Exploring energy-saving performance of radiative cooling roofs with a transient heat transfer model

Abstract

Radiative cooling coatings, serving as building roof exterior surfaces, reflect most solar energy and dissipate heat mainly through the atmospheric window. A roof transient heat transfer model was proposed considering the atmospheric transmittance and spectral properties of radiative cooling coatings within the longwave range by coupling one-dimensional transient heat transfer and a spectral-dependent method. The atmospheric radiation was calculated based on the dew point temperature. Field measurements were conducted to verify the model reliability. The simulation and experimental data matched well. The validated model was employed to analyze the energy-saving performance of radiative cooling coatings in diverse climate zones in China (Guangzhou, Shanghai and Beijing). The results showed that the radiative cooling coatings (solar reflectance β = 0.85 and thermal emissivity ε = 0.95) achieved excellent cooling performance, although they could cause a penalty during the heating period. Compared with conventional exterior surfaces (β = 0.2 and ε = 0.9), the radiative cooling coatings reduced the annual load (the sum of the cooling and heating loads) of roofs by 84.9%, 30.3% and 2.4% in Guangzhou, Shanghai and Beijing, respectively, with roof insulation thicknesses of 40, 80 and 100 mm, respectively. Sensitivity analysis revealed that the solar reflectance determined the roof annual load in Guangzhou, while the insulation thickness determined the roof annual load in Shanghai and Beijing. The numerical model developed herein could be used to analyze the energy-saving potential of radiative cooling roofs with different spectral properties and provide guidance for effectively applying radiative cooling coatings on building roofs.