As a heat absorber, I decided to use carbon. It's not very efficient, because it is basically a black body radiator. This means that its radiation absorbtion and emission coefficients are nearly the same, so the hotter it gets, the more power is again lost to radiation. Modern solarthermal absorbers have a much lower emission coefficient than their absorption coefficient, so they can reach higher temperatures with lower solar radiation.

But thats good enough for our application, as the regions where there is a need for this kind of water treatment aren't known for their cold climate.

To let the charcoal powder stick to the wooden baseplate, we need a binder. I used a simple flour-sugar-water glue and added charcoal powder to it. You need about a 3/4 liters of this paint to cover the whole panel. So to make the paint I used 1/2 liters of water, 5 spoons of flour, 5 spoons of sugar and 20 spoons of charcoal powder and boiled the mixture for a minute.

Then I applied the paint evenly to the panel in two layers and let it dry for a day. I had very good results with this absorber, without any cover it got to about 70 degrees celsius in half an hour of sunlight.