Current status and progress of trough solar thermal power generation technology

Trough solar thermal power generation uses a trough concentrating mirror to concentrate sunlight on a line. A tubular collector is installed on this line to absorb solar energy and heat the heat transfer medium, and then use steam. Power cycle to generate electricity. The parabolic concentrator's parabola is one-dimensionally tracked by the sun, with a concentrating ratio of 10 to 100 and a temperature of 400 °C. In the mid-1980s, trough solar thermal power generation technology has been developed. At present, California has installed a 354 MW trough concentrating thermal power station. The working medium is heat transfer oil, and the heat exchanger can produce heat transfer oil. Superheated steam close to 400 °C drives the turbine to generate electricity.

The most important feature of trough solar thermal power generation technology is the use of a large number of parabolic trough concentrators to collect solar radiant energy and convert light energy directly into heat energy, which turns the water into high temperature and high pressure steam through a heat exchanger. Push the turbine to generate electricity. Because solar energy is uncertain, a conventional fuel-assisted boiler is added to the heat transfer medium for emergency use.

The disadvantages of trough solar thermal power generation are:

(1) Although the light collection efficiency of such a linear focusing system is improved due to single-axis tracking, it is difficult to achieve dual-axis tracking, resulting in an average of 30% loss of light per cosine effect per year.

(2) The trough type solar thermal power generation system has a huge structure and is difficult to stand in the windy and high wind and sand areas of China.

(3) Since the surface of the linear heat sink is completely exposed in the light receiving space, the heat treatment cannot be performed. Although the vacuum layer is designed to reduce the loss caused by convection, the radiation loss still increases with the increase of temperature.

(1) Collecting heat pipe

The heat collecting tube is a key component of the trough solar thermal power generation heat collecting system, which can convert the direct solar radiation energy collected by the mirror into heat energy, and the temperature can reach 400 °C. The inner layer of the heat collecting tube currently used is a stainless steel tube, and the outer layer is a glass tube and a metal bellows at both ends. The inner tube is coated with a selective absorbing coating to achieve maximum absorption of concentrated solar direct radiation and minimal infrared wave re-radiation. The glass-metal seal at both ends is sealed with the metal bellows to provide high temperature protection and seal the internal space to maintain vacuum. Reducing the convection and conduction heat loss of the gas, coupled with the application of a selective absorption coating - minimizes the radiant heat loss of the vacuum collector. On the other side, the metal bellows is welded to the internal heat absorbing tube. These bellows with elastic connection function can compensate for the difference in thermal expansion and contraction between the inner metal tube and the outer glass tube during the temperature rise and cooling of the heat absorption tube. The focused direct solar radiant energy can be converted to heat at the surface of the collector tube, transferred to a heat transfer medium, and heated to a maximum temperature of 400 °C. The external glass tube can be used as an additional protection to prevent the infrared wavelength energy from radiating outward to reduce the heat loss. The outside of the glass tube is covered with an anti-reflection coating, so that the solar radiation energy passes through the glass tube.

China has been developing vacuum collector tubes since the mid-1980s, which has overcome many technical difficulties and established a vacuum collector tube production base. Such as Beijing Solar Energy Research Institute, Huangming Solar Energy Group, etc., the production of heat collecting tubes through laboratory testing and testing, can meet the relevant technical requirements, compared with international similar products, operating indicators can meet the relevant technical requirements of the international industry. Individual technical indicators are also superior to foreign technology. However, most of the heat collecting tubes developed and produced in China are only used for experimental research, and lack of certain practical experience in engineering.

(2) Collecting the mirror surface

3. The collector mirror used in trough solar thermal power generation is made of ultra-white glass. While ensuring a certain focus accuracy, it also has good resistance to wind, acid and alkali, and UV resistance. The mirror surface is made of low-iron glass. The rigidity, hardness and strength can withstand the harsh environment and extreme climatic conditions in the field. The back of the glass is coated with a protective film to prevent aging. Due to the low iron content. This kind of glass has good solar radiation transmittance.

(3) Support structure and control system

The support structure is used to fix the trough parabolic concentrating mirror, and the control system is used for one-dimensional tracking of the heat collecting array. Obtain effective solar radiant energy. The design of the support structure requires computer simulation studies, wind tunnel experiments and actual operations, and is designed with full consideration of optimal mechanical, optical and mechanical properties and minimum cost.

