Silicon-monoxide (SiO) film on aluminum (Al) substrate is known as one of the spectral selective radiating materials that utilizes selective infrared emission through the atmospheric window (8-13 $mu@m). Nevertheless, the radiative cooling power of them is rather low comparing to that of black body which is ranging from 60 to 100 W/m$+2$/, depending on the model of the atmosphere and the humidity. In other words, the sky radiator must have a considerably large are film with homogeneous thickness. This is the reason why it is difficult to put the radiative cooling idea to practical use. To attempt a solution of this problem, we have examined some silicon composite films and/or their multilayers on aluminum substrate by numerical calculation. The main results are following: (1) (SiO $PLU Si)/SiO film with adequate thickness shows the best result to obtain the high power radiative cooling. The radiative cooling power at room temperature (300 K) is expected to be about 20% larger, and the obtainable temperature drop to be about 10% larger than that of SiO single film on Al. (2) The difficulty of deposition of the coating with large area and homogeneous thickness can be overcome by use of porous SiO stacked film whose optical properties are not so influenced by the film thickness. If we couple the above mentioned materials with cover films of high reflectance for solar radiation and high transmittance in atmospheric window region, the radiative cooling may be more realistic even under direct daylighting.
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