Experimental characterization of the diffuse incident angle modifier of solar thermal collectors: Improving consistency between test methods

Diffuse solar irradiance is essential for modeling solar energy conversion devices, especially solar thermal collectors. The globally accepted ISO 9806:2017 standard defines a thermodynamic model and two test methods to determine its parameters, the steady-state test (SST) and the quasi-dynamic test (QDT). Although both methods are generally considered equivalent, discrepancies between the values of the diffuse incident angle modifier (IAM) have been reported, representing an area for improvement. This study advances the experimental characterization of diffuse IAM, specifically improving the compatibility between SST and QDT methods. Using the ISO 9806:2017 model as a baseline, two alternative diffuse IAM models are introduced and experimentally evaluated with data from a flat plate collector and an evacuated tube collector, covering different technologies. Model 1 extends the SST diffuse IAM model to QDT and treats diffuse irradiance in a global manner, while Model 2 treats diffuse irradiance from sky and ground separately, requiring an additional solar measurement. The evaluation shows that both proposed models improve the consistency between test methods. As the performance differences between these two new models are minimal, Model 1 is the recommended option as its implementation is simpler.