Sunflower-Inspired Zero-Energy-Consumption Thermal Switching for Dual-Mode Photochromic Smart Window

The optimization of building energy efficiency largely governs the process of urban sustainable development. The design of smart windows can adaptively respond to external conditions by regulating internal environments, thereby reducing indoor energy consumption. However, switching between different environments (such as hot and cold conditions) incurs external energy costs (e.g., mechanical or electrical energy usage). Drawing inspiration from the sunflower's habit of 'unfurling at sunrise and closing at sunset', herein, a zero-energy, thermal-triggered automatic switching device was designed for a dual-mode photochromic smart window with the upper layer of a copper-doped WO₃ photochromic film and the lower layer of a polyurethane two-way shape memory polymer. In hot environments, an approximately 4.86 °C lower temperature is realized between a prototype house equipped with this device (with a maximum indoor temperature of 47.34 °C) compared with a house without this device (52.20 °C), which confirms the film's solar heat rejection effect. In cold conditions, the device curls to allow sunlight entry, maintaining a comfortable indoor environment. It exhibits superior recycling performance (over 200 cycles). Its exceptional thermal management capabilities and automatic switching enable optimal temperature control by selecting the appropriate mode. This zero-energy thermal management device contributes to achieving low-carbon and environmentally friendly objectives.