Performance Analysis Of Supercritical CO₂ Solar Thermal Power System

Integrating with supercritical CO₂（S-CO₂）Brayton cycle thermal energy storage provides solar thermal power（STP）plants advantages of environmental friendliness,flexible dispatching,compact structure,potential of using air cooling and high solar-to-electricity efficiency.However,the current technology in molten salt solar tower receiver can hardly meet the heat source temperature requirement of S-CO₂cycle（>650°C）.It is necessary to develop a suitable heat transfer and storage technology and investigate the performance of integrated STP system.This thesis proposes an air-cooled S-CO₂ Brayton cycle solar tower power system based on an upper bubble fluidized bed（UBFB）particle receiver.A comprehensive system model is established,including the full working conditions models of a solar field,a UBFB particle receiver,two-tank particle storage subsystem and six layouts of S-CO₂ Brayton cycle.On this basis,this thesis investigates the effects of cycle layouts,climate conditions and subsystems characteristics on the integrated system performance,the appropriate system layout and the optimal values of key design parameters.Main contributions are as follows:In terms of the system design performance integrated with the UBFB particle receiver and different S-CO₂ Brayton cycles,the design solar-electricity efficiency of the integrated system can reach 30.64%and the thermal efficiency of the UBFB particle receiver can reach more than 85%at the design temperature of 700°C for the receiver outlet particles and20°C for the cooling air.The integrated system design solar-electricity efficiency ranking is consistent with the thermal-electricity efficiency ranking of different cycle layouts.Under the same design boundary conditions,the thermal-electricity efficiency rankings of six S-CO₂Brayton cycle layouts are:intercooling>recompression>partial cooling>split expansion>pre-compression>simple regeneration S-CO₂ Brayton cycle.In terms of cycle off-design operation,the off-design performance ranking of different cycles is different from that under design conditions and is related to the type of off-design conditions.When the air temperature increases,the off-design performance of complex cycle layouts deteriorates significantly,especially for the intercooling and partial cooling S-CO₂ Brayton cycles.Simple regeneration and recompression S-CO₂Brayton cycle perform more stable among six cycle layouts,thus they are more suitable cycles developed in dry and hot areas.In terms of the key parameters optimization,results show that the design incident radiation flux（IR）in the range of 1200～1500 k W/m²maximizes the integrated system design and annual solar-electricity efficiency.Meanwhile,improving the heliostat beam quality and power cycle efficiency contribute to higher optimal IR and overall system efficiency.However,in the optimal IR range,design particles temperature at the receiver outlet（T_p）plays the opposite effect on the design performance and annual performance of the integrated system.Increasing T_p helps to improve the integrated system design efficiency but reduces the system annual efficiency and power generation.Considering the yearly performance of the integrated system,there is no need to increase T_p higher than 750°C.Based on the analysis of the integrated system thermo-economic performance under two typical climatic conditions,it can be figured out that the solar resource distribution resource has a greater impact on the integrated system capacity factor and Levelized Cost of Electricity（LCOE）as comparing to the air temperature distribution.Compared with the recompression S-CO₂ STP system,the simple regeneration S-CO₂STP system leads to a 4%lower design solar-electricity efficiency,but a higher annual capacity factor of 1.11%～5.59%,a higher annual solar-electricity efficiency of 1.22%～3.01%and a lower LCOE of 0.34（?）/k Wh～0.78（?）/k Wh.In conclusion,the simple regeneration S-CO₂ STP system integrated with the UBFB particle receiver contributes to the best annual thermo-economic performance under the calculation conditions in this paper.This is because its lower power cycle costs and less degradation in off-design performance at high air temperatures.The component-level system model developed in this paper provides an explicit method to analyze the optimal design conditions and integrated cycle form for a given site by considering the system performance under full working conditions.