Natural circulation solar water heating system is the most widely used in developing countries, and it is also one of the most popular solar thermal utilization technology products in the world. This system uses fluid (water) as the heat-carrying medium. When the fluid is heated, its temperature rises, its specific gravity becomes lighter, and its density becomes smaller. It automatically generates the physical phenomenon of thermosiphon convective circulation heat transfer; and uses this as the cyclic power. Carry out solar light-heat conversion to obtain solar radiant heat. This kind of system only needs to meet one prerequisite, that is, the system only needs to be equipped with a high-position heat preservation circulating hot water storage tank that is conducive to temperature stratification and a circulating pipeline connecting the collector and the water storage device, and ensure that the entire system is in place. The open state connected to the atmosphere can ensure the natural occurrence and effective operation of the thermosiphon convective circulation heat exchange phenomenon. The device shown in Figure 1 is a typical natural circulation solar hot water system. This system generally consists of a (group) solar flat plate collector, a high-position insulation circulating hot water storage tank, a set of float valve control feed water tank and corresponding tripod system (including collector tripod and insulation water tank tripod ) And other four major assembly components. In the thermal insulation circulating hot water storage tank, there should generally be 7 pipe joint holes: cold water inlet, hot water outlet, upper circulation hole, lower circulation hole, drain hole, vent hole, overflow hole, etc. The solar flat plate collector is connected with the heat preservation circulating hot water storage tank through the upper circulation pipe and the lower circulation pipe to realize the diagonal circulation loop connection of both sides.
The natural circulation solar hot water system does not require more intervention measures, and does not consume any auxiliary energy, except for the need to set up a float valve to supply the water tank to control the water level. After the system is connected to the cold and hot water pipes, it can run automatically and safely and stably for a long time. The system has a high degree of automation, and the solar light-heat exchange efficiency is much higher than that of the boring system.
The power of the natural circulation system for temperature difference convection heat exchange comes from the specific gravity of the cold and hot heat-carrying medium fluid in the communicating container. Due to the uneven heating at both ends of the communicating container, the temperature difference between the fluid molecules is caused, and the specific gravity of different temperatures leads to the appearance of different thermosiphon pressure heads. In a natural circulation system, the thermosiphon force obtained by the system can be obtained by the following formula:
HT =h×(r2－r1) (1)
Where: HT——thermosiphon pressure (kg/㎡);
r2——The specific gravity of cold water (kg/m3);
r1——The specific gravity of hot water (kg/m3);
h——Average height difference (m), that is, the height difference between the center line of the collector and the center line of the heat preservation and heat storage circulating water tank.
Obviously, when the temperature difference of Δr = r2-r1 is constant, the thermosiphon pressure head HT depends on the value of h. Only when the HT value is large enough to overcome the total head loss of the system, the natural circulation or thermosiphon phenomenon will occur. In other words, whether the natural circulation system can operate depends on the minimum temperature difference that the heat transfer medium fluid can achieve and maintain, the average height difference between the water tank and the collector, the water resistance of the system circulation pipeline, and the overall system Objective factors such as the quality of the insulation effect.
As for the light-to-heat conversion efficiency (η) of the natural circulation system, it is determined by the percentage between the useful heat absorbed by the collector and the total solar radiant energy projected on the collector:
η =[ G×(T2－T1)/ A • I ]×100% ——(2)
In the formula: G——Total water capacity of the system;
T1——The average temperature of the water tank holding water at sunrise of the system;
T2——The average temperature of the water tank at sunset;
A——The effective heat collection area of the collector;
I——Total daily solar radiation.
According to the two formulas (1) and (2), we can draw some points that should be paid attention to when designing and constructing a natural circulation system: a single natural circulation system, the combined structure of the system should not be too large, and the exposed upper and lower circulations The pipe should be as short as possible. When other factors remain the same, an overly large system symbolizes the need to install a thermally insulated circulating hot water storage tank with a higher potential and a larger volume, a larger collector array and a more complex circulating pipe network system. According to many years of practical experience, the general natural circulation shared hot water system, the collector area should be controlled within 40 square meters, and the total volume of the water tank is about 3t. As far as possible, the parallel method of collectors can effectively reduce the water resistance along the way, but each row of parallel collectors should be controlled to no more than 5~6 flat plate collectors, because too many parallel combinations will make the middle section of the parallel connection. The heat collector has a high-temperature stagnation phenomenon of uneven flow and unbalanced water resistance, which affects the hot water production of the entire hot water system and the improvement of the system’s thermal cycle efficiency.
