Performance of the PTR in the winter of 2021. (A) A schematic of the experimental setup. A PTR with the hierarchical Cu foam side facing the sky and a conventional solar absorber are set as controls. The foam, polypropylene (PP) box, and polyethylene (PE) film are used to minimize heat loss to the surroundings. (B) A whole day temperature profile of the samples. Shaded area: 18 ± 3°C. Our PTR realizes a near-flat temperature curve, highly desirable in practical applications. In contrast, the temperature of the conventional solar absorber severely fluctuates. (C) Nighttime heating power enabled by the PTR. (D) Cycling performance of the PTR. The temperature profiles from 0:00 to 8:00 every day are presented. The temperatures of the conventional solar absorber and ambient are also shown for comparison.