Study On Heat And Mass Transfer Of A Shallow Multistage Fluidized Bed Heat Exchanger Based On SCO₂ Brayton Cycle System In CSP

Large-scale thermal energy storage is an important advantage of concentrating solar power（CSP）.It’s a trend to improve efficiency and reduce cost by utilizing suitable high-temperature heat transfer fluids in CSP.Particles with particle temperatures capable of reaching 800℃and higher,is a kind of dominant thermal energy storage material with high working temperature,lower cost and environment friendly.CSP system based on solar particle receiver has great application research value.The improvement of the design level of solar receiver makes it possible to enable the use of higher efficiency power cycles.Supercritical carbon dioxide（SCO₂）Brayton cycle,as a closed cycle,with the turbine inlet temperature exceeding 700℃and the pressure exceeding 20MPa,has advantages of compact size,low water consumption and high thermal efficiency.The SCO₂ Brayton cycle system with solar particle receiver is proposed to further improve turbine inlet parameters through indirectly heat SCO₂.Particle-SCO₂ heat exchanger becomes the key component connecting concentrating system and power cycle system.The fluidized bed heat exchanger has excellent heat and mass transfer performance and lower pressure loss.It is suitable as a structural form of a SCO₂ heat exchanger.A shallow multistage fluidized bed heat exchanger cold test is established.The mass transfer characteristics are explored,which provide guidance of structure and operating parameters for the thermal test.Without reliable heat exchanger performance models,the design and cost analysis of the system is subject to large uncertainty.Therefore,a model on a SCO₂ fluidized bed heat exchanger is proposed to provide guidance for the design of particle-SCO₂ heat exchanger.At the same time,numerical simulation based on Euler-Euler Two Fluid Model is utilized to calculate the heat transfer between a bubbling bed and an immersed horizontal tube,which provide a numerical basis for the design and operation parameters of the SCO₂ fluidized bed heat exchanger.The content of the research includes the following three parts:1.In the 1st part,a shallow multistage fluidized bed heat exchanger cold test is designed and built.The critical fluidization velocity of the particle is about 0.10m/s.The fluidization curves of different static bed height are compared.The particle flow characteristics of only discharge-no feed condition and start on empty bed are analyzed.The results show that the fluidized bed has good mass transfer characteristics.2.In the 2nd part,a SCO₂ fluidized bed heat exchanger model is established.The tubes number,tube size,particle size and gas temperature are considered to analyze a 100kW heat exchanger.The results show that:Considering tube materials cost and CO₂ pressure loss,the outer diameter of tube,wall thickness and the number of tube bundles are suggested as 10 mm,2.9 mm and 97,respectively.Smaller particle size is selected for a low critical fluidize velocity to promote thermal efficiency and reduce fan power.Under given operating conditions,the thermal efficiency is 99.64%.3.In the 3rd part,numerical simulation based on Euler-Euler Two Fluid Model is utilized to calculate the heat and mass transfer between a bubbling bed and an immersed horizontal tube.Two different effective thermal conductivity models are compared.Realistic predictions of heat transfer between a tube-wall and the fluidized bed.The relationship between the instantaneous heat transfer coefficient and gas-solid flow behavior is analyzed.The effects of superficial gas velocity,particle size and bed temperature on the wall-bed heat transfer coefficient are investigated.The distribution characteristics of time-average heat transfer coefficient around tube wall are studied.