Interesting idea, but if you don't mind I'd like to poke some holes in it.
First off, Gary did do
some testing to see the efficiency hit from going with copper-copper to copper-aluminum, and there was only a ~4% efficiency loss. For the price of a copper-aluminum panel, you can easily make one more panel to make up the difference in heat output and still be way under cost for copper-copper of the same output.
Second, you want a parabolic lense to properly reflect the sunlight at the pipe. A semi-circle will not do nearly as well.
Third, PVC will melt at the temps a solar hot water panel sees, especially during stagnation.