“Result would be high efficiency on the surface exposed to sunlight.”
That would be the end face of the fiber. For a 1mm dia fiber, the area is small.
Just thinking out loud here and I apologize as this is not my field of engineering (Civil Engineers build targets), but I would think that you would need to lay the fiber bundle in a small, highly reflective tray similar to a oversize rain gutter so that you can focus the light on to the fiber array.
But I am sure they already thought of this...