Forced circulation solar hot water system

Forced circulation solar hot water system

The forced circulation type solar water heater refers to relying on the power generated by the mechanical drive equipment to apply continuous operating pressure to the heat transfer fluid to drive the heat transfer fluid to carry out a macro-circulation movement for the purpose of heat exchange, and to continuously transfer the solar collector The solar radiant heat obtained by the heating medium is effectively absorbed, transferred and stored in the solar heat utilization equipment in the heat accumulator. According to the operating needs of the forced circulation system, it can generally be divided into two types of forced circulation systems, direct contact and indirect contact. The so-called “direct contact type” means that the heat transfer medium is the heat carrier fluid (such as domestic hot water or hot air) that is ultimately supplied to the user for direct use; while the “indirect contact type” uses high heat capacity, high boiling point, and Other fluid media with good flow properties and superior frost resistance are used as heat transfer working fluids. After the heat transfer working medium obtains heat in the solar collector, it is forced to drive into the heat preservation hot water storage tank, and through the secondary heat exchange in the heat storage tank, the heat is directly used to heat the heat preservation hot water storage The tank is prepared to provide the thermal fluid for direct use by the user. In other words, in an indirect system, the heat transfer medium and the thermal fluid provided by the system to the user are completely different types of substances. There must be a process of secondary heat exchange between them, so the thermal efficiency of the system is obviously much lower than that of the direct system. However, the indirect system is more conducive to protecting the purity and safety of the water because it reduces the process of circulating heat exchange in the collector. For forced circulation systems, whether direct or indirect solar water heaters are used, in theory, the entire system components such as solar collectors, pipe and valve fittings, circulating pumps, and pressure-bearing heat preservation hot water exchange tanks need to be under different bearing conditions. Work under pressure. Therefore, the entire system needs to be equipped with pressure-bearing and corrosion-resistant pipes, valves and other parts and accessories. It can be said that doing a good job in leak-proofing and pressure-bearing the whole system is the first priority of a forced circulation system. Because the forced circulation system is equipped with a dedicated circulation power and control system, the operation of the entire system is also inseparable from auxiliary energy. Due to the guaranteed supply of conventional energy, the forced circulation system generally also has the functions of all-weather heating and anti-icing. With the above-mentioned related functions, the overall cost and daily operating cost of the forced circulation system are much higher than that of the conventional natural circulation system. Figure 1 shows a set of the simplest “direct forced circulation” solar water heating system.

Forced circulation solar hot water system
Figure 1 Diagram of forced circulation solar water heating system

Generally speaking, we define a system in which the same fluid medium is repeatedly exchanged for heat energy under the action of an external force more than twice as a “forced circulation” system. It also relies on external forces (such as the potential energy of the high-level water tank or the pressure of the tap water system) to promote the “solid-liquid” heat exchange between the fluid medium and the heat-carrying solid in the forced flow process; if the fluid molecules are only obtained through the heat exchange pipeline at one time The system of heat, we still call it “DC system” by convention. The biggest feature of the forced circulation system is that a circulating pump that continuously provides circulating power to the heat transfer medium must be installed on the circulating pipeline; the system uses the circulating pressure provided by the circulating pump to force the heated fluid to be repeatedly circulated. hot. In addition, in order to save energy, the system needs to be located at the outlet of the heated working fluid of the collector (ie the outlet of the hot water circulation pipe) and the outlet of the heat sink of the cold water area at the bottom of the heat preservation and storage tank (ie the cold water circulation pipe). Two temperature sensor probes (T1 and T2) are preset respectively. When (T1-T2) ≥ 3℃~5℃, the control system will automatically start the circulating pump to forcibly suck the low-temperature working fluid (cold water) at the bottom of the heat preservation hot water storage tank and squeeze it into the solar heat collector. Heating cycle heat exchange. When (T1-T2) ≤ 3℃, the control system will automatically cut off the power supply, so that the working fluid placed in the core of the solar collector is in a state of sun-absorbing heat, waiting for the water temperature to continue to be heated by solar radiation to ≥ 3℃ When the temperature is above 5℃, the circulating pump is restarted for forced circulation heat exchange to save the energy consumption of the circulating pump. Because the forced circulation system is equipped with a power circulation device, the system has great flexibility in layout and installation, that is, the thermal insulation hot water storage tank is no longer required to be placed on the roof, or even higher than the collector or The place where the water is used. Among the various types of solar hot water systems currently in use, the forced circulation system is the first to realize the long-distance separation of collection and storage, and the thermal insulation hot water storage tank can be entered into the house. According to the requirements of the building load, the solar hot water installed in the designated location is concealed. system. But it is also a hot water system that is completely inseparable from auxiliary energy participation in heat exchange cycle operation and system forced water supply and drainage operation management.

To design a forced circulation system, first of all must calculate the circulating water resistance of the pipeline, only to ensure that the power of the circulating pump (H) is equal to or greater than the elevation difference between the inlet and outlet of the collector (h1); the head loss of the collector itself (h2); The total head loss of the pipeline (hw); and the sum of the local water resistance (hj) caused by other pipe valve fittings in the pipeline, the system can effectively circulate heat exchange work:

     H ≥ h1+h2+hw+hj (1)

The required power (N) of the circulating pump of the forced circulation system is:

           N= rQH/75η (2)

In the formula: r——The specific gravity of water (kg/m3);
Q——Flow rate (m3/s);
η——The total mechanical efficiency of the pump;
For high-end villa residences in areas with large hot water supply, high water quality standards, hot water temperature quality and heating guarantee rate, poor winter sunshine, and long low-temperature freezing periods, the forced circulation system is general All use indirect contact antifreeze working fluid to carry out secondary heat exchange; and at the same time, a set of electric heating temperature automatic control system with over-current, over-voltage and leakage protection devices is configured in the heat-preserving and pressurized hot water storage tank , To ensure that the hot water system can achieve the goal of 24-hour domestic hot water supply. Figure 2 is a schematic diagram of a villa type forced circulation system with indirect secondary circulation heat exchange and auxiliary electric heating function.