For a natural circulation system that requires a large heat collection area and hot water output, it can be divided into parts and reorganized into several relatively independent sets of subsystems to operate to ensure the flexible layout and normality of the entire large system Running. Or it can be assembled into a number of small systems with natural circulation and constant temperature water (replenishment), and then the standard hot water of each of its small systems is concentrated in a total thermal insulation hot water storage tank that can be installed in a low-level concealed manner according to the load-bearing requirements of the building. Meet the actual needs of more users. Because this small system of natural circulation and constant temperature water release (replenishment) is not only more flexible to build, but also can effectively improve the overall thermal efficiency of the large system and the total output of hot water. Figure 3 is a diagram of the natural circulation “fixed temperature water release” system; and Figure 4 is a natural circulation “fixed temperature water replenishment” system diagram.
The so-called natural circulation constant temperature discharge water and natural circulation constant temperature replenishment, although both belong to natural circulation systems, the difference between the two systems is: First, the natural circulation constant temperature discharge system must have a constant temperature discharge control valve installed in the circulating water tank On the hot water outlet pipe of the natural circulation constant temperature replenishment system, the solenoid valve or electric valve that functions similarly must be installed on the cold water inlet pipe of the circulating water tank; The natural circulation system is the same. It is realized by a set of conventional float valve replenishment tanks, while the natural circulation constant temperature replenishment system omits the float valve replenishment tank and is directly controlled by a temperature sensor embedded near the hot water outlet of the thermal insulation circulating water tank. The electric valve installed on the cold water inlet pipe, when the sensor feels that the temperature at the outlet of the water tank is lower than the set temperature, it will immediately cut off the cold water entering the water tank, stop squeezing hot water, and the system will turn to normal natural circulation operation. . As we all know, the control valve system with large heat capacity is installed on the cold water pipe, which can effectively reduce system heat loss, improve system efficiency, reduce equipment cost and extend the service life of pipe valve parts. Considering from this perspective, it is obvious that the constant temperature water make-up system has less heat loss than the constant temperature water discharge system, and the equipment investment and use costs are also lower, so there are more desirable points in the system design.
The common point of the above two systems is: the function of the high-position heat preservation circulating hot water storage tank of the original large natural circulation system can be divided into two, and the system only needs to retain a small volume (≤ 200L~300L), which is only equivalent to one A high-position insulation circulating water tank for household use. Separate the large water tank, which is mainly responsible for heat preservation and storage, and install it on the load-bearing structure of nearby buildings in a low position to achieve a certain degree of separation of collection and storage, reduce the height of the center of gravity of the water tank, eliminate visual pollution, and reduce potential safety hazards.
For a constant temperature discharge (make-up) water system, the thermal efficiency of the system is directly related to the efficiency factor of the collector. The sealing performance of the collector, the heat transfer efficiency of the plate core, the size of the heat capacity of the plate core, and the reasonableness of the system setting temperature may directly affect the thermal efficiency achieved by the system. In areas where the solar intensity is not high, if the structure of the collector itself is defective, the heat capacity of the plate core is too small, and the system design is unreasonable, the heat generation effect of the system will not be ideal. However, in any case, the use of large-scale or super-large natural circulation systems to reduce the height of the center of gravity of large-scale thermal insulation circulating hot water storage tanks, simplify the system structure, and improve the overall light-to-heat conversion efficiency of the system is still the same. Kind of the best choice.
