Understanding the principle and classification of solar thermal power generation

Release time：2018-12-29 18:25Number of visits：168times

What is solar thermal power generation, solar thermal power generation refers to the use of large arrays of parabolic or disk-shaped mirrors to collect solar thermal energy, through the heat exchanger to provide steam, combined with the traditional turbo-generator process, to generate electricity. Solar Thermal Power Generation Technology, which avoids the expensive silicon photoelectric conversion process, can greatly reduce the cost of solar power generation. Moreover, this form of solar energy utilization has an advantage over other forms of solar energy conversion, namely that the water heated by solar energy can be stored in huge containers, it can still power a turbine a few hours after sunset. Principle of photothermal power generation

Photothermal power generation technology is a new energy application technology which is different from photovoltaic power generation. It is a process of converting solar energy into thermal energy and then into electrical energy. The heat transfer medium is heated to hundreds of degrees high temperature by solar heat energy collected by condensing mirror and so on. The heat transfer medium generates high temperature steam after passing through the heat exchanger, which drives the steam turbine to generate electricity. The heat transfer medium here is mostly heat conducting oil and molten salt. In general, the whole system is divided into four parts: Heat collection system, heat transfer system, heat storage and heat exchange system, power generation system. Heat collecting system: The heat collecting system consists of a light collecting device, a receiver, a tracking mechanism and other parts. If the collector is the core of the whole photothermal power generation, then the concentrator is the core of the collector. The focusing device is a focusing mirror or a Heliostat, etc. . The reflectivity and the focus deviation can affect the efficiency of power generation. At present, the efficiency of domestic condenser can reach 94% , which is similar to that of foreign condenser. The collection system collects solar energy and converts it into heat energy. Heat transfer system: The heat transfer system is mainly to transfer the heat collected by the heat collection system. The heat energy is transmitted to the heat storage system by heat transfer medium. The heat transfer medium is mostly heat conducting oil and molten salt. In theory, Molten salt is hotter, more efficient and safer than heat-conducting oil. Heat transfer systems generally consist of preheaters, steam generator, superheaters, and reheaters. The basic requirements of heat transfer system are: low loss of heat transfer pipeline, low power of heat transfer medium pump, low cost of heat transfer. In the heat transfer process, the shorter the heat transfer pipeline, the smaller the heat loss. Thermal storage and heat exchange system: In my opinion, thermal power generation technology in thermal storage and heat exchange system fully reflects the advantages of photovoltaic power generation technology. To store the heat of the Sun. Can generate electricity at night, also can be based on the local electricity load, adapt to the power grid dispatch generation. A heat storage device usually consists of a vacuum-insulated or heat-insulating material-coated heat storage device. The requirements of heat storage medium in heat storage system are: high energy storage density, rich source and low price, stable performance, no corrosion, good safety, large heat transfer area, good heat conduction performance of heat exchanger, good viscosity of heat storage medium. At present, all kinds of new technologies and materials for heat storage are being studied in our country, and some experts suggest using low-cost solid materials such as ceramics to reduce the cost of power generation. POWER GENERATION SYSTEMS: generators used in solar thermal power systems include steam turbines, gas turbines, low boiling point steam turbines, and Stryn generators. These generating units can be selected according to the thermal energy level at the inlet of the turbine and the conditions of heat, steam pressure and so on. For large-scale photo-thermal power generation system, gas turbine can be used when the working temperature is above 800 °C, because its temperature grade is basically the same as that of thermal power system For Low power or low temperature solar power systems, a low boiling point steam turbine or Stirling engine can be used. Most of the steam turbines currently in use are air-cooled. Although the power generation system of photothermal technology is similar to the thermal power generation system, there are some differences, which requires the turbine to have the characteristics of frequent start-stop, fast start-up, low load operation and high efficiency. Solar photothermal power generation classification, according to the focus of different ways and structures, photothermal technology can be divided into tower, trough, dish, fresnel four. 1. Tower Power Generation System, the tower power generation system is a point-focus system, which uses the array of Heliostat field formed by large-scale Heliostat to reflect the solar radiation to the heat sink placed on the top of the tower, heating the heat transfer medium, make it produce steam directly or generate steam after heat exchange, in order to drive the steam turbine power generation. The tower system has the advantages of short heat transfer path, small heat loss, high condensing ratio and high temperature, but the tower system must be utilized on a large scale, requires high land occupation and large single investment, and adopts a two-axis tracking system, the control system of the mirror field is more complex. Trough solar thermal power generation system is called trough parabolic mirror solar thermal power generation system. It is a series-parallel arrangement of several trough parabolic concentrators to heat the working medium, superheated steam is produced, which drives the turbine electric generator to generate electricity. 3. Dish System, dish system is also a point focus system, it should be the solar thermal power system is the world's first solar photovoltaic thermal power system. The dish system, also known as the parabolic mirror Stryn system, consists of a number of mirrors with a parabolic mirror that receives heat from the heat transfer medium in the receiver at the parabolic focal point, to drive the Stirling engine to generate electricity. The focusing ratio of dish system is very high, from hundreds to thousands can be achieved, focusing temperature can even reach more than 1000 °C, high efficiency, for the ground slope requirements are more flexible. However, the cost advantage is still lacking, and the technology still needs to be perfected. Dish system is more suitable for isolated power stations in remote areas. The utility model can be used by a single unit or multiple units in parallel, and is suitable for small-scale power generation. 4. FRESNEL power generation system, fresnel power generation system works like trough photothermal power generation, but using fresnel structure of the condenser to replace the parabolic mirror. This makes it relatively cheap, but correspondingly less efficient. Such systems have a low quality of heated water vapor due to the fact that the concentrator is only a few tens of times, so that the annual power generation efficiency of the whole system can only reach about 10% , its construction and maintenance costs are also relatively low.

Advantages of photothermal power generation 1, excellent power quality, can be directly barrier-free grid. Solar photothermal power generation works in the same way as conventional fossil energy generation in terms of thermal power generation, either by converting thermal energy into electrical energy through a Rankine cycle, a Brayton cycle or a STIrling cycle, with direct output of alternating current, the technology of power transmission is relatively mature and stable, so it is more convenient to connect with the domestic power grid at present, and the power quality is good. 2, can store energy, can adjust peak, realize continuous power generation. The shape of the load curve of the power grid is similar to the natural curve of solar power generation during the day. The load rises with time in the morning and decreases with time in the afternoon. Therefore, solar power generation is the natural peak-shaving load of the power grid, the load proportion can be determined according to the maximum load difference between day and night in the power grid, which generally accounts for 10-20% of the total load, that is to say, it can buffer and smooth power generation through heat transfer, and it can cope with the transient instability of solar energy, practice has proved that the efficiency and economy of heat storage is significantly better than that of electricity storage and pumped storage. Solar thermal power plants equipped with special heat storage devices can not only replenish energy when starting up and when the clouds are low to cloudy, ensure the stable operation of the units, and even realize uninterrupted power generation 24 hours after sunset, at the same time, according to load, power grid demand for peak load regulation, dispatch. 3. The cost advantage is prominent under the scale effect. Because the conversion link is the same as thermal power, solar thermal power generation and thermal power also have significant scale effect, better than wind power and photovoltaic. With the development of technology and the expansion of industrial scale, the cost of solar thermal power generation will soon be close to or even lower than that of conventional fossil energy generation. 4. Clean and pollution free, help reduce carbon emissions. Although photovoltaic is clean power generation, the silicon wafer production link is high energy consumption and high pollution, and solar thermal power generation does not need to refine heavy metals, rare metals and silicon, production and power generation link are pollution-free, is the real clean energy.