The Research On Heat Transfer Characteristics Of Solar Cylindrical Rod Receiver And Cylindrical Rod/Water Spiral Coil Heat Exchanger

The fourth generation of solar thermal power generation technology uses 750℃supercritical CO₂（abbreviated as s CO₂,the same below）working medium to generate electricity,power generation efficiency can be increased by 30%,the price can be reduced to0.45 yuan/k Wh.800℃high temperature receiver and supercritical CO₂ heat exchanger equipment are in the core position in the whole system,which outlet temperature and thermal efficiency directly affect the photothermal-electric conversion efficiency of the whole thermal power system.However,the existing 800℃high temperature receiver system and the s CO₂heat transfer system have some problems:The curtain type particle receiver is easy to be blown out by the wind.The high temperature flow channel of the spherical flow receiver is easy to deform and the strength of its internal support structure is very high.As a discontinuous flow medium,particles also lead to a smaller comprehensive heat transfer coefficient between particles and the coil wall of the heat exchanger in the particle/s CO₂ heat transfer system,high cost of heat exchanger.In order to effectively overcome the existing problems of particle receiver,sphere receiver and particle/s CO₂ heat exchanger,a cylindrical rod absorber is used as a working medium and a cylindrical rod receiver is proposed.Through experimental research,the basic structure feasibility of the cylindrical rod receiver is verified,the warming law of the cylindrical rod is understood,and the influence of the solar normal direct radiation irradiance and the mirror reflectance of the solar furnace on the temperature rise rate of the cylindrical rods are explored.In addition,a cylindrical rods/s CO₂ spiral coil heat exchanger is proposed to solve the heat transfer problem between high temperature endothermic cylindrical rods and s CO₂ on the basis of the experimental study of cylindrical rod receiver.In this paper,the coupled heat transfer among high temperature cylindrical rod,spiral coil,annular interlayer air and fluid in the heat exchanger is studied.A one-dimensional unsteady heat transfer model of a cylindrical rod/water spiral coil heat exchanger with water as the heat transfer fluid was established and verified by experiments.This study is of great significance for the optimization design,performance prediction and operation strategy of spiral coil-cylindrical rod heat exchanger with s CO₂ as heat transfer fluid in solar thermal power generation system.The main findings of this paper are as follows:（1）According to the experimental data in the second chapter,the maximum temperature of the cylindrical rod can reach more than 900℃in a certain time and under the condition of low DNI.304 stainless steel cylindrical rod is used as the heat absorber to meet the demand of800℃for the fourth generation of solar thermal power generation.In addition,when the average DNI is 745W/m² and the heat absorption duration is 200s,the average heat absorption efficiency of the cylinder heat absorber is 74.6%.（2）According to the analysis of the surface morphology of the endothermic cylindrical rod after high temperature concentration,the surface damage of the endothermic cylindrical rod is very serious,and the damage degree of the exposed side is greater than that of the backlit side.When the cumulative heat absorption duration of the cylinder with the highest concentration temperature is 120min,the average bulge height of the middle region on the backlit side of the cylinder is 250μm,while the bulge degree measured by light is very uneven,and the maximum bulge height is more than 300μm.（3）The experimental study of the heat transfer characteristics of the cylindrical rod/water spiral coil heat exchanger found that the surface of the cylindrical rod is coated with high emissivity paint,which can greatly increase the heat transfer of the cylindrical rod/water spiral coil heat exchanger.When the mass flow is 15.8g/s,compared to the cylindrical rod surface without high emissivity paint.The temperature difference between import and export of spiral coil can be increased by 93.3%.When the flow rate in the spiral coil is 0g/s,the temperature of the spiral coil wall rises rapidly to 260.1℃.（4）A one-dimensional unsteady state model was established based on the analysis of the experiment and heat transfer process of the cylindrical rod/water spiral coil heat exchanger,and the simulation heat transfer model was verified by the experimental data.The simulation results are in good agreement with the experimental values,and the maximum relative error is5.3%.（5）The simulation results of the cylindrical rod/water spiral coil heat exchanger show that the surface temperature and emissivity of the cylindrical rod have a greater influence on the surface radiation heat transfer coefficient and the convective heat transfer coefficient between the inner wall of the spiral coil and the heat transfer fluid.However,it has little effect on the natural convection heat transfer coefficient of the air interlayer of the cylindrical rod/water spiral coil heat exchanger.The cylindrical rod radius has little effect on the above three heat transfer coefficients.As the inlet flow of the spiral coil increases,the convective heat transfer between the inner wall of the spiral coil and the heat transfer fluid increases,and the radiation heat transfer coefficient of the cylindrical rod surface decreases accordingly.In addition,the radiant heat transfer coefficient on the surface of the cylindrical rod is much larger than the natural heat transfer coefficient in the air interlayer.