Research On Modeling And Tracking Control Strategy Of Concentrating Photovoltaic System

Concentrating photovoltaic power generation technology has become the focus of large photovoltaic power stations due to its features of low cost and high efficiency.As the third generation technology is mainly upgraded and optimized from three aspects: the concentrator with high concentrator efficiency and high uniformity,the concentrator solar cell with high conversion efficiency,and the tracking device with high tracking accuracy.Although compared with the previous two generations,concentrating photovoltaic technology has made great innovation,the tracking device of photovoltaic system generally adopts the traditional tracking method based on step size,not considering the relationship between the power generation and power consumption of the system.As a result,the system is unable to maximize the net generating capacity of the system,reducing low efficiency.Thus,this paper mainly focuses on the third aspect: the tracking device,and proposes a dual-axis tracking strategy with alternating residual variable frequency based on the theme of “energy saving and high efficiency”.Based on the basic model of solar motion tracking,this strategy is designed completely,combining with the output characteristics of concentrated solar cells.The new control strategy is mainly designed in the following three steps:The basic model of the solar motion is established,which is divided into two types: the model of solar trajectory and the model of solar radiation.In order to accurately track the sun,the position of the sun is transferred from the time angle coordinate system to the horizontal coordinate system by coordinate transformation.According to the geographical characteristics such as height angle,azimuth angle and so on,photovoltaic module’s irradiance of both the horizontal plane and the inclined plane can be calculated.aThe simulation results show that the model of solar trajectory can accurately describe the changes of the sun’s movement at the observation point,which provides a reference standard for accurate tracking of the tracking device.At the same time,the establishment of the solar irradiance model lats a foundation for analyzing the output characteristics of the triple-junction concentrated solar cell.Secondly,this paper selects triple-junction concentrated solar cell to solve the problem of low conversion efficiency of single-junction silicon cell and thin-film cell.Starting from the effect of solar spectrum on different subjunction battery materials,the output curve under the influence of irradiance and temperature was simulated in matlab simulation environment.The result shows that when the concentration ratio is 400,the conversion efficiency of the battery is the highest,in which condition the temperature will increase with the increase of irradiance,resulting in the decrease of system efficiency.In other words,even if the photovoltaic modules receive more irradiation,the maximum power output of the photovoltaic cells will show a trend of decline.This conclusion provides a theoretical basis for designing the new tracking control strategy.Traditional tracking control strategy often has the following three problems: the step size of fixed step size tracking is difficult to set,lag tracking causes the tracking devices’ motion always lag behind the sun,two motors consume too much power when synchronous tracking.Aiming at the problems in the traditional tracking control strategy,a new tracking strategy alternating residual variable frequency is proposed.Based on the model of solar trajectory,this strategy takes the "optimal irradiance value" as the first measurement condition and the "allowable deviation angle" as the second measurement condition,controlling the directional motor by comparing the algebraic relationship between the allowable deviation angle and the misfit angle.The simulation shows that the control strategy can effectively reduce the frequency of the tracking system.In the summer solstice of 2018,the total moving number of the dual axial motor is 57,which is 40.63% lower than that of the fixed-step strategy,and 28.75% lower than that of the traditional variable frequency strategy.Therefore,compared with the traditional control strategy,this strategy can reduce the number of motor rotation,reduce energy consumption and maximize the net power generation of concentrated photovoltaic system while ensuring the output power.