For water heating we use a flat plate collector. This is easy to build. It works with diffuse radiation and does not need to follow the sun during the day. The discharge of the boiled water from the system is controlled by the difference in density between steam and water. The water heats up in the collector to about 100°C. As soon as a small amount of the water evaporates in the collector, the density of the water-steam mixture in the collector drops sharply. This causes the steam to flow out together with the heated water. Cold water flows into the collector and the process starts all over again.
Thanks to this principle, the device can boil up to 50 litres of water per day. The material costs for the system are about 200 €. A more detailed description of the operating principle and the dimensioning of the system and individual components can be found in the following publication:
Dietl, Jochen ; Engelbart, Hendryk ; Sielaff, Axel (2015). A Novel Type of Thermal Solar Water Disinfection Unit. http://tuprints.ulb.tu-darmstadt.de/4460/.