| The main challenges of large-scale solar energy utilization are low energy conversion and utilization efficiency,and the grid-connection due to the DNI fluctuation.The concentrated solar power(CSP)station coupled with a thermal energy storage system may solve the difficulty of grid integration and fluctuation.Compared with traditional heat transfer fluids,the use of solid particles as heat transfer fluid and thermal storage medium has the merits of a wide operating temperature,stable thermal properties and driving the supercritical CO2 power cycle.These advantages make it become a promising technical route to utilize solar energy on a large scale.The solid particle solar receiver is a key energy conversion device and the CSP station power generation efficiency is determined by its performance directly.However,most of the conceptions are still under lab-scale research and have various defects.The traditional gas-solid fluidized bed with excellent heat and mass transfer has drawn more and more attention from researchers.The countercurrent fluidized bed(CCFB)has the potential to combine both merits of co-current upflow fluidized and Downer,such as higher solids holdup,longer particles residence time,lower solids reflux and stable operation and so on.Therefore,CCFB may provide a new technical route for the thermal absorption of solid particles for high-temperature.To make up for the insufficiency of existing technologies,a novel high temperature,efficient,and stable solid particle solar receiver was proposed in this paper and a series of experiments and theoretical analysis was carried out.Firstly,a visualized gas-solid CCFB for ambient temperature was established.Investigations on the hydrodynamics of CCFB and its influence factors have been comprehensively studied,including various operating conditions,gas inlet arrangement,particles diameter and tube diameter.The experimental result showed that the solids holdup could exceed 9%for particles and tube diameter with 113.5 μm and 40 mm respectively.Besides,it is demonstrated that the CCFB could operate stably under extensive working conditions and the upward gas prolongs the residence time of particles and improves the solids holdup.Based on PV6M gas-solid optical fiber probe,it is the first time to reveal the solids holdup distribution and its evolution for gas-solid countercurrent.Moreover,the voltage signal of the optical fiber probe was used to analyze the characteristics of particles cluster by the threshold method.The series of studies on the CCFB hydrodynamics could fill the research insufficiency in the field of gas-solid two-phase flow and provide significant guidance to the following hightemperature solid particle solar receiver design.Based on the previous study on the CCFB in cold mold,a high-density gas-solid countercurrent fluidized bed particle solar receiver was proposed in this paper and a single tube experimental setup was constructed for high temperature.The side solids feeder was adopted to achieve the solids mass flow self-adjusting and reduce the dependence on auxiliary equipment.A middle frequency electromagnetic heating furnace with a power range of 10-40 kWel was applied in this testing.The nonlinear heat transport mechanism for gas-solid countercurrent flow under high temperature and large temperature differences was obtained.The experimental results indicated that the high-density CCFB receiver could operate stably in the fluidized state as gassolid countercurrent flow with bubbling and the average solids holdup of～40%was achieved,resulting in a particle temperature increase ranging between 101-312℃.The global wall-to-bed heat transfer coefficient with a range of 31 1-1481 W/(m2·K).It is demonstrated that the upward gas could improve the wall to bed heat transfer process and make the suspension temperature more uniform.Furthermore,the effects of the input heat flux fluctuation on the high-density CCFB receiver were studied to reveal its operating characteristics.The results showed that the high-density CCFB receiver has abilities of self-adjusting and anti-interference.The feasibility of the high-density CCFB receiver was primarily demonstrated based on a series of comprehensive hotmold experiments.An equipment scale and a particle scale numerical model were established based on the computational particle fluid dynamics and particle-resolved direct numerical simulation theory respectively.The characteristic of particle flow in the high-density CCFB receiver was studied through the computational particle fluid dynamics equipment scale model.The axial/radial solids holdup distribution and axial particle velocity profile were obtained.Besides,the characteristics of upward gas bubbles,frequency and time fraction,were investigated based on analyzing in-time solids holdup signal.The numerical results showed that the equipment scale model could capture the evolution of the gas-solid two-phase flow and obtain the process of bubble formation and rupture.Besides,the upward bubbles affect the solids holdup and axial particles velocity directly.Based on the particle scale model,the characteristics of particle stress and heat transfer and its influence factors of particle were studied,including wall heat flux,distance between particle and wall,two particles array and dynamic boundary conditions.The numerical results indicated that the CD,loc and Nu of the near-wall particle is negatively correlated with Reloc.The thickness of the temperature boundary layer is increased with the wall heat flux,and the CD,loc is almost constant and the Nu is still decreased when the particle breaks away from the temperature boundary layer.Furthermore,the influence of particles vertical array is more obvious than the particles horizontal array.Based on multiscale numerical models,the heat transport phenomenon and principle in the high-density CCFB receiver were revealed from different aspects and levels,and some parameters are difficult to obtain by experiments were replenished.It is beneficial to expand the breadth and depth of this paper.In this thesis,a high-density gas-solid countercurrent fluidized bed particle solar receiver was put forward based on the previous investigation of the hydrodynamics of the CCFB.It is the first time to achieve high-density suspension fluidization under high-temperature gas-solid counterflow.It helps to develop the control method of gassolid two-phase flow pattern.The results of this thesis improve the dispersion particle fluidized heat transport mechanism and the multi-scale numerical modelling. |
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