Forced circulation solar hot water system
Figure 2 Structure diagram of the auxiliary energy system with forced circulation indirect heating

The advantage of this system is that it completely eliminates the heating impact that climate change may bring to the solar water heating system, especially the damage that may be caused by the low temperature frost damage to the solar water heating system. The system can realize a stable supply of domestic hot water 24 hours a day, 7 days a week. This kind of system uses a special antifreeze working fluid to collect heat from the antifreeze cycle of the collector. Therefore, it is necessary to add a set of pressure-bearing and heat-dissipating pure copper coils corresponding to the circulating heat release in the heat preservation hot water storage tank; in order to pass the solar radiation heat collected by the working fluid from the collector through the secondary cycle The heat exchange is transferred to the heat preservation hot water storage tank to heat the cold water in the water tank. This system that uses antifreeze working fluid to perform secondary circulation heat exchange on the water tank is what we call an indirect contact forced circulation hot water system. When the working fluid undergoes an uninterrupted heat absorption and heat release cycle in the system, according to the physical principle of thermal expansion and contraction, a series of pressure changes caused by temperature increase and decrease will inevitably occur. In order to eliminate, or constantly rebalance, the physical changes in the pressure, temperature, and density of the working fluid that occur randomly due to changes in fluid morphology, as well as the damage and other adverse effects that may be caused to the fluid circulation pipeline, so In the circulation pipeline of this system, an expansion tank with supporting pressure must be installed before the circulation pump. At the same time, it is necessary to install a safety valve at the outlet of the thermal working medium of the solar collector and the expansion tank, so that the system can automatically remove the high-pressure gas generated by the high-temperature gasification due to the working medium in an abnormal state at any time. Immediate release to ensure that the system does not affect safe operation due to air lock. In the hot water storage tank of this kind of system, a set of independent, parallel-operated electric heating devices must generally be installed. When the water temperature monitoring probe embedded near the hot water outlet in the upper half of the water tank detects that the water temperature near the hot water outlet is lower than the minimum normal temperature set by the system, it will immediately start the electric heater to heat the upper half of the water tank. The hot water that does not meet the standard in the waiting area is electrically heated to ensure the temperature quality of the water. In addition, in the system control program, protection functions are set to prevent low-temperature freezing damage cycle and high-temperature shutdown protection, and so on. In order to realize “valve open, hot water comes” and reduce the waste of heat dissipation and clean water resources in the system pipeline, for the forced circulation system with long hot water supply pipeline, it is generally at the end of the hot water output main pipe of this system. , Install a hot water circulating pump with a temperature controller. When this circulating pump temperature probe on the hot water output main pipe senses that the water temperature in the hot water output pipe is lower than the set temperature, it will automatically start the hot water circulating pump to save the heat and water in the hot water output main pipe , Re-pump the heat preservation hot water storage tank for secondary heating until the hot water temperature in the end of the hot water output main pipe returns to the original set temperature state, and then stop the machine to ensure that it is at any point along the hot water supply main pipe. All hot water points can enjoy the instant service of “valve open, hot water coming”. Obviously, the heat preservation hot water storage tank of this kind of indirect “secondary cycle heat exchange” system must also be a hot water storage tank with pressure-bearing, anti-rust and high-efficiency heat preservation functions. In order to ensure that several water distribution points in the whole villa can get hot water normally, the system adopts the top water method of squeezing hot water with cold water, using the pipeline pressure of tap water to force hot water to each one through a pressurized water tank Water points. One of the main problems of this forced circulation system with secondary circulation heat exchange is that the system’s heat conversion efficiency is low. The reason is very simple, because the thermal efficiency of the system is equal to the product of the light-to-heat conversion efficiency of the solar collector and the secondary heat exchange efficiency in the water tank. Assuming that the two heat exchange efficiency can reach 60%, then the total efficiency of the system is only: 60%×60%=36%. The heat dissipation pipeline of the secondary circulation system is long, and the pipeline system is equipped with a number of heat loss components such as circulation pumps, expansion tanks, and safety valves with high heat capacity, and each component will inevitably cause different degrees of heat dissipation loss. As a result, it is difficult to improve the overall efficiency of the entire system. From this, we can see the inherent advantages and disadvantages of this forced circulation system with a secondary circulation tropical electric heating device and the complexity of the system structure. Due to differences in national economic development and material consumption levels, a considerable proportion of solar water heaters used in Western countries in Europe and the United States are so-called indirect contact forced circulation hot water systems that use antifreeze working fluid for secondary heat exchange. However, this kind of solar hot water system, which has been very popular in Western countries, is in fact unrealistic and unnecessary if it is to be promoted and applied to the same degree in the vast majority of developing countries. Except for a few exceptions for high-end households such as high-end villas and high-standard large-scale residences, for the general public with large populations and limited living environment and economic and technical conditions in developing countries, not every developing country has just become rich. Every well-off home has a goal that needs to be pursued. The old road taken by developed countries in Europe and the United States should be properly understood and understood from the perspective of energy conservation, emission reduction, and protection of mankind’s green ecological home. It has developed a new generation of high-tech products with better functions, more reasonable structure, more stable performance, higher utilization rate of solar light and heat resources, and more conservation of conventional energy resources. This is not only the historical responsibility of our practitioners, but also the glorious duty that we should assume.