[Daox]

The cans will probably make your collector throw off less heat.

What you have in a solar collector is pretty simple. You have a window X by X large. Sun hits the material inside the collector and heats it up. What you want to do is efficiently move the heat away from that material. In this case you're using air to remove the heat. So, you essentially want some form of heat sink which means lots of surface area. But, you also don't want to restrict airflow. I see your cans restricting airflow. IF they had good surface contact with the downspout, they could help spread the heat out and give more surface area for the air to pick up heat off of. But, since they're just sitting inside the downspout I don't see them transferring much heat away from it and into the air.