In order to store water at night or on cloudy days, a water storage tank is needed. A very simple way to use gravity - a thermosyphon system. The principle of the thermosiphon system is that cold water has a higher specific gravity than warm water and the heavier ones will sink. Therefore, the collector is always installed below the storage tank so that the cold water in the tank reaches the collector through a descending pipe. If the collector heats the water, the water will rise again and reach the tank through the rising water pipe at the upper end of the collector. The tank-pipe-collector cycle ensures that the water is heated until it reaches equilibrium temperature. The consumer can then use the hot water at the top of the tank and use the cold water at the bottom instead of any water used. The collector then heats the cold water again. Due to the large temperature difference at higher solar irradiances, warm water rises faster than at lower irradiances. Therefore, the water circulation is almost perfectly adapted to the solar irradiance. The storage tank of a thermosiphon system must be located above the collector, otherwise the cycle will back up at night and all the water will cool. In addition, the cycle does not work properly at very small height differences. In areas with high solar irradiance and flat-roofed buildings, the storage tank is usually installed on the roof.
A thermosiphon system is very economical to operate as a domestic hot water system and the principle is simple, requiring neither a pump nor a controller. However, thermosiphon systems are usually not suitable for large systems, i.e. systems with collector surfaces of more than 10 m2. In addition, in buildings with sloping roofs, it is difficult to place the tank above the collector, and single circuit thermosiphon systems are only suitable for frost-free areas.