Daytime passive radiative cooling by ultra emissive bio-inspired polymeric surface

摘要

Saharan silver ants can maintain their body temperature below ambient air due to unique triangular shaped hairs that enhance solar reflection and thermal emission through a transparent window that lies in the atmosphere. Applying this thermoregulatory prismatic structure to polydimethylsiloxane (PDMS), highly emsissive in the 8-13 mu m spectrum, we present a geometrically modified polymer-based daytime passive radiative cooler. The selective thermal emitter was fabricated based on the optimized prismatic structure from Finite Difference Time Domain (FDTD) simulations. The average emissivity within the 8-13 mu m spectrum was enhanced to 0.98 by the gradient refractive index effect, while the average solar reflectivity in the visible and near-infrared spectrum was measured to be 0.95. The net radiative cooling power is estimated to reach 144 W/m(2), exceeding records of previously reported radiative coolers. Last, in Hong Kong's hot and humid climate, a field test successfully demonstrated cooling by 6.2 degrees C below the temperature of ambient air corresponding to a net cooling power of 19.7 W/m(2) in a non-vacuum setup during the peak daytime with shading. This is the largest temperature reduction observed in a tropical region for daytime passive radiative cooling. Our work presents an alternative method to enhance passive thermal emission and may facilitate its world wide application in eco-friendly space cooling.