The difference between flat panel solar water heaters and vacuum tube solar water heaters is currently becoming increasingly popular in the domestic market for low-temperature solar energy utilization, and is also being recognized and valued by consumers and governments. Solar collectors mainly include flat plate collectors and combined pipe collectors, both of which are mature in technology. The flat plate collector was invented in the late 17th century and began in-depth research and large-scale application after 1960; Tubular collectors originated in China and have been around for over a decade now. They are further divided into ordinary non pressure collectors, heat pipe collectors, U-shaped pipe collectors, and so on. 2. Ordinary non pressurized combined pipe collectors and scaling solutions. Combined pipe collectors are widely promoted due to their simple process, low price, and good winter insulation of vacuum tubes. However, ordinary combined pipe collectors also have some shortcomings. One reason is that water is directly heated inside the vacuum tube, which heats up quickly and easily evaporates the water in the header box (when the pipeline system is unreasonable or the control system is malfunctioning, the water in the vacuum tube decreases, and the upper inner tube is exposed to sunlight. When the temperature difference between the upper and lower inner tubes exceeds the impact limit of the glass's tolerance temperature difference, the inner tube will explode, causing system leakage. The reason is generally due to stress concentration and microcracks in the glass, leading to explosion. The second reason is that there is water in the vacuum tube for a long time and the temperature rises quickly. When the concentration of calcium and magnesium ions in the water is high, the inner wall of the tube is prone to scaling, and some flocculent substances are also easy to accumulate inside the tube (especially in the vertically inserted tube collector, where the dirt cannot be automatically discharged, and the horizontally inserted tube collector is Causing a decrease in the collection efficiency due to the removal of a large portion; Thirdly, when the winter ambient temperature is below zero degrees, the water in the outdoor heat collection system is prone to freezing, causing pipeline damage; The commonly used solution now is to not only maintain insulation, but also install an electric heating strip and perform anti freezing cycles under the pipeline. This means that in winter, when heat cannot be collected, electricity and heat must be consumed to prevent freezing. The vacuum tubes of the aforementioned combined pipe heat collector have scaling problems. The current solutions mainly include: regularly cleaning the vacuum tubes according to the water quality situation; Or purify and soften tap water, add chemicals, magnetize it, etc. Purification and softening can be achieved by using filtration devices to remove large particulate pollutants such as sediment, and by using reverse osmosis membranes or resin ion exchange methods to remove calcium and magnesium ions. The commonly used method in the current market is silicon phosphorus crystal POWER-PHOS for scale and corrosion prevention. Its mechanism is that silicon phosphorus crystals effectively inhibit and interfere with the normal growth of calcium carbonate crystals in water, control the rate of crystal nucleus formation, and prevent them from depositing and forming scale; At the same time as this action, complexation or chelation reactions occur on calcium, magnesium, and iron that have formed and adhered to the inner wall of the tube, and then the final dispersion effect is achieved through Brownian motion or water flow. In addition, the p-p component in trace amounts of silicon phosphorus crystals isolates the chemical reaction between dissolved oxygen in water and metal ions, preventing the further reaction of ferrous hydroxide to form iron hydroxide. That is, passivation effect. A amorphous protective film that can automatically repair is generated on the inner wall of the water pipe. This strong hydrophilic molecular film blocks the corrosion of dissolved oxygen in water, thereby suppressing the entire corrosion process. According to different water quality, the concentration of silicon phosphorus crystals is generally controlled between 5-10ppm, which means that medication should be added regularly in the system. At present, magnetization is mainly achieved through electronic descaling devices (i.e. electronic water processors). The principle of scale inhibition and descaling is to use electronic integrated circuits to generate high-frequency electromagnetic fields, so that the water flowing through the descaling device absorbs electromagnetic energy and changes its physical structure without changing its original chemical composition. This disrupts the binding bonds of the original large aggregates and turns them into highly active single or small aggregates of water, promoting the orderly arrangement of water molecules, enhancing the dipole distance of water, and strengthening the decomposition effect of water on scaling ions and their components. The old scale gradually peels off until it is removed, while the new scale is not easy to form and changes the sedimentation state of sediment in the water, the physical properties of various ions, and the living environment of microorganisms such as bacteria and algae, thus effectively playing a role in scale removal, anti scaling, sterilization, and algae killing. After being processed by high-frequency electromagnetic fields, the electrons of water molecules are activated, and their energy levels increase. According to the