Accurately Quantifying Clear-Sky Radiative Cooling Potentials: A Temperature Correction to the Transmittance-Based Approximation

Abstract

Theoretical calculations of the cooling potential of radiative cooling materials are crucial for determining their cooling capability under different meteorological conditions and evaluating their performance. To facilitate these calculations, accurate models of long-wave infrared downwelling atmospheric irradiance are needed. However, the transmittance-based cosine approximation, which is widely used to determine radiative cooling potentials under clear sky conditions, does not account for the cooling potential arising from heat loss to the colder reaches of the atmosphere itself. Here, we show that use of the approximation can lead to >10% underestimation of the cooling potential relative to MODTRAN 6 outputs. We propose a temperature correction to the transmittance-based approximation, which accounts for heat loss to the cold upper atmosphere, and significantly reduces this underestimation, while retaining the advantages of the original model. In light of the widespread and continued use of the transmittance-based model, our results highlight an important source of potential errors in the calculation of clear sky radiative cooling potentials and a means to correct for them.

Daytime Radiative Cooling under Extreme Weather Conditions

“…Although the cooling power higher than this value can be obtained if a concentrator structure is used, [ 30,66 ] this amount of longwave imbalance represents the fundamental limit on the available power by radiative cooling for each region and its sky condition. [ 32 ] The downwelling measured by the Pyrgeometer is directly related to the conventionally defined sky temperature, via . The corresponding sky temperature , between 16.9–18.0 °C, is shown as the brown curve in the bottom panel of Figure 4b.…”

“…However, radiative cooling in hot and humid climates remains a challenge because radiative cooling depends strongly on the regional climatic conditions, in particular the sky emissivity. [10,26,[29][30][31][32] The sky emissivity depends on the transparency of the "sky window" (from 7.9 to 14 mm, or a smaller range within this interval, from 8 to 13 mm, is often used). [10,12] One of the most determining factors for the sky window transparency is known to be the water vapour content in the air of the region, correlated with the local humidity.…”

“…Furthermore, it was pointed out that downward atmospheric emission may be overestimated in MODTRAN simulations. [ 32 ] Therefore, while the sky emissivity obtained using the MODTRAN simulation results may provide a reasonable estimate for a stationary and uniform sky, it is not considered suitable for a dynamic and cloudy sky. To this end, we present a method to characterize a dynamic sky to understand the radiative cooling performance in the equatorial tropical regions.…”

Radiative cooling and indoor light management enabled by a transparent and self-cleaning polymer-based metamaterial

“…The radiative cooling power of the PMMM film is simulated using the mathematical model by Raman et al 21 , 59 . The radiative cooling powers of the PMMM and glass samples are plotted against the temperature difference between the radiative cooling sample and the ambient (i.e., T RC − T a ) in Fig.…”

“…Notably, we have integrated a refined atmospheric model recently developed by ref. 59 . to enhance the accuracy of our calculations, particularly regarding atmospheric thermal radiation.…”