Mesoporous-silica-glazed LCST-type smart windows for saving heating and cooling energy in winter and summer

Abstract

Thermochromic smart windows alter their optical properties from an off-mode clear state to an on-mode light-blocking state at specific transition temperatures, saving building cooling energy during hot weather. However, to ensure applicability in regions with four distinct seasons, smart windows must save energy in both winter and summer. To address this, we introduce a mesoporous silica (MPS) thin film on the surface of a thermochromic smart window utilizing a poly(N-isopropylacrylamide) aqueous solution exhibiting lower critical solution temperature behaviors. In the clear state, the MPS film provides anti-reflection properties, increasing light transmittance by approximately 4% compared with that of the smart window without the MPS film. The film also enhances the solar transmittance contrast (∆Tsol) between the off- and on-mode states compared to that of the uncoated smart window. Under simulated sunlight with 1000 W m-2 (summer conditions), a miniature house equipped with MPS-glazed and uncoated smart windows achieves an interior temperature 1.7 °C lower than that of the house with conventional double-glass windows, reducing accumulated thermal energy by approximately 26%. The MPS-glazed smart window also has a shorter switching time by an average of 29 s than that of the uncoated smart windows. Under an illumination intensity of 500 W m-2 (winter conditions), the MPS-glazed smart window achieves an interior temperature 0.6–1.3 °C higher than that of the other (smart) windows, increasing accumulated thermal energy by 8.5–21%. Therefore, the MPS-glazed smart windows could contribute to both heating and cooling energy savings during the winter and summer seasons.