Are all-glass vacuum tubes the most advanced solar collectors of our time?

Are all-glass vacuum tubes the most advanced solar collectors of our time?

The evacuated tube solar collector is another solar collector developed in parallel with the hot box type flat plate collector. There is no “blood relationship” or generational distinction between the two. Just like cattle and horses, they are both animal powers that were domesticated and selected by human ancestors to serve them. It is hard to say who is stronger than the other. Each has its own strengths and weaknesses: there is absolutely no need to rank them among them. . Evacuated tube collectors can be divided into: glass endothermic vacuum tubes (also known as all-glass vacuum tubes) and metal endothermic vacuum tubes (or glass-metal vacuum tubes, including heat pipe vacuum tubes, U-shaped metal vacuum tubes, etc.) Tube vacuum tubes, concentric metal sleeve vacuum tubes, heat storage vacuum tubes, straight-through vacuum tubes, etc.) two categories.

In the 1970s and 1980s, well-known experts such as Director Gong Bao of Beijing Solar Energy Institute and Professor Zhiqiang Ku of Tsinghua University successively introduced the early production technology of heat pipe vacuum tubes and all-glass vacuum tubes from Europe, America and Australia. Then, on the basis of their foundation, Beitai Institute and Tsinghua Sunshine respectively carried out technical development of in-depth production process for both, and realized new technology industrialization production lines with complete independent intellectual property rights in China respectively. Compared with the production process of the two vacuum tubes, the process of the all-glass vacuum tube is relatively simple, and the price is relatively more competitive. Using all-glass vacuum tubes, on the basis of the existing “circular horizontal water tank plus flat plate collector” household compact solar water heater in China at that time, a corresponding in-line “circular horizontal water tank plus vacuum tube” type household solar water heater was developed.

This kind of domestic water heater is mainly composed of three standard components: all-glass vacuum tube, circular horizontal heat preservation circulating water tank and stamped plate-type tripod, which is simpler in structure and lower in metal consumption than the flat-type. As long as the three major components are standardized and molded into large-scale assembly line production, the production cost of compact household solar water heaters can be reduced very low on the premise of more guaranteed product quality and interchangeability, thereby achieving better economic benefits. Since the 1990s, the production technology of all-glass vacuum tubes has been widely popularized in China, and mass production has been realized in the north and south of the Yangtze River. Therefore, the all-glass vacuum tube in-line household solar water heaters stand out and become popular throughout the country.

As of incomplete statistics in 2010, China has built more than 2,000 all-glass vacuum tube coating production lines, with an annual output of more than 400 million all-glass vacuum tubes and an annual assembly of 46 million m2 of vacuum tube water heaters. After entering the 21st century, the all-glass evacuated tube solar water heater has accounted for about 1/3 of the market share in the Chinese market from the initial stage with simple non-circulating water heaters and flat-panel water heaters represented by “circular horizontal water tank plus flat-plate collector”. It has skyrocketed all the way, occupying more than 95% of the national household solar water heater market share, and has almost become synonymous with Chinese household solar water heaters. Among the solar water heaters in the world, 97% of all-glass vacuum tube solar water heaters are used by the Chinese.

Apart from China, only a few developing countries such as India, Brazil, South Africa, and Thailand import a small amount of vacuum tubes from China. The fundamental reason for this abnormal phenomenon is that although the all-glass evacuated tube collector uses a vacuum method between the inner and outer tubes to effectively reduce the convection heat loss between the tube core and the tube shell, and on this basis The standardized production of the basic unit of the collector is realized; however, the hot water in the inner tube of the glass vacuum tube and the cold water in the high-level circular horizontal thermal insulation hot water storage tank are the channels for macro convection circulation heat exchange, but this kind of “single” The “hole-cecal-like” all-glass evacuated tube structure is complicated.

Through the actual use of hundreds of millions of all-glass vacuum tubes, it was found that this all-glass vacuum tube also has a series of technical problems that have not been properly solved:

First, in-line vacuum tube solar water heaters are difficult to achieve the separation of collection and storage, and the low-level concealed installation of the thermal insulation and heat storage circulating water tank; the entire system and the facade of modern buildings are more difficult to achieve coordinated anchoring installation of building integration.

Second, the vacuum tube inserted obliquely into the water tank is prone to water leakage at the interface of the water tank.

Third, the “single-hole cecum-shaped” vacuum tube has the characteristics of high temperature and heat collection, which leads to serious scaling of the inner tube wall; so that the average use of each vacuum tube is less than 2 to 3 years, and the heat collection efficiency is significantly reduced.

Fourth, the “single-hole cecal tube” inserted upward into the middle of the water tank, through the same nozzle for the heat exchange of cold and hot water, will inevitably destroy the fluid temperature field distribution in the nozzle area, causing fluid particles of different temperatures to be separated. The infiltration and mixing increase the resistance of convective circulation heat transfer between the two, resulting in the slow heating of the whole system, and it is difficult to establish a rank temperature difference circulation heat exchange mechanism and water temperature stratification in the water tank, which affects the improvement of the thermal efficiency of the entire system.

