Solar collector surfaces with wavelength selective radiation characteristics

The use of porous materials through which a coolant is forced for the protection of surfaces exposed to high temperature gas streams has been discussed in a number of recent papers. The knowledge of the radiation properties of these materials is required if the designer is to be able to predict the porous wall temperatures to determine whether metallurgical limitations have been exceeded. With this application in mind, measurements were previously reported for the absorptivity for solar radiation of a number of porous surfaces.¹ Since that time total normal emissivity data have been obtained for the same surfaces. In viewing these results, it was discovered that these surfaces combined high absorptivity for solar radiation with a low emissivity value, and consequently their use as solar collectors is suggested. It is the purpose of this paper to describe the experimental apparatus used to obtain the radiation data and to compare the recent emissivity measurements with the solar absorptivity data for the same surfaces. For specified conditions, an efficiency is defined which allows a quantitative comparison of these with other surfaces as solar collectors.