In the early stages of the development of solar water heaters in China, a solar water heater called “DC” appeared. The characteristic of this water heater is that the cold water is heated to the use temperature at the same time by the collector simultaneously when it flows through the serpentine tube or the collector of the series structure. The system relies on adjusting the flow rate and flow rate of the cold water and the length of the heating path to roughly regulate the temperature of the produced water. The system has a simple structure, high heat exchange efficiency, can realize the separation of collection and storage, and the thermal insulation hot water storage tank is placed in a low position. Moreover, as long as there is the sun, hot water can be obtained immediately, and the hot water output of the system is proportional to the intensity of sunlight. The hot water tank contains only hot water, and the system does not have the problem of mixing hot and cold water. With the technological development of solar energy equipment manufacturing and the improvement of temperature control equipment stability technology, this type of hot water system has been widely promoted and applied in recent years, and is welcomed by the market.
The early appearance of the small DC water heater system is simpler in structure. According to the direction of cold water entering the collector, it is generally divided into two types of solar water heaters, namely, the upper-inlet and lower-inlet solar water heaters, as shown in Figure 1. A “upflow direct-flow solar water heater”.
In the heat collector of the “upflow water heater”, there is a layer of corrosion-resistant fiber fabric; or silicone rubber dampers. When the cold water flows from the make-up tank above the heat collector, the flow is adjusted by the flow control valve Later, when slowly injected into the collector, under the damping action of the wave-shaped flow channel such as the corrosion-resistant fiber fabric, the cold water slowly permeates and flows downward to increase the time to stay in the collector to receive the transfer heat exchange; the water molecules are in the collector. Under the action of gravity, in the process of continuous downward flow, full heat exchange with the metal wall of the collector under the sun’s rays is carried out. When it flows to the lower outlet of the collector, the temperature has basically reached the use temperature, so it enters directly The heat preservation hot water storage tank under the collector is reserved for use.
The direct-flow water heater that enters the cold water from the downward direction and the reverse upward into the heat collector, as shown in Figure 2, is generally called a “thermosiphonic water heater”. In this system, the flow direction of cold water is just opposite to that of the upward flow. Under the control of the maximum water level pressure of the make-up water tank, the cold water enters the collector from the lower port in the reverse direction, and the water flows through the serpentine metal pipe in the collector from bottom to top in the opposite direction. The heat transferred from the metal pipe wall of the collector continues to heat up. When it reaches the hot water outlet pipe at the top of the system, it is level with the control height of the make-up water tank. If the water temperature is still low at this time and the cold water density is still high, turn it over. However, the highest point of the thermosyphon; the water in the collector is forced to stop moving and continue to be heated by the collector; after the water temperature reaches above the set temperature, the density of the water becomes smaller and the volume becomes larger. The water level in the hot water outlet pipe is higher than the highest control point of the thermosiphon, which generates a thermosiphon phenomenon, and the up-to-standard hot water in the collector is automatically pumped to the low-level heat preservation storage under the cooperation of the cold water potential and pressure of the make-up water tank. Keep in the hot water tank for later use. After the subsequent water temperature decreases, the volume shrinks, and the fluid is blocked by the siphon again and the flow is cut off; the system enters the state of boring and warming again. The biggest problem of the above two types of small DC water heaters is that the water temperature cannot be accurately and quantitatively controlled. In particular, the upper water inlet system must be controlled by the user according to the temperature and solar conditions of the day, artificially adjust the flow rate and flow of the cold water, so that the system can operate effectively. Before sunset in the afternoon, users have to cut off the cold water source in time to avoid a large amount of cold water being mixed into the heat preservation hot water storage tank, causing the entire tank to drop in temperature and become unusable.
