Zwitterionic hydrogel smart windows: Radiative cooling, privacy protection and energy savings

Current thermochromic smart windows based on lower critical solution temperature (LCST) behavior can achieve solar regulation and energy saving by modulating solar transmission at the cost of visibility at higher temperature. To tackle this issue, a new concept of smart window based on the upper critical solution temperature (UCST) characteristic of zwitterionic hydrogel has been developed, giving daytime luminous transparency, all day radiative cooling and nighttime privacy protection. This design rule is suitable for energy saving smart windows for regions where cooling demand is dominant. The UCST smart window panel provides moderate luminous transmission (T_lum) of 37.1 % and low near-infrared transmission (T_NIR) of 20.3 % during the daytime, all day near unity long-wave infrared emissivity (Ɛ_LWIR) of 0.96 facing outdoor to promote radiative cooling with outer space, low Ɛ_LWIR 0.19 facing indoor to suppress heat transfer, and low visible transmission (T_vis) of 1.4 % at night mode for privacy protection. Energy-saving simulations demonstrate that compared to commercial low-emissivity (low-E) windows in three climate zones (zone 0, 1, 2), the UCST smart window achieves an impressive annual energy-saving up to 30.7 %. This work introduces a novel conceptual design specifically tailored for climate zones where cooling demands prevail, offering a combination of moderate luminous transparency, suppressed NIR transmission, excellent radiative cooling with outdoor environment and reduced heat transfer with indoor combining with an added privacy protection feature.