Tell us about yourself!
I looked at the picture and first thing I noticed was that there was too much water in the pipes and insufficient surface made of plastic to really heat it up. I'd say you need it to have smaller diameter pipes and more of them. Problem is that gets heavy on the roof and provides lots more opportunities for leaks. Still I like the concept. Same concept as an undersized radiator on a car or auto AC system. Not enough surface to release or absorb heat, so it doesn't work well enough
The drainage thing you mentioned is a good point. I wouldn't notice that until I finally got the thing together and leak proof with smaller pipes and then wonder why it isn't draining so good. Fine line to any do-it yourself projects.