Design of photovoltaic tracking system combining astronomical algorithm and inclination sensor feedback
Solar tracking of photovoltaic modules can be realized by combining photovoltaic modules and single-axis tracking photovoltaic brackets. Based on the data provided by GPS, a photovoltaic tracking system combining astronomical algorithm and inclination sensor feedback is proposed. The system adopts closed-loop control mode, introduces inverse tracking mode on the basis of positive tracking mode, and also includes fixed mode and manual mode, which will adopt different working modes according to different conditions, and can adjust the working state of flat single-axis tracking photovoltaic bracket according to different geographical location, date and weather; Its tracking Angle error in the positive tracking stage is within ±1°, which can avoid shadow blocking photovoltaic modules in the morning, evening and extreme weather, helping to improve the overall economic benefits of photovoltaic power plants.
0 Introduction
Weather conditions, day and night alternation, seasonal change, geographical differences and other natural environmental factors will have a greater impact on photovoltaic power generation, so it is crucial to improve the power generation efficiency and work stability of photovoltaic power generation system. By changing the inclination Angle of the photovoltaic module, the photovoltaic module can be directly facing the direct sunlight as far as possible, which is one of the effective ways to improve the power generation of the photovoltaic module [1]. By combining the photovoltaic module and the uniaxial tracking photovoltaic bracket, the astronomical algorithm is used to determine the position of the sun according to the data provided by GPS. The solar tracking of photovoltaic modules can be realized by controlling the inclination Angle of photovoltaic modules on the photovoltaic bracket through the action of electric push rod. However, this tracking method is relatively simple and adopts open-loop control mode, which has the disadvantages of low precision and accumulated error. Based on the data provided by GPS, a photovoltaic tracking system combining astronomical algorithm and inclination sensor feedback is proposed in this paper. The closed-loop control mode is adopted in the system, and the inverse tracking mode is introduced on the basis of the positive tracking mode to solve the problem of shadow occlusion caused by excessively large inclination of photovoltaic modules. The system has four main working modes, which will adopt different working modes according to different conditions, and can adjust the working state of the flat single-axis tracking photovoltaic bracket according to different geographical locations, dates, weather and other conditions.
1. Design of working mode of photovoltaic tracking system
The GPS module in the photovoltaic tracking system described in this paper is used to provide longitude, latitude, date, time and other data [2]. After calculating the real-time position of the sun through astronomical algorithm, the system adjusts its theoretical tracking Angle accordingly, so as to change the inclination Angle of the photovoltaic module installed on the flat single-axis tracking photovoltaic bracket, and uses the inclination sensor to feedback the actual tracking Angle of the system in real time. When the actual tracking Angle is consistent with the theoretical tracking Angle, the solar tracking of the photovoltaic module is realized. This closed-loop control method can eliminate the accumulated error through the feedback amount of the inclination sensor, so that the photovoltaic tracking system can still have a high accuracy when running for a long time. The control principle diagram of the photovoltaic tracking system described in this paper is shown in Figure 1.
The photovoltaic tracking system mainly has four working modes: positive tracking mode, inverse tracking mode, fixed inclination mode and manual mode, and will choose different working modes according to different situations.
2. Hardware circuit design
The hardware circuit of the photovoltaic tracking system is designed based on STM32 single chip microcomputer, and the hardware circuit is mainly composed of power supply unit, communication board, control unit, motor, drive unit, etc. [9]. Among them, the communication board is used to collect the data provided by GPS and the data measured by the wind speed and direction instrument and other information, and the data is processed and transmitted to the control unit, the control unit obtains the theoretical tracking Angle through the corresponding calculation, and compares the Angle with the actual tracking Angle fed back by the inclination sensor. In order to improve the function of the photovoltaic tracking system, the communication board also includes buttons for snow prevention, component cleaning and other operations, and the control unit also includes a button for enabling manual mode, clock chip and other components. The frame diagram of the hardware circuit is shown in Figure 5.
The control unit adopts the STM32F0x chip produced by ST Group, which is responsible for collecting, analyzing and calculating the feedback data of GPS, tilt sensor, wind speed and direction indicator. The STM32F0x series is based on the ultra-low power ARM Cortex-M0 processor core, which is widely used in economic control systems due to its high speed, low cost and low power consumption.
When MCU internal clock is used, the internal clock will be affected by the external crystal vibration, and the ambient temperature has a great influence on the external crystal vibration, which eventually leads to a certain error in the time of the internal clock. Therefore, in the design of the photovoltaic tracking system, an additional high-precision clock chip DS3231 was added, which contains an integrated temperature compensation crystal oscillator, and has a strong ability to adapt to external temperature changes. In addition, the backup battery can provide the battery life after the power failure of this clock chip, and the memory chip in the clock can store the time information after the power failure, thus ensuring the continuity of the time after the resumption of the photovoltaic tracking system.
The whole circuit system requires the power supply unit to provide suitable and stable power. For example, after the 220 V AC is converted to 24 V DC by the external AC/DC power supply unit, the output voltage of +5 V can be obtained by the LM2596 switching integrated voltage regulator chip and the isolated power supply, and the output voltage of 3.3V can be obtained by the SPX1117 low voltage differential voltage regulator chip. Then power the chip components with different voltage requirements. In addition, the control unit controls the forward and reverse operation of the drive motor through the H-bridge circuit [10], and uses the drive chip to monitor the current of the motor.
