Sammanfattning: The overall yield factor of solar thermal collectors is strongly affected by the Incident Angle Modifier (IAM). Costly measurement procedures are indispensable and make it often impossible to collect a sufficient number of accurate base points to make sure that the performance of the collector is known under any irradiance condition. Especially asymmetric shapes are subject of investigation, as models of biaxial IAM approximations are much in question. A ray tracing simulation tool based on the commercial software OptiCAD is developed in this work and calculations performed and analyzed. A graphical user interface facilitates modeling of the most common collector types. Emphasis is put on complete collectors rather than simplifications. The resulting IAM graphs are compared with measured data and existing biaxial approximation methods. Former is performed with the help of a diffuse model, which evaluates collector performance under diffuse condition based on the calculated beam IAM values.



Moreover, achieved data are implemented in SPF’s software Polysun and yearly solar fractions of a simple district hot water system evaluated. At last, a parametric study discusses the influence of the various material properties on the optical performance of a collector. The results show that an introduction of an accurate universal mathematical description of any IAM seems to be impossible and is affirmed by the comparison of calculated with biaxial approximated values. The great variety of geometric and material properties makes each collector’s curve unique. Further, the traditional IAM definition of flat plate collector may not be applied directly to tube collectors, as it does not consider irradiation e....

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