Thermosiphon is a type of passive solar water heating that utilizes convection to heat water. Water passes through a closed-loop circuit that is vertical and thus needs no mechanical devices or pumps.
It is convection that is key to the proper function of a passive thermosiphon system. Convection, from solar heated water, forces the hot water up to the top of the loop while gravity pulls the cooler water down. This constant exchange of heat and energy creates a natural pumping system. Water for household use is then drawn into the home from the top of the loop.
Solar collectors may heat the water directly (i.e., water passes directly through the collector) or indirectly (i.e., a heat transfer fluid, such as antifreeze, is used to exchange heat between the collector and the water in the thermosiphon loop).
Gravity is essential for the proper functioning of a thermosiphon system. Therefore, orientation of the system is of vital importance. Furthermore, because the flow and interchange of hot and cold water is vital, water must be stored in a tank at the top of the collector. In this way cool water is able to flow down and warm water forced up into the tank, completing the cycle.
Thermosiphon / Integral Collectors