In the design of modern solar hot water system, the direct current system is almost no longer applied to a small single-family system, but is used to construct a large-scale system with a group of tens of square meters of components. This kind of large-scale DC system first combines the standard components of the collector into a system with no more than 3 to 5 flat plates in each group by using a parallel connection method, and then according to the cold water pressure, cold water temperature, sunshine intensity, and climate characteristics of the system in the installation area , The required water temperature and other empirical parameters, and then according to actual needs, the standard parallel collector components are connected in series to form a direct-flow water heater collector array; this is used to artificially control the length of the cold water heating path, the flow rate and flow rate of the system In order to achieve the purpose of controlling the temperature of the produced water and the hourly hot water output in advance, so as to improve the daily hot water output of the system and the light-to-heat conversion efficiency of the system. In order to ensure the quality of the temperature of the water produced by the system heating, it is generally necessary to pre-bury the collector on the collector adjacent to the hot water outlet in the direct current system near the end of the process, and the last group of the main runner in a parallel system. Install a high-sensitivity temperature sensor probe to dynamically monitor the temperature of the flowing water at any time and control the actual temperature of the hot water produced. When the temperature of the flowing water is lower than the set water temperature by 3℃~5℃, the control system will automatically close the cold water inlet electric valve or solenoid valve, and switch the system to the heating state; when the water temperature returns to the set temperature value, Restart the cold water inlet electric valve or solenoid valve to restore the running of the water flow that heats up the temperature of the DC system along the way. The direct-flow system has a simple structure and high thermal efficiency; the heat preservation hot water storage tank can realize the separation of collection and storage, and it can be installed at a low position; however, because water is a poor conductor of heat, when the system finds that the heating water temperature is lowered, the cold water source is cut off, forcing the system After turning to the sallow state, if the temperature probe is installed at an improper location, the control system will not receive the signal of the water temperature rise for a long time, which will delay the normal operation of the system. Therefore, the direct current system in areas with poor solar radiation intensity, especially the thermal insulation effect of the collector is not good, the heat capacity of the collector is small, and the climate is cloudy with flickering and sunny, diffuse reflection of the sun. The effect is relatively poor when used under the following conditions. In order to overcome the above shortcomings, we can add a so-called “tube-plate natural circulation system integrated solar collector-water heater” to the traditional “DC solar hot water system”; The circulatory system uses it to provide instantaneous recovery of sunlight and the information that the system’s water temperature has reached the standard, so as to wake up the control system immediately and re-enter the normal working state. The system with the above-mentioned functions is called “DC-Natural Circulation Constant Temperature Water Replenishment System”. This technically transformed, high-efficiency, large-scale, direct-flow-natural circulation constant-temperature replenishment solar hot water system is shown in Figure 3; it has a higher light-to-heat conversion efficiency and a lower system cost. The prototype of this type of system was first effectively used in large-scale hot water projects in Yunnan Province in 1978; after continuous improvement and improvement, it was successively promoted and applied to many large-scale solar energy projects in Yunnan Province. Shared hot water system and received good reviews. Practice has proved that the heat production efficiency of the system, the simplicity, flexibility, reliability of the system structure, and the excellent cost performance are far superior to the simple direct-flow system of the same scale structure and the traditional natural circulation system and forced circulation system of the same water production volume. .
In Figure 3, a direct-flow solar hot water system is connected in parallel by two standard plates first, and then connected in four stages in series; in order to improve the temperature control sensitivity of constant temperature replenishing water, it is necessary to connect a piece of the same raw material in series on the last stage of the system “Tube-plate natural circulation system integrated solar collector-water heater” manufactured. This type of collector-water heater realizes a plate, which is a flat plate-type collector-water heater with natural circulation heat exchange function; therefore, after the direct current system is repeatedly heated by four stages in series, it is finally monitored that the water temperature is not up to the standard, and the temperature is not up to the standard. After the control switch cuts off the cold water make-up valve, and the system is converted to a dull state, the water in all parallel and series plate cores in the system stops normal flow. At this time, only the water in the last flat plate-type water heater can continue to follow the law of natural circulation convection heat transfer. Under the diffuse state of sunlight, as long as there is a temperature difference, effective convection circulation can be continuously performed without interruption. Heat; and instantly transmit the temperature change information to the temperature sensor located at the other end of the header on the board; after the sensor senses that the temperature of the board core has risen to the artificially set standard temperature, the cold water supply valve will It was restarted immediately; the system immediately resumed normal operation. The original slow-response direct current system, after the introduction of new system integration technology for transformation, can greatly improve the sensitivity of the direct current fixed-temperature replenishment system temperature control, and on the premise of ensuring the quality of the system’s water supply temperature, the system’s light-to-heat conversion efficiency is improved ; Simplify the system structure design and system assembly work installation engineering quantity and system cost. This type of system was successfully developed by a company to form a new product of “flat-plate natural circulation system integrated solar-water collector”. After being incorporated into large-scale field promotion and application, it has achieved a series of obvious social Economic benefits. At present, the system is also widely used in high-rise building floor network mutual aid system and other industrial and agricultural low-temperature heat utilization projects.