The incidence angle modifier (IAM) of a solar thermal collector for diffuse irradiance is usually determined under the simplifying assumption of isotropic sky and ground radiance. It is applied as one constant collector parameter, independent from slope or weather conditions. The simulation model introduced here considers the varying anisotropy of sky radiance. To create realistic distributions, the approach of Brunger and Hooper is used. Three modes are possible: Mode 1 calculates separate IAMs for anisotropic sky (for every time step) and isotropic ground. Mode 2 calculates separate IAMs for isotropic sky and isotropic ground (once per simulation). Mode 3 uses a user-specified isotropic IAM-value for the collector hemisphere. The model is applied to a stationary, double-covered process heat flat-plate collector with one-sided CPC booster reflector (RefleC). This collector shows a biaxial and asymmetric IAM for direct irradiance. It is found that, compared to anisotropic modeling, the simplified isotropic model is undervaluing the annual output of this collector by 13.7 % for a constant inlet temperature of 120 °C in Wurzburg, Germany. At 40 °C inlet temperature the undervaluation is 9.3 %. For the basis flat-plate without reflector the undervaluation is 7.5 % at 120 °C and 3.3 % at 40 °C. An annual irradiation distribution diagram shows that this is due to an underestimation of diffuse irradiation from directions with high direct irradiation. Detailed results reveal that for RefleC the IAM for anisotropic diffuse sky radiance can vary by up to approx. 25 percentage points during one day. It is concluded that isotropic modeling of diffuse irradiance can be expected to significantly undervalue the annual output of all non-focusing solar thermal collectors. Highest relevance is found for high collector slopes, complex IAMs and at low-efficiency operation. The optimal collector slope is almost not affected. Accuracy of existing models can be increased by applying Mode 2.
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