Multi-gradient energy-saving smart windows with thermo-response and multimodal thermal energy storage†

Buildings, especially installed windows, account for a large proportion of global energy consumption. The research trend of smart windows leans towards multi-functional integration, concurrently achieving solar modulation and thermal management. However, sometimes a one-time performance switch cannot meet demands, making the design of multi-gradient adjustable smart windows particularly important. The combination of the temperature-responsive optical properties of hydroxypropyl cellulose (HPC), the high specific heat capability of water (sensible heat storage) and the solid–liquid phase transition of κ-carrageenan (latent heat storage) is proposed first and can be used to prepare the thermo-responsive hydrogel and multi-gradient energy-saving smart window with thermo-response and multimodal thermal energy storage (MGES smart window) quickly without long-term polymerization. The MGES smart window has excellent solar modulation capability (ΔT_lum = 82.72% and ΔT_sol = 68.65%) together with outstanding specific heat absorption ability (c = 4.2 kJ kg⁻¹ K⁻¹) and phase transition heat (ΔH = 1.23 kJ kg⁻¹), showing superior energy saving and conserving performance. In demonstrations, the MGES smart windows can reduce the surface and indoor temperature by more than 15 °C and 10.6 °C compared with normal windows. Simulations suggest that they can cut off 45.1% of building energy consumption. To sum up, the MGES smart windows realize multi-aspect adjustment of energy, opening up a new avenue for green buildings.