  As an emerging electricity-free cooling technology, radiative cooling employs
outer space as the heat sink. With this, a sky-facing thermal emitter is
usually required. Due to the black-body radiation limit at ambient temperature,
the maximum cooling power density for a single-faced radiative cooling device
is ~156.9 W/m2. Here we report a double-sided radiative cooling architecture
using graded nanocomposite metamaterials (GNM) designed for a vertically
aligned thermal emitter. This GNM structure possesses an optical absorption of
over 90% throughout the solar spectrum, and exceeds 90% reflection in the
mid-infrared spectral region. With this configuration, both sides of a planar
thermal emitter can be used to perform radiative cooling and a record cooling
power density beyond 280 W/m2 was realized in a single thin-film thermal
emitter. Under the standard pressure, we realized a temperature reduction of 14
degree Celsius below the ambient temperature in the laboratory environment, and
over 12 degree Celsius in the outdoor test.
