The forced circulation solar hot water system is a system in which a water pump is installed on the pipeline between the collector and the storage tank as the circulating power for the water in the system; At the same time, the useful energy benefits of the collector are continuously stored in the water storage tank by heating water. 
During the operation of the system, the start and stop of the circulation pump must be controlled, otherwise it will waste both electrical energy and thermal energy. Usually, temperature difference control is more common, and sometimes both temperature difference control and photoelectric control are applied simultaneously. 
Temperature difference control is the use of the temperature difference between the outlet water temperature of the collector and the bottom water temperature of the storage tank to control the operation of the circulating pump. 
After sunrise in the morning, the water inside the collector is heated by solar radiation, and the temperature gradually increases. Once the temperature difference between the outlet of the collector and the bottom of the water storage tank reaches the set value (usually 8-10 ℃), the temperature difference controller sends a signal to start the circulation pump, and the system begins to operate; When encountering cloudy days or before sunset in the afternoon, the solar irradiance decreases and the temperature of the collector gradually decreases. Once the temperature difference between the outlet water temperature of the collector and the bottom water temperature of the storage tank reaches another set value (usually 3-4 ℃), the temperature difference controller sends a signal to turn off the circulation pump and the system stops running. 
When using hot water, there are also two methods for taking hot water: the top water method and the water drop method. 
The top water method is to replenish cold water (tap water) to the bottom of the water storage tank and use hot water from the upper layer of the water storage tank; The drowning method relies on the gravity of the hot water itself to fall from the bottom of the storage tank for use. Under forced circulation conditions, due to the sufficient mixing of water in the storage tank, there is no obvious temperature stratification, so both the top water method and the bottom water method can obtain hot water from the beginning. Compared with the falling water method, the advantage of the top water method is that the spraying of hot water under pressure can improve the comfort of users, and there is no need to consider the issue of replenishing water to the storage tank; The disadvantage is that the cold water entering from the bottom of the water storage tank will mix with the hot water inside the tank. The advantage of the drowning method is that there is no mixing of cold and hot water, but the disadvantage is that hot water falls by gravity, which affects the comfort of the user, and the problem of replenishing the water storage tank must be considered every day. 
In a dual loop forced circulation system, the heat exchanger can be either an immersion heat exchanger placed inside the storage tank or a plate heat exchanger placed outside the storage tank. Compared with immersion heat exchangers, plate heat exchangers have many advantages: firstly, they have a larger heat transfer area, smaller heat transfer temperature difference, and less impact on system efficiency; Secondly, the plate heat exchanger is installed in the system pipeline, which provides greater flexibility and facilitates system design and layout; Thirdly, plate heat exchangers have been commercialized and standardized, with easy quality assurance and good reliability.