Highly solar modulated and robust PNIPAM/HEMC smart windows†

Energy-efficient windows are considered as one of the most promising energy saving strategies for buildings. An ideal thermochromic smart window should possess high stability, high transmittance and outstanding mechanical properties. However, existing smart window technologies could not meet these requirements simultaneously. Herein, composite hydrogels containing hydroxyethyl methyl cellulose and thermoresponsive poly(N-isopropylacrylamide) were synthesized and applied to smart windows. These hydrogels exhibited excellent solar modulation (ΔT_sol = 74.97%), luminous transmittance (T_lum = 84.98%), and mechanical properties with a reversible compression ratio of up to 74%. Besides, they offered faster response time and superior thermal stability compared to traditional hydrogels. It was demonstrated that the composite hydrogels retained exemplary optical properties after 80 heating–cooling cycles, and PNIPAM/HEMC smart windows showed superior indoor temperature control capability. Compared with normal glass, a typical office building with the produced sample was calculated to show annual energy reductions of 19.2, 28.2, 74.8, and 49.7 kW h m⁻² in Beijing, Hong Kong, Bangkok, and Kuala Lumpur, respectively. This study demonstrated a reliable strategy for PNIPAM windows that is promising to save building energy.