principle of energy conservation, the molecular potential energy is lost, the potential decreases, and the potential difference between water molecules and the contactor wall decreases or even disappears. Causing a decrease in the free activity of ions in water, inhibiting the dissociation of metal on the vessel wall, and providing anti-corrosion effects on the equipment. The advantages of vacuum tube heat pipe collectors and vacuum tube U-shaped pipe collectors are that there is no direct water flow inside the vacuum tube, reducing the probability of tube burst (even if the tube bursts, there is no liquid leakage). Currently, most manufacturers have direct water flow in the header box, which causes scaling in the header box and a decrease in the thermal conductivity efficiency of the heat pipe head due to scaling. It is difficult to disassemble the heat pipe after a malfunction occurs; Water flowing inside the U-shaped tube can cause scaling inside the tube, reduce heat transfer efficiency, and even cause blockage inside the U-shaped tube to lose its heat collection effect; The solution is to purify and soften the water, add chemicals, and magnetize it, which can alleviate the above problems to a certain extent. The best way is to use antifreeze as the thermal conductivity medium in the system. The history of flat plate collectors has been a long time, and their technological applications are also very mature. The advantages are: its strong pressure bearing capacity, the system is made into a fully enclosed system, which avoids oxygen and impurities in the air from entering the heat collection system, and has high reliability; Using antifreeze as a thermal conductive medium, thoroughly prevent freezing in winter, and the heat collection system does not require auxiliary heat sources to avoid energy consumption; The heat absorbing area is large, and the heat absorbing sheet accounts for the vast majority of the building contour area occupied by the collector. It is one of the best types of collectors to achieve the combination of solar energy and buildings; Disadvantages: The production process is more complex, the cost is higher, and the collection efficiency is greatly affected by the environmental temperature compared to the combined collector. It is higher in summer and lower in winter. The heat absorption strip coating technology of flat plate collectors requires high requirements. Due to their long-term exposure to air and long-term contact with oxygen and water molecules in the air, their aging resistance is extremely high (flat plate collectors cannot be evacuated like vacuum tubes, and the requirements for the film layer are far higher than those of vacuum tubes). In order to improve the insulation performance of flat plate collectors, vacuum glass has already been used for flat plate collectors. However, due to the immature vacuum glass process and high cost, this technology has not yet been applied on a large scale. The research and application of selective absorption materials in China has gone through several generations, from non selective ordinary black paint to selective lead sulfide and metal oxide coatings, from black nickel, black chromium to aluminum anodized coatings, and so on; At present, the absorption surface of flat plate collectors in China mainly adopts anodized coloring on aluminum strips and selective coating on copper strips. The current coating production technology for selective absorption coatings of aluminum nitrogen aluminum materials by intermittent magnetron sputtering on vacuum tubes has developed with the emergence of vacuum tube collectors. Due to the poor weather resistance of this coating, it is not suitable for the use of flat plate collectors. In addition, developed countries internationally, especially Europe, have two main characteristics in the production of selective absorption coatings. One is the use of vacuum coating technology, and the other is the use of winding continuous coating method. BATEC, a Danish company, produces black chromium absorption coatings by using continuous electroplating on copper strips. The optical properties and weather resistance of the products are ideal; Several companies in Germany that produce selective coatings, such as INTERPANE, TINOX, and ALANOD, all use vacuum or continuous production methods to produce absorption coatings. Huisi Sunshine has been committed to the development of flat plate collectors for several years. The core component, the heat absorbing copper strip absorption coating, uses German TINOX technology. It vaporizes titanium in a vacuum environment, adds nitrogen and oxygen to the vaporized material of titanium, and generates nitrogen oxide titanium through chemical reactions, which is cold condensed and deposited onto the metal strip. In addition, quartz vaporizes to form a second layer of anti reflective layer and provides protection for the nitrogen oxide titanium layer. TINOX coating is the only coating that uses titanium and quartz instead of nickel and chromium to date. During the production process, it does not pollute water sources or air, making it the most environmentally friendly coating; The absorption rate of TINOX coating is as high as 95%, and the emissivity is about 4%; Due to its excellent environmental and aging resistance, the TINOX coating can still maintain over 95% of its initial performance after 25 years of use. The flat plate collector cover glass is made of ultra white low iron tempered glass, which is more resistant to hail, has strong solid impact ability, high fatigue aging resistance, and high transparency; The frame is assembled with a solid shell or aluminum alloy profile, with high strength. The collector is not