Passive radiative cooling of non-sky-facing surfaces: Downward-facing and double-sided radiative cooling

Passive radiative cooling is a promising zero-energy thermal-management technology, but its deployment remains limited by two persistent challenges: most radiative coolers are designed for sky-facing surfaces, and conventional single-sided designs provide relatively low cooling power density. In this work, we experimentally and theoretically investigate downward-facing and double-sided passive radiative cooling by mounting a cooler above an angled, spectrally selective planar mirror. The key novelty is the use of a fluorine-doped tin oxide coated glass pane that is highly reflective in the infrared while transparent to solar radiation. This pane acts as a mirror for thermal radiation, redirecting bottom-side emission from the cooler upward, without concentrating solar radiation onto the cooler as in earlier designs. Under rooftop test conditions relevant to real-world deployment, we demonstrate passive cooling from a downward-facing surface and show that the double-sided configuration can achieve a temperature drop relative to ambient temperature that is up to 90% greater than that of a sky-facing single-sided radiative cooler. Cooling performance depends on cardinal orientation and increases with mirror angle. Furthermore, a day–night reversal in relative device performance passively aligns cooling with thermal demand: the double-sided device cools more strongly during the day but becomes slightly warmer than the single-sided device at night, although both remain below ambient temperature. The results of this study establish a straightforward, practical strategy for extending passive radiative cooling to non-sky-facing surfaces and increasing effective cooling per unit footprint; potential applications include photovoltaic back-side cooling, thermoelectric power generation, building-integrated cooling, and space-constrained thermal-management systems.