Solar Engineering of Thermal Processes, 4th Edition

Chapter 4

Radiation Characteristics of Opaque Materials

This chapter begins with a detailed discussion of radiation characteristics of surfaces. For many solar energy calculations only two quantities are required, the solar absorptance and the long-wave or infrared emittance, usually referred to as just absorptance and emittance. Although values of these two quantities are often quoted, other radiation properties may be the only available information on a particular material. Since relationships exist between the various characteristics, it may be possible to calculate a desired quantity from available data. Consequently, it is necessary to understand exactly what is meant by the radiation terms found in the literature, to be familiar with the type of information available, and to know how to manipulate these data to get the desired information. The most common type of data manipulation is illustrated in the examples, and readers may wish to go directly to Section 4.5.

The names used for the radiation surface characteristics were chosen as the most descriptive of the many names found in the literature. In many cases, the names will seem to be cumbersome, but they are necessary to distinguish one characteristic from another. For example, both a monochromatic angular-hemispherical reflectance and a monochromatic hemispherical-angular reflectance will be defined. Under certain circumstances, these two quantities are identical, but in general they are different, and it is necessary ...