There has been continued recent interest in radiative sky cooling of coated flat surfaces that are able to passively attain sub-ambient temperatures. As the lowest incoming infrared radiation from a clear sky occurs at the zenith, a surface which sees mainly this region of the sky will receive much lower levels of sky radiation than one which views the whole sky, since the near-horizon contains significantly more incoming radiation. Two approaches to extra cooling are thus angular selectivity, which limits oblique outgoing as well as incoming radiation, and macroscopic reflectors which block oblique incoming sky radiation, while directing most outgoing emitted radiation towards the near zenith. This work focuses on the second of these techniques. We maximise cooling potential via coated 3D printed structures which can passively maintain a thermal reservoir below ambient temperature throughout the night and day. Novel design methods are used to fabricate and test structures which maximise outgoing thermal radiation from a surface, while minimising its illumination by incoming radiation from the sky and sun. Preliminary results gave 10°C below ambient both day and night during a Sydney spring. 3D printing allows the production of complex designed mirror cones with relatively low thermal conductivity. Post processing of the 3D printed structures allows the desired surface textures and optical properties to be created.
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