Ray-tracing Simulation of Incident Angle Modifiers (IAM) of Solar Thermal Collectors

Abstract: The overall yield factor of solar thermal collectors is strongly affected by the IncidentAngle Modifier (IAM). Costly measurement procedures are indispensable and make it oftenimpossible to collect a sufficient number of accurate base points to make sure that theperformance of the collector is known under any irradiance condition. Especially asymmetricshapes are subject of investigation, as models of biaxial IAM approximations aremuch in question.A ray tracing simulation tool based on the commercial software OptiCAD is developed inthis work and calculations performed and analyzed. A graphical user interface facilitatesmodeling of the most common collector types. Emphasis is put on complete collectorsrather than simplifications. The resulting IAM graphs are compared with measured dataand existing biaxial approximation methods. Former is performed with the help of a diffusemodel, which evaluates collector performance under diffuse condition based on thecalculated beam IAM values. Moreover, achieved data are implemented in SPF’s softwarePolysun 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 onthe optical performance of a collector.The results show that an introduction of an accurate universal mathematical descriptionof any IAM seems to be impossible and is affirmed by the comparison of calculated withbiaxial approximated values. The great variety of geometric and material propertiesmakes each collector’s curve unique. Further, the traditional IAM definition of flat platecollector may not be applied directly to tube collectors, as it does not consider irradiationentering from the side.Parametric studies show that collector’s inconstant angular absorptance influences overallIAM most, followed by reflector properties if present. In addition, diffuse irradiance atthe measuring site causes inaccuracy as well. Most collectors perform better with purebeam irradiance.