Fifth, the vacuum tubes installed through the horizontal port-to-port horizontal connection of the longitudinal headers not only increase the resistance of the hot and cold water huff and puff heat exchange of each vacuum tube, but also cause different temperature working fluids between the nozzles of the adjacent butt-jointed vacuum tubes. The intermixing and interference of fluid particles leads to the confusion of the temperature field distribution of the entire header system, and the disorder of the heat exchange order of the temperature difference cycle affects the establishment of the natural circulation heat exchange order of the system and the effective improvement of the convective circulation efficiency.

Sixth, when the temperature difference is large and the sunshine is strong, when the “top water method” is used to take the hot water in the thermal insulation water tank of the in-line vacuum tube water heater, after the vacuum tube is stimulated by cold water, due to the uneven cooling and heating, it is very easy to occur. No matter how big the system is, as long as there is a problem with one vacuum tube, the water in the entire connected system will spurt out, and the “Flooded Seventh Army” will immediately be paralyzed.

Seventh, in the vacuum tube high-level thermal insulation circulating water tank, there is a non-circulating “dead water” and dirt sedimentation area at the bottom of the water tank below the vacuum tube nozzle. The system does not have the function of simple remote control of sewage, and the water quality of the entire system is hygienic and safe over time. .

Figure 1 - Falling vacuum tube water heater accident scene, Figure 2 - Vacuum tube water heater roof collapse scene
Figure 1 – Falling vacuum tube water heater accident scene, Figure 2 – Vacuum tube water heater roof collapse scene

Eighth, the high-rise thermal insulation water tank of the vacuum tube water heater also causes the problem of high center of gravity of the system and too long overturning arm; it reduces the stability, safety and wind resistance of the system; it causes visual pollution to the installation building. In addition, the plate-type tripod is stamped and formed with a metal sheet, which has large lateral wind resistance and poor bending strength; especially for the vacuum tube hot water system installed with reflectors, the rear fulcrum of the vertical tripod is too close to the center of gravity of the high-level water tank, and any direction The slight swing caused by the strong wind can easily cause the center of gravity of the high-level water tank to move out of the rear fulcrum, and the unfortunate overturning accident of “disaster from the sky” is inevitable.

Ninth, the operating temperature of the vacuum tube water heater system cannot be regulated: the vacuum tube is in a high-intensity heat collection in a “stuffy” state for a long time. The higher the tube temperature, the greater the conduction and radiation heat loss, and the system’s photothermal conversion efficiency will inevitably decrease. For domestic solar water heaters, what we are looking for is the maximum water production at the operating temperature, not the maximum water temperature. Conversion efficiency, and used as advertising slogans to mislead consumers and promote pseudoscience.

The above-mentioned series of problems began in 1975 when the American Owens-Illinois took the lead in stretching the thermos bladder into a vacuum tube, and has continued to this day, and has not been properly solved for decades. From the perspective of solar water heating system engineering, these problems caused by congenital structural defects of heat collecting components will inevitably affect the perfect and efficient operation of the entire system. If these cross-century “old and difficult” problems continue to not be properly solved from theory to practice for a long time, as well as the final overcoming and improvement of the technical performance of related systems, it is hoped that in a wider range of solar heat utilization, the Wider application is not realistic.

In addition, there is still the same phenomenon of homogenization of vacuum tube water heaters in the country. A considerable proportion of manufacturers, in order to meet the needs of market competition, take advantage of the psychological characteristics of consumers that “look at the appearance without paying attention to the connotation, and pay attention to the price without paying attention to the quality; buy low-priced fancy products, do not buy simple and durable goods”; use similar-sized vacuum tubes; Assemble water heaters of the same structure; in the case of similar performance, the same number of tubes and similar water tank volumes; only depending on the color of the tripod, different colors, and a variety of beautiful names; or adopt the method of sticking the same brand label as you like, Selling counterfeit and shoddy products allows consumers to mislead consumers and engage in excessive price competition when they cannot verify, measure, distinguish, confirm, asymmetric information, and opaque qualifications.

This kind of business strategy of harming others and benefiting oneself is common in the solar industry across the country, which has seriously damaged the image and reputation of the industry. It has reached the point where it is not enough to protect the interests of the solar industry and is not conducive to the popularization and improvement of solar science and technology; The healthy development of industry and consumer markets and technological progress are harmful rather than beneficial.

The explosive growth of all-glass vacuum tubes in China for more than 20 years, and the “cold confrontation” that has been delayed in the world, especially in Europe and the United States, the fundamental reason for the two days of ice and fire is because of the bright spring and the snow, and there must be few. , or was it a congenital deformity that was rejected by it? The benevolent sees benevolence, the wise sees wisdom, and there is no need to make a fuss in the industry. However, if there is no convincing and simple solution to the above-mentioned series of cross-century drawbacks of the vacuum tube system for a long time, it will be difficult to at least be confident when promoting its products to new customers in China and abroad. This is the unavoidable historical responsibility of scientific and technological workers in China’s solar energy industry and the technical problems that must be completely solved within a limited time.

Read more: What is new technology for vacuum tube system integration?