Forced circulation and DC solar water heating system
Forced circulation solar hot water system
The forced-circulation solar hot water system is to install a water pump on the pipeline between the collector and the storage tank as the circulating power of the water in the system; at the same time, the useful energy of the collector is collected by heating the water and continuously stored in the storage tank. Compared with the natural circulation system, the forced circulation system uses external force to force the water in the collector and the storage tank to circulate. Therefore, the remarkable feature of this system is that the position of the water storage tank is not restricted by the position of the collector, and can be arranged arbitrarily, which can be higher or lower than the collector. The system uses a water pump to mix the hot water produced by the heat collector after absorbing solar radiation with the cold water in the water storage tank, so that the water temperature of the water storage tank gradually rises.
According to the different heat exchange methods of the heat transfer working medium, the forced circulation system is divided into direct forced circulation and indirect forced circulation.
1. Direct forced circulation
Figure 1 shows a direct forced circulation system. In this system, a drainage tank is sometimes added to drain the water in the collector and pipeline when the water pump fails or stops working. There is no heat exchanger in the direct system. Because the forced circulation system relies on the water pump as the circulation power, the system circulates at a fixed large flow rate, so the water in the storage tank is fully mixed during operation, and it can be considered that there is no temperature stratification in the storage tank. Because the temperature stratification in the storage tank is destroyed in the forced circulation solar water heating system, the annual average efficiency of the system is 3%~5% lower than that of the natural circulation solar water heating system.
2. Indirect forced circulation
As shown in Figure 2, the solar heat collector is connected to the upper circulation pipe, the lower circulation pipe and the heat exchange coils in the water storage tank to form a closed loop. A system that heats a certain heat transfer working medium (such as water, ethylene glycol, acetone, etc.) in the circulation loop, and uses the heat transfer working medium to heat water to the user through a heat exchanger. In this way, the heat is transferred, and the user does not participate in the circulation of water, which can ensure the quality of the water. Antifreeze liquid can be added to the heat transfer working medium to prevent the water pipe from freezing and breaking. However, indirect heat transfer through the heat exchanger will lose part of the heat.
DC solar water heating system
The direct current solar water heating system is developed on the basis of natural circulation and forced circulation, as shown in Figure 3. During operation, when the water in the collector is heated to a predetermined upper temperature limit, the electrical contact thermometer at the outlet of the collector will immediately send a signal to the controller, open the solenoid valve, and the tap water will push the hot water that reaches the predetermined temperature out of the collector and flow into the water storage tank. When the electric contact thermometer measures the predetermined lower temperature limit, the solenoid valve is closed, and the system is turned on and off in this way to continuously obtain hot water.
The DC system has many advantages: First, compared with the forced circulation system, no pump is required; second, compared with the natural circulation system, the water storage tank can be placed indoors; third, compared with the circulation system, available hot water is obtained earlier every day, and as long as there is a clear period, a certain amount of available hot water can be obtained; fourth, it is easy to realize the design of the system for emptying and antifreezing at night in winter. The disadvantage of the DC system is that it requires reliable variable flow electric valves and controllers, which makes the system complicated and increases investment. The DC system is mainly suitable for large-scale solar water heating systems.