A Thermosyphon Solar Water Heating System takes advantage of the fact that hot water rises and cool water sinks. As water is heated it expands and so, as gravity pulls down the relatively heavier cool water molecules, the warmer molecules rise up.
The diagram below shows how this works in a real world thermosysphon solar water heating system. Such a system offers the advantage that no pump or controller is required to run everything and so it is very simple and less liable to failure.
During the day water in the solar collector (flat panel or evacuated tube) is heated by the sun. As the water heats up, it rises pushing up into the insulated tank. Cooler water at the bottom of the tank is then pushed down to re-enter the bottom of the solar collector where it is heated further. This cycle continues throughout the day with the water stored in the tank getting hotter and hotter.
Pictured above is a shematic of a more advanced type of thermosyphon solar water heater which couples a standard electric immersion element heated hot water tank together with the solar heating system and its tank. Therefore, when there is not enough sunshine to meet your hot water requirements, the electric heater will turn on and heat up the water (in the backup tank) further.
To keep this a purely renewable energy water heating system, a wind turbine can be used as described in our article: Wind Turbine Water Heating using a low voltage immersion heater element. When it is cold and windy the water will be heated by wind power, and when it is still and sunny the water will be heated by solar power. (A second, mains powered, heating element could be added to guarantee unlimited hot water in all seasons and weather conditions.)