Flat plate solar heaters are some of the oldest technologies used for water. They have advanced over time to increase efficiency. This has mostly been by changing the absorber surface through better selective paints as well the absorber material itself.
As opposed to evacuated tubes which are a series of single independent solar absorbers, flat plate panels offer a single gross area which acts as the heater.
These panels work very well with clean water directly circulating in the capillaries. However, caution has to be taken with salty water or sources with high mineral contents. Direct connection in such cases blocks the capillaries over time. Heating with a heat transfer fluid is recommended to avoid premature failure of the panels.