Spectrally selective Janus window for switchable passive cooling and solar heating in energy-efficient buildings

Spectrally selective windows utilize the photothermal characteristics of nanoparticles to achieve the separation of visible and near-infrared (NIR) radiation so as to improve building energy efficiency in summer. However, traditional photothermal windows are difficult to fully shield NIR energy, generating additional indoor heat, and increasing cooling load. Photothermal windows prevent sunlight from entering the room during winter, resulting in additional heating energy consumption. Here, the Janus window is proposed for transmitting visible light while achieving a switch between passive cooling and solar heating. The window has 57.8% visible transmittance and 8.1% NIR transmittance, achieving the decoupling of visible light and NIR radiation. Meanwhile, the asymmetric structure with an emissivity range of 32.5%-80.4% in the 8-14 µm atmospheric window effectively regulates the indoor thermal gain between heating and cooling modes. Compared to double-pane glass, the Janus window can lower indoor temperatures by 9°C in cooling mode. By rotating the window to adjust the positions of the low radiative heat dissipation layer and the solar heat absorption layer, the Janus window achieves a temperature rise of 4°C relative to cooling mode. The Janus window provides a new strategy, to the best of our knowledge, for improving energy efficiency, promoting the development of building windows, solar cells, and greenhouse farmland.