Solar thermal utilization is generally divided into three areas based on operating temperature: below 80°C is the low-temperature area, 80°C to 250°C is the medium-temperature area, and 250°C to 800°C is the high-temperature area. The heat utilization technology in the low-temperature field is relatively mature at home and abroad. It mainly provides domestic water. Its products are mainly flat-plate or vacuum tube solar collectors. The heat utilization technology in the medium-temperature field can be used for regional building heating, air conditioning and refrigeration, seawater desalination and For some industrial heat applications, it can be said that solar thermal utilization is currently the technology with the greatest development potential and the most widely used applications. It is also one of the effective ways to effectively solve the current energy shortage, environmental pollution, and global warming.

　　In the high-temperature field, it is mainly solar thermal power generation technology. Solar thermal power generation technology collects sunlight to a solar collection device through reflectors, uses solar energy to heat the heat transfer medium in the collection device, and then generates steam through heat exchange to drive a steam turbine to drive a generator to generate electricity. There are three types of solar thermal power generation systems: trough type, tower type and dish type. Among them, the trough type has been commercially used on a large scale abroad and the technology is relatively mature. The structure of the trough system solar collector tube is a glass-metal sealed solar collector tube. It uses solar energy to replace traditional fossil fuels and reduce pollutant emissions. It is a clean energy power generation technology. Table 1 shows the medium-temperature application fields of solar energy, and Figure 2 shows the high-temperature thermal power generation technology.

　　1 Current research status in the field of medium-temperature solar energy

　　In order to meet the needs of medium-temperature solar energy applications and make greater contributions to the low-carbon economy, Linuo and Tsinghua University jointly developed a new all-glass product for medium-temperature applications—medium-temperature solar vacuum collector tubes. This product is made of titanium that has reached the international advanced level. As a film material, the current application temperature of gold coating is 80℃~160℃, and special vacuum maintenance technology is used to solve the vacuum maintenance problem of all-glass vacuum collector tubes working at high temperatures for a long time. This product has the characteristics of high thermal efficiency, high temperature resistance, anti-attenuation, long life, and high cost performance. It marks the transition of solar energy from the era of low-temperature hot water application to the era of thermal energy application, laying the foundation for the development of medium-temperature heat utilization technology at home and abroad.

　　In order to increase the operating temperature of solar energy, expand the scope of solar energy applications, improve the thermal performance of solar collectors, and extend the service life of solar collectors, Linuo has developed a new product - medium-temperature solar collectors. This solar collector uses medium-temperature solar vacuum collector tubes, inorganic heat pipes and non-tracking concentrator technology. It can work in an environment of 150°C. It has the characteristics of high heat collection efficiency, long service life and low cost. Its technical level has reached the international leading level. , which has laid the foundation for the engineering and industrial scale of solar energy in air conditioning and refrigeration, seawater desalination, industrial heating, agricultural drying and other fields, and has very broad development prospects.

　　Linuo has made great progress and breakthroughs in medium-temperature applications in industry. The world's first CPC medium-temperature solar industrial thermal system developed by Linuo can achieve more than 10% energy replacement of the original 10t boiler. At the same time, Linuo achieved breakthrough development in the low-carbon pavilion, China's first civil demonstration project of solar medium-temperature thermal utilization, built by Linuo. Linuo has also actively carried out the promotion of medium-temperature applications of solar energy based on the original demonstration projects, mainly targeting the fields of solar air conditioning and solar boiler systems, among which solar boiler systems have been widely promoted.

　　1.1 Solar air conditioner

　　The system designed in the solar air conditioning system that combines medium-temperature CPC solar collectors with single-effect lithium bromide absorption refrigerators under construction can realize automatic operation of the system in different seasons and different working conditions by switching valves and water pumps. switch. During the year-round operation of the system, it can be divided into summer, winter and transitional seasons according to the building's different needs for cooling, heating and hot water: cooling is mainly used in summer, heating is mainly used in winter, while the building's demand for domestic hot water is Throughout the four seasons of the year. According to the quality of the sun's rays, the system's working modes can be divided into three types: full solar energy, solar energy + auxiliary electric heating, and heat pump.

　　Solar radiation energy increases in summer, allowing air conditioning systems driven by solar energy to generate more cooling capacity. The cooling capacity of solar air conditioners increases with the increase of solar radiation energy, which exactly matches people's urgent demand for air conditioners in summer. This system can be used as heating in winter, reducing the use of coal and environmental pollution. The system can provide hot water for daily use such as showering. Figure 2 is the schematic diagram of the solar air conditioning system, and Figure 3 is the installation effect diagram.

　　1.2 Solar boiler

　　Our country is the country that produces and uses the most boilers in the world. The number of boilers is about 500,000, of which about 80% are coal-fired boilers. Coal consumption accounts for 1/3 of the country's coal production. The combustion efficiency is very low, only 45%~ 70%, more than half of which are small boilers with lower efficiency. How to reduce the use of industrial fossil fuels, which consume the most energy, has become an effective way to solve the current situation. The medium-temperature solar collector developed by Linuo fills this gap and makes solar energy applications possible in the industrial field. Figures 4 and 5 both show solar boiler solutions.

