Study On Wind Pressure And Force Of Heliostat Structure Under Full Angle Wind Direction

Tower solar thermal power generation system is a system that reflects sunlight into the collector at the top of the heat absorption tower and transforms solar energy into electricity.Tower solar thermal power generation system is usually composed of heliostats,heat sbsorption tower,heat energy and electric energy conversion system,and auxiliary supporting system.Heliostats are one of the main components of a tower solar thermal power station,and heliostats not only occupy most of the site but also account for at least half of the investment of the power station.Heliostats are mostly built in open and gentle areas,often accompanied by strong winds.Because of its own characteristics,the heliostat structure is sensitive to wind load and has poor wind resistance stability.Under the action of strong wind,heliostat is easily destroyed.Therefore,wind resistance design of the heliostat can not be ignored.In addition,it is necessary to ensure that the sunlight is accurately projected to the distant collector during the working process of the heliostat,and its deformation must be controlled within a certain range.Therefore,the extreme values of the base shear,moment,axial force and wind pressure are key parameters in the wind resistant design.The study of the extreme value of load has been widely concerned in structural engineering,but researchers have done less research on heliostats in this respect.So the extreme value of wind load of the heliostat to be analyzed and studied has great significant.Based on the sunlight angle change,the heliostat adjusts the elevation angle and azimuth angle by using the control system to ensure that the reflected light can be accurately cast into the collector.There are many working conditions for heliostat by combining the different azimuth angles and elevation angles.The heliostat stress condition and the wind pressure of the mirror under different working conditions are very different.The wind tunnel test and field measurement of heliostat are needed to be tested many different working conditions by previous researchers,which is not only cumbersome and complicate in the process,but also needed to consider lots of working conditions with huge analysis workload,and its research results are still insufficient.But how to solve the above problems needs further research.The force(moment)of the heliostat column and the wind pressure on the heliostat mirror under wind are studied in this paper.At the same time,the method which can simplify the wind tunnel test of the heliostat and improve the test efficiency is verified.The main research contents and findings are as follows:(1)Working conditions have been selected according to the uniform design method,and wind load coefficient values are obtained by the wind tunnel test.The regression equations have been determined,and the rationality and accuracy of the regression equations are determined by evaluating the indicators.Wind load coefficient regression values are calculated by regression equations.The accuracy of the uniform design method can be comprehensively determined by comparing statistical error indicators between test and regression values.The regression equations of uniform design method are reasonable with high regression results accuracy.This means the uniform design method can be used to minimize the required test conditions and the test efficiency can be improved without reducing the data accuracy.(2)The probability characteristics of the force(moment)of the heliostat column are analyzed,and whether the the force(moment)of the heliostat column obeys the criterion for judgement of the Gaussian distribution is obtaining.The main reason for the non-Gaussian characteristics of the force(moment)of the heliostat column under some working conditions is explained by the central limit theorem.Then the peak factor method,the generalized extreme value distribution(GEV)method and generalized Pareto distribution(GPD)method in this paper are used to calculate the positive and negative extreme values of the force(moment)of the heliostat column.It is found that the GPD method is the most suitable method to calculate the force(moment)extreme value.(3)Through wind tunnel test,the variation law of mean and peak wind load of single heliostat is obtained.The variation law of mean and peak base shear,mean and peak axial force,mean and peak base overturning moment is same.The peak wind load is much larger than the mean wind load,and the working condition of maximum value of the peak wind load does not to be corresponded to the working condition of the maximum value of the mean wind load,so the distribution of the maximum value of the mean and peak wind load is necessary to be considered.And the critical wind load and the corresponding working conditions which affect the structure of heliostat are determined.(4)Through wind tunnel test,the mean wind pressure distribution of single heliostat is given.The variation law of the mean wind pressure with azimuth angle and elevation angle is studied by 10 representative pressure taps which is selected.The distribution of mean wind pressure under the most unfavorable working conditions of single heliostat is given.Then the distribution of the local shape coefficient of the mirror surface under the most unfavorable working condition of the heliostat is studied.The weak position of the mirror surface is given,and the suggested value is provided for the calculation of the wind load.(5)The probability characteristics of the wind pressure of the heliostat are analyzed,and whether the the wind pressure of the heliostat obeys the criterion for judgement of the Gaussian distribution in the literature is verified.The result show that it is reasonable to use the criterion for judgement in the literature to judge whether the wind pressure on the heliostat mirror surface obeys Gaussian distribution under different working conditions in this paper.Then the peak factor method,the generalized extreme value distribution(GEV)method and generalized Pareto distribution(GPD)method in this paper are used to calculate the positive and negative extreme values of the wind pressure of the heliostat.It is found that the GPD method is the most suitable method to calculate the wind pressure extreme value.(6)The extreme wind pressure is calculated by the GPD method,then the distribution of extreme wind pressure under different working conditions is analyzed,and the variation law of the extreme wind pressure of a pressure tap is obtained.Therefore,the mirror surface can be divided into two areas according to its variation law.The variation law of extreme wind pressure in the same area under all working conditions is similar,and the variation law of extreme wind pressure in different areas is obviously different.Then the applicability and accuracy of peak factor method,GPD method and GEV method for calculating extreme wind pressure under full angle wind direction are also compared.It is found that the GPD method is the most suitable method to calculate the wind pressure extreme value under full angle wind direction.