comparison of tubing for heat collection

This article reaches some interesting conclusions on tubing types heat collection for solar.

solar tubing heat transfer comparison.

Here is another one that reaches a surprising conclusion.  Various plastic tubing including  PEX tubing is nearly as good as good as copper for solar collectors.  When I originally found this, I found 2 other articles that reached similar conclusions.(have since had a crash and lost the other 2 bookmarks but there were multiple sources.)  Provide the tubing was thin wall like the common plastic tubing varieties and surrounded by the hot air environment inside a collector the combined forms of heat transfer mean that they are nearly as good as copper for heating water in a collector in spite of way lower thermal conductivity.  Cost savings over copper is great and risk saving where the PEX will potentially take freezing without breaking are both valuable.