Control technology and research progress in the ho

2022-10-16
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Control technology and research progress in trough solar thermal power generation Abstract: This paper briefly describes the development status of trough solar thermal power generation system at home and abroad, and introduces the specific structure of the system in detail. Because the trough solar thermal power generation system is a strong nonlinear system, high-quality control is very important for the operation of the system. This paper makes a detailed analysis and Research on the key control problems: solar tracking control, temperature and pressure control system, and makes a comprehensive overview of the application status of various control methods in it

key words: trough solar thermal power generation; Temperature control; Tracking control

book classification number: tk513 document identification code: b

0 introduction

with China's increasing attention to energy security and ecological environment, the investment in renewable energy research and development is also increasing. Among them, solar thermal power generation technology is a large-scale technology that can be commercialized in the near future. It is not only the hot spot of energy technology development, but also the focus of international solar technology development. After continuous research in some developed countries, various forms of solar thermal power generation systems have been developed, and the single-stage capacity has developed from kW to MW. According to different types of collectors, concentrating solar thermal power generation systems can be divided into three categories: trough system, tower system and dish system [1]

trough solar thermal power generation technology is the most mature utilization technology of solar thermal power generation at present. At present, only trough solar thermal power generation has realized commercial operation. Luz company began to develop this kind of thermal power generation system in 1980 and realized commercial operation five years later. The total installed capacity of the solar thermal power plant consisting of nine trough systems, which has been in operation in California since 1991, has reached 354mw, with an annual power generation of 10twh. So far, it has operated well. Since 1998, the European Framework Program has funded Spain to develop a new generation of parabolic condensers to improve the efficiency of trough solar thermal power generation systems and reduce system costs [2]

the research on solar thermal power generation technology in China started late and has been limited to key projects of small components and materials, and the research and development is far behind some developed countries. In recent years, the research on solar thermal power generation in China has made some progress. In 2007, the first 70 kW tower type solar thermal power generation system in China presided over by academician Zhang Yaoming passed the appraisal and acceptance, marking a "groundbreaking step" in the field of solar thermal power generation in China until June 15. On the basis of comprehensive research, the Guangzhou Institute of energy, Chinese Academy of Sciences, has carried out relevant research in the field of trough solar thermal power generation and made preliminary progress

the research and development and application of trough solar thermal power generation system involves the knowledge of many disciplines and fields, and has a number of key technologies, including the development of concentrators and absorbers, the development of high-efficiency heat exchange system, thermal energy storage technology, control technology and so on. Because of the importance of control technology to the safe and efficient operation of the whole power generation system, this paper mainly studies the trough solar thermal power generation system and its control technology

1 trough solar thermal power generation system

the block diagram of acurex trough solar thermal power generation system built in Spain is shown in Figure 1 [3]. With the help of the trough shaped paraboloid concentrator, the sunlight is focused and reflected on the receiving heat collecting pipe, and the sunlight is focused and reflected on the receiving heat collecting pipe through the heat carrier in the pipe. The water is heated into steam through the heat carrier in the pipe to drive the steam turbine to generate electricity. As a backup when the solar energy is insufficient, the system is equipped with an auxiliary combustion furnace, which uses natural gas or fuel oil to generate steam

Figure 1 acurex trough solar thermal power generation system block diagram

the trough solar direct steam power generation system shown in Figure 2 has three working modes, which are called through mode, injection mode and cycle mode [4]. The water in the through mode directly drives the steam turbine to generate electricity after preheating, evaporation and overheating. This mode is the simplest, with less investment, but complex control. In the injection mode, water is injected from different places of the collector pipe. The normal operation of this mode requires the necessary test system. Due to the complexity of the system and the large system investment, this mode is no longer competitive. Circulation mode is the most competitive way at present. A water vapor separation device is installed at the end of the evaporation section of the heat collection pipeline. The water injected into the evaporation section is greater than the water that can be evaporated by the system. The excess water is pumped to the inlet section of the heat collection loop through the separator. The steam enters the superheat section of the collector pipe through the water vapor separator. This mode is highly controllable, but the existence of circulating pipelines and water vapor separation devices increase the investment of the system

Figure 2 trough solar direct generation steam power generation system

the above trough solar thermal power generation system has three circuits: one is the heat transfer liquid circuit, one is the steam circuit, and one is the cold water circuit. The heat transfer liquid usually adopts synthetic oil, which absorbs the heat of the sun to produce high temperature, and then generates the required steam in the steam circuit through the heat exchanger to drive the steam turbine to generate electricity. In order to reduce investment and improve system efficiency, a new trough solar thermal power generation system has emerged in Spain, as shown in Figure 2. The lifting piston system combines the heat transfer liquid circuit and steam circuit in Figure 1 into one, and uses water as the heat transfer medium to directly generate steam to drive the steam turbine to generate electricity, thus eliminating the need for heat exchanger devices and simplifying the system design

Figure 3 working mode of trough solar direct generation steam power generation system

to improve the efficiency and normal operation of trough solar thermal power generation system, there are two control problems involved. One is the automatic tracking device, which requires that the trough concentrator be aligned with the sun at all times to ensure the maximum absorption of solar energy from the source. According to statistics, the energy obtained by tracking is 37.7% higher than that obtained by non tracking. The other is to control the temperature and pressure of the heat transfer liquid circuit to meet the requirements of the steam turbine and realize the normal power generation of the system. For these two control problems, scholars at home and abroad have carried out research and made some research progress