A question that needs to be considered and answered after the new system is built up after rectification and processing is to separate it and place it in a large water tank at the side. What should I do if there is a considerable amount of overnight heat dissipation water that cannot be used up in the same day? ? There are two ways to solve this problem: First, install an electric heating device near the hot water outlet of the large water tank, but install the temperature sensor at a position slightly lower than the electric heater. If the hot water produced by the system mixes with the water in the water tank on the same day, and the water temperature near the hot water outlet of the water tank still does not reach the operating temperature, the system will automatically start the electric heater, and the part that needs to be provided to the user immediately Warm water is raised to the use temperature to alleviate the urgent need. When the water temperature in this area reaches the standard, the power supply will be cut off automatically. Second, between the large water tank and the closest small system, add a set of low-temperature heat dissipation residual water secondary circulation temperature increase control system. When the temperature sensor installed at the bottom of the large water tank finds that the temperature of the water in the water tank is lower than the operating temperature After setting, the system automatically cuts off the cold water inlet solenoid valve of the closest set of small system. At the same time, start a set of small water pumps that can pump the radiating waste water from the large water tank into the closest set of small system circulating small water tanks, and switch the small system constant temperature control power signal to control the small water pump to ensure that the large water pump Before the water storage in the water tank returns to the set temperature, the small system only undertakes the secondary temperature increase of the remaining water from the heat dissipation of the large water tank. The system will not switch back to the original operating state until the temperature of the entire water storage in the large water tank returns to normal. Practice has proved that one of the above two schemes can effectively solve the problem of increasing the temperature of the residual water in the large water tank overnight.
Strictly speaking, any combination of systems has its own advantages and disadvantages. As far as the system combination method is divided into parts, it is absolutely impossible to generalize dogmatically. Don’t think that as long as you connect small systems in series and parallel, you will inevitably get a larger system with higher efficiency. For example: According to the information, a certain unit needs to build a set of bathing equipment with a scale of 40L/person×80 people/day=3200L/d. The design plan specifies that a complete set of household vacuum tube solar water heaters should be used as the basic component combination system. However, since the water tank capacity of each water heater of this type is only 225 (L/unit), a total of this type of water heater needs to be configured according to the water consumption: 3 200÷225 ≈15 units. So I simply wanted to divide 15 vacuum-tube household solar water heaters of the same type into three groups to form a common large system. The engineering design proposal proposes: 5 household vacuum tube water heaters are connected in series as a group, and 15 vacuum tube water heaters are combined into three series units. Each unit uses a common floating ball supply water tank to control the row of 5 household vacuum tubes. The cold water of the water heater system is refilled. Then three series sub-systems are combined in parallel to form a large vacuum tube hot water collector array. The design scheme puts the hot water produced by the three-row sub-system the day before through the so-called “water volume control type” unified control valve, postponed to open for one day, (but after it is opened, it will not be closed) one day later, three parallel sub-systems The hot water in the water is uniformly input into a large water tank with a total volume of 3T for the thermal insulation and heat storage of the project. In the future, according to the changes in the water level in the large water tank of the project, the three series-connected vacuum tube systems will randomly and automatically supply the hot water produced by each Engineering large water tank. An auxiliary electric heater with a power of 24kW is installed in the heat preservation hot water storage tank (bottom) of the project, and then the heat preservation water tank of the project uniformly supplies shower hot water to the collective shared bathing room with a total of 20 shower heads. It is said that the system ensures the supply of hot water in the shared shower room. Regarding the design scheme of the natural circulation direct current system, which is called “reliable operation, low failure rate, good effect, and worthy of promotion” by the designer, the issues that need to be discussed are: (1) Equipped with double-volume heat preservation hot water storage tank Is your hot water system economically reasonable? (2) Through the three float valve water level control boxes, can the group control the household vacuum tube water heater array (a total of 15 units) combined in parallel through the low-position inlet and outlet in a row and series connection in real time and effectively can ensure that the All the hot water produced by each household vacuum tube solar water heater that day can be efficiently replaced by the direct current method to the large water tank for thermal insulation and storage of the project, and to ensure the normal heat of the shower water for 80 people/day Water supply? (3) How high is the heating efficiency of this solar system? To test the actual operation effect of this system, in fact, as long as the power switch of the 24kW electric heater in the thermal insulation water tank of the project is cut off under the premise of ensuring the normal supply of cold water pressure, it is continuously measured for several sunny days and collected to the project through the water volume control method every day. The average production water temperature of the actual total production water in the large thermal insulation hot water storage tank may be able to draw an objective and correct conclusion on the actual operation effect of this system and the actual available heat output of the solar energy of the system.