General Incident Angle Modifier Model for Quasi-Dynamic Testing of Solar Thermal Collectors
Abstract
The thermal performance of solar collectors is commonly evaluated using standardized procedures, with ISO 9806 being the most widely applied. This standard defines two testing methods: steady-state testing (SST) and quasi-dynamic testing (QDT). While SST requires clear-sky conditions, QDT allows testing under variable weather conditions but introduces challenges in modelling the incident angle modifier (IAM) for beam radiation. Several IAM models have been proposed within the framework of the QDT method, yet no consensus exists on the most suitable one, leading laboratories to rely on empirical choices for each specific technology, which compromises result comparability. To address this limitation, a general IAM model was recently proposed and successfully applied to both flat-plate and evacuated tube collectors, improving accuracy across a wide range of incidence angles, though at the cost of an increased number of fitting parameters. This paper builds upon that work by evaluating the model’s applicability to evacuated tube collectors with CPC reflectors and by performing a sensitivity analysis of the angular discretization step, which directly influences the number of fitting parameters. The results show that the model is suitable for application to this technology, advancing its general applicability, and that an angular step of 10°, as recommended by ISO 9806, is sufficient to accurately represent IAM curves for evacuated tube collectors while limiting the number of fitting parameters and maintaining manageable parameter identification procedure.
Keywords
solar thermal collectors, incident angle modifier, ISO 9806