This is a very simple design for a batch-type solar water heater that uses a thermosiphon loop to move water between a solar thermal collector and a storage barrel. While it’s not the most efficient method for solar water heating and works rather slowly, it effectively demonstrates the operation of a thermosiphon and its connection to hot water storage.
The system can provide useful amounts of hot water for an outdoor shower, washing garden produce, keeping a biogas generator warm, or any number of other uses you can think of. Because it uses water, the system must be drained when the weather turns cold.