I have built half a dozen solar hot water heaters, active solar systems with no pumps. They all worked well, I built some in the late 70s and one a couple of years ago. Copper is the best but it will not tolerate freezing water. If you will be using a thermosiphon (no pumps) system and you are sure you will drain your system before the first hard freeze go with copper. Personally, I'm interested in pex and might build one to see how it holds up mostly because of the freezing issue. The last system I built for an organic farm (not for profit) and it keeps busting out either the line or a sweated fitting. Also, I use glass from discarded patio doors and build the collectors to fit them. They are free, often easy to find, easy to use and the most efficient. I used a fiberglass product called Calwall in 1979 which lasted 10 years but turned an ugly yellow, cutting down the efficiency, I'm sure.
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