Solar thermal collector

A solar thermal collector of the type including a collector body having an absorber surface adapted to be exposed to sunlight and be heated thereby and a having a liquid flow passage there though adjacent said absorber surface, characterized in that said collector body is formed of body portions of material selected from polymer and polymer composite and adhesively bonded together at two or more sets of complementary bonding surface portions with spaces between said sets, a substantially continuous said liquid flow passage being defined in adjacent surfaces of said body portions in said spaces, said liquid flow passages being of a flattened cross section, whereby the transverse cross section of the flow passage has a dimension parallel to the absorber surface greater than the dimension perpendicular to the absorber surface passage in a ratio of not less than 3:1 parallel dimension:perpendicular dimension, said bonding surface portions having complementary surface profiles selected to provide a bond section across said bonding portion which is longer than the width of said bonding surface portion.