Daytime Clear-Sky Radiative Cooling Potential Map of the Contiguous United States

The outer space can be approximated as a blackbody at a temperature about zero Kelvin, which makes it an effective heat sink. Radiative cooling is to use this heat sink to dissipate heat from objects to outer space. The radiative cooling potential of objects on the Earth surface is lower than that in outer space, because the presence of atmosphere hinders the radiative heat transfer to the outer space. However, the atmosphere can be viewed as nearly transparent in a spectrum window ranges from 8 to 14 micrometers, through which the objects on Earth could reject heat to the outer space. During the daytime, the cooling is challenging because the absorption of solar irradiance. To achieve passive cooling during the daytime, the radiative cooling devices need be to designed to have low absorptance in the solar spectrum and high emisstance in the atmospheric window spectrum. Therefore, the optical design of the radiative cooling devices requires detailed balance between the incoming thermal radiation from the sky (DLW) plus shortwave solar radiation (DSW) and the outgoing emissive power from the coolers in order to calculate the equilibrium temperatures and cooling potentials.

The cooling potential of radiative cooling devices is dependent on the ‘transparency’ of the atmospheric window (related to DLW), while the ‘transparency’ is a function of constituent concentration in the atmosphere, because water vapor, carbon dioxide, aerosols and clouds all `block’ the window. Previous studies have shown that the broadband value of DLW at a given location is strongly correlated to the relative humidity and temperature at the surface level. In the solar shortwave spectrum, the broadband value of DSW is strongly correlated to the concentration of water vapor and aerosols in the atmosphere, which can be calculated by local values of temperature, relative humidity and aerosol optical depth.

Here we use a combination of meteorological measurements and spectral models to discuss the radiative cooling potential, in seven distinct spectrum bands. In the atmospheric longwave spectrum (wavelength greater than 4 μm), an efficient two-flux model is used to compute the band sky emissivity. In the shortwave spectrum (wavelength smaller than 4 μm), an efficient Monte Carlo radiative model is used to compute the transfer of solar irradiance in the atmosphere. With the spectral atmospheric LDW and SDW, the cooling power of different materials used for passive cooling can be computed by performing surface thermal balances. The equilibrium temperatures and cooling potentials are found to be functions of air temperature, relative humidity and aerosol loading at the ground level. The computed cooling power is high for hot and dry conditions (~140 W/m2), and substantially lower for mild temperatures and high humidities (no cooling can be achieved). By applying the methodology to 900 stations in the Automated Surface Observing System (ASOS), monthly passive cooling potential map is generated for the Contiguous of United States. Locations with hot and dry conditions are favorable for passive cooling applications while locations with cool and humid conditions barely have passive cooling potentials. Since clouds will `block’ the atmospheric windows, the cooling power will be reduced with the presence of clouds.