Simulation On Optical-thermal Performance Of Receiver Based On Sparse Heliostat Field Near User Side

In order to alleviate the shortage of heating energy in urban areas and the environmental pollution caused by the existing heating system,using urban and rural land to build a mediumscale concentrating heat collection system is presented,which can be used as the charging node of the renewable energy heating network,thus can realize agriculture-light,riverside-light and fishery-light integration.Based on the MATLAB software,optical and thermal properties of the receiver under the condition of low-coverage heliostat fields are studied.The main contents and conclusions are as follows:(1)The optical efficiency model of the sparse heliostat field is proposed,and the distribution law of the optical efficiency of the heliostat field in typical days,throughout the year and at different latitudes is analyzed.The results show that the optical efficiency on typical days and throughout the year gradually rise,reach the maximum optical efficiency at 12 noon,and then gradually decrease.The annual average optical efficiency distribution law is similar to the typical day and the error is small,so the typical daily average method can be used instead of the daily average method to reduce the calculation time.Narrow north-south boundary heliostat field layout has a higher average annual optical efficiency.Reducing the vertical distance from the north and south borders of the heliostat field to the tower,and increase the length of the east-west border of the heliostat field,which helps to obtain a more stable optical efficiency throughout the year.When the heliostat spacing is selected as 10 m,it achieves higher ground coverage of the heliostat field and higher optical efficiency.(2)A method of calculating the energy flux density distribution on the receiver surface which optimizes calculation time is proposed,and the heliostats with different coordinate positions are analyzed.The results show that the calculation method without directly obtaining the energy flux density of all grid points on the receiving surface reduces the calculation time by 68.47% and effectively improve the calculation efficiency of the energy flux density.The small-scale heliostat has higher truncation efficiency and better spot uniformity.The energy flux density of the imaging spot on the receiving surface is normally distributed.The energy flux density distribution at the same time depends on the azimuth angle and the distance between the heliostat and the receiver.Under the same azimuth angle,the closer the distance,the denser the energy flux density distribution and the greater the peak energy flux density.The spot energy in the morning is mainly distributed in the right area of the receiving surface,and the spot energy in the afternoon is mainly distributed in the left area of the receiving surface.(3)The mathematical model of receiver is established by the micro-element analysis method and the equivalent thermal resistance method,and the influence of the temperature distribution of the heat absorption surface of the receiver,the temperature distribution of the molten salt and the key parameters on the heat transfer characteristics of the receiver is studied.The results show that along the flux direction of the molten salt,the average temperature and maximum temperature of the heat absorbing tube wall and the temperature of the molten salt in the heat absorbing tube gradually increase.The average wall temperature rises from 578.01 K to 1129.25 K,the highest wall temperature rises from 583.08 K to 1138.22 K,and the molten salt fluid temperature rises from 573 K to 798.11 K.The larger the inner diameter and wall thickness of the heat absorption tube,the higher the wall temperature and the temperature of the molten salt;the greater the flux rate of molten salt in heat absorption tube,the lower the wall temperature and molten salt temperature;the greater inlet temperature of the molten salt,the wall temperature and molten salt temperature also increase.(4)The ray tracing method is used to establish a model of the ground shading area and sunshine duration of the sparse heliostat field and the shading area and sunshine duration of the sparse heliostat field in typical days,throughout the year and at different latitudes are studied.The results show that the annual average shading rate and the monthly average shading rate are both in a "U" shape,and the corresponding time between morning and afternoon is symmetrical about 12 noon.As the latitude increases,the shading rate gradually increases,and the shading rate rises more obviously.The error between the all-clear weather model and the typical weather model is about 10%,so the all-clear weather model can be used to replace the typical weather model to reduce the calculation time.