3. Test and analysis of experimental data
According to the above formula, the theoretical tracking Angle of PV tracking system with inverse tracking mode and without inverse tracking mode can be calculated in one day. The time of calculation was taken as May 7, 2019, the place was chosen as Zhangjiagang City, Jiangsu Province (120°E, 31°N), the width of photovoltaic modules was 1.2m, and the minimum east-west distance between adjacent photovoltaic supports was 2.5m. Plot the time-theory tracking Angle curve.
In the non-inverse tracking mode, the theoretical tracking Angle changes from 180° at night to about 90° after sunrise (05:20), which indicates that the tracking system adopts a positive tracking mode, and the photovoltaic module receives direct sunlight at this time. Then with the change of time, after sunset (18:40), the theoretical tracking Angle is fixed at 185°, which indicates that the photovoltaic bracket once again enters the night state. In the inverse tracking mode, after sunrise (05:20), the tracking system enters the inverse tracking mode from 05:25 to 07:35 after a short adjustment phase. After that, due to the limit protection of the photovoltaic module by the control unit and the limit module, the theoretical tracking Angle remains static when it reaches 135°, so a smooth straight line appears. Between 08:45 and 15:00, the theoretical tracking Angle changes linearly and rises to the maximum value, which indicates that the tracking system adopts the positive tracking mode at this time. At 15:00 ~ 16:20, also due to the reason of limit protection, the theoretical tracking Angle was maintained at 225°, and then, after the tracking system went through the inverse tracking mode for a period of time again, the theoretical tracking Angle dropped to 185°, and the photovoltaic bracket entered the night state.
In order to verify the reliability of the control unit of the PV tracking system under load operation, the field test of the control unit under load when the PV tracking system adopts the inverse tracking mode was carried out in the outdoor experimental area of a science park in Zhangjiagang City, Jiangsu Province (120°E, 31°N), and the actual tracking Angle of the PV tracking system was statistically analyzed. The test date is May 7, 2019, the width of the photovoltaic modules is 1.2 m, and the minimum east-west spacing of the adjacent photovoltaic supports is 2.5m. The time-actual tracking Angle curve of the PV tracking system on the test day is shown in Figure 7.
The actual tracking Angle of the PV tracking system with the inverse tracking mode was compared with the theoretical tracking Angle, and the Angle error histogram of the actual tracking Angle relative to the theoretical tracking Angle was drawn, as shown in Figure 8. As can be seen from FIG. 8, the tracking Angle error range between the actual and theoretical tracking angles in the reverse tracking stage (05:20 ~ 07:30 and 16:25 ~ 18:35) is ±3°, while the tracking Angle error range of the positive tracking stage (07:30 ~ 16:25) is within ±1°. It meets the design requirements of tracking accuracy within ±10° in "Standard Outline of Single-axis Tracking System for Solar photovoltaic power generation System".
The main reasons for the tracking Angle error are: the adjustment process of the electric push rod takes time, and the theoretical tracking Angle is also changing in real time, resulting in a large tracking Angle error; When running with load or when there is wind load, it will have a slight impact on the photovoltaic support structure, and even make the photovoltaic support slightly shake; Artificial errors during installation and measurement of sensors and measuring tools.
4 Conclusion
In this paper, a photovoltaic tracking system combining astronomical algorithm and inclination sensor feedback is proposed, which can achieve high precision sun tracking and overcome the influence of the change of sunlight on the photovoltaic tracking system. In view of the working state in the actual environment, the system also adds the inverse tracking mode, fixed inclination mode, manual mode and other working modes. The circuit design of the system is based on STM32 single chip microcomputer, and the test results show that when the photovoltaic tracking system adopts the inverse tracking mode for sun tracking, the tracking Angle error in the positive tracking stage is within ±1°, and the tracking Angle error in the inverse tracking stage is within ±3°, which can effectively improve the overall economic benefit of the photovoltaic power generation system.
Reference
[1] LONG Zhenhua, WANG Jinhui, PENG Xiaohong, et al. Research on Protocol Analysis Algorithm of Beidou /GPS Dual-mode Positioning System [J]. Modern Electronic Technology, 2016, 39(1): 6-8.
[2] LU G J. Research on Sun Tracking control system based on GPS [D]. Beijing: North China Electric Power University, 2013. (in Chinese)
[3] ZHU Guodong, WANG Chenglong, MA Jun, et al. Research on a high-precision Solar tracking control Device [J]. Journal of Sensing Technology, 2018, 31(6): 830-835.
[4] ZHANG Zhen, Chen Xin, HAN Xuedong. Simulation Research on Shadow Avoidance Technique of Flat Single-axis Tracking System [J]. Electrotechnical Engineering, 2018(23): 17-20.
[5] WANG Shitao. Research on solar tracking system and application based on Inverse tracking technology [D]. Harbin: Harbin Institute of Technology, 2016. (in Chinese)
[6] GUAN Yanling, ZHANG Hao, YAN Xuzhou, et al. In-situ experimental study on the effect of dust covering on the performance of photovoltaic modules [J]. Journal of Solar Energy, 2016, 37(8): 1944-1950.
[7] SHEN Yan, WANG Qi, KANG Xiaokang, et al. Simulation of body size coefficient and optimal spread Angle of solar photovoltaic panel array [J]. Special Structures, 2018, 35(6): 27-32.
[8] TAO Heng, WANG Qi. Research on Dual-path Tracking System Based on PLC [J]. Inverter World, 2018(4): 125-129.
[9] LI Yahui, WANG Qi, LI Rongmin, et al. Design of solar power generation Device based on MSP430 single chip Microcomputer [J]. Mechanical and Electrical Equipment, 2013, 30(4): 99-103.
