Performance optimization of multi-stage thermochromic windows: Experimental and numerical analysis of daylight and thermal regulation in Tianjin

Windows, key building components, serve as the main channels for natural light and heat to enter interiors. Hydrogel-based thermochromic smart windows regulate light/heat entry via transmittance control. However, traditional single-stage thermosensitive smart windows (SWⅠ) abruptly opacify at the transformation temperature (LCST) of hydrogel, which triggers sudden indoor dimming and visual discomfort. Therefore, a novel multi-stage structure is proposed to provide occupants time to adapt to dimness and cut indoor lighting load. Experimental results showed that the indoor light and heat environment can be optimized by adjusting the LCST matching position in the multi-stage structure. Specifically, relative to SWⅠ with an LCST corresponding to the middle hydrogel of the three-stage structure (SWⅢ), the average indoor temperature ( T avg ) of SWⅢ increased by 0.26 °C and the available indoor natural light ( day-UDI 100–2000 lux ) increased by 28.17 %. In contrast, compared to SWⅠ with an LCST corresponding to the upper hydrogel of the SWⅢ, the T avg of SWⅢ decreased by 1.04 °C and the day-UDI 100–2000 lux decreased by 10.16 %. Furthermore, annual simulation results indicated that the optimal LCST for SWⅠ in Tianjin was 28 °C. The optimal SWⅢ had an LCST in its middle that was consistent with SWⅠ-28, which prolonged the indoor thermal comfort time by 1 h and reduced the unfavorable illumination by 60.44 % at 12:00 noon on a typical summer day than SWⅠ-28. This study provides valuable insights and a practical reference for the application of thermosensitive smart windows.