Solar Geysers, or solar water heaters, operate on a principle of natural thermal convection. This is a process which causes warm water to rise, and cooler water to drop to the lowest point.
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.
Thus the water circulates naturally through the system.