The Solar Ray Tracing Model
The solar load model’s ray tracing algorithm can be used to predict the direct illumination energy source that results from incident solar radiation. It takes a beam that is modelled using the sun position vector and illumination parameters, applies it to any or all wall or inlet/outlet boundary zones that you specify, performs a face-by-face shading analysis to determine well-defined shadows on all boundary faces and interior walls, and computes the heat flux on the boundary faces that results from the incident radiation. The solar ray tracing model includes only boundary zones that are adjacent to fluid zones in the ray tracing calculation. In other words, boundary zones that are attached to solid zones are ignored. The resulting heat flux that is computed by the solar ray tracing algorithm is coupled to the ANSYS FLUENT calculation via a source term in the energy equation.
DO Irradiation Model
The solar load model’s discrete ordinates (DO) irradiation option provides you with an easy means of applying a solar load directly to the DO model. Unlike the ray tracing solar load option, the DO irradiation method does not compute heat fluxes and apply them as heat sources to the energy equation. Instead, the irradiation flux is applied directly to semi-transparent walls (which you specify) as a boundary condition, and the radiative heat transfer is derived from the solution of the DO radiative transfer equation.
I hope now you would have understood the physics and the difference between the two models. Hope this helps you to solve your problem.