All-optical seasonal energy saving windows

Adaptive thermal regulation is crucial for year-round energy savings in buildings, especially in mid-latitude regions. Here, leveraging the seasonally varying angles of direct sunlight, we report a microprism-array sheet that enables seasonal energy saving windows. Guided by geometric optics, the asymmetric microprism-array sheet, featuring a silver coating on the sky-facing facets, exhibited an average transmittance of 10% in summer and 53% in winter at predominant sunlight angles, compared to 30% and 38% of a standard tint film, respectively. During outdoor testing in South Korea (37°N), a south-facing window integrated with the microprism-array sheet reduced floor temperatures by up to 5°C during direct sunlight hours, relative to a tinted window. Notably, in east-facing windows, the microprism-array sheet transitioned from heating to cooling around 9:00 a.m., highlighting its potential for modulating seasonal thermal management.