2 sun tracking system

parabolic cylindrical trough focusing collector can only collect the direct light of the sun, but can't do anything about the scattering part. Therefore, the concentrating system of the collector must make the optical axis point to the sun, that is, track the sun. Because the sun is always in motion, and the natural weather changes at any time, the design of all-weather automatic sun tracking device has become a difficulty

the solar thermal power generation tracking system can be divided into two axis tracking system and single axis tracking system according to the position relationship between the incident light and the main optical axis. Two axis tracking requires the direction of the incident light and the main optical axis to be consistent; Uniaxial tracking only requires the incident light to be in the plane containing the main optical axis and focal line. Two axis tracking is designed according to the changes of solar altitude angle and declination angle. It has the most ideal optical performance and is the best tracking method, which can make the incident light consistent with the direction of the main optical axis and obtain the most solar energy. However, the equipment structure is complex and the manufacturing and maintenance costs are high. The single axis tracking type only requires that the incident light be located in the plane containing the main optical axis and focal line, and the structure is simple. The single axis tracking is generally preferred where the tracking accuracy is not high or the sun is abundant

the ways of tracking the sun can be roughly divided into two categories: photoelectric tracking and tracking according to the apparent solar motion trajectory. The advantages of photoelectric tracking are high sensitivity and convenient structural design, while the disadvantages are that it is greatly affected by the weather. According to the solar trajectory tracking, the sun trajectory is predicted mathematically, the sun trajectory is calculated accurately in theory, and the actuator is driven to track the sun according to a certain program. The disadvantage is that there is cumulative error

in article [5], the low boiling point working fluid is used as the sensing element, and the hydraulic motion is used to realize the automatic tracking of the single axis sun. Article [6] adopts an equal area five quadrant photoelectric detection device composed of two concentric circles. When the light is perpendicular to the photocell, its projection spot just completely covers five photocells after passing through a lens. When the light is at an included angle with the photocell, the projection spot must shift, and the shift of the spot on the photocell is converted into the change of photocurrent difference, realizing automatic tracking of the sun. Article [7] starts with the analysis and calculation of the law of solar azimuth change, uses astronomical time to provide accurate and reliable automatic tracking signals, uses servo motor to drive the concentrator to realize the automatic tracking of azimuth angle, and reflects the slow change of pitch angle on a specific cam surface according to its nonlinear law, and drives the cam to advance or retreat through the one-dimensional translation formed when the lead screw rotates, so as to realize the tracking of pitch angle. This one-dimensional drive two-dimensional tracking method has the new characteristics of simple structure and low cost. Article [8] combines the sensing system with the program control system, adopts the principle of timing method for program control, and uses the sensor to detect and locate the concentrator in real time, so as to eliminate the cumulative error caused by mechanical structure and other factors, and uses the stepping motor to realize the tracking of altitude angle and azimuth angle. The spring wheel automatic sun tracker invented by Professor wangcunyi of Shaanxi Normal University realizes the generalized time angle automatic tracking and generalized declination tracking, and has the advantages of low cost, no power consumption, high precision, large carrying capacity and so on, reaching the international leading level

3 temperature and pressure control system

in order to ensure the stable and normal operation of the solar thermal power generation system, the main control goal of the system with heat exchanger such as acurex is to achieve the constant temperature of the heat transfer oil at the outlet of the pipeline under different operating conditions by adjusting the flow rate of the heat transfer liquid. For the direct generation steam power generation system, the control goal is to achieve constant temperature and pressure steam output at the outlet of the pipeline according to the needs of the steam turbine generator, so that the change of solar radiation will only affect the quantity of steam at the outlet, but not the quality of steam. Due to the large variation range of solar radiation caused by turning off the power supply, the corresponding flow rate also needs to be adjusted greatly, and the dynamic characteristics (response time, delay, etc.) of the whole heat transfer loop have also changed considerably. Therefore, the conventional control method based on the simplified model of the system has poor control effect for this kind of system with a wide range of operating points. It is precisely because of the strong nonlinear characteristics of the transmission system that various modern control theories have been put into use

article [9] adopts the parameter self-tuning control method to realize the constant control of the output oil temperature. Through the step response experiment of the system, a simplified single input single output first-order transfer function model with time delay for the output oil temperature with respect to the input flow rate is established. There are three model parameters in the transfer function, which are estimated by least square regression algorithm, and then the parameters of PI controller are self-tuning based on pole assignment algorithm. The influence of solar radiation and input oil temperature on output oil temperature is compensated by parallel feedback structure and series feedforward according to the steady-state relationship. Simulation and experimental results show that the prospect is very decadent, and the series feedforward compensation method is more conducive to system identification. Article [10] adopts the multi model indirect adaptive control method. According to the operating range of the system, five different system operating points are selected, and the linear ARMAX model is used to model the system at each operating point. The switching between different models is determined by the value of the characteristic variable steady-state flow rate. LQG control is adopted respectively

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