The horizontal ball valve will create a pressure differential between the input and output of the panels which then creates the flow through the panels. Even if it is completely open, there should still be enough pressure difference to cause at least a little bit of flow through the panels.

The vertical valve should only be needed if you want to isolate the panels and shut them off. However, it should be left fully open when running the panels because that path already has more head loss than the bypass route so water flow will always favor the bypass unless the horizontal valve is partially closed.