Over 780 million people don't have access to clean drinking water. It is contaminated by germs, parasites and chemicals, leading to many easily preventable diseases. There are both high-tech and low-tech water treatment devices, who are either very expensive, difficult to set up and operate, or not very effective in removing pathogens.
In my little project, I'll design and build a device, that’s cheap to build, easy to operate and very effective. It will work like a solar thermal collector, boiling the water flowing through it. Additionally it doesn’t absorb the UV-radiation from the sun, so that no heat resistant germs can grow inside the pipes, even while the device is not in use.
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.
To keep the heat from convecting back out of the panel, there must be some kind of transparent cover for it. Usually you'd use glas or special acrylic glas, but that would be too expensive for such a project. So I decided to cut out the straight segment of PET water bottles and iron it. To really get the segment as flat as in the picture, you need to put the segment between two cotton cloth and carefully iron it on the highest level with steam and pressure.
For the project you need about 25-30 pieces.
After you have enough to cover the panel, you can glue the pieces together with silicone, or weld them together with a soldering iron. For the latter method you need to put a piece of baking paper between the pieces and the soldering iron.
After finishing that, you should have a nice big plastic cover for the panel, which you can fix to the panel frame with some nails and silicone glue.