Switchable bilayer energy saving window for multispectral photothermal modulation

Multi-modal energy saving windows is promising for passive thermoregulation in buildings through adaptive optical modulation. Herein, a switchable bilayer energy saving window (BESW) with multispectral photothermal management property was proposed for building energy efficiency. The BESW comprises a diffuse transmission radiative cooling window (DTRCW) and a solar transmission photo-thermal window (STPTW), thus it could set cooling and heating modes according to the need. The two modes of the BESW can be switched by manual flipping. The similar visible transmittance of DTRCW (83.1 %) and STPTW (92.7 %) enabled the BESW enough natural lighting in all states. Meanwhile, in the cooling mode, owing to the low near-infrared transmittance (53.2 %), high infrared selective emission ratio (1.84) and high haze (67.4 %) of DTRCW, the BESW could reduce the temperature by passive radiative cooling while reducing the discomfort caused by sunlight glaring. Conversely, the high near-infrared transmittance (93 %), low infrared selective emission ratio (0.83) and low haze (0.2 %) of the STPTW endowed the BESW with remarkable solar heating in the heat mode. Hence, the contrast optical properties between the DTRCW and STPTW enabled the BESW with multispectral photothermal management performance. The thermal regulation and energy saving properties of the BESW in different areas were demonstrated by outdoor measurements and energy saving evaluation. Moreover, both DTRCW and STPTW were facilely fabricated and had potential to be mass produced, while the surface hydrophobicity and durability of both DTRCW and STPTW were also confirmed. The proposed novel photothermal modulation window provides great prospects for the next-generation green buildings.