At Thermosifonic system (low pressure), we refer to hot water circulation using natural flow. In solar water heaters, hot water goes to the storage tank, following the rule of physics (gravity), which states that mass of hot water is lighter and thus tends to rise above the mass of cold water, which is heavier.
Therefore, hot water flows to the storage tank, which for this reason is located higher than the collectors. This movement of water occurs throughout the day.