Solar Engineering of Thermal Processes, 4th Edition

6.16 Collector Characterizations

Based on the theory outlined in the previous sections and the laboratory measurements that support the theory, several methods for characterizing collectors can be noted. These characterizations, or models, have various numbers of parameters and are thus of varying complexity, and they serve different purposes. At one extreme very detailed models include all of the design features of the collector (plate thickness, tube spacing, number of covers and cover material, back and edge insulation dimensions, etc.). At the other extreme is a model that includes only two parameters, one that shows how the collector absorbs radiation and the other how it loses heat. (The simplest model would be a one-parameter model, a single efficiency; this is essentially useless as the efficiency is dependent on the collector operating temperature and ambient temperature, so that in most collector applications the efficiency changes with time.)

For collector design (i.e., specification of the details of the design, such as plate thickness, tube spacing, etc.) and for detailed understanding of how collectors function, detailed models are appropriate. The most complete steady-state model includes all of the design parameters entering the terms in Equation 6.7.6, that is, those that determine , , and . To this can be added an analysis of transient behavior, which can be a single ...