Dual-Layered Titanium Oxide-Acrylic Film on Carbon Black for Passive Radiative Cooling below Ambient Temperature under Direct Solar Irradiance

Abstract

Cooling is an energy-intensive process; and with the effects of global warming, space conditioning poses more load on the scarce and costly energy available for consumption in facilities. Thus, the use of a passive sub-ambient diurnal radiative cooler, which is an effective natural space cooling technology, can help reduce the total energy demand for buildings. A diurnal passive radiative cooler of 40 × 40 cm, has been designed, fabricated, and experimentally investigated. The cooler comprises a photonic solar reflector and thermal emitter made of titanium dioxide embedded in an epoxy resin that reflects 96% of insolation and emits strongly in the 8-13 µm atmospheric window. Design calculations were made under Owerri climatic conditions. The results obtained showed that when the cooler was exposed to direct insolation well above 950 W/m2 , it achieved a drop in temperature of 1-2 °C below ambient between 6 am and 9 am. As the solar radiation increased, the temperature of the cooler increased until it reached 41 °C, which was above the ambient air temperature (30 °C). The cooler temperature decreased as the solar radiation decreased, dropping below the ambient temperature from 5:45 pm, by 3 °C during the remaining hours of the investigation. The photonic radiative cooler has an estimated cooling power of 56.8 W/m2 under a clear night sky and achieved sub-ambient cooling during the early and late hours of the day under low solar radiation. Therefore, passive cooling through the photonic approach offers prospects for energy efficiency. Further works may have the prospects of achieving improved passive sub-ambient daytime cooling.