Bilayer porous polymer for efficient passive building cooling

Passive building cooling without any electricity input are highly desirable in pursuing low energy consumption and environment protection. However, widespread adoption of existing techniques is restrained by the complex system design or low cooling power. Herein, we propose an efficient passive cooling approach with a bilayer porous polymer film, which comprises a hygroscopic hydrogel and a hydrophobic top layer with hierarchical pores. The hydrogel implements evaporative cooling in the daytime and regenerates itself at night. The top layer protects and radiatively cools the hydrogel, which enhances the cooling power during day and helps the hydrogel regeneration at night. With the synergistic effect, the bilayer film attains a remarkable sub-ambient temperature drops of ~7 °C and an effective cooling power of ~150 W·m−2 under direct sunlight, showing great potential for low-cost, efficient and scalable passive building cooling.