Natural circulation solar water heating system refers to a system that uses solar energy to make the heat transfer working medium in the system naturally circulate and heat between the collector and the storage tank or between the collector and the heat exchanger. The power of the system circulation is the thermosiphon effect caused by the density change caused by the liquid temperature change. Due to the large resistance of the indirect system, the thermosiphon effect cannot provide sufficient pressure head, so the natural circulation system is generally a direct system.

During the operation of the system, the water in the collector is heated by solar radiant energy, the temperature increases, and the density decreases. The heated water gradually rises in the collector and enters the upper part of the water storage tank from the upper circulation pipe of the collector; At the same time, the cold water at the bottom of the water storage tank flows into the bottom of the collector through the lower circulation pipe;

In this way, after a period of time, the water in the water storage tank forms obvious temperature stratification, and the upper water reaches the usable temperature first, until the water in the entire water storage tank can be used.

Commonly used natural circulation systems can generally be divided into two types: natural circulation (see Figure 1) and natural circulation with constant temperature release function (see Figure 2). The disadvantage of the natural circulation system is to ensure the level difference between the water storage tank and the heat collector, which is not very beneficial for building integration, especially the sloped roof, which not only has difficulties in installation and construction, but also affects the appearance of the building. In this system, the greater the density difference of the cycle, the faster the cycle speed, and vice versa, the slower the cycle. When the solar radiation stops, the cycle gradually ends. Therefore, in the natural circulation hot water system, the thermosiphon head is the key factor. In this system, the greater the height difference between the water storage tank and the heat collector, the greater the thermosiphon head, but the temperature difference of the water and the height difference between the water storage tank and the heat collector are often not very large, therefore, the circulating power of the system is often limited. The natural circulation fixed-temperature water release type is an improvement of the natural circulation hot water system, which divides the hot water storage tank of the natural circulation system into two water tanks. The water tank used for the heat collection cycle is small in size and easier to be heated. When the temperature of the small water tank reaches the set temperature, the water is put into the large water tank, so the large water tank is always hot water. The water tanks of these two systems must be higher than the collectors, the location is not easy to arrange, and it is not suitable to be integrated with the building design.

The advantage of the top-water method is to make full use of the characteristics of the upper layer of water being heated first, and the user can get hot water from the beginning; the disadvantage is that the cold water entering from the bottom of the storage tank will be mixed with the hot water in the storage tank, reducing the available hot water. The advantage of the falling water method is that there is no mixing of hot and cold water, but the disadvantage is that the hot water must be drained from the bottom of the storage tank before the hot water can be taken, so it wastes water and heat.

The advantages of the natural circulation system are simple structure, reliable operation, no power required, and low cost; the disadvantage is that in order to maintain the necessary thermosiphon head, the water storage tank must be placed above the collector, which sometimes brings some troubles to the building layout, structural load-bearing and system installation. The natural circulation system is mainly suitable for domestic solar water heaters and small and medium-sized solar water heating systems.