　　This system is mainly used in low-pressure and low-temperature steam places with discontinuous gas consumption and small gas consumption. It adopts two closed pressure-bearing systems. The solar heat collection system is an independent closed cycle system. When the working medium flows through the heat collection system, it absorbs the heat of the solar energy to increase its temperature, uses natural convection or uses a circulation pump to force circulation into the steam generator, and releases the heat to the internal water through the internal heat exchanger; temperature After being reduced, it is recycled to the solar hot water system for secondary heating, and the heat is transported back and forth to the steam generating package. The temperature of the water inside continues to rise after receiving heat. When the temperature reaches saturation, it vaporizes in an open manner. As the heat enters, the amount of vaporization becomes larger and larger, achieving the purpose of supplying steam.

　　The production cost of the system is low, the system installation is simple, and there is no need to consider the system pressure problem. Due to the high efficiency of direct heating, it can operate normally all year round and has good thermal insulation effect.

　　2 Core technology of solar thermal power generation

　　Trough solar thermal power generation technology is to arrange multiple trough-type parabolic concentrating collectors in series and parallel to focus sunlight onto a high-temperature power generation tube located at the focus. The heat transfer medium flowing through the high-temperature power generation tube obtains energy. Directly or indirectly generate high-temperature steam to drive a steam turbine to drive a generator to generate electricity. This technology has the advantages of large scale, long life, and low cost, and is very suitable for commercial grid-connected power generation. It is currently the most mature large-scale commercial solar thermal power generation technology.

　　The core component of the trough solar thermal power generation system is the high-temperature vacuum solar collector tube, which is composed of stainless steel tubes covered with selective absorption coatings, cover glass tubes, bellows and other components. Figure 6 shows the structure of the high-temperature vacuum solar collector tube. A vacuum is drawn between the cover glass tube and the stainless steel tube to reduce heat loss of the collector tube and prevent oxidation of the selective absorption film outside the stainless steel tube. The length of a single collector tube is about 4000mm, the outer diameter of the stainless steel heat-absorbing tube is 70mm, and the outer diameter of the cover glass tube is 125mm.

　　1—Metal heat absorber tube; 2—High temperature solar selective absorption coating; 3—Vacuum interlayer; 4—Cover glass tube; 5—AR coating; 6—Getter; 7—Non-evaporable getter; 8—glass metal sealing ring; 9—corrugated pipe; 10—exhaust tail nozzle

　　Figure 6 Structural diagram of high temperature vacuum solar collector tube

　　The key technologies of high-temperature vacuum solar collector tubes mainly include: high-temperature solar selective absorption coating technology, glass and metal sealing technology and vacuum maintenance technology.

　　2.1 High temperature film technology

　　The temperature of the working fluid in the trough solar power generation system is as high as 390°C~400°C. Ordinary solar selective absorption coatings cannot meet the high temperature usage requirements. Therefore, Linuo and Tsinghua University jointly developed a high-temperature vacuum solar collector tube suitable for use in high-temperature vacuum solar collectors. Solar selective absorption coating, excellent coating performance: absorption ratio ≥ 0.95, emission ratio ≤ 0.14 at 400°C.

　　2.2 Glass and metal sealing technology

　　High-temperature vacuum solar collector tube, the heat-absorbing tube is made of stainless steel tube, and the outer tube is made of glass tube. Due to this special structure, the sealing technology of glass and metal has always been a key technology in its research and development. The sealing technology directly affects the life of the collector tube, thermal performance, and even the operational safety and operation of the entire solar thermal power plant. cost.

　　There are two types of sealing between glass and metal, one is non-matching sealing and the other is matching sealing. Matching sealing means that the linear expansion coefficients of glass and metal are similar within a certain temperature range (difference <10%); non-matching sealing means that the linear expansion coefficients of glass and metal have a large difference within a certain temperature range. (Difference>10%). The non-matching sealing method requires special processing of the metal part, that is, the processing of the metal cutting edge. The thinnest can be 0.03mm~0.04mm, which is difficult to process. The glass and the metal are easy to deform during the sealing process, and the finished product is sealed. It has low strength and is easily damaged especially in high temperature environments.

　　Linuo has developed a new type of borosilicate glass, whose linear average expansion coefficient is controlled within the range of (5.2±0.1)×10-6K-1, and can be matched and sealed with Kovar alloy. The glass has strong weather resistance, the critical value of mechanical impact strength is ≥8J, the solar transmittance is ≥0.92 (AM1.5), the glass ring cutting grade is not lower than grade B, it has good wettability with Kovar alloy, and the sealing point can withstand It can withstand hot and cold shock temperature difference of 240℃ without damage, and the tensile strength is ≥2MPa.

　　2.3 Vacuum maintenance technology

　　Compared with ordinary solar collectors, high-temperature vacuum solar collectors have undergone earth-shaking changes in terms of product components, production processes and product working conditions. Linuo has more than ten years of experience in R&D and production of ordinary collector tubes, and began to do technical reserves and pre-research work on this difficult problem many years ago, which gives Linuo a great advantage in the research and development of high-temperature vacuum solar collector tubes.

　　Linuo has carried out in-depth cooperation with Tsinghua University and other universities and scientific research institutions, devoted itself to research and development, and tackled the technical difficulties of high-temperature vacuum solar collector tubes. Currently, Linuo has produced high-temperature vacuum solar collector tubes with excellent performance and achieved the product's Mass production.

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