Heat Transfer Characteristics Of Heat Storage Particles For Solar Thermal Power Generation

Receiver is an important component of solar thermal power generation,which has a significant impact on the thermal efficiency of solar thermal power generation system.The operating temperature range of the molten salt receiver limits the further improvement of the efficiency of the molten salt receiver.Due to its high temperature stability and chemical stability,and its working temperature can reach more than 1000℃,the new generation of solid particles is expected to replace molten salt system and become the mainstream heat transfer and storage medium for solar thermal power generation with large-scale heat storage capacity in the future,which can be used in supercritical carbon dioxide solar thermal power generation system with higher power generation efficiency.In this paper,the heat transfer characteristics of particles used in solar thermal power receiver are studied from both experimental and numerical aspects,and the heat and mass transfer law of heat storage particles is deeply analyzed.In this paper,quartz sand particles(mainly composed of SiO2),silicon carbide particles(mainly composed of SiC)and brown corundum particles(mainly composed of Al2O3)are selected as the objects.Firstly,the particle size,specific heat capacity and thermal conductivity of the particles are measured through experiments.In this paper,the experimental system of convective heat transfer of particles is built,and the convective heat transfer characteristics of the above three kinds of particles are studied by using the experimental system.The effects of mass flow rate,particle size and type on the convective heat transfer characteristics of particles are analyzed,and a new experimental correlation for convective heat transfer of particles between flat plates is constructed.At the same time,the corresponding numerical simulation model is built for the thermal experimental system,and the numerical simulation research on the heat transfer characteristics of particles is carried out to characterize the distribution of velocity field and temperature field in the particle heat exchanger.The results show that the specific heat at constant pressure increases with the increase of temperature.The specific heat range of quartz sand is 0.24-0.58(J/(g·℃)),silicon carbide is 0.31-0.76(J/(g·℃)),and brown corundum is 0.67-1.03(J/(g·℃))between 20℃ and 100℃.Under the isothermal condition,the specific heat of brown corundum is the largest,that of quartz sand is the smallest,and that of silicon carbide is the largest.The thermal conductivity of particles is affected by particle size.The larger the particle size is,the smaller the thermal conductivity is.Among the three kinds of particles,the thermal conductivity of silicon carbide is the highest,and that of quartz sand is the lowest.The thermal conductivity of 80 mesh silicon carbide is 0.1185,and that of 40 mesh quartz sand is 0.1086 w/(m·k).The experimental results show that the mass flow rate and particle size have significant effects on the convective heat transfer coefficient.When the mass flow rate increases or the particle size decreases,the wall temperature decreases and the convective heat transfer coefficient increases.At the same particle size,the convective heat transfer coefficients of SiC particles,brown corundum particles and quartz sand particles decrease in turn.On the basis of the experimental study,the calculation results are compared with the experimental correlation which is suitable for the heat transfer characteristics of rectangular channel.It is found that the large particles fit better,but the deviation of the heat transfer characteristics of small particles is larger.Therefore,a new experimental correlation which is suitable for the heat transfer characteristics of particles between plates is obtained.In this paper,the heat transfer model of the experimental platform is established,and the distribution of velocity field and temperature field in the particle heat exchanger is analyzed by CFD numerical simulation method.The results show that the particle temperature increases gradually along the flow direction of the particles.After entering the heating section,the particle temperature increases rapidly and linearly.After leaving the heating section,the particle temperature almost remains unchanged.Along the radial direction,the particle temperature decreases gradually from the wall to the axis,and the fluid near the wall has higher heat transfer degree and higher temperature.The particle flow velocity increases rapidly with the increase of the distance from the wall.By comparing the calculated results with the experimental values,the variation trend of the two is basically consistent,and the error range is within 10%.The experimental correlation can be used in the design of particle heat receiver.