Passive Radiative Daytime Cooling BY – ANKUSH KUMAR SINGH Introduction Passive radiative cooling can be considered as a renewable energy source, which can pump heat to cold space and make the devices more efficient than ejecting heat at earth atmospheric temperature. Passive radiative cooling is a method of cooling objects with radiation energy exchange with cold outer space, which is at 3K. To exchange radiative heat with cold outer space, radiative cooler is required to emit radiation in atmospheric window 8–13 μm and reflect elsewhere, i.e., solar radiation and atmospheric radiation. Energy Exchange The ideal radiative cooler decoupled from the atmosphere can achieve equilibrium temperature of outer space, i.e., 3K, but the actual theoretically limit by considering atmospheric radiation and suppressing parasitic heat gain attain an average 37 ᵒC below ambient in. The more actual scenarios have humidity and cloud in the environment and need consideration. Solar reflectance and LWIR emittance In the solar spectrum (0.3–2.5 μm), the absorptance should be 0 (100% reflectance) to avoid solar heating. In the atmospheric LWIR transmission window (8–13 μm), the emittance should be 1 to effectively radiate heat to outer space through this partially transparent atmospherewindow. In other mid-infrared wavelengths (2.5–8 μmand > 13 μm), the emittance should be 0 to avoid the overheating of the atmospheric irradiation at a higher temperature than that of the PDRC coating. Reflectance A mismatch between the refractive index causes scattering of light in material and backscatters for the slab thickness. Any change in refractive index leads to some reflection and this concept can be utilized for maximum reflection of varying refractive index alternatively in dielectric stack multiple layers. Other than layered media, randomly distributed particles scatter the light and lead to diffuse reflection or backscattering. Non-Radiative heat transfer Fabrication of PDRC materials Thermal mid-infrared emittance To achieve better PDRC performance in sub-ambient conditions, diverse materials have been proposed to improve the mid-infrared emittance where λ =8–13 μm. Inorganic dielectric materials can achieve spectrally selective infrared emittance because of the wide absorption band of metalO bonds.37,48 In addition, Al2O3, TiO2, and SiO2 are stable and chemically inert materials, and inorganic coatings inherently exhibit superior mechanical robustness and retainability, thereby guaranteeing a long lifespan in exterior environments. Inorganic PDRC has lack of flexibility,water-repellence, and antiabrasion. Effect of environmental parameters The high humidity or cloud cover blocks the atmospheric window in 8–13 μm range due to the presence of water vapor, which absorbs/emits in the specified atmospheric window, leads to radiative cooler nearly ineffective. Convective effects - The effect of wind over a radiative cooler can be suppressed by a wind-shield and depends directly on change in wind velocity for daytime and nighttime radiative cooling. Applications Space Cooling Water Cooling Harvesting Water from atmosphere Electricity Generation
