Thermochromic smart windows with highly regulated radiative cooling and solar transmission

Highlights: • Smart window to regulate both solar transmittance and radiative cooling. • Switchable emissivity to prompt cooling in summer and warm keeping in winter. • High luminous transmittance, good solar regulation. • Better energy efficiency than commercial low-E glass across climate zones. Radiative cooling (RC) is a technique that spontaneously radiates long-wave infrared (LWIR) to the cold outer space, which provides cooling power to buildings, however only preferred in hot seasons. RC has been widely researched on walls and roofs but rarely on windows while windows are the least energy-efficient part of buildings. Moreover, in the current smart window design, consideration of tunable RC is missing. For the first time, we proposed an ideal smart window with a switchable front side LWIR emissivity (ε_Front) and solar modulation ability (ΔT_sol). Such window needs not only to have high luminous transmission (T_lum) and ΔT_sol, the two major conventional performance indexes but also possess a switchable ε_Front to cater for the variable seasonal thermal performance requirements and energy-saving demands. We further fabricated a tunable emissivity thermochromic (TET) smart window with large ΔT_sol (51%) and T_lum (72%) and switchable ε_Front (0.95–0.1) to prove the efficacy of the proposed ideal window. Compared with current smart window technology solely regulating solar transmission and other RC materials with fixed emissivity, the TET smart window panel gives a wide tunability in the selective solar spectrum for dynamic climate conditions in energy-saving buildings, achieving largely enhanced energy saving performance globally