These pages cover an experimental solar collector for heating water that uses aluminum heat absorbing fins and copper pipes for conveying the heat transfer fluid.
Copper pipe solar water heater.
The key to good performance in this design is a good thermal connection between the aluminum fin and the copper pipe a lot of attention is given to providing this good thermal bond.
Resembles corrugated flexible stainless steel pipe used in gas lines it has recently found its way into solar water heating application.
Flexible high grade stainless steel tube 316l or 304.