Tower-type CSP design specifications contain five key technical issues

Tower solar thermal power generation system is to build a tall central absorption tower on the open ground. An absorber is installed on the top of the tower. A certain number of heliostats are installed around the tower, and the sunlight is collected by the heliostat. A high temperature is generated in the cavity of the receiver at the top of the tower, and the working medium passing through the absorber is heated to generate high-temperature steam to drive the steam turbine to generate electricity. That is, the tower solar thermal power generation system utilizes a plurality of planar reflection arrays to reflect solar radiation onto a solar receiver placed on the top of the high tower, to heat the working medium to generate superheated steam, and to drive the steam turbine generator set to generate solar energy for solar thermal power generation. Converting light energy into heat, and then generating electricity through traditional thermodynamic cycles. The tower solar thermal power generation system mainly consists of a mirror field and a fixed-day system, an endothermic and heat transfer system, a heat storage system, and a conventional island power generation system. The mirror field and the fixed-day system track the sun and accurately reflect the sunlight onto the heat sink. A collector located on the tower converts the specularly reflected high heat flux radiant energy into the thermal energy of the working fluid.

The heat collecting system includes a single mirror, a concentrating device, a receiver, a tracking mechanism and the like.

The heat transfer system is mainly the heat energy collected by the heat collecting system. The heat transfer medium is used to deliver thermal energy to the thermal storage system. The heat transfer medium is mostly water, heat transfer oil and molten salt.

Photothermal power generation technology fully demonstrates the advantages of comparative photovoltaic power generation technology in heat storage and heat exchange systems. The solar heat is about to be stored. It can generate electricity at night, or it can adapt to the grid to generate electricity according to the local power load. The heat storage device is often composed of a vacuum heat insulator or a heat accumulator coated with a heat insulating material. The requirements for heat storage medium in the heat storage system are: high energy storage density, abundant source and low price, stable performance, no corrosion, good safety, large heat transfer area, good heat transfer performance of heat exchanger, and heat storage medium. Better viscosity. At present, China is studying various new technologies and materials for heat storage, and some experts have proposed the use of low-cost solid heat storage such as ceramics to achieve the effect of reducing power generation costs.

For steam turbine generator sets used in large-scale solar thermal power generation systems, since the temperature level is basically the same as that of the thermal power generation system, a conventional steam turbine can be used; the corresponding demineralized water system and auxiliary auxiliary circulating water system still need to be configured. The cooling method currently used by the condensing device is mostly air-cooled. Although the power generation system of photothermal technology is similar to the thermal power generation system, there are certain differences. Therefore, it is required to have the characteristics of frequent start and stop, fast start, low load operation and high efficiency.

Like the commissioning of traditional thermal power plants, tower-type CSP is also based on the system for sub-system debugging and complete startup commissioning:

1. As with traditional power plants, common power and chemical water production must be completed, and the entire construction begins normally.

2. Installation of mirror field and fixed day system and debugging of automatic control. As the energy source of the photothermal power plant, the mirror field needs to complete the single mirror control system and the commissioning and debugging of the actuator after the completion of the single mirror installation; after the mirror installation of the mirror field is completed, the date of the entire mirror field is fixed. The system's tracking and debugging, and the debugging of the mirror field automation, including the proportion of the mirror input in the power plant startup process, self-protection against harsh natural conditions, the test of the periodic self-test function of the mirror field, and the regular cleaning of the later operation.

3. The heat transfer system is currently divided into a single loop and a two loop heat transfer system.

3.1 Single circuit Taking hydraulic working fluid as an example, the hydrothermal tower thermal power generation technology sends the feed water to the heat absorber of the top of the tower through the feed water pump, and is directly heated and evaporated in the heat absorber to generate saturated steam to drive the steam turbine to generate electricity. The machine system generates electricity; or another superheated steam heat absorber is added at the top of the tower to superheat the high pressure steam and then drive the turbine generator system to generate electricity. This single loop is similar to a conventional thermal power system. Before the test run, the system needs to carry out corresponding water flushing and blowpipe operation of the entire steam pipeline to prevent the pipeline impurities from entering the steam turbine and causing damage to the steam turbine.

3.2 Two-circuit heat transfer system is mainly divided into molten salt and compressed air according to different heat transfer medium of heat collecting field. The main components of the currently used binary molten salt are NaNO3 and KNO3. The system flow is that the cold-melt salt at 290 °C is taken from the cold storage tank to the heat absorber at the top of the tower, heated to 565 ° C, and then returned to the hot-melt salt storage tank by gravity, and then extracted by the hot salt pump. The steam generator system generates high temperature and high pressure steam to drive the steam turbine power generation system to generate electricity. The key to the commissioning of this system includes the stable operation of the molten salt pump, the low temperature solidification of the molten salt cycle, the initial salt and molten salt of the molten salt, and the thermal insulation of the molten salt tank system. Because the molten salt is irreversible in the system once it is solidified, it is destructive to the system. Therefore, the stable control of the molten salt pump is generally designed as a variable frequency control. A boiler feed water regulating valve similar to a conventional power plant is added to the upper tower pipeline, and the molten salt temperature at the outlet of the collector is strictly controlled by the flow rate. The problem of low temperature solidification of molten salt circulation is generally more than 200 degrees Celsius according to the melting point of the molten salt. In order to avoid the solidification of the heat sink and the molten salt of the pipeline after the sun goes down, it takes a lot of energy. At sunset, the molten salt in the entire pipeline is recovered to the molten salt tank; when the system is re-run the next day, the entire pipeline is preheated by surrounding the resistance wire through the molten salt pipeline, and then melted into the melt after reaching the predetermined temperature. salt. The heat absorber is preheated by the heliostat system. Before the system is started, some of the heliostats are aligned with the heat absorber. After the temperature rises above 260 °C, the molten salt and the operating system are charged, thereby avoiding molten salt. Solidified in the system.

