HowThermosyphoning systems work:
Cold water enters the heat collector (vacuum collector tube) where it is warmed by the sun. As the water molecules warm up, they become less compacted and therefore less dense. In effect, these molecules become lighter and therefore rise to the highest position. This warmer water then replaces the "heavier" cold water which in turn drops to the lowest position. The process of hot water siphoning through the system is termed Thermo-siphoning. Thus the water circulates naturally through the system.
Solar geysers can either be gravity fed, or pressurized. With gravity fed systems, the tank needs to be relatively high up, whereas pressurized don't need to be. All solar geyser storage tanks are insulated, allowing the water to stay hot overnight, and for an extra day or so, in case of bad weather.