You would need a fan to move the air. The warm air is at the highest point of the piping, and so will just sit there.
Actually, I just looked at the picture again. I didn't notice the slope to the ceiling on the wall side. You should get some air flow that way. The problem is that only a certain amount of light/heat comes in through the window, so you aren't getting any more heat than you get through the window itself. You are circulating the air somewhat, but a small fan would do the same thing.