The initial salt and salt-in operation of the molten salt, after the entire mirror field and the heat collecting system have the operating conditions, the salt needs to be introduced into the system, and the initial salt is a powdery solid, which is initially heated by steam until solid-state melting. The salt is melted to a liquid state of 220 ° C (melting point), and is pumped by a molten salt preparation pump and then sent to an electric heating molten salt furnace, heated to 290 ° C and sent to a cold-melt salt tank for use by the system. After the corresponding quantity is prepared, after the cold-melting salt tank reaches a certain liquid level, the cycle of the solar heat collecting zone is started, and the molten salt is heated to a higher temperature (up to 565 ° C) through the collector, and is circulated and stored in the hot-melt salt tank. in. When the prepared molten salt reaches a sufficient amount, a part of the high-temperature molten salt is sent to the hot-melt salt tank, and then the electric valve is opened to the molten salt preparation tank, the high-temperature molten salt is placed in the molten salt preparation tank, and a certain amount of solid molten salt is added. It is mixed with high-temperature molten salt and melted into a liquid molten salt of 290 °C. Due to the increase of molten salt, two molten salt preparation pumps are opened and pressurized, and then passed through an electric heating molten salt furnace to a cold-melt salt tank. Thereby, the steam heating molten salt furnace and the electric heating molten salt furnace can be deactivated, and the molten salt is prepared by solar heating. With the gradual increase of the prepared liquid molten salt, the solar heat collecting zone can be gradually added to the cycle until all the solar heat collecting zones are put into circulation, and finally the molten salt preparation of the heat storage portion is completed. At this point, the entire first molten salt into the salt work is completed. Due to the low-temperature solidification characteristics of the molten salt, the condensation protection of the binary salt is necessary because once the crystal appears in the tank, it cannot be melted again, and after a large area of ​​solidification, the power station can no longer operate, and the destruction of the entire tank is irreversible. of. First, the foundation at the bottom of the molten salt tank is divided into the following parts from the bottom to the top: concrete layer, heat insulation layer, foam layer and heat-resistant (refractory) layer. Secondly, through two methods, one is through the molten salt pump to carry out the circulation in the tank; the other is to heat the protection through the electric heater in the heat storage tank. There are usually multiple sets of electric heaters in each tank. When the temperature is lowered, the electric heating system is automatically put into operation.

3.3. Steam generator system

For the two-loop system, the steam generation system exchanges heat with hot-melt salt and water to generate high-temperature and high-pressure superheated steam to drive the steam turbine unit and drive the generator to work. The system mainly includes: a preheater, an evaporator, and a superheater. The heat exchangers are all shell-and-tube type, in which the tube side of the preheater and superheater is water/steam, the side of the evaporator tube is molten salt, and the corresponding shell sides are molten salt and saturated water, respectively. A feature of a solar power plant is that the system may require daily start and stop, including a steam generation system. Of course, when the power station does not have enough heat storage, the heat exchanger should be preheated when the system is restarted every day, so that the temperature of the heat exchanger is evenly distributed before it enters the normal working state, and the high-pressure steam is generated to preheat the steam turbine and Steam line. Of course, when the temperature and pressure reach the turbine start requirement, the turbine will enter the normal starting operation. The preheating of the system requires auxiliary steam, which is generated from the remainder of the heat storage system. If the system has a large-capacity heat storage system, the system will operate 24 hours in good weather conditions, regardless of the preheating process of the steam generator system. Therefore, the two-circuit system also involves the blowing operation of the steam system, which is achieved by the high-temperature and high-pressure steam generated by the steam generator.

3.4 conventional island

The main steam generated by the steam generator is taken out from the superheater and sent to the steam turbine for work. The spent steam after the work is discharged into the condenser. After the work, the steam turbine converts the heat energy of the steam into kinetic energy and drives the generator to generate electricity. The generator converts the kinetic energy into electrical energy. The steam exhaust of the low pressure cylinder of the steam turbine is condensed into water in the condenser, and the condensed water enters the condensate pump. The condensed water after the boosting passes through the shaft seal cooler, the low pressure heater, and finally enters the deaerator. The deaerator has heating and removal. Oxygen and water storage function. After the deaerated feed water is boosted by the feed water pump, it flows through the high pressure heater into two stages into the preheater of the steam generation system, and then enters the steam generator. Complete the entire cycle. The debugging work of the entire conventional island is consistent with the traditional thermal power, and will not be introduced here. After completing the above-mentioned commissioning and operation of each system, the entire system has the conditions for production and operation after the entire system has the commissioning conditions.