Field investigation of a photonic multi-layered TiO₂ passive radiative cooler in sub-tropical climate

Cost reduction and enhanced cooling performance are strongly demanded for daytime passive radiative cooling due to its attractive cooling strategy that does not require any energy input. Its potential application varies widely from air conditioning systems for buildings, photovoltaic cells, electronic device cooling and automobiles. However, recently proposed daytime passive radiative coolers are based on photonic structures which are high in cost. A relatively cheap metal oxide material, TiO₂, which lowers the cost but is highly emissive in the mid-infrared range has been used, also improving the cooling performance of the photonic daytime passive radiative cooler. An optimized TiO₂–SiO₂ alternating multi-layered photonic daytime radiative cooler with average emissivity of 0.84 within 8–13 μm while reflecting 94% of incident solar energy is developed. Its net cooling power is estimated to be 136.3 W/m² at ambient air temperature of 27 °C which shows an improvement of 90 W/m² compared to that of the HfO₂-SiO₂ photonic radiative cooler. Last, a field test has been conducted in Hong Kong's subtropical climate (i.e. relative humidity = 60–70%) to investigate its feasibility, and with the help of solar shading, successfully demonstrated temperature reduction of 7.2 °C with a net cooling power of 14.3 W/m² under direct sunlight.