afraid of air exposure and will not experience tube explosion; Using antifreeze as a thermal conductive medium, thoroughly prevent freezing in winter, and the heat collection system does not require auxiliary heat sources to avoid energy consumption; Flat plate collectors are not afraid of direct sunlight in summer and freezing in winter, and have strong adaptability to climate. The modular design of the collector can be combined into various sizes of collector arrays, making it more flexible to use. The analysis of the collection efficiency of 5 types of collectors, whether it is a flat plate collector or a combined pipe collector, the most influential factor on the collection efficiency is the heat absorbing body, i.e. the heat absorbing film layer. The absorption rates of the better vacuum tube film layer and the better flat plate heat absorbing film layer are both around 0.95; The entire contour area of the flat plate collector is almost occupied by the heat absorbing plate, but the vacuum tube collector must have a certain spacing due to the influence of the structure. Generally Φ The spacing between 47 pipes is 70mm, Φ The spacing between 58 pipes is 80mm, Φ The inner diameter of the 47 tube product is Φ 37, Φ The inner diameter of the 58 tube product is Φ 47. If the instantaneous efficiency limit of a collector without a reflector is not considered, such as reflection and refraction of light, Φ 47 tubes are: 0.95 x 0.9 x 37/70=0.452, Φ 58 tubes are: 0.95 x 0.9 x 47/80=0.502; If the gap between the vacuum tubes is not taken into account, the instantaneous efficiency limit of the collector is limited, Φ 47 tubes are: 0.95 x 0.9 x 37/47=0.673, Φ 58 tubes are: 0.95 x 0.9 x 47/58=0.693; The instantaneous efficiency limit of a flat plate collector is 0.95 × 0.9=0.855. According to the national standard, the instantaneous efficiency intercept of flat plate collectors is ≥ 0.72; Instantaneous efficiency intercept of vacuum tube collector (without reflector) ≥ 0.62; The instantaneous efficiency intercept of the vacuum tube collector (with reflector) is ≥ 0.52. Note: When calculating the heat collection area of a vacuum tube collector with a reflective plate, the gap between the vacuum tubes is included. When calculating the heat collection area of a vacuum tube collector without a reflective plate, the gap between the vacuum tubes is not included. The overall efficiency of a flat plate collector is more affected by environmental temperature than that of a vacuum tube collector. Generally speaking, in spring, summer, and autumn, its heat collection efficiency is higher than that of a vacuum tube collector, and in winter it is lower than that of a vacuum tube collector; Its instantaneous efficiency is shown in the following figure:
The abscissa value of its intersection position is 0.076, and the test data was obtained when the solar radiation intensity is 800W/㎡. Therefore:
Ti ta=0.076 × 800=60.8 ℃; That is to say, when the difference between the medium temperature inside the collector and the ambient temperature is 60.8 ℃, the instantaneous efficiency of the combined pipe collector and the flat plate collector is the same. When the temperature difference is less than 60.8 ℃, the efficiency of the flat plate collector is high, and when the temperature difference is greater than 60.8 ℃, the efficiency of the combined pipe collector is high, as shown in the following figure. 6 Conclusion
In recent years, domestic flat plate collectors have accounted for less than 15% of the market share, which is due to many reasons: (1) Some manufacturers use flat plate collectors as direct systems in winter, which cannot be antifreeze and must be emptied, so they cannot be used in winter and maintenance is complex; (2) All glass vacuum tube water heaters can be used year-round in most areas; (3) Due to technological innovation, the cost of all glass vacuum tubes has been significantly reduced, and production enterprises have rapidly increased, promoting the rapid expansion of the solar water heater market; (4) Domestic consumers have a much higher awareness of all glass vacuum tube water heaters than flat panel solar water heaters. Therefore, the current domestic market pattern of household water heaters is formed due to factors such as product characteristics and prices. It can be foreseen that in the low-end market of household water heaters, full glass vacuum tube water heaters will still be the main type.
The foreign solar energy market has always been dominated by flat panel collectors due to the different design concepts of solar energy systems abroad. Foreign systems generally adopt indirect and split type systems, which operate under pressure; This type of system generally has a high initial investment, but it has high reliability, low maintenance costs, water quality will not be polluted, and the entire system life is long.
For such systems, flat plate collectors demonstrate their own technological advantages:
(1) Flat plate collectors are most suitable for pressure systems; (2) The most suitable solar water heater for dual circulation; (3) Most conducive to achieving the integration of solar water heaters with buildings; (4) Long system lifespan and low maintenance costs;
(5) In most cases, more domestic hot water can be provided; (6) Flat panel collectors can conveniently solve the problem of overheating in solar heating systems during non heating seasons.
Conclusion: In solar energy system engineering, split solar water heaters, and places that require the integration of solar energy and buildings, flat panel collectors have significant advantages over all glass vacuum tube collectors in terms of system life, system maintenance, and other aspects.
