Optimized infra-red spectral response of surfaces for sub-ambient sky cooling as a function of humidity and operating temperature

摘要

The preferred surface spectral response for sub-ambient sky cooling varies according to the amount of water vapor in the atmosphere and the operating difference (T_a-T_s) between ambient and emitter surface temperatures. While all good candidates average high emittance from 7.9 urn to 13 urn, where the atmosphere is most transparent (the IR "sky window"), the preferred spectral response in the remainder of the Planck spectrum depends on a number of factors. Emittances E in studies to date have been near the two extremes of a high E ～ 0.85 to 0.95, and an E value between 0.3 to 0.4 for surfaces which emit strongly only in the sky window. Cooling rates and ideal spectral properties vary with operating conditions. The reasons behind this will be explained for select different coatings, using spectral densities for emitted outgoing radiation, which is T_s dependent, and the incoming radiation that is absorbed, which is fixed unless the atmosphere changes. Higher E surfaces always work best above and just below ambient but external factors that reduce incoming radiation from the atmosphere, including very low humidity or heat mirror apertures, extend this preference down to lower surface temperatures. Sky window spectrally selective coatings do not benefit as much because they already absorb little incoming radiation, but always have the potential to achieve very much colder temperatures if non-radiative heat gains are kept low.