Comparison of theoretical and isotropic models of incoming solar radiation for the tilted, fixed, flat-plate solar collector southoriented

Knowing the intensity of incoming solar radiation is the starting point for any further analysis of the thermal performance of (all) solar collectors. A large number of existing theoretical models (both isotropic and anisotropic) can be justified by the complexity of determining its components, because the total solar radiation for an arbitrarily oriented surface of the solar collector consists of: direct, diffuse and reflected solar radiation. In the first part of the paper, three theoretical and isotropic models of incoming solar radiation are presented in detail, for the tilted, fixed, flat-plate solar collector oriented to the south, presented in the works of the following authors: LUKIĆ-BABIĆ, WIDÉN-MUNKHAMMAR and DESPOTOVIĆ. In the second part of the paper, using the mentioned methods, the incoming solar radiation was calculated by comparing the obtained results, all with the aim of facilitating (by pointing out the advantages and disadvantages of their use) other researchers to select the appropriate solar model in theoretical calculations of the solar collectors.