A quantitative investigation on the cooling benefits of retrofitting building skylights with broadband-spectrallyum selectivity in China

Spectral selective design offers a promising solution for enhancing windows' energy efficiency. Most of the current spectral regulation strategies focus on facades windows, pursuing high Near-Infrared (NIR) reflectance and low infrared (IR) emissivity. However, in skylight scenarios, low IR emissivity is generally undesired, as it can hinder the radiative sky cooling. This study proposes a spectral regulation strategy specific for skylights, by developing the PDMS/ITO/PET flexible film. In addition to a high visible transmittance of 86 %, the polymeric film can reduce indoor solar heat gain by 20 %, and improve the radiative sky cooling performance by 16 %. An experimental demonstration conducted in Hong Kong revealed that retrofitting clear skylights with this film led to a decrease in indoor air temperature by 4.7 °C, demonstrating its superior energy-saving potential in hot regions. To further access it's feasibility in different climatic conditions, a transient model was developed and implemented across 371 cities in China. In different climatic zones of China, retrofitting clear skylight with the PDMS/ITO/PET film can induce indoor heat removal by 0.61 GJ/(m² year) to 1.34 GJ/(m² year). With its flexibility and energy-saving potential, it is foreseen that the PDMS/ITO/PET film may offer a sustainable solution for building skylight retrofitting.