Collector testing is required to evaluate the performance of solar collectors and compare different collectors to select the most appropriate one for a specific application. As can be seen from Sections 4.1-4.5, the tests show how a collector absorbs solar energy and how it loses heat. They also show the effects of angle of incidence of solar radiation and the significant heat capacity effects, which are determined from the collector time constant.
Final selection of a collector should be made only after energy analyses of the complete system, including realistic weather conditions and loads, have been conducted for one year. In addition, a preliminary screening of collectors with various performance parameters should be conducted in order to identify those that best match the load. The best way to accomplish this is to identify the expected range of the parameter AT/G for the load and climate on a plot of efficiency r as a function of the heat loss parameter, as indicated in Figure 4.11 (Kalogirou, 2004).
Collector efficiency curves may be used for preliminary collector selection. However, efficiency curves illustrate only the instantaneous performance of a collector. They do not include incidence angle effects, which vary throughout the year; heat exchanger effects; and probabilities of occurrence of Ti, Ta, solar irradiation, system heat loss, or control strategies. Final selection requires the determination of the long-term energy output of a collector as well as performance cost-effectiveness studies. Estimating the annual performance of a particular collector and system requires the aid of appropriate analysis tools such as f-chart, WATSUN, or TRNSYS. These are presented in Chapter 11, Section 11.5.
The collector performance equations can also be used to estimate the daily energy output from the collector. This is illustrated by means of Example 4.2.
Collector Test Results and Preliminary Collector Selection 237
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