simplicity, durability, easy applicability, and high selectivity make polysilazane a unique emitter for efficient prospective passive daytime radiative cooling structures.

Slippery Passive Radiative Cooling Supramolecular Siloxane Coatings

“…Passive radiative cooling polymer coatings (PRCPCs) represent one of the most facile and efficient energy-saving strategies for controlling spacing temperature. − They usually have heterogeneous micro/nanostructures that reflect solar light (wavelength range of 0.3–2.5 μm) but allow for thermal emission to the cold outer space through the atmospheric longwave infrared (LWIR) transparent window (wavelength: 8–13 μm) . Such heterogeneous structures can be created by introducing cavities or particles into the polymer matrices. ,, This idea has inspired the design of various PRCPCs with excellent cooling performance. − When great attention is focused on improving the coating’s cooling performance, many other aspects should also be considered for practical applications of these coatings. One aspect is the combination of additional desirable functions.…”

A tailored indoor setup for reproducible passive daytime cooling characterization

“…Previous reports showed that angles <60° have a dominant contribution to the emission of passive cooling materials. 4 , 27 , 28 Therefore, the convection shield made from XPS and LDPE has a cone-shaped inner part, with a zenith angle of 60° to reduce the thermal loss (conduction and convection) area while marginally blocking the pathway of thermal radiation from the sample. Conductive heat transport from each gas compartment to the next would eventually cool the air below the convection shield.…”

Tunable Photovoltaics: Adapting Solar Cell Technologies to Versatile Applications

“…[413] Hence, this offers a promising pathway to control of IR emission region through molecular-level design with appropriate chemical bonding. [414][415][416] Wang et al found experimentally that commercial encapsulated c-Si SCs can be cooled down by 2 °C in real environment using a 600 µm-thick pyramid-textured polydimethylsiloxane (PDMS) emitter without convection covering. This is due to a broadband emissivity close to 1 in mid-infrared range and an average transmittance of >90% in the absorption spectrum from 0.3 to 1.1 µm.…”