(4) Thermal storage system

Renewable energy sector. Energy storage technology has always been an urgent problem to be solved. However, solar thermal power generation already has a mature energy storage technology solution. This is also the future of solar thermal power generation, which is expected to catch up with other new energy sources such as wind power and photovoltaics. The energy storage technology can realize the energy storage problem of large-scale solar thermal power stations.

Foreign development status

The solar thermal power industry has experienced several ups and downs due to various reasons. As early as the beginning of the 20th century, there was research on solar thermal power generation. Due to the outbreak of the 2nd World War and the discovery of oil in the Near East, the utilization of solar energy was slow. Among them, due to the backward technology, low efficiency and high production cost of solar thermal power itself, it is also an important reason hindering its development. Until the oil crisis of the 1970s, the solar thermal industry was re-invigorated.

With the development of solar thermal power generation technology and scale, solar thermal power generation will have potential advantages in competition with conventional energy power generation. However, this technology is not yet fully developed and is not economically competitive. Therefore, to promote this technology, it is necessary to further reduce power generation costs, improve system efficiency, and automate plant operation, reducing operating costs from the current 3 cents/kWh to 0.8 cents/kWh. Therefore, the future research focus of trough solar power technology is:

1. Strengthen the investigation of solar energy resources at the project site;

2. Develop thermal energy storage technology for direct vaporization systems;

3. Improve the working temperature of the heat carrier;

4. Develop efficient heat-absorbing tube coating technology to increase the temperature of the heat collecting surface to 550-600 C.

Domestic development status

China's research on solar thermal power generation technology started late, and it was not until the 1970s that some basic research began. During the “seventh five-year” period, Xiangtan Motor Factory cooperated with American Space Power Corporation to develop two groups of 5 kW. Parabolic-focusing solar thermal generators, but because of the high price, and the lack of fundamental solutions to processes, materials, components and related technologies, they have not been promoted. The National “Eighth Five-Year Plan” planned a small-scale component and material research project, and built a small parabolic trough vacuum tube high-temperature heat collecting device in the Institute of Electrical Engineering of the Chinese Academy of Sciences. The successful operation of the LUZ trough solar thermal power station in California has attracted wide attention in China, and it is planned to introduce this type of unit in Lhasa, Tibet - a 35 MW LL Z trough solar thermal power station. At that time, due to the feasibility assessment, the power cost of the power station was estimated to be about 1.1 yuan / kWh, and the operating cost was 0.1 yuan / kWh. Compared with the energy cost of the coal-fired power station in Lhasa area of ​​0.8 yuan / kWh, there is still a certain advantage.

In general, China's research on solar thermal power is still relatively backward. The research level in the 1980s is only equivalent to the level of foreign countries in the 1960s. Although China's research on solar thermal power technology has paid considerable attention in recent years, it has also achieved a certain development, such as the first domestic solar thermal power demonstration power station (capacity 7 kW) built in 2005 in Jiangning District of Nanjing. However, there is a big gap with the level of international development. To this end, the state has arranged billions of funds in the 11th Five-Year Plan to develop solar thermal power generation technology. Taking into account the current state of the art in China, it is possible to prioritize the development of trough solar thermal power generation systems, or to combine solar power generation with small hydropower, solar power generation and wind power generation to form various joint systems, or to use - some energy storage equipment to reduce Dependence on climatic conditions.

In order to further improve the development of trough solar thermal power generation technology and enhance its competitiveness, the following measures can be taken:

First, it is an advanced concentrator designed. The structure is developed from a shaft unit to a truss unit. The length of a single column of the concentrator is increased to 100 m, so that a set of driving mechanisms can drive a longer concentrator array. At the same time, continuously optimize the concentrator material, glass thickness, etc., to minimize the weight of the whole machine.

Second, fully consider the influence of azimuth and elevation angles, using polar axis tracking technology to change the concentrating collector array from the original north-south horizontal position to the north-south tilt axis (inclination angle is related to latitude), thus Effectively receive solar radiant energy.

Third, it is to develop high-performance high-temperature vacuum tube receivers.

Fourth, it is a new trough power generation technology that develops direct water as a medium. By using this technology, a large number of heat exchangers can be replaced, thereby simplifying the system, improving efficiency, and reducing costs.

Fifth, it is to strengthen the reliability research, comprehensively consider the temperature, pressure, sealing and other related factors, and improve the sealing connection problem between the high-temperature vacuum receiver at the two ends of the concentrator array and the heat-conducting oil pipeline that is not fixed on the ground.