Many thanks for the opinions.

I'd guess that, just as copper or aluminum collectors would be more efficient in gaining solar energy they'd also be more efficient in shedding heat. Of course if I had access to a large number of truck radiators and fans....

But metal collectors would be far, far more expensive, and there'd be a greater risk of freeze damage as opposed to the plastic sort. Do any of you users out there have drain-down provisions for frosty nights, or are the collectors pitched so that they drain themselves?