Solar Analysis for Curved Surfaces

Hello. I would like to perform a solar radiation analysis for a curved arch with openings, as shown in the image below.

However, I received an error stating that the openings are not in the same plane as the curved arch.

I believe the cause is that the mesh is generated based on the finite element method, which prevents the pure curved surface from being accurately reproduced.

I would appreciate your guidance on how to model the curved arch without creating a mesh.

Note: The openings were created by subtracting a sphere from the curved arch.